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The National Academy of Sciences of Ukraine L. V. Pisarzhevskii Institute of Physical Chemistry Qualifying academic paper © L. V. Pisarzhevskii Institute of Physical Chemistry 2017 All rights reserved OVCHAROV MYKHAILO LEONIDOVYCH UDC 544.526.5, 544.653.3 THESIS

Photocatalytic Reduction of Carbon Dioxide by Metal (Cu, Au, Ag) – Containing Nanostructures Based on TiO2 and C3N4 Physical chemistry (02.00.04) Branch of knowledge – 102 (chemistry) Submitted to the Academic Council of L. V. Pisarzhevskii Institute of Physical Chemistry NAS of Ukraine in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The thesis contains results of own research. The use of the ideas, results and texts of other authors has a link to the corresponding source

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___________________________________________ (author`s signature, initials and last name)

Thesis Supervisor: Granchak Vasyl’ Mykhailovych, Doctor of Chemical Sciences (Habilitat), Senior Research Scientist. (last name, name, patronymic, scientific degree, academic title)

Kyiv – 2017

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Photocatalytic Reduction of Carbon Dioxide by Metal (Cu, Au, Ag) – Containing Nanostructures Based on TiO2 and C3N4 by Ovcharov Mykhailo Leonidovych Submitted to the Academic Council of L. V. Pisarzhevskii Institute of Physical Chemistry NAS of Ukraine on May, 12, 2017, in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

ABSTRACT The thesis deals of the relationships between the photocatalytic activity of nanostructured semiconductor materials based on TiO2 and C3N4 and its morphology in liquid–, gas–phase and electrochemical photo–induced processes of CO2 reduction, the investigation of features of the photocatalytic behavior of TiO2 and C3N4 porous samples modified by monometallic nanoparticles as well as bimetallic nanostructures with different spatial organization of the cocatalyst components. It is established that the photocatalytic activity of the samples of TiO2 significantly depending on its morphology in the process of CO2 reduction. The highest photoactivity was achieved by irradiation of mesoporous materials, due to the prolonged retention of reagents in the photocatalyst pores and the possibility of migration of photogenerated electrons in the system of contacting TiO2 nanocrystals, as well as TiO2 microspheres, due to the light penetration into microspheres, light’s refraction and scattering in microsphere’s volume. It is shown that the anode photoactivity of Ti/TiO2 electrodes, characterized by different structure (mesoporous, spongy, tube), increases in the row of meso– TiO2 < sp–TiO2 < t–TiO2. The high photoactivity of the photoanodes based on TiO2 nanotubes can be related to effective transfer of photogenerated electrons along the tube to the surface of titanium. The main product of photo–induced electrochemical CO2 reduction is oxalic acid, materials yield of which for 5 hours of radiation is 60±5%.

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It was found that the porous C3N4 samples, obtained by matrix synthesis, compared to the synthesized by volumetric pyrolysis of melamine C3N4 sample, exhibit a higher activity in the CO2 photoreduction to acetaldehyde and methane. This effect related due to the presence in such samples of a developed surface and porosity. It was shown the possibility of increasing of the TiO2 photocatalytic activity in CO2 reduction process by metal nanoparticles deposition on the semiconductor surface, the highest photoactivity inherent for TiO2/Cu composite due to properties of copper as electrode–catalyst for the electrochemical reduction of CO2 to СН4. It was demonstrated that during the deposition of nanoparticles Cu and Ag/Cu on MCF–C3N4 sample the rate of CH4 is increases, compared to the inital carbon nitride, but the formation of acetic aldehyde slow much down. This effect is explained to the presence of metal nanoparticles that can accumulate photogenerated electrons and promote the increased probability of multielectron CO2 reduction to methane and occurrence of the process by "carben" mechanism that does not include the formation of acetaldehyde stage. For the first time it was shown that the bimetallic nanocomposites TiO2/Au/Cu and TiO2/Ag/Cu, compared to monometallic TiO2/Au, TiO2/Ag and TiO2/Cu, at the ratio of components close to optimal (≈20% noble metal, ≈80% copper), exhibit much higher activity in the reaction of photocatalytic reduction of CO2 to methane. This could be explained due to better separation of photogenerated charges in bimetallic nanostructures and suppression of the electron–hole recombination due to the high electrical conductivity of the noble metals and its ability to act as effective transmits of photogenerated electrons to copper. It was established for the first time that the photocatalytic activity of the TiO2/Au/Cu and TiO2/Ag/Cu nanocomposites into the CO2 reduction to CH4 significantly depends on the order of the deposition of metals on the semiconductor surface. The highest photoactivity is inherent for composites with Cu bimetallic co– catalysts with "core/shell" type TiO2/Au@Cu and TiO2/Ag@Cu, obtained by initial deposition of noble metal nanoparticles, followed by deposition of a layer of catalytically active copper on them. Key

words:

photocatalysis,

carbon

dioxide,

metal–semiconductor

nanocomposites, titanium dioxide, bimetallic nanostructures, carbon nitride.

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ACKNOWLEDGEMENTS My years at the institute were years of personal growth. I learned a lot from my studying there: for the first time having plunged into the scientific world, it opened to me from all sides, which allowed me to significantly expand the breadth of my views on so many things. The gloomy post–Soviet ambiance and the harsh traditions to which all the leadership obeys unquestioningly, have harden my character, will and spirit. Absolute indifference of the directorate to my work, to my experiments and to my actions taught me to rely only on myself and fight for my place in life. The lack of research equipment at the Institute (except obsolete devices of the 1960s, which often break down) taught me how to establish a wide network of contacts with people who have access to the necessary instruments, and also contributed to understanding the value of time, which is allocated to the analysis of my samples. The arrogance and incredible ambition of most of the scientific workers surrounding me showed me how the scientist should not treat people. A lot of questions, not on the topics of my work, given at the seminars, helped to increase my erudition in related topics. The heavy and oppressive silence during my work often helped me to concentrate better while I reading publications and interpreting my experemental results. The lack of proper heating in a building at the winter contributed to an increase in the speed of my movement through the laboratory, as well as an increase in the solubility of carbon dioxide in water substrates. Doing a lot of "black work", which does not belong to scientific work (bureaucracy, transfer of heavy items, travel to different ends of the city, etc) gave me the confidence that I would be able to find a job not in the specialty in case of failure with a thesis. I'll want thank to my research advisor, Vasyl’ Granchak. I have greatly appreciated the intellectual freedom he has given me to explore research areas and questions I found interesting. The healthy indifference and the Buddhist calm of my thesis supervisor taught me to relate more easily to failures on my scientific path. I want to express my gratitude to Professor V.Ilyin for his intelligence, delicacy and help at any time, whenever I addressed to him. I am also grateful to PhD N. Shcherban for help and facilitate in carrying out this thesis. I would like to express special gratitude to A.Mishura, whose help was invaluable. His highest professionalism, high experimenter skills and deep knowledge have helped me many times over various difficulties. The greatest gratitude I want to express my family: to my wife Mariya and to my loving little Angel Maryna, who bring a smile to my face even when I am almost gave in. And I hope they will live in a cleaner world powered by green energy!

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TABLE OF CONTENTS List of Figures

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List of Tables

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List of Abbreviatios

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List of doctoral candidate's publications on the thesis content

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INTRODUCTION

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CHAPTER 1. REVIEW OF THE LITERATURE DATA

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1.1. CARBON DIOXIDE: THE MAIN PROPERTIES, APPLICATION AND LOCATION IN NATURE 1.2. CO2 PHOTOCATALYTIC REDUCTION WITH THE PARTICIPATION OF SEMICONDUCTORS

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1.3. MECHANISM OF CARBON DIOXIDE PHOTOCATALYTIC REDUCTION

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1.4. PHOTOELECTROCHEMICAL REDUCTION OF CARBON DIOXIDE

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CHAPTER 2. EXPERIMENTATION TECHNIQUES 2.1. REAGENTS AND MATERIALS 2.2. PREPARATION OF TIO2 AND C3N4 NANOSTRUCTURED SAMPLES AND COMPOSITES BASED ON THEM

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2.3. PREPARATION OF TI/TIO2 PHOTOELECTRODES

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2.4. METHODOLOGY OF EXPERIMENTS

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CHAPTER 3. INFLUENCE OF MORPHOLOGY OF TIO2 AND C3N4 PHOTOCATALYSTS ON ITS PHOTOACTIVITY IN THE PROCESSES OF CARBON

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DIOXIDE PHOTOREDUCTION. 3.1. THE IMPACT OF TiO2 SPATIAL STRUCTURE ON TITANIUM DIOXIDE ACTIVITY IN THE CO2 PHOTOREDUCTION PROCESS 3.1.1. Morphology and optical properties of mesoporous TiO2 3.1.2. Preparation, structure, and optical properties of the TiO2 microspheres and titanium dioxide nanorods 3.1.3. Investigation of the photocatalytic activity of TiO2 with

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different spatial forms in the process of liquid–phase CO2 reduction 3.2 FORMATION OF TI/TIO2 PHOTOELECTRODES WITH DIFFERENT SPATIAL ORGANIZATION AND RESEARCH OF ITS ANODE PHOTOACTIVITY IN THE

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PROCESS OF ELECTROCHEMICAL CO2 REDUCTION 3.2.1 Preparation, morphology, phase composition and optical properties of Ti/TiO2 photoelectrodes 3.2.2 Studies of the photocatalytic activity of Ti/TiO2 electrodes in the process of electrochemical reduction of CO2 3.3 MORPHOLOGY AND PHOTOCATALYTIC PROPERTIES OF C3N4 SAMPLES

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CHAPTER 4. INVESTIGATION OF THE CO2 PHOTOCATALYTIC REDUCTION PROCESS WITH THE PARTICIPATION OF METAL–SEMICONDUCTOR 124 NANOSTRUCTURES BASED ON POROUS TIO2 AND C3N4 SAMPLES 4.1. INFLUENCE OF MODIFICATION OF MESOPOROUS TIO2 BY Ag, Au AND Cu NANOPARTICLES ON ITS PHOTOCATALYTIC PROPERTIES 4.2. FORMATION OF THE C3N4/Cu NANOCOMPOSITE AND INVESTIGATION OF ITS PHOTOACTIVITY DURING THE GAS–PHASE CO2 REDUCTION

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CHAPTER 5. SYNTHESIS AND INVESTIGATION OF THE ACTIVITY OF PHOTOCATALYSTS BASED ON POROUS TIO2 AND C3N4, MODIFIED BY BIMETALLIC NANOPARTICLES, IN THE GAS–PHASE REDUCTION OF CARBON

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DIOXIDE 5.1. COMPOSITE TiO2/Au/Cu

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5.2. COMPOSITE TiO2/Ag/Cu

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5.3. INFLUENCE OF MCF–C3N4 MODIFICATION BY BIMETALLIC Ag@Cu NANOSTRUCTURES ON ITS ACTIVITY IN THE CO2 PHOTOREDUCTION 168 PROCESS GENERAL CONCLUSIONS

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REFERENCES

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LIST OF FIGURES Figure 1.1 Figure 1.2

Figure 1.3 Figure 1.4 Figure 1.5

Figure 1.6 Figure 2.1 Figure 2.2 Figure 2.3


Figure 3.1 Figure 3.2


Dependence of carbon dioxide solubility in the water from pressure p and temperature T. Schematic illustration of electronic processes in photocatalytic systems based on semiconductor particles during irradiation (A – electron acceptor, D – electron donor, А•‾ – anion–radical, D•+ – cation–radical, hν – quantum of light). The position of the VB and CB potentials (vs. NHE at pH = 7) and the value of Eg in polymorphous TiO2 modifications Change in the energy states of platinum depending on the size of the metal particles in the TiO2/Pt – CO2 system Band gaps and band positions of a) n–type semiconductors and b) p–type semiconductors relative to the redox potentials of various compounds involved in water splitting and CO2 reduction. Schematic illustration of the photocatalytic reduction of CO2 with H2O on the anchored titanium oxide species. Simplified scheme of the sol–gel formation of mesoporous titanium dioxide using gelatin as the pore–forming agent. Scheme for the synthesis of titanate nanotubes. Voltamperometric curves obtained by blowing the electrochemical cell with argon (1), CO2 for 1 minute (2), 2 minutes (3), 3 minutes (4), 4 minutes (5), 5 minutes (6) and after addition of small amounts of air (7). A copper electrode as a cathode was used, anode – glasscarbon. Schematic diagram of the preparation of PC sample for gas– phase reduction of CO2. Scheme of a laboratory installation for the process of photoinduced electrochemical reduction of CO2. Three–electrode scheme for the investigation of photoelectrochemical properties of semiconductor photoelectrodes. X–ray diffraction pattern of mesoporous TiO2 synthesized using gelatin. ICDD, card # 21–1272 (anatase). (a) the adsorption (curve 1) and desorption (curve 2) nitrogen isotherms obtained at 77 K with the meso–TiO2 sample; (b) the distribution of pores by the radius. Meso–TiO2 TEM microphotographs obtained at different magnification values. Meso–TiO2 high resolution TEM microphotographs

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31 39 42

49 63 65 70

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77 78

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Figure 3.5


Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.11




Diffuse reflection spectra of mesoporous titanium dioxide, processed according to the Kubelka–Munk ratio. Inset: spectra in the coordinates of the Tautz equation for indirect electronic transitions (D×hv)0.5. TEM microphotographs of TiO2 microspheres, obtained by the template method using polystyrene latexes as solid matrices. TEM micrographs of TiO2 hollow microspheres obtained by the template method. Insert: the electron diffraction pattern of the sp–TiO2 sample. High Resolution TEM microphotographs of initial titanate nanotubes (a) and anatase nanorods obtained by H2Ti3O7– nanotubes heat treatment at 500° C for 2h (b). The microphotographs of different scaling are presented. Diffractograms of titanate nanotubes before (1) and after (2) heat treatment. ICDD, cards # 21–1272 (anatase), # 41–192 (H2Ti3O7). Diffuse reflection spectra of anatase nanospheres (1) and nanorods (2), transformed by the Kubelka–Munk ratio. Inset: spectra in the coordinates of the Tautz equation for indirect interband electronic transitions (D×hv)0.5. Dependence of the methane formation rate in the process of liquid–phase photocatalytic reduction of CO2 from the spatial structure and phase composition of the PC. Chromatogram of the gaseous medium after irradiation of the aqueous suspension of TiO2 P25 for 2h. Diffractogram of the original titanium plate. ICDD, card #44– 1294 (Ti). SEM microphotographs of the titanium plate after ultrasonic treatment in isopropyl alcohol. SEM microphotographs of Ti/meso–TiO2 photoelectrode. TiO2 obtained by sol–gel method and deposited on a titanium plate by the immersion–stretching method. X–ray diffraction pattern of Ti/meso–TiO2 electrode. SEM microphotographs of Ti/sp–TiO2 photoelectrode. TiO2 obtained by electrochemical oxidation of a titanium plate in an aqueous solution of inorganic acids (HF:HNO3:H2O = 1:4:5) with subsequent calcination. X–ray diffraction pattern of Ti/sp–TiO2 electrode. Schematic presentation of the process of titanium plate anodizing in the presence of fluoride ions.

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Figure 3.20 A typical current–time dependence for electrochemical anodizing (12V vs. Ag/AgCl) of a titanium plate in a water– ethyleneglycol solution (C2H6O2 ÷ H2O = 9 ÷ 1) in the presence of ammonium fluoride (0.5% by weight). Figure 3.21 The diffractogram of an anodized titanium plate in a water– ethyleneglycol solution after calcination at 430° C for 4h. ICDD, anatase (card #21–1272) and rutile (card #21–1276). Figure 3.22 SEM microphotographs of Ti/tb–TiO2 photoelectrode after calcination. Figure 3.23 Diffuse reflection spectra of Ti/TiO2 photoelectrodes (1 – Ti/meso–TiO2, 2 – Ti/sp–TiO2, 3 – Ti/tb–TiO2), transformed by the Kubelka–Munk ratio. Inset: spectra in the coordinates of the Tautz equation for indirect interband electronic transitions (D×hv)0.5. Figure 3.24 Chronoamperograms of Ti/TiO2 photoelectrodes with different morphology in the LiBF4 electrolyte (0.1 M) aqueous solution, where 1 is a titanium plate, 2 – Ti/meso–TiO2, 3 – Ti/sp–TiO2, 4 – Ti/tb–TiO2. Figure 3.25 Typical linear sweep voltamperogram after deaerating the electrochemical cell with carbon dioxide (experimental conditions: a copper point electrode as a work electrode, counter electrode – glasscarbon, reference electrode – Ag/AgCl, LiBF4 electrolyte (0.1 M) aqueous solution). Figure 3.26 Chronoamperograms during irradiation of the Ti/sp–TiO2 electrode in the absence (1) and the presence of butyl– (2), n– propyl– (3), isobutyl– (4), ethyl– (5), isopropyl– (6) and methyl– (7) alcohols in the anode space of the separated electrochemical cell. Figure 3.27 Ti/tb–TiO2 electrode сhronoamperograms in the absence of electron donors in the anode space after deaerating the cathode space with argon (1), saturation of the cathode space of CO2 (2), and continuous carbon dioxide bubbling of the copper cathode (3). Figure 3.28 Ti/tb–TiO2 electrode сhronoamperogram during the process of photoinduced electrochemical reduction of CO2. Figure 3.29 High–angle XRD patterns (a) and small–angle XRD patterns (b) of obtained carbon nitride samples. Figure 3.30 Fourier transform infrared (FTIR) spectra of carbon nitride obtained via bulk and hard template synthesis. Figure 3.31 TGA curves of the obtained carbon nitride C3N4–MCF.

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Figure 3.32 Nitrogen ad(de)sorption isotherms (77K) for the obtained carbon nitride samples: C3N4–Bulk (1), C3N4–SBA–15 (2), C3N4– MCF (3). Insert: mesopore size distribution (BJH method, desorption branch of isotherm). Figure 3.33 SEM–images of carbon nitride samples: a, b – C3N4–Bulk, c, d – C3N4–SBA–15, e, f – C3N4–MCF. Figure 3.34 TEM–images of carbon nitride samples: a – C3N4–Bulk, b – C3N4–SBA–15, c – C3N4–MCF. Figure 3.35 Absorption spectra of carbon nitride samples (C3N4–Bulk (1), C3N4–SBA–15 (2), C3N4–MCF (3). Inset: spectra in Tauc coordinates for direct electron transitions. Figure 3.36 Irradiation time–dependent generation of acetaldehyde (a) and methane (b) for C3N4–samples. Figure 3.37 Chronoamperogram for the photocurrent as a function of time for C3N4–samples. Figure 3.38 Photocurrent–time curves for C3N4–MCF sample in the aqueous (a) and N,N–dimethylformamide (b) solution under deaeration by argon (1) and carbon dioxide (2). Figure 4.1 Photographs of the initial TiO2 and after Ag+ photoreduction. Figure 4.2 The absorption spectra of deaerated aqueous Na2SO3 solutions containing Ag+–ions with a immersed in it TiO2 film on the glass plate prior to irradiation (1) and after irradiation for 5 (2), 14 (3), 22 (4), 34 (5), 45 (6), 60 (7), 80 (8) min. Figure 4.3 The absorption spectra of sodium sulfite aqueous solution with an immersed TiO2 film a) in the presence of NaAuCl4 before (1) and after irradiation for 5 (2), 16 (3), 30 (4), 42 (5), 52 (6), 60 (7) min. b) in the presence of CuCl2 before (1) and after irradiation for 10 (2), 35 (3), 58 (4), 92 (5), 105 (6) min. Insets: SPR bands of gold and copper, respectively, isolated from the spectra (7, a) and (6, b). Figure 4.4 Diffractograms of meso–TiO2 (1) and metal–SC nanostructures TiO2/M on its basis, where M is Ag (2), Au (3), Cu (4). Inset: X– ray patterns with accumulation in the angular region 2Θ = 40– 47. ICDD, cards # 21–1272 (anatase), # 00–004–0783 (silver), # 04–0784 (gold), # 04–0836 (copper). Figure 4.5 HRTEM microphotographs of metal–SC composites: TiO2/Au (a), TiO2/Ag (b) and TiO2/Cu (c). Figure 4.6 TEM microphotographs of TiO2/Ag nanocomposite made in bright (a) and dark (b) field modes. Figure 4.7 Isotherms of adsorption (curve 1) and desorption (curve 2) of nitrogen, obtained at 77 K (meso–TiO2/Au sample). Inset: pores size distribution.

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Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.11

Figure 4.12



X–ray diffraction pattern of the TiO2/Cu/Cu2O composite obtained by holding an air TiO2/Cu sample at room temperature. ICDD, cards: #04–0836 (Cu), # 05–0667 (Cu2O). Influence of the amount of deposited metal in meso–TiO2/Ag (a), meso–TiO2/Au (b), meso–TiO2/Cu (c) composites on the amount of generated methane in processes of gase– (a) and liquid–phase (b) CO2 reduction for 60 min. Kinetic curves of CH4 accumulation upon irradiation of aqueous suspensions of meso–TiO2 in the presence of CO2 (1), and meso–TiO2/Au composite with saturation of only CO2 (2), and with additional H2 (3) injection into the system. Experimental conditions: volume of the solution is 2.0 ml, PC mass is 0.01 g, the amount of metal in the composite is 1% (mas.) H2÷CO2 ratio is ≈ 1÷1. Dependence of the amount of accumulated methane on the pH of the reaction medium in the process of liquid–phase CO2 PC reduction with the participation of meso–TiO2/Au. Dependence of the methane formation rate (during CO2 photoreduction with TiO2/Cu) on the ethanol concentration in the presence (1) and the absence (2) of CO2 in the system. X–ray diffraction pattern of MCF–С3N4/Cu composite. ICDD, cards #087–1526 (С3N4), #04–0836 (Cu). Spectrum of diffuse reflection of MCF–С3N4/Cu composite, transformed according to the Kubelka–Munk ratio. SEM–images of MCF–С3N4/Cu nanocomposite sample. Rates of formation of methane and acetic aldehyde during photoreduction of CO2 with water vapor using unmodified porous carbon nitride and metal–SC structure (MCF–C3N4/Cu) on its basis. X–ray diffraction pattern TiO2/Au/Cu composite. ICDD, cards #04–0784 (Au), #04–0836 (Cu). SEM–images of TiO2/Au/Cu nanocomposite. Molar ratio in bimetallic nanostructure Au:Cu ≈ 1: 3. Dependence of the rate of CH4 formation (in the process of gas–phase CO2 PC reduction) on the composition of the metallic component of the TiO2/Au/Cu composite in mole percent of the metal content. The composite obtained by co–precipitation of metals. The dashed line indicates the CH4 formation rate using unmodified TiO2.

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Figure 5.4

Figure 5.5

Figure 5.6


Figure 5.11

Figure 5.12


The rate of methane accumulation in the process of CO2 gas– phase photoreduction using mesoporous TiO2, as well as monometallic and bimetallic nanostructures based on it, different in metal deposition order on the surface of titanium dioxide. Influence of the air oxygen injection on the methane formation rate during irradiation of TiO2/Au@Cu nanocomposite in the presence of wet carbon dioxide. Molar ratio in bimetallic nanostructure is ≈ 1: 3. The spectra of diffuse reflection in the Kubelka–Munch coordinates of the output TiO2 (1) and after irradiation in the presence of Cu2+ (2), Ag+ (3) ions, and with the simultaneous content of metal ions in various molar ratios Ag ÷ Cu, namely: curve (4) ≈ 3 ÷ 1, curve (5) ≈ 1 ÷ 1, curve (6) ≈ 1÷ 3, curve (7) ≈ 1 ÷ 6. X–ray diffraction pattern TiO2/Ag/Cu composite. ICDD, card #04–0836 (Cu). Molar ratio Ag÷Cu ≈ 1÷6. SEM–images of TiO2/Ag/Cu nanocomposite. Molar ratio (Ag÷Cu) in bimetallic nanostructure is ≈ 1÷1.44. TEM–images of TiO2/Ag/Cu nanocomposite. Molar ratio (Ag÷Cu) in bimetallic nanostructure is ≈ 1÷1.44. Dependence of the rate of CH4 formation (in the process of gas–phase CO2 PC reduction) on the composition of the metallic component of the TiO2/Ag/Cu composite in mole percent of the metal content. Absorption spectra of TiO2/Ag@Cu (1) and TiO2/Cu@Ag (2) nanostructures obtained with different order of precipitation of metals. The molar ratio Ag÷Cu ≈ 1÷6. The rate of methane formation during gas–phase CO2 photoreduction using TiO2 modified by bimetallic nanostructures with simultaneous (Ag/Cu) and sequential (Ag@Cu) metal deposition. The molar ratio Ag÷Cu ≈ 1÷6. X–ray diffraction pattern of MCF–С3N4/Cu@Ag composite. ICDD, cards #087–1526 (С3N4), #04–0836 (Cu). Diffuse reflection spectrum of MCF–С3N4/Cu@Ag composite, transformed according to the Kubelka–Munk ratio. Experimental conditions: the total amount of metals in the composite is 1% (mass.), the molar ratio Ag÷Cu≈1÷6.

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LIST OF TABLES Table 1.1 Table 1.2

Table 1.3 Table 3.1

Table 3.2

Table 4.1

Table 4.2

Table 5.1

Table 5.2

Decomposition temperature of СО2 hydrates depending on pressure. The content of carbon monoxide in the gas phase as a function of temperature during the reduction of carbon dioxide by carbon Ratio of CO2 reduction products upon irradiation of oxide SC modified with metals. Potentials at copper cathode during investigation the activity of Ti/TiO2 electrodes with different surface morphology in the process of photooxidation of water. Potentials values at the copper cathode during the investigation of the activity of the Ti/sp–TiO2 SC–electrode in the process of alcohols photooxidation. Comparison of the specific surface area of TiO2–based samples before and after the modification by metal nanoparticles. Photoactivity of titanium dioxide and metal–SC nanostructures on its basis during liquid–phase reduction of carbon dioxide. Experiment conditions: І0 = 3.01∙1018 Einstein∙min–1, τirradiation – 60 min. The rate of CH4 accumulation in the process of gas–phase CO2 photoreduction using TiO2 modified by bimetallic structures with different metals precipitation order on the surface of the SC. Molar ratio Ag:Cu ≈ 1÷1.44. Quantity of products formed during gas–phase CO2 photoreduction with the MCF–C3N4/Ag@Cu sample.

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35 100

103

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LIST OF ABBREVIATIOS Eg

semiconductor bandgap

ЕCB

potential of the electrons of the semiconductor conduction band

ЕVB

potential of valence band holes of the semiconductor

ICDD

the International Centre for Diffraction Data

J

current density

VB

valence band

YC

yield by current

CB

conduction band

NHE

normal hydrogen electrode

NP

nanoparticle

SC

semiconductor

SPR

surface plasmon resonance

SEM

scanning electron microscopy

XRD

X–ray Powder Diffraction

TEM

transmission electron microscopy

TTIP

titanium tetraisopropoxide

UV

ultraviolet

PC

photocatalytic, photocatalyst

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List of doctoral candidate's publications on the thesis content

[1]

Ovcharov, M. L., et al. Photocatalytic reduction of CO2 using titanium dioxide and metal–semiconductor nanostructures made from titanium dioxide. Theor. Exp. Chem. 49, 172–177 (2013).

[2]

Ovcharov, M. L., et al. Photocatalytic reduction of carbon dioxide by water vapor on mesoporous titania modified by bimetallic Au/Cu nanostructures. Theor. Exp. Chem. 50, 53–58 (2014).

[3]

Ovcharov, M. L., et al. Photocatalytic reduction of CO2 on mesoporous TiO2 modified with Ag/Cu bimetallic nanostructures. Theor. Exp. Chem. 50, 175– 180 (2014).

[4]

Ovcharov, M. L., et al. Effect of the morphology of TiO2/Ti electrodes on photoactivity in the electrochemical reduction of carbon dioxide. Theor. Exp. Chem. 50, 218–225 (2014).

[5]

Ovcharov, M., et al. Hard template synthesis of porous carbon nitride materials with improved efficiency for photocatalytic CO2 utilization. Mat. Sci. Eng. B. 202, 1–7 (2015).

[6]

Ovcharov, M. L. (2011, October). Photocatalytic reduction of CO2 by metal– semiconductor nanostructures based on titanium dioxide. International Symposium "Nanophotonics–2011", Katsiveli, Crimea, Ukraine (S 43).

[7]

Ovcharov, M. L. (2013, March). Photocatalytic reduction of CO2 by mesoporous TiO2 and metal–semiconductor nanostructures based on it. XLIV annual conference of young scientists, postgraduates and students of IPhCh NAS of Ukraine, Kyiv, Ukraine (22–24).

[8]

Ovcharov, M. L. (2013, November). Photocatalytic reduction of CO2 by metal– semiconductor and ternary bimetallic nanoheterostructures based on mesoporous TiO2. IV International scientific conference "Nanoscale Systems: Structure, Properties, Technologies", Kyiv, Ukraine (S4–33).

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[9]

Ovcharov, M. L. (2014, March). Photocatalytic reduction of CO2 by water vapor with mesoporous TiO2 modified with bimetallic Au/Cu and Ag/Cu nanostructures. XLV annual conference of young scientists, postgraduates and students of IPhCh NAS of Ukraine, Kyiv, Ukraine (15–16).

[10] Ovcharov, M. L. (2016, March). Photocatalytic reduction of CO2 by nanostructured TiO2 and C3N4 and mono– and bimetallic composites on its basis. XLVII annual conference of young scientists, postgraduates and students of IPhCh NAS of Ukraine, Kyiv, Ukraine (14–15). [11] Ovcharov M.L. (2014, August). Modification of the surface of mesoporous TiO2 by Ag/Cu bimetallic nanostructures for improve the process of photocatalytic reduction of CO2. 2nd International research and practice conference Nanotechnology and Nanomaterials, 2014, Lviv, Ukraine (272). [12] Ovcharov M. (2016, March). Modification of titanium dioxide by mono– and bimetallic nanoparticles to improve the activity of the photocatalytic reduction of carbon dioxide by water vapor. 251st American Chemical Society National Meeting & Exposition "Computers in Chemistry", San Diego, CA. (482).

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INTRODUCTION Actuality of theme In recent years, the attention of many researchers has been attracted to the problem of activation of "small" molecules (CO, CO2, SO2, NO, etc.), which is connected both with the search for new alternative ways of obtaining various compounds, and with solving the problems of raw materials base, ecology, etc. Among the promising ways of "small" molecules transforming, the process of photocatalytic (PC) reduction of carbon dioxide to organic substances, or "artificial photosynthesis" becomes especially interesting. PC conversion of CO2, which can be controlled in the direction of valuable products obtaining (in particular methanol, methane, acetic aldehyde and a number of others), can be a promising source of organic compounds in the future. Today, methods for obtaining organic compounds from CO2 in photocatalytic systems in the presence of semiconductor (SC) materials are being studied quite actively. It is also attractive that the heterogeneous PC conversion of CO2 by SC can occur at room temperature and at atmospheric pressure. At the same time, it should be noted that the overwhelming majority of works, known from the literature, are devoted to massive samples of titanium dioxide. Nanocrystalline PCs (for example, TiO2 Evonik P25) are nonporous, which limits its ability to accumulate the initial reagents and significantly reduces the rate of of CO2 PC reduction with the participation of such materials. The number of articles devoted to the CO2 photoreduction by the participation of various nanostructured SC (TiO2, WO3, Cu2O, BiVO4 etc.), including mesoporous (with pores of 2–50 nm in diameter), is rather limited. Also, in the literature there are no systematic studies of the effect of modifying additives on the activity of these PCs, although it is known that formation of metal nanoparticles (NP) on the surface of PP can significantly increase its photoactivity. In such composite structures, metal particles can accept photographed electrons and thus reduce the probability of electron–hole recombination and help overcome the overvoltage of phase–to–phase electron

18

transfer to adsorbed substrates. On the other hand, the metal, being on the surface of the SC in the form of nanoscale particles, can act as a catalyst for the dark stages of the PC processes. Accordingly, such materials may prove promising for efficient photocatalytic systems of the CO2 conversion into organic substances. This is precisely what determines the relevance of this statement of work. Relations work with scientific programs, plans, themes The work was carried out in accordance with the research plans of the Department of Photochemistry of the L. V. Pisarzhevskii Institute of Physical Chemistry by topics “Physico–chemical foundations of the strategy for creating new ecologically important photocatalytic systems and processes for the activation of small molecules” (state registration #0107U000404) and “Development of the fundamental foundations for the creation of effective photocatalytic, photovoltaic and luminescent systems based on ultrafine particles of chalcogenides and metalates” (state registration #0113U003132). The purpose and objectives of the scentific research The aim of the work is to develop new photocatalytic systems involving nanostructured titanium dioxide and carbon nitride (C3N4) (nanotubes, nanorods, nanospheres, mesoporous samples), as well as metal and bimetallic composites based on them for neutralization and conversion of CO2. In order to achieve the goal of work, the following scientific tasks must be solved:  to obtain nanostructured PC on the basis of TiO2 and C3N4 and to investigate its morphological, spectral and photocatalytic properties in the reactions of liquid– and gas–phase CO2 photoreduction;  to obtain nanostructured TiO2 coatings on the surface of conductive substrates, to investigate its morphological and photoelectrochemical properties, depending on the preparative conditions for obtaining of SC photoelectrodes;  to develop the methods of photodeposition on the obtained SC samples of a number of metals (Cu, Ag, Au), to investigate the PC activity of the obtain

19

metal–SC composites in the CO2 photoreduction reaction under steady–state irradiation conditions, to elucidate the features of the change of the medium pH, the amount of precipitated metal and the presence of organic electron donors in the reactor volume on the efficiency of the CO2 photoreduction process;  synthesize metal–metal–SC nanocomposites, study its optical, structural and photocatalytic properties, the effect of the ratio of metals in the composition of bimetallic components on the activity of CO2 photoreduction; to determine the effect of the renewal sequence of the corresponding metal ions on the formation of bimetallic particles of different structures and to study the activity of the obtained samples during the photoreduction of CO2. The objects of investigation: nanostructured samples of titanium dioxide and carbon nitride, TiO2 coatings of different morphology on metal substrates; aqueous or hydroalcoholic suspensions of TiO2 and nanocomposites based on it; systems "PC/gas mixture" containing samples of TiO2, C3N4, or composites based on them, carbon dioxide and water vapor. The

subject

of

investigation:

the

surface

morphology,

optical,

photoelectrochemical and photocatalytic properties of nanostructured titanium dioxide films, TiO2 and C3N4 powders, metal–SC composites based on them, as well as bimetallic nanostructures with a metal component of various structures. Regularities of the CO2 photoreduction in the gas and liquid phases with water at room temperature and atmospheric pressure with the participation of the resulting PCs. Research methods: X–ray Powder Diffraction (XRD), adsorption–desorption studies of nitrogen, spectral methods (absorption and diffuse reflection in the ultraviolet (UV) and visible regions of the spectrum), scanning electron microscopy (SEM), high– resolution

transmission

electron

microscopy

chromatography, thermogravimetric analysis.

(TEM),

chronoamperometry,

gas

20

Scientific novelty of the thesis results. The dependence of the photocatalytic activity of TiO2 samples on its morphology in the process of CO2 reduction is established. The most active among studied samples are mesoporous materials and hollow microspheres, which are characterized by spatial order, developed surface and porosity. A number of photoelectrodes Ti/TiO2 of various morphologies are obtained. It is shown that in the processes of oxidation of water and alcohols, its photoactivity increases in the series: mesoporous film < sponge film < nanotubes. The use of electrodes with a nanotube structure in the process of photoinduced electrochemical reduction of CO2 leads to the formation of oxalic acid (yield 60 ± 5% by substance). A systematic study was made of the dependence of the photocatalytic activity of metal–SC composites on a number of factors, namely: the amount of metal on the surface of the SC, the pH of the medium, and the concentration of organic electron donor. The presence of optimal values of the CO2 photoreduction rate is established when the values of these factors vary. It was shown for the first time that carbon nitride obtained by the matrix method using porous silica (SBA–15 and MCF) shows a high activity in the photoconversion of carbon dioxide to acetaldehyde and methane with water compared to C3N4 obtained by voluminous pyrolysis of melamine. This effect is related both to the presence of a developed surface and porosity in the samples, and to the appearance in such materials of quantum–size effects that lead to an increase in the width of the SC band gap and, as a consequence, the growth of the energies and chemical potentials of photogenerated charges. A photochemical approach to the formation of bimetallic nanoparticles with different structures on the surface of a SC is proposed. The successive reduction of metal ions leads to the formation of a cocatalyst of the "core@shell" type. It was shown for the first time that the use of bimetallic TiO2/Au/Cu and TiO2/Ag/Cu nanocomposites leads to an increase in the rate of photochemical

21

formation of CH4 from CO2 and water in comparison with monomethallic TiO2/Au, TiO2/Ag and TiO2/Cu nanostructures. Such effect can be caused by a higher efficiency of the capture of electrons in the SC conduction band by a bimetallic cocatalyst, and also by the possibility of noble metals act as efficient conductors of photogenerated electrons to catalytically active copper.

22

Chapter 1 Review of the Literature Data 1.1 Carbon Dioxide: Main Properties, Application and Location in Nature

Physical properties of carbon dioxide. Under normal conditions, carbon dioxide – a colorless gas, odorless and tasteless with a molecular weight Mr(CO2) = 44.009, considerably heavier than air (relative density by air is D = 1.529) [1]. Carbon dioxide consists of symmetric triatomic linear nonpolar molecules with a structural formula О=С=О, with internuclear distance С–О 113 pm. In the CO2 molecule, the sp–hybridization of carbon atoms is realized: two σ–bonds are formed as a result of overlapping of two sp–hybridized orbitals of the carbon atom and two 2рх–orbitals of the oxygen atom. Two other localized π–bonds are formed as a result of the 2ру– and 2рz–orbitals overlapping of two oxygen atoms [1]. Liquid carbon dioxide is a colorless fluid that exists only under high pressure at a suitable temperature. Solid CO2 is a molecular crystals with a face–centered cubic lattice. The interpolation formula for the constant lattice of the crystal а0 (in Å) has the following form: а0 = 5.54 + 4.679·10–6Т2

(1.1)

The dielectric constant of carbon dioxide is approximately the same as the dielectric constant of nitrogen and oxygen. CO2 is a non–conducting medium, the specific electrical conductivity of liquid carbon dioxide depends on the purity of the liquid and varies from 10–16 to 10–14 S∙m–1 [3].

Physicochemical properties of carbon dioxide. One of the most important physico– chemical properties of carbon dioxide is its solubility. Gaseous CO2 is soluble in liquids. The solubility of CO2 in water is about 1:1 by volume, the saturated aqueous solution has a concentration of 0.04 M relative to

23

CO2, the pH of this solution is 3.7 [1]. Figure 1.1 shows the dependence of the solubility of gaseous CO2 in water from temperature and pressure [4].

Figure 1.1 Dependence of carbon dioxide solubility in the water from pressure p and temperature T.

In an aqueous solution of carbon dioxide, during cooling, hydrates (CO2∙8H2O) [3] are formed, the decomposition temperature of which depends on the pressure, that shown in Table 1.1.

Table 1.1 Decomposition temperature of СО2 hydrates depending on pressure Тdec, К

249

273

283

Рdec, kPa

112.2

1285

4671.6

The solubility of carbon dioxide in alcohol and aqueous–alcoholic mixture is approximately twice the solubility of CO2 in water. At 273 K, 6 kg∙m–3 СО2 dissolves in ethylalcohol, at 298 К – 4 kg∙m–3 [4]. Gaseous CO2 dissolves well in organic solvents (acetone, chloroform, etc.) The solubility of CO2 in aqueous solutions of inorganic substances is lower than in pure water, and decreases with increasing concentration of inorganic component. Liquid carbon dioxide is highly soluble in alcohols and ethers. Solid carbon dioxide is well soluble in low–temperature freezing

24

liquids, when mixed with diethyl ether, depending on the ratios of the quantities of substances, the temperature of the mixture can be brought to 175 K [3]. Chemical properties of carbon dioxide. СО2 refers to acid oxides, reacts with alkalis to form carbonates and hydrocarbons. Carbon dioxide enters an electrophilic substitution reaction, for example, with phenol [5]:

(1.2) and nucleophilic addition, for example, with magnesium compounds:

(1.3) CO2 does not burn and does not support combustion. In the medium of carbon dioxide only those elements whose affinity for oxygen is greater than carbon, for example, magnesium [1, 3] continues to burn: 2Mg + CO2 = 2MgO + C

(1.4)

In the presence of copper oxide (II) at an elevated temperature of 200°С, CO2 reacts with hydrogen to form methane: CO2 + 4Н2 = СН4 + 2Н2О

(1.5)

When CO2 is passed over hot coals is formed CO[3]: СО2 + С = 2СО

(1.6)

In the reaction (1.6), the equilibrium content of carbon monoxide in the gas mixture depends on the temperature, that shown in Table 1.2 [3]. Table 1.2 Content of carbon monoxide in the gas phase as a function of temperature during the reduction of carbon dioxide by carbon. СО2, % vol.

2

57.7

94

99.3

Т, К

723

973

1073

1273

25

Carbon dioxide is thermally stable, dissociated at very high temperatures. The percentage of dissociated CO2 at 2273 K is 2%, at 3173 K – 50%, at 5273 K – 99% [3]. In the case of dissolution of CO2 in water, carbonic acid is partially formed [1]: СО2 + Н2О = Н2СО3

(1.7)

The equilibrium of this interaction is very shifted to the left, therefore in solution most of the CO2 remains in the dissolved state. At that the concentration of hydrogen ions in aqueous solutions of CO2 practically does not depend on the pressure and content of carbon dioxide in water, with increasing the pressure of carbon dioxide from 100 to 2340 kPa and the concentration of CO2 in water from 2 to 80.5 g∙l–1, the pH of the aqueous medium consist 3.2–3.5 [3]. Carboxylic acid is a weak bicarbonate acid, in the aqueous solution it mainly dissociates with the formation of H+ and HCO3– ions. The heating leads to a shift in equilibrium aside of almost complete removal of CO2 from the solution [1]. Carbon dioxide in the chemical industry. The largest industrial consumers of CO2 are carbamide, soda ash and salicylic acid production [6]. Carbamide is formed by the interaction of CO2 with ammonia at a pressure of 20.3 MPa and at a temperature of 200° C: 2NH3 + CO2 = NH4COONH2 = (NH2)2CO + H2O

(1.8)

The world's volume of urea production is 110 million tons per year. The next big industry related to the use of CO2 as a raw material is the production of soda ash according to the Solve method: NaCl + NH3 + CO2 + H2O = NaHCO3 + NH4Cl

(1.9)

Further calcination of NaHCO3 yields soda ash. Salicylic acid is obtained by the carboxylation of phenol under pressure (the Kolbe–Schmidt reaction):

(1.10)

26

From salicylic acid get aspirin:

(1.11) CO2 in nature. CO2 in the Earth, the content of which in the lithosphere is 5.5∙1016 t [3], is present in the hydrosphere and air atmosphere (as of 2014, in the atmosphere of the Earth, the concentration of carbon dioxide fluctuates within 0.0397 – 0.0403 %vol). The role of CO2 in the life of the biosphere is to maintain the process of photosynthesis that occurs in plants. In air, carbon dioxide, in spite of its low concentration, affects the heat exchange of the planet with the environment, effectively absorbing and re–emitting infrared radiation at different wavelengths, and thus contributes to the formation of the planet's climate [7]. Natural sources of carbon dioxide in the atmosphere include volcanic eruptions, combustion of organic substances in the air and respiration of representatives of the animal world (aerobic organisms). Also, carbon dioxide is produced by some microorganisms as a result of fermentation, cellular respiration and in the process of organic remains rotting in the air. Anthropogenic sources of CO2 emissions to the atmosphere include: burning energy for heat production, electricity generation, transportation of people and goods. Certain types of industrial activity, such as the production of cement and the utilization of gases by burning them in torches, are considered as significant CO2 allocation [8]. According to recent studies, the current level of CO2 in the atmosphere is the highest in the last 800 thousand years and, perhaps, over the past 20 million years [9].

27

1.2 CO2 photocatalytic reduction with the participation of semiconductors Basic principles of semiconductor photocatalysis. Any PC process in systems involving reagents and SC PC begins with an act of absorption of light by a photosensitive reagent system [10]. The direction of electron transport in a system containing SC and two reagents with donor (D) and acceptor (A) properties, can be represented by the scheme shown in Figure 1.2. Absorption by a SC of a quantum of light (hν) with energy exceeding the band gap results width leads to the appearance of an electron–hole pair or exciton (e–...h+) (process 1). The thermal dissociation of such a quasiparticle leads to the formation of free charges – the electron of the conduction band (CB) e–CB and the hole of the valence band (VB) h+VB [11] (process 2): НП + hν → (e–…h+)

(1.12)

(e–…h+) → e–CB + h+VB

(1.13)

After the electron–hole pair formation, the necessary processes are the transfer of the electron from the SC CB to the reagent–acceptor (A) and the filling of the VB hole with the donor reagent (D) electron (process 3). As a result of the transfer of the electron, the acceptor becomes anion–radical (А•‾), and the donor is converted into a cation–radical (D•+) [12]. Subsequent reactions of primary reduction/oxidation products, А•‾ and D•+, lead to stable end products Ared and Dox.The unwanted side processes include: charge recombination (processes 4 and 5) and redox reaction between А•‾ and D•+ resulting in reproduction of the acceptor and the donor (process 6). The necessary condition for the functioning of the PC system on the basis of the SC crystals is the mutual correspondence of the energy characteristics of the components, according to which the potential SC CB should be in a more negative area in relation to the reduction potential of one of the reagents (electron–acceptor A), and the potential of VB – in a more positive region than the oxidation potential of the second reagent (electron donor D) (Figure 1.2) [11].

28

Figure 1.2 Schematic illustration of electronic processes in photocatalytic systems based on semiconductor particles during irradiation (A – electron acceptor, D – electron donor, А•‾ – anion–radical, D•+ – cation–radical, hν – quantum of light).

Photocatalytic reduction of CO2 with the participation of TiO2. SC PC system for CO2 reduction are studied more than 30 years: in the pioneering work of 1979 [13] the possibility of PC liquid–phase reduction of CO2 to formaldehyde, formic acid, methyl alcohol and trace amounts of methane was demonstrated with the participation of various SC powders. In recent years, a hypothesis has emerged about the important place of PC reaction of the carbon dioxide reduction with the participation of SC, which could occur 4–5 billion years ago, in the process of accumulation of primary biomass – the simplest organic molecules. Then there were optimal conditions for the occurrence of PC reactions with the participation of colloidal metal sulfide SC (ZnS [14], MnS [15], etc.) that could be formed in ocean waters saturated with hydrogen sulfide. High intensity of UV radiation, reducing atmosphere, high CO2 concentration in ocean waters created favorable conditions for CO2 PC reduction and the formation of the simplest organic compounds, which in future could participate in the synthesis of complex organic molecules [12]. The analysis of the literature concerning the photochemical conversion of carbon dioxide indicate that the properties of materials based on titanium dioxide have been studied in the most detail at present. TiO2 in nature is found in the form

29

of three minerals: anatase, rutile and brookite, which are polymorphic modifications, differing in the structure of the crystal lattice [16]. In the literature there are few data on the comparison of PC activity of all three crystalline modifications in the process of CO2 reducing. Thus, according to [17], the highest PC activity is inherent for anatase, for brookite there is a lower activity value, the lowest activity was shown by rutile. The main product of CO2 photoreduction by water vapor was CO, methane was found in small amounts only with the use of anatase and brookite. Irradiation of rutile did not lead to the formation of methane. Also, the authors proposed the modification of all TiO2 varieties by pre–heat treatment in the presence of helium at 220° C for several hours. Thus, it was possible to increase the CO2 photoreduction efficiency with the application of anatase and brookite by a factor of 10, while the activity of the treated brookite exceeded the photoactivity of the modified anatase. Such an effect, according to [17], is achieved due to an increase in the defectiveness level of titanium dioxide, expressed in the creation of oxygen vacancies and Ti3+ sites on the surface of anatase and brookite crystals. This can affect the activity of both dark and light reactions of CO2 activation.The maximum activity of the modified brookite can be caused by several reasons, namely, the facilitated formation of oxygen vacancies, the higher reaction rate of anion–radical СО2•‾ with adsorbed water or surface ОН– groups, as well as the possible change in the CO2 conversion mechanism by reducing of the CO and CH4 formation stages. Mixed phases based of nanosized titanium dioxide, except industrial TiO2 Evonic P25 (consisting of 20% of rutile and 80% anatase), in the process of PC reduction of CO2 are almost not studied, although it is known that the use of combinations of crystalline modifications of TiO2 plays an important role in increasing of the PC activity in the processes of photooxidation of organic compounds [18]. High PС activity of TiO2 P25 in combination with photostability, low toxicity, availability and cheapness stimulated research aimed at developing synthesis of TiO2 samples having a certain texture, phase composition, optical and

30

electrophysical properties [19]. The increased photoactivity of TiO2 P25, according to [20], is explained by the formation of composite particles whose core consist of anatase, and the shell of rutile. Contacting the crystallites of these polymorphous modifications leads to the appearance of a spatially charged layer that facilitates the transfer of photogenerated holes from the anatase to rutile, and further to the surface of the PC. At the same time there is a reciprocated motion of the electrons. Thus, the high PC activity of the TiO2 Evonic P–25 is a consequence of the high efficiency of the separation of photogenerated electron–hole pairs. According to the article [21], the main product of PC CO2 reduction in the presence of hydrogen with the participation of industrial TiO2 Degussa (Evonic) P25 is methane, as well as small admixtures of ethane and carbon monoxide at approximately the same concentration. However, in the absence of H2 in the reactor C2H6 was not formed. The authors of work [22] have shown that the main product of PC reduction of carbon dioxide with the participation of Evonic P25 is methane, and ethan was also found in small quantities, similar to the publication [23]. In the work [24], the authors investigated CO2 photoreduction at irradiation with a 75–watt light–emitting mercury lamp as a TiO2 P25 aqueous suspension and composites based on it with various doping admixtures (RuO2, Nb2O5, V2O5). The highest efficiency was shown by the sample of undoped titanium dioxide, however, the velocity of the process was low: yields of formic acid, formaldehyde and methanol were 1.46, 0.17 and 0.064 μmol∙h–1. Rutile NP modified with anatase nanorods (with (010) facets), are demonstrated high PC activity in the process of CO2 reduction to CH4 in comparison with individual anatase rods due to improved charge carriers separation [25]. It is also possible to increase the efficiency of CO2 PC reduction by using brookite in combination with anatase instead of rutile [26]. The highest activity in carbon dioxide photoreduction process with water vapor inherent to anatase– brookite mixture with a ratio of 75% –25%, respectively, the possible reasons of which the authors call:

31

1. A more negative value of the brookite CB potential (Figure 1.3), which facilitates the transfer of electrons from brookite to anatase, thereby increasing the separation rate of photogenerated charges. 2. With indicated anatase÷brookite ratio the formation of defects on the brookite is facilitated, which can increase the number of trapped electrons. 3. The distorted phase separation surface of titanium dioxide phases can facilitate interphase transfer of photogenerated electrons, thus reducing the degree of electron–hole recombination.

Figure 1.3 The position of the VB and CB potentials (vs. NHE at pH = 7) and the value of Eg in polymorphous TiO2 modifications.

The main product of PC CO2 reduction was carbon monoxide, methane was also registered in small quantities. At the same time, the efficiency of the most active sample exceeded the activity of the industrial TiO2 Evonic P25. Thus, the interaction between anatase and brookite is more effective than between anatase and rutile [26]. The efficiency of PC CO2 reduction was also investigated by comparing PC activity of single crystals of rutile TiO2 (100) (ends with Ti) and TiO2 (110) (ends with O) [27]. Careful kinetic experiments have shown that the surface of TiO2 (100) exhibits a much greater PC activity, compared with the surface of TiO2 (110). Thus,

32

the TiO2 (100) surface has enhanced reducing properties in the process of carbon dioxide photoreduction. Efficiency of CO2 PC reduction strongly depends on the ratio of H2O÷CO2 [28]. During irradiation of deposited on a porous glass titanium dioxide, the activity of the reduction process increases with the increase in the number of water vapor in the reaction medium; however, an excessive amount of water suppresses the reaction rate. PC activity of the CO2 reduction process also depends on the size of the titanium dioxide crystals. The authors of the publication [29] made a series of analyzes with crystallites of anatase, obtained by sol–gel method, with diameter from 4.5 to 29 nm. The results showed that the size of crystallites plays an important role in nano–PC based on TiO2. With a decrease in the size of the TiO2 NP, higher yields of the main reaction products – methanol and methane – were obtained. The optimal particle size corresponding to the highest outputs of both products was 14 nm. For a crystal of less than that size, the activity of the photoprocess is decreased, probably due to changes in the optical and electronic properties of TiO2 crystals. The optimum size of TiO2 particles, according to [29], is determined by the resulting effect of the competing effects of the surface area, the dynamics of charge carriers, and the absorption of light. Saladin [23] studied the effect of temperature on the efficiency of the CO2 PC reduction process in the presence of an aqueous suspension of TiO2 Evonic P25. Experiments were carried out at three temperature regimes: 298 K, 373 K and 473 K respectively. According to the results of [23], with an increase in the temperature of the reaction medium from 298 K to 373 K, the yield of the main product – methane – slightly increases from 7 to 9 μmol∙g–1∙h–1. Further increase of temperature to 473 K leads to an increase in the rate of accumulation of methane up to 10 μmol∙g–1∙h–1. Consequently, the temperature is characterized by a weak influence on the process of PC CO2 reduction.

33

Thus, it can be summarized that the PC activity of TiO2 during the CO2 reduction process depends on a large number of factors, namely: chemical composition, PC surface condition, titanium dioxide particle size, ratio of carbon dioxide and reducing agent concentrations, etc. However, even with the same chemical composition, TiO2 activity can vary widely, depending on the method and conditions of preparation of PC. The reasons for this may be the change of dispersion and porosity structure, crystallochemical forms and other factors that significantly affect the sorption properties of samples, the efficiency of the separation of photogenied charges, its recombination and the course of the following dark stages of PC CO2 reduction process.

Influence of modification of semiconductors by metals and bimetallic NP on the process of photochemical CO2 reduction. Despite a large–scale study of the CO2 photoreduction process with the participation of various TiO2 forms, effective values of conversion of carbon dioxide to useful products have not yet been achieved. There are several significant drawbacks in TiO2, namely: 1. Light absorption is limited to UV (which occupies a small part of the sunlight spectrum ( Rh> Pt> Au> Cu> Ru. In addition to CH4, there were also insignificant amounts of ethane, methanol, acetic (concentration increased with modification of TiO2 by rhodium, copper and gold) and formic acid [47]. However, the publication does not contain data on a mechanism for the such effects of cocatalysts depending on its electronic, structural, surface and other properties. An investigation of the Dzhabiev [48] showed that the modification of TiO2 and SrTiO3 by a number of metals leads to a significant change in the selectivity of

35

the CO2 photoreduction process during irradiation of aqueous suspensions of metal– SC composites saturated with CO2 (Table 1.3).

Table 1.3 Ratio of CO2 reduction products upon irradiation of oxide SC modified with metals [48]. Photocatalyst

Ratio of final products (CH4 : Н2 : СО)

Pt/TiO2

1.2 : 8.0 : 0.0

Pt/SrTiO3

1.0 : 1.0 : 0.0

Rh/SrTiO3

1.0 : 250.0 : 0.0

Ag/SrTiO3

1.0 : 7.0 : 15.0

Pt–Ru/SrTiO3

1.0 : 6.0 : 1.2

As shown in the analysis of literary data, in many other works as modifiers used noble metals, the position of the Fermi level which is below the position of SC CB, which thermodynamically allows the transfer of photogenied electrons from SC to metal [33]. Thus, irradiation of titanium dioxide with deposited metallic ruthenium leads to the reduction of CO2 by water to formaldehyde, formic acid and methanol [30], while the modified TiO2 activity is higher than an individual titanium dioxide. However, if the obtained TiO2/Ru is applied to highly dispersed silica, the activity of the PC completely disappears, in the opinion of the authors of the publication, due to the formation of Ti–O–Si bonds. One of the first investigations of PC based on TiO2, modified with platinum or palladium, was performed in an aqueous solution of carbonates/bicarbonates [44, 49], the main products being formaldehyde and formic acid. Anpo established the important role of platinum in increasing TiO2 activity and selectivity in relation to the methane formation [50]. The author carried out dispersion of titanium dioxide on Y– zeolites, which led to the formation of TiO2 with Ti in the tetrahedral, more active in the formation of methanol, coordination, and TiO2 with Ti in octahedral

36

coordination, which contributes to the formation of methane. Modification of any TiO2 sample with platinum and its subsequent radiation in the presence of water and carbon dioxide leads to the restoration of CO2 mainly to methane. It was shown [28] that the modification of nanodispersed titanium dioxide powders and TiO2 nanotubes by platinum leads to the formation of methane as the main product during the PC reduction of CO2. There is a dependence of the influence of the amount of deposited Pt on the SC surface on the activity of the PC. The most active, according to [51], is platinum content in the amount of 0.12% by weight. Reducing or increasing the amount of deposited modifier leads to a significant decrease of sample photoefficiency. The modification of TiO2 with palladium (0.5% by weight) [52] and subsequent composite irradiation with water, it is also observed CO2 reduction predominantly to methane with a small amount of CO. Long–term irradiation leads to the deactivation of PC, according to the authors, due to oxidation of palladium in PdO [52]. Detailed studies of the CO2 photoreduction (under UV irradiation λ = 254 nm) in the presence of water and TiO2/Au were carried out [53]. The utilization of unmodified TiO2 leads to the formation of methane only. The deposition of the gold NP on the SC surface leads to the appearance of ethane, methanol and formaldehyde in the reaction medium (besides CH4), wherein the composite PC retained its activity during irradiation with visible light (λ = 532 nm). This fact is explained by the authors by the possibility of visible light absorption due to the surface plasmon resonance of gold. The rate of PC product formation using TiO2/Au was exceeded more than 20 times compared to initial titanium dioxide [53]. Deposition on TiO2 powders (obtained by the sol–gel method) of silver in various concentrations leads to an increase the efficiency of CO2 photoreduction to carbon monoxide, methane and methanol [54], the most active was PC with a silver with mass fraction of 7%. Possible reasons for increasing TiO2 activity by modifying its with Ag NPs are:

37

- the creation of impurity levels within the titanium dioxide band gap, thereby shifting the TiO2 absorption edge towards the long–wave part of the spectrum. - The reduction of Ag+ on the TiO2 surface not in form of randomly located atoms, but of metallic clusters. According to [54], in this case, decreasing of the electron–hole pair recombination will be achieved due to the formation of a Schottky barrier in the region of the metal–SC contact, which increases the value of separation of photogenic charges. CO2 can be reduced to methanol in the presence of TiO2/Ag, stabilized in the cavities of the Nafion membranes [55]. CH3OH is also the final product of irradiation with UV–light of titanium dioxide modified with silver (samples deposited on fiber optic rods) in the presence of gaseous CO2 and H2O [30]. It is known that Cu is often used as a cathode in the electrochemical conversion of CO2 [56–59], because, due to its unique properties, it is possible to carry out a process of carbon dioxide electroreduction to hydrocarbons in aqueous electrolytes at room temperature. That's why copper is widely used as a metal co– catalyst in the process of CO2 PC reduction. At irradiation (λ = 254 nm) of 0.2 M aqueous NaOH saturated with CO2, in the presence of TiO2/Cu, methanol is formed in the system [60]. The amount of methyl alcohol increases with an increase of the content of copper deposited on TiO2 surface, and reaches a maximum (≈120 μmol∙gcat–1 for 6 hours of irradiation) with a copper fraction of copper of 2% mass relative to SC. Methanol can also be formed by irradiation of titanium dioxide modified with Cu2+ ions by impregnation [61]. Dispersion of titanium dioxide on mesoporous silica and subsequent irradiation of such PC in the presence of carbon dioxide and water vapor leads to the formation of CO [62]. The investigation of copper depositing on a TiO2–SiO2 surface with subsequent radiation under the same conditions showed that instead of methanol, carbon monoxide and a small amount of methane are formed. The highest activity showed a sample with copper content of 0.5% mass.

38

According to literature data [63], the preparation conditions of PC significantly affect on the TiO2/Cu photoactivity during the process of CO2 reduction by water vapour. The deposition of copper on TiO2 Evonic P25 with subsequent low– temperature treatment in various regenerating media forms a PC with different surface defects compositions and copper in different valence states: on the initial TiO2/Cu composite and after its heat treatment in helium and hydrogen media, Cu2+, Cu+ and Cu+/Cu0 states of copper dominate, respectively. Compared to unmodified titanium dioxide, treatment in H2 environment results in a 10–fold increase in the rate of CO accumulation, and almost 200–fold increase in the rate of CH4 formation. According to the authors, a significant increase in activity may be caused by the following factors [63]:  formation of surface defects that promote the CO2 adsorption;  the presence of copper in the Cu+/Cu0 state, which provides better separation of photogenied charges, since the presence of Cu+ centers leads to the accumulation of photoelectrons, and Cu0 – to the absorption of photogenied holes simultaneously. However, as recent studies show, pre–treatment by high pressure hydrogen promotes the formation of so–called "black TiO2", which is capable of absorbing visible light, and thus the PC activity of titanium dioxide is increased [64]. Copper NPs can act not only as centers for CO2 reduction, but also to interact with photogenerated VB holes [65]. It was shown that upon irradiation of TiO2/Cu aqueous suspension, saturated with carbon dioxide, methanol, formaldehyde, CO, and Cu2+–ions are formed in the reactor, thus confirming that copper also serves as an electron donor [65]. There is a very limited amount of literature data about the effect of the metal NPs size on the PC activity of composite in the process of CO2 photoreduction. A group of authors [66] received films from one–dimensional (1D) structured TiO2 nanocrystals coated with low–frequency controlled platinum (0.5–2 nm) by gas– phase deposition. TiO2/Pt films showed high activity in the process of PC reduction

39

of carbon dioxide by water vapor to methane under UV–irradiation (λ = 250–388 nm). The efficiency of the process depended on the size of the Pt NPs, the highest rate of CH4 accumulation (1361 μmol∙gcat–1·h–1) observed at the size of the platinum particles ≈ 1 nm. In such conditions, the quantum yield of the reaction is 2.41%, which is one of the largest values for the CO2 photoconversion according to the literature data. Wherein, the increase or decrease of metal particles leads to a diminution of the PC activity of the CO2 reduction to methane. The authors explain this effect by the energy features of the metal depending on its size [66] (Figure 1.4): for very small particles of platinum ( Cu > Ru > Fe > Co > Ni. Precipitation of bimetallic alloys at 1÷1 ratio on the surface of titanium dioxide nanotubes made it possible to

41

achieve a twofold increase in the total yield only in the case of the use of the Cu/Pt alloy. When TiO2 was modified with Ni/Pt and Co/Pt alloys, the activity of the obtained samples didn`t differ much from the activity of monometallic cobalt and nickel composites, and in the case of using an iron–platinum alloy, the efficiency of the process decreased practically to zero. A significant impact to the photocatalytic activity of composites is consist of variation in the ratio of the composition of metals in the alloy: the efficiency of the CO2 photoreduction increased more than twofold during the transition from the ratio 1÷1 to 1÷2 in the Cu/Pt alloy [70]. It should be noted that a similar change in the ratio of metals in the Ni/Pt alloy did not lead to a visible change in the velocity of the investigated process [69]. Bimetallic systems based on platinum and copper, however, as a "core–shell" type, photochemically deposited on the surface of TiO2 Evonic P25, greatly increase the activity of individual titanium dioxide during photocatalytic reduction of carbon dioxide to methane (~ 30 times) and carbon monoxide (~ 4 times ) [71]. A significant increase in reactivity by the authors is explained by the role of platinum as a mediator of photogenerated electrons to catalytically active copper, which acts as the center of the conversion of CO2 to CH4 and CO. Summarizing data on the modification of TiO2 by metals, it can be concluded that the deposition of metals on the surface of titanium dioxide significantly increases its activity in the process of CO2 photoreduction to CO and the simplest organic compounds due to the complicated complex interaction of many factors. It can be noted that the modification of SC by metals and its combinations is one of the key factors to improving the efficiency of PC CO2 reduction. СО2 photoreduction with other semiconductors. In addition to various SC systems based on titanium dioxide, a wide variety of other SC are also active in the PC reduction of carbon dioxide. Thus, in one of the first papers [13] concerning of CO2 photoreduction by water to formaldehyde and methanol, a large number of powder SC materials were investigated, upon irradiation of which the yield of CH3OH decreased in the following order: SiC, CdS, GaP, ZnO, TiO2 and WO3. The sequence of

42

CO2 photoreduction activity to formaldehyde increased in a row: SiC, GaP, TiO2, ZnO, CdS. For each SC, the authors found a direct correlation between the yield of methanol and the potential of SC CB. Thus, silicon carbide with the most negative CB level (Figure 1.5) was the best PC.

Figure 1.5 Band gaps and band positions of a) n–type semiconductors and b) p– type semiconductors relative to the redox potentials of various compounds involved in water splitting and CO2 reduction [73].

In subsequent studies of CO2 photoreduction with the participation of SC powders, the authors used experimental approaches using either aqueous suspensions of SC or surfaces that were coated with such materials. As shown by the article authors, during the irradiation (70–W high–pressure mercury lamp) a suspension of strontium titanate SrTiO3 through which the CO2 was passed at a rate of 164 ml / min, the formaldehyde and methanol formation velocities were 0.05 and 7.16 μmol∙h–1, respectively [72]. Irradiation of the WO3 suspension under similar conditions results of photoreduction of CO2 to HCOOH and CH3OH at rates of 5.04

43

and 0.21 μmol∙h–1, respectively. Other materials (TiO2, CaTiO3, BaTiO3/HgS, ZnO/TiO2, SiC) showed significantly lower rates of CO2 photoconversion [72]. C3N4 is a promising PC, which has attracted close attention since the notification about the possibility of obtaining hydrogen by water decomposition under irradiation of graphite–like carbon nitride (g–C3N4) with visible light [74]. C3N4 is characterized by high thermal stability, chemical resistance, low toxicity, the possibility of influencing on electronic properties by doping [75–76]. In addition, the band structure of carbon nitride, where the potentials of the CB and VB are –1.3 eV and 1.4 eV, respectively [77], contributes to both the reduction of CO2 and the oxidation of water simultaneously. Thus, it was shown that carbon dioxide can be reduced by irradiating of C3N4 in the presence of reducing agents to CO [78], CH4 and acetaldehyde [79], methanol and ethanol [80], formic acid [81]. At the same time, the efficiency of many PC processes involving C3N4 remains low. Today, large–scale studies are being conducted to increase the efficiency of carbon dioxide photoconversion by creating of various C3N4–based composites: metal–SC [82, 83], hybrid systems with metal–organic complexes [84, 85], organic acids [86]. In addition, the photocatalytic efficiency of C3N4 is limited by its low specific surface, due to harsh preparation conditions [87, 88]. For ZnO it is inherently a high activity in the process of photocatalytic reduction of CO2 to methanol, which exceeds the activity of TiO2 when the aqueous SC suspension is irradiated with laser r(wavelength of 355 nm) [35]. Thus, CH3OH was the main product of the reaction, insignificant amounts of acetic acid were also detected in the liquid phase, and methane and carbon monoxide in the gas phase. However, with prolonged irradiation of such a suspension, a decrease in the methanol yield is observed, which indicates the instability of such compound under the reaction conditions. The PC activity of zinc oxide during the reduction of CO2 depends on the morphology, microstructure, crystal size and crystal orientation. Thus, the photoactivity of two types of ZnO, obtained both by thermal decomposition of a

44

precursor (porous plates of zinc oxide) and hydrothermal deposition (hexagonal ZnO) was investigated [89]. Porous sample of zinc oxide showed a large photoactivity of CO2 reduction to CO and CH4 (product yields are 76.35 та 20.52 ppm·h–1 respectively), while the activity of hexagonal ZnO turned out to be much lower (СО – 44.68 ppm·h–1, СН4 – 1.57 ppm·h–1). Thus, it should be noted that the PC activity of a SC material is determined not only by the chemical composition, but also by the crystallographic and structural parameters that were acquired by the sample during its synthesis. Gallium oxide Ga2O3 is a promising PC of the CO2 reduction due to a significant negative value of the CB potential (–1.449 V [90] relative to NHE at pH = 7). According to the literature [90], high–porosity gallium oxide with macro– and mesopores, synthesized by the template method, shows a high, compared to the industrial sample β–Ga2O3, activity in the CO2 photoreduction by water vapor to carbon monoxide and methane. The values of the samples activities are correlated with the CO2 adsorption on its surfaces. It is known that the Eg value of CdS (2.42 eV) [91] and especially Bi2S3 (1.2 eV) [92] is much narrower compared to TiO2, and the values of the CB potentials are more negative (ECB = – 0.95 V for CdS relative to NHE at pH = 7 [93], ECB = –0.76 V for Bi2S3 [93, 94] relative to NHE at pH = 7) compared to titanium dioxide of anatase modification (ECB = – 0.53 for NHE at pH = 7 [10]). Thus, these SC materials, due to its properties, are widely studied in the process of PC reduction of carbon dioxide. CdS crystals with a modified thiol surface and without modifications are used in the process of CO2 photoreduction in various solvents [95]. The main products of the process using modified samples of cadmium sulfide and isopropanol as an electron donor were formic acid, carbon monoxide, the total yield of products after 7 hours of irradiation consist 2 μmol. Bi2S3 in combination with CdS exhibits a high PС activity during the СО2 photoreduction to methanol upon visible light irradiation [96]. Moreover, the addition of 15% (mass.) of Bi2S3 to CdS increased the yield of methanol three–fold compared to CdS, and 2–fold with compared to Bi2S3.

45

WO3–X (tungsten oxide) in the form of ultrathin nanorods is used for gas– phase photoreduction of CO2 by water vapor to methane [97]. BiVO4 shows a high yield of ethanol in the process of CO2 PC reduction under visible light irradiation, according to [98]. Synthesis of the PC by the hydrothermal method with microwave irradiation using two different surfactants (cetyltrimethylammonium bromide and polyethylene glycol) leads to the formation of two different crystalline phases – monoclinic (much more active, Eg = 2.24 eV) and tetragonal (Eg = 2.56 eV) [98]. Selective PC reduction of carbon dioxide to methanol is possible using bismuth tungstate [99]. Hollow Bi2WO6 microspheres exhibit photoactivity higher more than 25–fold compared to the industrial bulk sample of the PC, and its using in three cycles of the CO2 photoreduction process does not lead to a yield decrease of the final product. The increased PC activity of modified samples of bismuth tungstate in comparison with bulk samples is explained both by the presence of a large number of surface active centers and by the possibility of adsorbing on the surface a greater amount of carbon dioxide [99]. InTaO4 is widely used for the PC splitting of water to hydrogen under irradiation with visible light [100], while the photoactivity of such SC during the CO2 photoreduction is almost not investigated. There are data, that irradiation of a saturated CO2 aqueous solution of potassium hydrogen carbonate (at a concentration of 0.2 M) in the presence of polycrystalline InTaO4 leads to the formation of methanol in the system, but the process is extremely low– efficiency [101]. In recent years, PC based on niobium for photoreduction of carbon dioxide are widely studied. The samples of InNbO4 [102], KNb3O8 [103], HNb3O8 [103, 104], and NaNbO3 [105, 106] were obtained and investigated, but these SC are characterized by low activity, the main reduction product is CH4. Cu2O NPs deposited on silicon carbide were irradiated with visible light in the presence of gaseous CO2 and H2O [107]. The combined PC Cu2O/SiC was the most

46

active sample in the process of CO2 PC reduction to methanol in comparison with the individual components – copper (I) oxide and silicon carbide. The examples given above show the availability of SC with PC activity during the conversion of CO2 to useful compounds, which in some cases is higher compared to TiO2, especially in the visible region of the spectrum. However, the data presented in the literature were obtained under different experimental conditions, which makes it impossible to determine the most active PC SC system presently. The next important problem is the precise determination of the origin of the products formed in the reaction systems. This is especially important to the synthesis of PC from organic precursors, the residues of which during the experiments can produce CH3OH, CH4, CO or other products. In these cases, trace amounts of organic compounds can contribute a significant error in the determination of photoactivity, which makes it necessary to conduct additional studies in the absence of CO2 in order to prove the inactivity of such a system under deaeration with an inert gas. In addition, in many cases, using of organic electron donors leads to considerable difficulties of determining the amounts of final products that were formed directly from CO2.

47

1.3 Mechanism of carbon dioxide photocatalytic reduction

As mentioned earlier, CO2 is one of the most thermodynamically stable carbon compounds [33, 41, 56, 59]. PC reduction of carbon dioxide to hydrocarbons requires a large amount of energy for the dissociation of the C=O bonds and the formation of C–H bonds. This involves participation in the process of a certain number of electrons and a corresponding number of protons [108]. Since carbon atoms in the CO2 molecule are in the highest oxidation state, carbon dioxide can only be reduced with the participation of reducing agents, among which water is most attractive due to its availability, non–toxicity and relative cheapness. The reduction of carbon dioxide by water to organic compounds (eg methanol or methane) is a reaction with a high positive Gibbs energy change value [33, 34]: CO2 + 2H2O → CH3OH + 1.5 O2 ΔG0=702.2 kJ∙mol–1

(1.14)

ΔG0=818.3 kJ∙mol–1

(1.15)

CO2 + 2H2O → CH4 + 2 O2

In order to overcome these energy barriers, it is necessary to use an external energy source, for example, radiation by quanta of light with different wavelengths in the presence of a SC. CO2 activation is a key factor in the process of carbon dioxide reduction. The transfer of one electron to a free carbon dioxide molecule is a thermodynamically unfavorable process [10, 33–43]: CO2 + e– → CO2•– E0 = – 1.9 V vs NHE

(1.16)

However, the multielectronic CO2 recovery processes have significantly lower (regarding absolute scale) values of oxidation–reducing potentials, i.e they are more advantageous compared to the monoelectronic CO2 reduction [10, 30, 33–36, 40, 41, 44–46]. The formed products may be different, depending on the number of electrons and protons involved in the chemical reaction: CO2 + 2H+ + 2e– → CO+ H2O

E0 = – 0.52 V vs NHE

(1.17)

CO2 + 2H+ + 2e– → HCO2H

E0 = – 0.42 V vs NHE

(1.18)

CO2 + 4H+ + 4e– → CH2O + H2O

E0 = – 0.48 V vs NHE

(1.19)

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CO2 + 6H+ + 6e– → CH3OH + H2O

E0 = – 0.38 V vs NHE

(1.20)

CO2 + 8H+ + 8e– → CH4 + H2O

E0 = – 0.24 V vs NHE

(1.21)

Thus, the chemical reactions of CO2 reduction occur with the participation of a certain number of electrons and a similar number of protons. The selectivity of the final product of the process depends on the CO2 PC conditions and the thermodynamic reduction potentials [33]. Modern ideas about the mechanism of PC reduction of carbon dioxide are presented in a large number of articles, but there is no unified opinion on the possible stages and directions of the process. Anpo [109] first considered this question, reporting that irradiation by UV–light of titanium dioxide in the presence of CO2 and H2O leads, according to the data of the electron paramagnetic resonance (EPR), to the appearance in the system of C• radicals, hydrogen atoms and Ti3+ ions, and the intensity of the signals increased with increasing intensity of irradiation. Relative intensities of EPR signals strongly depend on the amount of CO2 and water on the catalyst surface: the intensity of the H–atoms signal increases with an increase of H2O percentage, and the intensity of the Ti3+ ions signal decreases. These results indicate that CH4, which was detected chromatographically, is formed by the reaction of С• radicals (formed from СО2) with H–atoms (formed as a result of the protons reduction), and H+ are formed from H2O molecules adsorbed on TiO2. The same author has suggested that most of the reduced carbon atoms can be on the PC surface in the form of atomic carbon, which can react with adsorbed water as well as surface OH groups, producing methane. Methanol can be formed as a result of the reaction of OH• radicals (due to its high reactivity, the presence of which was not proved by the authors in the article) with CH3• radicals and/or methane. Formation of ethylene and ethane, the presence of which was also fixed, is possible due to nteraction of СН3• radicals with each other. Subsequently [110], under the same conditions, the presence of radicals O2– in the reaction medium was shown. Thus, on the basis of the obtained data, a possible mechanism for the process of CO2 photoreduction was proposed, the scheme of which is shown at the Figure 1.6.

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Figure 1.6 Schematic illustration of the photocatalytic reduction of CO 2 with H2O on the anchored titanium oxide species [110].

At the initial stages of the process of irradiation of a semiconductor in the presence of CO2 and H2O, the generation of charged particles occurs [11]: SC + hν → e–(SC) + h+(SC)

(1.22)

The consumption of photogenied charge carriers results from a number of processes, namely: - recombination of charges e–(SC) + h+(SC) → hν

(1.23)

- oxidation with holes of water adsorbed on the PC surface h+ + H2Oads → OH• + H+

(1.24)

- subsequent transformations involving photoelectrons to produce reduction products. Formation of the final products, according to the majority of the literature data processed by us [27, 29, 43, 50, 51, 54, 62, 63, 70, 103, 104, 106, 111] occurs as follows: The interaction of H+ with SC CB electrons leads to the formation of H• radicals (reaction 1.25) that can react with each other to form molecular hydrogen (reaction 1.26), as well as to participate in subsequent reactions of CO2 reduction. H+ + e– → H•

(1.25)

2H• → H2

(1.26)

50

There is a literature data [21] according to which hydrogen can be dissociatively adsorbed on the PC surface with the formation of protons: H2 → 2Hads+ + 2e–

(1.27)

which by reaction (1.25) turn into radicals H• having high reduction activity. The interaction of CO2 with hydrogen radicals contributes to the formation of carbon monoxide: CO2 + 2H• → CO + H2O

(1.28)

Further, after passing the formation of carbon radicals CO + e– → C•O– + H• → C• + OH–

(1.29)

methane is formed by a number of successive reactions: C• + H• + e– → CH• + H•→ CH2 + H• →CH3•+ H•→ CH4

(1.30)

Dimerization of radicals CH3• contributes to the formation of ethane [21, 23], which can be protonated to methane: C2H6 + 2H• → 2CH4

(1.31)

A number of papers [15, 21, 23, 38, 98] proposed a mechanism for the carbon dioxide reduction by the initial formation of anion–radical CO2•– after the interaction of a CO2 molecule and a photogenic electron: CO2 + e– → O=C•–O–

(1.32)

According to [112], the interaction of anion–radicals CO2•– formed in environments with high values of the dielectric constant, with photoelectrons and protons, formed during the PC decomposition of water, leads to the formation of formic acid: O=C•–O– + H+ → O=C•–OH

(1.33)

O=C•–OH + H• → O=CH–OH

(1.34)

НСООН adsorbed on the PC in the interaction with CB electrons and H+ can be reduced to formaldehyde: O=CH–OH + H++ e– → O–C•H–OH

(1.35)

O–C•H–OH + H• → OH–HCH–OH

(1.36)

OH–HCH–OH – H2O → H–CH=O

(1.37)

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The final product – CH4 – is the result of the reactions of the successive reduction of СНОН to methanol and CH3OH to methane: H–CH=O + e– → H–CH•–O–

(1.38)

H–CH•–O– + H+ → H–CH•–OH

(1.39)

H–CH•–OH + H• → CH3–OH

(1.40)

CH3–OH + H• → CH3• + H2O

(1.41)

CH3• + H• → CH4

(1.42)

It should be noted that according to the proposed mechanism, the formation of the final product occurs after passing many successive reduction stages. Taking into account the possibility of oxidation of intermediates due to its interaction with VB holes, additional use of electron donors is required for the successful CO2 reduction to methane. Nevertheless, without specific interaction with the surface of the PC, the reduction of carbon dioxide by such a mechanism is unlikely, since the energy level of the redox pair СО2/СО2•– [33, 34] significantly exceeds ECB of most known SCs. Using of a high–intensity photon flux source (for example, a laser) allows to study the PC reduction of carbon dioxide at short intervals. Using of a high–intensity photon flux source (for example, a laser) allows to study the PC reduction of carbon dioxide at short intervals. An investigation of the laser (λ = 355 nm) irradiation of TiO2, ZnO and NiO aqueous suspensions has shown that the formation of products (predominantly methanol) in the reaction medium can occur not only by activation of carbon dioxide molecules but also by reduction of carbonic acid and CO32– anions [35]. It is known that H2CO3 can be reduced to formic acid due to the passage of multielectron processes [35]: H2CO3 + 2H+ + e– → HCOOH + H2O

(1.43)

Carbonate anions are also capable of participating in multi–electron processes of transformation: 2CO32– + 4H+ + 2e– → C2O42– + 4H2O

(1.44)

2CO32– + 4H+ + 2e–→ HCOO– + 2H2O

(1.45)

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Anions of oxalic acid, formed during the reduction of CO32– ions, can be reduced to HCOO– anions, which are precursors of methanol formation during CO2 photoreduction: C2O42– + 2H+ + 2e– → 2HCOO–

(1.46)

HCOO– + 5H+ + 4e– → CH3OH + H2O

(1.47)

According to [113], the formation of metal carbides is a key moment in the process of PC conversion of carbon dioxide into methane using of metal–modified SC. Thus, in the study of a sample of TiO2 Evonic P25, on the surface of which ruthenium was deposited, the photogeneration of Ru–C is initiated by SC irradiating to form an electron–hole pair, followed by the reduction of the CO2 present in the reaction medium: 4e– + CO2 (Ru)→ Ru–C + 2O2–

(1.48)

Holes of the SC VB can interact with hydrogen, which is additionally introduced into the reactor: 2H2 + 4h+VB → 4H+

(1.49)

The next stage of the process is the neutralization of intermediate particles: 4H+ + 2O2– → 2H2O

(1.50)

The formation of methane is the result of the interaction of ruthenium carbide and hydrogen: Ru–C + 2H2 → Ru + CH4

(1.51)

According to the authors of [113], the possibility of carbon dioxide conversion according to the above mechanism is confirmed by the passage of reactions (1.48) and (1.51) during cathodic reduction of CO2 to methane using a ruthenium electrode. Thus, to date, there is no unified view on the mechanism of the photocatalytic reduction of CO2. However, according to the literature, two main ways of forming the products of the process can be distinguished: 1. Carbon dioxide reduction through a series of organic compounds according to the scheme: CO2 → HCOOH → HCOH → CH3OH → CH4.

53

2. The formation of final compounds through the reduction of CO2 to CO and C followed by protonation of the intermediates: CO2 → CO → C → CH2 → CH4 However, as a result of very complex processes of multielectron transport and a large number of consecutive reactions, an understanding of the mechanism and selectivity of the СО2 photoconversion process is at the initial stages nowadays.

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1.4 Photoelectrochemical reduction of carbon dioxide

The ability of some semiconductors (TiO2, p–GaAs, etc.) to convert light energy into a photocurrent was used to CO2 reduction in aqueous solutions [10]. The efficiency of converting solar energy into electrical energy on single crystals of n– GaAs and p–InP reaches 12% [114] and up to 12.4% on n–CdSe single crystals [115]. For these SC, the width of the band gap is optimal (Eg = 1.2–1.6 eV) with respect to the solar spectrum, but the attempt to use them for photoelectrolysis of water was not successful [10]. Owing to the overvoltage required for cathodic and anodic reactions, additional energy is required in comparison with the energy calculated from thermodynamic data. To investigation the process of photoelectrochemical reduction of carbon dioxide on SC electrodes, both aqueous [115–124] and non–aqueous [125–129] solutions are widely used, the most common solvents being acetonitrile, polypropylene carbonate, dimethylformamide, dimethylsulfoxide and methanol. The greatest advantage of non–aqueous media is its CO2 solubility, which may exceed 7– 8 fold a similar figure in comparison with water [4, 128]. Water is widely used as a source of protons in aphotonic solvents, and it has been shown [130] that the addition of 1% water to acetonitrile does not affect the solubility of carbon dioxide, but a further increase of H2O concentration greatly reduces it. Dimethylformamide and dimethylsulfoxide are more effective solvents than polypropylenecarbonate and acetonitrile in a mixture with water (1% by vol.), since they are able to effectively suppress competing processes of hydrogen evolution [128]. Photoelectrochemical reduction of CO2 in an aqueous medium. In early works where the process of photoelectrochemical CO2 reduction was investigated, silicon based material such as p+/p–Si [123] or p–Si with a layer of polyaniline [117] was used as photoelectrodes. The authors showed that upon irradiation of photocathodes immersed in a carbon dioxide–saturated solution of sodium sulfate (0.5 mol/l) [123] or LiClO4 (0.1 mol/l) [117], carbon dioxide is reduced to formic acid

55

with a yield by current (YC) up to 21% [123] (28% [117]) and formaldehyde (19.4% [117]). The authors have established that under irradiating SC photocathodes with (111) p–CdTe or (100) p–InP [124] planes, indium phosphide exhibits greater selectivity in the process of CO2 reduction (YC ≈ 60%) compared to cadmium telluride (YC ≈ 38%), the main products of carbon dioxide reduction were CO and HCOOH. It was also shown that using carbonate salts as background electrolytes, the selectivity of the CO2 reduction to formic acid is greatly increased, but the formation of carbon monoxide practically does not occur. According to the authors, the CO is formed according to equation CO2•– + CO2 + e– → CO + CO32–

(1.52)

after the stage of formation of the radical anions CO2•–. Obviously, the presence of CO32– ions in such media can block the formation of carbon monoxide. Gallium arsenide is widely used in photoelectrochemical CO2 reduction to methanol. Thus, the formation of CH3OH occurs during photoreduction of CO2 with the participation of p–GaAs and p–InP in the electrolyte in the presence of sodium sulfate [116]. In the case of an electrode in the form of a p–GaAs single crystal at pH = 4.2, the yield of methanol during CO2 reduction achieves 10% at a current density J = 1.4∙10–4 A∙cm–2 [120]. Wherein, J increase leads to a decrease of the yield of CH3OH. During the CO2 photoreduction with GaAs in an electrolyte containing the redox system V2+/V3+ for photocorrosion protection, HCOOH forms with YC of 1% [10]. Photoelectrochemical synthesis of methanol in aqueous electrolyte saturated with CO2 under the solar irradiation is possible with the participation of SC CuO– Cu2O nanorods with high efficiency [122]. GaP is also a common SC, which is used in the process of CO2 photoreduction. CO2 photoelectroreduction in aqueous solutions with GaP or GaAs electrodes leads to the formation of formic acid with an admixtures of formaldehyde and methyl alcohol. The maximum YC reaches 80 % at a CO2 pressure of 8.5 atm and a potential of –1.0 V (vs. Ag/AgCl). Under irradiation with a monochromatic light of an aqueous

56

solution of sulfuric acid (0.5 mol / l) saturated with carbon dioxide, formaldehyde and methyl alcohol are formed on GaP at –1.5 V [120]. Selective formation of methanol is observed under sunlight irradiation og p–GaP electrode in the presence of carbon dioxide and pyridine [121] at a potential of –0.4 V (vs. Pt|Hg|Hg2Cl2|Cl−). Irradiation of the GaP photocathode with a mercury lamp at pH 6.8 results in the reduction of CO2 to HCOOH, CH3OH, and HCOOH [118]. Accumulation of formic acid and methanol at room temperature was observed for 18–90 h. The maximum yields of formaldehyde and methyl alcohol formation using monochromatic light (365 nm) are 5.6 and 3.6%, in the case of solar irradiation, the yields fall to 0.97 and 0.61, respectively. Methanol, CO and hydrogen were detected by irradiating sodium bicarbonate solution with GaP photocathode by mercury, xenon and tungsten lamps [119]. The addition of crown ethers to a solution of lithium carbonate during the photoreduction of CO2 by gallium phosphide increases both the photocurrent and the YC of HCOOH, CH3OH, and HCOOH. It was suggested that the metallic lithium, which is formed in the presence of crown ethers, interacts with CO2 to form СО2•– [116]. Some SC materials due to corrosion are not suitable for direct contact with the aqueous solution, which excludes its applying in an aquatic medium. Photoelectrochemical reduction of CO2 in organic solvents. According to the literature data processed by us, the main product of the CO2 potoreduction in non– aqueous media is carbon monoxide. The Bockris group carried out a considerable amount of work in studying the process of CO2 photoelectrochemical reduction using cadmium telluride as a photoelectrode. Thus, when a CdTe photocathode is irradiated with monochromatic light (λ = 600 nm) in a solution of dimethylformamide or CH3CN with small additions of water, carbon dioxide is reduced to carbon monoxide with a YC up to 70% [125]. Compared with other p– type SC (for example, p–Si, p–GaP, p–InP, p–GaAs), p–CdTe is distinguished by lower CO2 reduction potentials and high YC [128].

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Photoelectrochemical CO2 reduction can be achieved by irradiating a p–type photoelectrode (gallium arsenide and indium phosphide [126]) with a xenon lamp, while methanol was used as the electrolyte. Formation of carbon monoxide occurred using InP–electrode, YC was 41.5%. The efficiency of the reduction of carbon dioxide to formic acid ranged from 12 to 15% for both SC photocathodes. With a significant increase in the pressure of CO2 in the system, formation of CO with a YC of 90% (p–InP electrode) is observed [127]. The significant yield of carbon monoxide is explained by the formation and dense (due to the high pressure of carbon dioxide (40 atm.)) adsorption on the SC surface of an anion–radical complex (CO2)2•–. This leads to both selective formation of CO, and low activity of hydrogen production during photoelectrolysis. In addition, the adsorbed complex is responsible for stabilizing SC electrode materials at high pressures of carbon dioxide, even at J values exceeding 100 mA·cm–2. Acetic and formic acids are also capable of forming by photoelectrochemical CO2 reduction in methanol media [127] with YCs close to carbon monoxide. Effect of metal deposition on photoelectrodes on the of the CO 2 reduction process efficiency. As is known, various metal electrodes are used to reduce carbon dioxide in aqueous and non–aqueous media [130–132]. The catalytic properties of a metal can be transferred to SC by depositing metal films or particles on the photocathode surface. According to the data of the literature, the deposition of metals on the SC photoelectrode leads to an increase in the efficiency of photoelectrochemical reduction of carbon dioxide [133], while the distribution of the reduction products of CO2 in CH3OH can be related to the catalytic properties of metal electrodes during the electrochemical reduction of CO2 in methanol. Thus, the gradual precipitation of lead on p–InP leads to an increase in the activity of the formic acid formation, the maximum YC is 29.9% [132], while the deposition of Pb particles on the silicon photocathode not only reduces the hydrogen reduction activity, but also shifts the CO2 reduction potential by 100 mV [123]. Modifying the SC with gold or silver increases the efficiency of carbon dioxide reduction to carbon monoxide, the

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maximum YC values for Au/p–InP and Ag/p–InP electrodes were 69.9% and 80.4%, respectively. Modification of the SC electrode with nickel leads to the reduction of carbon dioxide to hydrocarbons (СН4 and С2Н4) with a low YC: 0.7% and 0.2%, respectively. No hydrocarbons were detected when the photoelectrode was modified with copper and palladium, while carbon monoxide emissions for Pd/p–InP (maximum YC 55%) and formic acid for Cu/p–InP (21%) were observed. Investigation of the effect of coating of the p–GaP photocathode by a metal film with a high hydrogen overvoltage value showed that using a lead film, the main CO2 reduction product is formic acid with YC of 42%, while the yield of the product on an uncoated cathode is an order of magnitude lower [120]. Applying a zinc film, the main product of reduction is carbon monoxide with YC of 18%. The process of photoelectrochemical CO2 reduction on a p–Si electrodes modified with gold, silver or copper [133] was investigated. Analysis of the products showed that an unmodified metal electrode reduces carbon dioxide to formic acid and carbon monoxide (II), while modification with silver or gold predominantly leads to the formation of CO, and copper to methane and ethylene. On boron–doped silicon photocathode, carbon dioxide is reduced to formic acid (28%) and formaldehyde (19%), while using TiO2 and titanates photoelectrodes doped with manganese, iron, cobalt or niobium (0.05–0.5% mass.), the main products are HCOOH and CH3OH [123]. The photoinitiation of the CO2 reduction reaction by water on the strontium titanate (111) facet leads to the formation of methane, as well as formaldehyde, hydrogen and oxygen. Selective reduction of carbon dioxide to formic acid occurs upon irradiation with visible light of a SC–electrode (p–type indium phosphide) doped with zinc [134]. The surface of such electrode is modified by a polymeric complex of ruthenium [(Ru(L–L)(CO)2]n, де L–L – diimine ligand. YC of this process is 34.3%, but the modification of the photoelectrode by changing the polymerization conditions of the ruthenium complex leads to an increase in this value to 62.3%, which can be

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explained by the formation of a more dense contact between the polymer component and SC.

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Concluding the review of the literature, it should be noted several main points. Attempts by researchers to reproduce the process of natural photosynthesis for carbon dioxide conversion have attracted attention for a long time. PC reduction of carbon dioxide to hydrocarbon fuels is an extremely complicated but promising area with a number of advantages: - carrying out the process at relatively mild conditions – ambient temperature and atmospheric pressure; - using as a source CO2 – greenhouse gas, which will help reduce the anthropogenic load on the environment; - activation of the process occurs with the participation of the clean and inexhaustible energy of the Sun; - photoreduction of carbon dioxide leads to the formation of hydrocarbon compounds, such as CH4, CH3OH, C2H6, etc., which can positively influence to the solution of the problem of the exhaustibility of fossil fuels (oil, coal, natural gas, etc.). Each approach considered for photochemical or photoelectrochemical CO2 conversion requires a catalyst to facilitate the formation and/or dissociation of chemical bonds. Despite an intensive study of this problem, a review of the literature shows that currently there is no PC for photoreduction of carbon dioxide with a high quantum yield, low overvoltage and high selectivity, which makes it impossible to use existing samples in large–scale systems of converting solar energy to chemical energy. Despite the fact that the researchers have established the possibility of CO2 photocatalytic reduction using various SC materials and composites based on it, systematic research of the effect of modifying additives on the activity of SC PC are very limited in the literature data, although this factor can substantially change the photoactivity of the SC material. Information on the change of the PC activity during the transition from mono–to bimetallic structures ("metal– metal–SC" type) is extremely limited, the use of which makes it possible to substantially improve the efficiency of CO2 photoreduction. Thus, further research

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aimed at obtaining new PCs with high catalytic activity and selectivity, as well as establishing the regularities and mechanism of the CO2 reduction process are priority and promising. Taking into account the results of the analysis of the literature data known today, in this thesis we focused on a systematic study of the PC activity of materials based on nanostructured (in particular, mesoporous) materials based on TiO2 (in the form of powders and nanostructured films) and C3N4 (powders), and metal–SC and bimetallic composites based on them in the processes of both photochemical and photoelectrochemical reduction of carbon dioxide, since such materials may prove promising for the development of new effective PCs for CO2 conversion.

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Chapter 2 Experimentation Techniques 2.1. Reagents and Materials AgNO3, CuCl2, Na[AuCl4], HCl, Na2SO3, NaOH, KCl, Na2CO3, H2SO4, LiBF4, titanium tetraisopropoxide (TTIP) 97% Ti[OCH(CH3)2]4, Na2HPO4, KH2PO4, gelatin, glycerol, HF, polyvinylpyrrolidone (Mr = 360000 g/mol), hydrazine hydrate N2H4 × H2O, ethylene glycol С2Н6О2, melamine, ethylcellulose, TiO2 Evonik P–25 (80% ‒ anatase, 20% ‒ rutile, SBET = 55 m2·g–1 [135]), distilled water, metallic titanium and copper, platinum foil, СО2 from balloon. To purify ethanol, 100 g of CaO was calcined in a muffle furnace at 900–950°C for 4–5 hours. The calcined CaO was transferred to a 2 L flask containing 1.0 to 1.2 L of alcohol, tightly closed, shaken and left for 8–12 hours, after which the alcohol was heated with CaO for 5 hours with reflux condenser to remove humidity and then distilled [136]. The average alcohol fraction was used for the experiments. Alcohols – butyl, n–propyl, isobutyl, isopropyl and methyl (5 ml ampoules, Sigma–Aldrich) – were used without further purification.

2.2. Preparation Of TiO2 and C3N4 Nanostructured Samples and Composites Based on It

Preparation of Mesoporous Titanium Dioxide Samples. Mesoporous titanium dioxide was prepared according to the procedure developed by [137], namely by TTIP hydrolysis (0.2 mol·l–1) in an acidic (2.0 mol·l–1 HCl) water–glycerin mixture (10 % glycerol), containing gelatin (16.5 % mass. vs. TTIP weight) at 40 °С. After complete solubilization of the precipitate formed in the initial stage of the sol–gel process, the solvent was evaporated, then dried residue was kept at 150 °C for 1 hour, and calcined sequentially for 1 hour at 350 °C and 1.5 hours at 450 °C (fig. 2.1).

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Figure 2.1 Simplified scheme of the sol–gel formation of mesoporous titanium dioxide using gelatin as the pore–forming agent.

The formation of mesoporous transparent TiO2 films on the glass was carried out by the method of slowly pulling out the glass plate from the precursor solution prepared similar to [138]. In a typical procedure, 1.6 g of gelatin was dissolved in 14.4 ml of distilled water at 50–55 °C, after which 2.0 ml of concentrated HNO3 was added. To the resulting mixture at 50 °C, 3.0 ml of TTIP was added dropwise under intense stirring. The resulting solution then diluted with 20 ml of distilled water. Prior to the deposition of TiO2 films, the glass plates were cleaned by sequential treatment with a solution of "piranha" (a mixture of concentrated H2SO4 and H2O2 in a volume ratio of 3:1), 5% Na2CO3 solution, water and ethanol, and then dried in a hot air stream. The cleaned glass plates were immersed in the precursor solution, after which they were removed at a speed of 10 mm/min. The resulting films were cooled to room temperature and annealed at 300°C in air. The application deposition/annealing procedure was repeated 6 times, after which the films were calcined at 450 ° C for 1 hour in air. Preparation of TiO2 films by screen–printing method. 0.45 g of ethylcellulose was dissolved in 7.3 g (9.0 ml) of n–butanol under heating and continuous stirring. Then, 1.8 g of glycerol was added, stirred until a homogeneous solution was formed and 0.9 g of Evonik P25 powder was added. The suspension was stirred, placed in an ultrasonic bath for 10 minutes, and again stirred at slight heat (≈ 50 °C). Paste was applied to the glass by the screen–printing method, which consists of uniformly spreading the paste of TiO2 using a glass rod on a plate on the sides of which the film

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"scotch" is glued. The films were dried at 70 °C for 30 minutes, then left for 12 hours in the air, after which they were calcined at 450 °C with air access for 1 hour. Heating to 450 °C was carried out gradually over 1 hour. The receiption of hollow anatase microspheres was carried out in accordance with [139]. For synthesis, functionalized polystyrene latexes (Sigma–Aldrich) with size of 1.1 μm were used. Matrix coatings were performed by controlled hydrolysis of TTIP in ethanol solutions. For this, 0.3 g of a 10% aqueous suspension of polystyrene microspheres in 12 ml of absolute ethanol was treated in an ultrasonic dispersion for 15 minutes. The total volume of ethylalcohol was adjusted to 24 ml and injected into solution of 0.01 g polyvinylpyrrolidone (M = 360000 g/mol) and 0.18 g of TTIP. The mixture then heated to 80 °C and held at this temperature for 2 hours at reflux condenser with stirring. After the completion of this procedure, the polystyrene matrices were removed by slowly heating the samples to 350 and 450 °C and retention at a preset temperature for 30 and 60 minutes, respectively. Titanate nanotubes obtaining. Titanate nanotubes were given to us by D. Bavykin (University of Southampton, Great Britain). Typical synthesis of samples consists of microcrystalline TiO2 hydrothermal treatment in a NaON solution at 110– 150 °C, followed by acid washing and final heat treatment. The scheme of this process is presented at Fig. 2.2.

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TіО2 An alkaline treatment of 5–20 M NaOH under stirring Hydrothermal treatment at 110–150 °С for 17–48 hours. Rinsing with distilled water and 0.1 M acid (H2SO4, HCl) to pH=7 Calcination of samples at 400 °C, 2 hours. Figure 2.2 Scheme for the synthesis of titanate nanotubes. Synthesis of bulk and porous samples of C3N4. C3N4–bulk was obtained via bulk pyrolysis of melamine according to [140]. Melamine was placed at the middle of the quartz tube. The heating on inner atmosphere (argon) was conducted at 300 °C for 1 h, then at 600 °C for 2 h. Finally, yellow carbon nitride powder was obtained. C3N4–MCF and C3N4–SBA–15 were obtained via matrix (hard–template) synthesis using MCF and SBA–15 correspondingly. After calcination MCF [16] and SBA–15 [17] were used as hard templates and nanoreactors for the synthesis of mesoporous carbon nitride by pyrolysis of melamine in the pores of the matrices, followed by separation of carbon nitride from silica. For this a weighted amount of the initial silica mesoporous molecular sieve (MMS) was mixed with an aqueous solution containing certain amounts of melamine and hydrochloric acid used for binding of melamine into salt in order to reduce its sublimation (estimated at 0.255 g of melamine and 0.17 ml of aqueous solution of concentrated hydrochloric acid respectively for 0.25 g SBA–15 with pore volume 1.0 cm3/g; amounts of the reagents for MCF were determined based on the calculation of the pore volume 1.8 cm3/g by multiplying the above quantities on 1.8).

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The resulting suspensions were stirred for 3 h at a room temperature and dried at 40 °C for 12 h. Then, obtained composites were subjected to pyrolysis as previously described for bulk carbon nitride. Silica components were removed by treatment of the obtained composites with 15% HF solution for 12 h. Carbon nitride was filtered, washed with water to neutral pH and dried at 100 °C. Photochemical synthesis of mono– and bimetallic semiconductor nanostructures. Metal–semiconductor nanocomposites, as well as composites with a bimetallic component, were formed during irradiation of deaerated aqueous suspensions TiO2 and C3N4 in the presence of metal ions (Ag+, AuCl4–, Cu2+). Bimetallic nanostructures with different order of deposition of metals on the surface of the meso–TiO2, depending on the position of the corresponding metals in the number of activity, were obtained either by successive photoinduced reduction of the ions of each of the metals, or by photoreduction of the excess amount of ions of the more active metal, followed by the galvanic substitution of the excess of less active metal. The process of obtaining composites was carried out in the absence of oxygen in the air, which was achieved by bubbling suspensions of argon. Experiments were carried out in glass vials of 5 cm3. Irradiation of suspensions was conducted by the focused light of the DRS–1000 lamp, the intensity of the light source was determined using a pheryoxalate actinometer [141]. Sodium sulfite as an electron donor was used. Photochemical processes were carried out at room temperature under intense stirring. After photochemical formation of the nanostructures, the samples were washed several times with distilled water and dried at a temperature of 30 °C in an argon stream. Investigation of optical and structural properties of SC materials and composites on its basis. Microphotographs of the samples were obtained by transmission electron microscopy using an electron microscope TEM–125K (SELMA) at an acceleration voltage of 100 kV; high–resolution photomicrographs were obtained on an electron microscope JEOL 3010–TEM at an acceleration voltage of 300 kV.

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Scanning electron microscopy studies were performed with a LMU Mira3 Tescan raster microscope at an acceleration voltage of 5–20 kV. The microscope was equipped with an Oxford X–MAX 80 mm prefix to obtain Energy Dispersive X–ray spectroscopy data. X–ray phase analysis of the samples was performed on a Bruker D8 Advance diffractometer using copper emission of K of copper with Cu = 0.1541 nm at a scan speed of 1 deg / min. The average particle size was calculated according to the Sherer formula [142]: τ = K∙λ∙(β∙cos Θ)–1 where

(1.6)

τ –particles size; К – shape factor, 0.9 for cubic crystallites, 1.075 for spherical ones; β – physical expansion of the diffraction reflection at half its height, in

radians; λ – wavelength; Θ – diffraction angle; To determine the specific surface area, a volumetric method of low– temperature ad(de)sorption of nitrogen using a gasometer GH–1 chromatograph was used. The spectra of the infrared spectroscopy method were recorded using the Perkin Elmer Spectrum One Fourier Spectrometer in the frequency range 400– 4000 cm–1. Diffuse reflectance spectra in the ultraviolet and visible regions were recorded on a Specord M40 equipped with an integrated sphere, in which MgO was used as the reflectance standard. UV–Vis spectra of carbon nitride samples were analyzed in the wavelength range of 200–800 nm at ambient temperature. Spectra of diffusion reflection was counted in absorption spectra using the Kubelki–Munch formula [143]. Elemental composition of obtained materials was determined using СHN– analyzer Carlo Erba 1106. The analysis method is based on the complete and

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instantaneous oxidation of the sample by "flash combustion" on the catalyst in an oxygen atmosphere. The resulting combustion gases pass through a reduction furnace, swept on the chromatographic column and then detected by the thermal conductivity detector. Adsorption of nitrogen was investigated by a volumetric method (77 K, up to 1 atm) on the analyzer of porous materials "Sortomatic 1990". The total area was estimated according to the Brunauer–Emmet–Teller equation [144]; the size of the mesopore was determined from the desorption isotherm branch using the Barrett– Joyner–Halenda method [145]. Thermographic investigations were carried out using a serial Q–1500 derivatograph in the temperature range from ambient temperature (20 °C) to 1000 °C. The heating rate of the sample and standard was 10°/min. Simultaneously, the curve of differential thermal analysis, thermogravimetric and differential thermogravimetric curves were recorded.

2.3. Preparation of Ti/TiO2 photoelectrodes

Synthesis of mesoporous titanium dioxide films (Ti/meso–TiO2). In a typical synthesis of the meso–TiO2 film, a sol–gel method was obtained by the TTIP hydrolysis in the presence of a triblock copolymer of ethylene oxide and propylene oxide Pluronic 123 as template and acetylacetone as a complexing agent for slowing the rate of Ti[OCH(CH3)2]4 hydrolysis. The molar ratio of the components of the reaction mixture TTIP : Р123 : AсAс : Н2О : С2Н5ОН : HCl= 1 : 0.04 : 0.5 : 10 : 41 : 0.2. Separately, a solution of Pluronic 123 was prepared in 6 ml of ethylalcohol, stirred for 1 hour. To 1.2 ml of concentrated hydrochloric acid and 0.26 ml of acetylacetone 6 ml of ethanol was added and stirred for 1 h and then gradually added a solution of template in ethanol. After this, the solution was stirred for an additional hour. Thus, precursors were prepared for applying films on a titanium plate.

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Films were deposited on titanium substrates by the immersion–stretching method with a pulling speed of 1.5 mm/s. After application, the films were hydrolyzed in air for 2 hours. After that it were calcined in a muffle furnace at 350, 400 (4 hours) and 500 °C (2 hours). The heating rate is 2 deg/min. During the experiments, six–layer coatings were used, which were obtained by successive application of the films after each layer was dried in air. Synthesis of electrodes with sponge structure (Ti/sp–TiO2). Photoelectrodes Ti/sp– TiO2 were obtained by anodic oxidation of the titanium plate according to the method developed by [146]. In a typical synthesis technique, a titanium plate (purity 99%, thickness of 0.5 mm, area 2.7 cm2) was treated in an ultrasonic bath with acetone for 15 minutes and dried in air at ambient temperature. The plate was then treated with an aqueous solution of nitric acid and hydrogen fluoride with a volume ratio of HF:HNO3:H2O = 1:4:5 for 20 seconds and washed with distilled water. Further, without drying, the plate was immersed in a solution of 0.2% (vol.) of hydrogen fluoride and titanium was anodized for 40 minutes in a glass cell using a two–electrode scheme with a potential difference of 20 V. As a cathode, platinum foil (S = 1 cm2) was used. The resulting samples were washed with distilled water and air dried at ambient temperature, followed by calcination in a muffle furnace at 450 °C for 1.5 h. Synthesis of semiconductor electrodes with tubular structure. TiO2 тanotubes on the surface of titanium (Ti/tb–TiO2) were formed according to the procedure developed [147]. A typical synthesis was the anodization of a titanium plate (99% purity, 0.5 mm thick, 2.7 cm2 area) immersed in 10 ml of electrolyte, which is a solution of water in ethylene glycol (10% by volume of distilled water) with addition of 0.05 g of ammonium fluoride. The titanium plate was pretreated with ultrasonic for 15 minutes in the presence of acetone. For the formation of TiO2 nanotubes, a tri–electrode scheme was used. As a cathode, a 1 cm2 platinum foil was used, as a silver chloride saturated electrode was used as the reference electrode. The anodizing time was 60 minutes at a potential difference of 12 V, after which the

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sample was washed with distilled water, dried at ambient temperature and calcined in a muffle furnace at 430 °C for 3 hours.

2.4. Methodology of Experiments

Implementation of the photocatalytic processes. Processes of liquid–phase PC reduction of carbon dioxide were carried out with stirring in closed 5–ml glass cylindrical vials with a membrane for reagent injection and sampling. The injection of CO2 into the system and the removal of oxygen were carried out by bubbling carbon dioxide gas through aqueous or aqueous–ethanol suspensions of the SC PC. To verification the presence of residual oxygen in the ballon, voltammetric studies of the O2 reduction peak intensity were carried during system deaeration with carbon dioxide. These results are shown at figure 2.3. As the solvent, dehydrated dimethylformamide was used, as electrolyte – tetrabutylammonium fluoride.

Figure 2.3 Voltamperometric curves obtained by blowing the electrochemical cell with argon (1), CO2 for 1 minute (2), 2 minutes (3), 3 minutes (4), 4 minutes (5), 5 minutes (6) and after addition of small amounts of air (7). A copper electrode as a cathode was used, anode – glasscarbon.

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Deaerating the system with high purity argon results in the appearance of a typical background voltammetric curve (1). The consecutive carbon dioxide injection into the cell for 1 to 5 minutes (fig. 2.3, curves 2 – 6) does not lead to significant alterations compared to purge argon curve. Adding a small amount of air (20 μl) to the volume of such a system leads to the appearance of a clear peak, which can be attributed to cathodic oxygen reduction [148]. Since the injected sample is 0.005% vol. of the total cell volume, this method allows us to suggest the actual absence of O2 in the initial CO2, ie. the gas from the ballon does not require additional purification from oxygen. Preparation of the sample for PC gas–phase reduction of carbon dioxide carried out according to the scheme shown at figure 2.4. A 0.01 g sample was carefully ground in an agate mortar, then it was pressed onto a metal substrate (1) and placed perpendicular to the incident light rays in a glass reactor (2) (V = 50 cm3) with a membrane (3) for reagent injection and sampling of the reaction mixture. Carbon dioxide from the ballon (4) (without further purification) set through a reducer (5) in a Drexel vessel (6) filled with bidistilled water to saturate CO2 with water vapour. The resulting gas mixture was purged through the reactor for 30 minutes to saturate the reaction medium with wet carbon dioxide. To additional gas flow turbulize, a magnetic rod (7) to the reactor brought in, which was rotationally driven by a magnetic stirrer (8).

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Figure 2.4 Schematic diagram of the preparation of PC sample for gas–phase reduction of CO2.

In order to remove possible organic impurities from carbon dioxide, the CO2 went through additional purification, for which a block A consisting of a two– chamber filter (9) filled with silicagel and molecular sieves 4A was installed in the system, then the gas was successively passed through a trap filled with a chromium mixture (10), and a trap with bidistilled water. All traps were equipped with ceramic dispersants of a gas. After purging, the reactor was sealed, fixed to a tripod, followed by irradiation with a focused by quartz lens light of a high–pressure mercury lamp DRSH–1000 with an intensity of І0 = 3.01·1018 Einstein·min–1.

Determination of product concentrations during photoreduction of CO 2. Concentrations of organic products of carbon dioxide reduction (methane, ethylene, acetylene, ethane, acetic aldehyde) in the gas phase were determined chromatographically using a chromatograph "CHROM 5" equipped with a glass column (Porapak Q) and a flame ionization detector. As a carrier gas, high purity argon was used.

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Methodology of photoelectrochemical CO2 reduction. The processes of photoelectrochemical reduction of carbon dioxide were carried out at the installation, the scheme of which is shown at fig. 2.5. For the experiments, a separated glass electrochemical cell (1) was used. The ion–exchange membrane MK–40 (2) was used as the septum, the aqueous solution of LiBF4 (0.1 mol·l–1), was used as an electrolyte, and the cathode was a copper wire (S = 7.5 cm2) (3). During photoelectrolysis, carbon dioxide, after a half–hour saturation of the cathode compartment, was continuously pumped through the enclosed area by a peristaltic pump, Velp Scientifica SP311 (4). As photoanodes (5), the obtained TiO2 coated plates with different morphologies were used, using the Elins P–8S potentiostat (6), an equilibrium potential was set at the anode with respect to the saturated silver chloride electrode (7). In the cathodic area of the electrochemical cell, using a similar reference electrode (8) and a voltmeter, the potential values on the copper cathode were recorded.

Figure 2.5 Scheme of a laboratory installation for the process of photoinduced electrochemical reduction of CO2.

Investigation of photoelectrochemical properties of SC electrodes. Three– electrode scheme was used for research (Fig. 2.6).

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Figure 2.6 Three–electrode scheme for the investigation of photoelectrochemical properties of SC photoelectrodes.

A main electrode (1) (investigated photoanode, or metal grids with a SC coating applied) and a counter electrode (2) (copper plate) were located in the glass reactor. As the reference electrode (3), a chlorine–silver electrode was used. 20 ml of the background salt (LiBF4, 0.1 M) (4) were bring in into the reactor. Chronoamperograms were recorded at an equilibrium potential. SC coatings on grids were obtained by applying 5 layers of isopropyl suspension of material (15 mg∙ml–1), which was prepared by triturationing the sample in an agate mortar and subsequent ultrasonic treatment.

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Chapter 3 Influence

of

Morphology

of

TiO2 And C3N4

Photocatalysts on Its Photoactivity in The Processes of Carbon Dioxide Photoreduction One of the important strategic approaches aimed at increasing the PC efficiency of systems based on SC is related to the purposeful formation of a material with a certain spatial organization of the PC in order to impart porosity, to increase the efficiency of spatial separation and further transport of photogenerated charge carriers, and to create conditions for more efficient absorption of light [12]. Various nanostructured SC are more active PCs for redox–transforms than non– porous nanodispersive materials [10]. Thus, the positive effect of the employment of porous TiO2 is achieved not only in gas–phase PC transformations, where the specific surface area and pore volume enact an important role, but also in many liquid–phase photoreactions where the porosity effect is not so obvious, as, for example, in the PC water splitting to hydrogen [138]. A characteristic feature of this reaction is the fact that aggregated porous TiO2 samples inherents a higher photoactivity than conventional colloidal nanoparticles and nanodispersed TiO2 powders with respect to it, as well as compact titanium dioxide [10, 12]. This chapter presents the results of investigating the optical, structural and PC characteristics of PCs on the basis of TiO2 and C3N4, in particular the effect of morphology and surface area on the amount of organic products obtained by PC and photoelectrochemical reduction of CO2. The main results presented in the chapter were published in [149 – 153].

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3.1. The Impact of TiO2 Spatial Structure on Titanium Dioxide Activity in the CO2 Photoreduction Process

3.1.1. Morphology and optical properties of mesoporous TiO2

In the diffractogram of the mesoporous TiO2 sample (meso–TiO2) (fig. 3.1), there is a set of reflexes typical of anatase modification of titanium dioxide (International Center for Diffraction Data (ICDD), anatase card).

Figure 3.1 X–ray diffraction pattern of mesoporous TiO2 synthesized using gelatin. ICDD, card # 21–1272 (anatase). Calculations carried out using the Sherer formula (1.6) for TiO2 reflexes (101), (004) and (200) showed that the average size of crystallites in samples of anatase was ≈ 10 nm. The nitrogen adsorption/desorption isotherms of the titanium dioxide sample are shown in fig. 3.2, a. The specific surface area of meso–TiO2, calculated from the Brunauer–Emmett–Teller equation, was 89.4 m2·g–1. On the curve of the pore volume distribution by size (figure 3.2, b), the maximum at ~ 3.8 nm (р/р0 = 0.45– 0.50) corresponds to the tensile strength of the liquid nitrogen meniscus (artifact).

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The real diameter of the mesopores for the titanium dioxide sample is calculated from the desorption branch by the Barrett–Joyner–Halenda method [145] for p/p0 > 0.5, the distribution maximum corresponds to a value of 6.6 nm. The mesopores volume is 0.14 cm3·g–1. The obtained titanium dioxide also has an extremely low volume of micropores, which is consist 0.0056 cm3·g–1. Such characteristics of the sample make it possible to classify it as a mesoporous material [154].

a

b

Figure 3.2 (a) the adsorption (curve 1) and desorption (curve 2) nitrogen isotherms obtained at 77 K with the meso–TiO2 sample; (b) the distribution of pores by the radius. According to the electron microscopic data, the meso–TiO2 sample is a aggregates consisting of separate anatase nanoparticles of ≈10 nm in size (fig. 3.3), which are not–tightly connected to each other and form a porous system.

Figure 3.3 Meso–TiO2 TEM microphotographs obtained at different magnification values.

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The presence of TiO2 NPs of about 10 nm in the obtained samples was demonstrated using high–resolution electron microscopy (figure 3.4).

Figure 3.4 Meso–TiO2 high resolution TEM microphotographs.

As can be seen from Fig. 3.4, the size of the vast majority of titanium dioxide particles hesitates within the range of 8–11 nm, that fully consistent with the XRD data. A detailed analysis of the photomicrographs confirms the presence of titanium dioxide of anatase modification, as evidenced by the characteristic interplanar distances [155]. For the reflex (101), according to the Wolf–Bragg equation [156] 2d∙sinΘ = n∙λ, where

(3.1)

d – interplanar distance; Θ – slip angle (breggian angle); n – the order of the diffraction maximum; λ – wavelength;

the calculated interplanar distance is 0.35 nm, which completely coincides with the high–resolution TEM data (figure 3.4). Figure 3.5 shows the diffuse reflection spectra of a meso–TiO2 sample obtained by processing using the Kubelka–Munk ratio.

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Figure 3.5 Diffuse reflection spectra of mesoporous titanium dioxide, processed according to the Kubelka–Munk ratio. Inset: spectra in the coordinates of the Tautz equation for indirect electronic transitions (D×hv)0.5.

As can be seen from the figure, mesoporous TiO2 practically doesn’t absorb light in the range λ> 400 nm. Analysis of the absorption spectra using the Tautz equation [157] α = A ∙ (hν – Eg)n∙(hν) –1, where

(3.2)

α –ТіО2 absorption coefficient А – constant hυ – energy of the light quantum Eg – band gap n – coefficient equal to 0.5 for allowed direct and 2 for allowed indirect

interband transitions

showed that the width of the band gap (the point of intersection of the tangent line to the linear site of the spectra with the abscissa axis, fig. 3.5, insert) of the obtained anatase samples, for which indirect electronic transitions [158] are inherent, is 3.05 eV. The obtained value of Eg is characteristic for the anatase modification of titanium dioxide.

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3.1.2. Preparation, structure, and optical properties of the TiO2 microspheres and titanium dioxide nanorods

As can be seen from the electron micrographs at fig. 3.6, samples of hollow TiO2 microspheres (sp–TiO2) obtained after removal of the template are characterized by regular spherical shape and shell integrity.

Figure 3.6 TEM microphotographs of TiO2 microspheres, obtained by the template method using polystyrene latexes as solid matrices.

The size of the TiO2 microspheres is approximately 1100–1200 nm, and its wall thickness is of the order of 100 nm. As can be seen from fig. 3.6, the main array of material present in the system is in the form of hollow microspheres. In the sample, only a small amount of material is present in the form of solid spheres measuring 150–200 nm, most likely formed by hydrolysis of TTIP on the surface of calcium oxide particles, which is used in the absolutization of ethanol, or titanium dioxide particles that could separate from the surface of microspheres during synthesis. With a higher–resolution photomicrograph obtained by the TEM method (figure 3.7), it can be seen that the walls of the resulting hollow microspheres consist of densely packed polycrystalline particles without a clearly expressed porous structure. According to the electron diffraction pattern, it was established that the phase composition of the resulting hollow microspheres corresponds to the

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crystalline modification of TiO2–Anatase (figure 3.7, insert). According to the results of the volumetric method of low–temperature ad(de)sorption of nitrogen, TiO2 microspheres are characterized by a specific surface of 115 m2·g–1.

Figure 3.7 TEM micrographs of TiO2 hollow microspheres obtained by the template method. Insert: the electron diffraction pattern of the sp–TiO2 sample.

One of the promising ways of synthesis of nanostructured titanium dioxide is based on the thermodynamic instability of titanate nanotubes (tb–H3Ti2O7) when they are maintained at high temperatures. Thus, heating leads to a slow conversion of titanate nanotubes into nanostructured TiO2, which is accompanied by a loss of nanotubular morphology [159–160]. In fig. 3.8 electron micrographs of the initial titanate nanotubes (a) and the material obtained after its thermal treatment at 500 °C for 2 hours (b) are shown. As can be seen, nanotubes have a length of several hundred nanometers and an internal diameter of ~ 5–6 nm. Heat treatment of it leads to the complete disappearance of the initial nanotubes and its transformation into solid nanorods (rd–TiO2). In this case, a sharp decrease in the specific surface area is observed (Sinitial nanot. = 200 m2·g–1, Snanorods = 65 m2·g–1), and the transition of the titanate phase (figure 3.9, curve 1) to the crystalline modification of titanium dioxide – anatase (figure 3.9, curve 2), which was confirmed by XRD data.

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а

б

Figure 3.8 High Resolution TEM microphotographs of initial titanate nanotubes (a) and anatase nanorods obtained by H2Ti3O7–nanotubes heat treatment at 500 °C for 2h (b). The microphotographs of different scaling are presented.

Figure 3.9 Diffractograms of titanate nanotubes before (1) and after (2) heat treatment. ICDD, cards # 21–1272 (anatase), # 41–192 (H2Ti3O7).

Figure 3.10 shows the diffuse reflection spectra of nanospheres (1) and nanorods (2). As can be seen, there is a bathochromic shift of the edge of the absorption bands during the transition from a nanorods to nanospheres, which which can be related with an increase in the size of anatase crystallites. The obtained materials have optical transparency in the range λ > 400 nm. However, the

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spectra differ insignificantly and are close to the previously given reflection spectrum of mesoporous titanium dioxide (Fig. 3.5).

Figure 3.10 Diffuse reflection spectra of anatase nanospheres (1) and nanorods (2), transformed by the Kubelka–Munk ratio. Inset: spectra in the coordinates of the Tautz equation for indirect interband electronic transitions (D×hv)0.5.

The analysis of the spectra in the coordinates of the Tauts equation for indirect interband electron transitions (figure 3.10, insert) shows that the studied samples of the nanosphere are characterized by Eg close to 3.13 eV. The nanorods obtained from titanate nanotubes Eg correspond to 3.33 eV. Eg increase in the case of the sample rd– TiO2 can be explained by the indication of quantum–size effects [11–12], which in turn is explained by the formation of nanosized titanium dioxide.

3.1.3. Investigation of the photocatalytic activity of TiO2 with different spatial forms in the process of liquid–phase CO2 reduction

Irradiation of aqueous suspensions of titanium dioxide saturated with carbon dioxide, by light with a wavelength of ≤ 375 nm, whose energy exceeds SC Eg, results in the appearance of predominantly gaseous methane in the system for all obtained samples (fig. 3.11). As nonporous (unmodified) titanium dioxide, TiO2 Evonik P25

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was used, which de facto acts as a standard under comparing the PC activity of SC nanomaterials.

Figure 3.11 Dependence of the methane formation rate in the process of liquid– phase photocatalytic reduction of CO2 from the spatial structure and phase composition of the PC.

It should be noted that the presence of any organic products of the CO2 reduction was not recorded as with the long–term maintenance of a system containing TiO2, water and carbon dioxide without irradiation, and irradiation in the absence of a PC. This confirms that the formation of final products occurs during PC reduction of CO2 on the surface of titanium dioxide. In our experiments, methane is the main, but not single, product of PC CO2 reduction detected in the gas phase. In the gas phase, the presence of ethylene, acetylene and ethane was also detected, but its concentrations were significantly lower. Analysis of the liquid phase of the system showed the absence of possible intermediate products of photoreduction of carbon dioxide – formic acid and formaldehyde. Methyl alcohol was also not detected. The possibility of methane formation in our system is obviously due to its high photochemical stability, which prevents its oxidation and promotes accumulation in the system. In particular, we have experimentally established that irradiation of TiO2 in the presence of oxygen from air and methane does not lead to

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photooxidation of СН4 during prolonged, 1.5–2 hours, irradiation of the system. As it turned out, the anatase rods exhibit the lowest activity among the investigated samples, much lower compared to industrial titanium dioxide Evonik P25, despite the highest value of Eg among the synthesized TiO2 samples, although it is known that PC reactions involving SC with quantum–size effects pass more intensively, than in the presence of larger crystals and a similar chemical composition [11]. Since the specific surface area under heat treatment of tb– H3Ti2O7 decreases significantly, it accordingly affects to the amount of adsorbed substrates In addition, the investigated commercial sample TiO2 P25, in contrast to nanorods, consists of a mixture of nanocrystalline phases of anatase (~ 80%) and rutile (~ 20%), which is one of the reasons for its high photocatalytic activity, due to the possibility of spatial separation of photogenerated charge carriers between anatase and rutile nanocrystals due to a small difference in the value of the CB potential of its TiO2 modifications [19, 20]. Despite the fact that samples of mesoporous titanium dioxide and hollow TiO2 microspheres consist only of anatase, it are characterized by spatial order, developed surface and high activity, exceeding P25, during PC reduction of carbon dioxide under current conditions (fig. 3.11). Differences in activity can be interpreted as a consequence of the presence of a porous structure in the obtained samples [137], which contributes to long–term retention of the reagents in the PC pores, as well as migration of photogenerated electrons through a system of contacting TiO2 nanocrystals, which should lead to a decrease the electron–hole recombination probability. In the case of the sp–TiO2 sample, except for the indicated advantages and a slight Eg increase (as compared to meso–TiO2), the possibility of a deep penetration of the light flux into hollow microspheres in combination with multiple light refraction and reflection in the volume increases the probability of light absorption, which leads to a significant growth of TiO2 photoactivity. Figure 3.12 shows a typical chromatogram recorded during irradiation of TiО2 P25 aqueous suspension.

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Figure 3.12 Chromatogram of the gaseous medium after irradiation of the aqueous suspension of TiO2 P25 for 2h.

As can be seen from fig. 3.12, in the process of carbon dioxide reduction, a set of alkanes, alkenes and/or alkynes is observed. Since no formic acid, formaldehyde and methanol were detected in the liquid medium, the probability of passing the process by the mechanism of the gradual CO2 reduction to methane through a series of intermediate organic products СО2 → НСООН → НСНО → СН3ОН → СН4

(3.3)

is extremely low, taking into account the high reduction properties of its compounds and the significant oxidation potential of VB holes, photogenerated in titanium dioxide. Similarly, the absence of oxalic acid in the liquid phase is explained, considering that one of the first stages of the reduction of carbon dioxide is the formation СО2•‾ anion radicals and its subsequent dimerization to С2О42–. However, oxalate ions are also strong reducing agents, therefore, after its possible formation, there will be a reversible oxidation of oxalic acid to carbon dioxide. The presence of light hydrocarbons in the gas phase (figure 3.12) indicates the passage of the process by a mechanism that involves the reduction of carbon dioxide to CO (reaction 1.28), carbon radicals (reaction 1.29) and its subsequent protonation with the formation of intermediates (reaction 1.30), which , combining in different combinations between themselves and the products of water

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decomposition (reactions 1.24 – 1.26), will contribute to the formation of a wide range of the final products. This is evidenced by the presence: - acetylene and acetylene 2СН2 → С2Н4,

(3.4)

2СН• → С2Н2,

(3.5)

- ethane, which can be formed both by dimerization of radicals СН3• [21] СН3•+ СН3•→ С2Н6,

(3.6)

and by the reaction of hydrogen addition to ethylene: С2Н4 + Н2 → С2Н6.

(3.7)

Obviously, that the highest concentration among reduction products is typically for methane – the simplest representative of hydrocarbons, the chemical stability of which allows it to accumulate in the system. A significantly smaller amount of C2 hydrocarbons is observed in the reaction medium, since for its preparation it is necessary preliminary availability of СН•, СН2 and СН3• intermediates, which increases the number of intermediate reactions and, accordingly, reduces the probability of the ethylene, acetylene and ethane formation. Thus, in this chapter there are have been considered the synthesis, as well as structural, optical and PC properties of SC materials based on TiO2. The investigated titania powders were prepared as template sol–gel synthesis using a hydrolysis reaction of TTIP in the presence of structurizing agents and calcination of titanate nanotubes. The obtained samples are anatase crystal modification of TiO2, which differ by the value of Eg, the specific surface area and the spatial ordering. The carried out research showed the availability of PC activity of such materials in the liquid–phase reduction of CO2 by water. The highest photoprocess intensity is characteristic for hollow microspheres. According to the analysis of the formed products of the reaction, an assumption is made about the possible direction of the mechanism of the CO2 photoreduction process.

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3.2 Formation of Ti/TiO2 Photoelectrodes With Different Spatial Organization And Research of Its Anode Photoactivity in The Process of Electrochemical CO2 Reduction

Photoelectrochemical CO2 reduction is a promising method of converting solar energy into a chemical energy [10, 12]. An intensive study of the process of obtaining organic compounds from carbon dioxide by photoelectrochemical methods using various SC electrodes is carry out nowadays [161]. At the same time, the data on a systematic study of the role of the SC photoelectrodes structure in the reaction of photoelectrochemical reduction of CO2 is almost absent. Taking into account these facts, we carried out a comparative study of the effect of the surface morphology of SC Ti/TiO2 photoelectrodes on its activity in the process of photoinduced electrochemical reduction of CO2.

3.2.1 Preparation, morphology, phase composition and optical properties of Ti/TiO2 photoelectrodes

The diffraction pattern of the initial metal plate (fig. 3.13) characterized by a set of reflexes corresponding to metallic titanium.

Figure 3.13 Diffractogram of the original titanium plate. ICDD, card #44–1294 (Ti).

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In fig. 3.14 shows SEM photomicrographs of the of a titanium plate, which, as can be seen, has a relatively smooth surface.

Figure 3.14 SEM microphotographs of the titanium plate after ultrasonic treatment in isopropyl alcohol.

For the surface of the TiO2 film, which was obtained by the immersion– stretching method, after calcination at 400 °C inherent a sufficiently developed surface (figure 3.15) due to utilization of the Pluronic P123 structure–forming templating agent during the synthesis.

Figure 3.15 SEM microphotographs of Ti/meso–TiO2 photoelectrode. TiO2 obtained by sol–gel method and deposited on a titanium plate by the immersion– stretching method.

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According to the XRD date (fig. 3.16), submitted titanium dioxide is a low– crystalline anatase without rutile impurities.

Figure 3.16 X–ray diffraction pattern of Ti/meso–TiO2 electrode.

One approach to improving the efficiency of SC–based systems is to impart a certain spatial organization to the PC. Among the simple and inexpensive approaches, the application of which leads to the formation of nanostructures with the desired parameters, are the methods of anodizing. Recently, vertically oriented nanotubes of titanium dioxide [162], obtained by the electrochemical oxidation of titanium have become widespread, which, due to its architecture, have a large surface area without concomitant reduction in geometric and structural ordering. The high spatial orientation of nanotube arrays facilitates the directed movement of photogenerated charges along the walls of the tubes, which can reduce the electron–hole recombination degree. It should be noted that the morphology of the systems formed during electrochemical oxidation strongly depends on the conditions of the process (in particular, the anodization voltage) and the parameters of the solutions (the presence of fluoride ions and its concentration, pH medium, water content in the

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electrolyte, etc). Thus, we investigated the difference in the morphology of the surface of titanium dioxide during it forming in both aqueous HF solution and water solution in ethylene glycol with the addition of ammonium fluoride. Figure 3.17 shows microphotographs of the TiO2 surface obtained by titanium anodizing in a solution of ammonium fluoride (0.2% vol.) during potential difference U = 20 V for 40 min. and after calcination in a muffle furnace.

Figure 3.17 SEM microphotographs of Ti/sp–TiO2 photoelectrode. TiO2 obtained by electrochemical oxidation of a titanium plate in an aqueous solution of inorganic acids (HF:HNO3:H2O = 1:4:5) with subsequent calcination.

Consideration of the surface of the SC electrode shown in Fig. 3.17, shows that for the morphology of such a titanium dioxide inherent not a geometric ordering of the tubular type, but a developed spongy structure with pores of diameter on the order of 50–70 nm.

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According XRD date (fig. 3.18), after the heat treatment, the phase composition of titanium dioxide of the Ti/sp–TiO2 electrodes is only anatase.

Figure 3.18 X–ray diffraction pattern of Ti/sp–TiO2 electrode.

The process of formation of anodic titanium oxide is a complicated complex process. Thus, according to literature data, under anodizing titanium plate is an initial step a compact layer of amorphous titanium dioxide [163–165] (1 in figure 3.19) is formed.

Figure 3.19 Schematic presentation of the process of titanium plate anodizing in the presence of fluoride ions.

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Reactions at the anode [165–167]: – oxidation of metal with the production of electrons and titanium ions: Ti → Ti4+ + 4e–;

(3.8)

2 H2O → O2 + 4H+ + 4e–;

(3.9)

– anodic oxidation of water – the interaction of titanium ions with OH– ions and oxygen O2– ions Ti4+ + 4ОН– → Ti(OH)4;

(3.10)

Ti4+ + 2 О2– → TiO2;

(3.11)

Ti(OH)4 → TiO2 + 2H2O;

(3.12)

Reactions at the cathode [164–167]: – formation of superoxide anions of oxygen: О2 +e– → О2–;

(3.13)

– formation of molecular hydrogen 8H+ + 8e– → 4H2;

(3.14)

– cathodic water reduction with formation of hydrogen and hydroxyl ions: 2 H2O + 2e– → H2 + 2ОН–.

(3.15)

The external anodic layer, in comparison with the interior one, has an excess of hydroxyl ions [165] and, according to [166], is Ti(OH)4. The interior layer, in which the process of dehydroxylation has carried out, is amorphous TiO2. Probably, there is a concentration gradient along the anode film is exists, which can be written as TiO2∙nH2O and shows the transition from "dry" to "wet" anode oxide. In the presence of fluoride ions, its interaction with the oxide phase, or, under the influence of an electric field, reaction with Ti4+ in the anode space to form water–soluble complexes [TiF6]2– are occurs [163–168]: TiO2 + 6F– + 4H+ → [TiF6]2– + 2H2O

(3.16)

Ti4+ + 6F– → [TiF6]2–

(3.17)

Ti(OH)4+ 6F– → [TiF6]2– + 4OH–

(3.18)

As a result of localized dissolution of TiO2 (equation 3.16), small pits are formed on the surface of the electrode, acting as pore formation centers (2 in figure

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3.19). The growth of pore lengths is due to the internal movement of the titanium dioxide layer to the bottom (bottom) pores (barrier layer). The rates of formation of the anodic oxide at the metal/oxide interface and its dissolution at the pore/electrolyte interface are equalized, after which the thickness of the barrier layer remains unchanged, while it moves into the depth of the metal, increasing the pore length (3 at figure 3.19). This is facilitated by a small radius of fluoride ions, which makes its suitable for transporting through the anode oxide layer towards the externally applied electric field [164–165]. Figure 3.20 shows a typical chronoamperogram for an electrochemical cell in which the process of formation of nanostructured titanium dioxide by the anodic oxidation of a titanium plate carrying out.

Figure 3.20 A typical current–time dependence for electrochemical anodizing (12V vs. Ag/AgCl) of a titanium plate in a water–ethyleneglycol solution (C2H6O2 ÷ H2O = 9 ÷ 1) in the presence of ammonium fluoride (0.5% by weight).

As can be seen from Figure 3.20, during the first stages of anodization after the application of the voltage, a sharp drop in the current density (point 1 in Figure 3.20) is observed, which, according to the literature [163, 165], can be explained by oxidation of the titanium plate to form an amorphous titanium dioxide phase. At the same time, cracks and pits begin to appear on the TiO2 layer surface due to its

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chemical and anodic dissolution, acting as nucleation centers for the pores. Reducing the thickness of the oxide layer at these points leads to a gradual increase in the current density. When the surface density of pits/cracks reaches the limit, the value of J ceases to increase and reaches a local maximum (figure 3.20, point 2). The current density gradually decreases with a corresponding increase of porous structure depth (point 3). At point 4, the length of the nanotube array reaches its maximum value and further remains practically unchanged. Analysis of the literature data [162–168] shows that for the formed TiO2 inherent amorphous structure generally, but there are literature data indicating the presence of TiO2 nanocrystallites in the tube walls, especially if the electrochemical process is carried out at high voltages [169–170]. Using the XRD method, we investigated the initial titanium plate before and after the anodizing process was completed. The resulting sample was thoroughly washed with distilled water and dried in the open air at ambient temperature. At the diffractogram of an non– annealing anodized plate, there are only reflexes inherent for titanium only are present (figure 3.14), which indicates the absence of a crystalline structure in the formed anodic oxide. Amorphous oxide material can be converted into anatase at temperatures above 280 °C or in anatase and rutile mixture (> 450 °C) by heat treatment in air. Calcination of an anodized titanium plate at a temperature of 430 °C for 3 hours in the presence of oxygen of air leads to the crystallization of amorphous TiO2 in predominantly anatase with impurities of rutile, as evidenced by the appearance of characteristic reflexes of titanium dioxide in diffractograms of samples obtained after thermal treatment (fig. 3.21). Noteworthy that the main orientation of the anatase is the (101) plane, while the other planes – (200), and especially (105) and (211) – are present only in a small amount. There are data [171] that crystallization of TiO2 can occures along the entire length of the tubes in only one plane – (101), which indicates the possibility of obtaining single–crystal tubes.

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Figure 3.21 The diffractogram of an anodized titanium plate in a water– ethyleneglycol solution after calcination at 430° C for 4h. ICDD, anatase (card #21– 1272) and rutile (card #21–1276).

The relative content of anatase fA was estimated by the expression [172]:

fA 

1 100% 1 IR 1  K IA

(3.19)

where IA – intensity of the peak (101) of the anatase at 2Θ = 25.3°; IR – intensity of peak (110) of rutile at 2Θ = 27.4°; К – coefficient, wherein К = 0.79 at fA > 0.2 and 0.68 at fA ≤ 0.2. According to the calculations, the amount of anatase in nanotubes was ≈ 75%. From the XRD data of Ti/TiО2 photoelectrodes, it can be seen that the intensity of the anatase reflex (101) decreases in the row tb–TiO2> sp–TiO2> meso– TiO2, which can be explained by a decrease in the degree of SC coating crystallinity. Figure 3.22 shows SEM microphotographs of the surface of TiO2 nanotubes electrochemically formed on a titanium and calcined in a muffle furnace. As can be seen, the anodization of the titanium plate in the water–ethylene glycol medium

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leads to the formation of vertically ordered, densely and uniformly located tubes of titanium dioxide with an internal diameter of 20–25 nm and a wall thickness of about 10 nm.

Figure 3.22 SEM microphotographs of Ti/tb–TiO2 photoelectrode after calcination.

The synthesized samples of titanium dioxide are characterized by high optical transparency in a wide spectral range of 380 < λ