Perovskite-type Oxides as the Catalyst Precursors for Preparing

Review Cite This: Ind. Eng. Chem. Res. 2018, 57, 1−17

pubs.acs.org/IECR

Perovskite-Type Oxides as the Catalyst Precursors for Preparing Supported Metallic Nanocatalysts: A Review Qilei Yang,†,‡ Guilong Liu,*,§ and Yuan Liu*,†,‡ †

Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China ‡ Collaborative Innovation Center of Chemical Science & Engineering, Tianjin 300072, P. R. China § College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, Henan, P. R. China ABSTRACT: Industrial and nanometallic catalysts are composed of an active component of metallic nanoparticles tailored by one or more additives and/or other metals, which should be in interaction and highly dispersed on a support. Therefore, it is a tough challenge to design and prepare such complicated catalysts. One promising way involves using perovskite-type oxides (PTOs, or written as ABO3 according to the composition) as the catalyst precursors. That is the subject of this Review, with the aim to provide inspiration for designing and developing supported metallic nanocatalysts. The contents include ABO3, partially substituted ABO3 (A1‑yA′yBO3, AB1‑xB′xO3, and A1‑yA′yB1‑xB′xO3), and PTOs supported on a support as the catalysts precursors. In PTOs, transition metal ions at A, A′, B, and B′ sites are confined in a PTO crystallite, and those ions are mixed uniformly at the atomic level, which is the basis for using PTOs as the precursors.

1. INTRODUCTION Perovskite-type oxides (PTOs), with the general formula of ABO3, are well-known catalysts for oxidation or reduction reactions in the field of heterogeneous catalysis, which show particularly good performance for applications involving high temperatures, owing to their high stability.1,2 In ABO3, the A sites are usually occupied by rare-earth, alkaline-earth, or other large ions, and the B sites are filled with transition metal cations, as shown in Figure 1.3 According to the Goldschmidt tolerance factor (t) as defined in eq 1, rA + rO t= 2 (rB + rO) (1)

Figure 1. Crystal structure of a cubic perovskite oxide with a chemical formula of ABO3. Adapted with permission from ref 3. Copyright 2015 American Chemical Society.

where rB, rA, and rO are the radius of the cation B, the cation A, and the anion O2−, respectively, the structure of the perovskite would be stable as long as the tolerance factor lies between 0.8 and 1. The ideal cubic structure is characterized by a tolerance factor of 1. Moreover, the A and B sites can be substituted by other cations, resulting in a general formula of A1‑yA′yBO3, AB1−xB′xO3, or A1‑yA′yB1‑xB′xO3, and 90% of the metallic elements in the Periodic Table can be stabilized in the lattice of the perovskite structure.1,2 Meanwhile, PTOs’ redox and surface properties can be adjusted by partially substituting the A and/or B sites, which may have an extraordinary effect on the catalytic performance. Reviews about PTOs as catalysts or catalyst supports, including the method of catalyst preparation and the surface properties of PTOs for oxidation reactions, reduction reactions, © 2017 American Chemical Society

hydrogenation and hydrogenolysis reactions,2,4 NO removal,5 steam reforming of bio-oil,6 photocatalysts,7−10 and electrocatalysts,11 can be found in the open literature. However, as for works on the use of PTOs as precursors for preparing metallic catalysts, only two reviews can be found, and they are concentrated on specific catalytic reactions and catalyst compositions, both by Valderrama et al.12,13 The reviews reported La 1 ‑ y Sr y CoO 3 , La 1 ‑ y Sr y Ni 0 . 4 Co 0 . 6 O 3 , and La0.8Sr0.2Ni1‑xCoxO3-based perovskite-type solid solutions as Received: Revised: Accepted: Published: 1

August 8, 2017 November 30, 2017 December 7, 2017 December 7, 2017 DOI: 10.1021/acs.iecr.7b03251 Ind. Eng. Chem. Res. 2018, 57, 1−17

Review

Industrial & Engineering Chemistry Research

Table 1. Summary of the Structure Evolutions of the Nonsubstituted ABO3, Partially Substituted PTOs, and PTOs Supported on a Support as the Catalyst Precursor after Reduction catalyst precursor

after reduction

preparation method Section 2.1: ABO3 → B/AOδ precipitation citrate sol−gel nanocasting

LaNiO3 SmCoO3 LaNiO3

Ni/La2O3 Co/Sm2O3 Ni/La2O3

La1‑yCeyNiO3 La1‑yCayNiO3

Ni/CeO2−La2O3 Ni/CaO−La2O3

La1‑yKyNiO3 La1‑ySmyNiO3

Ni/K2O−La2O3 Ni/Sm2O3−La2O3

La1‑ySryCoO3‑δ

Co/SrO−La2O3

LaCr1‑xNixO3 LaCoxNi1‑xO3 LaFe0.6Ni0.2RexO3+δ LaCu0.53Ni0.47O3 LaFe1‑x‑yNixCoyO3

Section 2.3: AB1‑xB′xO3 → B′/ABO3 or B−B′/AOδ or B−B′/ABO3 Ni/La2O3−LaCrO3 urea-based coprecipitation Ni−Co alloy/La2O3 citrate complexation Ni−Re−Fe alloy/La2O3−LaFeO3 the Pechini method Ni−Cu alloy/La2O3 citrate sol−gel Ni−Co/La2O3−LaFeO3 citric acid complexation

Section 2.2: A1‑yA′yBO3 → B/AOδ−A′Oδ self-combustion amorphous citrate decomposition citrate sol−gel modified citrate sol−gel method citrate sol−gel

reaction applied

ref

steam reforming of ethanol methane reforming by CO2 dry reforming of methane

15 20 29

steam reforming of glycerol steam reforming of ethanol

34 36

water−gas shift of reformate gas reforming of methane with CO2 and O2 partial oxidation of methane

37 38

CO2 reforming of CH4 steam reforming of ethanol dry reforming of methane dry reforming of methane steam reforming of ethanol

54 69 70 80 85

42

La0.9M0.1Ni0.5Fe0.5O3 (M = Sr, Ca) La1‑yCayFe0.7Ni0.3O3 La0.8Sr0.2Ni1-xCoxO3

Section 2.4: A1‑yA′yB1‑xB′xO3 → B−B′/AOδ−A′Oδ or B′/ABO3−A′Oδ−AOδ Ru−Ni alloy/CaO−LnOδ (LnOδ = La2O3, Sm2O3, or modified citrate method CO2 reforming of CH4 Nd2O3) Ni/SrO−La2O3−LaFeO3 Ni/CaO−La2O3−LaFeO3 modified EDTA-cellulose CO2 reforming of CH4 method Ni/K2O−LaFeO3 citric acid complexation steam reforming of ethanol Co−Ni/SrO−La2O3 sol−gel resin CO2 reforming of methane

103 12

LaNiO3/ZrO2 LaNiO3/CeO2

Section 3.1: ABO3/Support → B/AOδ−Support Ni/La2O3−ZrO2 citrate complexation Ni/La−Ce−O solid solution citrate complexation

CO methanation steam reforming of ethanol

105 106

YRh0.5Fe0.5O3/ZrO2 LaCo1‑xCuxO3/ZrO2 LaNi0.7Co0.3O3/ZrO2 LaCo0.7Cu0.3O3/SiO2 LaRuxNi1‑xO3/SiO2

Section 3.2: AB1‑xB′xO3/Support → B−B′/AOδ−Support Rh−Fe alloy/La2O3−ZrO2 citrate complexation Co−Cu alloy/La2O3−ZrO2 citrate complexation Ni−Co alloy/La2O3−ZrO2 citrate complexation Cu@Co/La2O3−SiO2 Co@Cu/La2O3−SiO2 citrate complexation Ru@Ni/La2O3−SiO2 citrate complexation

higher alcohol synthesis higher alcohol synthesis steam reforming of ethanol higher alcohol synthesis CO methanation

107 108 110 114 116

La1‑yCeyCo1‑xCuxO3/ZrO2 La1‑yYyCo1‑xCuxO3/SiO2

Section 3.3: A1‑yA′yB1‑xB′xO3/Support → B−B′/AOδ−A′Oδ−Support Cu−Co alloy/CeO2−La2O3−ZrO2 citrate complexation Cu−Co alloy/Y2O3−La2O3−SiO2 citrate complexation

higher alcohol synthesis higher alcohol synthesis

119 120

LnyCa1‑yRu0.8Ni0.2O3

catalyst precursors for CO2 reforming of methane, where the PTOs were prepared according to combustion and sol−gel resin methods. After reduction, La1‑ySryCoO3 was transformed to Co/La2O3−SrO, where SrO worked as an electronmodifying agent for Co and La2O3 can eliminate the deposited carbon. Similarly, reducing La0.8Sr0.2Ni1‑xCoxO3 resulted in the formation of bimetallic Co−Ni nanoparticles (NPs) highly dispersed on SrO−La2O3. The high dispersed Co−Ni, SrO, and La2O3 (converted to La2O2CO3 in the reaction process) together contributed to its high activity and good stability. This Review will summarize papers on the preparation of metallic nanocatalysts by using PTOs as the catalyst precursors up to the present, with an emphasis on the features of the prepared catalysts, the structure evolution, and the correlation between the structure and the catalytic performance. PTOs as catalyst precursors are based on the fact that the transition

93 96

metal ions are confined in a PTO crystallite and mixed uniformly at the atomic level; therefore, the metallic atoms/ ions would tend to be in interaction after reduction, and the active metallic NPs reduced from the PTOs would be highly dispersed. The content of this Review can be divided into the following parts: (1) ABO3 as the catalyst precursors. For example, Ni/La2O3 could be prepared by reducing LaNiO3, and the main advantages of using LaNiO3 perovskite as the precursor include (a) the formation of uniform and small particle size of metallic Ni NPs ( 0.1, La2NiO4, and NiO would be generated. The activity improvement for La1‑ySmyNiO3‑δ could be attributed to the inhibition of coke deposition, for that the Ni NPs are highly dispersed at the surface of a matrix of La2O3 and Sm2O3, on which the deposition of carbon could be suppressed. This is similar to the report by Resende et al.,39 where LaNiO3, La1‑yPryNiO3, and La1‑ySmyNiO3 were investigated as the precursors of the catalysts for steam reforming and oxidative reforming of acetic acid. La1‑ySryCoO3‑δ perovskite had been reported extensively for catalytic applications, such as for oxidation of carbon monoxide, propane, and methanol.40 Ao et al.41 prepared La1‑ySryCoO3 by co-precipitation method for Fischer−Tropsch (FT) synthesis. Increasing the partial substitution of lanthanum by strontium beyond y = 0.1 would change the stable rhombohedral structure of La1‑ySryCoO3 toward a less stable cubic symmetry, which is easier to be reduced while more inclined to be sintered during the reduction process. At high Sr substitution levels (y ≥ 0.2), the inactive compound of SrCO3 was formed, which covered the catalyst surface and further affected the activity. At low strontium substitution level (y = 0.1), the catalytic activity in FT synthesis was improved compared to LaCoO3, attributing to the improvement in reducibility without the negative effect of Co coverage by SrCO3. In addition, the regeneration of a catalyst is essential for practical application. Morales and co-workers42 reported the performance and stability of La0.5Sr0.5CoO3‑δ synthesized by the sol−gel citrate method for synthesis gas production by partial oxidation of methane. After the total reduction of La0.5Sr0.5CoO3‑δ, the catalyst was transferred into Co/La2O3− SrO, which showed high activity and very good stability. The same group43 also investigated the catalytic activity and stability of La0.6Sr0.4CoO3‑δ for hydrogen production by SRE and oxidative SRE. The catalyst was highly selective and stable for hydrogen production, which can be attributed to the following reasons: under reducing atmosphere the cobalt ions in the perovskite could be reduced to NPs of metallic cobalt; La2O3− SrO could promote the dispersion or inhibit the agglomeration of the Co NPs. What’s more, the precursor could be totally regenerated to the initial perovskite structure under thermal treatment in air. According to the XRD analysis as shown in Figure 5, the regenerated La0.6Sr0.4CoO3‑δ presented only diffraction peaks of the perovskite structure, suggesting that the catalyst precursor is a PTO. In addition, neither metallic cobalt nor cobalt oxide for the re-oxidized catalyst was detected by XRD, meaning perovskite structure can be regenerated by reoxidizing the Co/La2O3−SrO catalyst. Thus, the following cycle may take place for the catalyst precursor under redox conditions as shown in eq 2.

Figure 5. X-ray diffraction patterns for La0.6Sr0.4CoO3‑δ: (a) asprepared; (b) after reducing in 5% H2/Ar at 700 °C for 1 h; and (c) after regenerating in air at 900 °C for 12 h, and at 300 °C for 72 h. Phases: (○) cubic La0.6Sr0.4CoO3‑δ; (□) β-Co; (Δ) La2O3; (◇) SrO. Adapted with permission from ref 43. Copyright 2015 Elsevier.

There are some restrictions for the A sites substitution, including (1) the amount of the partial substitution may be not high in order to obtain pure perovskite phase and (2) the coverage of the active sites may be existed by the substituted species. The catalyst precursor of PTOs can be regenerated under thermal treatment at proper atmosphere, which may be an attractive feature for the practical application. 2.3. AB1‑xB′xO3. As for PTOs catalysts, the partial substitution at the B sites in ABO3 leads to changes in the perovskite structure, the oxygen mobility, and the redox properties of the perovskites, which have an effect on the catalytic activity.44,45 Therefore, the B sites substitution could provide a wide platform for preparing versatile catalysts. ABxB′1‑xO3 could transform into mainly three types of catalysts, including B′/ABO3, B−B′/AOδ, and B−B′/ABO3, according to the nature of B and B′ during reduction. Interestingly in B−B′ catalysts, B−B′ can be in the state of B−B′ alloy, metallic B and B′ NPs in close contact, and bimetallic B−B′ in core−shell structure, which make PTOs attractive and interesting for use in the design of advanced catalytic materials and the preparation of supported metallic nanocatalysts. 2.3.1. AB1‑xB′xO3 → B′/ABO3. Metallic NPs supported on a PTO as a kind of valuable new catalysts can be prepared by using partial substitution at the B sites. This requires two conditions for AB1‑xB′xO3: (1) the B′ ions can be easily reduced under the reduction atmosphere; (2) B ions are stable enough in reduction atmosphere at high temperature such as Fe, Cr, Mn, and so on. For example, Bourzutschky et al.46 and Van et al.47 reported the catalytic performance of Cu/LaMnO3 derived from LaMn1‑xCuxO3+γ and Cu/LaTiO3 from LaTi1‑xCuxO3 for CO hydrogenation. Before the reduction, copper ions were evenly dispersed in the lattice of LaMn1‑xCuxO3+γ or LaTi1‑xCuxO3, and in the reduction process, the copper ions moved onto the surface and were reduced into copper atoms, which can be highly dispersed on the support because of the much high dispersion of copper ions in the precursor. Thus, in the both catalysts, the active metal of Cu can be highly dispersed on the support, and the elimination of carbon can be accomplished by the oxygen vacancies on the support of the PTOs.

reduction

La 0.6Sr0.4CoO3 − δ XooooooooY CO/La 2O3−SrO oxidation

(2)

To summarize this section, the following can be concluded. Partial substitution at the A site (by Ce4+, Ca2+, Sr2+, Sm+, or + K ) in ABO3 could improve the dispersion of the active metal NPs and facilitate the interaction between the metal NPs and the support. On the other side, the substituted metal oxide can modify the oxide of AOδ to improve the catalytic performance, leading to higher carbon deposition resistance, better sintering resistance, and/or higher selectivity. 6

DOI: 10.1021/acs.iecr.7b03251 Ind. Eng. Chem. Res. 2018, 57, 1−17

Review


Figure 6. Schematic illustration of the possible structure of B and B′ supported on AOδ derived from AB1‑x B′xO3 precursor.

perovskite and a series of LaMn0.4‑xZnxCu0.6Oδ (x = 0, 0.1, 0.2, 0.3, or 0.4) catalysts with perovskite structure prepared by a sol−gel method for methanol synthesis from CO2 hydrogenation. Both La2CuO4 (perovskite-like structure) and LaMnO3 perovskite structure can be detected in the asprepared materials. With decreasing the ratio of Mn/Zn, more oxygen defects were formed, and the catalytic performance was improved owing to the formation of the Cu+−O−Zn2+ sites, which can activate hydrogen and carbon dioxide. According to the authors, the existence of Cu+ is important and can be attributed to the abundant defects of the perovskite structure and the strong interaction between different elements, including the strong interaction between La2CuO4 and LaMnO3. In summary, for B′/ABO3 derived from reducing AB1‑xB′xO3, the features include high dispersion of B′ on the support, a specific valence state of B′ which may be stabilized on the surface of the support, and the fact that the PTO support may adsorb and activate the reactant molecules. 2.3.2. AB1‑xB′xO3 → B−B′/AOδ. The supported metallic nanocatalysts are widely used in industry, while the extensively well-studied catalysts are usually mono metal catalysts. Recently, bimetallic or multimetallic nanocatalysts have attracted great attention.62,63 The traditional preparation methods for supported bimetallic alloy NPs mainly include impregnation and co-precipitation methods,64 which easily lead to the formation of the non-homogeneous distribution of the bimetallic NPs and/or the existence of the mono-metal particles. It is an effective way for preparing nanoscale bimetallic catalysts by using complex metal oxides as the precursors, such as hydrotalcite, spinels, and PTOs.65 When using hydrotalcite-like compounds and spinel oxides as the precursors to prepare supported bimetallic alloy NPs, the metal ions are restricted to divalent/trivalent ions and with size restriction. Using PTOs as the precursors is an alternative for preparing nanoscale bimetallic or multimetallic catalysts. Fine dispersion of bimetallic B−B′ on AOδ can be obtained by reducing AB1‑xB′xO3 perovskite, where B and B′ are any transitional metal ions which can be filled in the perovskite lattice. AB1‑xB′xO3 could be transformed into NPs of B and B′ on AOδ support in several possible configurations according to the nature of B and B′ as illustrated schematically in Figure 6: (1) B−B′ alloy NPs which required B and B′ are mutually soluble; (2) B and B′ as separate NPs but still in close contact; (3) B− B′ NPs with core−shell structure. As there is no relevant literature to report pure AB1‑xB′xO3 transform into B−B′ with core−shell structure supported on

LaFexNi1‑xO3 has surface and bulk/lattice oxygen vacancies, which should be beneficial for the elimination of the carbon deposited.48−51 Chen et al.52 prepared NiO/LaFexNi1‑xO3 catalysts by the citric complexing method for SRE and obtained very good activity and stability, owing to the well-dispersed NiO/Ni on the support and the elimination of the deposited carbon via the oxygen vacancies on the support of the PTO. Song et al.53 also studied Fe partial substituted Ni in LaNiO3 (LaNixFe1‑xO3) catalyst precursors prepared by wet impregnation method for dry reforming of methane. The stability of the perovskite structure is significantly enhanced by the partial substitution of Fe. Meanwhile, improved carbon resistance is observed in the partially substituted catalysts, which is attributed to the smaller particle size and better dispersion of Ni resulting from the strong metal−support interaction. The mobile oxygen in the perovskite matrix in the catalyst may enhance the conversion of CO2 during the methane reforming by CO2, and the perovskite matrix can interact with the supported metal NPs to improve the stability of the highly dispersed metallic NPs.54 In the published work, LaCr1‑xNixO3 was used as the precursor for methane reforming by CO2. Cr ion was selected as the B site ions because (1) LaCrO3 is stable at high temperature55 and (2) the size of Cr3+ (0.615 Å) is close to that of Ni3+ (0.600 Å), and hence the doped Cr may not cause a big distortion of the perovskite structure.56 After reduction, the catalyst is Ni/La2O3−LaCrO3, and the oxygen mobility in the oxygen matrix of LaCrO3 can activate CO2. Therefore, a better catalytic performance can be seen for Ni/ La2O3−LaCrO3. After the reduction of AB1‑xB′xO3, B′ NPs with specific chemical valence can be stabilized on the surface of ABO3. Jia and co-workers57,58 prepared a series of LaCr1‑xCuxO3 and LaMn1‑xCuxO3 catalysts by a sol−gel method used for methanol synthesis from CO2 hydrogenation. The pre-reduced Cu-based LaCr0.5Cu0.5O3 showed much better catalytic performance (XCO2 = 10.4% and SMeOH = 90.8%) at 250 °C than did Cu/ LaCrO3 prepared by regular wet-impregnation method (XCO2 = 4.8% and SMeOH = 46.6%). Similarly, LaMn1‑xCuxO3 also showed very good catalytic performance. The reasons were proposed as follows: after reduction, Cuα+ sites were stabilized on the PTO support of LaCrO3 or LaMnO3; in the catalysts, Cuα+ can activate hydrogen, CO2 could be activated on the PTO support, and the close contact of PTO with Cuα+ would enhance the synergistic effect between them to give a better catalytic performance. Zhan and co-workers59−61 arrived at a similar conclusion by studying La−M−Mn−Cu−O (M = Ce, Mg, Y, or Zn) 7


Review


owing to the fact that La2O3 has the ability to eliminate the deposited carbon. Fierro and co-workers74−79 reported that adding Ru into LaCoO3 could improve the stability of the catalyst at high temperature, such as for preferential oxidation of CO, oxidative reforming of diesel, and hydrocarbon oxidative reforming. During the reactions or pretreatments, La2O3- or La2O2CO3supported Co−Ru NPs were generated. The improved catalytic activity was attributed to the enhanced reducibility of LaCoO3 by Ru catalyzing and to the consequent improved formation of bimetallic Co−Rh NPs. Touahra et al.80 investigated LaCu0.53Ni0.47O3 prepared by using the sol−gel citrate method for dry reforming of methane. During the reaction, the catalyst was changed to bimetallic Ni− Cu/La2O2CO3, where Ni and Cu were in close contact, and the catalyst exhibited better resistance to sintering of metallic NPs and to carbon deposition as compared with the monometallic catalysts. Surendar et al.81 investigated LaCo0.99X0.01O3 perovskites (X = Au, Ag, Cu, or Pt) prepared by co-precipitation method for steam reforming of glycerol reaction. After reduction, LaCo0.99X0.01O3 transformed into X-doped Co/La2O3. The authors found that Cu and Pt are efficient dopants for producing smaller metallic cobalt NPs during the reduction process as compared to Ag and Au dopants incorporation of Cu and Pt in LaCoO3 promotes water−gas shift reaction, leading to enhancement in hydrogen yield. 2.3.3. AB1‑xB′xO3 →B−B′/ABO3. Similar to AB1‑xB′xO3 → B− B′/AOδ, in the route of AB1‑xB′xO3 → B−B′/ABO3, B−B′ could also be in the state of B−B′ alloy NPs, B−B′ as separate NPs but still in close contact, and B−B′ NPs with core−shell structures. The reported papers include B−B′ alloy and B−B′ NPs with core−shell structure. In the case of AB1‑xB′xO3 → B− B′/ABO3, ABO3 is required to be stable enough in reduction atmosphere at high temperature otherwise it would be reduced and transferred to B−B′/AOδ. Fang et al.82 prepared the PTO-supported cobalt oxides (Co3O4/LaFe0.7Cu0.3O3) as the catalyst precursor, where LaFe0.7Cu0.3O3 was prepared according to citrate complexation method and then Co3O4 was loaded on it by wet-impregnation. By reducing the resulted Co3O4/LaFe0.7Cu0.3O3, copper ions in the perovskite lattice were easily segregated by pre-reduction, which resulted in the formation of nanosized copper metal. Under the catalyzing by the Cu NPs, Co3O4 was reduced and located around Cu at higher temperature, which led to the formation of highly dispersed Cu NPs covered by metallic cobalt shell, resulting in the core−shell structure of Cu@Co supported on the PTO of LaFeO3. During the reaction, Cu−Co and La2O3 facilitated the formation of Co2C, and the phase of the catalyst precursor changed to Co2C−Cu/La2O2CO3− LaFeO3. The prepared catalysts were highly active, highly selective, and better stable for higher alcohols synthesis (HAS) from syngas. The high activity and selectivity of the catalyst were attributed to the presence of Cu-modified Co2C species, which were highly dispersed on the surface of La-doped LaFeO3. The good stability of the catalyst was attributed to the interaction between Co2C−Cu NPs and the support. The same group23 also prepared LaFeO3-supported Cu−Co bimetallic catalysts with high surface area and mesoporosity by using nanocasting method combined with impregnation. The prepared catalysts were used for HAS from syngas and showed good stability and high selectivity to higher alcohols. The elements in the catalysts were uniformly mixed and in

AOδ until now, while AB1‑xB′xO3 supported on SiO2 could be transformed into B−B′ with core−shell structure supported on AOδ-SiO2 by tuning the reduction conditions. The relevant content will be discussed in detail in Section 3.2.2. 2.3.2.1. B−B′ Alloy/AOδ. Bimetallic alloy catalysts are beneficial for the synergy of the two active metals. Both cobalt and nickel-based catalysts are very active for SRE in terms of C−C cleavage, and cobalt and nickel-based catalysts showed very high ethanol reforming activity and selectivity to hydrogen, but their stabilities are not good enough due to sintering of the active components and carbon deposition.66−68 Liu et al.69 prepared bimetallic Ni−Co alloy NPs by using PTOs of LaCoxNi1‑xO3 as the precursor according to citrate complexing method for SRE. The resultant catalysts of Ni−Co alloy supported on La2O3 presented high activity and good stability, and the bimetallic alloy catalysts presented much better ability, including resistance to sintering and carbon deposition than the monometallic catalysts as shown in Table 2. Zubenko et al.70 reported trimetallic alloy catalysts of Ni− Re−Fe alloy NPs supported on La2O3−LaFeO3 derived from LaFe0.6Ni0.2RexO3+δ precursor and found that the catalyst showed very good catalytic performance for dry reforming of methane, as shown in Figure 7. The carbon accumulation,

Figure 7. Structure evolution of LaFe0.6Ni0.2RexO3+δ catalysts. Adapted with permission from ref 70. Copyright 2017 Elsevier.

sintering, and evaporation of the Re-containing phases were restricted due to the strong metallic NPs-support interaction given by the in situ growth process of the NPs. 2.3.2.2. B−B′ Close Contact. Tien-Thao and co-workers22,28,71,72 prepared LaCo1‑xCuxO3 and Cu2O/LaCoO3 by a mechano-synthesis process known as reactive grinding and found that the copper outside the perovskite lattice favors the production of methanol and methane, while the copper in the perovskite lattice is advantageous to C2+ alcohol generation. In addition, the existence of strong cobalt−copper interaction in the perovskite lattice can enhance the metallic dispersion of cobalt and copper. Fang et al.73 also prepared Cu−Co bimetal catalysts derived from CuO/LaCoO3 perovskite structure by using citrate complexing method. Under reduction, copper ions in CuO were reduced to metallic Cu at low temperature, and then cobalt ions in LaCoO3 were reduced to metallic cobalt under the catalyzing by the metallic Cu NPs, and in the resultant catalyst of Cu−Co/La2O3, the metallic Cu and Co are in close contact. For the synthesis of higher alcohols from syngas, the prepared catalyst was highly active, attributed to synergistic catalyzing by Cu and Co, and showed good stability 8


Review


points were raised: in the catalysts metal alloys of Co−Fe were formed, and the anti-sintering ability of the metal NPs is very good. 2.3.4. Summary and Remarks on Sections 2.3.2 and 2.3.3. For ABO3 containing two kinds of B site ions, both of the ions can be reduced to metallic state. The focus of the studies is on the preparation of supported bimetallic nanocatalysts, owing to the fact that bimetallic nanocatalysts are of great interest and possess wide applications, while the preparation of bimetallic nanocatalysts is a challenge. The bimetallic nanocatalysts can catalyze reactions under synergistic catalyzing by the two metal, may tailor the catalyzing behavior of a metal by the other metal, may improve the resistance to sintering and carbon deposition, etc.; details on bimetallic catalysts can be seen from recent reviews on this subject.89−92 Using PTOs as a catalyst precursor is an efficient way for preparing bimetallic nanocatalysts, which possesses the features of generally studied bimetallic nanocatalysts. Besides, the bimetallic nanocatalysts derived from a precursor of PTO have a more character, the oxide originated from A site metal ions or from A and B site ions (such as La2O3 or LaFeO3, respectively) can be used as the promoter or support and tend to interact with the metallic NPs. 2.4. A1‑yA′yB1‑xB′xO3. A1‑yA′yB1‑xB′xO3 contains four metallic ions; therefore, the composition variation of the catalysts derived from reducing A1‑yA′yB1‑xB′xO3 should be much more than that from ABO3, and only one site of A or B replaced PTOs. Up to now, metallic catalysts prepared by using A1‑yA′yB1‑xB′xO3 as the precursor contain mainly B−B′/AOδA′Oδ and B′/ABO3-A′Oδ-AOδ according to the nature of A, A′, B, and B′. 2.4.1. A1‑yA′yB1‑xB′xO3 → B−B′/AOδ−A′Oδ. Goldwasser et al.93 studies a series of PTOs of LnyCa1‑yRu0.8Ni0.2O3 (Ln = La3+, Sm3+, or Nd3+) as catalyst precursors for carbon dioxide reforming of methane. After reduction LnyCa1‑yRu0.8Ni0.2O3 was transformed into Ru−Ni alloy NPs supported on CaO−LnOδ (LnOδ = La2O3, Sm2O3, or Nd2O3), and the substitution of Ca by Ln resulted in higher activity (Table 3 because of the fact that the substitution facilitated the reduction of nickel ions. Sutthiumporn et al.94 prepared a series of La0.8Sr0.2Ni0.8M0.2O3 (M = Bi, Co, Cr, Cu, or Fe) by a sol− gel process for CO2 dry reforming of methane. Among all the catalysts, Cu-substituted Ni catalyst showed the highest initial activity owing to the highest amount of accessible Ni, while the Ni NPs would agglomerate in the reaction process, thereby leading to a low stability. However, Fe-substituted Ni catalyst exhibited low initial activity due to the low reducibility of iron and nickel ions in the perovskite lattice, which showed high stability, owing to (1) the strong metal−support interaction which hinders the thermal agglomeration of the Ni NPs and (2) the presence of the abundant lattice oxygen species, which are active to react with CO2 to form La2O2CO3, and La2O2CO3 could react with the surface deposited carbon to produce CO, and thus the deposited carbon could be eliminated as shown in Figure 8. Morales et al.95 reported La1‑yCeyAl0.18Ni0.82O3 (y = 0, 0.1, 0.5, or 0.7) prepared by a self-combustion method and used successfully as precursors to prepare Ni nanoclusters. Perovskite structure was obtained for Ce-free and lower Ce content samples (y = 0, 0.1), whereas, at high Ce contents (y = 0.5, 0.7), a mixture of NiO and CeO2−La2O3 solid solution was obtained. The catalysts were used for aqueous-phase hydrogenation of xylose. In the catalysts, the rare earth ions were used to adjust the basicity/acidity, and the formation of

interaction; thus sintering of the NPs in the catalysts was suppressed, leading to very good stability. The high activity was mainly attributed to the high surface area and mesoporosity. Compared with Cu−Co/LaFeO3 prepared by conventional methods, Cu−Co/LaFeO3 with higher surface area and mesoporosity exhibited better activity and higher selectivity to higher alcohols, which agrees with the work by Kim et al.21 They reported two mesoporous PTOs of LaFe0.7Cu0.3O3 and LaCo0.7Cu0.3O3, both of which showed very good catalytic performance for HAS from syngas. Oemar et al.83 investigated LaNixFe1‑xO3 perovskite catalyst for hydrogen production via steam reforming of tar using toluene as a model compound. LaNi0.8Fe0.2O3 catalyst showed the best performance in terms of activity and stability, attributing to the presence of highly dispersed Ni-rich Ni−Fe NPs, the strong metal support interaction, and the low carbon deposition rate. The synergy between Ni and Fe atoms in the bimetallic Ni−Fe NPs is crucial for the high activity of LaNi0.8Fe0.2O3 catalyst, the strong interaction between the metallic NPs and support prevented the sintering of the NPs, and the oxygen vacancies on the surface of the PTO support could eliminate the deposited carbon. Wang et al.84 prepared Fe−Ni@Ni/LaFeO3−La2O2CO3 by reducing NiO-Fe2O3/LaFe1‑xNixO3, where Fe−Ni@Ni was metallic NPs composed of nickel core and Fe−Ni alloy shell. For comparison, NiO/LaFeO3 was prepared, and after reduction, it was Ni/LaFeO3. For CO methanation, Ni/ LaFeO3 is a little more active, while Fe−Ni@Ni/LaFeO3− La2O2CO3 is much more stable and showed very good resistance to carbon deposition. In another work, the same group85 replaced iron ions partly with nickel ions in LaFeO3 resulted in LaFe1‑xNixO3, then replaced part of iron ions with cobalt ions in LaFe1‑xNixO3 generated LaFe 1‑x‑y Ni x Co y O 3 . After reduction of LaFe1‑x‑yNixCoyO3, Ni−Co/LaFeO3−La2O3 formed, and the catalyst showed excellent resistance to sintering and carbon deposition, which is obviously better than the monometallic catalysts. Characterization results indicated that Ni−Co NPs were in the state of solid solution alloy. For FT synthesis catalysts, the elements of Co and Fe are the most widely adapted active components. Bedel et al.86 prepared LaCoxFe1‑xO3 perovskites as catalyst precursors for FT synthesis using a sol−gel method. Depending on the ratio of Co/Fe the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO3 is maintained for the mixed perovskite when x > 0.5, the orthorhombic phase of LaFeO3 is found for x < 0.5. Interestingly the orthorhombic structure is active for FT synthesis, attributing to that Co−Fe alloy NPs supported on LaFeO3 was generated by reducing the orthorhombic perovskites (x < 0.5); while rhombohedral perovskite (x ≥ 0.5) cannot be used as the precursors due to that Co3+ was reduced into Co2+ other than the active metallic Co. By the way, the oxygen vacancies on the surface of the PTO support could eliminate the coke deposited. Then, the same group made another interesting work,87 a series of La1‑yCo0.4Fe0.6O3‑δ were prepared by a sol−gel method, actually, the composition was γ-Fe2O3−LaCoxFe1−xO3, and the characterization results indicated that it is in the form of the magnetic nanocores of γ-Fe2O3 surrounded by the PTO shell. The catalyst is more active than that of LaCoxFe1‑xO3, owing to that the support of γ-Fe2O3 improved the dispersion of LaCoxFe1−xO3. Escalona et al.88 studied LaFe1‑xCoxO3 for CO hydrogenation to produce hydrocarbons, and similar view9


Review


deposition was attributed to the carbon elimination ability of CaO or SrO. Similar works were also reported by Khalesi and Sutthiumporn,97,98 who attributed the improvement on the resistance to carbon deposition to that the substitution of Ca at A sites enhanced the basicity and increased the content of the oxygen vacancies. Urasaki et al.99 developed a series of catalysts of Co supported on LaBO3 (B = Al, Cr, Mn, or Fe) and found that Co/LaAlO3 showed the highest activity and stability for SRE. Furthermore, partial substitution of La3+ by Sr2+ in LaAlO3 resulted in further remarkable improvement on activity and hydrogen yield. It was proposed that the enhanced mobility of lattice oxygen in La1‑ySryAlO3‑δ allows more frequent participation in the oxidation of intermediates over metallic cobalt, leading to both high catalytic activity and stability of Co/ La1‑ySryAlO3‑δ in comparison with Co/LaAlO3. Chen et al. 1 0 0 prepared La 1 ‑ y Ca y Fe 1 ‑ x Ni x O 3 and La1‑ySryFe1‑xNixO3 perovskites with citrate complexing method for SRE. After reduction, Ni NPs highly dispersed on CaO- or SrO-doped LaFeO3 were formed, and the catalyst showed very good performance, including very good resistance to carbon deposition. The same group101 also simultaneously employed La1‑yCayFe0.7Ni0.3O3 in the reactions of SRE and oxidative SRE to produce hydrogen. For this kind of catalysts, a reduction− oxidation cycle was proposed to explain the excellent resistance to sintering and to carbon deposition, as shown in eq 3.

Figure 8. Schematic diagram of the proposed mechanism of dry reforming of methane reaction over the reduced La0.8Sr0.2Ni0.8M0.2O3 perovskite catalyst. Adapted with permission from ref 94. Copyright 2012 Elsevier.

perovskite was beneficial to the homogeneous mixing of all the ions, after reduction the high dispersion of metal nickel was favored. 2.4.2. A1‑yA′yB1‑xB′xO3 → B′/ABO3−A′Oδ−AOδ. Yang et al.96 prepared La0.9M0.1Ni0.5Fe0.5O3 (M = Sr or Ca) perovskite catalysts by the modified EDTA-cellulose method, and the influence of partial substitution of La by Sr/Ca was investigated for methane reforming by CO2. After reduction, the PTOs transformed to Ni NPs supported on SrO−La2O3−LaFeO3 or CaO−La2O3−LaFeO3 as M = Sr or Ca, respectively. It was found that the perovskite catalysts demonstrated superior carbon resistance during the reaction, catalyst with the composition of La0.9Ca0.1Ni0.5Fe0.5O3 exhibiting the best catalytic performance among the prepared catalysts as shown in Table 3. The schematic diagrams of the reaction mechanism for synthesis gas production over La0.9M0.1Ni0.5Fe0.5O3 (M = Sr, Ca) are shown in Figure 9. The very good resistance to carbon

reduction

La1 − yCa yFe1 − xNixO3 XooooooooY Ni/CaO−La1 − yCa y − δFeO3 oxidation

(3)

Reducing La1‑yCayFe0.7Ni0.3O3 generated Ni NPs highly dispersed on CaO−La1‑yCay‑δFeO3, and oxidation made the highly dispersed Ni NPs back into the perovskite lattice. What’s more, in the process for oxidative SRE, both reductive agents of CO and H2 and oxidative agents of O2 and H2O exist; therefore, the incorporation into/separation out of the nickel element likely be cycled, i.e., the forward and backward reaction in eq 3 was taking place. Thus, the sintering of Ni NPs under oxidative SRE was depressed effectively (Figure 10). Besides, the oxygen in the feed is helpful to the elimination of deposited carbon. It seems promising for overcoming the problems of the active component sintering and carbon deposition in SRE by regulating the redox ability of the PTOs and the feed composition. Replacing the element of Co with Ni, resulted in La 1 ‑ y Ca y Fe 1 ‑ x Co x O 3 , 1 0 2 which w orked similar t o La1‑yCayFe1‑xNixO3. The regeneration of the PTOs was worked in the reaction process, thus the recycling, the regeneration/ reduction was repeated. The similar regeneration of PTOs was stated in Section 2.2 above.

Figure 9. Schematic diagrams of the reaction mechanism for dry reforming of methane over La0.9M0.1Ni0.5Fe0.5O3 (M = Sr, Ca). Adapted with permission from ref 96. Copyright 2015 Elsevier.

Figure 10. Mechanism illustration for the reduction−oxidation cycle of Ni in the perovskite structure and the effect of oxygen addition in the SRE on metallic Ni sintering. Adapted with permission from ref 101. Copyright 2011 Elsevier. 10


Review


better stability. In Ni/La2O3−ZrO2 derived from LaNiO3/ ZrO2, Ni NPs are in close contact with La2O3, for that both Ni NPs and La2O3 were originated from a crystallite of LaNiO3. Thus, the Ni NPs were interacted and confined by the La2O3, improving the sintering resistance. Additionally, the carbon deposited on Ni NPs could be eliminated by La2O3 effectively, for that Ni and La2O3 are in close contact. However, for Ni− La2O3/ZrO2 prepared by the traditional impregnation method, in most cases Ni NPs are away from La2O3; therefore, La2O3 hardly works, as shown in Figure 11.

Carbon deposition is the critical problem for catalysts deactivation for SRE. Several techniques have been reported for improving the ability to prevent carbon deposition, such as adding alkaline to balance the acidic sites on the support, using support which can generate oxygen vacancies and use bimetallic catalysts. Zhao et al.103 tried to improve the ability to prevent carbon deposition by doping additives and prepared La1‑yKyFe0.7Ni0.3O3 (y = 0, 0.05, and 0.1) with perovskite structure by citric acid complexation method. After reduction, K-doped Ni/LaFeO3 was obtained, highly dispersed in both the catalyst of metal nickel NPs and the dopant of K2O. The prepared K-doped catalyst exhibited very good activity, high selectivity to hydrogen, and very good stability, as shown in Table 2. The doping of K improved the ability to prevent sintering of the metal nickel particles, which may be attributed to the barrier roles of K2O between metal nickel particles. The very good resistance to carbon deposition was attributed to the oxygen vacancies on the PTO of LaFeO3 support and the high dispersion of Ni NPs. 2.4.3. Summary and Remarks. A1‑yA′yB1‑xB′xO3 should include the features of both A site substitution and B site substitution. Each of those has been summarized above, such as high dispersion of metallic B′ NPs, A′Oδ works as promoter and using the property of the ABO3 (oxygen vacancies, interaction with metal NPs, etc.). However, the reports on A1‑yA′yB1‑xB′xO3 as catalyst precursor are not many, among the references new and interesting information that the B′ site element may be cycled between B′ ions in the perovskite lattice and the highly dispersed B NPs on the surface of the perovskite crystallite, thus showing excellent resistance to sintering.

Figure 11. Structural evolution of (a) LaNiO3/ZrO2 and (b) NiO− La2O3/ZrO2 prepared with general impregnation. Adapted with permission from ref 105. Copyright 2016 Royal Society of Chemistry.

Compared with Ni−La2O3/ZrO2 prepared by the traditional impregnation method, the citrate complexing method could make all of the metal ions to be homogeneously dispersed, leading to the formation of single LaNiO3 perovskite after calcination. As for traditional impregnation method, NiO and La2O3 were formed beside a few grains of LaNiO3 after calcination. During reduction, the Ni NPs released from NiO are away from La2O3, which does not favor the interaction between Ni and La2O3. Therefore, Ni NPs are inclined to sintering during the reaction process. Loading LaNiO3 with perovskite structure on CeO2 showed another interesting feature.106 Reducing LaNiO3/CeO2 generated Ni NPs supported on La−Ce−O solid solution. The Ni NPs on La−Ce−O are highly dispersed owing to the following two reasons: the one, Ni NPs came from LaNiO3, and in LaNiO3 nickel ions are evenly dispersed; the other, LaNiO3 was dispersed on the support of CeO2. La−Ce−O inclined to interact with Ni NPs and La−Ce−O is good at eliminating the deposited carbon; therefore, the catalyst is very resistant to sintering and to carbon deposition. The reaction applied in this paper is steam reforming of ethanol. 3.2. AB1‑xB′x O3/Support → B−B′ /AOδ−Support. 3.2.1. B−B′ Alloy. For Rh−Fe catalysts prepared by traditional impregnation for HAS from syngas, the existence of mono iron and rhodium is inevitable, as a result methane and other hydrocarbons were the main product. Han et al.107 loaded YRh0.5Fe0.5O3 with perovskite structure on ZrO2 by using the citrate complexing method and used for HAS from syngas to overcome this problem. By reducing YRh0.5Fe0.5O3, Rh−Fe alloy NPs were highly dispersed on Y2O3-doped ZrO2, by using PTOs as the catalyst precursors the formation of mono Fe and Rh were avoided, and the catalyst exhibited very high selectivity to ethanol, as shown in Table 4. Liu and co-workers108,109 loaded a series of LaCo1‑xCuxO3 with perovskite structure on the surface of ZrO2 by using the citrate complexation method. The resulted LaCo1‑xCuxO3/ ZrO2 tends to generate Cu−Co alloy NPs supported on ZrO2 under reduction condition. In the reduced catalyst of Co−Cu/

3. PTOs/SUPPORT The specific surface area of pure PTOs is rather small, so after reduction the size of the metallic NPs is large, generally larger than 10 nm. In the long-time reaction, the active species of the metallic NPs would be further sintered inevitably. An effective way of overcoming this problem is to load the PTOs on a support with high specific surface area, such as SiO2, ZrO2, CeO2, and so on. Although the specific surface areas of ZrO2 and CeO2 are not sufficiently high, they incline to interact with the supported metallic NPs and may activate some reactant; therefore, they are good candidates as the supports of PTOs.104 The design scheme has the following advantages: (1) the support possesses a high specific surface area, thus the NPs would be highly dispersed and the sintering of active metallic NPs on the support would be restricted owing to the interaction and confinement of the oxide derived from reducing the PTOs; (2) after the reduction of ABO3/support, B would mix with the oxide of A uniformly and highly dispersed on the support, for that A and B are evenly distributed at the atomic level in the precursor of ABO3, the uniform mixing of B and the oxide of A favors the interaction between B and the oxide of A; (3) the active B NPs can be tailored, promoted and/or confined by the substitutions at A sites and/or B sites. 3.1. ABO3/Support → B/AOδ−Support. Sintering of active metallic NPs and carbon deposition are two critical problems for many reactions of carbon-containing compounds. Si et al.105 prepared a series of LaNiO3/ZrO2 catalysts according to the citrate complexing method. Reducing LaNiO3/ZrO2 generated Ni/La2O3−ZrO2. Compared with Ni−La2O3/ZrO2 prepared by the traditional impregnation method, Ni/La2O3−ZrO2 derived from LaNiO3/ZrO2 showed higher CO conversion, higher CH4 selectivity as well as much 11


Review


Table 4. Catalytic Performances for Higher Alcohol Synthesis of PTOs and PTOs Supported on Support as Precursor reaction conditions catalyst LaCo0.7Cu0.3O3 YRhO3/ZrO2 YRh0.5Fe0.5O3/ZrO2 30%LaCo0.7Cu0.3O3/ZrO2 30%LaCo0.7Cu0.3O3/ZrO2 20%LaCo0.7Cu0.3O3/SiO2 30%LaCo0.7Cu0.3O3/SiO2 30%La0.95Ce0.05Co0.7Cu0.3O3/ ZrO2 30%La0.8Y0.2Co0.7Cu0.3O3/SiO2 a

T (°C)

P (MPa)

300 290 290 310 270 330 290 270

1000 psi 4 4 3 4 3 3 4

290

3

catalytic performance

GHSV (mL (gcat h)−1) −1

15 000 h 3000 3000 3900 6000 3900 3900 6000 3900

H2/ CO

CO conversion (%)

total alcohol selectivity (%)

C2+ alcohols (wt%)a

ref

2 2 2 2 2 2 2 2

16.0 20.5 34.1 35.3 9.1 32.1 13.8 11.7

38.1 39.8 43.7 43.4 45.6 39.5 50.9 51.6

53.8 27.7 81.6 82.3 69.0 66.1 67.4 71.0

22 107 107 108 119 114 120 119

2

9.2

51.7

70.4

120

The proportion of higher alcohols (C2+ alcohols) in the total alcohol.

Figure 12. Schematic representation of the structural evolution of the LaNi0.7Co0.3O3/ZrO2 catalyst before and after reduction. Adapted with permission from ref 110. Copyright 2016 Elsevier.

Figure 13. Structure evolution of LaCo0.7Cu0.3O3/SiO2 during the reduction process. Adapted with permission from ref 114. Copyright 2014 Elsevier.

ZrO2−La2O3, the Cu−Co alloy NPs are uniformly mixed with La2O3, for both copper and cobalt ions came from the nanograins of LaCo1‑x Cu xO 3 . The Co−Cu/ZrO 2−La 2 O3 catalyst exhibited good activity, excellent selectivity to C2+ alcohols, and very good stability for HAS. The excellent selectivity to C2+ alcohols is attributed to the formation of Cu− Co alloy NPs on ZrO2, and the very good stability is ascribed to the interaction between Cu−Co NPs and La2O3 and the carbon eliminating effect of La2O3. For SRE, the deactivation of the catalysts caused by carbon deposition and/or sintering of the active components is a challenge. Zhao et al.110 designed and prepared the Ni−Co alloy catalysts using a PTO of LaNi1‑xCoxO3 as the precursor to overcome this problem. In the precursor, PTO of LaNi1‑xCoxO3 was loaded on the surface of ZrO2 by using the citrate

complexation method, and after the reduction LaNi0.7Co0.3O3/ ZrO2 was transformed into Ni−Co/ZrO2−La2O3, where Ni− Co is highly dispersed alloy NPs as shown in Figure 12. The bimetallic catalyst exhibited very good catalytic performance for SRE, including high activity, high selectivity to hydrogen, and very good stability, as listed in Table 2, due to the synergistic catalysis of cobalt and nickel. This is similar to the works of LaNiO3/ZrO2105 and LaNiO3/SiO2,111 while the focus is the formation of Ni−Co alloy in the work.110 There are a few attempts to combine PTOs with carbon materials such as carbon nanotubes (CNTs), graphene, and so on. For instance, Weidenkaff et al.112 prepared Ln1‑yAyCoO3 (Ln = Er, La; A = Ca, Sr)/CNTs composite material and used it as an oxygen electrode in zinc/air batteries. Niu et al.113 prepared the bimetallic Cu−Co alloy NPs supported on a 12


Review


Figure 14. Schematic illustration of the structural evolution of Ni@Ru/La2O3−SiO2 from LaRuxNi1‑xO3/SiO2. Adapted with permission from ref 116. Copyright 2017 Elsevier.

and whether Cu and Co are in the form of alloy or another state of bimetal is likely dependent on the conditions of the treatments. Loading NPs with base metal core and noble metal shell on a support should be attractive, which can lower the cost of noble catalysts. Our group116 loaded bimetallic Ni@Ru NPs with Ni core and Ru shell on SiO2 by using LaRuxNi1‑xO3/SiO2 as the precursor and used for CO methanation, as shown in Figure 14, and the catalyst showed very good catalytic performance. The explanations of structural variation by the authors are as follows: (1) A nanocrystallite of LaRuxNi1‑xO3 likes a much large molecule, in which the elements are uniformly dispersed at the atomic level including ruthenium and nickel. At the atmosphere of H2, ions of Ru3+ would be reduced at first, then the ion of Ni3+ would be reduced subsequently, and the resulting Ni atoms would be inclined to overlay on the NPs of Ru, due to that ion of Ni3+ were reduced under the acceleration of metallic Ru via the hydrogen spillover. In the reduction process, La3+ can hardly be reduced, which should be in the state of La2O3. Thus, Ru@Ni/La2O3−SiO2 was obtained. (2) Subsequently, the atoms of Ru would be migrated onto the surface of the Ni−Ru NPs under the treatment of CO at high temperature, because of that the specific surface energy of metallic Ru is lower in CO atmosphere, obtaining Ni@Ru/ La2O3−SiO2, where La2O3 can work as an additive for the catalyst. 3.3. A1‑yA′yB1‑xB′xO3/Support → B−B′/AOδ−A′Oδ−Support. The reactions on the interface of a solid catalyst are very complicated, and in many cases, the target reaction is catalyzed under the synergy of several components. Taking HAS from syngas as an example, the formation of Cu−Co alloy or closely contacted Cu−Co NPs is important, for that higher alcohols are generated under the synergy of Cu and Co atoms, while metallic Cu and Co could produce methanol and methane, respectively. Therefore, to lower the possibility of methanol and methane generation, additives are generally used to tailor the state of Cu and Co NPs. Besides, carbon deposition is a problem, adding promoters is a choice to relieve it. For example, alkali metals (e.g., K or Na) and rare earth metals (e.g., La, Ce, or Y) can suppress the over hydrogenation,117,118 while some transition metals, such as Mn can improve the C−C chain growth. Thus, additives would be more than one. Furthermore, the additives and Cu−Co NPs are required to be in close contact to intensify their interaction and to avoid unwanted effect, and all the actives and additives should be dispersed on a support to avoid aggregation. Thus, to design and prepare a good catalyst for HAS is difficult. Using PTOs as the precursor may be a promising route.

composite consisting of graphene sheet and LaFeO3 with perovskite structure by using hydrothermal combined with impregnation method, and the catalyst showed very good catalytic performance for HAS. To load a PTO on the support of carbon materials is a little complex, because of that transitional metal oxides may react with carbon materials at high-temperature, and the regular method for preparing PTOs usually contains a process of hightemperature treatment. 3.2.2. B−B′ Core−Shell and Closely Contacted Bimetal B− B′. Reducing LaNi1‑xCoxO3/ZrO2 would make Ni−Co alloy NPs supported on La-doped ZrO2, attributed to the facts that (1) both nickel and cobalt ions are uniformly mixed in the PTO of LaNi1‑xCoxO3 and (2) both nickel and cobalt ions are confined in a crystallite of the PTO. While some pairs of B−B′ may not be fit for forming alloys due to their nature of B and B′, the interaction between them would make core−shell NPs or closely contact bimetallic NPs. Liu et al.114 prepared meso-macroporous SiO2-supported LaCo0.7Cu0.3O3 by citrate complexation method and used for HAS. After reduction, bimetallic NPs of Cu−Co supported on La2O3−SiO2 were made. The bimetallic NPs were characterized in a core−shell structure of Cu@Co or Co@Cu, and whether it is Cu@Co or Co@Cu can be adjusted by tuning the reduction condition as shown in Figure 13. The reasons are as follows as stated by the authors. When LaCo0.7Cu0.3O3/SiO2 was heated from room temperature to around 300 °C in H2, Cu ions were reduced into Cu0, and the catalyst changed to Cu−LaCoO3/ La2O3−SiO2. The three kinds of NPs Cu, LaCoO3, and La2O3) should be packed together and supported on SiO2. When the temperature increased to around 580 °C, Co0 began to generate from the reduction of cobalt ions in LaCoO3. Co0 was generated under the catalyzing of the formerly formed Cu0; therefore, Cu was surrounded by Co, and Cu@Co/La2O3− SiO2 was obtained. As for the route marked by the red arrows, in which LaCo0.7Cu0.3O3/SiO2 was heated from room temperature to 600 °C in N2 and then changed the flowing gas to H2. In this situation, copper and cobalt ions could be reduced simultaneously without any catalyzing. Thus, the catalyst changed to nanoparticles of Cu, Co, and La2O3 supported on SiO2, and the NPs of Cu and Co should be in close contact owing to confinement of the PTO crystallites. When the reduction process prolonged, sintering would take place, and Cu would segregate on the surface due to its lower surface free energy, generating Co@Cu/La2O3−SiO2. In some cases,108,109 Cu−Co alloy NPs were dispersed on ZrO2 by reducing LaCo1‑xCuxO3/ZrO2. It is known that copper and cobalt are not fit to form alloy from their phase diagram.115 While at nano size, the phase diagram may be not applicable, 13


Review

Industrial & Engineering Chemistry Research Song et al.119 loaded a PTO of La1‑yCeyCo1‑xCuxO3 on ZrO2 by citrate complexation method and used for HAS from syngas. Compared with the catalyst LaCo 0 . 7 Cu 0. 3 O 3 /ZrO 2 , La0.95Ce0.05Co0.7Cu0.3O3/ZrO2 showed better catalytic performance, specially the selectivity to total alcohols was higher than 50% and the mass fraction of higher alcohols in the total alcohols was about 70% as shown in Table 4. The results indicated that reducing La0.95Ce0.05Co0.7Cu0.3O3/ZrO2 could lead to the formation of bimetallic Cu−Co NPs highly dispersed on La- and Ce-doped ZrO2. Meanwhile, some of the Ce4+ ions would enter the lattice of ZrO2 to form Zr−Ce− O solid solution, and a strong interaction existed among CeO2, ZrO2, and Co, resulting in an enrichment of cobalt at the interface between Co and cerium-containing oxide. The result is similar to the work reported by Hong et al.,120 in which La1‑yYyCo1‑xCuxO3 on SiO2 prepared by citrate complexation method was used for HAS from syngas. Hong also found Cu− Co alloy NPs in close contact with La2O3 and Y2O3 can be highly dispersed on SiO2 after reduction of La1‑yYyCo1‑xCuxO3/ SiO2, and the catalyst showed better performance for HAS compared with that of LaCo0.7Cu0.3O3/SiO2 (Table 4). Even though preparing metallic catalysts by reducing PTOs has many advantages, there are also some limitations. First, for the elements which are stable enough in perovskite lattice under reduction atmosphere, the formation of metallic catalysts seems impossible by reducing PTOs. Second, not all of metal elements in the Periodic Table can enter into the crystal lattice of the perovskite and form perovskite structure according to the Goldschmidt tolerance factor (t), which should be another limitation. Finally, due to the special structure, the content of doping elements in the perovskite is limited in some cases, and the adjustment of the ratio of two/three elements in bimetallic or trimetallic systems is not arbitrary. 3.4. Summary and Remarks. The technique of using PTOs on a support as a catalyst precursor possesses the same characteristics and features as using PTOs as the precursor, as stated in Sections 2.1−2.3, but supported PTOs as the precursor have another attractive advantage, which is confinement. The PTOs on a support are highly dispersed nanocrystallites, meaning that the metallic ions of A, A′, B, and B′ in A1‑yA′yB1‑xB′xO3 are confined in the crystallites with uniform mixing, which favors the interaction between them. By using this property, clusters composed of metallic species and metal oxides could be constructed on a support, which is a much promising way for designing and preparing metallic nanocatalysts. In the view of this preparation method, maybe other complexed oxides, such as spinel supported on a support could also be used to the preparation of metallic catalysts.

much promising for practical applications, while those can be used for academic studies. By selecting the ions at A and B sites and the substituted ions, the wanted composition of a catalyst could be constructed, including active metal of monometal, bimetal, or multimetal, promoters and support, via which insight into the nature of the interaction, synergy, etc., may be obtained. Using supported PTOs as the catalyst precursor is a promising technique for designing and constructing practical catalysts. Considering a lot of metallic ions can be accommodated in the PTOs’ lattices, this technique for the preparation of nanometallic catalysts by reducing PTOs could be extensively applied in many fields.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuan Liu: 0000-0001-9785-8799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work by National Natural Science Foundation of China (NSFC) (Nos. 21576192 and 21376170) is gratefully acknowledged.

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REFERENCES

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4. CONCLUSIVE REMARKS The characters of the catalysts derived from the precursors of PTOs have been summarized at the end of each section, so it is better not to repeat here. The reactions reported for the prepared catalysts mainly include CO2 and CO methanation, steam reforming of hydrocarbons (ethanol, glycerol, acetic acid, and toluene), hydrogenation of CO or other chemicals, higher alcohols synthesis from syngas, methanol synthesis from CO2 + H2, and partial oxidation of methane. All the attempts indicate that to design and to prepare nanometallic catalysts by using PTOs as the precursors are attractive and promising. From the reports on ABO3 and the PTOs substituted with A and/or B sites as the precursors, restricted by their relatively small specific surface area, the prepared catalysts may be not 14


Review


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