Catalysis Science & Technology

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Putla Sudarsanam,a Brendan Hillary,a Dumbre K. Deepa,a Mohamad Hassan Amin,a Baithy Mallesham,b. Benjaram M. Reddyb and Suresh K. Bhargava*a. 5.

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Catalysis Science & Technology

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DOI: 10.1039/C5CY00525F

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Highly efficient cerium dioxide nanocubes-based catalyst for low temperature diesel soot oxidation: cooperative effect of cerium- and cobalt-oxides

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

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A variety of catalytic materials based on alkali metal-titanates and -zirconates, supported noble metals, and metal oxides have been studied for diesel soot oxidation.4 Noble metal-based -------------------------------------------------------------------------------

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Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, VIC 3001, Australia. E-mail:[email protected]; Tel: +61 3 9925 2330 b Inorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical Technology, Uppal Road, Hyderabad, 500 007, India. † Electronic Supplementary Information (ESI) available: experimental details, characterization results, and comparative assessment for activity of various CeO2-based catalysts. See DOI: 10.1039/b000000x.

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Diesel engines are promising power sources due to their high fuel efficiency, low CO2 emissions, and remarkable durability compared to equivalent gasoline powered engines.1 However, soot emitted from diesel engines causes huge environmental and human health problems including lung cancer.2 These concerns limit the broad range applications of diesel engines in the industrial sector. Significant efforts have therefore been made for the elimination of soot. The combination of a diesel particulate filter (DPF) and catalytic materials is an efficient technological option for diesel soot elimination.2a,b The function of a DPF is to capture the soot emitted from diesel engines, which is ultimately removed by catalysts via oxidation processes. It is clear that the DPF must be regenerated frequently to capture the soot, and to consequently enhance the engine efficiency, which is highly dependent on the catalyst performance. The catalyst must be active in the 473–773 K range, which is the working temperature of a typical diesel engine.3 The design of catalyst systems with exceptional soot oxidation activity at a low temperature range has therefore attracted much global attention.

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catalysts are found to show high soot oxidation activity at lower temperatures.1c,d,4a However, noble metals are expensive and emit large amounts of hazardous sulphates, which limit their use in industrial processes.2b,4b Alternatively, investigating the catalytic efficiency of cerium dioxide (CeO2) for soot oxidation has gained paramount research interest due to its interesting physicochemical properties and low cost.2a,b,e CeO2 is a commercially employed component in auto-exhaust catalytic converters for the elimination of toxic gases, like carbon monoxide, nitrogen oxides, and unburned hydrocarbons.2e It is a well-known fact that the ability of cerium to shift from Ce4+ to Ce3+, creating oxygen vacancy defects, plays a favourable role in soot oxidation. However, the performance of CeO2 is quite low compared with the desirable diesel engine working temperature range.5 Several strategies have been reported to improve the catalytic efficiency of CeO2, such as varying the ceria morphology, addition of transition metal oxides, and doping metal ions into the CeO2.1c,d,6,

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Co3O4 promoted CeO2 nanocubes have been found to exhibit an outstanding catalytic activity for the oxidation of diesel soot at low temperatures (50% soot conversion = 606 K). This remarkable performance is attributed to the superior reducible nature of cerium oxide and the preferential exposure of CeO2 (100) and Co3O4 (110) facets. A probable mechanism based on the cooperative effect of the cerium- and cobalt-oxides has been proposed, offering new possibilities for the design of promising materials for catalytic soot oxidation.

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Putla Sudarsanam,a Brendan Hillary,a Dumbre K. Deepa,a Mohamad Hassan Amin,a Baithy Mallesham,b b a 5 Benjaram M. Reddy and Suresh K. Bhargava*

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In particular, the design of CeO2 nanomaterials with controlled morphologies is of special interest due to favourable shapedependent properties.6a,6d-f For example, CeO2 nanocubes have [journal], [year], [vol], 00–00 | 1

Catalysis Science & Technology Accepted Manuscript

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been found to exhibit superior redox and catalytic properties, which is attributed to preferentially exposure of highly reactive (100) planes. It has been demonstrated that the energy required for creating reactive oxygen vacancy defects on the (100) surface is lower than those on the (110) and (111) surfaces. Consequently, the CeO2 nanocubes have been employed as effective materials in many important catalytic applications. Hence, in this work CeO2 nanocubes with an average diameter of 32 ± 2 nm were synthesized by a hydrothermal method without using any organic substrates (ESI†). Cobalt oxide (CoOx) was deposited onto the surface of CeO2 nanocubes using a wetimpregnation method (ESI†). CoOx is used extensively as a promising alternative to precious metals in oxidation catalysis. For this reason, CoOx was selected as a promoter in this study. The efficiency of the developed catalysts was studied for diesel soot oxidation by a TG method (ESI†). Several advanced analytical techniques were used to understand the structural, chemical, and electronic properties of the materials and their role in diesel soot oxidation (ESI†).

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The CeO2 cubes show XRD peaks at 2θ values of 28.26, 32.83, 47.25, 56.03, 69.2, 76.51, and 78.91°, indicating the presence of fluorite-structured CeO2 (Fig. 1A).7 A small XRD peak was observed at ~36.5° for the CoOx/CeO2 sample, which indicates the presence of cubic spinel Co3O4. The XRD peaks of the CoOx/CeO2 are shifted to lower angles compared with that of pure CeO2 (Fig. 1B). This observation is evident from the estimated lattice parameters (Table S1). The radii of Co ions (Co2+ = 0.075 nm and Co3+ = 0.061 nm) are much smaller than that of Ce4+ ions (0.097 nm).8 If Co ions replace Ce4+ ions in the CeO2 lattice, a lattice contraction must be observed due to the smaller ionic radii of Co ions with respect to Ce4+. Interestingly, a lattice expansion is noticed after the addition of CoOx, indicating that no Co ions are doped into the CeO2 lattice. It has been demonstrated that the conversion of Ce4+ to Ce3+ is a probable reason for the lattice expansion.9 The radius of Ce4+ (0.097 nm) is smaller than that of Ce3+ (0.114 nm), hence there is an expansion in the lattice during the reduction of Ce4+ to Ce3+. The N2 adsorption-desorption isotherms can be classified as Type IV isotherms with H1-type hysteresis, which are characteristic of mesoporous materials (Fig. S1, ESI†). The BET surface area of the CeO2 and Co3O4/CeO2 samples is found to be 30 and 27 m2/g, respectively. The BJH analysis data indicates broad pore size distributions with average pore diameters of 14.04 and 16.28 nm for CeO2 and CoOx/CeO2, respectively (Fig. S2 and Table S1, ESI†). The pore volumes for CeO2 and CoOx/CeO2 are found to be 0.165 and 0.170 cm3/g, respectively (Table S1, ESI†). The Raman spectrum of the CeO2 cubes shows a sharp peak at ~464 cm-1 (Fig. S3, ESI†), indicating the presence of the Ramanactive F2g mode of fluorite-structured CeO2, in line with the XRD results (Fig. 1A).2b The CoOx/CeO2 sample shows five Raman activate modes of Co3O4.10 Appearance of bands at around 195, 520, and 618 cm-1 are indicative of tetrahedral sites (CoO4) with F2g symmetry, while the band at ~480 cm-1 is assigned to Eg symmetry in Co3O4. Another Raman band at ~687 cm-1 indicates octahedral sites (CoO6) with A1 symmetry. The F2g band of CeO2 cubes is changed significantly after the addition of CoOx. In 2 | Journal Name, [year], [vol], 00–00

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particular, the intensity of the F2g band of the CoOx/CeO2 sample is much lower compared with that of CeO2 cubes. This unusual observation reveals a higher disorder in the anion sublattice.11 Additionally, the F2g band of the CoOx/CeO2 sample is shifted towards lower wavenumbers with respect to CeO2 cubes. This band shift is mainly due to variations in the M–O vibration frequency. In general, vibrations are slower for the expanded lattice and rapid for the contracted lattice, and consequently, the F2g band shifts to lower and higher wavenumbers, respectively.6b Therefore, the lattice expansion, as evidenced by XRD studies (Fig. 1B), is the key reason for the F2g band shifting towards lower wavenumbers. Fig. 1C shows the Ce 3d XPS spectra of the samples. As shown in Fig. 1C, the Ce 3d spectra are complex and fitted with eight peaks corresponding to four pairs of spin-orbit doublets.12 The doublets (v ~882.05 eV, u ~900.45 eV), (u// ~906.97 eV, v// ~888.26 eV), and (u/// ~916.34 eV, v/// ~897.94 eV) indicate the Ce4+ state, whereas the doublet (u/ ~902.19 eV and v/ ~884.34 eV) is indicative of Ce3+ state. Appearance of these peaks is evidence of both Ce3+ and Ce4+ in the synthesized samples. The intensity ratio of the u/ and v/ peaks to the total area of Ce 3d peaks (ICe3+/ITotal) is used to determine the surface concentration of Ce3+ ions: a higher ratio (ICe3+/ITotal) indicates a higher Ce3+ concentration.12 The estimated ICe3+/ITotal values (0.1660 and 0.1099 for CoOx/CeO2 and CeO2, respectively) reveal that the CoOx/CeO2 sample has a higher number of Ce3+ ions compared with pure CeO2. EELS analysis has been also performed to estimate the relative strength of Ce3+ and Ce4+ in the developed catalysts. The observed M4 and M5 peaks indicate the 3d3/2→4f5/2 and the 3d5/2→4f7/2 transitions in CeO2, respectively, which are associated with spin-orbit splitting.13 The relative intensity of the M5 and M4 peaks (IM5/IM4) reflects the redox properties of CeO2: a higher ratio indicates a higher reducible nature. The estimated IM5/IM4 values for CeO2 and CoOx/CeO2 samples are 0.691 and 0.798, respectively. These results reveal that the reducible nature of the ceria is enhanced after the addition of CoOx, in line with the XPS studies (Fig. 1C). H2-TPR analysis was performed to gain more insight into the reducible behavior of the catalysts (Fig. S4, ESI†). Two reduction peaks are found for the CeO2 sample, while the CoOx/CeO2 sample has several peaks. The assigned α (1143 K) and β peaks (867 K) reveal the bulk and surface reduction of CeO2, respectively, which are shifted to a lower temperature for the CoOx/CeO2 sample.14 This observation confirms the highly reducible nature of the CoOx/CeO2 sample compared with that of CeO2, and supports the observations made from the XPS (Fig. 1C) and EELS studies (Fig. 1D). It is therefore confirmed that the presence of more Ce3+ ions is the reason for the observed lattice expansion in the CoOx/CeO2 sample (Fig. IB and Table S1, ESI†). The observed peaks at around 567 and 681 K can be assigned to Co3+ to Co2+ (γ) and Co2+ to Co (δ) transformations, respectively.15 The Co 2p spectrum shows two peaks at ~795.41 and 780.26 eV for CoOx/CeO2 sample, which are attributed to Co 2p1/2 and Co 2p3/2, respectively (Fig. S5, ESI†). The estimated value of the spin-orbit splitting is 15.15 eV, confirming the presence of a Co3O4 phase in the CoOx/CeO2 catalyst.16 The presence of a This journal is © The Royal Society of Chemistry [year]

Catalysis Science & Technology Accepted Manuscript

DOI: 10.1039/C5CY00525F

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Fig. 2 (A) TEM, (B) magnified TEM, (C) HRTEM, (D) particle size distribution of CeO2 cubes, (E) HRTEM of the CoOx/CeO2 and (F) & (G) STEM-EELS elemental mapping images of the CoOx/CeO2 sample: green-Ce and red-Co (scale bar is 20 nm). This journal is © The Royal Society of Chemistry [year]

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Fig. 3 (A) Soot conversion (%) versus temperature (K) for CeO2 cubes, Co3O4/CeO2 cubes, Co3O4/conventional CeO2, conventional CeO2 and un-catalysed (tight contact) and (B) CeO2 cubes and Co3O4/CeO2 cubes (loose contact). The catalytic efficiency of the CeO2 nanocubes and Co3O4 promoted CeO2 cubes was studied for soot oxidation under both tight (3A) and loose contact (3B) conditions.1a,2b,6a,6b For comparison, the soot oxidation efficiency of conventional CeO2 synthesized by a precipitation method and impregnated Co3O4 on conventional CeO2 was also studied under identical conditions (Fig. 3A). It was clear from Fig. 3A that soot oxidation starts in the range of 570-600 K for highly active catalysts and is completed at a temperature of ∼1000−1100 K for the less active catalysts (Fig. S12, ESI†). For comparison, the T50 values (the temperature at which 50% of soot conversion was achieved) of the samples are estimated. The CeO2 nanocubes exhibit a high catalytic performance (T50 ~723 K) with a huge temperature difference of 113 K with respect to conventional CeO2 (T50 ~836 K). This observation demonstrates that the morphology of CeO2 plays a key role in the elimination of soot. On the other hand, it is interesting to note that the soot conversion is very low (i.e., T50 ~886 K) under un-catalyzed conditions. This observation clearly indicates the necessity of the ceria-based catalysts for the oxidation of soot at low temperatures. The efficiency of the CeO2 cubes was significantly improved after the addition of Co3O4 (T50 ~606 K). The difference between T50 values of Co3O4/CeO2 cubes and CeO2 cubes was found to be 117 K, revealing the promoting role of Co3O4 on the catalytic performance of CeO2 Journal Name, [year], [vol], 00–00 | 3

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The TEM image of the as-synthesized CeO2 clearly shows the cubic morphology (Fig. S9, ESI†). Fig. 2 (A & B) shows TEM images of the CeO2 cubes after calcination at 773 K. It was found that most of the CeO2 cubes are in uniform size with an average diameter of 32 ± 2 nm (Fig. 2D). The lattice fringes of the CeO2 nanocubes are clearly visible with a d-spacing of 0.27 nm, which is attributed to the CeO2 (100) plane (Fig. 2C & Fig. S10, ESI†). The Co3O4/CeO2 sample possesses (110) and (111) faces of Co3O4 with a lattice spacing of 0.286 and 0.467 nm, respectively (Fig. 2E & Fig. S10, ESI†). The estimated average diameter of Co3O4 particles is 8 ± 2 nm. Interestingly, most of the Co3O4 species are present along the edges and corners of the CeO2 cubes (Fig. 2E, 2F, 2G & Fig. S10, ESI†). The estimated Ce and Co mass contents from the TEM-EDS analysis are very close to the actual values of 90 and 10 wt.%, respectively (Fig. S11, ESI†).

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Co3O4 phase in the CoOx/CeO2 catalyst is also evidenced from the EELS analysis (Fig. S6, ESI†).17 The O 1s spectra exhibit two peaks at ~529.13 and 530.84 eV, corresponding to the O2 ions in CeO2 or Co3O4 and surface adsorbed oxygen, respectively (Fig. S7, ESI†). The observed broad FT-IR band centred at ~557 cm-1 and a sharp band at ~665 cm-1 can be assigned to the Co–O stretching vibrations of cubic spinel Co3O4 (Fig. S8, ESI†).17

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nanocubes for soot oxidation. Similar results were also found under the loose contact condition (Fig. 3B). The achieved T50 values for CeO2 nanocubes and Co3O4/CeO2 nanocubes are 835 and 794 K, respectively. It was also found that the Co3O4/CeO2 cubes show a higher catalytic efficiency (T50 = 606 K) compared with that of Co3O4/conventional CeO2 (T50 = 660 K) (Fig. 3A). This observation reveals the synergetic effect of Co3O4 and CeO2 nanocubes in the oxidation of diesel soot. We have made an attempt to correlate the performance (T50 values) of the Co3O4/CeO2 catalyst with that of different CeO2-based materials reported in literature (Table S2, ESI†). It was found that the Co3O4/CeO2 catalyst shows the highest soot oxidation performance, indicating the significance of the present work. It has been demonstrated that reduced cerium oxide plays a favourable role in soot oxidation through active oxygen formation.6a,18 Ceria is reduced by carbon soot at the soot/ceria interface creating oxygen vacancy defects. The gas phase oxygen adsorbs on the surface of reduced ceria, resulting in the generation of active oxygen species. The active oxygen species contribute to the oxidation of soot by a spillover mechanism. XPS, H2-TPR and LEES studies reveal that the reducible nature of the CeO2 nanocubes is significantly improved after the addition of Co3O4. In many catalytic applications, including soot oxidation, CeO2 nanocubes with well-defined (100) planes are usually more active than conventional ceria with exposed (111) planes because of the lower energy required for formation of oxygen vacancies.6a This reason explains the high catalytic efficiency of the CeO2 nanocubes compared with conventional CeO2 in soot oxidation (Fig. 3A). Several studies have also reported that Co3+ species present in Co3O4 [Co2+(Co3+)2O4] act as active sites for oxidation reactions, including CO oxidation, ethylene oxidation, CH4 oxidation, and soot oxidation.19 The (110) planes of Co3O4 are mainly composed of Co3+ ions. HRTEM studies confirmed that Co3O4 species with (110) planes are present at the corners and edges of the CeO2 nanocubes (Fig. 2E & Fig. S10, ESI†). Based upon the above observations, a possible mechanism has been proposed for soot oxidation: CeO2 nanocubes act as a source for the formation of active oxygen species and the CeO2-Co3O4 interface acts as an adsorption site for soot (Scheme 1). The active oxygen species can readily oxidize the soot particles at the CeO2-Co3O4 interface.

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The authors thank the RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to their instruments used in this study. We thank Dr. Matthew Field, RMIT University for his immense help for technical assistance for characterization.

References

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Scheme 1. Proposed mechanism for soot oxidation over heteronanostructured Co3O4/CeO2 catalyst.

Conclusions It was found that the addition of Co3O4 to CeO2 nanocubes significantly enhances its catalytic efficiency for diesel soot oxidation. It was proposed that soot oxidation occurs at the CeO2Co3O4 interface with preferentially exposed CeO2 (100) and Co3O4 (110) planes. The outstanding activity of the promoted CeO2 nanocubes is expected to bring new opportunities in the design of efficient CeO2-based catalysts for diesel soot oxidation.

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Catalysis Science & Technology Accepted Manuscript

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Catalysis Science & Technology

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TOC Graphic Co3O4 promoted CeO2 nanocubes have been found to exhibit a remarkable catalytic activity for low temperature soot oxidation, which is attributed to the superior reducible nature of cerium

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oxide and the preferential exposure of reactive CeO2 (100) and Co3O4 (110) facets.

Catalysis Science & Technology Accepted Manuscript

DOI: 10.1039/C5CY00525F