CeO2 Catalysis

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Reaction Sensitivity of Ceria Morphology Effect on Ni/CeO2 Catalysis in Propane Oxidation Reactions Xuanyu Zhang, Rui You, Dan Li, Tian Cao, and Weixin Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11536 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Reaction Sensitivity of Ceria Morphology Effect on Ni/CeO2 Catalysis in Propane Oxidation Reactions Xuanyu Zhang, Rui You, Dan Li, Tian Cao and Weixin Huang* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, P. R. China. Corresponding Author *Tel.: 008655163600435. Email: [email protected]

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Abstract: CeO2 nanocubes (c-CeO2), nanoparticles (p-CeO2), nanorods calcined at 500 °C (rCeO2-500) and 700 °C (r-CeO2-700) were used as supports to synthesize a series of Ni/CeO2 catalysts for the propane combustion and oxidative dehydrogenation of propane (ODHP) reactions. The Ni-CeO2 interaction greatly promotes the reducibility of CeO2, but CeO2 morphology-dependent Ni-CeO2 interaction was observed to form different speciation of Ni species (Ni2+ dissolved in CeO2, highly dispersive NiO, NiO aggregate) and oxygen species (strongly-activated oxygen species, medially-activated oxygen species, weakly-activated oxygen species) in various Ni/CeO2 catalysts. Ni-CeO2 interaction is stronger in Ni/c-CeO2 catalysts than in other Ni/CeO2 catalysts. Different morphology-dependences of Ni/CeO2 catalysts in propane combustion and ODHP reactions were observed. The Ni/r-CeO2-500 catalyst with the largest strongly-activated oxygen species is most catalytic active in the propane combustion reaction while the Ni/c-CeO2 catalyst with the largest amount of weakly-activated oxygen species exhibits the best catalytic performance in the ODHP reaction. Thus, the CeO2 morphology engineering strategy is effective in finely tuning the metal-CeO2 interaction and the reactivity of oxygen species to meet the requirements of different types of catalytic oxidation reactions.


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1. Introduction Ceria (CeO2) has been extensively studied as catalysts and catalyst supports in a wide array of catalytic reactions, particularly in catalytic oxidation reactions, due to its high oxygen storage capacity (OSC) and Ce4+/Ce3+ redox cycle.1-3 Recently, morphology engineering of catalyst nanoparticles is emerging as a novel strategy to tune their catalytic performances without changing catalyst compositions and meanwhile to establish their structure-performance relations.4-12 Various uniform CeO2 nanocrystals including octahedra, cubes and rods have been successfully synthesized13-15 and thus the morphology effect of CeO2 nanocrystals on their catalytic performances have been much explored.16-19 For examples, strong CeO2 morphology effects were observed for CO oxidation catalyzed by CeO2 nanocrystals and correlated to the morphology-dependent reducibility and oxygen vacancy structures/concentrations.13-15,20,21 Interestingly, a recent work reported an opposite facet sensitivity of CeO2 nanocrystals in oxidation and hydrogenation catalysis,22 in which the CeO2 (100) surface predominantly exposed in CeO2 nanocubes is optimal for the CO oxidation reaction while the CeO2 (111) surface prevalent in conventional polyhedral CeO2 particles dominates in the C2H2 selective hydrogenation reaction. The metal-CeO2 interaction in CeO2-supported catalysts plays a key role in determining catalyst structures and catalytic performances.23 CeO2 morphology-dependent metal-CeO2 interaction has also been much observed to strongly affect the structures and catalytic performances of CeO2-supported catalysts.24-38 CeO2 nanorods enclosed by {110} and {100} crystal planes were found to be most active for gold stabilization and activation, resulting in the higher catalytic activity of Au/CeO2 nanorods catalyst than other CeO2-supported Au catalysts.24 The CeO2 morphology-dependent local structures of VOx species, VOx-CeO2 interaction and


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catalytic activities in the oxidative dehydrogenation of isobutene reaction of VOx/CeO2 catalysts were observed and related to the oxygen vacancy concentrations of CeO2 support.29-31 An interesting morphology-dependent Nb(V)-surface oxygen vacancy of CeO2 redox reaction was observed to occur in NbOx/CeO2 catalysts, resulting in the formation of Nb(IV) species.35,36 Beyond the chemical states of the catalysts, DRIFTS methods was also used to reveal different chemical pathways and intermediates on morphologically structured CuO/CeO2 catalysts.37 Ni/CeO2 catalysts have been studied as catalysts for water-gas shift reaction,39 selective catalytic reduction of NO with NH3,40 preferential CO oxidation in excess H2,41 methane combustion,42-44 oxidative dehydrogenation of light alkanes45,46 and reforming reactions.38,47-49 The CeO2 morphology effect on NiO/CeO2 catalysts were previously examined in several reactions.50-53 However, systematic studies still lack, especially for CeO2 rods whose surface structures were found to vary with calcination temperatures.54,55 We recently employed CO and CO2 chemisorption to probe the surface structures of various CeO2 nanocrystals,56 in which CeO2 rods calcined at 500 and 700 °C were found to mainly expose {110}+{100} and {111}+{110} facets, respectively. In this paper, the CeO2 morphology effect on Ni/CeO2 catalysts for both the propane combustion and oxidative dehydrogenation of propane (ODHP) reactions were systematically studied with CeO2 nanocubes (c-CeO2), nanoparticles (p-CeO2), nanorods calcined at 500 °C (r-CeO2-500) and 700 °C (r-CeO2-700) as the supports. The CeO2 morphology-dependent Ni-CeO2 interaction was observed, in which the Ni/c-CeO2 catalyst exhibits stronger Ni-CeO2 interaction than other Ni/CeO2 catalysts. Meanwhile, the CeO2 morphology effect on Ni/CeO2 catalysis in propane oxidation reactions exhibits the interesting reaction sensitivity, in which the Ni/r-CeO2-500 catalyst with the largest strongly-activated oxygen species is most catalytic active in the propane combustion reaction while the Ni/c-CeO2


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catalyst with the largest amount of weakly-activated oxygen species exhibits the best catalytic performance in the oxidative dehydrogenation of propane to propylene (ODHP). 2. Experimental Section All chemical reagents with A.R. grade and gases (C3H8 (> 99.99%), O2 (> 99.999%) and Ar (> 99.999%)) were used as received. The synthesis of CeO2 cube and rod precursor followed the Mai et al.’s recipe.14 Typically, 1.96 g Ce(NO3)3⋅6H2O (99.0%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 40 mL ultrapure water (resistance > 18 MΩ) and 16.88 g NaOH (96.0%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 30 mL ultrapure water. The NaOH solution was added dropwise into the Ce(NO3)3 solution under stirring at room temperature. The mixed solution was adequately stirred for additional 30 minutes at room temperature and then transferred into a 100-mL Teflon bottle. The Teflon bottle was tightly sealed and hydrothermally treated in a stainless-steel autoclave. To prepare CeO2 cubes, the hydrothermal treatment proceeded at 180 °C for 24 h. After cooling, the obtained precipitate was collected, washed with ultrapure water, and dried in vacuo at 80 °C for 16 h. Then the acquired yellow powder was calcined in muffle oven at 500 °C for 4 h to acquire CeO2 cubes. To prepare CeO2 rods, the hydrothermal treatment proceeded at 100 °C for 24 h. After cooling, the obtained precipitate was collected, washed with ultrapure water, and dried in vacuo at 80 °C for 16 h. Then the acquired precursor was calcined in muffle oven at 500 and 700 °C for 4 h to obtain CeO2 nanorods calcined at 500 °C (r-CeO2-500) and 700 °C (r-CeO2-700), respectively. The CeO2 nanoparticles were purchased from Sigma-Aldrich. Ni/CeO2 catalysts were prepared by the wet impregnation method. Typically, a calculated amount of Ni(NO3)2⋅6H2O (98.0%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 0.5


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mL ultrapure water at 50 °C and then 0.5 g CeO2 nanocrystals were added. The mixture was stirred at 50 °C for 30 min, then dried at 110 °C for 12 h, and finally calcined in muffle oven at 400 °C for 4 h to prepare Ni/CeO2 catalysts denoted as xNi/CeO2, where x represents the actual weight fraction of Ni relative to the CeO2 support calculated from ICP-AES results. Compositions of catalysts were analyzed with a Perkin Elmer Optima 7300 DV inductively coupled plasma-atomic emission spectrometer (ICP-AES). BET specific surface areas were measured on a Micromeritics Tristar II 3020 M analyzer in a N2 atmosphere and the samples were degassed at 300 oC prior to the measurement. Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kɑ radiation (λ = 0.15406 nm) operating at 40 kV and 50 mA. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images were recorded on JEOL-2010 and JEOL-2100F high-resolution transmission electron microscopes with the electron acceleration energy of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα radiation (hν = 1486.7 eV). The likely charging of samples was corrected by setting the adventitious carbon 1s binding energy to 284.8 eV. H2-temperature programmed reduction (H2TPR) experiments were performed on a Micromeritics Autochem 2920 apparatus equipped with TCD detector and connected to an online mass spectrometer (HIDEN QIC-20). All the samples were used as synthesized without any pretreatments. Typically, 50 mg catalyst was placed in a quartz reactor and heated in a flow of 5% H2/Ar mixture (flow rate: 30 mL/min) at a heating rate of 10 °C/min. The O2-temperature programmed oxidation (O2-TPO) experiments were conducted on the same apparatus measured with the mass spectrometer. Typically, 50 mg as-synthesized catalyst and 50 mg used catalyst after ODHP reaction were heated in a flow of 2.5% O2/He


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mixture (flow rate: 30 mL/min) at a heating rate of 10 °C/min, respectively. Laser Raman spectra were obtained using an inVia confocal Raman microscope spectrograph with the excitation lines at 532 nm (Visible Raman spectra) and 325 nm (UV Raman spectra). Catalytic performances in the ODHP reaction were evaluated with a fixed-bed flow reactor. 200 mg catalyst (60-80 mesh) mixed with 200 mg silicon carbide (~60 mesh) were placed in a quartz reactor (i.d. = 8 mm). The reaction gas consisting of C3H8, O2 and Ar (C3H8:O2:Ar = 6:5:39) was fed at a GHSV of 15,000 mL⋅h-1⋅g-1. The catalyst was heated to the desired reaction temperatures at a rate of 2 °C/min and the temperature was measured with a thermocouple fixed to the inner surface of the catalyst bed. After the reaction reached the steady state, the composition of effluent gas was analyzed with an online SHIMADZU GC-2014 gas chromatography equipped with a Porapak Q column attached to a TCD detector for the separation and detection of O2, CO and CO2 and a SH-Rt 7 Alumina BOND/KCl capillary column attached to a FID detector for the separation and detection of hydrocarbons. The propane conversion and propene selectivity were calculated as the following: C3H8 conversion = ([C3H8]inlet − [C3H8]outlet) / [C3H8]inlet × 100% C3H6 selectivity = [C3H6]outlet / ([C3H8]inlet − [C3H8]outlet) × 100% Catalytic performances in the C3H8 combustion reaction were evaluated with the same fixedbed flow reactor with the feed gas (flow rate: 50 mL/min) consisting of 0.2% C3H8, 2% O2 and 97.8% Ar 200 mg catalyst (60-80 mesh) diluted with 200 mg silicon carbide (~60 mesh) was used. The propane conversion was calculated as the following: C3H8 conversion = ([C3H8]inlet − [C3H8]outlet) / [C3H8]inlet × 100%


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3. Results and Discussion Compositions of Ni/CeO2 catalysts were determined by ICP-AES, from which the weight fraction of Ni relative to the CeO2 support was calculated (Table 1). Table 1 also summarizes BET specific surface areas of various CeO2 and Ni/CeO2 catalysts. r-CeO2-500, c-CeO2, r-CeO2700 and p-CeO2 exhibit BET specific surface areas respectively of 81, 39, 51 and 38 m2/g, and the loading of Ni does not change the BET specific surface areas much. Figure 1 shows the XRD patterns of various CeO2 and Ni/CeO2 catalysts. All samples show representative diffraction patterns arising from the cubic fluorite phase structure of CeO2 but no diffraction patterns corresponding to NiO or Ni phases. This suggests the formation of either highly dispersive or amorphous Ni-containing species. Agreeing with our previous results,34,35 the crystallinity of CeO2 nanocrystals follows the order of p-CeO2 > c-CeO2 > r-CeO2-700 > r-CeO2-500. The loading of Ni barely deteriorates the crystallinity of CeO2 supports. Figure 2 A-D show representative TEM and HRTEM images of r-CeO2-500, c-CeO2, r-CeO2700 and p-CeO2 nanocrystals. Agreeing with the previous reports,14,33 r-CeO2-500 exhibits quite uniform rod morphologies with the diameter of ~8 nm and the lengths of 20-200 nm and mainly the lattice fringes of CeO2 {100} and {110} facets. Comparing r-CeO2-500, r-CeO2-700 also exhibits uniform rod morphologies with the similar diameter of ~8 nm and the short lengths of 10-100 nm, but mainly the lattice fringes of CeO2 {111} and {110} facets are identified. These observations are consistent with previous results54-56 that the dominantly exposed facets evolve from the {100}+{110} facets on r-CeO2-500 to the {111}+{110} facets on r-CeO2-700. c-CeO2 exhibits a uniform cubic morphology with the edge lengths of 10-30 nm enclosed by the {100} facets. p-CeO2 consists of nanoparticles with irregular shapes. Fig. 2 E-H show representative TEM and HRTEM images of 2.5Ni/r-CeO2-500, 2.6Ni/c-CeO2, 2.5Ni/r-CeO2-700 and 2.6Ni/p-


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CeO2 samples. The loading of Ni does not change the morphologies of c-CeO2, r-CeO2-700 and p-CeO2 much, but makes r-CeO2-500 short and round. These observations agree with our previous results of Ag/r-CeO2 and NbOx/r-CeO2

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, suggesting that the rod structure of r-

CeO2-500 nanocrystals seem not very stable during the processes of metal impregnation and recalcination. However, the observed CeO2 lattice fringes in Ni/CeO2 samples are same to those in the corresponding CeO2 supports. No lattice fringes from Ni or NiO could be identified, agreeing with the above XRD results that Ni-contained species are highly dispersed on CeO2 supports. An EDS mapping image of 2.6Ni/c-CeO2 (Figure S1) also demonstrates the well dispersion of Ni element on c-CeO2 support. Figure 3 shows Ce 3d, O 1s and Ni 2p3/2 XPS spectra of CeO2 and Ni/CeO2 samples. The Ce 3d XPS spectra could be adequately deconvoluted into eight components, in which the marked v’ and u’ components respectively correspond to the 3d3/2 and 3d5/2 features of Ce3+ 3d final state while the v/v’’/v’’’ and u/u’’/u’’’ components respectively correspond to the 3d3/2 and 3d5/2 features of multiple Ce4+ 3d final states.57,58 The O 1s XPS spectra could be adequately deconvoluted into three components at about 529.1, 530.6 and 531.8 eV that could be assigned respectively to the lattice oxygen of CeO2 (O’),58 the adsorbed oxygen species on CeO2 (O’’)59 and the hydroxyl or/and carbonates groups on CeO2 (O’’’)60,61. The surface Ce3+ contents of various samples were estimated by the integrating XPS peak ratios of the Ce3+ 3d components to the Ce 3d spectra in the Ce 3d XPS spectra, listed as Ce3+/Cetotal in Table 1. The Ce3+/Cetotal ratios of CeO2 nanocrystals follow the order of r-CeO2-500 enclosed with {100} and {110} facets > c-CeO2 enclosed with {100} facets > r-CeO2-700 enclosed with {111} and {110} facets > p-CeO2 enclosed with {111} facets. The Ce3+/Cetotal ratios of all CeO2 nanocrystals increase upon the loading of Ni, but do not vary much with the Ni loadings. At a given Ni loading, the


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Ce3+/Cetotal ratios of Ni/CeO2 catalysts follow the same order as those of CeO2 nanocrystal supports. The Ni 2p3/2 XPS spectra of Ni/CeO2 catalysts display major peaks with binding energies between of 854.6-857.0 eV assigned to supported Ni2+ species and their shake-up satellites at higher binding energies.62-64 The Ni 2p XPS spectra of both Ni/r-CeO2-500 and Ni/r-CeO2-700 catalysts exhibit similar Ni 2p3/2 binding energies slightly shifting from 856.6 to 855.8 eV with the Ni loading increasing. The Ni 2p XPS spectra of Ni/p-CeO2 catalyst also exhibit Ni 2p3/2 binding energies slightly shifting from 856.6 to 855.8 eV with the Ni loading increasing, but a second component at 854.6 eV appears in 2.5Ni/p-CeO2 catalyst. The Ni 2p XPS spectra of 0.5Ni/c-CeO2 and 1.0Ni/c-CeO2 catalysts locate at 857.0 eV while that of 2.5Ni/c-CeO2 catalyst consists of two components at 857.0 and 855.8 eV. Ni2+ species supported on CeO2 with stronger Ni2+-CeO2 interaction was reported to exhibit a higher Ni 2p3/2 binding energy and the following order was established: the Ni 2p3/2 binding energy of Ni2+ forming the Ni-O-Ce bond (Ni2+ dissolved in CeO2 with substitution for bulk Ce4+ or with location at interstitial sites)42,53 > the Ni 2p3/2 binding energy of highly dispersive NiO > the Ni 2p3/2 binding energy of NiO aggregate (854.6 eV).62 Thus the Ni2+ species exhibiting Ni 2p3/2 binding energies of 856.6-857.0 eV could be assigned to the Ni2+ species in the Ni-O-Ce structure, and the Ni2+ species exhibiting a Ni 2p3/2 binding energy of 855.8 eV could be assigned to the highly dispersive NiO species. Thus, among all Ni/CeO2 catalysts, Ni/c-CeO2 catalysts exhibit the strongest Ni-CeO2 interaction, giving the Ni-O-Ce species with the highest Ni 2p3/2 binding energy. The appearance of NiO aggregate in 2.5Ni/p-CeO2 catalyst could be likely due to the small BET surface area of p-CeO2. These XPS results agree with previous DFT calculation results of the adsorption energies of Ni


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atoms on various CeO2 surfaces that follow an order of CeO2(100) (-1.490 eV) > CeO2(110) (0.322 eV) > CeO2(111) (-0.185 eV).65 Oxygen vacancy concentrations in various CeO2 and Ni/CeO2 catalysts were probed with visible and UV Raman spectroscopies. In both visible (Figure 4) and UV Raman spectra (Figure 5), CeO2 nanocrystals exhibit a strong F2g vibrational peak around 460 cm−1 and a weak oxygen vacancy defect-induced (D modes) vibrational peak around 590 cm−1.66 The integrating peak area ratios (ID/IF2g) were calculated to estimate oxygen vacancy concentrations in CeO2. The ID/IF2g value of CeO2 nanocrystals is much larger in the UV Raman spectra than in the visible Raman spectra. Due to the strong absorbance of CeO2 in the UV light region, UV Raman spectroscopy is more surface sensitive than visible Raman spectroscopy.67 Thus, oxygen vacancies in CeO2 nanocrystals are enriched within the surface and near-surface regions. Either in the visible Raman spectra or in the UV Raman spectra, the ID/IF2g values of r-CeO2-500 and c-CeO2 nanocrystals are larger than those of r-CeO2-700 and p-CeO2 nanocrystals. These results are consistent with previous DFT calculation and experimental results that the oxygen vacancy formation energy of CeO2 surfaces follows the order of (110) > (100) > (111) and surface oxygen vacancies are more stable on CeO2 (100) and (110) surfaces while the subsurface/bulk oxygen vacancies are more stable on CeO2(111) surface.19,20,33-35 In the Raman spectra of Ni/CeO2 catalysts, no characteristic bands relating to Ni-O vibration could be observed. The calculated ID/IF2g values of Ni/CeO2 catalysts in the visible Raman spectra increase obviously with the Ni loading, demonstrating that the loading of Ni promotes the reduction of CeO2. However, the calculated ID/IF2g values in the UV Raman spectra do not increase much. These observations suggest that the increased oxygen vacancies in CeO2 of Ni/CeO2 catalysts should mostly locate in the bulk region beyond the detection depth of UV Raman spectroscopy.


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As shown in Figure 6, all CeO2 nanocrystals exhibit a broad reduction peak attributed to lowtemperature surface reduction and high-temperature subsurface reduction in H2-TPR profiles.20 The initial reduction temperature of various CeO2 nanocrystals follows the order of r-CeO2-500 < c-CeO2 < r-CeO2-700 ≈ p-CeO2, and r-CeO2-500 with the best reducibility also exhibits the highest H2 consumption (Table 1). The Ni/CeO2 catalysts exhibit much stronger H2 reduction peaks at much lower reduction temperatures than the corresponding CeO2 catalysts, suggesting that the loading of Ni significantly enhances the reducibility of CeO2. Figure S2 show the O2TPO-MS and H2-TPR-MS profiles of r-CeO2-500 and 2.5Ni/r-CeO2-500 catalysts. Only a weak CO2 peak was observed at around 120 °C and likely resulted from the desorption of weakly adsorbed CO2 on catalysts or the decomposition of surface carbonate species. Thus the CO2 desorption trace should not contribute to the H2-TPR profiles shown in Figure 6. The total H2 consumption of Ni/CeO2 catalysts was calculated by integrating the H2 reduction peaks, and the H2 consumption of CeO2 supports in the Ni/CeO2 catalysts were calculated by subtracting the total H2 consumption with the theoretically-calculated H2 consumption of Ni2+ reduction. The results are listed in Table 1. It can be seen that the H2 consumption of CeO2 supports in the Ni/CeO2 catalysts are much larger than that of bare CeO2 supports and increases with the Ni loading. Meanwhile, both the total H2 consumption and the H2 consumption of CeO2 supports of Ni/CeO2 catalysts at a given Ni loading follow the order of r-CeO2-500 > r-CeO2-700 ≈ c-CeO2 > p-CeO2. All H2 TPR profiles of Ni/CeO2 catalyst consist of three major low-temperature reduction peaks labeled as the α1, α2 and β peaks and a high-temperature weak and broad reduction peak labelled as the γ peak. The α1 and α2 reduction peaks were previously attributed to the reduction of surface oxygen species related to the oxygen vacancies associated with the formation of Ni-O-


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Ce structure on CeO2,42,57 such as surface adsorbed oxygen species (O2- or O-), and the β reduction peak was previously attributed mainly to the reduction of highly dispersive NiO species.68 We performed peak deconvolution analysis of all H2 TPR profiles, from which the H2 consumption of α1, α2, β and γ peaks were acquired (Figure S3 and Table 1). It was found that the α1+α2 H2 consumption amount does not change much with the Ni loading for all catalysts. This agrees with the assignment that both reduction peaks arise from the reduction of surface oxygen species related to the oxygen vacancies associated with the formation of Ni-O-Ce structure in CeO2 and indicates that the formation of Ni-O-Ce structure on CeO2 of Ni/CeO2 catalysts should saturate at Ni loading below 0.5Ni/CeO2. This is reasonable since such a Ni species corresponds to the Ni2+ dissolved within CeO2. We found that ratio of the α1+α2 H2 consumption amount of Ni/CeO2 catalysts to that the H2 consumption amount of corresponding bare CeO2 supports follows an order of Ni/r-CeO2-500 >> Ni/ r-CeO2-700 > Ni/p-CeO2 > Ni/cCeO2, in consistence with the order of the fraction of exposed {110} facets. This indicates that the oxygen vacancies formed due to the Ni-O-Ce structure should be most extensive on CeO2 {110} facets. The H2 consumption amount of β reduction peak associated with the reduction of highly dispersive NiO species increases with the Ni loading in Ni/CeO2 catalysts, however, for most catalysts, the values are larger than corresponding theoretically-calculated H2 consumptions of Ni2+ reduction. Thus, this peak should contain the CeO2 reduction associated with the reduction of highly dispersive NiO species. We propose that metallic Ni resulted from NiO reduction can activate lattice oxygen in CeO2 whose reduction also contributes to the β reduction peak. It can be seen that the H2 consumption amount of β reduction peak of 0.5Ni/r-CeO2-500 is much smaller than the corresponding theoretically-calculated H2 consumptions of Ni2+ reduction. This


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might imply the formation of a large amount of Ni-O-Ce structure in Ni/r-CeO2-500 catalysts, which agrees with their large α1 and α2 reduction peaks. The γ reduction peak of Ni/CeO2 catalysts was not discussed much previously. This peak is present for all Ni/CeO2 catalysts, but its H2 consumption amount does not change much with the Ni loading, meanwhile, it is very small comparing with those of the α1, α2 and β reduction peaks. We attribute the γ reduction peak to the reduction of Ni2+ in Ni-O-Ce structure and the associated CeO2 reduction. It can be seen that the H2 consumption amount of γ reduction peak in Ni/rCeO2-500 catalysts is much larger than in other Ni/CeO2 catalysts, in consistence with their richest Ni-O-Ce structure. In addition to the α1, α2, β and γ reduction peaks, Ni/c-CeO2 catalysts exhibit another reduction peak (labelled as the γ’ peak) while other Ni/CeO2 catalysts do not. The γ’ reduction peak lies between the β and γ reduction peaks and its H2 consumption amount does not change much with the Ni loadings of Ni/c-CeO2 catalysts. The above XPS results show that Ni/c-CeO2 catalysts exhibit the Ni-O-Ce species with the strongest Ni-CeO2 interaction among all Ni/CeO2 catalysts. We thus assign the γ’ reduction peak to the reduction of CeO2 activated by the very strongly-interacting Ni-O-Ce structure of Ni/c-CeO2 catalysts. On the basis of above discussion, we define herein the α1+α2, β, and γ+γ’ reduction peaks in H2 TPR profiles of Ni/CeO2 catalysts respectively to the strongly-activated, medially-activated, and weakly-activated oxygen species. Our comprehensive characterization results demonstrate morphology-dependent Ni-CeO2 interaction and oxygen activation in Ni/CeO2 catalysts. The Nir-CeO2-500 interaction leads to the formation of the largest amounts of Ni-O-Ce structure, surface oxygen vacancies and strongly-activated oxygen species while the Ni-c-CeO2 interaction


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leads to the formation of the Ni-O-Ce structure with the strongest Ni-CeO2 interaction and the largest amount of weakly-activated oxygen species. The Ni-O-Ce structure with strong Ni-CeO2 interaction might weaken the neighboring Ce-O bonds and activate involving lattice oxygen in CeO2, but it needs further studies how strong the Ni-CeO2 interaction within the Ni-O-Ce structure should be to exert such an effect. Catalytic performances of various CeO2 and Ni/CeO2 catalysts were examined in both propane combustion and ODHP reactions. Figure 7 plots the steady-state C3H8 conversion and C3H6 selectivity of all CeO2 and Ni/CeO2 catalysts as a function of reaction temperature, and Figure S4 shows their catalytic stability. In the propane combustion reaction (Figure 7 A1-D1), the CeO2 catalysts become active above 300 °C, and their catalytic performances are enhanced significantly by the loading of Ni. At a given Ni loading, Ni/r-CeO2-500 is most catalytically active. Figure 7A2-D2 shows catalytic performances in the ODHP reaction in the low temperature region up to 300 °C. Our previous work shows that the ODHP reaction catalyzed by CeO2 and CeO2-based catalysts should occur on the catalyst surfaces at low temperatures (below 400 °C).36 A blank test using silicon carbide was also carried out and a C3H8 conversion of 0.5% was observed at 550 °C. The main products of ODHP reaction are C3H6, CO2 and other minor products are CH4, CO, and the calculated carbon balances are always higher than 93.7% for all catalysts (listed in Table S1). Taking the catalytic performance of r-CeO2-500 at 300 °C as an example, the conversion of C3H8 is 18.5%, and the selectivities of C3H6, CO2, CH4 and CO are 3.9%, 90.3%, 0.2% and 3.8%, respectively. At reaction temperatures above 350 °C (Figure S5-


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S7), nearly 100% O2 conversion were observed for Ni/CeO2 catalysts and propane cracking with coke formation likely occurred, producing CO2, CH4, C2H6 and H2. Among the bare CeO2 samples, r-CeO2-500 shows the highest C3H8 conversion in ODHP reaction at the low temperatures whereas c-CeO2 shows negligible catalytic activity. However, the C3H6 selectivity is very low and CO2 is always the major product. The loading of Ni on rCeO2-500, r-CeO2-700 and p-CeO2 decreases the C3H8 conversions but greatly increases the C3H6 selectivities; however, the loading of Ni on c-CeO2 increases both the C3H8 conversions and the C3H6 selectivities. The highest C3H6 yield here was acquired over 2.6Ni/c-CeO2 catalyst at 300 °C with a C3H8 conversion of 21.3% and a C3H6 selectivity of 36.2%. The O2 conversions for CeO2 and Ni/CeO2 catalysts in ODHP reaction below 300 °C were below 80% (Figure S6), excluding the non-oxidative dehydrogenation of propane reaction under this condition. Based on the number of loaded Ni atoms, we calculated the turnover frequency (TOF) of Ni/CeO2 catalysts in propane combustion reaction and ODHP reaction at 300 °C, and the results were summarized in Table S3. Among all investigated catalysts, 0.5Ni/r-CeO2-500 catalyst shows the highest TOF of 0.00086 s-1 in propane combustion reaction while 0.5Ni/c-CeO2 catalyst shows the highest TOF of 0.014 s-1 in ODHP reaction. The morphology of used 2.5Ni/r-CeO2-500 and 2.6Ni/c-CeO2 catalysts in propane combustion reaction at 600 °C and ODHP reaction at 500 °C were examined with TEM and HRTEM (Figure S8). CeO2 in the used catalysts well maintain their original shapes after propane combustion reaction up to 600 °C, while after ODHP reaction up to 500 °C, CeO2 rods in the used 2.5Ni/rCeO2-500 catalyst becomes shorter and CeO2 cubes in the used 2.6Ni/c-CeO2 catalyst exhibit rounder corners. Lattice fringes of CeO2 {111}, {110} and {100} facets were observed in the


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used 2.5Ni/r-CeO2-500 catalyst, and lattice fringes of CeO2 {100} facets dominate in the used 2.6Ni/c-CeO2 catalyst. No lattice fringes from Ni or NiO and few coke formation were observed on the used catalysts. The carbon balances were calculated to be all higher than 92.1%. XPS spectra (Figure S9) indicate no NiCx or any carbides formations on these used Ni/CeO2 catalysts. O2-TPO profiles (Figure S10) show only a few CO2 production at the temperature below 300 °C over the used 2.5Ni/r-CeO2-500 catalyst. All these characterization results suggest no obvious formation of cokes or carbides on Ni/CeO2 catalysts during the ODHP reaction with the employed reaction conditions. Thus, various CeO2 nanocrystals exhibit similar morphology-dependent catalytic activity in C3H8 combustion and ODHP reactions in which r-CeO2-500 is most active while c-CeO2 is least active. Both C3H8 combustion and ODHP reactions below 400 °C catalyzed by CeO2 and CeO2based oxide catalysts were demonstrated to follow the Mars van Krevelen (MvK) mechanism in which the oxidation of propane by surface lattice oxygen of CeO2 is the rate-limiting step.36 The highest catalytic activity of r-CeO2-500 could be attributed to both its largest specific surface area and the smallest oxygen vacancy formation energy of CeO2 (110) surface among CeO2 (111), (100) and (110) surfaces.69 However, various Ni/CeO2 catalysts exhibit different morphology-dependent catalytic performances in C3H8 combustion and ODHP reactions. Ni/rCeO2-500 is most active in catalyzing C3H8 combustion while Ni/c-CeO2 is most active in catalyzing ODHP reaction. These results indicate that C3H8 combustion and ODHP reactions catalyzed by Ni/CeO2 catalysts should follow reaction mechanisms involving different oxygen species. Likely correlations between the calculated C3H8 combustion rates at 250 °C in C3H8 combustion and the C3H6 formation rates at 300 °C in ODHP reaction of various Ni/CeO2


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catalysts and the amount of different oxygen species estimated from H2 TPR results were comprehensive examined (Figure 8, Figure S11-S13). It was found that the C3H8 combustion rates of various Ni/CeO2 catalysts are not proportional to the amount of any individual H2 TPR peak but do correlate well with the amount of α1+α2 reduction peak. This demonstrates that the strongly-activated oxygen species on Ni/CeO2 catalysts, i.e., the surface adsorbed oxygen species (O2- or O-) on oxygen vacancies, is the active oxygen species for C3H8 combustion at 250 °C. The strongly-activated oxygen species on Ni/CeO2 catalysts is more reactive than the surface lattice oxygen species on CeO2 catalysts, giving a higher activity of Ni/CeO2 catalysts in C3H8 combustion reaction. As summarized in Table S2, the line slope of the C3H8 combustion rate versus the amount of α1+α2 reduction peak is the largest for Ni/c-CeO2 catalysts, suggesting the highest intrinsic activity of the strongly-activated oxygen species on Ni/c-CeO2 catalysts. However, due to its largest amount of strongly-activated oxygen species at a given Ni loading for Ni/CeO2 catalysts, Ni/r-CeO2-500 catalysts exhibit the largest C3H8 combustion rate. Although very reactive, the surface adsorbed oxygen species (O2- or O-) on oxygen vacancies should not be stable and their surface coverages should sensitively depend on the reaction temperature and reactant composition. In the ODHP reaction, the C3H8 combustion reaction over Ni/r-CeO2-500, Ni/r-CeO2-700 and Ni/p-CeO2 catalysts are suppressed comparing with that over corresponding CeO2 catalysts. This indicates that the surface coverages of strongly-activated oxygen species on these Ni/CeO2 catalysts should be quite low due to the high C3H8:O2 ratio employed in the ODHP reaction and thus their contributions to the observed C3H8 conversion should be minor at low temperatures. The C3H6 formation rates at 300 °C in ODHP reaction of various Ni/CeO2 catalysts are not proportional to the amount of individual α1 or α2 H2 TPR peak, but proportional to the amount


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of individual β and γ (except Ni/c-CeO2 catalysts) H2 TPR peak, and the C3H6 formation rates of Ni/c-CeO2 catalysts are proportional to the amount of γ’ and γ+γ’ H2 TPR peaks. Thus, both the medially-activated oxygen species (β peak) and weakly-activated oxygen species (γ+γ’ peaks) are on Ni/CeO2 catalysts responsible for the C3H6 formation, but the line slope of the C3H6 formation rate versus the amount of γ+γ’ reduction peak of each Ni/CeO2 catalyst is much larger than the corresponding line slope of the C3H6 formation rate versus the amount of β reduction peak (Table S2); meanwhile, the line slope of the C3H6 formation rate versus the amount of γ+γ’ reduction peak of Ni/c-CeO2 catalyst is much larger than those of other Ni/CeO2 catalysts (Table S2). This suggests that the weakly-activated oxygen species on Ni/CeO2 catalysts is more intrinsic active in the ODHP reaction than the medially-activated oxygen species and that the weakly-activated oxygen species on Ni/c-CeO2 catalysts is more intrinsic active in the ODHP reaction than the medially-activated oxygen species on other Ni/CeO2 catalysts. The very strong Ni-CeO2 interaction within Ni/c-CeO2 catalysts leads to the formation of additional weaklyactivated oxygen species corresponding to the γ’ reduction peak, and the 2.6Ni/c-CeO2 catalyst exhibits the largest amount of the weakly-activated oxygen species and subsequently the highest C3H6 formation rate. The above results exemplify the concept of morphology-engineering strategy for both fundamental understanding of complex heterogeneous catalysis and innovation of efficient catalysts.6,7 Employing uniform CeO2 nanocrystals with various morphologies, the surface lattice oxygen species, strongly-activated oxygen species, and weakly-activated oxygen species are shown to be the active species respectively for CeO2-catalyzed C3H8 combustion and ODHP reactions, Ni/CeO2-catalyzed C3H8 combustion reaction, and Ni/CeO2-catalyzed C3H8 ODHP reaction. It can thus be deduced that the surface lattice oxygen species of CeO2 is capable of


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influencing the reaction pathways of propane oxidation reactions. The participation of O of ceria activated by Ni towards the activation of C3H8 in the ODHP reaction points to a Mars van Krevelen-type mechanism. Meanwhile, the CeO2 morphology affects the Ni-CeO2 interaction and the speciation of oxygen species in Ni/CeO2 catalysts. Among the investigated Ni/CeO2 catalysts, 2.5Ni/r-CeO2-500 catalysts exhibit the largest amount of strongly-activated oxygen species and the highest C3H8 combustion rate in C3H8 combustion reaction while 2.6Ni/c-CeO2 catalyst exhibits the largest amount of the weakly-activated oxygen species and the highest C3H6 formation rate in ODHP reaction. Thus, the CeO2 morphology can effective tune the metal-CeO2 interaction and the reactivity of oxygen species of CeO2 to meet the requirements of different types of catalytic oxidation reactions. 4. Conclusions In summary, CeO2 morphology-dependent Ni-CeO2 interaction and catalytic performances of Ni/CeO2 catalysts in the C3H8 combustion and ODHP reactions were studied. CeO2 morphologydependent Ni-CeO2 interaction leads to the formation of different Ni and oxygen speciation in various Ni/CeO2 catalysts. Ni-CeO2 interaction is stronger in Ni/c-CeO2 catalysts than in other Ni/CeO2 catalysts. Various Ni/CeO2 catalysts exhibit different morphology-dependences in propane combustion and ODHP reactions. Among the investigated Ni/CeO2 catalysts, 2.5Ni/rCeO2-500 catalysts exhibit the largest amount of strongly-activated oxygen species and the highest C3H8 combustion rate in C3H8 combustion reaction while 2.6Ni/c-CeO2 catalyst exhibits the largest amount of the weakly-activated oxygen species and the highest C3H6 formation rate in ODHP reaction. These results successfully identify the active oxygen species on Ni/CeO2 catalysts in propane combustion and ODHP reactions and demonstrate CeO2 morphology


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engineering as an effective strategy to tune the metal-CeO2 interaction and the reactivity of oxygen species to meet the requirements of different types of catalytic oxidation reactions. Supporting information. Calculated carbon balance of ODHP reaction at 300 and 350 °C, slopes of fitted lines between reaction rates and H2 consumptions, peak-fitted H2-TPR profiles, C3H8 conversion and C3H6 selectivity in ODHP reaction above 350 °C, O2 conversion in ODHP reaction, product distribution in ODHP reaction at 400 °C, relationships between catalytic performance and various oxygen species. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel.: 008655163600435. Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by National Basic Research Program of China (2013CB933104), National Natural Science Foundation of China (21525313, 21761132005), MOE Fundamental Research Funds for the Central Universities (WK2060030017) and Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES (1) Trovarelli, A. Catalytic Properties of Ceria and CeO2-Containing Materials. Catal. Rev. 1996, 38, 439-520.


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(2) Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Fundamental and Catalytic Applications of CeO2-Based Materials. Chem. Rev. 2016, 116, 5987−6041. (3) Sun, C.; Li, H.; Chen, L. Nanostructured Ceria-Based Materials: Synthesis, Properties, and Applications. Energy Environ. Sci. 2012, 5, 8475−8505. (4) Zhou, K.; Li, Y. Catalysis Based on Nanocrystals with Well-Defined Facets. Angew. Chem. Int. Ed. 2012, 51, 602-613. (5) Huang, W. Crystal Plane-Dependent Surface Reactivity and Catalytic Property of Oxide Catalysts Studied with Oxide Nanocrystal Model Catalysts. Top. Catal. 2013, 56, 1363-1376. (6) Huang, W. Oxide Nanocrystal Model Catalysts. Acc. Chem. Res. 2016, 49, 520-527. (7) Huang, W.; Sun, G.H.; Cao, T. Surface Chemistry of Group IB and Related Oxides. Chem. Soc. Rev. 2017, 46, 1977-2000. (8) Wang, S.; Zhao, L.; Wang, W.; Zhao, Y.; Zhang, G.; Ma, X.; Gong, J. Morphology Control of Ceria Nanocrystals for Catalytic Conversion of CO2 with Methanol. Nanoscale 2013, 5, 5582−5588. (9) Liu J.; Meeprasert, J.; Nammuangruk, S.; Zha, K.; Li, H.; Huang, L.; Maitarad, P.; Shi, L.; Zhang, D. Facet-Activity Relationship of TiO2 in Fe2O3/TiO2 Nanocatalysts for Selective Catalytic Reduction of NO with NH3: In Situ DRIFTs and DFT Studies. J. Phys. Chem. C 2017, 121, 4970−4979. (10) Han, J.; Meeprasert, J.; Maitarad, P.; Nammuangruk, S.; Shi, L.; Zhang, D. Investigation of the Facet-Dependent Catalytic Performance of Fe2O3/CeO2 for the Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. C 2016, 120, 1523−1533. (11) Romero-Núñez, A.; Díaz, G. High Oxygen Storage Capacity and Enhanced Catalytic Performance of NiO/NixCe1-xO2-δ Nonorods: Synergy between Ni-Doping and 1D Morphology. RSC Adv. 2015, 5, 54571-54579. (12) Zhao, X.; Huang, L.; Namuangruk, S.; Hu, H.; Hu, X.; Shi, L.; Zhang, D. Morphologydependent performance of Zr-CeVO4/ TiO2 for selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 2016, 6, 5543−5553. (13) Zhou, K.; Wang, X.; Sun, X.; Peng, Q.; Li, Y. Enhanced Catalytic Activity of Ceria Nanorods from Well-Defined Reactive Crystal Planes. J. Catal. 2005, 229, 206-212. (14) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109, 24380-24385. (15) Liu, X.; Zhou, K.; Wang, L.; Wang, B.; Li, Y. Oxygen Vacancy Clusters Promoting Reducibility and Activity of Ceria Nanorods. J. Am. Chem. Soc. 2009, 131, 3140-3141. (16) Zhang, D.; Du. X.; Shi, L.; Gao, R. Shape-Controlled Synthesis and Catalytic Application of Ceria Nanomaterials. Dalton. Trans. 2012, 41, 14455-14475. (17) Qiao, Z.-A.; Wu, Z.; Dai, S. Shape-Controlled Ceria-based Nanostructures for Catalysis Applications. ChemSusChem 2013, 6, 1821-1833. (18) Trovarelli, A.; Llorca J. Ceria Catalysts at Nanoscale: How Do Crystal Shapes Shape Catalysis. ACS Catal. 2017, 7, 4716-4735. (19) Huang, W.; Gao, Y. Morphology-Dependent Surface Chemistry and Catalysis of CeO2 Nanocrystals. Catal. Sci. Technol. 2014, 4, 3772-3784. (20) Wu, Z.; Li, M.; Overbury, S. H. On the Structure Dependence of CO Oxidation over CeO2 Nanocrystals with Well-Defined Surface Planes. J. Catal. 2012, 285, 61-73.


22

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Wu, Z.; Li M.; Howe, J.; Meyer III, H. M.; Overbury, S. H. Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595-16606. (22) Vilé, G.; Colussi, S.; Kurmeich,F.; Trovarelli, A.; Pérez-Ramírez, J. Opposite Face Sensitivity of CeO2 in Hydrogenation and Oxidation Catalysis. Angew. Chem. Int. Ed. 2014, 47, 2884-2887. (23) Liu, J. Advanced Electron Microscopy of Metal-Support Interactions in Supported Metal Catalysts. ChemCatChem 2011, 3, 934-948. (24) Si, R.; Flytzani-Stephanopoulos, M.; Shape and Crytal-Plane Effects of Nanoscale Ceria on the Activity of Au-CeO2 Catalysts for the Water-Gas Shift Reaction. Angew. Chem. Int. Ed. 2008, 120, 2926-2929. (25) Lv, J.; Shen, Y.; Peng, L.; Guo, X.; Ding, W. Exclusively Selective Oxidation of Toluene to Benzaldehyde on Ceria Nanocubes by Molecular Oxygen. Chem. Commun. 2010, 46, 59095911. (26) Guan, Y.; Hensen, E. J. M. Cyanide Leaching of Au/CeO2: Highly Active Gold Clusters for 1,3-Butadiene hydrogenation. Phys. Chem. Chem, Phys. 2009, 11, 9578-9582. (27) Kalamaras, C. M.; Americanou, S.; Efstathiou, A. M. “Redox” vs “Associative Formate with –OH Group Regeneration” WGS Reaction Mechanism on Pt/CeO2: Effect of Platinum Particle Size. J. Catal. 2011, 279, 287-300. (28) Feng, L.; Hoang, D. T.; Tsung, C. K.; Huang, W.; Lo, S. H.-Y.; Wood, J.B.; Wang, H.; Tang, J.; Yang, P. Catalytic Properties of Pt Cluster-Decorated CeO2 Nanostructures. Nano Res. 2011, 4, 61-71. (29) Wu, Z.; Schwartz, V.; Li, M.; Rondinone, A. J.; Overbury, S. H. Supported Shape Effect in Metal Oxide Catalysis: Ceria-Nanoshape-Supported Vanadia Catalysts for Oxidative Dehydrogenation of Isobutane. J. Phys. Chem. Lett. 2012, 3, 1517-1522. (30) Wu, Z.; Li, M.; Overbury, S. H. A Raman Spectroscopic Study of the Speciation of Vanadia Supported on Ceria Nanocrystals with Defined Surface Planes. ChemCatChem. 2012, 4, 1653-1661. (31) Wu, Z.; Rondinone, A. J.; Ivanov, I. N.; Overbury, S. H. Structure of Vanadium Oxide Supported on Ceria by Multiwavelength Raman Spectroscopy. J. Phys. Chem. C 2011, 115, 25368-25378. (32) Liu, L.; Yao, Z.; Deng, Y.; Gao, F.; Liu, B.; Dong, L. Morphology and Crystal-Plane Effects of Nanoscale Ceria on the Activity of CuO/CeO2 for NO Reduction by CO. ChemCatChem. 2011, 3, 978-989. (33) Gao, Y.; Wang, W.; Chang, S.; Huang, W. Morphology Effect of CeO2 Support in the Preparation, Metal-Support interaction, and Catalytic Performance of Pt/CeO2 Catalysts. ChemCatChem. 2013, 5, 3610-3620. (34) Chang, S.; Li, M.; Hua, Q.; Zhang, L.; Ma, Y.; Ye, B.; Huang, W. Shape-Dependent Interplay between Oxygen Vacancies and Ag-CeO2 Interaction in Ag/CeO2 Catalysts and Their Influence on the Catalytic Activity. J. Catal. 2012, 293, 195-204. (35) Liu, Y.; Luo, L.; Gao, Y.; Huang, W. CeO2 Morphology-Dependent NbOx-CeO2 Interaction, Structure and Catalytic Performance of NbOx/CeO2 Catalysts in Oxidative. Appl. Catal., B 2016, 197, 214-221. (36) You, R.; Zhang, X.; Luo, L.; Pan, Y.; Pan, H.; Yang, J.; Wu, L.; Zheng, X.; Jin, Y.; Huang, W. NbOx/CeO2-rods Catalysts for Oxidative Dehydrogenation of Propane: Nb-CeO2 Interaction and Reaction Mechanism. J. Catal. 2017, 348, 189-199.


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Page 24 of 38

(37) Yao, S.; Xu, W.; Johnston-Peck, A.; Zhao, F.; Liu, Z.; Luo, S.; Senanayake, S.; MartinezArias, A.; Liu, W.; Rodriguez, J. Morphological Effects of the Nanostructured Ceria Support on the Activity and Stability of CuO/CeO2 Catalysts for the Water-Gas Shift Reaction. Phys. Chem. Chem. Phys. 2014, 16, 17183− 17195. (38) Liu, Z.; Grinter, D. C.; Lustemberg, P. G.; Nguyen-Phan, T.-D.; Zhou, Y.; Luo, S.; Waluyo, I.; Crumlin, E. J.; Stacchiola, D. J.; Zhou, J.; et al. Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking. Angew. Chem., Int. Ed. 2016, 55, 7455−7459. (39) Saw, E. T.; Oemar, U.; Tan, X. R..; Du, Y.; Borgna, A.; Hidajat, K.; Kawi, S. Bimetallic Ni-Cu catalyst Supported on CeO2 for High-Temperature Water-Gas Shift Reaction: Methane Suppression Via Enhanced CO Adsorption. J. Catal. 2014, 314, 32-46. (40) Maitarad, P.; Han, J.; Zhang, D.; Shi, L.; Namuangruk, S.; Rungrotmongkol, T. Structure-Activity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies. J. Phys.Chem. C 2014, 118, 9612-9620. (41) Chagas, C. A.; Souza, E. F.; Manfro, R. L.; Landi, S. M.; Souza, M. M. V. M.; Schmal, M. Copper as Promoter of the NiO-CeO2 Catalyst in the Preferential CO Oxidation. Appl. Catal., B 2016, 182, 257-265. (42) Shan, W.; Luo, M.; Ying, P.; Shen, W.; Li, C. Reduction Property and Catalytic Activity of Ce1-xNixO2 Mixed Oxide Catalysts for CH4 Oxidation. Appl. Catal., A 2003, 246, 1-9. (43) Pakulska, M. M,; Grgicak, C. M.; Giorgi, J. B. The Effect of Metal and Support Particle Size on NiO/CeO2 and NiO/ZrO2 Catalyst Activity in Complete Methane Oxidation. Appl. Catal., A 2007, 332, 124-129. (44) Yisup, N.; Cao, Y.; Feng, W. L.; Dai. W. L.; Fan, K. N. Catalytic Oxidation of Methane over Novel Ce-Ni-O Mixed Oxide Catalysts Prepared by Oxalate Gel-Coprecipitation. Catal. Lett. 2005, 99, 207-213. (45) Liu, Y. M.; Wang, L. C.; Chen, M.; Xu, J.; Cao, Y.; He, H. Y.; Fan, K. N. Highly Selective Ce-Ni-O Catalysts for Efficient Low Temperature Oxidative Dehydrogenation of Propane. Catal. Lett. 2009, 130, 350-354. (46) Solsona, B.; Concepcion, P.; Hernandez, S.; Demicol, B.; Nieto, J, M, L. Oxidative Dehydrogenation of ethane over NiO-CeO2 Mixed Oxides Catalysts. Catal. Today 2012, 180, 51-58. (47) Xu, S.; Yan, X.; Wang, X.; Catalytic Performances of NiO-CeO2 for the Reforming of Methane with CO2 and O2. Fuel. 2006, 85, 2243-2247. (48) Srinvias, D.; Satyanarayana, C. V. V.; Potdar, H. S.; Ratanasamy, P. Structural Studies on NiO-CeO2-ZrO2 Catalysts for Steam Reforming of Ethanol. Appl. Catal., A 2003, 246, 323-334. (49) Wang, Z.; Shao, X.; Larcher A.; Xie, K.; Dong, D.; Li, C. Z. A Study on Carbon Formation over Fibrous NiO/CeO2 Nanocatalysts during Dry Reforming of Methane. Catal. Today 2013, 216, 44-49. (50) Zou, W.; Ge, C.; Lu, M.; Wu, S.; Wang, Y.; Sun, J.; Pu, Y.; Tang, C.; Gao, F.; Dong, L. Engineering the NiO/CeO2 Interface to Enhance the Catalytic Performance for CO Oxidation. RSC Adv. 2015, 5, 98335-98343. (51) Du, X.; Zhang, D.; Shi, L.; Gao, R.; Zhang, J. Morphology Dependence of Catalytic Properties of Ni/CeO2 Nanostructures for Carbon Dioxide Reforming of Methane. J. Phys. Chem. C 2012, 116, 10009-10016.


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(52) Moraes, T. S.; Neto R. C. R.; Ribeiro M. C.; Mattos, L. V.; Kourtelesis, M.; Verykios, X.; Noronha, F. B. Effects of Ceria Morphology on Catalytic Performance of Ni/CeO2 Catalysts for Low Temperature Steam Reforming of Ethanol. Top. Catal. 2015, 58, 281-294. (53) Pal, P.; Singha, R. K.; Saha, A.; Bal, R.; Panda, A. B. Defect-Induced Efficient Partial Oxidation of Methane over Nonstoichiometric Ni/CeO2 Nanocrystals. J. Phys. Chem. C 2015, 119, 13610-13618. (54) Tan, J. P. Y.; Tan, H. R.; Boothroyd, C.; Foo, Y. L.; He, C. B.; Lin, M. ThreeDimensional Structure of CeO2 Nanocrystals. J. Phys. Chem. C 2011, 115, 3544-3551. (55) Ta, N.; Liu, J.; Chenna, S.; Crozier, P. A.; Li, Y.; Chen, A.; Shen, W. Stabilized Gold Nanoparticles on Ceria Nanorods by Strong Interfacial Anchoring. J. Am. Chem. Soc. 2012, 134, 20585-20588. (56) Chen, S.; Cao, T.; Gao, Y.; Li, Dan.; Xiong, F.; Huang, W. Probing Surface Structures of CeO2, TiO2 and Cu2O Nanocrystals with CO and CO2 Chemisorption. J. Phys. Chem. C 2016, 120, 21472-21485. (57) Mahammadunnisa, S.; Reddy, P. M. K.; Lingaiah, N.; Subrahmanyam. NiO/Ce1-xNixO2-δ as an Alternative to Noble Metal Catalysts for CO Oxidation. Catal. Sci. Technol. 2013, 3, 730736. (58) Santos, V. P.; Carabineiro, S. A. C.; Bakker J. J. W.; Soares, O. S. G. P.; Chen, X.; Pereira, M. F. R.; Orfao, J. J. M.; Figueiredo, J. L.; Gascon, J. Kapteijn, F. Stabilized Gold on Cerium-Modified Cryptomelane: Highly Active in Low-Temperature CO Oxidation. J. Catal. 2014, 309, 58-65. (59) Ni, J.; Chen, L.; Lin, J.; Schreyer, M. K.; Wang, Z.; Kawi, S. High Performance of MgLa Mixed Oxides Supported Ni Catalysts for Dry Reforming of Methane: The Effect of Crystal Structure. Int. J. Hydrogen Energy 2013, 38, 13631-13642. (60) Torrente-Murciano, L.; Gilbank, A.; Puertolas, B.; Garcia, T.; Solsona, B.; Chadwick, D. Shape-Dependency Activity of Nanostructured CeO2 in The Total Oxidation of Polycyclic Aromatic Hydrocarbons. Appl. Catal., B 2013, 132, 116-122. (61) Xu, D.; Cheng, F.; Lu, Q.; Dai, P. Microwave Enhanced Catalytic Degradation of Methyl Orange in Aqueous Solution over CuO/CeO2 Catalyst in the Absence and Presence of H2O2. Ind. Eng. Chem. Res. 2014, 53, 2625-2632. (62) Hou, Z.; Yokota, O.; Tanaka, T.; Yashima, T. Characterization of Ca-Promoted Ni/ɑAl2O3 Catalyst for CH4 Reforming with CO2. Appl. Catal., A 2003, 253, 381-387. (63) Uhlenbrock, S.; Scharfschwerdt, C.; Neumann, M.; Illing, G. Freund, H-J. The Influence of Defects on the Ni 2p and O1s XPS of NiO. J. Phys.: Condens. Matter 1992, 4, 7973-7978. (64) Song, H.; Xu, X. W.; Song, H. L.; Jiang, N. Zhang, F. Y. Synthesis of an YttriumModified Bulk Ni2P Catalyst with High Hydrodesulfurization Activity. Catal. Commun. 2015, 63, 52-55. (65) Li, W. Q.; Goverapet Srinivasan, S.; Salahub, D. R.; Heine, T. Ni on the CeO2(110) and CeO2(100) Surfaces: Adsorption vs. Substitution Effects on the Electronic and Geometric Structures and Oxygen Vacancies. Phys. Chem. Chem. Phys. 2016, 18, 11139-11149. (66) Lin, J.; Li, L.; Huang, Y.; Zhang, W.; Wang, X.; Wang, A.; Zhang, T. In Situ Calorimetric Study: Structural Effects on Adsorption and Catalytic Performances for CO Oxidation over Ir-in-CeO2 and Ir-on-CeO2 Catalysts. J. Phys. Chem. C 2011, 115, 16509-16517. (67) Luo, M. F.; Yan, Z. L.; Jin, L. Y.; He, M. Raman Spectroscopic Study on the Structure in the Surface and the Bulk Shell of CexPr1-xO2-δ Mixed Oxides. J. Phys. Chem. B 2006, 110, 13068-13071.


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(68) Wang, Y.; Zhu, A.; Zhang, Y.; Au, C. T.; Yang, X.; Shi, C. Catalytic Reduction of NO by CO over NiO/CeO2 Catalyst in Stoichiometric NO/CO and NO/CO/O2 Reaction. Appl. Catal., B 2008, 81, 141-149. (69) Nolan, M.; Parker, S. C.; Watson, G. W. The Electronic Structure of Oxygen Vacancy Defects at the Low Index Surfaces of Ceria. Surf. Sci. 2005, 595, 223-232.


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Table 1. BET surface areas (SBET), XPS results and H2 consumption value of various CeO2 and Ni/CeO2 catalysts. H2 consumption value (µmol g-1)

XPS catalyst

SBET (m2/g)

Ce3+/Ce total(%)

Total H2 consumption

Peak fitting results

n(H1)a

n(H2)b

n(H3)c

α1

α2

β

γ

γ’

r-CeO2-500

81

12.1

509.9

-

509.9

-

-

-

-

-

0.5Ni/r-CeO2-500

77

15.1

674.3

84.6

589.7

253.4

289.3

41.1

90.5

-

1.1Ni/r-CeO2-500

73

15.3

924.7

188.5

736.2

290.2

270.4

266.2

97.9

-

2.5Ni/r-CeO2-500

80

15.7

1256.4

425.2

831.2

301.5

234.6

586.3

134.0

-

c-CeO2

39

10.2

344.5

-

344.5

-

-

-

-

-

0.5Ni/c-CeO2

38

10.9

447.2

85.1

362.1

90.4

81.4

74.4

49.4

151.6

1.0Ni/c-CeO2

36

11.1

559.3

169.7

389.6

103.9

83.8

168.5

43.3

159.8

2.6Ni/c-CeO2

36

11.4

988.4

442.9

545.5

88.4

102.6

578.2

46.5

172.7

r-CeO2-700

51

9.5

343.8

-

343.8

-

-

-

-

-

0.5Ni/ r-CeO2-700

51

10.4

471.1

85.8

385.3

144.0

137.3

155.8

34.0

-

1.0Ni/ r-CeO2-700

46

10.6

563.9

168.0

395.9

154.7

147.9

231.0

30.3

-

2.5Ni/ r-CeO2-700

39

10.8

1018.7

442.9

575.8

109.0

222.7

646.8

40.2

-

p-CeO2

38

7.8

301.3

-

301.3

-

-

-

-

-

0.5Ni/p-CeO2

37

9.2

358.4

84.6

273.8

43.6

180.9

97.1

36.8

-

1.1Ni/p-CeO2

34

9.5

514.6

187.7

326.9

87.4

144.6

242.2

40.4

-

2.6Ni/p-CeO2

30

9.9

943.2

443.7

439.3

33.4

173.9

692.2

43.7

-

a

n(H1) represents the total amount of H2 consumption of CeO2 at ca. 100-600 oC and Ni/CeO2 catalysts at ca. 100-500oC. (H2 consumption values were calculated by integrating the H2-TPR peak areas calibrated by Ag2O reference) b n(H2) represents the theoretical H2 consumption values of Ni species as Ni2+ based on the actual Ni amount calculated from ICP-AES. c n(H3) = n(H1)-n(H2) represents the H2 consumption values of CeO2 supports in Ni/CeO2 catalysts.


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Page 28 of 38

Figure captions Figure 1. XRD patterns of various CeO2 and Ni/CeO2 catalysts. Figure 2. Representative TEM and HRTEM images of (A1 and A2) r-CeO2-500, (B1 and B2) cCeO2, (C1 and C2) r-CeO2-700, (D1 and D2) p-CeO2, (E1 and E2) 2.5Ni/r-CeO2-500, (F1 and F2) 2.6Ni/c-CeO2, (G1 and G2) 2.5Ni/r-CeO2-700 and (H1 and H2) 2.6Ni/p-CeO2. Figure 3. Ce 3d, O 1s and Ni 2p3/2 XPS spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) cCeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2. Figure 4. Visible Raman spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) c-CeO2 and Ni/cCeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts. Figure 5. UV Raman spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) c-CeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts. Figure 6. H2-TPR proﬁles of (A) r-CeO2-500 and Ni /r-CeO2-500, (B) c-CeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts. Figure 7. Catalytic performances of CeO2 and Ni/CeO2 catalysts in the propane combustion reaction (A1-D1) and the oxidative dehydrogenation of propane reaction (A2-D2). Figure 8. (A) Relationship between the propane oxidation rate at 250 °C in the propane combustion reaction and the H2-consumption values of (ɑ1+ɑ2) peak for various Ni/CeO2 catalysts (B) Relationship between the propene formation rate in the ODHP reaction at 300 °C and the H2-consumption values of γ and (γ + γ’) peaks for various Ni/CeO2 catalysts. (C)


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Relationship between the propene formation rate at 300 °C in the ODHP reaction and the H2consumption values of peak γ’, γ, (γ’ + γ) for various Ni/c-CeO2 catalysts.


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Figure 1. XRD patterns of various CeO2 and Ni/CeO2 catalysts.


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Figure 2. Representative TEM and HRTEM images of (A1 and A2) r-CeO2-500, (B1 and B2) cCeO2, (C1 and C2) r-CeO2-700, (D1 and D2) p-CeO2, (E1 and E2) 2.5Ni/r-CeO2-500, (F1 and F2)2.6Ni/c-CeO2, (G1 and G2) 2.5Ni/r-CeO2-700 and (H1 and H2) 2.6Ni/p-CeO2.


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Figure 3. Ce 3d, O 1s and Ni 2p3/2 XPS spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) cCeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2.


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Figure 4. Visible Raman spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) c-CeO2 and Ni/cCeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts.


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Figure 5. UV Raman spectra of (A) r-CeO2-500 and Ni/r-CeO2-500, (B) c-CeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts.


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Figure 6. H2-TPR proﬁles of (A) r-CeO2-500 and Ni /r-CeO2-500, (B) c-CeO2 and Ni/c-CeO2, (C) r-CeO2-700 and Ni/r-CeO2-700, and (D) p-CeO2 and Ni/p-CeO2 catalysts.


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Figure 7. Catalytic performances of CeO2 and Ni/CeO2 catalysts in the propane combustion reaction (A1-D1) and the oxidative dehydrogenation of propane reaction (A2-D2).


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Figure 8. (A) Relationship between the propane oxidation rate at 250 °C in the propane combustion reaction and the H2-consumption values of (ɑ1+ɑ2) peak for various Ni/CeO2 catalysts (B) Relationship between the propene formation rate in the ODHP reaction at 300 °C and the H2-consumption values of γ and (γ + γ’) peaks for various Ni/CeO2 catalysts. (C) Relationship between the propene formation rate at 300 °C in the ODHP reaction and the H2consumption values of peak γ’, γ, (γ’ + γ) for various Ni/c-CeO2 catalysts.


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TOC GRAPHICS


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CeO2 Catalysis

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