Au and Pt Remain Unoxidized on a CeO2-Based Catalyst during the Water-Gas Shift Reaction

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ "This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/jacs.1c10481 ." Au and Pt remain unoxidized on CeO2 based catalyst during WGS reaction Tomas R. Reina1,2*, Miriam Gonzalez-Castaño1*, Victor López-Flores1,3, L. Marcela Martínez T.1 , Andrea Zitolo3, S. Ivanova1, Wenquian Xu4 , Miguel Angel Centeno1, Jose Antonio Rodriguez4 and Jose Antonio Odriozola1,2 corresponding authors: [email protected] ; [email protected]; *authors share equal contributions as first authors 1Inorganic Chemistry Department and Materials Science Institute, University of Seville- CSIC, 41092 Sevilla, Spain. 2Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, UK. 3Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, B.P. 48, 91192 Gif-sur-Yvette, France 4Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973. Abstract The active form of Au and Pt in CeO2 based catalysts for the WGS reaction is an issue that although it has been widely studied remains unclear. On one hand, ionic species might be responsible of weakening the Ce-O bonds increasing the oxygen mobility and WGS activity. On the other hand, the close contact of Au or Pt atoms with CeO2 oxygen vacancies at the metal/CeO2 interface might provide the active sites for an efficient reaction. In this work, by using in-situ X-ray absorption spectroscopy, we demonstrate that both Au and Pt remain unoxidized during the reaction. Remarkable differences involving the dynamics established by both species under WGS atmospheres were recognized. For the pre-reduced Pt catalyst, an increase of the conversion coincided with a re-structuration of the Pt atoms into cuboctahedrical metallic particles without significant variations on the overall particle size. Contrary to the relatively static behavior of Pt0, Au0 nanoparticles exhibited a sequence of particle splitting and agglomeration whilst maintaining a zero oxidation state, even though not being located in a metallic environment during the process. High WGS activity was obtained when Au atoms were surrounded by oxygen. The fact that Au preserves its unoxidized state indicates that the chemical interaction between Au and oxygen must be necessarily electrostatic being such electrostatic interaction fundamental for a top performance in the WGS process. Keywords: Pt; Au; metal; Water Gas Shift reaction; active phase INTRODUCTION Hydrogen is recognized as one of the most promising future fuels since it is a clean, renewable and highly efficient energy carrier 1. The purification of H2 by CO removal processes is one of the major challenges to face in order to definitely deploy a "hydrogen economy" as a real energy alternative 2. In this respect, an old chemical process as the water gas shift (WGS, 𝐶𝑂 + 𝐻2𝑂 ⇌ 𝐻2+𝐶𝑂2) reaction has received a renewed interest and it is under intensive investigation 2–5. The development of efficient heterogeneous catalysts for this process is inseparably associated to the success of hydrogen-based technologies. Among the studied catalysts, Pt- and Au-CeO2 based systems have been broadly employed exhibiting remarkable activity in the WGS 6–8. In general terms, it is well accepted that CO is absorbed on the metal nanoparticles while H2O is activated in the oxygen vacancies of the cerium oxide and the subsequent steps proceed on the metal-CeO2 interface 9,10. Nevertheless, what it is not so clear is the nature of the catalytically active Au and Pt species. Despite the vast literature behind Au and Pt WGS catalysts this issue remains controversial 11–13. Flytzani-Stephanopoulos and coworkers have published that the active species of Au and Pt in nanostructured CeO2 materials are nonmetallic (cationic) species strongly associated with the surface CeO2 oxygen groups 14,15. After applying a leaching process, they remove metallic Au species and they ended up with high effective WGS catalysts with a very low amount of noble metal where the active species are Auδ+. They concluded that metallic Au nanoparticles are un-important in the WGS reaction over Au- CeO2 15. Opposite observations have been however reported for both noble metals establishing M0-Ov-Ce3+ as the main WGS active sites 16–18. Thus, investigations reported by Kim and Thompson contradict the Au cationic as active WGS species 19. Even though using the same leaching methodology that in ref. 15, the unleached catalysts were much more active that the leached ones [19]. Karpenko et al. 20 found that after the leaching process both cationic and metallic Au species remain in the catalysts. Similar disagreements concerning the oxidation state of active Pt species are reported being both PtOx and Pt0 species the active phases 21,22. Namely, studies conducted by Ding et al. 23 on Pt supported catalysts proposed Pt0 as the WGS active site whilst Pt+ species were mere spectators. These results challenge the idea of cationic metal species and situate the active site on unoxidized Au and Pt nanoparticles. The diversity in author opinions regarding both active species seems to be a never-ending topic. Despite its complexity, this question has to be addressed since it is crucial to understand and design efficient Au and Pt based catalysts for the WGS reaction. In this work, we have shed some light to close the debate. Using in-situ XAS experiments on iron-doped Au/ and Pt/ CeO2 catalysts, the nature of active Au and Pt species has been identified. Despite the fact that only metallic Au and Pt particles are present in the whole reaction, during the WGS the close interaction of Au with oxygen from CeO2 results on an incipient AuOx formation where Au remains as Au0. Our observations unifies previous hypotheses demystifying the nature of the active phase in the WGS process. EXPERIMENTAL Ceria based supports were prepared over commercial -Al2O3 (Sasol) by wetness impregnation using their corresponding metal precursors Ce(NO3)4 • 6H2O and Fe(NO3)3 • 9H2O (Aldrich) in order to obtain 15 wt.%/CeO2/Al2O3 and 15 wt.%Ce0.8Fe0.2O2/Al2O3 supports. Once impregnated, the obtained solids were dried and calcined at 450°C for 4h. The obtained supports were labelled CeAl and CeFeAl, respectively. For Au and Pt catalysts, different synthesis methods were employed with the aim of achieving metal content ca. 2 wt.%. with comparable metal dispersions 24. Thus, Pt catalysts were synthetized via incipient wetness impregnation using the tetrammonium nitrate platinate solution as Pt precursor slightly modified with acetic acid 1M 25. Instead, Au catalysts were prepared by direct anionic exchange method using HAuCl4 as Au precursor. The catalysts were calcined at 350°C. Over the three different supports: -Al2O3, 15 wt.%/CeO2/Al2O3 and 15 wt.%Ce0.8Fe0.2O2/Al2O3, the obtained Pt and Au catalysts were labelled correspondingly, Pt/Al, Au/Al, Pt/CeAl, Au/CeAl, Pt/CeFeAl and Au/CeFeAl. The chemical composition of the prepared catalysts was evaluated by X-ray microfluorescence spectrometry and the measurements were conducted in an EDAX Eagle III spectrophotometer equipped with a Rh cathode. The catalytic activity of the samples was evaluated in a tubular flow reactor at 4000 h-1 using feed streams composed by 4.5 vol.%CO + 30 vol.% H2O (N2 balanced). Operando XAS spectra were collected on Pt L3-edge, Au L3-edge and Ce L3-edge during the WGS reaction. Details of each experiment are described in the Supplementary Information. RESULTS The chemical composition of both noble metals catalysts employed in this study is presented in Table S1. Full details of catalysts preparation and characterization can be found elsewhere 24,26. The catalytic activity of the Pt/ and Au/CeO2 catalysts for the WGS reaction is depicted in Fig. 1. The graph evidences the impact of the noble metal nature and the role of CeO2 in the WGS reaction. It is also evident that the support has a deeper influence on Au catalysts than in Pt ones, although it must be noticed that at low temperatures, Au catalysts are always more active than the corresponding Pt ones. Whilst the Pt intrinsic ability for performing water splitting and CO oxidation renders the Pt/Al2O3 catalyst an active system for the WGS reaction, Au/Al2O3 is hardly active pointing the need of an "active support" like CeO2 in which oxygen vacancies enable the water activation, which is the controlling reaction step 27. In view of the later, the creation of oxygen vacancies on CeO2 is desirable to achieve high catalytic performances. Actually, the oxygen vacancies of CeO2 play a multirole on the reaction. On one hand, they are the reactive sites for water splitting and on the other hand, they act as preferential nucleation points for metal deposition allowing high metallic dispersion and improving the catalytic performance 28,29. This interaction O-vacancy/metallic center involves a charge transfer from the structural defects towards noble metal atoms, modulating the activity of the Au and Pt based catalysts 24. Previous reports demonstrated that the addition of small amounts of iron increases the oxygen vacancies population on CeO2 based systems resulting in a remarkable activity in the WGS process [18, 22]. Fig. 1 also demonstrates the superiority of the iron doped system which is able to achieve very high CO conversions at 300 ºC. Iron-doped CeO2 based catalysts can be considered as an interesting approach towards the new generation of WGS catalysts and therefore it was selected to face the main question of this paper: What is the nature of the active Au and Pt species during the WGS? Figure 1. Catalytic activity of Pt and Au supported catalysts (4.5%CO+30%H2O; 4000 h-1) Fig. 2A and 2B shows Pt-L3 and Ce-L3 XANES spectra obtained for the Pt/CeFeAl catalyst during the pre-reduction treatment (not conducted for Au/CeFeAl catalyst). The evolution of the formal oxidation state of Pt and Ce atoms during the pre-reduction treatment are shown in Table S2. Before the introduction of H2, the Pt-L3 XANES of the Pt/CeFeAl sample (Fig. 2A) shows the characteristic PtO sharp rising absorption peak at 11564 eV underlining the existence of oxidized Pt atoms. This absorption peak, denominated the “white line”, directly accounts for the vacancies on 5d orbital when Pt species are oxidized 30. These PtOx species are reduced by H2 starting from room temperatures as indicated by the decrease of the intensity of the white line until a totally reduced Pt state is reached. The different XANES line shapes characteristic of Ce3+ and Ce4+ species 19 permitted us to discern the evolution of the Ce-L3 XANES absorption with the temperature when exposed to H2 rich streams. At 25 ºC, the Ce-L3 XANES spectrum show two absorption peaks. The lower energy absorption peak is commonly attributed to the absorption from 2p level to the 5d with the 4f orbital remaining unoccupied, resulting in a Ce [2p54f05d1]O[2p6] state configuration 31,32. The second absorption characteristic of Ce4+ appears at higher energies and it is assigned to an electron transfer from the oxygen valence band to the CeO2 4f shell leaving a hole in the oxygen valence band, with a final Ce[2p54f15d1]O[2p5] state configuration 31,32. The Ce-L3 XANES spectrum obtained prior the H2 contact significantly changes when the temperature is increased. Meanwhile, the Ce4+ absorption bands intensity decreases, a new peak is observed and related to the appearance of Ce3+ species. For the Ce3+ species, in which the 4f shell is partially occupied, the observed transition is associated to the absorptions from 2p to 5d, with the initial and final configuration states for the electron transfer corresponding to Ce[2p64f15d0]O[2p6] and Ce[2p54f15d1]O[2p6] 31,32. Figure 2. XANES spectra obtained during the H2 pre-activation treatment for: a) Pt-L3 absorption edge; b) Ce-L3 absorption edge Fig. 3 shows the normalized background subtracted XAS Pt-L3 and Au-L3 spectra under WGS atmospheres. For the pre-reduced Pt catalyst, the Pt and Ce formal oxidation state and defects concentration remain almost unchanged whatever the reaction temperature (Table S3, S4 and S5, respectively). In agreement with the studies of Ding et al. 23, the XANES data revealed Pt0 as the major active site in Pt/CeO2 catalysts under WGS reaction conditions. In a similar way, the Au-L3 XANES spectra resembled that of the metallic Au for all the studied temperatures. Indeed, no evidence of the white line fingerprints of oxidized metals with partially occupied d-band is noticed 33. Furthermore, there is no displacement of the absorption edge position towards higher energies that would correspond to an oxidized state. Therefore, it must be concluded that Au remains as Au0 during the WGS. Contrary to the observations of Flytzani and coworkers 15, in our Au-based sample, Au preserves a formal oxidation state of (0) from the very beginning of the WGS reaction. However, structural changes that take place in the process facilitate the contact between Au and lattice oxygen from CeO2, resulting in the existence isolated Au atoms surrounded by O. The Au- O contact is in line with the theory of Auδ+ as active goldspecies. Experimental evidences in support of this close Au-O contact are given by the analysis of EXAFS data (Fig. 3). Figure 3. XAS spectra obtained during WGS reaction: A) XANES Pt-L3; B) XANES Au-L3; C) EXAFS Pt-L3; D) EXAFS Au-L3 In-situ Pt-L3 and Au-L3 EXAFS spectra (Fig. 3C and 3D) evidenced the different behaviors observed for Pt and Au metal. An EXAFS model was built and fitted to the experimental data by using the FEFF6 and FEFFIT 2.54 software. Detailed information of the used model can be found in the Supporting material section. In short, Pt particles were modelled assuming that Pt atoms form epitaxial squared layers growing on top of a Ce (100) surface, with different covering proportions, in which the Pt atoms placed at the bottom occupied positions which initially should be occupied by oxygen atoms of the CeO2 lattice. The Au atoms EXAFS spectra were fitted with a linear combination of (1) the local environment of isolated Au atoms in direct contact with the CeO2 (100) and (110) surfaces at specific sites, and (2) Au nanoparticles, composed of the local environments of (2.1) Au bulk in regular fcc crystal structure and (2.2) Au at the surface of the nanoparticle. The proportion of each local environment in the model is given by the Au dispersion rate and the size of the nanoparticles. Moreover, rather good fittings were obtained in the in-situ Pt-L3 EXAFS spectra for Pt nanoparticles particularly at high-R providing relevant information about the Pt particle geometry. Within this region, the apparent absence of multiple-scattering signals supports the conclusion that the Pt atoms form quite small nanoparticles. Setting as starting scenario a tri-layered Pt particle, it was obtained in all the cases almost a pure bilayer, with only spurious atoms coming to the third in some cases. The Pt nanoparticle size does not exhibit meaningful variations with the temperature (Table S6). This may arise from the fact that the transformations are made during the previous H2 reduction, and once the sample is in a reductive environment, it does not change significantly. In very good agreement to particle sizes observed HR-TEM micrographs (Fig. S5), the average lateral length estimated for the Pt nanoparticles is 1.2 nm, which is about 5-6 atoms per side. Certain differences were though noticed on the shape of the Pt particle as the temperature increased from 150 to 200 °C. The intermediate layer of Pt particle showed the same size as the bottom one only at the lowest temperature (150 °C). As the temperature increased up to 200 °C, the intermediate layer became slightly larger than the bottom one. Furthermore, the coordination distances for Pt-Pt shells seem to follow a slight trend as the temperature rises, contracting the lattice at higher temperatures (Table S7 and S8). Being a small nanoparticle, the distances are expected to be smaller than for the bulk Pt. Such decrease on the Pt-Pt shell distances might be related to the epitaxial growth onto a slightly smaller lattice hence affecting the distances between the Pt atoms. The coordination distances for the Pt-Ce shells also experiment a transition between 150 ºC and 200 ºC (Table S9 and S10). At the lowest temperature, the height from the CeO2 surface to the Pt bottom layer is 0.23 Å above the original position at the center of the tetrahedral holes. This is expected, as the Pt atom size is larger than the O2-ions. However, this height is changed to about 0.11 at 200 ºC, and then maintained around that value at higher temperatures. This abrupt decrement on the Pt-Ce shells agrees with a Pt particle reconstruction in which an (1) Song, C. Fuel Processing for Low-Temperature and High-Temperature Fuel Cells: Challenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today 2002, 77 (1–2), 17–49. https://doi.org/10.1016/S0920-5861(02)00231-6. (2) Trimm, D. L. Minimisation of Carbon Monoxide in a Hydrogen Stream for Fuel Cell Application. Appl. Catal. A Gen. 2005, 296 (1), 1–11. https://doi.org/10.1016/j.apcata.2005.07.011. (3) Farrauto, R.; Liu, Y.; Ruettinger, W.; Ilinich, O.; Shore, L.; Giroux, T. Precious Metal Catalysts Supported on Ceramic and Metal Monolithic Structures for the Hydrogen Economy. Catal. Rev. - Sci. Eng. 2007, 49 (2), 141–196. https://doi.org/10.1080/01614940701220496. (4) Petallidou, K. C.; Kalamaras, C. M.; Efstathiou, A. M. The Effect of La 3 + , Ti 4 + and Zr 4 + Dopants on the Mechanism of WGS on Ceria-Doped Supported Pt Catalysts. Catal. Today 2014, 228, 183–193. https://doi.org/10.1016/j.cattod.2013.10.081. (5) Farrauto, R. J. New Catalysts and Reactor Designs for the Hydrogen Economy. Chem. Eng. J. 2014, 238, 172–177. https://doi.org/10.1016/j.cej.2013.07.004. (6) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Senanayake, S. D.; Barrio, L.; Liu, P.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. Gold, Copper, and Platinum Nanoparticles Dispersed on CeO x/TiO2(110) Surfaces: High Water-Gas Shift Activity and the Nature of the Mixed-Metal Oxide at the Nanometer Level. J. Am. Chem. Soc. 2010, 132 (1), 356– 363. https://doi.org/10.1021/ja9087677. (7) Andreeva D., Idakiev V., Tabakova T., Ilieva L., Falaras P., Bourlinos A., Travlos A. Low-temperature water-gas shift reaction over Au/CeO2 catalysts. Catal Today 2002, 72, 51-57. https://doi.org/10.1016/S0920-5861(01)00477-1 (8) González-Castaño, M.; Reina, T. R.; Ivanova, S.; Martínez Tejada, L. M.; Centeno, M. A.; Odriozola, J. A. O2-Assisted Water Gas Shift Reaction over Structured Au and Pt Catalysts. Appl. Catal. B Environ. 2016, 185, 337–343. https://doi.org/10.1016/j.apcatb.2015.12.032. (9) Daly, H.; Goguet, A.; Hardacre, C.; Meunier, F. C.; Pilasombat, R.; Thompsett, D. The Effect of Reaction Conditions on the Stability of Au/CeZrO4 Catalysts in the Low- Temperature Water-Gas Shift Reaction. J. Catal. 2010, 273 (2), 257–265. https://doi.org/10.1016/j.jcat.2010.05.021. (10) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Pérez, M. Activity of CeOx and TiOx Nanoparticles Grown on Au(111) in the Water-Gas Shift Reaction. Science (80-. ). 2007, 318 (December), 1757–1760. (11) Bonda, G.; Thompson, D. Formulation of Mechanisms for Gold-Catalysed Reactions. Gold Bull. 2009, 42 (4), 247–259. (12) Ratnasamy, C.; Wagner, J. Water Gas Shift Catalysis. Catal. Rev. - Sci. Eng. 2009, 51 (3), 325–440. https://doi.org/10.1080/01614940903048661. (13) Jacobs, G.; Davis, B. H. Low Temperature Water-Gas Shift: Applications of a Modified SSITKA-DRIFTS Method under Conditions of H2 Co-Feeding over Metal/Ceria and Related Oxides. Appl. Catal. A Gen. 2007, 333 (2), 192–201. https://doi.org/10.1016/j.apcata.2007.07.029. (14) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts. Science (80-. ). 2003, 301 (5635), 935–938. https://doi.org/10.1126/science.1085721. (15) Yang, M.; Li, S.; Wang, Y.; Herron, J. A.; Xu, Y.; Allard, L. F.; Lee, S.; Huang, J.; Flytzani-stephanopoulos, M. Catalytically Active Au-O ( OH ) - Species Stabilized by Alkali Ions on Zeolites and Mesoporous Oxides. 2014, 346 6216 1498-1501 (16) Li, Y.; Kottwitz, M.; Vincent, J. L.; Enright, M. J.; Liu, Z.; Zhang, L.; Huang, J.; Senanayake, S. D.; Yang, W. C. D.; Crozier, P. A.; Nuzzo, R. G.; Frenkel, A. I. Dynamic Structure of Active Sites in Ceria-Supported Pt Catalysts for the Water Gas Shift Reaction. Nat. Commun. 2021, 12 (1), 1–9. https://doi.org/10.1038/s41467-021-21132-4. (17) Gonzalez Castaño, M.; Reina, T. R.; Ivanova, S.; Centeno, M. A.; Odriozola, J. A. Pt vs. Au in Water–Gas Shift Reaction. J. Catal. 2014, 314, 1–9. https://doi.org/10.1016/j.jcat.2014.03.014. (18) Kalamaras, C. M.; Dionysiou, D. D.; Efstathiou, A. M. Mechanistic Studies of the Water- Gas Shift Reaction over Pt/Ce XZr1-XO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant. ACS Catal. 2012, 2 (12), 2729–2742. https://doi.org/10.1021/cs3006204. (19) Kim, C. H.; Thompson, L. T. On the Importance of Nanocrystalline Gold for Au/CeO2 Water-Gas Shift Catalysts. J. Catal. 2006, 244 (2), 248–250. https://doi.org/10.1016/j.jcat.2006.08.018. (20) Karpenko, A.; Leppelt, R.; Plzak, V.; Behm, R. J. The Role of Cationic Au3+ and Nonionic Au0 Species in the Low-Temperature Water-Gas Shift Reaction on Au/CeO2 Catalysts. J. Catal. 2007, 252 (2), 231–242. https://doi.org/10.1016/j.jcat.2007.09.017. (21) Liu, P.; Rodriguez, J. A. Water-Gas-Shift Reaction on Metal Nanoparticles and Surfaces. J. Chem. Phys. 2007, 126 (16). https://doi.org/10.1063/1.2722747. (22) González, I. D.; Navarro, R. M.; Wen, W.; Marinkovic, N.; Rodriguéz, J. A.; Rosa, F.; Fierro, J. L. G. A Comparative Study of the Water Gas Shift Reaction over Platinum Catalysts Supported on CeO2, TiO2 and Ce-Modified TiO2. Catal. Today 2010, 149 (3– 4), 372–379. https://doi.org/10.1016/j.cattod.2009.07.100. (23) Ding, K.; Gulec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G. D.; Marks, L. D.; Stair, P. C. Identification of Active Sites in CO Oxidation and Water-Gas Shift over Supported Pt Catalysts. Science (80-. ). 2015, 350 (6257), 189–192. https://doi.org/10.1126/science.aac6368. (24) Gonzalez Castaño, M.; Reina, T. R. R.; Ivanova, S.; Centeno, M. a. A.; Odriozola, J. a. A. Pt vs. Au in Water-Gas Shift Reaction. J. Catal. 2014, 314, 1–9. https://doi.org/10.1016/j.jcat.2014.03.014. (25) Romero-Sarria, F.; Garcia-Dali, S.; Palma, S.; Jimenez-Barrera, E. M.; Oliviero, L.; Bazin, P.; Odriozola, J. A. The Role of Carbon Overlayers on Pt-Based Catalysts for H2-Cleanup by CO-PROX. Surf. Sci. 2016, 648, 84–91. https://doi.org/10.1016/j.susc.2015.12.017. (26) Reina, T. R.; Ivanova, S.; Delgado, J. J.; Ivanov, I.; Idakiev, V.; Tabakova, T.; Centeno, M. A.; Odriozola, J. A. Viability of Au/CeO2-ZnO/Al2O3 Catalysts for Pure Hidrogen Production via WGS. ChemCatChem 2014, 6, 1401–1409. https://doi.org/10.1002/cctc.200)). (27) González-Castaño, M.; Ivanova, S.; Laguna, O. H.; Martínez T., L. M.; Centeno, M. A.; Odriozola, J. A. Structuring Pt/CeO2/Al2O3 WGS Catalyst: Introduction of Buffer Layer. Appl. Catal. B Environ. 2017, 200, 420–427. https://doi.org/10.1016/j.apcatb.2016.07.039. (28) Hernández, W. Y.; Romero-Sarria, F.; Centeno, M. A.; Odriozola, J. A. In Situ Characterization of the Dynamic Gold-Support Interaction over Ceria ModifiedEu3+. Influence of the Oxygen Vacancies on the Co Oxidation Reaction. J. Phys. Chem. C 2010, 114 (24), 10857–10865. https://doi.org/10.1021/jp1013225. (29) Bobadilla, L. F.; Santos, J. L.; Ivanova, S.; Odriozola, J. A.; Urakawa, A. Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water − Gas Shift Reaction by Operando DRIFTS and Ultraviolet − Visible Spectroscopy. ACS Catal. 2018, 8, 7455−7467. https://doi.org/10.1021/acscatal.8b02121. (30) Bera, P.; Priolkar, K. R.; Gayen, A.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Jayaram, V.; Subbanna, G. N. Ionic Dispersion of Pt over CeO2 by the Combustion Method: Structural Investigation by XRD, TEM, XPS, and EXAFS. Chem. Mater. 2003, 15 (10), 2049–2060. https://doi.org/10.1021/cm0204775. (31) Linganiso, L. Z.; Pendyala, V. R. R.; Jacobs, G.; Davis, B. H.; Cronauer, D. C.; Kropf, A. J.; Marshall, C. L. Low-Temperature Water-Gas Shift: Doping Ceria Improves Reducibility and Mobility of o-Bound Species and Catalyst Activity. Catal. Letters 2011, 141 (12), 1723–1731. https://doi.org/10.1007/s10562-011-0720-1. (32) Barrabés, N.; Föttinger, K.; Llorca, J.; Dafinov, A.; Medina, F.; Sá, J.; Hardacre, C.; Rupprechter, G. Pretreatment Effect on Pt/CeO 2 Catalyst in the Selective Hydrodechlorination of Trichloroethylene. J. Phys. Chem. C 2010, 114 (41), 17675– 17682. https://doi.org/10.1021/jp1048748. (33) López-Cartes, C.; Rojas, T. C.; Litrán, R.; Martínez-Martínez, D.; De La Fuente, J. M.; Penadés, S.; Fernández, A. Gold Nanoparticles with Different Capping Systems: An Electronic and Structural XAS Analysis. J. Phys. Chem. B 2005, 109 (18), 8761–8766. https://doi.org/10.1021/jp050184+. (34) Sayle, D. C.; Maicaneanu, S. A.; Watson, G. W. Atomistic Models for CeO2(111), (110), and (100) Nanoparticles, Supported on Yttrium-Stabilized Zirconia. J. Am. Chem. Soc. 2002, 124, 38, 11429–11439 (35) Molinari, M.; Parker, S. C.; Sayle, D. C.; Islam, M. S. Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria. J. Phys. Chem. C 2012, 116, 7073–7082. TOC Graphic