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1. IntroductionCeria (CeO2) is the most significant of the oxides of rare-earth elements in industrial catalysis with its reducibility being essential to its functionality in catalytic applications. The complexity of real (powder) catalysts hinders the fundamental understanding of how they work. For instance, in the case of metal nanoparticles on ceria supports, it is known that metal-oxide interactions can dramatically influence catalytic activity. To unravel the effect of the support, well-defined ceria-based model catalysts are prepared experimentally or created theoretically and studied. The basic questions that need to be answered are then, how does the presence of a ceria support modifies catalytic activity?, what is the effect of metal loading?, and what is the role of both metal and oxide in the reaction mechanism?In this work, recent results on ceria-supported Ni, Co and Cu model catalysts will be discussed, 1-3 as examples of catalysts for methane dry reforming (DRM: CH4+CO2 --> 2H2+2CO). Also, the Ni/ceria system is considered for the water-gas shift reaction (WGS: CO+H2OCO2+H2 ) to produce hydrogen.4 The emphasis is here put on theoretical studies and special attention is given to the effects of ceria as catalyst support.2. Models and methodsMethane decomposition and water dissociation are frequently cited as the most difficult steps for the DRM and WGS processes, respectively. We apply the spin-polarized density functional theory+U approach to investigate the dissociative adsorption of CH4 on Co, Ni and Cu nanoparticles deposited on the stoichiometric CeO2(111) and reduced Ce2O3(0001) surfaces, as well as the dissociative adsorption of H2O on Ni/CeO2(111). A value of 4.5 eV was used for the Hubbard U-like term. We model the metal nanoparticles using single atoms and small tetrahedral clusters. The results are compared to those for the extended metal surfaces. To locate the transition-state (TS) structures we employ the CI-NEB method. The theoretical studies are performed in combination with experiments using ambient pressure XPS and catalytic tests.3. Results and discussionThe ability of ceria to stabilize oxidized species (MOx: Co2+ and Ni2+) on the stoichiometric CeO2 surfaces, by relocalizing electrons on localized f-states, and metallic ones (M0: Co0, Ni0) on the reduced CeO2-x support, is essential for catalytic activity in the DRM reaction. Methane dissociation (CH4  CH3  CH2  CH  C) occurs already at room temperature on MOx/CeO2. With increasing temperature (700 K), in the presence of CH4, oxygen vacancies (C + Osurf  COgas + Vac) are formed on the support, which is accompanied by the MOx/CeO2  M0/CeO2-x transformation. The CH4 binding on M0/CeO2-x is increased compared to that on MOx/CeO2 (cf. Fig.1), increasing the probability of reaction on M0/CeO2-x, and addition of CO2, results in its dissociation at the oxygen vacancies, and a catalytic cycle is established without coke deposition if the metal loading is low. The Co/ceria system is the most active with a barrier for methane dissociation becoming negligible as the MOx/CeO2  M0/CeO2-x transformation takes place (cf. Fig.1). The marked differences in activation barriers relate to the ability of the metals to form strong M-H bonds.Fig. 1. Left panel: Catalytic activity for DRM. Right panel: Reaction energy profile for the CH4-->CH3+H on Cu4, Co1, and Ni1 clusters on CeO2(111) as well as Co1 and Ni1 on Ce2O3(0001).As for the H2O dissociation on Ni/CeO2(111) systems for WGS, computed activation barriers reveal an enhanced reactivity of Ni species in direct contact with the ceria support as compared to those not in contact. At the origin of this support effect is the stabilization of Ni2+ species on the ceria support.4. ConclusionsWe have established computational models for metal/ceria systems that are consistent with experimental knowledge for experimental model catalysts, as well as powder catalysts, and thus help to bridge the gap between them. An oxide support can modify the electronic properties of an admetal in substantial ways, making its chemical properties very different from those of the corresponding bulk metal as exemplified by ability of Ni2+ species on CeO2(111) to cleave CH bonds at RT and OH bonds. It is also seen that the metal loading can be crucial for catalytic activity and stability. Thus, choosing the right metal-oxide combination and manipulating metal-oxide interactions, as well as controlling the effects of metal loading, an improved catalytic activity can be obtained.AcknowledgementWe thank Zongyuan Liu, Ramón A. Gutiérrez, John J. Carey, Michael Nolan, Robert M. Palomino, Mykhailo Vorohta, David C. Grinter, Pedro J. Ramírez, Vladimir Matolín, H. Fabio Busnengo, Javier Carrasco, Jing Zhou, Dario J. Stacchiola, Ethan J. Crumlin, Si Luo, Iradwikanari Waluyo, Thuy-Duong Nguyen-Phan, David López-Durán, Tomá? Duchoň, and Jaime Evans for their contributions to this work.References[1] 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.; Carrasco, J.; Busnengo, H. F.; Ganduglia-Pirovano, M. V.; Senanayake, S. D.; Rodriguez, J. A. Angew. Chem., Int. Ed. 2016, 55, 7455−7459.[2] Lustemberg, P. G.; Ramírez, P. J.; Liu, Z.; Gutiérrez, R. A.; Grinter, D. G.; Carrasco, J.; Senanayake, S. D.; Rodriguez, J. A.; Ganduglia-Pirovano, M. V. ACS Catal. 2016, 6, 8184 ? 8191.[3] Liu, Z.; Lustemberg, P. G.¸ Gutiérrez, R. A.¸ Carey, J. J.; Palomino, R. M.; Vorokhta, M.; Grinter, D. C.; Ramírez, P. J.; Matolín, V.; Nolan, M.; Ganduglia-Pirovano, M. V.; Senanayake, S. D.; Rodriguez, J. A. Angew. Chem. Int. Ed. 2017 DOI: 10.1002/anie.201707538).[4] Carrasco, J.; López-Durán, D.; Liu, Z.; Duchoň, T.; Evans, J; Senanayake, S. D.; Crumlin, E. J.; Matolín, V; Rodriguez, J. A.; Ganduglia-Pirovano, M. V. Angew. Chem. Int. Ed. 2015, 54, 3917?3921.