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Title

Ni/ceria catalysts: Are they bifunctional? ‒ The role of the ceria support

AuthorsLópez-Durán, David ; Rodriguez, J. A.; Ganduglia-Pirovano, M. V. ; Carrasco Rodríguez, Javier
Issue Date18-Apr-2013
CitationReducible oxide chemistry, structure and functions COST Action CM1104 (2013)
AbstractCeria (CeO2)-supported heterogeneous catalysts exhibit a remarkable activity and selectivity in many chemical processes, which has opened exciting perspectives in catalysis research. The superior performance of ceria is mainly associated with the easy conversion between the Ce3+ and Ce4+ oxidation states [1]. In addition, ceria usually enhances the catalytic properties of late transition metals. Recent work has proven the high activity and stability of Ni-CeO2 systems for the water-gas shift (WGS) reaction (CO+H2O→CO2+H2) at small Ni coverages [2]. The strong support-metal interaction experienced by Ni1 adatoms compared to larger metal clusters explains the observed chemical properties and catalytic performance of these systems towards CO oxidation [3], although the full WGS mechanism is yet unclear. In general, a bifunctional mechanism is assumed where CO adsorbs on the metal and oxygen vacancies on the ceria surface act as active sites for water dissociation [2,4], which is believed to be the limiting step of the whole WGS process. Nevertheless, we show here that this is not necessarily the case, since the dissociation of water on top of ceria supported Ni1 adatoms is comparable easy as on the reduced support. The oxide support will play an indirect role in the catalytic process by modifying the chemical activity of the nickel. We studied Ni1/CeO2(111) and Ni4/CeO2(111) model catalysts employing density-functional theory (DFT) with the DFT+U approach and compare with Ni(111) and CeO2(111). We showed that the C−O bond strength follows the trend: Ni(111)<Ni4/CeO2(111)<Ni1/CeO2(111) [3], which provides an explanation of the Ni coverage dependence reported for the CO methanation reaction on Ni/CeO2(111) catalysts and its high efficiency for the WGS reaction at small Ni coverages [2]. In addition to CO oxidation, we have also investigated the dissociation of water on these systems. Our findings reveal that Ni1/CeO2(111) is a better candidate to carry out the water dissociation step than reduced CeO2(111) as previously thought. In particular, we put forward the idea that water dissociation on surface oxygen vacancies on CeO2(111) results in too strongly bound OH species not available to form WGS reaction intermediates, whereas OH species produced upon water dissociation on the Ni1/CeO2(111) system are less strongly bound and readily available to continue the WGS reaction pathway.
DescriptionTrabajo presentado en el Meeting COST Action CM1104-Reducible oxide chemistry, structure and functions, celebrado en Upsala (Suecia) del 6 al 8 de noviembre de 2013.
URIhttp://hdl.handle.net/10261/185672
Appears in Collections:(ICP) Comunicaciones congresos
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