A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Abstract

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO 2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO 2 and the formation of H 2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO 2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied.