Nature of Ptn/CeO2 (111) Surface under Water-Gas Shift Reaction Conditions: A Constrained ab Initio Thermodynamics Study

The growth pattern of small Pt-n (n = 1-10) clusters has been investigated on the stoichiometric and partially reduced ceria (111) surfaces using density functional theory. On both surfaces, the metal packing was cubic-closed packed, and starting from metal clusters as small as Pt-10, the (111) facet of the metal cluster was clearly recognizable. The main focus of this article is on identifying a realistic catalyst model for the Pt/ceria surface under realistic water-gas shift (WGS) reaction conditions. As a result, the interactions of O, CO, and H species with our chosen catalyst model (Pt-10/CeO2(111)) have been studied. By applying the constrained ab initio thermodynamic simulation method, we find that under WGS reaction conditions, oxygen vacancies and vacancy clusters are thermodynamically stable, while oxygen adatoms are not stable on the Pt cluster. As expected, Pt atoms of the Pt-10/CeO2(111) catalyst model, that are not in contact with the ceria surface, are susceptible to be covered by CO molecules. Presence of these CO ad-molecules does not change the redox behavior of the ceria surface significantly. Interestingly, adsorbed CO molecules increase the hydrogen adsorption energy at the Pt atoms at the TPB' and decrease CO adsorption at the TPB. As a result, we find that, under experimentally relevant temperatures and partial pressures of CO and H-2, it is more likely for a hydrogen atom to adsorb at the TPB than a CO molecule. Finally, we studied the effect of coadsorbed hydrogen atoms on the ceria surface (hydroxylated ceria surface) on the nature of our catalyst model. The presence of H adatoms can considerably change the redox behavior of the ceria surface in a reducing environment by destabilizing the oxygen vacancy clusters at relatively low temperatures (400-700 K). However, the presence of coadsorbed CO molecules on the Pt cluster likely compensate for this destabilizing effect.