(330f) Theoretical Investigation of the Water-Gas Shift Reaction At the Three-Phase Boundary of Ceria (111) Supported Platinum Clusters

This work is focused on the investigation of the catalytic activity of the three-phase boundary (TPB) of Pt supported CeO₂(111) model catalysts for the water-gas shift (WGS) reaction. We investigated the growth pattern of small Pt_n (n=1-10) clusters on the stoichiometric and partially reduced CeO₂(111) surfaces. Then, we performed constrained ab initio thermodynamic calculations for the Pt₁₀/CeO₂(111) surface to identify a meaningful catalyst surface model. 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. Pt-atoms 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. However, adsorbed CO molecules increase the hydrogen adsorption energy and decrease the CO adsorption energy at the Pt atoms at the TPB. In this way, it is possible to have a high coverage of H atoms at the boundary sites of the Pt cluster even at high partial pressures of CO. Finally, we studied the effect of co-adsorbed hydrogen atoms on the ceria surface (hydroxylated ceria surface) on the nature of our catalyst model. 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, presence of co-adsorbed CO molecules on the Pt cluster likely compensate for this destabilizing effect. After characterizing our catalyst model surface, we studied various reaction pathways at the boundary sites of our Pt₁₀/CeO₂(111) model with 2 to 3 vacancies, three adsorbed CO molecules on the atop sites of Pt₁₀, and six H atoms adsorbed on the ceria surface. Specifically, we focused the redox, associative carboxyl and carboxyl pathway with redox regeneration.