(139c) QUANTUM MODELING of H2 and H2 O CHEMISTRY ON Pt/α-Al2O3

The H₂ and H₂O reactions on Pt/Al₂O₃ surfaces are encountered in a number of processes, including hydrogen combustion, preferential oxidation of hydrogen in the presence of CO, the water gas-shift reaction, etc. In order to delineate the mechanisms of this group of reactions. we performed first principles Density Functional Theory calculations of (O, H, O₂, H₂, OH, and H₂O) adsorption, dissociation, and surface diffusion processes on αAl₂O₃ and Pt/αAl₂O₃ surfaces. Our results indicate the O₂ dissociation is not thermodynamically or kinetically favored on the αAl₂O₃ (0001) surface. However, both H₂ and H₂O can dissociate, forming hydroxyls with oxygen atoms in the second atomic layer. H₂O prefers to molecularly adsorb to atomic Pt and the barrier to further dissociation is four times larger than on the clean Al₂O₃ surface. Once dissociated, the oxygen species can diffuse locally but encounter a large barrier to long-range surface diffusion in the absence of defects or other species. Understanding these processes is put in context in order to exploit the ?inverse spillover effect' which occurs when reactant and environmental species chemisorb or dissociate on the support forming mobile species that migrate to the catalytically active particle and further promote combustion