Catalytic water formation on platinum: A first-principles study

The study of catalytic behavior begins with one seemingly simple process, n amely the hydrogenation of O to H2O on platinum. Despite the apparent simpl icity its mechanism has been much debated. We have used density functional theory with,gradient corrections to examine microscopic reaction pathways f or several elementary steps implicated in this fundamental catalytic proces s. We find that H2O formation from chemisorbed O and H atoms is a highly ac tivated process. The largest barrier along this route, with a value of simi lar to1 eV, is the addition of the first H to O to produce OH. Once formed, however, OH groups are easily hydrogenated to H2O with a barrier of simila r to0.2 eV. Disproportionation reactions with 1:1 and 2:1 stoichiometries o f H2O and O have been examined as alternative routes for OH formation. Both stoichiometries of reaction produce OH groups with barriers that are much lower than that associated with the O + H reaction. H2O, therefore, acts as an autocatalyst in the overall H O formation process. Disproportionation w ith a 2:1 stoichiometry is thermodynamically and kinetically favored over d isproportionation with a l:I stoichiometry. This highlights an additional ( promotional) role of the second H2O molecule in this process. In support of our previous suggestion that the key intermediate in the low-temperature H 2O formation reaction is a mixed OH and H2O overlayer we find that then is a very large barrier for the dissociation of the second H2O molecule in the 2:1 disproportionation process. We suggest that the proposed intermediate is then hydrogenated to H2O through a very facile proton transfer mechanism .