Theoretical investigation of water formation on Rh and Pt surfaces

Catalytic water formation from adsorbed H and O adatoms is a fundamental re action step in a variety of technologically important reactions involving o rganic molecules. In particular, the water-formation rate determines the se lectivity of the catalytic partial oxidation of methane to syngas. In this report we present a theoretical investigation of the potential-energy diagr am for water formation from adsorbed O and H species on Rh(111) and Pt(111) surfaces. The study is based on accurate first-principles calculations app lying density-functional theory. Our results are compared to the potential- energy diagram for this reaction inferred from experimental data by Hickman and Schmidt [AIChE. J. 39, 1164 (1993)]. The calculations essentially repr oduce the scheme of Hickman and Schmidt for water formation on Rh(111) with the important difference that the OH molecule is significantly more stable than assumed by Hickman and Schmidt. On Pt(111) surfaces, however, the cal culations predict a barrier to OH formation very similar to that found on R h(111). In particular, the calculated barrier to OH formation of about 20 k cal/mol seems to contradict the small 2.5 kcal/mol barrier assumed in the H ickman-Schmidt scheme and the observed large rate of water formation on Pt. A possible explanation for the apparent discrepancy between the large calc ulated barrier for OH formation on Pt and the experimentally observed rapid formation of water even at low temperatures is that the active sites for w ater formation on Pt are at "defect" sites and not on the ideally flat terr aces. A similar conclusion has been reached by Verheij and co-workers [Surf . Sci. 371, 100 (1997); Chem. Phys. Lett. 174, 449 (1990); Surf. Sci. 272, 276 (1991)], who did detailed experimental work on water formation on Pt su rfaces. Analyzing our results, we develop an explicit picture of the intera ction processes governing the formation of OH groups. This picture rational izes the calculated weak dependence of OH formation on substrate material. An important conclusion from this work is that "good" catalysts for the par tial oxidation of hydrocarbons should resist defect formation at their surf aces. (C) 2000 American Institute of Physics. [S0021-9606(00)70222-4].