The mechanism of forming H2O from H2 and O2 over a Pt catalyst via direct oxygen reduction

Density functional theory (DFT) was used with the B3LYP gradient-corrected exchange-correlation functional to study the mechanism for the reaction of H-2 + (1)/(2) O-2 -> H2O over a Pt catalyst via direct oxygen reduction. Within these studies we first examined the binding characteristics and energetics for each likely intermediate chemisorbed on the Pt(111) surface, modeled by a 35 atom cluster: O, H, O-2, H-2, OH, OOH, H2O. Then, the pathways for the dissociation processes of the various intermediates on the Pt-35 cluster were calculated. For convenience in comparing different reaction steps, these energetics were used to calculate heats of formation (Delta H-f), which were combined with the dissociation barriers. Two main reaction pathways were found for the formation of H2O from H-2 and O-2: center dot (OO-Dissociation: Here O-2 adsorbs on the surface, dissociates, and finally reacts with H sequentially to firstly form OH and then water. The rate-determining step (RDS) for this mechanism is the O-ad + H-ad -> OHad with a barrier of 31.66 kcal mol(-1) and not the dissociation of O-2(ad) (barrier of 15.02 kcal mol(-1)). center dot OOH-Formation: Here O-2 reacts firstly with H-ad to form OOHad, which then dissociates to form OHad and O-ad (the RDS, with a barrier of 17.13 kcal mol(-1)), which finally reacts with another H-ad to form water. Thus, under gas-phase conditions, the OOH-Formation mechanism is found to be the most favorable.