A theoretical study of the effects of the charge state and size of gold clusters on the adsorption and dissociation of H2.

The adsorption and dissociation of H(2) on the neutral and charged gold clusters Au(n) (m)(m=0,+/-1; n=1-6) is investigated using the density functional theory PW91 functional. H(2) interacts very weakly with Au(n) (-1), whereas the interaction with Au(n) (+1) is relatively strong. The binding energies on neutral clusters are between those on the cationic and anionic systems. The binding energy decreases monotonically with the size increase of the cationic clusters while it goes up first and then goes down on the neutral systems with the maximum value of 0.78 eV at Au(3). Au cations show no propensity for the dissociation barrier reduction and are thermodynamically unfavorable for the dissociation. For the first time we find that H(2) dissociation involves valley-ridge inflection points on some clusters. Our results indicate that H(2) dissociates facilely at low temperatures on both neutral and cationic Au(4) and Au(5). The phenomenon that H(2) dissociation was not observed experimentally is not due to the higher dissociation barrier and weak binding of H(2). We also show that the coordination number of the Au atom may not play a determining role in H(2) dissociation.