Multistep reactions of water with small Pdn clusters: A first principles study

Abstract

Multistep dissociative chemisorption reactions of water with Pd 4 and Pd 7 clusters were studied using density functional theory. The adsorption energies and referred adsorption sites from water molecule ( H 2 O ) to partially dissociative ( H 2+ O and OH + H ), then to fully dissociative ( O + H + H ) configurations are carefully determined. It is found that the adsorption energies of three dissociative reactions are 5–6 times larger than that of water molecule. Atop sites of Pd 4 and Pd 7 clusters are found to be the most stable sites for the adsorbed H 2 O molecule. For the coadsorption cases of partially and fully dissociated products, H 2 and OH molecules preferably tend to bind at the low coordination (atop or bridge) sites, and O and H atoms prefer to adsorb on the high coordination (hollow) sites. It is also found that the most favorable adsorption sites for the molecular adsorbates ( H 2 O , H 2 and OH ) are adjacent to the Pd atoms with the largest site-specific polarizabilities. Therefore, site-specific polarizability is a good predictor of the favorable adsorption sites for the weakly bound molecules. The different directions of charge transfer between the Pd clusters and the adsorbate(s) is observed. Furthermore, the processes of the adsorption, dissociation, and the dissociative products diffusion of H 2 O are analyzed.