Water adsorption and dissociation on the Au(321) stepped surface

Abstract

The H(2)O adsorption and dissociation on the zigzag stepped Au(3 2 1) surface was investigated using density functional theory and a periodic supercell approach. The water molecule interacts directly with the lowest coordinated gold atoms on the surface with one of its hydrogen atoms pointing towards the (1 1 1) terraces below. The adsorption energy with respect to the separated H(2)O and metal clean slab is -0.23 eV. The co-adsorption of the OH and H species is also more favorable with the fragments adsorbed nearby the step (bridge sites). The calculated interaction energy, with respect to gaseous water and to the clean slab, is 0.96 eV. The dissociation of water is endothermic by 1.19 eV and the barrier of the reaction is 1.33 eV. Finally, the rate constant for this reaction was determined for two different temperatures from transition state theory. The reaction rate constants are 1 x 10(-4) s(-1) and 2 x 10(0) s(-1) at 150 and 300 degrees C evidencing that the reaction of water dissociation is slow on the Au(3 2 1) surface. Importantly, these results suggest that the high performance of some Au-based catalysts in the water gas shift reaction (WGSR) is not related only with the presence of low-coordinated atoms, and that the support nature and structure, and support-gold interaction must have also an important role in the catalytic activity.