Slab model studies of water adsorption and decomposition on clean and X- (X = C, N and O) contaminated Pd(111) surfaces

Abstract

To explore the effect of surface contaminants on water chemistry at metallic surfaces, adsorption and decomposition of water monomers on clean and X/Pd(111) (X = C, N and O) surfaces are investigated based on density functional theory calculations. It is revealed that H₂O binds to Pd(111) surface primarily through the mixing of its 1b₁ with the Pd 4d_z² state. A charge accumulation between the oxygen atom of water and the bound Pd atom is calculated, which is found to be relevant to the H₂O–Pd interaction. Water adsorption results in a reduction of surface work function and the polarization of the X 2p states. The O–H bond scission of H₂O on the clean Pd(111) is an energy unfavorable process. In the case of X-assisted O–H bond breaking on X/Pd(111) surfaces, however, the reaction barrier tends to be lower than that on the clean surface and decreases from C/Pd(111) to O/Pd(111). In particular, water decomposition is found to become feasible on O/Pd(111), in agreement with the experimental observations. The calculated barrier is demonstrated to be correlated linearly with the density of X 2p states at the Fermi level. A thorough energy analysis demonstrates that the following geometrical and electronic factors favor the barrier reduction on X/Pd(111) with respect to water decomposition on clean Pd(111): (i) the less deformed structure of water in TS; (ii) the decreased bonding competition between the fragments OH and H. The remarkable decrease of the barrier on O/Pd(111) is revealed to be due to the largest stabilization of the split H atom and the least deformation of water in the TS.