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

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 1b(1) 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.