An ab initio study of surface electrochemical disproportionation: The case of a water monolayer adsorbed on a Pd(111) surface

We present a theoretical study of the structural response of model water monolayers on a Pd(1 1 1) surface upon various electrochemical conditions. Whereas the water molecules pointing toward the vacuum are mostly electrochemically inactive, those lying parallel to the surface undergo an oxidative adsorption under oxidizing conditions, and those pointing toward the surface show a reductive adsorption under reducing conditions. The oxidative adsorption is shown to result from the interaction of the H2O 1b(1) orbital with the metal surface dz(2) band which becomes bonding only under oxidative conditions. The reductive adsorption arises from the interaction of the H2O 4a(1)-2b(2) orbitals with the dz2 band of the Pd surface. This electronic analysis of electrochemical effects is further validated using the Fukui function associated with the water monolayers. The Fukui function is shown to be a powerful tool for studying electrochemical effects as it is directly linked with the electrochemical bond reorganization and other parameters like the surface capacitance. The electrochemical stability diagram for these water monolayers is computed using a new correction for the DFT electrochemical calculations and compared to previous results. The phase transformation from a positively charge H-up to a negatively charged H-down phase is here confirmed with, however, a potential shifted up to 4.5 V compared to our previous report (4.3 V). The neutral phase is found to consist in a positively charged H-up phase and a negatively charged H-down phase. This phenomenon is confirmed by large super-cell calculations with different ratio of H-up and H-down phases. The stability of the mixed charged phases is then rationalized in terms of an electrochemical disproportionation whose origin and consequences are further discussed in regards to the results previously obtained on the basis of the ice rules. (C) 2012 Published by Elsevier B.V.