Theoretical studies on the adsorption and decomposition of H2O on Pd(1 1 1) surface

To provide information about the chemistry of water on Pd surfaces, we performed density functional slab model studies on water adsorption and decomposition at Pd(1 1 1) surface. We located transition states of a series of elementary steps and calculated activation energies and rate constants with and without quantum tunneling effect included. Water was found to weakly bind to the Pd surface. Co-adsorbed species OH and O that are derivable from H2O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H2O and OH, respectively. Calculated kinetic parameters indicate that dehydrogenation of water is unlikely on the clean regular Pd(1 1 1) surface. The barrier for the hydrogen abstraction of H2O at the OH covered surface is approximately 0.2-0.3 eV higher than the value at the clean surface. Similar trend is computed for the hydroxyl group dissociation at H2O or O covered surfaces. In contrast, the O-H bond breaking of water on oxygen covered Pd surfaces, H2Oad + Oad → 2OHad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OHad → H2Oad + Oad, is also found to be very feasible with a barrier of ∼0.1 eV. These results show that on oxygen-covered surfaces production of hydroxyl species is highly likely, supporting previous experimental findings.