Theoretical studies on the adsorption and decomposition of H 2O on Pd(1 1 1) surface

Abstract

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 H 2O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H 2O 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 H 2O 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 H 2O or O covered surfaces. In contrast, the O–H bond breaking of water on oxygen covered Pd surfaces, H 2O ad + O ad → 2OH ad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OH ad → H 2O ad + O ad, 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.