Theoretical study of the decomposition and hydrogenation of H ₂O₂ on Pd and Au@Pd surfaces: Understanding toward high selectivity of H₂O₂ synthesis

Three possible pathways for the conversion of hydrogen peroxide to water on Pd and Au@Pd catalysts are investigated with periodic density functional theory calculations: (1) the decomposition of H₂O₂ (H ₂O₂ ↔ H₂O + O), including the dissociation of H₂O₂ to two OH groups (H₂O₂ ↔ 2OH) and the disproportionation of two OH groups to water and oxygen (OH + OH ↔ H₂O + O); (2) the hydrogenation of the OH group to water (OH + H ↔ H₂O); and (3) the direct hydrogenation of H ₂O₂ to water (H₂O₂ + 2H ↔ 2H₂O). The results show that the decomposition of H₂O ₂ and the hydrogenation of OH groups are two available channels for the formation of water, and the former plays a main role. A key step in the overall process is the dissociation of H₂O₂, which is facile and irreversible. The direct hydrogenation of H₂O₂ to water has a very high activation barrier and is unlikely to occur. The competitions between the dissociation of H₂O₂ and the release of H₂O₂ on Pd and Au@Pd surfaces are analyzed. The high selectivity of H₂O₂ synthesis cannot be explained simply by the relatively increased barrier for H₂O₂ dissociation on the Au@Pd surface. Actually, the less active Au atoms on the Au@Pd surface weaken the interaction of the metal surface with H ₂O₂, and thus suppress the dissociation of H ₂O₂, and, on the other hand, facilitate the release of H₂O₂. The opposite effects of Au atoms on the dissociation and release of H₂O₂ move the balance to the release side, which is responsible for the high H₂O₂ selectivity of the Au@Pd catalysts. The effects of the unreacted H atoms are also considered. It is found that the H atoms coadsorbed on Pd and Au@Pd surfaces can decrease the interaction between the metal surfaces and H₂O₂ as well and, consequently, facilitate the release of H₂O₂ and suppress the dissociation of H₂O₂.