Theoretical Study of the Decomposition and Hydrogenation of H2O2 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 (H2O2 → 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 H2O2 to water (H2O2 + 2H → 2H2O). The results show that the decomposition of H2O2 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 H2O2, and thus suppress the dissociation of H2O2, 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.