Effects of hydrogen and propylene presence on decomposition of hydrogen peroxide over palladium catalysts

Reaction rates for H₂O₂ decomposition in a methanol solution were measured over Pd/SiO₂ catalysts in the presence of gas-phase N₂, H₂ and propylene. The H₂O₂ decomposition rates were higher in the presence of H₂ and lower in the presence of propylene compared to those under N₂, which acted as an inert gas. For interpretation of the experimental results, H₂O₂ decomposition reactions were evaluated with density functional theory calculations on four Pd(1 1 1) surfaces: (1) clean, (2) with pre-adsorbed hydrogen, (3) with molecularly pre-adsorbed propylene, and (4) with dissociatively pre-adsorbed propylene. The computational results show that the mechanism of H₂O₂ decomposition can proceed through O-O and O-H bond splitting reactions. The O-O bond splitting reactions are energetically preferable because they are exothermic and have lower activation energies. However, in the absence of H₂, the endothermic O-H bond splitting reactions with higher activation energies are required for the formation of molecular decomposition products: O₂ and H₂O. In contrast, in the presence of H₂, the O-H bond splitting reactions are no longer required, and the H₂O₂ decomposition can proceed only through the facile O-O bond splitting reaction in H₂O₂. The formed surface OH species can readily react with H species from dissociative H₂ adsorption, forming H₂O as the only product. This change in the reaction mechanism from the O-H bond splitting to the O-O bond splitting explains the experimentally observed higher H₂O₂ decomposition rates in the presence of H₂. The lower H₂O₂ decomposition rates in the presence of propylene are due to blocking of Pd surface sites by hydrocarbon species formed on propylene adsorption. Although propylene does not directly participate in H₂O₂ decomposition, it adsorbs on Pd more strongly than H₂O₂ and, therefore, forms surface spectator species that reduce the number of Pd active sites.