Reaction mechanism of direct H 2O 2 synthesis from H 2 and O 2 over Pd/C catalyst in water with H + and Br − ions

Abstract

Kinetic analyses including mass transfer processes were applied for the title reaction and the H 2O 2 decomposition. Adsorption of Br − ion on the catalyst was also measured. A hydride-hydroperoxy species H-M-OOH (M, the metal surface) was proposed as the key intermediate in the H 2O 2 formation, the direct H 2O formation, and the H 2O 2 decomposition. The H + added would accelerate Br − adsorption, reductive elimination and β-elimination of the H-M-OOH species, and desorption of surface H 2O 2 species. Two types of metal surface sites were suggested, one of which had a high degree of coordinative unsaturation and catalyzed the direct H 2O formation and the H 2O 2 decomposition. Both reactions would be inhibited by adsorption of the Br − and H + pairs. Another had a low degree of unsaturation and was active for the H 2O 2 formation. Sulfur poisoning study revealed that several surface Pd atoms were inactivated at once by adsorption of a sulfur atom for H 2–O 2 reaction.