Reaction mechanism of direct H2O2 synthesis from H2 and O2 over Pd/C catalyst in water with H+ and Br- ions

Kinetic analyses including mass transfer processes were applied for the title reaction and the H2O2 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 H2O2 formation, the direct H2O formation, and the H2O2 decomposition. The H+ added would accelerate Br- adsorption, reductive elimination and beta-elimination of the H-M-OOH species, and desorption of surface H2O2 species. Two types of metal surface sites were suggested, one of which had a high degree of coordinative unsaturation and catalyzed the direct H2O formation and the H2O2 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 H2O2 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. (C) 2011 Elsevier Inc. All rights reserved.