Direct synthesis of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ in an aqueous acidic medium over halide-modified Pd/Al₂O₃ catalysts

Abstract

Direct synthesis of H₂O₂ from its elements was carried out in an acidic aqueous reaction medium over halide-modified oxidized and reduced Pd/Al₂O₃ catalysts under very mild conditions (at 27 ° C and atmospheric pressure). The halide ions were introduced into the catalyst by incorporating halide ions into supported Pd/γ -Al₂O₃ catalyst or via depositing halide ions on the support (γ-Al₂O₃) prior to Pd deposition. The H₂O₂ decomposition and hydrogenation over the corresponding catalysts were also carried out under the reaction conditions similar to those employed for the H₂O₂ synthesis in order to elucidate the factors strongly affecting the H₂O₂ yield/selectivity in the direct H₂O₂ process. The performance of halide-modified Pd/Al₂O₃ catalysts in the direct H₂O₂ synthesis revealed that halide insertion in the catalyst system prior to or after Pd deposition on the support had comparable qualitative effect on the H₂O₂ formation. Both the Pd oxidation state and the nature of the halide ions had strong influences on the H₂ conversion (in direct H₂O₂ synthesis process) and H₂O₂ decomposition and/or hydrogenation reaction. While the effect of Pd oxidation state on the H₂O₂ formation was significant for the catalytic system containing F^- and Cl^- ions, the influence of the Pd oxidation state was found less important for the catalyst system containing Br- ions; the H₂O₂ formation selectivity increased significantly due to the presence of Br^- ions, irrespective of the Pd oxidation state. The nature of the H₂O₂ destruction pathway (i.e. hydrogenation and/or decomposition) in the presence of hydrogen over halide-modified Pd/Al₂O₃ catalysts was found to be strongly dependent upon the nature of the halide ions incorporated in the catalyst during halide-modification of the catalyst.