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

Abstract Direct synthesis of H 2 O 2 from its elements was carried out in an acidic aqueous reaction medium over halide-modified oxidized and reduced Pd/Al 2 O 3 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-Al 2 O 3 catalyst or via depositing halide ions on the support (γ-Al 2 O 3) prior to Pd deposition. The H 2 O 2 decomposition and hydrogenation over the corresponding catalysts were also carried out under the reaction conditions similar to those employed for the H 2 O 2 synthesis in order to elucidate the factors strongly affecting the H 2 O 2 yield/selectivity in the direct H 2 O 2 process. The performance of halide-modified Pd/Al 2 O 3 catalysts in the direct H 2 O 2 synthesis revealed that halide insertion in the catalyst system prior to or after Pd deposition on the support had comparable qualitative effect on the H 2 O 2 formation. Both the Pd oxidation state and the nature of the halide ions had strong influences on the H 2 conversion (in direct H 2 O 2 synthesis process) and H 2 O 2 decomposition and/or hydrogenation reaction. While the effect of Pd oxidation state on the H 2 O 2 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 H 2 O 2 formation selectivity increased significantly due to the presence of Br − ions, irrespective of the Pd oxidation state. The nature of the H 2 O 2 destruction pathway (i.e. hydrogenation and/or decomposition) in the presence of hydrogen over halide-modified Pd/Al 2 O 3 catalysts was found to be strongly dependent upon the nature of the halide ions incorporated in the catalyst during halide-modification of the catalyst.