Direct oxidation of H₂ to H₂O₂ over Pd/CeO₂ catalyst under ambient conditions: influence of halide ions

Abstract

Direct oxidation of H₂ by O₂ to H₂O₂ and decomposition/hydrogenation of H₂O₂ have been investigated over Pd/CeO₂ catalyst in presence of different halide ions. All reactions were carried out in an aqueous acidic (0.03 M H₃PO₄) medium at room temperature (25°C) and atmospheric pressure. When different halide ions were added directly to the reaction medium or incorporated in the catalyst, the activity/selectivity of Pd/CeO₂ catalyst in the H₂ to H₂O₂ oxidation changed significantly, depending upon the nature of the halide ions. Different ammonium halides impregnated reduced Pd/CeO₂ catalyst calcined under inert (flowing nitrogen) and oxidizing (static/flowing air) gaseous atmospheres revealed that bulk oxidation state of Pd and the nature of the halide ions incorporated into the catalyst cooperatively control the activity/selectivity of catalyst in H₂ to H₂O₂ oxidation. Both H₂O₂ decomposition and hydrogenation were found to affect strongly depending upon the nature of the halide ions present in the reaction medium or in the catalyst. Among the different halides, bromide was found the most effective promoter for enhancing the H₂O₂ yield/selectivity in H₂ to H₂O₂ oxidation, irrespective of the Pd oxidation state in the catalyst.