Decomposition and/or hydrogenation of hydrogen peroxide over Pd/Al₂O₃ catalyst in aqueous medium: Factors affecting the rate of H₂O₂ destruction in presence of hydrogen

Abstract

Hydrogen peroxide destruction by its decomposition and/or hydrogenation to water in the presence of H₂ over Pd(5 wt.%)/Al₂O₃ catalyst in an aqueous reaction medium, similar to that used in the H₂-to-H₂O₂ oxidation, has been thoroughly investigated at different reaction conditions. The H₂O₂ destruction is strongly influenced by the oxidation state of Pd in the catalyst and also by the presence of different halide anions (viz. F^-, Cl^-, Br^- and I^-) in the acidic aqueous medium or in the catalyst, depending upon the concentration of halide anions. The cations associated with halide anions have, however, a little or no influence on the H₂O₂ destruction. The iodide anions strongly poisoned the catalyst, even at their very low concentration. The fluoride anions have only a small effect on the H₂O₂ destruction. The chloride or bromide anions drastically inhibit the rapid H₂O₂ decomposition, but promote the slower H₂O₂ hydrogenation. The H₂O₂ destruction reactions are strongly influenced by the halide anion and acid concentrations. The presence of acid (protons) plays a very important role in drastically reducing the H₂O₂ destruction, particularly in the presence of chloride anions. Both in the presence and absence of Cl^- or Br^- anions, the H₂O₂ destruction activity of the catalyst is markedly reduced because of the oxidation of Pd but it is drastically increased due to the presence of H₂. A plausible reaction mechanism for the rapid H₂O₂ decomposition and slower H₂O₂ hydrogenation reactions, prevailing under the different reaction conditions, has also been discussed.