Direct oxidation of hydrogen to hydrogen peroxide over Pd(or PdO)/Al₂O₃ in aqueous reaction medium: influence of different acids and halide anions in reaction medium on formation and destruction of H₂O₂

Abstract

Effects of different mineral acids (viz. H₂SO₄, H₃PO₄, HNO₃, HCl, HBr, and HI), acid (H₃PO₄) concentration, different halide anions (viz. F^-, Cl^-, Br^-, and I^-), and halide anion concentration in aqueous reaction medium on the H₂O₂ formation (in H₂-to-H₂O₂ oxidation) and/or on the H₂O₂ destruction (by H₂O₂ decomposition and hydrogenation) activities of Pd (or PdO)/Al₂O₃ catalyst (at 300 K and atmospheric pressure) have been thoroughly investigated. Among the different halide anions, Br^- anions are most effective for promoting the H₂O₂ formation and inhibiting the H₂O₂ destruction by both the H₂O₂ decomposition and/or hydrogenation. The cations associated with the halide anions, however, have only a little or no influence on both the H₂O₂ formation and destruction. The concentration of the different halide anions has a strong influence on the H2 conversion and H₂O₂ formation and destruction activities of the Pd/Al₂O₃. The H₂O₂ formation activity in the presence of Br^- or Cl^- anions is highest at the optimum concentration of halide (about 1.0 mmol/dm³). It is also highest at the optimum concentration of phosphoric acid (between 0.1 and 0.3 mol/dm³). In general, the H₂O₂ formation is increased with decreasing the H₂O₂ destruction activity of the catalyst, indicating a close relationship between the two. Br^- anions act as an excellent catalyst promoter for Pd/Al₂O₃ catalyst, but they show only a small promoting effect for PdO/Al₂O₃ catalyst. The next choice for halide promoter for Pd/Al₂O₃ catalyst is Cl^- anions. F^- and I^- anions are, however, catalyst inhibitor and strong poison, respectively, for the H₂O₂ formation. At the same concentration, Br^- anions are more effective than Cl^- anions for inhibiting the H₂O₂ destruction reactions over Pd/Al₂O₃ catalyst. For both halide promoters (Cl^- and Br^-), the net H₂O₂ formation is controlled by the H₂O₂ hydrogenation rather than by the H₂O₂ decomposition.