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

Effects of different mineral acids (viz. H 2 SO 4 , H 3 PO 4 , HNO 3 , HCl, HBr, and HI), acid (H 3 PO 4 ) concentration, different halide anions (viz. F - , Cl-, Br - , and I - ), and halide anion concentration in aqueous reaction medium on the H 2 O 2 formation (in H 2 -to-H 2 O 2 oxidation) and/or on the H 2 O 2 destruction (by H 2 O 2 decomposition and hydrogenation) activities of Pd (or PdO)/Al 2 O 3 catalyst (at 300 K and atmospheric pressure) have been thoroughly investigated. Among the different halide anions, Br - anions are most effective for promoting the H 2 O 2 formation and inhibiting the H 2 O 2 destruction by both the H 2 O 2 decomposition and/or hydrogenation. The cations associated with the halide anions, however, have only a little or no influence on both the H 2 O 2 formation and destruction. The concentration of the different halide anions has a strong influence on the H 2 conversion and H 2 O 2 formation and destruction activities of the Pd/Al 2 O 3 . The H 2 O 2 formation activity in the presence of Br - or Cl- anions is highest at the optimum concentration of halide (about 1.0 mmol/dm 3 ). It is also highest at the optimum concentration of phosphoric acid (between 0.1 and 0.3 mol/dm 3 ). In general, the H 2 O 2 formation is increased with decreasing the H 2 O 2 destruction activity of the catalyst, indicating a close relationship between the two. Br - anions act as an excellent catalyst promoter for Pd/Al 2 O 3 catalyst, but they show only a small promoting effect for PdO/Al 2 O 3 catalyst. The next choice for halide promoter for Pd/ Al 2 O 3 catalyst is Cl- anions. F - and I - anions are, however, catalyst inhibitor and strong poison, respectively, for the H 2 O 2 formation. At the same concentration, Br - anions are more effective than Cl- anions for inhibiting the H 2 O 2 destruction reactions over Pd/Al 2 O 3 catalyst. For both halide promoters (Cl - and Br - ), the net H 2 O 2 formation is controlled by the H 2 O 2 hydrogenation rather than by the H 2 O 2 decomposition.