Formation from direct oxidation of H₂ and destruction by decomposition/hydrogenation of H₂O₂ over Pd/C catalyst in aqueous medium containing different acids and halide anions

Abstract

Direct oxidation of H₂ by O₂ to H₂O₂ and decomposition/hydrogenation of H₂O₂ (at 27176;C and atmospheric pressure) over Pd/C catalyst in an aqueous acidic reaction medium have been thoroughly investigated using different mineral acids (viz. 0.1N H₂SO₄, H₃PO₄, H₃BO₃, HNO₃, HCl, HBr or HI) and/or halide anions (viz. F^-, Cl^-, Br^-or I^-), covering their wide concentration range (0-24 mmol/dm³), in the reaction medium. In the absence of any externally added halide, appreciable H₂O₂ formation in the H₂ oxidation occurred only when the medium contained HCl. However, the rates of H₂O₂ decomposition and hydrogenation are greatly reduced because of the presence of any acid; the influence on the H₂O₂ decomposition was, however, much larger for the halo acids. In the presence of acid (0.1N H₃PO₄ or H₂SO₄), the H₂-to-H₂O₂ formation reaction is greatly enhanced by the externally added halide anions up to their optimum concentrations. However, above the optimum halide anion concentration, it is inhibited more than the H₂-to-water formation (parallel) reaction, causing a decrease in both the H₂O₂ yield and selectivity. Whereas, the H₂ conversion and H₂O₂ destruction activities of the catalyst are decreased continuously with increasing the halide anion (except F^-) concentration. Among the halides, chloride is the best halide promoter for Pd/C catalyst in the H₂-to-H₂O₂ oxidation. At the optimum Cl^- concentration (5.4 mmol/dm³), both the H₂ conversion and H₂O₂ yield are passed through a maximum and the H₂O₂ decomposition is greatly inhibited with increasing the phosphoric acid concentration. In the absence of either the chloride anions or the acid (or both) in the reaction medium, only a little or no H₂O₂ is formed in the H₂ oxidation and also the rate of H₂O₂ destruction is very fast, particularly in the presence of H₂; the rapid destruction of H₂O₂ is mainly due to its decomposition rather than its hydrogenation. The best H₂-to-H₂O₂ oxidation results are obtained at the optimum concentrations of both the acid and halide anions. In the presence of acid and chloride (or bromide) promoter, the H₂O₂ hydrogenation dominates the H₂O₂ destruction and hence the net H₂O₂ formation is mainly controlled by the H₂O₂ hydrogenation.