Role of chloride or bromide anions and protons for promoting the selective oxidation of H₂ by O₂ to H₂O₂ over supported Pd catalysts in an aqueous medium

Abstract

The influence of protons (mineral acid) and different halide anions (F^- , Cl^- , Br^- , or I^- ) in the aqueous reaction medium on H₂O₂ formation in the direct oxidation of H₂ to H₂O₂ (at 27°C) over different supported Pd catalysts (i.e., Pd supported on carbon, alumina, gallia, silica, zirconia, or ceria) in their reduced form, and also on the activity of the catalysts for the destruction of H₂O₂ by its decomposition and/or hydrogenation reactions at the conditions close to those used in H₂ to H₂O₂ oxidation, was thoroughly investigated to gain insight into the roles of protons and halide promoters. Among the halide anions, only the chloride and bromide anions (particularly for the Pd/C and metal oxide-supported Pd, respectively), when added to acidic reaction medium, act as a catalyst promoter to drastically enhance H₂O₂ formation selectivity or H₂O₂ yield. But the presence of protons is crucial to this promoting action. The promoting action of the halides is attributed mainly to the large decrease in the H₂O₂ decomposition and hydrogenation activities of the catalyst resulting from its poisoning of the two consecutive H₂O₂ destruction reactions. It also may be due to the inhibition caused by the halide promoters for water formation directly from H₂ oxidation. The cations associated with the halide anions play only a minor role or no role in controlling the conversion or selectivity in H₂ to H₂O₂ oxidation and H₂O₂ conversion reactions. A plausible mechanism for H₂ to H₂O₂ oxidation in the presence of chloride or bromide anions in acidic aqueous medium, explaining the beneficial roles of the halide anions and protons for promoting the selective oxidation reaction, is proposed.