Influence of nature/concentration of halide promoters and oxidation state on the direct oxidation of H₂ to H₂O₂ over Pd/ZrO₂ catalysts in aqueous acidic medium

Abstract

The nature/concentration of halide promoters and influence of the Pd oxidation state on the promoted reaction system has been investigated on the direct H₂O₂ process over a 2.5 wt.% Pd/ZrO₂ catalyst in an aqueous acidic reaction medium. The oxidation state of Pd had a profound influence on the H₂O₂ synthesis process. Interestingly, the nature of the halide determined the magnitude/type of influence the Pd oxidation state exerted on the overall process. While the effect of the oxidation state on the H₂O₂ yields was large for the reaction systems containing F^- or no halide, the effect was significantly smaller for the reaction systems containing Br^-; and Cl^-. The nature of the halide also strongly influenced the H₂O₂ synthesis process. Br^-strongly enhanced the H₂O₂ yields, while F^- had a negative influence on the H₂O₂ yields. The ability of the halides to enhance the H₂O₂ process was found to strongly depend on its propensity to suppress the secondary H₂O₂ decomposition reaction. The influence of Br^- and Cl^-concentration studies revealed that the optimum halide concentration for the direct H₂O₂ synthesis process was dependent on the nature of the halide. While the maximum in H₂O₂ yields for the Br^- containing reaction medium corresponded to a concentration of ~0.9 mmol/dm³ (KBr) the maximum for the Cl^- containing solution was obtained at ~1.5 mmol/dm³ (KCl). Such knowledge is crucial from the viewpoint of optimization (catalyst/reaction system screening studies) of the direct H₂O₂ process. The qualitative trends (H₂O₂ selectivity/yield) observed in case of the incorporated halide catalysts were similar to those observed with halides in reaction medium over the Pd/ZrO₂ catalyst.