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.