Influence of nature/concentration of halide promoters and oxidation state on the direct oxidation of H<SUB>2</SUB> to H<SUB>2</SUB>O<SUB>2</SUB> over Pd/ZrO<SUB>2</SUB> catalysts in aqueous acidic medium

The nature/concentration of halide promoters and influence of the Pd oxidation state on the promoted reaction system has been investigated on the direct H<SUB>2</SUB>O<SUB>2</SUB> process over a 2.5 wt.% Pd/ZrO<SUB>2</SUB> catalyst in an aqueous acidic reaction medium. The oxidation state of Pd had a profound influence on the H<SUB>2</SUB>O<SUB>2</SUB> 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 H<SUB>2</SUB>O<SUB>2</SUB> yields was large for the reaction systems containing F<SUP>-</SUP> or no halide, the effect was significantly smaller for the reaction systems containing Br<SUP>-</SUP>; and Cl<SUP>-</SUP>. The nature of the halide also strongly influenced the H<SUB>2</SUB>O<SUB>2</SUB> synthesis process. Br<SUP>-</SUP>strongly enhanced the H<SUB>2</SUB>O<SUB>2</SUB> yields, while F<SUP>- </SUP>had a negative influence on the H<SUB>2</SUB>O<SUB>2</SUB> yields. The ability of the halides to enhance the H<SUB>2</SUB>O<SUB>2</SUB> process was found to strongly depend on its propensity to suppress the secondary H<SUB>2</SUB>O<SUB>2</SUB> decomposition reaction. The influence of Br<SUP>-</SUP> and Cl<SUP>-</SUP>concentration studies revealed that the optimum halide concentration for the direct H<SUB>2</SUB>O<SUB>2</SUB> synthesis process was dependent on the nature of the halide. While the maximum in H<SUB>2</SUB>O<SUB>2</SUB> yields for the Br<SUP>-</SUP> containing reaction medium corresponded to a concentration of ~0.9 mmol/dm<SUP>3</SUP> (KBr) the maximum for the Cl<SUP>-</SUP> containing solution was obtained at ~1.5 mmol/dm<SUP>3</SUP> (KCl). Such knowledge is crucial from the viewpoint of optimization (catalyst/reaction system screening studies) of the direct H<SUB>2</SUB>O<SUB>2</SUB> process. The qualitative trends (H<SUB>2</SUB>O<SUB>2</SUB> selectivity/yield) observed in case of the incorporated halide catalysts were similar to those observed with halides in reaction medium over the Pd/ZrO<SUB>2</SUB> catalyst