Factors influencing decomposition of H 2O 2 over supported Pd catalyst in aqueous medium

Abstract

Since H 2O 2 decomposition can result in selectivity/yield loss in the direct H 2O 2 synthesis process from H 2 and O 2 over supported Pd catalysts, it is important to have an enhanced understanding about the factors affecting the H 2O 2 decomposition reaction. Herein, detailed studies have been undertaken to investigate the influence of different factors, such as (a) nature and concentration of acid in reaction medium, (b) nature and concentration of halide in presence and absence of acid in reaction medium, (c) pretreatment procedures and (d) catalyst modification by incorporation of different halides, on the H 2O 2 decomposition reaction over a 5% Pd/C catalyst in aqueous medium at 25 °C. This study has shown that the H 2O 2 decomposition activity is profoundly influenced by all the above factors. The effectiveness of the acids in suppressing the H 2O 2 decomposition activity decreased in the following order: hydroiodic acid > hydrobromic acid > hydrochloric acid ≫ acetic acid > phosphoric acid > sulfuric acid > perchloric acid. The ability of the acid to decrease the H 2O 2 decomposition activity was found to very strongly depend on the nature of its associated anion. Halides, such as iodide, bromide and chloride were particularly effective in suppressing the H 2O 2 decomposition activity. Oxidation pretreatment of the catalyst was found to strongly suppress its H 2O 2 decomposition activity, while a reduction treatment was found to promote its activity. A gradual decrease in the H 2O 2 decomposition activity of the catalyst was observed with each successive usage due to in situ sub-surface oxidation of Pd by H 2O 2. Halide incorporation either via the reaction medium or prior catalyst modification had a similar qualitative effect on the H 2O 2 decomposition activity.