Direct H₂-to-H₂O₂ oxidation over highly active/selective Br-F-Pd/Al₂O₃ catalyst in aqueous acidic medium: Influence of O₂/H₂ ratio and other reaction conditions on the H₂O₂ formation

Abstract

The influence of the O₂/H₂ mole ratio in the gaseous feed and also those of other reaction conditions [viz. concentration of H₃PO₄ (0-5 mol/dm³), temperature (5-50 ° C), gas (H₂ and O₂) space velocity (5.8-23.4 h-1) and reaction time (0.1-8 h)] on the H₂O₂ formation in the H2-to-H₂O₂ oxidation over the Br(1 wt%)-F(1 wt%)-Pd(5 wt%)/Al₂O₃ catalyst in an aqueous acidic (H3PO4) medium have been thoroughly investigated. The effects of the O₂/H₂ ratio, reaction temperature and acid concentration on the destruction of H₂O₂ by its decomposition and/or hydrogenation reactions over the catalyst in the acidic reaction medium have also been studied. The net H₂O₂ formation (H₂O₂ yield) over the catalyst passed through a maximum with increasing the acid concentration, the temperature or the O₂/H₂ feed ratio. However, it decreased markedly with increasing the gas space velocity or the reaction period. The H₂O₂ decomposition and hydrogenation activities of the catalyst increased appreciably with increasing the reaction temperature and decreased with increasing the acid concentration. The H₂O₂ destruction during the H₂-to-H₂O₂ oxidation increased with increasing the concentration of H₂ (relative to that of O₂) due to the increased H₂O₂ hydrogenation rate over the catalyst. The net H₂O₂ formation in the H2-to-H₂O₂ oxidation decreased sharply with increasing the initial amount of H2O2 present in the reaction mixture. The presence of H₂O₂ and the higher H₂/O₂ ratios have detrimental effects on the net formation of H₂O₂.