Direct oxidation of H₂ to H₂O₂ over Br and F-promoted Pd/Al₂O₃ in aqueous acidic medium: influence of the concentration of Br and F and the method of incorporation of the two halogens in the catalyst on their beneficial synergetic effect on the net H₂O₂ formation

Abstract

Influence of the addition of F (0-1.1 mmol/g) in the pre-brominated Pd/Al₂O₃ (Br loading = 0.13 mmol/g) and also that of Br (0-1.3 mmol/g) in the pre-fluorinated Pd/Al₂O₃ (F loading = 0.53 mmol/g) on the net H₂O₂ formation in the H₂-to-H₂O₂ oxidation (with H₂/O₂ mole ratio of 1.0) over the halogenated catalysts in aqueous acidic (0.1 M H₃PO₄) medium have been investigated. In both the cases, the highest synergetic effect (resulting in the highest net H₂O₂ formation) produced by the addition of other halogen is observed at its optimum concentration (viz. 0.13 mmol Br/g in the fluorinated Pd/Al₂O₃ and 0.13 mmol F/g in the pre-brominated Pd/Al₂O₃). The beneficial synergetic effect of the two halogens is also found to be strongly influenced by the way of incorporation of the two halogens in the catalyst. The enhancement in the net rate of H₂O₂ formation due to the synergetic effect for the different methods of Pd/Al₂O₃ halogenation is in the following order: simultaneous bromination and fluorination < first fluorination and then bromination < first bromination and then fluorination. The fluorination (at optimum concentration of 0.13 mmol F/g) of the pre-brominated (0.13 mmol Br/g) Pd/Al₂O₃ led to the best catalyst for the H₂-to-H₂O₂ oxidation with 100% H₂ conversion and 78% H₂O₂ yield/selectivity. The destruction of H₂O₂ by its decomposition and/or hydrogenation over the halogenated catalysts has also been studied.