Direct oxidation of H₂ to H₂O₂ over Pd-based catalysts: influence of oxidation state, support and metal additives

Abstract

The influence of oxidation state (reduced and oxidized), support (ZrO₂, Ga₂O₃, CeO₂, SiO₂, H-β, ThO₂, CeO₂-ZrO₂, BPO₄ and Pd/Al₂O₃) and precious metal additives (Au, Pt, Rh and Ru) on the direct H₂O₂ synthesis has been investigated over Pd-based catalysts. For all the supports investigated herein, the oxidized Pd catalysts showed significantly superior H₂O₂ yields as compared to their reduced counterparts. The effect of the Pd oxidation state was found to be more important for determining the H₂O₂ yields than the particle size and surface area properties of the catalyst system. An excellent correlation was observed between the H₂O₂ selectivity and H₂O₂ decomposition activity of the oxidized Pd catalysts. The oxidized Pd catalysts showed almost an order of magnitude lower H₂O₂ decomposition activity than the reduced catalysts. The H₂O₂ yield in the H₂ to H₂O₂ oxidation passed through a maximum with increase in the Au concentration. Although a similar effect was also observed in case of Pt, Au was found to be a superior promoter for the direct H₂O₂ synthesis process. Addition of Rh and Ru was found to be detrimental for the H₂O₂ yields; these results can be explained in terms of increased H₂O₂ decomposition activity and/or enhanced H₂ to H₂O reaction activity in their presence.