Au–Pd supported nanocrystals as catalysts for the direct synthesis of hydrogen peroxide from H2 and O2

Abstract

The direct synthesis of hydrogen peroxide with Au–Pd catalysts is described and discussed: in particular, the roles of the support and promoters. Catalysts prepared by co-impregnation on various supports with calcination at 400 °C were stable and could be re-used several times without loss of metal. Catalysts calcined at lower temperatures were found to be unstable and could not be successfully re-used. Au–Pd/carbon and Au–Pd/silica catalysts gave the highest rate of H₂O₂ production, and the order of reactivity observed for the support materials investigated is: carbon ∼ silica > TiO₂ > Al₂O₃. Bimetallic Au–Pd particles on TiO₂ and Al₂O₃ were found to exhibit a core–shell structure, Pd being concentrated on the surface. It is considered that the Au–Pd/silica catalysts have a similar core–shell morphology based on X-ray photoelectron spectroscopy studies, whereas, in contrast, the calcined Au–Pd/carbon catalysts are observed to be homogeneous alloys. TEM studies showed that the silica contains impurities of carbon and that the Au–Pd alloys all preferentially interact with these carbonaceous impurities, hence resulting in a rather similar catalytic performance to that of the carbon supported Au–Pd catalysts. The origin of the enhanced activity for the silica and carbon supported catalysts is a result of higher H₂ selectivity for the formation of hydrogen peroxide, which is due to the surface composition and size distribution of the nanoparticles. The effect of promoters is investigated, and it is shown that addition of Br⁻ and PO₄³⁻ is deleterious under our conditions, which contrasts markedly with Pd catalysts for which such species are essential. Furthermore, we show the acid solution formed by CO₂ in water increases the rate of H₂O₂ synthesis, and thereby the CO₂ diluent in our experiments acts as green in situ acid promoter.