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

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 degrees 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 similar to 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 H-2 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.