Direct formation of H2O2 from H2 and O2 over a Pd/SiO2 catalyst: the roles of the acid and the liquid phase

The direct formation of H2O2 from H2 and O2 was carried out over a Pd/SiO2 catalyst in a medium of ethanol or water acidified with either H2SO4 or HCl. The H2SO4/ethanol system is the most favorable for peroxide formation. Both the proton and the anion, in the case of Cl−, promote the net formation of the peroxide. Protons inhibit the reduction of H2O2 by H2, and chloride ions limit the direct reduction of O2 to water, presumably by blocking Pd ensembles. Sulfate ions, being noncoordinating ligands, do not serve this function; therefore the H2SO4/water system is a poor medium for producing the peroxide. By contrast, the H2SO4/ethanol system is believed to be effective because in the presence of O2, acetate ions are formed from ethanol, and these ions block Pd ensembles in the same manner as chloride ions.