Direct formation of H 2O 2 from H 2 and O 2 over a Pd/SiO 2 catalyst: the roles of the acid and the liquid phase

Abstract

The direct formation of H 2O 2 from H 2 and O 2 was carried out over a Pd/SiO 2 catalyst in a medium of ethanol or water acidified with either H 2SO 4 or HCl. The H 2SO 4/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 H 2O 2 by H 2, and chloride ions limit the direct reduction of O 2 to water, presumably by blocking Pd ensembles. Sulfate ions, being noncoordinating ligands, do not serve this function; therefore the H 2SO 4/water system is a poor medium for producing the peroxide. By contrast, the H 2SO 4/ethanol system is believed to be effective because in the presence of O 2, acetate ions are formed from ethanol, and these ions block Pd ensembles in the same manner as chloride ions.