EXAFS in situ: The effect of bromide on Pd during the catalytic direct synthesis of hydrogen peroxide

The direct synthesis of hydrogen peroxide is catalyzed by palladium catalysts supported over different solids. With the aid of a suitable plug-flow reaction cell, we carried out a preliminary X-ray absorption fine structure spectroscopy (EXAFS) characterization of a palladium catalyst supported on the commercial resin K2621 under reaction conditions (in situ). Whereas the catalyst, which in the dry catalyst presents metal Pd and PdO when fresh, is practically unaffected by the reaction medium (methanol) or by the reaction mixture (CO2/H-2/O-2, 86/10/4, v/v) it undergoes an apparent reduction of part of Pd0 to metal Pd and some metal leaching during the reaction in the presence of bromide ions. These findings suggest the role of bromide ions as enhancers of the selectivity of palladium catalysts in the direct synthesis of hydrogen peroxide could not be limited to the selective blocking of the sites responsible for the undesired formation of water, but could also entail phase modifications of the active metal.