Form of the catalytically active Pd species during the direct synthesis of hydrogen peroxide

Abstract Direct synthesis of H 2 O 2 could produce H 2 O 2 at lower cost than the Riedl–Pfleiderer process, which would enable the broader use of H 2 O 2 for industrial oxidations. The addition of inorganic acids and halides to the solvent increase H 2 O 2 selectivities on Pd nanoparticles but also dissolve Pd and produce dynamic mixtures of complexes including heterogeneous (Pd 0 ) and homogeneous (Pd 2+ ) species, any of which may contribute to H 2 O 2 formation. We combine kinetic measurements and in operando UV–vis spectroscopy to determine how H 2 O 2 rates and selectivities depend on concentrations of these forms of Pd. Introducing HCl to the solvent increases the concentration of Pd 2+ complexes by oxidizing Pd 0 nanoparticles. The rates of primary H 2 O 2 formation and H 2 O 2 selectivities do not depend directly on the population of the Pd 2+ species for catalysts tested in water or methanol. These results suggest that Pd nanoparticles form H 2 O 2 and detectable homogeneous complexes do not act as catalysts.