Theoretical study of size effects on the direct synthesis of hydrogen peroxide over palladium catalysts

Abstract

The direct synthesis of H2O2 over Pd catalysts is a promising alternative to the industrial Riedl–Pfleiderer process. We studied the size effects of Pd catalysts ranging from subnano clusters to extended surfaces using first-principles calculations. Compared to the high-coordinated terrace sites of extended Pd surfaces, the low-coordinated terrace and edge sites of clusters showed lower H2O2 selectivities. Bader charge analysis proved that low-coordinated terrace sites of clusters donated more electrons to adsorbed O2 than the extended surfaces, thus enhancing the undesired dissociation of O2. Accordingly, with the decline in the size of Pd nanoparticles (NPs), the increase in low-coordinated active sites results in the drop in H2O2 selectivity. Moreover, both site coordination and site geometry play critical roles in the performance of Pd active sites. Our findings provide a molecular understanding of size effects of Pd NPs on this reaction, which is consistent with the reported experimental results.