Feeble Single-Atom Pd Catalysts for H₂ Production from Formic Acid

Single-atom catalysts are thought to be the pinnacle of catalysis. However, for many reactions, their suitability has yet to be unequivocally proven. Here, we demonstrate why single Pd atoms (Pd_SA) are not catalytically ideal for generating H₂ from formic acid as a H₂ carrier. We loaded Pd_SA on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on amino-functionalized silica (SiO₂-NH₂/Pd_SA) was far higher than that of the thiol-based catalysts (SiO₂-S-Pd_SA and SiO₂-NHCS₂-Pd_SA), while the single-atom stability of SiO₂-NH₂/Pd_SA against aggregation after the first catalytic cycle was the weakest. In this case, Pd aggregation boosted the reaction yield. Our experiments and calculations demonstrate that Pd_SA in SiO₂-NH₂/Pd_SA loosely binds with amine groups. This leads to a limited charge transfer from Pd to the amine groups and causes high aggregability and catalytic activity. According to the density functional calculations, the loose binding between Pd and N causes most of Pd's 4d electrons in amino-functionalized SiO₂ to remain close to the Fermi level and labile for catalysis. However, Pd_SA chemically binds to the thiol group, resulting in strong hybridization between Pd and S, pulling Pd's 4d states deeper into the conduction band and away from the Fermi level. Consequently, fewer 4d electrons were available for catalysis.