Feeble Single-Atom Pd Catalysts for H 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) are not catalytically ideal for generating H from formic acid as a H carrier. We loaded Pd on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on amino-functionalized silica (SiO-NH/Pd) was far higher than that of the thiol-based catalysts (SiO-S-Pd and SiO-NHCS-Pd), while the single-atom stability of SiO-NH/Pd 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 in SiO-NH/Pd 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 SiO to remain close to the Fermi level and labile for catalysis. However, Pd 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.