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

Abstract

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 H 2 from formic acid as a H 2 carrier. We loaded Pd SA on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on aminofunctionalized silica (SiO 2 −NH 2 /Pd SA ) was far higher than that of the thiolbased catalysts (SiO 2 −S−Pd SA and SiO 2 −NHCS 2 −Pd SA ), while the single-atom stability of SiO 2 −NH 2 /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 SiO 2 −NH 2 /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 SiO 2 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.

Metal–Support Interaction in Pt Nanodisk–Carbon Nitride Catalyst: Insight from Theory and Experiment

“…Likewise, Pt with different particle sizes on ceria (CeO 2 ) was demonstrated to have particle-size-dependent charge transfer properties that can affect the catalytic impact [ 16 ]. We also previously demonstrated that Pd nanoparticles’ electronic state can be significantly influenced by the support’s surface and its functional groups, significantly altering the catalytic activity [ 8 , 17 ].…”