Controlling the Coordination Environments of Atomically Dispersed Catalysts via Reversible Ligand Exchange for Oxygen Reduction Reaction

Abstract

Atomically dispersed catalysts (ADCs) or single-atom catalysts have attracted tremendous attention because of their 100% of metal atom efficiency and unique catalytic behaviors that are distinct from bulk or nanoparticle catalysts. In ADCs, the coordination structures around central metal atoms affect their catalytic activity and selectivity significantly. However, the control of coordination environments has been achieved empirically. In this work, we have developed a low-temperature ligand exchange method that allows for reversibly controlling the coordination structure of the metal center in the ADCs, which consequently modulate the oxidation state of the metal center and its catalytic activity and selectivity for oxygen reduction reaction (ORR) in a reversible manner. The CO-ligated atomically dispersed Rh catalyst exhibited ca. 30-fold higher ORR activity than the NHx-ligated catalyst, whereas the latter showed three times higher H2O2 selectivity than the former. Post-treatments of the catalysts with CO or NH3 allowed the reversible exchange of CO and NHx ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. The reversible ligand exchange reactions were generalized to Ir- and Pt-based catalysts.