Kinetic Limitations of Single-Atom Catalysts in Hydrazine Oxidation

Abstract Single-atom catalysts (SACs) have attracted considerable interest, displaying impressive performance for a wide plethora of electrocatalytic reactions. However, the important question whether isolated sites are always beneficial remains unresolved. Here, by using the hydrazine oxidation reaction (HzOR) as a model reaction, we examine how isolated single-atom metal sites restrict the adsorption mode of intermediates to the monodentate configuration, which leads to weak intermediate stabilization and poor reaction kinetics. This contrasts with bulk metal sites where availability of the bidentate adsorption configuration offers better intermediate stabilization and dehydrogenation kinetics. As a result, Ir, Pt, and Pd SACs were found to require high HzOR overpotentials of 0.90, 1.21, and 1.19 V at 3 mA cm–2, respectively. Comparatively, their Ir, Pt, Pd nanoparticle counterparts with similar metal loading exhibit considerably lower overpotentials of only 0.35, 0.83, and 0.64 V at 3 mA cm–2, respectively. Taken together, our work contributes towards a more complete understanding of SACs, emphasizing the importance of considering site structure and the mechanistic demands of the reaction.