Fully Exposed Iridium Clusters Enable Efficient Hydrogenation of N-Heteroarenes

Fully exposed cluster catalysts (FECCs) can not only provide the catalytic sites with multiple metal atoms as in nanoparticles (NPs) but also maintain a full atomic utilization efficiency as in single-atom catalysts (SACs), making them a bridge linking metal SACs and NPs. Due to these particular characteristics, FECCs have been widely studied and have exhibited enhanced catalytic performance in dehydrogenation and oxidation reactions. However, their application in hydrogenation was rarely reported, and the reaction mechanism has yet to be understood. Herein, we report that fully exposed Ir clusters anchored on the sulfur-doped carbon support (Irn/SC) are an efficient catalyst for hydrogenation of N-heteroarenes and exhibit much higher activity and stability compared to the counterpart Ir SAC (Ir1/SC) and Ir NPs catalysts. Especially, we elucidate the hydrogenation reaction mechanism on Irn/SC and disclose how it differs from that on Ir1/ SC. Both experimental studies and density functional theory calculations reveal that the distinct geometric and electronic structures of Irn/SC enable the preferable dissociation of H2 molecules and improve the hydrogenation of quinoline via the classic Horiuti-Polanyi mechanism. In contrast, the absence of metal-metal bonds and the oxidized state of Ir atoms in Ir1/SC result in the difficulty in hydrogen activation and the hydrogenation of quinoline into dihydroquinoline via the Langmuir-Hinshelwood mechanism. This work demonstrates the particular usefulness of fully exposed metal clusters in hydrogenation and provides an insightful understanding of the reaction mechanism.