Sub-Nanometer Ru Clusters on Ceria Nanorods as Efficient Catalysts for Ammonia Synthesis under Mild Conditions

The wide availability of hydrogen from renewable energy has triggered increasing interest in developing green ammonia synthesis under mild conditions, which relies on the development of efficient catalysts. Here, highly dispersed sub-nanometer Ru clusters on CeO2 nanorods (Ru clusters/CeO2) which lack B5 sites were successfully synthesized and investigated as catalysts for ammonia synthesis. Structure analysis results reveal that the sub-nanometer Ru clusters on the CeO2 support have structures much different from that of the Ru nanoparticle counterpart. The Ru clusters/CeO2 catalyst exhibits higher activity than the Ru nanoparticle counterpart on CeO2, even outperforming many typical benchmark Ru-based ammonia synthesis catalysts. Moreover, a highly stable performance was achieved over the Ru clusters/CeO2 catalyst in a relatively long period of over 200 h. Kinetic analysis, H–D exchange, and nitrogen isotopic exchange results reveal that the sub-nanometer Ru cluster species can facilitate the activation and dissociation of hydrogen and nitrogen molecules, mainly responsible for its superior activity and intrinsic activity in ammonia synthesis. This work offers insight into the new kind of active site of Ru-based catalysts for ammonia synthesis, which may help to guide the design of efficient Ru cluster catalysts for ammonia synthesis under mild conditions.