Atomically dispersed supported metal catalysts are an emerging class of catalysts, offering opportunities for efficient utilization of expensive noble metals. Controlling their catalytic performance and stability at high metal loadings under reactive environments at elevated temperatures are the main challenges. Here, we synthesized supported Ir complexes by the reaction of Ir(C2H4)2(acac) and Ir(CO)2(acac) (acac = CH3COCHCOCH3) with various supports having a wide range of electron-donor character and coated them with various imidazolium ionic liquids (ILs). Detailed characterization demonstrated the control of catalytic performance by choice of the environment surrounding metal—both the support and a sheath of IL molecules. Next, we evaluated the changes in the coordination environment of Ir complexes on each support by continuous-scan X-ray absorption spectroscopy during a temperature programmed reduction (TPR) treatment to 120 °C in flowing pure H2. Data illustrated that the iridium complexes become more stable as their electron density increases. In regard to overcoming the limitation associated with the limited metal loadings, we used a support in the form of a reduced graphene aerogel (rGA). The high density of surface sites and the high surface area of the rGA allowed us to increase the metal loading beyond 10 wt% to prepare both site-isolated supported Ir complexes and supported Ir clusters of only a few atoms.

Meeting the challenges with atomically dispersed supported metal catalysts: Tuning catalytic performance and improving stability at high metal loadings

ACS Spring 2022

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