Ir Single Atoms and Clusters Supported on α-MoC as Catalysts for Efficient Hydrogenation of CO2 to CO

The conversion of CO2 into CO via the reverse water gas shift (RWGS) reaction has recently attracted considerable attention owing to the increase in atmospheric CO2 emissions. However, metal supported catalysts easily undergo sintering and become inactive at high temperatures. To fabricate highly active and stable catalysts, molybdenum carbide (MoxC), with properties similar to those of precious metals, has been extensively investigated. In particular, it has been demonstrated that face-centered cubic alpha-MoC can strongly interact with support metals, rendering it an attractive candidate as a catalyst for the RWGS reaction. Furthermore, it has been previously demonstrated that metallic Ir, with unique electronic properties and a low CO desorption barrier, is active for the RWGS at low temperatures (250-300 degrees C). Accordingly, in this study, a system of Ir species and alpha-MoC was constructed using a solvent evaporation self-assembly method. The catalytic performance of the Ir/MoC catalysts for the RWGS reaction was considerably superior to that of pure alpha-MoC over a wide temperature range (200-500 degrees C) owing to the synergistic effect of Ir and alpha-MoC. The optimal 0.5%Ir/MoC catalyst yielded a CO2 conversion of 48.4% at 500 degrees C, 0.1 MPa, and 300000 mL<middle dot>g(-1)<middle dot>h(-1), which was comparable to the equilibrium conversion (49.9%). The CO selectivity and spacetime yield of CO over 0.5%Ir/MoC reached 94.0% and 423.1 mu mol<middle dot>g(-1)<middle dot>s(-1), respectively, which were higher than most of the previously reported values. Moreover, 0.5%Ir/MoC retained its catalytic properties over 100 h and demonstrated excellent stability at high temperatures. Several characterization methods were used to demonstrate that the Ir species supported on alpha-MoC substrates were highly dispersed. The strong metal-support interaction between Ir and alpha-MoC, which occurred via electron transfer, considerably improved the stability of the Ir/MoC catalysts. For the Ir/MoC catalysts with Ir loadings > 0.2 wt% (wt%: mass fraction), Ir single atoms (Ir-1) and clusters (Ir-n) coexisted to create Ir-n-Ir-1-C-Mo synergistic sites between Ir and alpha-MoC. The number of Ir-1 species and size of Ir-n species of 0.5%Ir/MoC were higher and smaller, respectively, than those of the other Ir/MoC catalysts. This conferred 0.5%Ir/MoC an optimal electron density, which contributed to the remarkable adsorption and activation of CO2 and H-2 during the RWGS. In situ diffuse reflectance infrared Fourier transform spectroscopy experiments revealed that the RWGS reaction mechanism occurred via a formate pathway. Although the formation of Ir-n-Ir-1-C-Mo synergistic sites did not affect the reaction mechanism, the generation and decomposition of formate intermediates were distinctly promoted. Therefore, the catalytic performance of Ir/MoC was effectively improved by the synergistic effect. This study provides a guide for designing efficient and stable catalysts for CO2 utilization.