Abstract:

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 (Mo_xC), with properties similar to those of precious metals, has been extensively investigated. In particular, it has been demonstrated that face-centered cubic α-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 ℃). Accordingly, in this study, a system of Ir species and α-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 α-MoC over a wide temperature range (200–500 ℃) owing to the synergistic effect of Ir and α-MoC. The optimal 0.5%Ir/MoC catalyst yielded a CO₂ conversion of 48.4% at 500 ℃, 0.1 MPa, and 300000 mL·g⁻¹·h⁻¹, which was comparable to the equilibrium conversion (49.9%). The CO selectivity and space-time yield of CO over 0.5%Ir/MoC reached 94.0% and 423.1 μmol·g⁻¹·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 α-MoC substrates were highly dispersed. The strong metal-support interaction between Ir and α-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% (mass fraction), Ir single atoms (Ir₁) and clusters (Ir_n) coexisted to create Ir_n-Ir₁-C-Mo synergistic sites between Ir and α-MoC. The number of Ir₁ 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 H₂ 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-Ir₁-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.

Key words: Reverse water gas shift reaction, Metal-support interaction, Synergistic effect, Molybdenum carbide, Iridium catalyst