CO2 Capture and Reduction to CO in the Presence of CO over In–Cs/ZrO2 Dual-Functional Materials

Abstract

CO₂ exhaust mixed with large amounts of CO from the steel and petroleum industries, as a major source of greenhouse gases, can be a promising renewable carbon resource. However, the reduction of CO₂ under excess CO is thermodynamically limited by the reverse water gas shift (RWGS) reaction. Herein, we report a CO₂ capture and reduction (CCR) system for continuous conversion of CO₂/CO mixture into high-concentration CO. Screening tests of various dual functional materials (DFMs) show that indium and cesium-coloaded ZrO₂ (In–Cs/ZrO₂) serves as the best DFM for the selective reduction of CO₂ to CO under cyclic feeds of CO₂/CO ↔ H₂ at 350 °C. Operando IR and model reaction tests show that the higher CO yield of In–Cs/ZrO₂ compared to the other noble metal (Pt, Pd, Ru, Rh)-based DFMs was due to the lower activity of In–Cs/ZrO₂ in the disproportionation of CO into carbon and CO₂. Continuous cyclic reactions with an In–Cs/ZrO₂-based double-reactor under the CO-excess (10% CO₂/80% CO/10% He ↔ 100% H₂) conditions at 550 °C for 1000 min (250 cycles) showed a CO yield of 51% (based on the total amount of inlet CO₂). Operando In K-edge X-ray absorption near-edge structure (XANES) and UV–vis spectroscopy show that In₂O₃ species are reduced by H₂ to yield reduced In species that are reoxidized by CO₂ to regenerate In₂O₃. However, the main CO formation route is not based on the redox mechanism. Operando IR analysis with modulation excitation spectroscopy (MES) showed that Cs species are responsible for CO₂ capture and carbonate formation, while reduced In species are responsible for the H₂ reduction of carbonates to CO via formate intermediates.