Visible-light deep reduction of low concentration CO₂ over composite catalyst Ru/CoAl-LDH and the performance enhancement mechanism

Under the dual-carbon goal, the resource utilization of CO₂ driven by clean and renewable solar energy has become an important research topic. However, the previous reports have mostly used high-purity CO₂ as the research object, while the CO₂ concentration in the flue gas emitted by coal-fired power plants is only 3%−15%. To avoid the high-energy CO₂ enrichment process, photocatalytic directional conversion of low concentration CO₂ into high-valued fuels or chemicals has important scientific significance for energy saving, emission reduction and its resource utilization. Cobalt-aluminum layered double hydroxide (CoAl−LDH) was firstly prepared by coprecipitation-hydrothermal method and visible-light catalysts Ru/CoAl-LDH were constructed by loading ruthenium nanoparticles onto the surface of CoAl-LDH via surface impregnation coupled with hydrogen heat treatment. The unique surface composition and structural characteristics of Ru/CoAl-LDH composites are conductive to implement deep photoreduction of low concentration CO₂ using H₂O as the hydrogen source. Structural composition and micro-morphology of the composite catalysts Ru/CoAl-LDH were determined by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and ultraviolet-visible diffuse reflection spectroscopy. The results indicate that the loaded Ru species is zero valence state of metal Ru. Loading Ru has no effect on the nano-lamellar morphology of CoAl-LDH, but can significantly improve the photoresponse performance of composite catalysts. By using Ru/CoAl-LDH as photocatalysts, H₂O as electron donor and hydrogen source, and 10% CO₂/N₂ mixture as simulated flue gas, the effect of Ru loading amount on the productivity of CO₂ reduction products and the selectivity of deep reduction products were investigated under visible light irradiation. 1.6% Ru/CoAl-LDH exhibited the optimal CO₂ photoreduction performance. After 3 hours of visible light irradiation, the productivity and selectivity of deep reduction product methane reached 452.4 μmol/g and 86.3%, which were 10.4 and 3.3 times of single CoAl-LDH, respectively. Meanwhile, the performance enhancement mechanism on deep photoreduction of low concentration CO₂ was explored by using CO₂ adsorption isotherms, in-situ XPS, transient photocurrent and impedance spectroscopy. The —OH groups on the surface of CoAl-LDH facilitate selective adsorption of composite catalysts for low concentration CO₂. Excellent H₂O oxidation performance of CoAl-LDH can provide sufficient in-situ hydrogen source for deep photoreduction of CO₂, without the use of H₂ having explosive risk. As the photoelectron acceptor, the loaded Ru can not only enhance the separation and migration efficiency of photogenerated charges, but also implement multi-electron reduction as active reductive sites of CO₂. Therefore, the synergistic effect of CoAl-LDH and cocatalyst Ru is the primary reason for the improvement of low concentration CO₂ deep photoreduction performance. The composite catalysts Ru/CoAl-LDH realize the effective coupling of visible-light water oxidation and low concentration CO₂ deep reduction, providing important theoretical guidance for the construction of essentially safe and low-energy consumptive CO₂ conversion system. It also provides a new idea for the resource utilization of CO₂ from coal flue gas.