Plasma-catalytic CO₂ Hydrogenation To CH₃OH And CH₃CH₂OH:Study Of The Reaction Mechanism

CO₂ conversion is one of the key issues to achieve carbon peaking and carbon neutrality goals,but the conventional catalytic CO₂hydrogenation to CH₃OH must be operated under high temperature and pressure.Previous research in our group found that plasma and metal catalysts can initiate the CO₂hydrogenation to CH₃OH under mild conditions,but the reaction mechanism remains primitive.In this thesis,the reaction mechanism of CO₂hydrogenation to CH₃OH on the surface of Fe and Cu-based catalysts has been systematically studied.Furthermore,the role of H₂O molecule on regulating product selectivity has been explored by experiments and DFT calculations.Fe₂O₃/γ-Al₂O₃catalyst shows a synergistic effect with plasma in CO₂hydrogenation to CH₃OH,achieving 12%CO₂conversion and 58%CH₃OH selectivity at nearly 80 ℃ and atmospheric pressure.The characterization results of Fe-based catalysts revealed that the enhanced CO₂conversion and CH₃OH selectivity are attributed to the chemisorbed oxygen species on Fe₂O₃/γ-Al₂O₃.Furthermore,the chemisorption characterization results showed that the catalysts with stronger CO₂adsorption capacity exhibit a higher CO₂conversion and CH₃OH selectivity.Based on catalyst characterization and chemisorption results,a possible reaction mechanism of CO₂hydrogenation to CH₃OH driven by coupling of chemisorbed oxygen（O_β）species and plasma,i.e.,synergistic of Formate pathway and RWGS pathway,has been proposed.Experiments were combined with DFT calculations to investigate the reaction mechanism of plasma-catalyzed CO₂hydrogenation to CH₃OH over Cu-based catalysts,achieving 10%CO₂conversion with 50%CH₃OH selectivity.The CH₃OH selectivity further rose to 65%with H₂O addition（1:1 H₂O/CO₂ratio）.The Cu/γ-Al₂O₃catalyst was systematically characterized,which ensured us to build a Cu₁₃/γ-Al₂O₃model for further DFT calculations.DFT calculation results show that the plasma species can regulate the catalyst surface reactions via the Eley-Rideal mechanism,which accelerates the hydrogenation process and promotes the generation of the key intermediates.Furthermore,H₂O molecules can promote the CH₃OH desorption by competitive adsorption over the Cu₁₃/γ-Al₂O₃surface,thus improving CH₃OH selectivity.The product selectivity of plasma-catalyzed CO₂hydrogenation can be regulated by H₂O molecules.High content of H₂O enhanced CH₃CH₂OH selectivity by promoting C-C coupling reaction.With 2:1 H₂O/CO₂ratio,55.7%CH₃CH₂OH selectivity was achieved with 78%total alcohols selectivity over Cu₂O/CeO₂catalysts.Isotope tracing experiments show that H₂O molecules dissociated and participated in the plasma catalytic CO₂hydrogenation reaction.DFT calculations show that OH species（from H₂O dissociation）adsorbed on the catalyst surface are capable of promoting HCO-HCO coupling reaction,and CH₃CH₂OH can be formed through multi-step hydrogenation reactions.