Mechanistic exploration of non-thermal plasma-enhanced Cu-catalyzed CO₂ hydrogenation to methanol

Combining CO₂ and green hydrogen to produce methanol can simultaneously realize CO₂ conversion and green hydrogen storage. Methanol can be used as a green low-carbon fuel or industrial platform product for large-scale application, which is of great significance to promote the further development of Carbon Capture, Utilization and Storage (CCUS) technology. Non-thermal plasma (NTP) can activate CO₂ under mild conditions for hydrogenation reaction and obtain specific products such as methanol by combining with catalysts to tailor the reaction routes, but the underlying reaction mechanism is still unclear. Based on this, the reaction mechanism and process coupling laws of the NTP-enhanced CO₂ hydrogenation with assistance of Cu/γ-Al₂O₃ for methanol production were investigated by combining hydrogenation experiments in a dielectric barrier discharge (DBD) reactor with continuously-pulsed discharge plasma simulations. The experiments investigation showed that 18.74% CO₂ conversion and 36.28% CH₃OH selectivity could be achieved by combining NTP and 10% Cu/γ-Al₂O₃ at 80 ℃ and 0.1 MPa. The on-line discharge parameters monitoring and in-situ emission spectroscopy (OES) showed that the localized discharge was enhanced by Cu/γ-Al₂O₃, which increased the average electron energy and density, thus promote the CO and H generation and their surface consumption reaction, resulting in the weakening of spectral intensity. Furthermore, the sensitivity and ROP analyses indicated that the active substances such as H and CO in NTP promoted methanol generation efficiently by alternating the corresponding L-H routes, which usually had higher energy barriers, via E-R reactions such as CO₂(S)+H→COOH(S), CO+H(S)→HCO(S), CO(S)+H→HCO(S), and CH₃O(S)+H→CH₃OH(S). The reaction pathways analysis revealed that the formate (HCOO*) pathway was the main pathway for methanol generation on the Cu/γ-Al₂O₃ surface, where the reaction CH₃O(S)+H(S)→CH₃OH(S)+Cu(S) was the rate-limiting step. The generation of CO(S) via the CO₂(S)→COOH(S)→CO(S) in the RWGS+CO hydrogenation pathway and its rapid desorption played a vital role in reducing CH₃OH selectivity. Uncertainty analysis demonstrated that although increasing the CO₂ adsorption rate could effectively improve its conversion, but would not increase CO selectivity when H(S) was insufficient. The optimal CO₂ and H₂ adsorption rate ratio was predicated to be γ_H2/γ_CO2=7～8. Improving CO(S) adsorption stability and enhancing H₂ electron collisional dissociation to promote H generation could promote reactions of CO(S)→HCO(S) and CH₃O(S)→CH₃OH(S), thus facilitated a CO₂ conversion of 27.4% and CH₃OH selectivity of 51% synergistically.