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

Abstract

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.

Keywords

• plasma catalysis
• co2 hydrogenation
• green methanol
• kinetic calculation
• emission spectrum