Influence of Acidic Sites and Cu Loading on Plasma-Catalytic CO2 Hydrogenation to Methanol

Abstract

Plasma-catalytic CO₂ hydrogenation offers a promising route to methanol synthesis under near-ambient temperature and pressure by enabling nonequilibrium activation of CO₂ and H₂. However, the catalyst surface properties that govern methanol selectivity and yield under dielectric barrier discharge (DBD) plasma conditions remain insufficiently understood. Here, we investigate how support acidity and Cu functionality influence activity, selectivity, and surface reaction pathways over Cu-loaded silica–alumina catalysts. A series of silica–alumina supports containing 1–70 wt % SiO₂ were evaluated under identical DBD plasma conditions. CO₂ conversion and methanol formation exhibited a volcano-type dependence on silica content, with S30, a silica-alumina support containing 30 wt % SiO₂, giving the highest single-pass methanol yield of 8.7% among the Cu-free supports. NH₃ temperature-programmed desorption, CO₂ temperature-programmed desorption, pyridine diffuse reflectance infrared Fourier transform spectroscopy, and acidity-perturbation experiments indicate that methanol formation is not governed by total acidity or CO₂ uptake alone, but by a favorable acid environment combining predominantly Lewis acidity, relatively high medium-strength acid-site density, and low to moderate basicity. Introducing Cu increased both CO₂ conversion and methanol selectivity across the silica–alumina series, while preserving S30 as the optimal support. Under optimized conditions, S30–5Cu achieved a single-pass methanol yield of approximately 12.4% at 30 °C and near-atmospheric pressure. In situ plasma-coupled Fourier transform infrared spectroscopy supports a surface pathway in which adsorbed carbonate or bicarbonate species are progressively hydrogenated to formate and methoxy-like intermediates prior to methanol formation. The optimized S30–5Cu catalyst maintained relatively stable performance over 30 h and showed slightly improved performance when the reactor size was more than doubled. These results establish support acidity and Cu-acid cooperation as key design parameters for selective plasma-catalytic CO₂ hydrogenation to methanol.