Low-Temperature CO2 Methanation Driven by Plasma: Influence of Catalyst Dielectric Constant and Hydrogen Spillover Capacity

Abstract

Plasma catalysis methanation capably converts CO2 into CH4 at low temperatures, but how to regulate physicochemical properties of the catalysts toward efficient nonthermal plasma activation remains unclear. In this work, plasma-catalytic CO2 methanation over nickel catalysts with different supports (γ-Al2O3, ZrO2, SiO2, TiO2, and CeO2) and metal loadings have been systematically studied in a dielectric barrier discharge reactor. Ni/CeO2 coupled with plasma exhibited excellent CO2 conversion (84.8%) and CH4 selectivity (90.7%) even at 180 °C (catalyst bed), being followed by Ni/SiO2 and Ni/ZrO2. The catalyst dielectric constant was a key parameter impacting CH4 yield, via alerting the plasma density by changing spatial electric field, but it is not the only dominant factor. The hydrogen spillover capacity inspired by surface oxygen vacancies (Ni/CeO2, Ni/ZrO2) or hydroxyls (Ni/SiO2), collaboratively enhanced by the hydrogen affinity of nickel under low temperature, was demonstrated to primarily impact CO2 methanation activity under plasma activation. In-situ FT-IRs and optical emission spectra evidenced that plasma efficiently enhanced activation of CO2 to form carbonates and bicarbonates over Ni/CeO2 under low temperatures where CO2 could hardly be activated by only thermal catalysis. Plasmatron surface reaction analysis, aiming to discern the functionality of surface-adsorbed hydrogen and plasma-activated hydrogen, further confirmed that plasma-induced hydrogen species are critical for accelerating the conversion of key surface intermediates such as CO2* and HCO3*. A stepwise CO2 hydrogenation pathway with carbonate, formate, and methoxy as key intermediates, being accelerated by nonthermal plasma, has been also elucidated in this study.