Thermal and Plasma-Assisted CO2 Methanation over Ru/Zeolite: A Mechanistic Study Using In-Situ Operando FTIR

CO2 methanation is an attractive reaction to convert CO2 into a widespread fuel such as methane, being the combination of catalysts and a dielectric barrier discharge (DBD) plasma responsible for synergistic effects on the catalyst’s performances. In this work, a Ru-based zeolite catalyst, 3Ru/CsUSY, was synthesized by incipient wetness impregnation and characterized by TGA, XRD, H2-TPR, N2 sorption and CO2-TPD. Catalysts were tested under thermal and plasma-assisted CO2 methanation conditions using in-situ operando FTIR, with the aim of comparing the mechanism under both types of catalysis. The incorporation of Ru over the CsUSY zeolite used as support induced a decrease of the textural properties and an increase of the basicity and hydrophobicity, while no zeolite structural damage was observed. Under thermal conditions, a maximum CO2 conversion of 72% and CH4 selectivity above 95% were registered. These promising results were ascribed to the presence of small Ru0 nanoparticles over the support (16 nm), catalyst surface hydrophobicity and the presence of medium-strength basic sites in the catalyst. Under plasma-catalytic conditions, barely studied in similar setups in literature, CO2 was found to be excited by the plasma, facilitating its adsorption on the surface of 3Ru/CsUSY in the form of oxidized carbon species such as formates, aldehydes, carbonates, or carbonyls, which are afterwards progressively hydrogenated to methane. Adsorption and surface reaction of key intermediates, namely formate and aldehydic groups, was observed even on the support alone, an occurrence not reported before for thermal catalysis. Overall, similar reaction mechanisms were proposed for both thermal and plasma-catalysis conditions.