Chemical engineering aspects of plasma-assisted CO2 hydrogenation over nickel zeolites under partial vacuum

Ni-zeolites (ZSM-11 and USY) were prepared for application in carbon dioxide hydrogenation. Structure, physical properties and texture were analyzed by XRD, N-2 sorptiometry, SEM and TEM images. The zeolites nickel reducibility (determined by in-situ TPR-MS) was directly related to the catalytic activity of zeolites. Glow-discharge plasma-assisted CO2 hydrogenation was carried out under partial vacuum using a packed-bed catalytic reactor under two different configurations: In-Plasma Catalysis (IPC) and Post-Plasma Catalysis (PPC). CO2 hydrogenation into CO (CO2 + H-2 -> CO + H2O) and CO2 methanation (CO2 + 4H(2) -> CH4 + 2H(2)O) reactions were more efficient when the reaction was assisted by plasma using the IPC mode. In IPC mode, a novel phenomenon was observed: a considerable amount of methane was released from the catalyst after plasma extinction. This phenomenon was explained combining the operando IR plasma technique and the analysis of the carbon balance during the plasma-assisted CO2 hydrogenation reaction. Both configurations (IPC and PPC) were studied under different volumetric flow rates and the minimum lifetime of excited species generated within the plasma discharge for methane production was determined. The lifetime of the more active species which are responsible for methane production was lower than 67 ms. Under the operation conditions used in this work, methane production was only possible when CO2 hydrogenation was performed in IPC mode. CO2 hydrogenation was found to be more energy efficient when the reaction was carried out under plasma assistance compared to conventional heating. Higher CO2 conversions, CO and CH4 yields were achieved with respect to conventional heating at lower temperatures when plasma assistance was used.