Effect of Na and K impurities on the performance of Ni/CeZrO_x catalysts in DBD plasma-catalytic CO_2 methanation

摘要

Carbon dioxide hydrogenation to synthetic fuels and value-added molecules has been lately proposed as a promising technology to stabilize anthropogenic greenhouse gas emissions. Alkali compounds are often present in biomass pyrolysis/gasification syngas or can remain on the surface of the catalyst upon its synthesis. Their presence conditions the performance of the catalyst used in the CO2 hydrogenation reaction. This extends to plasma-activated catalysis since alkali compounds can moreover alter the electric features of the catalytic material. In the present work, non-thermal plasma (NTP)-assisted CO2 methanation was performed over alkali metal (Na and K)-containing Ni/CeZrOx catalysts. The influence of the presence of such alkali compounds on the physicochemical, dielectric and catalytic behavior of these materials during the on-plasma operation was assessed. The presence of Na and K impurities resulted in decreased CO2 conversions, lower selectivity to CH4 and increased power consumption. Their interaction with both the support and the active phase led to a more difficult reduction of the Ce-species, as well as to an increase in the dielectric constant of the packed bed. Higher dielectric constant contributed to increased energy dissipation and favored the strong ionization of the gas contained in the reactor gap, resulting in CO formation as a consequence of direct CO2 splitting. Finally, the impeeded reducibility of the Ce-species led to CO2 adsorption in the form of polydentate and bridged carbonates which were not able to participate in the catalytic cycle, resulting in poorer overall catalytic performance and in the accumulation of O and C-species within the support.