On the interaction of CO2 with Ni-Al catalysts

Knowledge of the CO2 interaction with the catalyst material is of great importance in the development of efficient catalysts for the methanation of CO2. In this study, the CO2 adsorption modes on and interaction with Ni-Al catalysts are systematically investigated by coupling FT-IR and CO2-TPD measurements next to pulsed and static CO2 chemisorption. The knowledge that is derived allows for a concise characterization of the CO2 interaction with Ni-Al catalyst by means of CO2-TPD. The strength of this technique in elucidating structure-activity relations of Ni-Al catalysts for CO2 methanation is demonstrated. For the study, catalysts with varying Ni loading are synthesized via co-precipitation and incipient wetness impregnation. CO2 adsorption under flow conditions close to room temperature leads to the formation of carbonate species on the catalyst. After elongated exposure of the catalyst to CO2, typically present under static conditions, Ni-CO species are also formed. The density of weak basic sites forming bicarbonate decreases with increasing Ni content for precipitated catalysts, while medium and strong basic sites forming bidentate, monodentate and bridged/organic-like species increase. Bicarbonate is the most abundant species on impregnated samples upon CO2 adsorption under flow conditions. Only a minor influence of the Ni loading on the catalyst basicity is observed for the impregnated samples. TPD results also indicate that CO2 adsorption on Ni-Al catalysts is an activated process. A more complete picture of the CO2 interaction with Ni-Al catalysts is obtained when starting TPD measurements at temperatures as low as 218 K. Bicarbonate, bidentate and monodentate carbonate exhibit desorption signals at 325, 416 and 500 K, respectively, while bridged/organic-like species show a broad desorption pattern between 550 and 750 K. An additional desorption signal is observed at 260 K. Pulsed CO2 chemisorption reveals that strong adsorption sites are filled first. The uptake of strongly chemisorbed CO(2 )determined from pulsed adsorption is significantly lower than corresponding values from static chemisorption.