Kinetic study and deactivation phenomena for the methanation of CO2 and CO mixed syngas on a Ni/Al2O3 catalyst

This study presents a detailed kinetic and deactivation analysis of a 24 wt.% Ni/Al2O3 catalyst for the hydrogenation of CO2 and CO to CH4, focusing the attention on the CO2 and CO co-methanation. More than 300 reaction conditions were tested on a fixed-bed reactor obtaining 907 observations. Among them, 852 measurements were used to derive the kinetic parameters in an isothermal reactor model. Power-law models accurately describe CO2 or CO methanation, but fail to predict co-methanation due to preferential adsorption of CO. On the contrary, a three-reactions Langmuir-Hinshelwood-Hougen-Watson model (model M4) successfully described it together with the different hydrogenation pathways. Experimental and literature insights suggest that CO2 adsorption occurs via either dissociative or H-assisted associative mechanism, and then, the high H* coverage favors its conversion into CH4 via the so-called dissociative formyl (CHO*) route. On the contrary, the exergonic CO adsorption increases the CO* coverage promoting the dissociative carbon (C*) route. In addition, C* species are responsible for the higher deactivation rates in CO methanation due to the formation of nickel carbides and coking. Long-term stability tests revealed several deactivation phenomena. CO2 methanation induced mild sintering, while CO methanation led to a significant decrease in stability. Notably, co-methanation improved stability at low temperature by suppressing nickel carbide formation. Contaminants like O2 and C2H4 decreased the stability due to re-oxidation and coking, respectively, while poisons like H2S deactivated the catalyst irreversibly. Power-law deactivation models were developed to predict the activity loss, supporting the potential scale-up of CO2 and CO methanation processes.