CO₂ methanation on transition-metal-promoted Ni-Al catalysts: Sulfur poisoning and the role of CO₂ adsorption capacity for catalyst activity

Co-precipitated and promoted Ni-Al catalysts, specifically Mn- and Fe-doped systems, rank among the most active and thermostable catalysts for the CO₂ methanation reaction. However, little is known about the resistance of those catalysts against sulfur poisoning and the exact reasons for activity enhancement. In order to resolve these questions, a co- precipitated Ni-Al benchmark catalyst with a Ni loading of 41 wt% was promoted by up to 5 wt% of Mn, Fe, Co, Cu and Zn. CO₂ methanation activity and stability against sulfur poisoning was evaluated by in situ poisoning with 5 ppm of H₂S and ex situ poisoning with liquid (NH₄)₂S. Characterization results obtained from XRD, TPR, N₂ physisorption, H₂ and CO₂ chemisorption contributed to derive structure- activity relationships. All promoted samples show a superior resistance versus H₂S poisoning, which is correlated to H₂S adsorption on promoter phases, protecting active Ni sites. Based on the adsorption properties of spent in situ poisoned samples, the individual CO₂ uptake of the Ni⁰ and the promoter phase were identified and correlated to CO₂ methanation activities of ex situ poisoned samples. Enhanced activities of Mn- and Fe-doped samples are ascribed to CO₂ adsorption on promoter phases and subsequent conversion to CH₄. In contrast, CO₂ adsorbed on Cu is converted to CO, causing severe catalyst deactivation. Regarding activity, Co and Zn have insignificant impact. Apparent activation energies of all samples are similar and in the range of 81-92 kJ/mol. Sulfur poisoning and promoter-induced activity changes are therefore ascribed to structural rather than electronic effects for the investigated promoter loadings.