CO₂ methanation over Fe- and Mn-promoted co-precipitated Ni-Al catalysts: Synthesis, characterization and catalysis study

The methanation reaction of CO₂ is in discussion to be a sustainable pathway to address future questions arising from limited primary energy feedstock and the accumulation of CO₂ in the atmosphere. Therefore, the development of highly active and thermostable catalysts for this reaction is an indispensable matter of research. For this reason, an equimolar NiAlO_x benchmark catalyst (44 wt.% Ni loading) was synthesized and modified by doping with Fe or Mn up to 10 wt.% of promoter by co-precipitation at constant pH 9. Their activity and stability performances in the CO₂ methanation reaction were evaluated by comparing the conversion versus temperature characteristics before and after an aging period of 32 h at 500 °C. Material characterization studies comprising BET, XRD, in situ IR spectroscopy, XPS, H₂ and CO₂ chemisorption, and EPR/FMR contributed to derive structure-activity relationships and to obtain a deeper understanding of the catalytic behavior. Promotion with Mn led to a significant enhancement of the catalytic activity. This is assumed to be caused by a higher density of medium basic sites and an enhanced CO₂ adsorption capacity on the activated catalyst related to interactions between Mn oxide species and the mixed oxide phase, in combination with a stabilization of the Ni surface area at moderate Mn loadings. Promotion with Fe increased the thermal stability of the catalyst, which is attributed to the formation of a Ni-Fe alloy during catalyst activation. For both phenomena, the optimum molar Ni to promoter ratio for co-precipitated catalysts was found to be around 5.