Targeted Fe-Doping of Ni−Al Catalysts via the Surface Redox Reaction Technique for Unravelling its Promoter Effect in the CO₂ Methanation Reaction

In promoted catalyst systems, the location of dopants is of very high interest to investigate promoter effects. A Ni/Al₂O₃ catalyst (w_Ni=11 wt. %) prepared by deposition-precipitation and a co-precipitated NiAlO_x (n_Ni/n_Al=1) catalyst are modified with Fe by means of the surface redox reaction technique and tested for activity under differential and integral conditions and for thermal stability (aging at 500 °C, 8 bar, 32 h) in the methanation reaction of CO₂. By applying detailed material characterization studies comprising H₂ and CO₂ chemisorption, ICP-OES, XRD, STEM-EDX, FMR and BET, it is shown that the surface deposition techniques can be used to selectively deposit Fe in the vicinity of Ni nanoparticles. Doping with Fe leads to an increase of the catalytic activity, attributed to electronic effects through the formation of surface Ni−Fe alloys, and, for the co-precipitated Ni−Al catalyst, to an enhancement of the apparent thermal stability at higher Fe loadings, which is assumed to be caused by a dynamic variation of Ni, Fe, and Al interactions depending on the reaction conditions.