Enhanced activity of co-precipitated NiFeAlO_x in CO₂ methanation by segregation and oxidation of Fe

Commercial application of the CO₂ methanation reaction demands for the development of catalysts that feature an enhanced stability to increase catalyst lifetime. By time-resolved aging studies, we show that the improved deactivation resistance of co-precipitated NiFeAlO_x catalysts compared to NiAlO_x is obtained by a temporal increase of the intrinsic catalytic activity of NiFeAlO_x, provoked by aging at elevated temperature and pressure in thermodynamic equilibrium. Detailed structural characterization of reduced and aged catalysts resolves that aging triggers the partial segregation of (γFe,Ni) nanoparticles initially formed during activation, accompanied by the oxidation of Fe. Thereby, the intrinsic catalytic activity increases, which can be explained by the generation of redox-active Fe²⁺ sites that offer an additional reaction pathway for CO₂ activation. The deactivation behavior of NiFeAlO_x can be described by a superimposition of activity increase related to Fe²⁺ site formation and decrease due to the loss of active sites by sintering processes.