Role of Gas Dropouts in CO₂ Methanation over MOF-Derived Ni₃Fe@C Catalysts: An In Situ XAS and PDF Study

Abstract

With industrial chemical processes facing the challenges of renewable energy supply, catalysts are needed that withstand fluctuations of operating conditions. For methanation reactions using hydrogen from electrocatalytic water splitting, dropouts of hydrogen are amongst the realistic scenarios. While Ni-based catalysts are the most widely used, bimetallic Ni/Fe-based catalysts recently emerged as superior. A new method of preparing highly active metallic catalysts is the decomposition of metal-organic frameworks. Even though it is difficult to control the particle size distribution and the homogeneity of the formed nanoparticles with this method, the carbonaceous features present between the nanoparticles permit avoiding the use of support, leading to a high-loading catalyst with superior stability. Here, we investigate with in situ X-ray absorption spectroscopy and pair distribution function analysis the structural details of a Ni₃Fe@C methanation catalyst derived from a metal-organic framework during its activation and catalysis under H₂-dropout conditions. Despite the similarity of these phases, it was possible to identify two fcc phases of Ni₃Fe and Ni coexisting during catalytic cycling, with a Fe₂NiO₄ spinel phase appearing during dropouts. This indicates oxidation of the particle surface in the absence of hydrogen, which can be fully recovered by reactivation in pure hydrogen atmosphere, providing high stability of the catalyst during an industrially relevant dropout scenario.