Revealing the characteristics of supported Ni-Mn based mixed metal oxide catalysts: Operando insights into their activation and CO2 methanation behavior

Power-to-gas technologies, such as CO₂ methanation, enable to mitigate man-made climate change. For this process to be viable, it is essential to use an active, selective and stable catalyst. This study addresses these requirements by creating a novel Ni-Mn_yO_x/Al₂O₃ (0 ≤ y ≤ 1) catalyst that uses Mn not as a dopant but creates a joined Ni-Mn mixed metal oxide (MMO) phase on a Al₂O₃ support. To identify the ideal composition, we compare different Mn contents y from 0 to 1 and determine an Mn/Ni ratio of 0.375 as the minimum for the formation of the supported Ni-Mn MMO phase. Compared to the self-synthesized, literature-based state-of-the-art type NiO_x/Al₂O₃ based CO₂ methanation catalysts the Ni-Mn MMO based catalysts achieve about 30 % higher Ni specific methane formation rates. We carefully characterize the catalysts phase composition, surface area, active metal surface area, reducibility, surface basicity and elemental analysis. Additionally, we present a detailed in situ X-ray absorption spectroscopy (XAS) study and phase characterization on the reduction process for catalytic activation of the calcined catalysts to draw conclusions on the final activated state. It was shown that the Ni-Mn mixed metal oxide phase changes upon activation into metallic Ni supported on a Ni-depleted Ni-Mn MMO phase. Additionally, the behavior of both catalysts under reaction conditions was investigated using operando XAS, phase analysis and Diffuse Reflectance Infrared Fourier Transform Spectroscopy. Based on this, a tentative reaction mechanism was proposed which includes the possibility of additional CO₂ activation pathways on the Ni-Mn MMO phase.