Evolution of Low Z-Elements in a Ni/MgFeAlO₄ Catalyst during Reaction: Insight from In Situ XRS

Ni/MgFeAlO4 displays promising activity and stability in the dry reforming of methane. In order to establish a correlation between its performance and structural changes, an in-depth analysis of the Ni/MgFeAlO4 catalyst's constituent elements is conducted using in situ techniques, specifically combined X-ray emission spectroscopy (XES) and X-ray Raman scattering (XRS) spectroscopy. This investigation spans sequential treatments involving H-2 reduction, CO2 reoxidation, and methane dry reforming. XES valence-to-core signals for Fe and Ni retrace the applied treatments. After the reaction, Ni is fully metallic, while Fe shows a mixed metal-oxide state. Comparing these XES spectroscopic data with ab initio simulations indicates that dry reforming of methane does not form metal carbide. C K-edge XRS data recorded after the reaction do not protrude above the noise, proving the capability of the Ni/MgFeAlO4 catalyst to mitigate carbon deposition. O K-edge spectra exhibit variations corresponding to each treatment, effectively replicated through simulations with semiempirical screening parameters. These parameters represent electron transfer between oxygen and neighboring 3d transition metal elements and offer insights into the acid-base properties of the material. Furthermore, the Al L-2,L-3-edge spectra provide valuable information about the impact of treatments on the spinel arrangement, indicating an increasing tetrahedral occupancy of Al3+ in the support. By employing a comprehensive approach that combines XRS with XES, this study provides a holistic understanding of the evolution of low Z-elements, O and Al, alongside Fe and Ni, in the Ni/MgFeAlO4 catalyst throughout the reduction, reoxidation, and methane dry reforming processes.