Tuning Mg-Fe-O solid solutions towards optimized exsolution of active sites for thermal catalytic decomposition of methane

The catalytic decomposition of methane (CDM) is a plausible means to convert methane to turquoise hydrogen with simultaneous carbon sequestration, in the form of solid carbon nanomaterials (CNMs). The key to a cost-effective CDM process is a high-performance and low-cost catalysts. Mg-Fe-O CDM catalysts have shown outstanding CDM performance in comparison with Fe, whilst being affordable and easy to produce. By optimizing the Fe: Mg ratios (3: 1, 2: 1, 1: 1, 1: 2, 1: 3) of Mg-Fe-O catalysts to render the best activation and exsolution behavior of fresh catalysts containing partial solid solutions. The structural–functional relationships relevant to CDM are established by characterizing the fresh catalysts, the spent catalysts, as well as catalysts sampled at various stages of CDM by using BET, XRD, TEM-EDS, H₂-TPR, Raman and TG. The results are interpreted with the help of computationally calculated phase diagrams of the Mg-Fe-O system at the operating conditions of interest. Notably, the catalyst with a Fe: Mg ratio of 1 offers the optimal CDM performance in terms of methane conversion (maximum 48%) and carbon yield (8.6 g_C/g_Fe), attributed to the efficient exsolution of well dispersed Fe⁰ particles from the MgO-FeO solid solution matrix. As the CDM reaction proceeded, Fe/MgO catalysts gradually become coated with deposited CNMs, ultimately leading to the CDM process cessation. The results exemplify the importance of understanding the redox behavior of solid solutions for designing cost-effective CDM catalysts with superior catalytic performance.