Tuning Mg-Fe-O Solid Solutions Towards Optimized Exsolution of Active Sites for Thermal Catalytic Decomposition of Methane

Abstract

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. 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, whilst being affordable and easy to produce. This research elucidates the relationship between the formulation of the Mg-Fe-O catalysts, their catalytic structures, and their catalytic performance towards CDM. This is achieved by optimizing the Fe: Mg ratios of Mg-Fe-O catalysts to render the best activation and exsolution behavior of the solid solution precatalysts. The structural-functional relationships relevant to CDM were established by characterizing the fresh catalysts, the spent catalysts, as well as catalysts sampled at various stages of CDM. The resultswere 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 CH4 conversion, H2 yield and carbon (CNMs) yield, attributed to the efficient exsolution of well dispersed Fe0 particles from the MgO-FeO solid solution matrix. As the CDM reaction proceeded, the Fe-based catalyst was gradually disintegrated by the rapid growth of the CNMs (predominately defective CNTs and CNOs), which ultimately encapsulate the active sites and terminate the reaction. The results exemplify the importance of understanding the redox behavior of solid solutions for designing cost-effective CDM catalysts with superior catalytic performance.

Keywords: Fe-based catalyst, MgFe2O4 phase, methane decomposition, hydrogen, CNTs, CNOs