Regeneration of Ni-Aluminate Spinel Catalysts for Dry Reforming of Methane— Insights from Exsolution-Dissolution Dynamics and In Situ STXM/Ptychography Analysis

Dry reforming of methane (DRM) is a process that converts CO₂ and CH₄ into synthesis gas (CO and H₂) in a 1:1 ratio, which can potentially be used to produce value-added hydrocarbons. Although Ni-based catalysts are primarily used for DRM reactions, the commercialization of DRM has not been realized due to the lack of stability of these catalysts. Rapid deactivation of the catalyst is primarily caused by carbon deposition on the catalyst surface and sintering of supported Ni metal clusters/particles. Recent interest in DRM catalysts and processes has been concentrated on overcoming these issues in both research and industry. Despite optimization efforts that can reduce coke formation to some extent, completely suppressing coke formation is extremely difficult, and overcoming catalyst deactivation remains a significant challenge. We have recently developed exsolution-based Ni-aluminate spinel (AB₂O₄) catalysts with high surface areas and well-defined mesoporous structures. The exsolution phenomenon offers three distinct advantages for heterogeneous catalysts: (1) During the reduction process, active metal cations diffuse from the parent oxide and exsolve as metal nanoparticles. These active metal species are uniformly distributed on the parent oxide surface and gradually grow, significantly enhancing catalytic activity. (2) The active metal species are anchored on the parent oxide, effectively inducing strong metal-support interactions (SMSI), which provide resistance to coke formation and sintering. (3) The exsolved active metal species possess reversible properties, allowing them to be re-dissolved and re-exsolved from the host oxide, which can be utilized in the catalyst regeneration process. Our study has experimentally demonstrated that the size of the exsolved active metal nanoparticles and the porous structure of the catalysts remain unchanged after several exsolution-dissolution cycles at high temperatures. This indicates not only the feasibility of sustained DRM reactions but also the potential to minimize catalyst performance loss during regeneration steps. Furthermore, to gain an in-depth understanding of the 'exsolution-dissolution' phenomena in spinel-based catalysts, we have introduced <i>in-situ</i> TEM and <i>in-situ</i> STXM/ptychography analytical techniques. By tracking and monitoring the oxidation states and morphology of active metals under various temperature conditions at in-situ, we have proposed the most suitable conditions for the 'exsolution-dissolution' process. Especially, <i>in-situ </i>STXM/ptychography experiments, which directly observe the oxidation state changes of active metals and coke decomposition during exsolution-dissolution, are expected to provide comprehensive insight into our spinel-based catalysts along with various catalyst characterization results. These findings will lay the foundation for a significant leap forward toward the commercialization of DRM processes.