Oxygen mobility and redox reversibility as key descriptors of CO2 storage and in situ regeneration: Insights from unsupported NiBa and NiK bifunctional catalysts

In this work, a comparative study of two unsupported bifunctional Ni-based catalysts, NiBa and NiK, is presented to elucidate the intrinsic mechanisms governing the CO 2 -SR process and to identify the key physicochemical properties controlling catalyst performance and stability. The catalysts were synthesized by coprecipitation using an atomic Ni/M ratio of 10:1, while colloidal SiO 2 was incorporated during preparation as a stabilizing matrix to minimize attrition and enhance surface development. An integrated experimental approach combining XPS, ATR and FT-IR spectroscopies, low-temperature CO adsorption, isothermal TGA-MS, transient O 2 pulse experiments, and in situ infrared analysis were performed. The results revealed marked differences in the surface reactivity and oxygen exchange dynamics of both catalysts. NiBa exhibited a highly reactive surface enriched with non-lattice oxygen species, promoting the formation of thermally stable bidentate and bridging carbonates during CO 2 adsorption. The incorporation of Ba 2 ⁺ enhanced surface basicity and facilitated lattice oxygen mobility, enabling low-temperature CO oxidation through Mars–van Krevelen-type pathway while maintaining highly reversible redox cycling behavior. In contrast, NiK showed weaker CO 2 adsorption, lower oxygen exchange capacity, and progressive redox deactivation under cyclic operation. Furthermore, NiBa demonstrated a remarkable ability to incorporate and activate oxygen species, releasing CO 2 and H 2 O even in the absence of external reductants, confirming its role as an efficient oxygen-buffer material with high redox accessibility for cyclic CO 2 valorization systems. Beyond a conventional characterization study, transient experiments combined with in situ infrared analysis provided mechanistic insights into the intrinsic pathways governing the CO 2 -SR process.