In Situ Spectroscopic Study of CO₂ Capture and Methanation over Ni-Ca Based Dual Functional Materials

Carbon dioxide capture and reduction (CCR) to CH₄ using dual-functional materials (DFMs) have recently attracted significant attention as a promising strategy for carbon capture and utilization. In this study, we investigate the mechanism of CCR to CH₄ over Al₂O₃-supported Ni-Ca DFMs (Ni-Ca/Al₂O₃) under cyclic feeds of model combustion exhaust (2.5 % CO₂+0 or 10 % O₂/N₂) and H₂ at 500 °C. Various spectroscopic analyses, including time-resolved in situ X-ray diffraction and X-ray absorption spectroscopy, were conducted during CO₂ capture and the subsequent H₂-reduction steps. Based on these analyses, we propose a mechanism of CCR to CH₄ over Ni-Ca based DFMs. During the CO₂ capture step, the Ni⁰ species underwent complete oxidation in the presence of O₂ to yield NiO. Subsequently, CO₂ was captured through the interaction between the CaO surface and CO₂, resulting in the formation of CaCO₃ layers on the CaO particles. When the gas flow was switched to H₂, NiO was partially to provide Ni⁰ sites, which acted as active sites for H₂-reduction of the adjacent CaCO₃ layers to yield CaO and gas-phase products, CH₄ and H₂O.