Elucidating the Roles of O ₂ /H ₂ O in the Deactivation of Ni-CaO Dual Functional Material Pellets for Integrated CO ₂ Capture and Reverse Water–Gas Shift Reaction

Integrated CO2 capture and utilization (ICCU) via the reverse water–gas shift (RWGS) reaction presents a promising pathway for mitigating CO2 emissions. Ni-CaO dual function materials (DFMs) are highly effective for this process, yet their stability in realistic, impurity-laden flue gas remains a challenge. In this work, the individual role of steam and synergistic roles of the O2/steam in the deactivation of Ni-CaO DFM pellets for ICCU-RWGS are elucidated. Steam alone enhances the initial CO2 capture capacity and CO yield by promoting CaO hydration, pore development, and surface basicity, while the promoting role of steam is independent of its concentration. However, over multiple cycles, H2O accelerates performance decay by inducing severe sintering of both Ni and CaO crystals and the loss of active sites. O2 exposure, while suppressing CO2 capture by diminishing surface basicity, enhances Ni reducibility and benefits the subsequent RWGS reaction. The coexistence of O2 and H2O induces a complex interplay. The promoting role of steam can partially counteract the negative impact of low-concentration O2 on the CO yield, but high-concentration O2 irreversibly degrades the CO2 capture capacity. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis reveals that steam shifts the CO2 adsorption pathway from carbonate to bicarbonate formation, while O2/H2O co-exposure favors the carbonate formation pathway for CO2 capture and effectively suppresses the methanation side reaction in the RWGS stage. These results provide fundamental insights into the deactivation mechanisms of Ni-CaO DFM pellets under realistic conditions, offering critical guidance for the design of highly efficient DFMs for industrial ICCU-RWGS applications.