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 CO 2 capture and utilization (ICCU) via the reverse water–gas shift (RWGS) reaction presents a promising pathway for mitigating CO 2 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 O 2 /steam in the deactivation of Ni-CaO DFM pellets for ICCU-RWGS are elucidated. Steam alone enhances the initial CO 2 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, H 2 O accelerates performance decay by inducing severe sintering of both Ni and CaO crystals and the loss of active sites. O 2 exposure, while suppressing CO 2 capture by diminishing surface basicity, enhances Ni reducibility and benefits the subsequent RWGS reaction. The coexistence of O 2 and H 2 O induces a complex interplay. The promoting role of steam can partially counteract the negative impact of low-concentration O 2 on the CO yield, but high-concentration O 2 irreversibly degrades the CO 2 capture capacity. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis reveals that steam shifts the CO 2 adsorption pathway from carbonate to bicarbonate formation, while O 2 /H 2 O co-exposure favors the carbonate formation pathway for CO 2 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.