Synergistic interaction between Ni and K2CO3 on γ-Al2O3 for integrated direct CO2 capture and methanation under varying humidity conditions

Abstract

Integrated direct air capture and methanation is an emerging strategy that integrates CO2 capture, storage and its conversion within a single catalytic system. This study presents a Ni/K2CO3/γ-Al2O3 material for the direct air capture (DAC) and subsequent methanation. While Ni serves as the active phase for CO2 hydrogenation, K2CO3 also acts as an effective sorbent for low-concentration CO2. The combined Ni and K2CO3 supported on alumina catalyst exhibited a CO2 capture capacity of 2.35 mmol·g−1, approximately three times higher than K2CO3/γ-Al2O3–demonstrating that Ni not only facilitates methanation but also promotes the formation of additional CO2 capture sites. The enhancement of CO2 capture and its conversion to CH4 is attributed to the reductive decomposition of K2CO3 during H2 pretreatment, forming KO− superbase species that serve as active sites for CO2 capture. Furthermore, K2CO3 inhibited strong Ni–Al2O3 interactions, thereby suppressing the formation of inactive NiAl2O4 and enabling the dispersion of reducible Ni oxide. In addition, the effect of ambient moisture was investigated by varying relative humidity (RH). The best performance, achieved with 10 wt% Ni and 30 wt% K2CO3 supported on γ-Al2O3, exhibited the highest CO2 uptake of 2.35 mmol g−1 at 30 °C under 30–50% RH, while the maximum CO2 utilization efficiency of 35% was achieved at 76% RH. In-situ DRIFTS analysis revealed that increasing RH favors the formation of *HCO3 species, which play a key intermediate role in the methanation pathway, thereby enhancing the CH4 productivity. These findings provide valuable insight into the roles of metal–sorbent interactions and moisture effects, which critically govern material performance in a practical integrated direct air capture and methanation system. © 2025 Elsevier B.V.