CO2 Adsorption and Methanation over Ni/ZSM-5 Catalysts: Mechanistic Insights from FTIR Spectroscopy

The development of materials capable of combining efficient CO2 capture with catalytic conversion is an important objective in carbon utilization strategies. In this work, a series of Ni/ZSM-5 catalysts prepared by different methods and characterized by different nickel loadings and speciation was investigated by FTIR spectroscopy, complemented by structural characterization and catalytic CO2 hydrogenation measurements, to establish the CO2 coordination mode and the relationship between nickel speciation, molecular CO2 adsorption, and catalytic performance. FTIR spectroscopy demonstrates that molecular CO2 is preferentially adsorbed on Ni2+ cations. Detailed investigation of the low-loaded catalyst, in which nickel is predominantly present as highly dispersed, mainly exchanged Ni2+ cations, shows that adsorption initially proceeds through end-on Ni2+–CO2 species, which progressively transform into geminal Ni2+(CO2)2 complexes with increasing coverage. The adsorption geometry was established by applying the isotopic linkage isomerism (ILI) approach using 13C16O2–13C18O2 isotopic mixtures. In contrast, CO2 adsorption on Ni+ sites formed upon reduction with CO is negligible, whereas these sites strongly bind CO, a key intermediate in CO2 hydrogenation. FTIR experiments performed under CO2/H2 mixtures reveal the formation of surface formate species at low temperatures, followed by the appearance of adsorbed and subsequently gaseous CO. For the catalysts containing larger amounts of nickel, in which less electrophilic and more readily reducible Ni species are additionally present, methane was also detected in the gas phase, and the onset of the reaction processes shifted to lower temperatures. The FTIR observations closely parallel the catalytic behaviour. Overall, the results demonstrate that isolated Ni2+ cations provide effective sites for molecular CO2 adsorption, whereas its subsequent catalytic hydrogenation requires more readily reducible nickel species capable of generating metallic sites under reaction conditions.