Structure Sensitivity of CO₂ Hydrogenation on Ni Revisited

Despite the large number of studies on the catalytic hydrogenation of CO₂ to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO₂ hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO₂ to CO are different from those for the subsequent hydrogenation of CO to CH₄. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO₂ hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO₂ hydrogenation and demonstrated that direct dissociation of CO₂ to CO is followed by the conversion of strongly bonded carbonyls to CH₄. These findings provide essential insights into the much debated structure sensitivity of CO₂ hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.