Unraveling the tunable selectivity on cobalt oxide and metallic cobalt sites for CO2 hydrogenation

Unraveling the correlation between the structure of the active sites and the reaction pathway is of utmost importance to designing selective catalysts for efficient conversion of greenhouse gas CO2. In this contribution, the tunable selectivity of CO2 hydrogenation to CO or CH4 is achieved in response to the regulation of valance state of active cobalt species. We combined multiple in situ spectroscopic characterizations to show that the small CoO particles highly dispersed in the mesopores and interacted with SBA-15 support prefer to keep the oxidized state (+2). These CoO species is stable in bulk and the surface is partially reduced under the reactive conditions. More CoO sites exposed in this catalyst lead to the higher performance of reverse water gas shift reaction, whereas more metallic cobalt sites derived from the large CoO particles on regular SiO2 mainly promote CO2 methanation. In situ FT-IR experiments demonstrated that adsorbed formate dominates on CoO and serves as the key intermediate for the formation of CO, while the adsorbed carboxylate intermediate observed on Co0 site is dissociated into adsorbed CO and then further hydrogenated to CH4. Stronger CO and H2 adsorption and activation on Co0 sites lead to the high CH4 selectivity, whereas weakly bonded CO prefers to desorb from CoO site, resulting in the high CO selectivity. This work discloses the tunable selectivity and offers an opportunity for rational design of efficient cobalt catalysts aiming for selective CO2 hydrogenation.