Origin of Structure Sensitivity in CO 2 Reduction over Ni/CeO 2 : From Single Atoms to Clusters

Ni-based catalysts exhibit great potential for the reverse water–gas shift (RWGS) reaction to convert waste CO2 to valuable CO, however, minimizing the formation of undesirable CH4 and understanding the structure sensitivity remain challenges. Herein, Ni with sizes varying from isolated single atoms to clusters (∼1.3 nm) on CeO2 nanorods were synthesized, characterized, and tested for CO2 reduction at 400 °C. In contrast to structure insensitive of CO2 conversion on Ni with sizes >1 nm reported in literature, both CO2 conversion and CO/CH4 formations are structure sensitive from isolated single atom to cluster. That is, the CO2 conversion rate increases by ∼15 times, CO selectivity decreases while CH4 selectivity increases with increasing Ni size, with RWGS to CO (intrinsic rate of 10.0 molCO2·gNi–1·h–1) being the exclusive reaction on isolated Ni sites while sequential methanation to CH4 being the exclusive reaction on Ni clusters at long space time. In comparison to Ni clusters, the isolated Ni sites on CeO2 weaken the H2 dissociation and spillover ability as well as the strength of CO binding, resulting in increased Ea and H2 reaction order while exclusively catalyzing the RWGS. Infrared spectroscopy and density functional theory calculations suggested that the reaction on isolated Ni sites follows the carboxyl pathway with formate being a spectator. The structure sensitivity of CO2 reduction is originated from the strongly reduced H2 dissociation ability with decreasing Ni size from clusters to single atoms, which however appears little affected by Ni with sizes >1 nm. The results shed light into the structure sensitivity of CO2 reduction on Ni sizes and may offer guidance for the rational design of Ni-based catalysts to achieve selective and efficient RWGS reaction.