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 CO 2 to valuable CO, however, minimizing the formation of undesirable CH 4 and understanding the structure sensitivity remain challenges. Herein, Ni with sizes varying from isolated single atoms to clusters (∼1.3 nm) on CeO 2 nanorods were synthesized, characterized, and tested for CO 2 reduction at 400 °C. In contrast to structure insensitive of CO 2 conversion on Ni with sizes >1 nm reported in literature, both CO 2 conversion and CO/CH 4 formations are structure sensitive from isolated single atom to cluster. That is, the CO 2 conversion rate increases by ∼15 times, CO selectivity decreases while CH 4 selectivity increases with increasing Ni size, with RWGS to CO (intrinsic rate of 10.0 mol CO2 ·g Ni –1 ·h –1 ) being the exclusive reaction on isolated Ni sites while sequential methanation to CH 4 being the exclusive reaction on Ni clusters at long space time. In comparison to Ni clusters, the isolated Ni sites on CeO 2 weaken the H 2 dissociation and spillover ability as well as the strength of CO binding, resulting in increased E a and H 2 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 CO 2 reduction is originated from the strongly reduced H 2 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 CO 2 reduction on Ni sizes and may offer guidance for the rational design of Ni-based catalysts to achieve selective and efficient RWGS reaction.