It is vital to study catalytic properties of atoms with different charge states at the atomic level. To achieve this, we fabricated three kinds of Ni catalysts: well-defifined cationic Ni atoms snugged in monolayered CuO, metallic Ni atoms supported on monolayered CuO and the Ni-O-Ni species which include the cationic Ni and metallic Ni. Here, we investigate catalytic mechanisms of Ni catalysts on monolayered CuO by carrying out scanning tunneling microscopy (STM) combined with noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. The first two Ni single-atom catalysts can realize the CO oxidation with different catalytic mechanisms. The Ni cations are chemically inert for CO adsorption even at 70 K but highly active toward O2 dissociation at room temperature. The adsorbed O atoms are active to oxidize incoming CO molecules from the gas phase into CO2, which follows the Eley−Rideal mechanism. The CuO-monolayer-supported metallic Ni atoms oxidize incoming CO molecules into CO<sub>2</sub> with its adjacent lattice O atom depleted, which follows the Mars−van Krevelen mechanism. The Ni-O-Ni catalyst with prominent cooperative effect of Ni cation and metallic Ni atom is efficient for the reduction of CO<sub>2</sub>. This study helps understand the chemistry of Ni catalysts with different charge states, which is of great importance in heterogeneous catalysis.

Impact of charge state on catalytic-pathway of Ni catalysts on monolayered CuO

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