Charge transfer characteristics of Au adsorption on CeO<sub>2</sub>(111) surface

Au/CeO<sub>2</sub>(111), as an important catalyst system, has demonstrated excellent catalytic performances in a variety of fields such as the catalytic oxidation and the water-gas shift reactions. In order to reveal in depth the Au/CeO<sub>2</sub>(111) catalytic mechanism, especially to understand the interaction of the active components on an atomic scale, in this work, the adsorption properties on the Au/CeO<sub>2</sub>(111) surface are investigated by calculating the adsorption energy, differential charge density, Bader charge, and the density of states by using density functional theory (DFT+<i>U</i>). First, five adsorption sites of Au/CeO<sub>2</sub>(111) are identified in the planar region of CeO<sub>2</sub>(111), and the most stable adsorption configuration is found to be located at the bridging position between surface oxygen atoms (the oxygen-oxygen bridging site), which suggests that Au interacts more closely with the oxygen-oxygen bridging sites. Further, the differential charge density and Bader charge reveal the charge transfer mechanism in the adsorption process. Specifically, the Au atoms are oxidized into Au<sup>+</sup>, while the Ce<sup>4+</sup> ions in the second nearest neighbor of Au are reduced to Ce<sup>3+</sup>, and the adsorption process is accompanied by a charge transfer phenomenon. Au also exhibits a unique adsorption behavior in the CeO<sub>2</sub>(111) step-edge region, where a highly under-allocated environment is formed due to the decrease in the coordination number of atoms in the step edge, which enhances the adsorption of Au in a highly under-allocated environment. The adsorption of Au at the step edge is enhanced by the lower coordinated environment due to the reduced coordination number of the atoms at the step edge. By comparing four different types of step structures (Type I, Type II, Type II*, and Type III), it is found that the higher adsorption energy of Au at Type II* site and that at Type III site are both mainly due to the lower coordinated state of Ce atoms at these sites. Charge transfer is also particularly pronounced at the Type III sites. It is also accompanied by electron transferring from Au to Ce<sup>4+</sup> ions, making Type III the preferred adsorption site for Au atoms. By constructing a more comprehensive Au/CeO<sub>2</sub> model, this study breaks through the previous limitation of focusing only on planar adsorption and reveals the adsorption mechanism of Au/CeO<sub>2</sub> at the edge of the step, which provides a new perspective for understanding in depth the catalytic mechanism of Au/CeO<sub>2</sub>(111).