Au Atoms and Dimers on the MgO(100) Surface:  A DFT Study of Nucleation at Defects

Abstract

The adsorption of Au atoms at the surface of MgO and the formation of Au dimers have been studied by means of first principles DFT supercell calculations. Au atoms have been adsorbed on flat MgO terraces and monatomic steps but also at point defects such as oxygen vacancies (F centers) or divacancies. Very low barriers for diffusion of Au atoms on the MgO(100) terraces have been found. Atom diffusion is stopped only at strong binding sites such as the F and F+ centers (adsorption energy E(a) = 3-4 eV), divacancies (E(a) = 2.3 eV), or, to less extent, steps (E(a) = 1.3 eV). The combination of two Au adatoms with formation of a dimer is accompanied by an energy gain, the dimer binding energy, E(b), between 2 and 2.4 eV for all sites considered, with the exception of the paramagnetic F+ center where the gain is negligible (0.3 eV). The dimerization energy on the surface is not too different from the bond strength of Au2 in the gas phase (2.32 eV). Thus, defects sites on MgO do not have a special role in promoting or demoting Au dimerization, while they are essential to trap the diffusing Au atoms or clusters. Calculations on Au3 formed on an F center show that the cluster is fluxional.

Models in Catalysis

“…6a). By a full analysis of the spectral angular dependence and the additional super-hyperfine interaction with O of an 17 O enriched MgO film, the Au atoms were shown to adsorb on top of oxygen ions in agreement with theoretical predictions [46][47][48]. DFT calculations showed, that these observations are indicative of polarization of the singly occupied 6s orbital of neutral Au atoms away from the MgO surface [40].…”

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…However, various DF results showed that the energetic preference of two atomic species to form homonuclear dimers (reactions 1) on isolated neutral F s defect sites of the MgOA C H T U N G T R E N N U N G (001) surface (Cu, [14,27,31] Ag, [14] Au, [14,32] Pd [33] ) is not significantly larger or even smaller than that for the dimerization involving only regular MgOA C H T U N G T R E N N U N G (001) sites (Table 5). For M 1 /O 2À + M 1 /F s , we calculated reaction energies of À138 (Cu), À152 (Ag) and À204 kJ mol À1 (Au).…”

Nucleation and growth on defect sites: experiment–theory comparison for Pd/MgO(001)

“…Since the only real difference between these studies and our experimental data set [6] is the method of surface preparation, it is now logical to explore whether there are other surface defects, and possibly other surface processes such as dimer mobility, that could explain these reproducible results. A recent theoretical study has made a serious start in this direction [37].…”