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

Abstract

Cu4, Ag4, and Au4 species adsorbed on an MgO(001) surface that exhibits neutral (Fs) and charged (Fs+) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas‐phase rhombic‐planar structure of the coinage metal tetramers is only moderately affected by adsorption. In the most stable surface configuration, the plane of the tetramers is oriented perpendicular to the MgO(001) surface; one metal atom is attached to an oxygen vacancy and another one is bound to a nearby surface oxygen anion. A very similar structural motif was recently found on defect‐free MgO(001), where two O2− ions serve as adsorption sites. Following the trend of the interactions with the regular MgO(001) surface, Au4 and Cu4 bind substantially stronger to Fs and Fs+ sites than Ag4. This stronger adsorption interaction at oxygen vacancies, in particular at Fs, is partly due to a notable accumulation of electron density on the adsorbates. We also examined the propensity of small supported metal species to aggregate to adsorbed di‐, tri‐ and tetramers. Furthermore, we demonstrated that core‐level ionization potentials offer the possibility for detecting experimentally supported metal tetramers and characterizing them structurally with the help of calculated data.

Structure of Ag Clusters Grown on Fs‐Defect Sites of an MgO(1 0 0) Surface

“…The ground‐state configuration c1 corresponds to the cluster plane oriented along the [1 0 0] direction and has C 2 v symmetry. Also in this case, a 45° rotation to the configuration c2 is accompanied by a bending of the cluster, so that one of the two lower coordinated vertices adheres to one oxygen ion of the surface nearest neighbor of the vacancy; this configuration has been previously found in reference 50. The transition between two analogous configurations c2 is represented by the saddle point c3 with C 2 v symmetry.…”

“…For Ag N with N =2–4, due to the small size of the metal clusters, we performed a biased DF search by locally optimizing a limited number of physically reasonable configurations, both in the gas phase and when adsorbed on the defected surface. The configurations of the adsorbed Ag N clusters with N =2, 4 have been reported in previous work49, 50 with the exception of configuration (c1) of the tetramer (see below). The present results are in qualitative (and often quantitative) agreement with these earlier investigations, and are briefly reported in the following to set the stage for larger clusters and the sake of completeness.…”

“…Cluster fragmentation : In analogy with previous work,49, 50, 77, 96 we have considered the energetics of processes corresponding to the fragmentation of the metal cluster into two pieces: the former still adsorbed on the defect, the latter adsorbed on the regular MgO(1 0 0) terrace. These processes can be important to understand the detrapping mechanism from this defect, and the process of Ostwald ripening of small clusters by bigger ones observed in MBE experiments at sufficiently high temperatures 31.…”

Density Functional Studies on the Structure and Reverse Hydrogen Spillover in Au6 Cluster Supported on Zeolite

“…Selected geometric parameters of the adsorbed clusters are summarized in Table 1. The structure of supported Au 6 is significantly deviated from its planar triangular structure observed in the gas phase, in contrast to the adsorption of Au 4 on MgO(001) surface where the geometry of the adsorbed cluster changes very little compared to their gas phase values [24,25]. As shown in Fig.…”

Characterization of optical spectra of interacting systems: Application to oxide‐supported metal clusters

“…The QM partition was described at the all-electron level using the linear combination of Gaussian-type orbitals fitting-functions density-functional method [25] and the exchange-correlation functional as suggested by Becke and Perdew (BP86) [29]. For further computational details and results on metal species adsorbed at MgO(001), the reader is referred to previous works [21][22][23][24].…”