Oxygen Molecule Activation on Single-Atom Catalysts with Cu, Ag, and Au: A Cluster Model Study

–2 1 MSi O n n + (M = Au, Ag, Cu; n = 1, 2, 3) clusters were used as a cluster model to study the activation of oxygen molecules on single-atom catalysts. Structures of –2 1 MSi O n n + clusters were studied by density functional calculations with global optimization. For each n, the most stable structures are quite similar for different metal types, and the oxygen molecule prefers to be adsorbed onto M atoms. It is found that the activation degree of oxygen is higher on clusters with non-noble metal Cu than that of Ag or Au containing clusters, by comparing the changes of O-O bond length and vibrational frequency, natural charge population analysis, Fuzzy bond order analysis, and energy barriers of O 2 dissociation. CO oxidation was used as a probe reaction to study the reactivity of Cu-containing clusters, and it is found that the reactivity decreases with the increase of the size of silicon-oxygen clusters. Our results give a new aspect to understand the reaction mechanism of non-precious metal single-atom catalyst for oxygen activation with high efficiency.


Introduction
Oxidation reaction has traditionally been a hot topic in chemistry [1] [2] [3] [4] [ 5], in which the selection of a proper oxidant is of great importance [6] [7]. In recent years, with the development of the concept of green chemistry, people are more inclined to seek environmentally friendly, cheap, and efficient oxidants [8] [9] [10] [11]. Molecular oxygen (O 2 ), the environmentally friendly oxidizing agent is undoubtedly attractive, which is highly valued by chemists because of its cheapness [12] [13] [14], easy availability, and non-pollution of the product to the environment. However, O 2 is quite stable at room temperature due to the tightly bound O-O bond. So it is a key step to select a suitable catalyst to activate Many research groups have studied the mechanism of it. Safonova and coworkers [15] studied the oxygen activation mechanism on ceria-supported copper-oxo species using time-resolved X-ray absorption spectroscopy. It was found that in this system oxygen activation involves copper-oxo species in close interaction with ceria. Laura and co-workers [16] compared oxygen activation catalyzed by pure gold cluster Au 13 and Au 12 M (M = Ag, Cu, Ir) cluster, respectively.
It found that the activation energy barrier for the O 2 dissociation of Au 12 M cluster is lower nearly 1 eV than that of Au 13 cluster. Manzoor  proposed the concept of single-atom catalysts (SACs) in their work in 2011 [18]. Because the active center in single-atom catalysts only contains a single metal atom, the atomic utilization of precious metals has been raised to the limitation. Since then, SACs have been an important subject of research in the catalysis field [19]. Due to the complexity of actual catalysts, there are still many unrevealed mechanisms in surface catalytic reactions even for SAC systems. Clusters of finite atoms are easy to deal with experimentally and computationally, the study of reasonable cluster models can be used as a bottom-up strategy to understand complex systems and processes [20] [21]. The structure of single-atom catalysts is uniform, and the cluster composed of active single atom and several surrounding atoms can be used as a reasonable model to study the reaction mechanisms.
In the previous work of our group [22], we found that 3) clusters perform well in the catalytic activation of methane. In order to further study the reactivity of MSi O n n+ , which M could be Au, Ag, and Cu. For all the Cu-group metals, the outermost electron configuration is the same: a closed outermost d-shell and a single s-valence electron. Therefore, Cu, a non-precious element, containing clusters may exhibit similar properties to clusters with gold atoms [23]. Based on that, the adsorption and dissociation of O 2 on the clusters are discussed. It was found that Cu-containing clusters performed better than Au-or Ag-containing clusters. Therefore, further calculations on CO oxidation, which is used as a probe reaction, are only performed on Cu-containing clusters.

Calculation Method
All our DFT calculations were performed using Gaussian 09 program suite [24].
A Fortran code [25] based on a genetic algorithm and DFT calculations was developed to generate sufficient and reasonable initial structures of MSi O n n+ clusters, which have also been successfully applied to some other clusters [26]- [31]. TZVP basis sets [32] for C, O, and Si atoms and the D95V basis sets combined with the Stuttgart/Dresden relativistic effective core potentials (denoted as SDD in Gaussian software) for Au atom were adopted [33]. Diffusion functions are essential for anionic clusters. Therefore, we modified the original TZVP and SDD basis sets with additional diffusion functions according to the approach proposed by Truhlar et al. [34] [35]. In this approach, the smallest exponents of s and p functions in the original basis sets are divided by 3 and then used as the exponent of the additional diffusion function. TPSS functional [36] was used in this work, since it had been tested to perform well for Au-Si-O systems [22].
Both singlet and triplet states were considered for the oxygen adsorption systems. The spin-crossing points of triplet and singlet states during the oxygen adsorption process were located by utilizing sobMECP software [37]. Natural

MSi O + Clusters
The calculated most stable structures of  Table 1.   MSi O n n+ clusters are shown in Figure 2 and some data are listed in Table 2. The triplet states of the adsorption structures for each cluster are considered at first, since the ground state of O 2 is a triplet. On Au/Ag-containing clusters, O 2 takes end-on adsorption coordination to the metal atom, while on the Cu-containing clusters, O 2 takes side-on coordination to Cu. After the adsorption, the electronic state may be changed due to spin flip. Therefore, the singlet states are also considered. In all the singlet state structures, O 2 takes side-on coordination on metal atoms. A spin-crossover may occur in the potential energy surfaces of singlet and triplet states when O 2 adsorbs onto the metal atoms in The minimum energy crossing points (MECP) were determined by the sobMECP program combined with the Gaussian 09 package, and the conversion from the triple-state to the singlet state is shown in Figure 2. The triplet state is less stable than the singlet state for n = 1, 2 clusters, and the conversion from the triplet to the singlet state is quite easy, with low energy barriers (i.e., the relative energy of MECP with respect to the triplet state) as about 0.2 eV for Au-containing clusters and less than 0.1 eV for Ag/Cu-containing clusters. For the relatively larger clusters (n = 3), the triplet state is more stable and the conversion from the triplet to the singlet need additional energy, and the Journal of Materials Science and Chemical Engineering         with CO, as shown in Figure 5(b). Differently, the reaction path of CuSiO -O has the lowest energy barrier and therefore the highest oxidation ability toward CO. These three paths are compared with our previous work in which 1,2 AuVO + were used to catalyse the reaction of CO to CO 2 by O 2 . [39] The difference between the two works is CO attack site. In our work, CO prefers O site of the activated oxygen, while CO prefers mental site in 1,2 AuVO + system. Rate-determining step (RDS) barrier means a lot when study reaction. The RDS barrier of previous work is from 1.100 eV to 2.470 eV, while that is from 0.817 eV to 0.992 eV in this work.

Conclusion
The geometric and electronic properties of anionic