Theoretical Investigation of Surface Oxidation of NiO/Au Core-Shell Catalyst∗

The surface oxidation of a Au@NiO core-shell catalyst is the key to its catalytic activity. In this density functional theory (DFT) study, we explain why the Au core promotes the oxidation of the NiO shell. The Au@NiO catalyst is represented by a slab model in which NiO(001) layers are placed on Au(001) substrates (NiO/Au). Ni vacancy and oxygen adsorption are examined as the surface oxidized state. Spin density analysis, as well as differential charge density analysis, shows that the oxidation of NiO/Au depletes the electrons in the Au–O antibonding orbitals at the interface, instead of creating holes in NiO. Thus, the highly-oxidized state of the NiO shell is stabilized by its interaction with the Au core. [DOI: 10.1380/ejssnt.2018.242]


I. INTRODUCTION
With high transparency and rigidity, polymethyl methacrylate (PMMA) is sometimes called the Queen of Plastics [1]. From an industrial point of view, it is very important to synthesize its monomer, methyl methacrylate (MMA), at low cost. In the usual scheme, aldehydes are oxidized to form carboxylic acids, which are reacted with alcohols to undergo dehydration condensation. A much more efficient process is aerobic oxidative esterification, in which oxidation and condensation occur simultaneously using oxygen from air. The reaction mechanism of aerobic oxidative esterification with Au@NiO core-shell catalyst is shown in Fig. 1. For aldehydes other than α, β-unsaturated aldehydes, several aerobic oxidative esterification methods have been developed, e.g., an organometallic chemical reaction with Pd [2,3]. However, it is very difficult to carry out the direct reaction on α, β-unsaturated aldehydes, including methacrolein, because side reactions such as CO desorption significantly decreases the selectivity of the target compound [1,4]. Currently, practical catalysts that are capable of the direct synthesis of MMA are limited to PdPb [1] and Au nanoparticles wrapped with NiO (Au@NiO) [5]. The Au@NiO core-shell catalysts also have appealing characteristics such as high durability because Au, which is easily sintered, is completely covered with NiO. Therefore, Au@NiO is an excellent practical catalyst for the MMA synthesis.
It has been experimentally shown that NiO, not Au, is exposed on the surface of the Au@NiO catalysts. In addition, the NiO shell is nonstoichiometric, containing more oxygen than Ni. This oxygen-rich state is important to the special catalytic activity of this catalyst [5]. It is known that the O-rich state is realized because of the Au core, but the underlying mechanism is unclear. It is also unclear how the O-rich state influences the ac- tivity and selectivity. In this study, we address the first problem on the basis of density functional theory (DFT) calculations for NiO/Au slab models. Note that it is a well established approach to adopt oxide/metal thin films as models for metal@oxide catalysts [6][7][8]. Several interesting studies were carried out about NiO/Au [9], FeO/Pt [10][11][12], ZrO 2 /Cu [13], MgO/Ag [14][15][16], and MgO/Mo [17].
According to experiments, the Au@NiO catalysts exhibit high activity when the diameter is 2-3 nm and when the molar ratio of Au : Ni is 2 : 8 [5]. These facts indicate that the core size and the shell thickness should be similar, close to 2 atomic layers each. Thus, we model Au@NiO by placing 2-3 layers of NiO on 2-3 layers of Au. As the oxidized states of NiO, slab models with a Ni vacancy or an O 2 adsorbate are considered.

II. COMPUTATIONAL DETAILS
DFT calculations using periodic slab models were carried out to analyze the reason for the oxidation of NiO on the Au@NiO surface. We used VASP [18][19][20] and Advance/PHASE [21] as the calculation package. PBE [22] was used as an exchange correlation functional, and a plane-wave basis set was used as the basis functions. In order to handle core electrons, we used the PAW method [23,24]. To correctly calculate NiO, which is a strongly correlated material, we used DFT + U [25]. The parameter of U − J = 5 eV was adopted [9]. For integration over the reciprocal space, we used a 3 × 3 × 1 k-point mesh with horizontal shifts, as well as Gaussian smearing with σ = 0.05 eV. NiO(001), Au(001), and NiO (001) Fig. 2. There was a sufficient distance in the z direction. In all models, Au and O are directly bonded at the (001) interface, as was found earlier [9].

A. Ni vacancy energy
First, we compared the stability of a Ni vacancy in unsupported NiO and NiO/Au. The relative stability was evaluated using the following formulae: (1) Figure 3 shows the calculation results that were obtained. It can be seen that the Ni vacancy is stabilized more stable than 1 eV by joining NiO to Au. Moreover, it was revealed that the Ni vacancy is more stable in the layer closer to Au. These results indicate that there are many more Ni vacancies in this catalyst than in ordinary NiO.
In particular, Ni vacancies are more likely to be found in the layer closer to the Au core. In addition, the opti- mized structure in Fig. 4(a) shows that when a Ni vacancy is present in the layer immediately below the surface, the O above the vacancy rises 0.3Å. This protruded O may contribute to the reaction as a strong base site.

B. O2 adsorption energy
We calculated the adsorption energy of O 2 on NiO/Au and that on unsupported NiO using the following formulae: Here, O 2 is adsorbed sideon to unsupported NiO, whereas O 2 bridges two Ni sites on NiO/Au. The adsorption energies were calculated to be −0.08 eV (to unsupported NiO) and −0.39 eV (to NiO/Au). Thus, this indicates that oxygen is more easily adsorbed to Au@NiO than it is to the usual NiO.

C. Electronic structure
In order to understand the reason for which the oxidation of NiO is stabilized only when it is supported on Au, we prepared spin-density distribution plots and differential charge-density plots. The latter were obtained by calculating ρ(NiO/Au) − ρ(NiO) − ρ(Au). Here, ρ(NiO/Au) is the charge density of a model obtained by optimizing the structure of a NiO/Au heterojunction. ρ(NiO) is the charge density obtained by extracting only NiO from the structure of NiO/Au, and fixing the coordinates of all nuclei. In the same way, ρ(Au) is the calculated charge density obtained by extracting only Au and fixing the coordinates of all the nuclei. Figure 5 shows the spindensity distribution plot of unsupported NiO with a Ni vacancy. When Ni 2+ is reduced and extracted as a neutral species, the p orbitals of the surrounding O are depleted of electronic charge. In other words, when Ni vacancies are present, the surrounding oxygen becomes radical-like. This result is consistent with earlier result [26]. Figure 6 shows the differential charge-density distribution plots around the Ni vacancy. From this figure, it is understood that the charge density of the orbitals of oxygen, which has become radical, is increased by the presence of Au. In contrast, it can be seen that the charge density of the orbital of Au and O in the z direction decreases at the junction interface. Because the Au-O distance at the interface decreases by 0.3Å when Ni vacancies are introduced, the orbital in which the charge decreases is considered as the antibonding orbital of O and Au. The system is stabilized by the charge transfer from Au-O antibonding orbitals to the radical orbitals generated by Ni vacancy. Figure 7 shows the spin density of bridging O 2 on unsupported NiO and that on NiO/Au. When the Au layer is absent, there are two radical orbitals of oxygen. However, when the Au layer is present, there is only one radical orbital. The O-O bond length of 1.34Å, together with the spin disappearance, indicates a super-oxo state created by charge transfer to the π * z orbital. Figure 8 shows the differential charge density for O 2 /NiO/Au. From this figure, it can be seen that the charge density of the π * z orbital of adsorbed oxygen increases when the Au layer is present. In addition, it is understood that the sources of The orbitals under the Fermi surface of gold and nickel oxide are shown in Fig. 9. These orbitals can be characterized as antibonding orbitals between Au and O. The removal of electronic charge from these orbitals results in a decrease in the charge density at the interface, as shown in Figs. 6 and 8. The oxidation of NiO is promoted because the interface orbital of NiO and Au supplies an electronic charge.
Au and NiO interact through Au/O contacts. The overlaps of orbitals between Au and O form bonding and anti-bonding orbitals. The d orbital of Au, which is the localized orbital of Au, is perfectly filled and the O anion has a closed shell. Because of repulsion between the closed shells, Au and NiO can bond only via weak interactions such as van der Waals interactions. However, when NiO/Au is oxidized, the Au-O anti-bonding orbitals are depleted instead of O of NiO. As a result, the Au-O distance decreases, and fewer holes are generated in NiO. In other words, the role of Au is to interact with O to form high-energy orbitals that can supply excess electronic charge. This makes the NiO surface easily oxidized. Thus, the two properties of Au are key to this effect: the closed d shell and the ability to bond directly with O of NiO.
Moreover, when a Ni vacancy is present in the layer one level below the surface, the oxygen above the vacancy rises 0.3Å above the surface, presumably forming a strong base site. Because Ni vacancies are preferentially created at the interface, this effect should be most pronounced when the NiO shell is about two layers in thickness. This is in agreement with the size at which Au@NiO exhibits high activity, as mentioned earlier. In addition, the bond length of O 2 (1.34Å) and the spin-density plot imply that adsorbed oxygen becomes a super-oxo species by the acceptance of an electron. We believe that the strong base site and the super-oxo species are key to the unique catalytic activity of Au@NiO. We are currently investigating a reaction mechanism that involve them by DFT.

IV. CONCLUSION
Using DFT calculation, we studied about the mechanism of the surface oxidation of Au@NiO core-shell catalyst. When Au and NiO are joined, it becomes easier for charges to flow from the orbitals of the junction interface, making it easier to oxidize NiO. As shown in Fig. 10, The source of the charge is the antibonding orbital of gold and oxygen. Thus, the origin of the facile oxidation of the NiO shell of Au@NiO can be traced to the ability of Au to bond directly to O and the complete filling of its d shell.