First-Principles Study on Adsorption and Decomposition of NOx on Mo (110) Surface

Based on the density functional theory, the adsorption and decomposition of NOx (x� 1, 2) on Mo (110) surface are studied with first-principles calculations. Results show that the stable structures of NO2/Mo (110) are MoNO2 (T, μ-N), MoNO2 (H, μ-N, O, O′), MoNO2 (S, η-O, O′), and MoNO2 (L, η-O, O′). +e corresponding adsorption energies for the structures are −3.83 eV, −3.40 eV, −2.81 eV, and −2.60 eV, respectively. Besides, the stable structures of NO/Mo (110) are MoNO (H, μ-N), MoNO (H, μN, O), and MoNO (H, η-N) with the corresponding adsorption energies of −3.75 eV, −3.57 eV, and −3.01 eV, respectively. N and O atoms are easily adsorbed at the hollow sites on Mo (110) surfaces, and their adsorption energies reach −7.02 eV and −7.70 eV, respectively. +e preferable decomposition process of MoNO2 (H, μ-N, O, O′) shows that the first and second deoxidation processes need to overcome energy barriers of 0.11 eV and 0.64 eV, respectively. All these findings indicate that NO2 is relatively easy to dissociate on Mo (110) surface.


Introduction
NO x (x � 1, 2) gas widely exists in the process of industrial exhaust and automobile exhaust emission. It is a major cause of air pollution. It does not only cause a series of environmental problems, such as photochemical pollution, ozone layer destruction, haze, and other pollution but also causes considerable harm to human health. In order to reduce the harm of NO x to humans and the environment, the removal and conversion of NO x (adsorption, decomposition, desorption, etc.) had always been a hot research topic. Presently, the mechanism of transition metal surface and NO x reaction is a hot topic in both experimental and theoretical simulation [1][2][3][4][5][6][7][8][9][10][11].
Using the technologies of electron stimulated desorption ion angular distribution (ESDIAD), electron energy loss spectroscopy (EELS), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED), the researchers analyzed the NO 2 [33], NO [34,35], and N 2 O [34] dissociative adsorption on Mo (100) and Mo (110). It indicates that NO 2 is easy to decompose to adsorbed NO + O at the temperature of 100∼150 K, while it is further decomposed into N 2 and O at the temperature of 250 K, showing Mo surface has the good catalytic ability for NO x removal and conversion.
However, the theoretical calculations on the role of NO x on transition metals surfaces and their alloys surfaces are still limited. Some basic problems in the experimental studies, such as the final adsorption structures and decomposition paths of NO x on the surfaces of transition metals and their alloys, have not been fully understood. For such microscopic processes, the experimental tools are not feasible. e firstprinciples calculation based on the density functional theory as a powerful tool can be used to investigate the reaction mechanisms of NO x with transition metals surfaces.
In this study, we report our findings about the adsorption and decomposition of NO x on Mo (110) surface with first-principles calculations. e goal of this study is to find out the most possibly dissociative process and the most stable adsorption structure of NO x on the Mo (110) surface.

Computational Method
e software used for the theoretical calculation is the Vienna ab initio simulation package (VASP) for total energy calculation based on the density functional theory [36][37][38].
e software package is a first-principles quantum mechanics and molecular dynamics composite package. It calculates the total energy and electronic structure with a plane wave as the basis function. All electron projector augmented wave (PAW) is used to deal with the interaction between ion real and valence electrons [39,40].
is is because the paw method is more accurate than other pseudopotentials such as ultra-soft pseudopotential (USPP), so the paw pseudopotential provided by the VASP is used in this paper. A Methfessel-Paxton [41] electronic energy smearing of 0.2 eV is used in the self-consistent calculations. For the exchange-correlation energy function, the Perdew-Burke-Enzerhoff (PBE) functional and generalized gradient approximation (GGA) is used. Spin polarization and the correction of dipole moment are considered in the calculation process [42]. e surface structure of Mo (110) is simulated by a slab normal to Z direction. e repeated slab is composed of 7 layers of molybdenum (Mo) atoms, with 4 layers for the substrate in which the positions of Mo atoms are fixed. e other remaining 3 layers of Mo atoms can relax their positions to optimize the total energy of the system when other species of atoms or molecules are adsorbed on the outer surfaces of the layers. A vacuum region with a thickness larger than 10Å is inserted between the adjacent crystal layers to avoid interference between the crystal layers. e periodic supercell p (2 × 2) of the system is used to calculate the adsorption of NO 2 , NO, N, and O. e selfconsistent calculation is carried out according to the irreducible k-point automatically generated by the Monkhorst-Pack scheme [41]. To optimize the total energy of the whole system, k-point grid sizes of (21 × 21 × 21) and (4 × 4 × 1) are used alternatively. In the calculation, the cutoff energy of plane wave expansion is taken as 400 eV. By changing the sampling point density and cut-off energy in K space to test the convergence, these settings are sufficient to ensure the accuracy of the calculation.
ere are four possible positions for the adsorption, namely top (T for short), long bridge (L for short), short bridge (S for short), and hole (H for short), as shown in Figure 1(a). e adsorption energy E ads is defined by the following expression In the abovementioned expression, E (adsorbate + slab) is the total energy of the optimized system with atoms adsorbed. E (slab) is the energy of the clean substrate surface and E (adsorbate) is for the gas phase adsorbed by the substrate. According to this definition, the adsorption energy is negative, which means that the process is exothermic. e surface energy σ is calculated using the equation where E relax , E unrelax , and E bulk represent the relaxed surface total energy, unrelaxed surface total energy, and the bulk total energy, respectively. A and N represent the surface area of the slab and the number of atoms in the cell, respectively.
To study the decomposition of NO x , the climbing image nudged elastic band (CI-NEB) [43,44] is used to search the transition state (TS). In this way, the path between the TSs is determined with the minimum energy. Practicably, eight images are set between the initial state (IS) and the final state (FS) for searching and locating the minimum energy paths (MEPs) of the decomposition reaction. (110) Surface. Before studying the NO x adsorption, let us study the structure of bulk Mo and clean Mo (110) surface at first. After the optimization in the calculation, the lattice parameter of the crystal molybdenum with body-centered cubic (BCC) structure is 3.146Å, which is in good agreement with the experimental results (∼3.15Å) [45,46] and other calculated data (∼3.16Å) [47,48]. e p (2 × 2)-layer crystal model of 7-layer Mo (110) is used to simulate the clean Mo (110) surface shown in Figure 1(b). It is found that the relaxation between the first and second layers, noted by Δd 12 , and the relaxation between the second and third layers, noted by Δd 23 , are −4.95% and 0.75%, respectively. e relaxation between the third and fourth layers is calculated to be Δd 34 � 0.26%, which is too small compared to Δd 12 and Δd 23 and can be ignored. e surface energy σ calculated for clean Mo (110) is 2.94 J/m 2 and the work function (W) is 4.57 eV, respectively. e data obtained above in this study are in good agreement with other reported values [49][50][51][52] and experimental measurements [53].

Gas-Phase NO 2 and NO Molecules.
e gas-phase NO 2 and NO molecules are simulated. After optimization, the N-O bond length of the NO molecule is 1.172Å. e bond length of the NO 2 molecule is 1.212Å, and the angle of O-NO is 133.8°. Table 1 lists bond length, bond angle, asymmetric stretching ] a , symmetric stretching ] s , and bending frequencies ] b . It can be seen that the calculated data in this paper are in good agreement with the experimental values.

N and O Atoms Adsorption on Mo (110) Surface.
Generally, NO 2 molecules are decomposed to be NO, N, and O. In order to study the possible decomposition and adsorption process of NO 2 on the surface of Mo (110), it is necessary to understand many possible stable adsorption structures including NO 2 /Mo (110), NO/Mo (110), N/Mo (110), and O/Mo (110). As discussed above, the four different adsorption sites, namely, T, S, L, and H, are considered. e symbols η and µ represent that the molecular plane of NOx is perpendicular and parallel to the substrate, respectively. e following definitions are the same.
At the same time, because of the symmetry of the molecular structure, it is also necessary to consider the possibility of multiple placements of adsorbed molecules during research. As shown in Figure 2, five adsorbed modes of NO 2 adsorption are considered. In this way, based on the detailed consideration of adsorption coordination and placement modes, all possible stable adsorption structures, including adsorption energies, stable adsorption sites, and adsorption geometries (bond length and bond angle) of NO, N, and O on Mo (110), are finally obtained.
First, let us consider the adsorption of N and O atoms on the Mo (110) surface. Figure 3 and Table 2 Table 2) between the N atom and substrate. It can be concluded that, the closer the distance between the N atom and substrate, the stronger the binding.
Let us study the adsorption of O atoms on the Mo (110) surface. It can be seen from Figure 4 and Table 3 that         Figure 5 and Table 4 that NO/Mo (110)

Deoxidation Process of NO 2 on Mo (110) Surface.
Next, we will focus on the deoxidation process of NO 2 on Mo (110) surface. Generally, the interaction between NO 2 and Mo (110) surface is carried out according to the following steps: Step 1: NO 2(gas) ⟶ NO 2 (ads) Step 2: NO 2 (ads) ⟶ NO (ads) + O (ads) Step 3: NO (ads) + O (ads) ⟶ N (ads) + 2O (ads) e climbing configuration elastic band method is used to study the decomposition process of NO 2 on Mo (110) surface. e CI-NEB method requires determining the initial and final states of the reaction. So in the deoxidation process of the first part (Step 2), the most stable structure of NO 2 /Mo (110) is selected as the initial state, that is, MoNO 2 (T,   Site Advances in Condensed Matter Physics at the hollow position, and O was placed at the supercell p (2 × 2). After calculations, it is found that only one structure of LM2-1 is stable (the specific structure as shown in Figure 7). Figure 7 shows the possible potential energy surface (PES) of the first step deoxidation process constructed.    For the second deoxidation process (Step 3), the initial state is the final state of the previous step, namely, LM2-1 and LM2-2. For the determination of the final state, it is also necessary to determine the coadsorption of N and 2O. After calculation, it is found that there are two stable structures, namely, LM3-1 and LM3-2 (as seen in Figure 8). Path 3: Figure 8(a) is the potential energy surface of the second constructed deoxidation process in step 3. It can be seen from the figure that the process needs to experience the transition state of TS3 from LM2-1 to LM3-1. e heat release in the whole process is 2.13 eV and the height across the potential barrier is 0.66 eV. In  Advances in Condensed Matter Physics this path, the N-O bond is broken and the atoms of oxygen and nitrogen are adsorbed at the hollow sites, respectively. Path 4: For the second deoxidation process of NO 2 on Mo (110) surface, NO(ads) + O(ads) ⟶ N(ads) + 2O(ads), another possible path from LM2-1 via the transition state of TS4 into LM3-2 is exothermic by 2.32 eV with the calculated barrier of 0.64 eV, as shown in Figure 8(b). In TS4, the N-O bond is elongated to be 1.83Å. After the TS4, the dissociating oxygen crosses the top site, short bridge site, and finally adsorbs at the hollow site.
In summary, the preferable reaction pathway of NO 2 (gas) + slab ⟶ LM1-2 ⟶ TS1 ⟶ LM2-1 ⟶ TS4 ⟶ LM3-2 is calculated to be exothermic by 5.11 eV, with the first, second deoxidation activation barriers of 0.11 eV, 0.64 eV, respectively. ese results have shown that NO 2 molecule can dissociate completely on the perfect Mo (110) surface, which is in agreement with the experiment [33,34]. It also indicates that Mo (1 1 0)

Conclusions
Based on the density functional theory, the adsorption and decomposition of NO x (x � 1, 2) on the Mo (110)  Data Availability e raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.