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

Authors: Yunmi Huang; Haijun Luo; Changkun Dong
Journal: Advances in Condensed Matter Physics
 (2021)

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## Abstract

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, μ1-N), MoNO2 (H, μ3-N, O, O′), MoNO2 (S, η2-O, O′), and MoNO2 (L, η2-O, O′). The 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, μ1-N), MoNO (H, μ2-N, O), and MoNO (H, η1-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. The preferable decomposition process of MoNO2 (H, μ3-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.


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## Body

## 1. Introduction

NOx (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 NOx to humans and the environment, the removal and conversion of NOx (adsorption, decomposition, desorption, etc.) had always been a hot research topic. Presently, the mechanism of transition metal surface and NOx reaction is a hot topic in both experimental and theoretical simulation [1–11].Molybdenum (Mo) and Mo-based catalysts exhibit excellent catalytic activity in many industrial areas, such as hydrodenitrogenation (HDN) [12–14] and hydrodesulfurization (HDS) of hydrocarbons, hydrogen evolution reaction (HER) [15–17], Fischer–Tropsch (F–T) synthesis [18, 19], and solid oxide fuel cell (SOFC) [20, 21]. Accordingly, a fundamental research focus on the interactions of nitrogen oxides with Mo and Mo-based surfaces is helpful to understand the reaction mechanism between them.Some experimental studies research on the interaction between NO2 and well-defined surfaces of Pt (111) [22–24], Ru (100) [25], Rh (111) [26], Ag (111) [27–29], Pd (111) [28, 30], and Au(111) [31, 32]. The results show that the NO2 can be completely dissociated on Rh (111), Pd (111), Pt (111), Ru (100), and Ag (111), and it is adsorbed in the molecular form on Au (111). At the same time, these experiment results indicate that the interaction between NO2 and metal surface can be generated by N atom or O atom.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 NO2 [33], NO [34, 35], and N2O [34] dissociative adsorption on Mo (100) and Mo (110). It indicates that NO2 is easy to decompose to adsorbed NO + O at the temperature of 100∼150 K, while it is further decomposed into N2 and O at the temperature of 250 K, showing Mo surface has the good catalytic ability for NOx removal and conversion.However, the theoretical calculations on the role of NOx 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 NOx on the surfaces of transition metals and their alloys, have not been fully understood. For such microscopic processes, the experimental tools are not feasible. The first-principles calculation based on the density functional theory as a powerful tool can be used to investigate the reaction mechanisms of NOx with transition metals surfaces.In this study, we report our findings about the adsorption and decomposition of NOx on Mo (110) surface with first-principles calculations. The goal of this study is to find out the most possibly dissociative process and the most stable adsorption structure of NOx on the Mo (110) surface.


## 2. Computational Method

The software used for the theoretical calculation is the Vienna ab initio simulation package (VASP) for total energy calculation based on the density functional theory [36–38]. The 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]. This 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].The surface structure of Mo (110) is simulated by a slab normal toZ direction. The 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. The 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 10 Å is inserted between the adjacent crystal layers to avoid interference between the crystal layers.The periodic supercellp (2 × 2) of the system is used to calculate the adsorption of NO2, NO, N, and O. The self-consistent 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 cut-off 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.There 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). The adsorption energy Eads is defined by the following expression(1)Eads=Eabsorbate+slab−Eslab+Eadsorbate.Figure 1
Model of Mo (110) surface (a) schematic diagram of possible adsorption coordination (b) schematic diagram of relaxation and other parameters.
(a)(b)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.The surface energyσ is calculated using the equation(2)σ=1AErelax−12Eunrelax+NEbulk,where Erelax, Eunrelax, and Ebulk 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 NOx, 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.


## 3. Results and Discussion

### 3.1. Bulk Mo and Clean Mo (110) Surface

Before studying the NOx 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 3.146 Å, which is in good agreement with the experimental results (∼3.15 Å) [45, 46] and other calculated data (∼3.16 Å) [47, 48]. The 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 Δd12, and the relaxation between the second and third layers, noted by Δd23, are −4.95% and 0.75%, respectively. The relaxation between the third and fourth layers is calculated to be Δd34 = 0.26%, which is too small compared to Δd12 and Δd23 and can be ignored. The surface energy σ calculated for clean Mo (110) is 2.94 J/m2 and the work function (W) is 4.57 eV, respectively. The data obtained above in this study are in good agreement with other reported values [49–52] and experimental measurements [53].


### 3.2. Gas-Phase NO2 and NO Molecules

The gas-phase NO2 and NO molecules are simulated. After optimization, the N–O bond length of the NO molecule is 1.172 Å. The bond length of the NO2 molecule is 1.212 Å, and the angle of O–NO is 133.8°. Table 1 lists bond length, bond angle, asymmetric stretching νa, symmetric stretching νs, and bending frequencies νb. It can be seen that the calculated data in this paper are in good agreement with the experimental values.Table 1
The structural parameters and vibrational frequencies of gas-phase NO2 and NO molecules.
NO2NOcalExp[2]calExp[5]r (Å)1.2121.1931.1721.150θ (deg)133.8134.1νa (cm−1)1686161819171904νs (cm−1)13481318νb (cm−1)733750


### 3.3. N and O Atoms Adsorption on Mo (110) Surface

Generally, NO2 molecules are decomposed to be NO, N, and O. In order to study the possible decomposition and adsorption process of NO2 on the surface of Mo (110), it is necessary to understand many possible stable adsorption structures including NO2/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. The symbols η and µ represent that the molecular plane of NOx is perpendicular and parallel to the substrate, respectively. The 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 Figure2, five adsorbed modes of NO2 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.Figure 2
Schematic diagram of possible adsorption states of adsorbed NO2 molecules.First, let us consider the adsorption of N and O atoms on the Mo (110) surface. Figure3 and Table 2 show that there are four adsorption structures of N adsorption on Mo (110) surface, namely, MoN (H, μ1-N), MoN (L, μ1-N), MoN (S, μ1-N), and MoN (T, η1-N). The corresponding adsorption energies are −7.02 eV, −6.98 eV, −6.16 eV, and −4.68 eV, respectively. It is found that the structure of MoN (H, μ1-N) is the most stable, and MoN (T, η1-N) is less than that of MoN (H, μ1-N). The structures of MoN (H, μ1-N) and MoN (L, μ1-N) have the shortest average distances (h in 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.Figure 3
Adsorption structures of N atom on Mo (110) surface (the upper and lower layers are a top view and side view, respectively, the same below).Table 2
Structural parameters of N atoms adsorption on Mo (110) surface.
SiteEads (eV)dN-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoN (H, μ1-N)−7.021.9661.250−1.81.10.1210.103MoN (L, μ1-N)−6.981.9491.251−2.11.20.0380.109MoN (S, μ1-N)−6.161.9061.376−2.11.10.1520.001MoN (T, η1-N)−4.681.7321.704−2.20.870.0280.108h is the average binding height with respect to the first Mo layer; Δdij is the average interlayer spacing relaxations between the i and the j layer; Z1 and Z2 represent the buckling in the first and second layer, respectively. The same definition is used in all subsequent tables.Let us study the adsorption of O atoms on the Mo (110) surface. It can be seen from Figure4 and Table 3 that MoO (H, μ1-O) is the most stable structure. The adsorption energies of the four adsorption structures MoO (H, μ1-O), MoO (L, μ1-O), MoO (S, μ1-O), and MoO (T, η1-O) are calculated to be −7.71 eV, −7.57 eV, −7.02 eV, and −6.32 eV, respectively. These data show that the adsorptions are strong. The adsorption of N and O atoms on the Mo (110) surface behaves similar to the adsorption of N and O atoms on the W (111) surface and the Fe (111) surface [1, 2].Figure 4
Adsorption structures of O atoms on the Mo (110) surface.Table 3
Structural parameters O atom adsorption on the Mo (110) surface.
SiteEads (eV)dO-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoO (H, μ1-O)−7.712.0251.189−1.80.670.1640.099MoO (L, μ1-O)−7.571.9911.200−1.80.710.0850.109MoO (S, μ1-O)−7.021.9601.464−2.70.730.1780.006MoO (T, η1-O)−6.321.7342.100−1.40.510.5470.057


### 3.4. Adsorption of NO on Mo (110) Surface

In order to study the dissociation of NO2 on the Mo (110) surface, it is very important to explore the interaction between NO and Mo (110). Similar to the abovementioned studies, four adsorption positions on the surface of Mo (110) are still used for NO adsorption. It can be seen from Figure 5 and Table 4 that NO/Mo (110) mainly has the following five structures: MoNO (H, μ1-N), MoNO (T, η1-O), MoNO (T, η1-N), MoNO (H, μ2-N, O), and MoNO (H, η1-N), where MoNO (H, μ1-N) is the most stable structure, and its adsorption energy is −3.75 eV. The second and third stable structures are MoNO (H, μ2-N, O) and MoNO (H, η1-N) with corresponding adsorption energies of −3.57 eV and −3.01 eV, respectively. From these structures, it is found that NO acts mainly through N atoms with Mo (110) surface.Figure 5
Adsorption structures of NO molecule on the Mo (110) surface.Table 4
Structural parameters of NO adsorption on the Mo (110) surface.
SiteEads (eV)dN-O (Å)θ (deg)dN-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoNO (H, μ1-N)−3.751.45715.31.9921.226−1.51.10.0360.128MoNO (T, η1-O)−0.921.20190.02.0022.189−3.51.20.2790.038MoNO (T, η1-N)−2.781.19490.01.8782.040−2.91.10.2700.051MoNO (H,μ2-N,O)−3.571.39432.72.0081.260−0.590.630.2370.136MoNO (H, η1-N)−3.011.25781.92.1111.338−3.01.10.1220.058θ is the include angel of NO-surf.


### 3.5. Adsorption of NO2 on Mo (110) Surface

NO2 molecule has a curved structure, and the inner angle of O–N–O is 133.8°. Calculations show that NO2 takes effect mainly through N atoms on the Mo (110) surface, which is similar to the adsorption of NO. According to Figure 6 and Table 5, NO2/Mo (110) mainly has the following stable structures: MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), MoNO2 (S, η2-O, O′), MoNO2 (L, η2-O, O′), MoNO2 (T, η1-O), MoNO2 (T, η2-O, O′), MoNO2 (L, η1-N), and MoNO2 (S, η1-N). As can be seen from Figure 6, MoNO2 (T, μ1-N) and MoNO2 (H, μ3-N, O, O′) are the most and second stable structures, and the adsorption energies are −3.83 eV and −3.40 eV, respectively. In both structures, NO2 is obliquely adsorbed on the surface. MoNO2 (S, η2-O, O′), and MoNO2 (L, η2-O, O′) are also two stable structures with corresponding adsorption energies of −2.81 eV and −2.60 eV, respectively.Figure 6
Adsorption structure of NO2 molecule on the Mo (110) surface.Table 5
Structural parameters of NO2 adsorption on the Mo (110) surface.
SiteEads (eV)dN-O (Å)φ (deg)θ (deg)d (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoNO2 (T,μ1-N)−3.831.141107.533.62.1811.441−0.621.020.1200.014MoNO2 (H,μ3-N,O,O′)−3.401.372113.841.32.1131.587−1.060.560.2350.088MoNO2 (S,η2-O,O′)−2.811.292111.289.82.0692.1340.811.090.2080.040MoNO2 (L, η2-O,O′)−2.601.298114.490.03.1292.127−1.950.370.2070.106MoNO2 (T, η1-O)−1.741.173111.690.01.8752.177−1.191.040.4360.059MoNO2 (T, η2-O,O′)−2.371.320106.290.02.6701.998−3.61.100.2160.031MoNO2 (L, η1-N)−2.061.310120.390.02.2201.612−2.020.990.2020.099MoNO2 (S,η1-N)−2.231.404124.690.02.0731.2550.0250.970.1710.042φ is the included angle of O1-N-O2, θ is the inner angle of NO2-surf.


### 3.6. Deoxidation Process of NO2 on Mo (110) Surface

Next, we will focus on the deoxidation process of NO2 on Mo (110) surface. Generally, the interaction between NO2 and Mo (110) surface is carried out according to the following steps:Step 1: NO2(gas) ⟶ NO2 (ads)Step 2: NO2 (ads) ⟶ NO(ads) + O(ads)Step 3: NO(ads) + O(ads) ⟶ N(ads) + 2O(ads)The climbing configuration elastic band method is used to study the decomposition process of NO2 on Mo (110) surface. The 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 NO2/Mo (110) is selected as the initial state, that is, MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), and MoNO2 (S, η2-O, O′) as the initial state of the first step deoxidation process, and the three structures are named LM1-1, LM1-2, LM1-3, respectively. In order to determine the structure of the final state NO(ads) + O(ads), it can be seen from the adsorption of NO and O on Mo (110) surface that they are the most stable structures at the hollow position. Therefore, NO was placed 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.Path 1: As shown in Figure7(a), the most possible pathway of the first deoxidation process of NO2 on Mo (110) surface starts from LM1-2 to LM2-1 via the transition states TS1 and TS1′. The heat release in the whole process is up to 2.79 eV. During this process, it needs to cross two transition states with energy barriers of 0.11 eV and 0.50 eV, respectively. The activated N–O bond length changes from 1.372 Å in the initial state to 1.419 Å in TS1, then extend to 2.709 Å in TS1′ and finally to be 2.741 Å in final state LM2-1. The bond angle of O–N–O changes from 113.7° in the initial state to 110.39° in TS1. The activated oxygen atom gradually crosses the short bridge position and finally reaches the hollow position.Path 2: Another possible pathway of the first deoxidation process is explored from LM1-3 to LM2-1. The heat release in the whole process reaches 3.38 eV. It needs to cross the transition states TS2 and TS2′. The potential barriers are 0.76 eV and 0.73 eV, respectively. The activated N–O bond length changes from 1.372 Å in the initial state to 1.419 Å in TS2, then extend to 2.709 Å in TS2′ and finally to be 2.741 Å in final state LM2-1. The bond angle of O–N–O changes from 113.2° in the initial state to 113.1° in TS2. The activated oxygen atom gradually moves from the top position to the hollow position.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 Figure8).Path 3: Figure8(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. The heat release in the whole process is 2.13 eV and the height across the potential barrier is 0.66 eV. In 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 NO2 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 1.83 Å. After the TS4, the dissociating oxygen crosses the top site, short bridge site, and finally adsorbs at the hollow site.Figure 7
Potential energy surface of NO2 molecule in the first step deoxidation process of Mo (110).
(a)(b)Figure 8
Potential energy surface of NO2 molecule in the second step deoxidation process of Mo (110).
(a)(b)In summary, the preferable reaction pathway of NO2(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. These results have shown that NO2 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) surface exhibits good catalytic activity to decompose NOx as well as other gas, such as H2S [54, 55].


## 3.1. Bulk Mo and Clean Mo (110) Surface

Before studying the NOx 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 3.146 Å, which is in good agreement with the experimental results (∼3.15 Å) [45, 46] and other calculated data (∼3.16 Å) [47, 48]. The 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 Δd12, and the relaxation between the second and third layers, noted by Δd23, are −4.95% and 0.75%, respectively. The relaxation between the third and fourth layers is calculated to be Δd34 = 0.26%, which is too small compared to Δd12 and Δd23 and can be ignored. The surface energy σ calculated for clean Mo (110) is 2.94 J/m2 and the work function (W) is 4.57 eV, respectively. The data obtained above in this study are in good agreement with other reported values [49–52] and experimental measurements [53].


## 3.2. Gas-Phase NO2 and NO Molecules

The gas-phase NO2 and NO molecules are simulated. After optimization, the N–O bond length of the NO molecule is 1.172 Å. The bond length of the NO2 molecule is 1.212 Å, and the angle of O–NO is 133.8°. Table 1 lists bond length, bond angle, asymmetric stretching νa, symmetric stretching νs, and bending frequencies νb. It can be seen that the calculated data in this paper are in good agreement with the experimental values.Table 1
The structural parameters and vibrational frequencies of gas-phase NO2 and NO molecules.
NO2NOcalExp[2]calExp[5]r (Å)1.2121.1931.1721.150θ (deg)133.8134.1νa (cm−1)1686161819171904νs (cm−1)13481318νb (cm−1)733750


## 3.3. N and O Atoms Adsorption on Mo (110) Surface

Generally, NO2 molecules are decomposed to be NO, N, and O. In order to study the possible decomposition and adsorption process of NO2 on the surface of Mo (110), it is necessary to understand many possible stable adsorption structures including NO2/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. The symbols η and µ represent that the molecular plane of NOx is perpendicular and parallel to the substrate, respectively. The 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 Figure2, five adsorbed modes of NO2 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.Figure 2
Schematic diagram of possible adsorption states of adsorbed NO2 molecules.First, let us consider the adsorption of N and O atoms on the Mo (110) surface. Figure3 and Table 2 show that there are four adsorption structures of N adsorption on Mo (110) surface, namely, MoN (H, μ1-N), MoN (L, μ1-N), MoN (S, μ1-N), and MoN (T, η1-N). The corresponding adsorption energies are −7.02 eV, −6.98 eV, −6.16 eV, and −4.68 eV, respectively. It is found that the structure of MoN (H, μ1-N) is the most stable, and MoN (T, η1-N) is less than that of MoN (H, μ1-N). The structures of MoN (H, μ1-N) and MoN (L, μ1-N) have the shortest average distances (h in 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.Figure 3
Adsorption structures of N atom on Mo (110) surface (the upper and lower layers are a top view and side view, respectively, the same below).Table 2
Structural parameters of N atoms adsorption on Mo (110) surface.
SiteEads (eV)dN-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoN (H, μ1-N)−7.021.9661.250−1.81.10.1210.103MoN (L, μ1-N)−6.981.9491.251−2.11.20.0380.109MoN (S, μ1-N)−6.161.9061.376−2.11.10.1520.001MoN (T, η1-N)−4.681.7321.704−2.20.870.0280.108h is the average binding height with respect to the first Mo layer; Δdij is the average interlayer spacing relaxations between the i and the j layer; Z1 and Z2 represent the buckling in the first and second layer, respectively. The same definition is used in all subsequent tables.Let us study the adsorption of O atoms on the Mo (110) surface. It can be seen from Figure4 and Table 3 that MoO (H, μ1-O) is the most stable structure. The adsorption energies of the four adsorption structures MoO (H, μ1-O), MoO (L, μ1-O), MoO (S, μ1-O), and MoO (T, η1-O) are calculated to be −7.71 eV, −7.57 eV, −7.02 eV, and −6.32 eV, respectively. These data show that the adsorptions are strong. The adsorption of N and O atoms on the Mo (110) surface behaves similar to the adsorption of N and O atoms on the W (111) surface and the Fe (111) surface [1, 2].Figure 4
Adsorption structures of O atoms on the Mo (110) surface.Table 3
Structural parameters O atom adsorption on the Mo (110) surface.
SiteEads (eV)dO-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoO (H, μ1-O)−7.712.0251.189−1.80.670.1640.099MoO (L, μ1-O)−7.571.9911.200−1.80.710.0850.109MoO (S, μ1-O)−7.021.9601.464−2.70.730.1780.006MoO (T, η1-O)−6.321.7342.100−1.40.510.5470.057


## 3.4. Adsorption of NO on Mo (110) Surface

In order to study the dissociation of NO2 on the Mo (110) surface, it is very important to explore the interaction between NO and Mo (110). Similar to the abovementioned studies, four adsorption positions on the surface of Mo (110) are still used for NO adsorption. It can be seen from Figure 5 and Table 4 that NO/Mo (110) mainly has the following five structures: MoNO (H, μ1-N), MoNO (T, η1-O), MoNO (T, η1-N), MoNO (H, μ2-N, O), and MoNO (H, η1-N), where MoNO (H, μ1-N) is the most stable structure, and its adsorption energy is −3.75 eV. The second and third stable structures are MoNO (H, μ2-N, O) and MoNO (H, η1-N) with corresponding adsorption energies of −3.57 eV and −3.01 eV, respectively. From these structures, it is found that NO acts mainly through N atoms with Mo (110) surface.Figure 5
Adsorption structures of NO molecule on the Mo (110) surface.Table 4
Structural parameters of NO adsorption on the Mo (110) surface.
SiteEads (eV)dN-O (Å)θ (deg)dN-Mo (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoNO (H, μ1-N)−3.751.45715.31.9921.226−1.51.10.0360.128MoNO (T, η1-O)−0.921.20190.02.0022.189−3.51.20.2790.038MoNO (T, η1-N)−2.781.19490.01.8782.040−2.91.10.2700.051MoNO (H,μ2-N,O)−3.571.39432.72.0081.260−0.590.630.2370.136MoNO (H, η1-N)−3.011.25781.92.1111.338−3.01.10.1220.058θ is the include angel of NO-surf.


## 3.5. Adsorption of NO2 on Mo (110) Surface

NO2 molecule has a curved structure, and the inner angle of O–N–O is 133.8°. Calculations show that NO2 takes effect mainly through N atoms on the Mo (110) surface, which is similar to the adsorption of NO. According to Figure 6 and Table 5, NO2/Mo (110) mainly has the following stable structures: MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), MoNO2 (S, η2-O, O′), MoNO2 (L, η2-O, O′), MoNO2 (T, η1-O), MoNO2 (T, η2-O, O′), MoNO2 (L, η1-N), and MoNO2 (S, η1-N). As can be seen from Figure 6, MoNO2 (T, μ1-N) and MoNO2 (H, μ3-N, O, O′) are the most and second stable structures, and the adsorption energies are −3.83 eV and −3.40 eV, respectively. In both structures, NO2 is obliquely adsorbed on the surface. MoNO2 (S, η2-O, O′), and MoNO2 (L, η2-O, O′) are also two stable structures with corresponding adsorption energies of −2.81 eV and −2.60 eV, respectively.Figure 6
Adsorption structure of NO2 molecule on the Mo (110) surface.Table 5
Structural parameters of NO2 adsorption on the Mo (110) surface.
SiteEads (eV)dN-O (Å)φ (deg)θ (deg)d (Å)h (Å)Δd12/d0 (%)Δd23/d0 (%)z1 (Å)z2 (Å)MoNO2 (T,μ1-N)−3.831.141107.533.62.1811.441−0.621.020.1200.014MoNO2 (H,μ3-N,O,O′)−3.401.372113.841.32.1131.587−1.060.560.2350.088MoNO2 (S,η2-O,O′)−2.811.292111.289.82.0692.1340.811.090.2080.040MoNO2 (L, η2-O,O′)−2.601.298114.490.03.1292.127−1.950.370.2070.106MoNO2 (T, η1-O)−1.741.173111.690.01.8752.177−1.191.040.4360.059MoNO2 (T, η2-O,O′)−2.371.320106.290.02.6701.998−3.61.100.2160.031MoNO2 (L, η1-N)−2.061.310120.390.02.2201.612−2.020.990.2020.099MoNO2 (S,η1-N)−2.231.404124.690.02.0731.2550.0250.970.1710.042φ is the included angle of O1-N-O2, θ is the inner angle of NO2-surf.


## 3.6. Deoxidation Process of NO2 on Mo (110) Surface

Next, we will focus on the deoxidation process of NO2 on Mo (110) surface. Generally, the interaction between NO2 and Mo (110) surface is carried out according to the following steps:Step 1: NO2(gas) ⟶ NO2 (ads)Step 2: NO2 (ads) ⟶ NO(ads) + O(ads)Step 3: NO(ads) + O(ads) ⟶ N(ads) + 2O(ads)The climbing configuration elastic band method is used to study the decomposition process of NO2 on Mo (110) surface. The 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 NO2/Mo (110) is selected as the initial state, that is, MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), and MoNO2 (S, η2-O, O′) as the initial state of the first step deoxidation process, and the three structures are named LM1-1, LM1-2, LM1-3, respectively. In order to determine the structure of the final state NO(ads) + O(ads), it can be seen from the adsorption of NO and O on Mo (110) surface that they are the most stable structures at the hollow position. Therefore, NO was placed 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.Path 1: As shown in Figure7(a), the most possible pathway of the first deoxidation process of NO2 on Mo (110) surface starts from LM1-2 to LM2-1 via the transition states TS1 and TS1′. The heat release in the whole process is up to 2.79 eV. During this process, it needs to cross two transition states with energy barriers of 0.11 eV and 0.50 eV, respectively. The activated N–O bond length changes from 1.372 Å in the initial state to 1.419 Å in TS1, then extend to 2.709 Å in TS1′ and finally to be 2.741 Å in final state LM2-1. The bond angle of O–N–O changes from 113.7° in the initial state to 110.39° in TS1. The activated oxygen atom gradually crosses the short bridge position and finally reaches the hollow position.Path 2: Another possible pathway of the first deoxidation process is explored from LM1-3 to LM2-1. The heat release in the whole process reaches 3.38 eV. It needs to cross the transition states TS2 and TS2′. The potential barriers are 0.76 eV and 0.73 eV, respectively. The activated N–O bond length changes from 1.372 Å in the initial state to 1.419 Å in TS2, then extend to 2.709 Å in TS2′ and finally to be 2.741 Å in final state LM2-1. The bond angle of O–N–O changes from 113.2° in the initial state to 113.1° in TS2. The activated oxygen atom gradually moves from the top position to the hollow position.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 Figure8).Path 3: Figure8(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. The heat release in the whole process is 2.13 eV and the height across the potential barrier is 0.66 eV. In 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 NO2 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 1.83 Å. After the TS4, the dissociating oxygen crosses the top site, short bridge site, and finally adsorbs at the hollow site.Figure 7
Potential energy surface of NO2 molecule in the first step deoxidation process of Mo (110).
(a)(b)Figure 8
Potential energy surface of NO2 molecule in the second step deoxidation process of Mo (110).
(a)(b)In summary, the preferable reaction pathway of NO2(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. These results have shown that NO2 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) surface exhibits good catalytic activity to decompose NOx as well as other gas, such as H2S [54, 55].


## 4. Conclusions

Based on the density functional theory, the adsorption and decomposition of NOx (x = 1, 2) on the Mo (110) surface were calculated by the first principle. The results show that the stable structures of NO2/Mo (110) coordination are MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), MoNO2 (S, η2-O, O′), and MoNO2 (L, η2-O, O′). The corresponding adsorption energies are −3.83 eV, −3.40 eV, −2.81 eV, and −2.60 eV, respectively. The stable structure of NO/Mo (110) are MoNO (H, μ1-N), MoNO (H, μ2-N, O), and MoNO (H, η1-N) and the corresponding adsorption energies are −3.75 eV, −3.57 eV, and −3.01 eV, respectively. N and O are easily adsorbed at the hollow site of the Mo (110) surface, and their adsorption energies are −7.02 eV and −7.70 eV. The study on the preferable decomposition process of NOx (x = 1, 2) on Mo (110) shows that the potential barriers of the first and second deoxidation processes of MoNO2 (H, μ3-N, O, O′) are 0.11 eV and 0.64 eV, respectively. All these findings indicate that the Mo (110) surface exhibits good catalytic activity to decompose NOx.



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*Source: 3264737-2021-12-30.xml*