Study of the NH₃-SCR Mechanism on LaMnO₃ Surfaces Based on the DFT Method

Abstract

LaMnO₃ with perovskite structure is a SCR de-NO_x catalyst with good performance at low temperatures. In this paper, the SCR reaction process on the 010 surface of LaMnO₃ catalyst was studied by DFT method, to guide the development of catalysts and their effective application. The results obtained through research indicate that both E-R and L-H mechanisms exist on the catalyst surface. The NH₃ molecule can be absorbed on L acid and then oxidized by lattice oxygen to form NH₂. Then, NH₂ can react with the NO molecule to form NH₂NO and decompose to N₂ and H₂O. The NH₃ can also be absorbed with hydroxyl to form NH₄⁺, it can also react with NO to form NH₂NO and then decompose. The NH₄⁺ also can react with NO₃⁻ which is formed by NO oxidized when O₂ is present, to participate in the rapid SCR process.

1. Introduction

NO_x (Nitrogen oxide) is a common air pollutant widely present in nature and human industrial activity [1]. It not only does harm to people health, but also causes great damage to the ecological environment [2]. Due to the extensive combustion of fossil energy such as coal, a large amount of NO_x is discharged into the atmosphere every year [3]. According to statistics, more than 75% of NOx in the atmosphere is produced by human activities, and this value is increasing year by year [4]. To meet the challenge of NO_x emissions, many countries and international regions have limited NO_x emissions and proposed various de-NO_x technologies [5]. SCR (selective catalytic reduction) is one of the most widely used de-NO_x technologies. It is mature, stable, and efficient with activity reaching 80–95%, and can meet de-NO_x requirements under different conditions in various production contexts [6].

SCR refers to the process of reducing NO to N₂ with reducing gases such as NH₃ and CO under the action of a catalyst; the reaction equation is known to be $4\mathrm{NO}+4{\mathrm{NH}}_3+{\mathrm{O}}_2\to 4{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$ [7]. The catalyst is the core component, and the quality of catalyst directly determines the de-NO_x performance. At present, V₂O₅-WO₃/TiO₂ catalysts are widely used in commercial activities. The active temperature window of this kind of catalyst is 300–400 °C, which provides high de-NO_x efficiency and stability [8]. However, the high active temperature is not suitable for de-NO_x in the flue-gas tail, and the crystal form of anatase TiO₂ is unstable, which hinders its regeneration application. Furthermore, V₂O₅ has biological toxicity [9]. Therefore, it is imperative to actively develop environmentally friendly de-NO_x catalysts with high efficiency at low temperatures.

Perovskite materials have the advantages of good structural stability, high-temperature sintering resistance, strong chemical adsorption capacity, rich acid–base sites, easy regulation of redox properties, rich reserves, and low price [10]. To date, many researchers have studied the structure–activity relationship of perovskite catalysts and their SCR properties [11]. Perovskite is the general name given to oxides with stable cubic ABO₃ structure, the A-site ion is a rare earth metal or alkaline earth metal cation with large radius and the B-site ion is a transition metal cation [12].

LaMnO₃ perovskite oxides, in which La as the A site plays a supporting role while Mn as the B site is the main active site, have strong stability and good SCR activity. They have great potential as low-temperature catalysts, which has attracted the attention of many scholars [13]. Zhang tested the de-NO_x performance of LaMnO₃ catalysts, and the results showed that LaMnO₃ perovskite exhibited excellent catalytic activity in the SCR of NO using NH₃, and the NO conversion could reach 78% at 250 °C, increasing along with the temperature in the range of 100–300 °C [14]. Guo et al. prepared LaMnO₃ catalysts and carried out modification and activity testing. The results showed that the modified catalyst demonstrated excellent activity with 90% NO_x conversion at 135 °C and more than 90% NOx conversion in the temperature window of 135–260 °C [15]. Wang et al. prepared LaMnO₃ supported iron ore catalysts by citric acid complex impregnation and tested SCR activity, and the results showed that the NO_x conversion efficiency was up to 98% at 180 °C [16]. These results fully demonstrate the good catalytic performance of SCR. However, the mechanism of the NO_x conversion reaction with the LaMnO₃ catalyst is still not clear yet.

Understanding the reaction mechanism on the catalyst surface is of great significance to the development of the SCR catalyst. In general, there are two kinds of NH₃-SCR reaction mechanisms. One is the L-H (Langmuir–Hinshelwood) mechanism, in which NH₃ and NO are adsorbed on the catalyst surface, and react to form products, and the other is E-R (Eley–Rideal) mechanism in which NH₃ is mainly adsorbed on the catalyst surface and then reacts with gaseous or weakly adsorbed NO [17,18]. Yang et al. studied the mechanism of a Fe-Mn mixed oxide catalyst with a spinel structure by in situ drift experiment, and reported that both the L-H mechanism and the E-R mechanism were active on the catalyst surface, the E-R mechanism dominant at high temperature and the L-H mechanism at low temperature [19]. Zhang et al. studied Mn based perovskite catalysts with Bi and La at position A, and found that the perovskite catalyst had strong Lewis acid (L-acid) and rich concentration of surface oxygen, which may have been the reason for the strong catalytic activity [20]. Further study by Zhang et al. on the mechanism of the LaMnO₃ catalyst showed that the excellent de-NO_x performance of LaMnO₃ lies in the large NH₃ adsorption capacity and rich nitrate or nitrite species. They proposed an L-H mechanism in which gaseous NH₃ and NO are adsorbed in the form of NH₄⁺ ions and nitrosyl species, respectively, and then oxidized to nitrite and nitrate. Then, NH₄⁺ reacts with active nitrite to produce unstable ammonium nitrite, and finally N₂ [14]. However, some researchers believe that the two mechanisms exist at the same time [21]. The study of LaBO₃ perovskite material (B = Mn, Ni, Fe, CO) by Shi et al. showed that its catalytic activity has no significant relationship with its reduction ability, but follows the same law as the adsorption capacity of NH₃ [22]. It can be considered that the adsorption capacity of NH₃ plays an important role in NH₃-SCR. However, it should be noted that in the field of perovskite NH₃-SCR de-NO_x, the mechanism research is still relatively simple, the catalytic processes of perovskite materials in different systems are different, and no consensus has been reached so far [23].

With the development of theoretical chemistry and computer technology, first principles and density function theory (DFT) have been introduced in the field of catalyst mechanism research by various researchers. Soyer et al. [24] used a quantum chemical method to study the mechanism on the V₂O₅ catalyst surface. Their results showed that the B acid site on the catalyst surface is the main reaction center, NH₃ combines to form NH₄⁺, and reacts with NO to finally produce N₂ and H₂O. Yang et al. [25] studied the reaction pathway on the CeO₂/TiO₂ catalyst surface by applying DFT, and their results showed that NH₃ is first coordinated and adsorbed on the surface L acid site in molecular form, then activated by dehydrogenation to generate NH₂, and then reacts with NO to generate N₂ and H₂O, following the E-R mechanism. Li et al. [26] studied the reaction on the Mn/γ-Al₂O₃ surface, the results showing that there were L acid and B acid sites on the catalyst surface at the same time, and that NH₃ can be adsorbed on the L acid site, and then dehydrogenated to NH₂. The NH₂ fragment reacts with NO to produce NH₂NO which is then decomposed to produce N₂ and H₂O. At the same time, NH₃ is also adsorbed on the B acid site to form an NH₄⁺ cation, then it can react with NO₂ to form NH₄NO₂, and finally decompose to form N₂ and H₂O, which shows that there are both E-R and L-H mechanisms on the Mn/γ-Al₂O₃ surface. Anstrom et al. [27] also studied the mechanism on the surface of vanadium oxide catalyst and reported that NH₂NO is the main intermediate product. These experiments fully prove the correctness of the first principle and the DFT calculation. At present, there is no definite conclusion on which mechanism is dominant on different catalyst surfaces, but the results obtained through DFT can help to a certain extent reveal it and make up for the deficiency of calculations.

Scholars are also conducting research on the first principle and DFT of perovskite catalyst. Zhang et al. calculated the atomic charges of BiMnO₃ and LaMnO₃ through DFT and found that the smaller O charge and higher electronegativity brought stronger acidity, promoted the adsorption and activation of NH₃ on the surface, and increased low-temperature NH₃-SCR activity [20]. Yan et al. studied the CO-SCR reaction process on the surface of LaMnO₃ catalyst, obtained the basic steps, activation energy barrier, and other parameters, and determined the surface reaction path [28]. Meanwhile, DFT-based study on NH₃-SCR surface reaction on LaMnO₃ catalyst has not been reported.

In conclusion, LaMnO₃ catalyst with perovskite structure is a good low-temperature SCR de-NO_x catalyst, but the research on its mechanism is relatively shallow. Using a mature and reliable DFT method to study the reaction process is helpful to reveal its surface reaction mechanism and guide the development and application of low-temperature de-NO_x catalysts. Therefore, in this study, the DFT method was applied to study the surface reaction mechanism, and parameters including the molecular adsorption process, elementary reaction steps, and activation energy were measured. The adsorption behavior and reaction process were studied, and the most probable reaction path on the catalyst surface was identified.

2. Computational Methods and Models

In this paper, the LaMnO₃ catalyst as research object is established; its structure is shown in Figure 1. Figure 1a shows the supercell of the LaMnO₃ catalyst, and Figure 1b shows its 010 surface. The LaMnO₃ catalyst is an orthorhombic structure with a Pnma space group [28], shown in Figure 1a, where the pale blue spheres represent La elements, occupying the A position and playing the role of the supporting structure, and the purple indicates that the Mn ion is the main reactive site. The (010) surface of LaMnO₃ is particularly thermodynamically stable [29]. Therefore, the 010 surface of LaMnO₃ catalyst was selected as the active surface to establish the model and calculations. Experiments have shown that the 010 surface with Mn as the terminal has better stability than the La-terminated surface [30]. Moreover, Mn as the active site can play a better catalytic role when exposed to the surface. A surface with Mn is generally used as the terminal in practical applications [31]. Therefore, the Mn-terminated 010 surface of LaMnO₃ catalyst was established in this research model, as shown in Figure 1b. Figure 1b shows a p(2 × 2) supercell, Mn cations are exposed to the surface as the main active site. The established model was a periodic plate structure. To simplify the calculation, we chose a 2 × 1 model for calculation [32]. Due to the number of layers of the grid having little effect on the results, a three-layered structural grid was established, the bottom atoms fixed and the two surface layers relaxed and optimized [33]. A vacuum layer with 30 Å was added to the lattice to eliminate the interaction between adjacent layers.

The Dmol3 program package was employed in the calculation process. In order to describe the electron exchange potential energy, the GGA (generalized gradient approximation) function was selected [34]. The internal electrons of Mn and La atoms were processed using the effective nuclear potential (ECP) method to facilitate the calculation [35]. The numerical orbit basis was selected as the basic function to solve the DFT equation. The number of atomic orbitals is controlled by the double numerical basis set of polarization function (DNP) [36]. The cut-off numerical basis set value was 4.4 Å.

In the SCR process, small molecules such as NH₃ can be absorbed on the surface, and the absorption strength is evaluated by the adsorption energy, which is defined in the equation [37]:

$$E_{ads}=E_{slab+molecule}-E_{slab}-E_{molecule}$$

(1)

where E_ads, $E_{slab+molecule}$, $E_{slab}$, and $E_{molecule}$ represent the adsorption energy, total energies of the substrate plus small molecules, the energy of substrate surface, and the energy of small molecules, respectively.

In the reaction process, there are intermediate, final, and transition states in each elementary reaction. The transition state of each reaction step was obtained by the linear synchronous transition quadratic synchronous transition (LST-QST) method, and only one virtual frequency was ensured [38]. The Brillouin zone sampling was set to (3 × 3 × 1) on a Monkhorst–Pack k-point grid. The convergence criteria were defined as atomic force 4.0 × 10⁻³ Hartree/Å, maximum displacement 5.0 × 10⁻³Å, total energy variation 2.0 × 10⁻⁵ Hartree, and the self-consistent field (SCF) tolerance was 1.0 × 10⁻⁵ Hartree [37].

The activation energy barrier (Ea) is defined by the following equation [39]:

Ea = E_TS − E_IM

(2)

where E_TS and E_IM represent the total energy of transition state and intermediate state, respectively.

3. Results and Discussion

3.1. Reaction of L Acid

On the surface of LaMnO₃, there are Mn cations which are active sites that distribute L acids, and these sites can accept lone pair electrons provided by MH₃. Zhang’s experiment fully proved this [14]. Figure 2 shows the adsorption configuration of NH₃, and the corresponding parameters are listed in

Table 1. The parameters of optimized NH₃ adsorption configuration.

Mn–N Bond Length (Å)	Angle of N–H (°)	Mulliken Charge of NH3 (a.u)	Adsorption Energy (kJ/mol)
2.107	109.782	0.327	−97.9

.

In this situation, the N atom of NH₃ bonds to the Mn site and conforms to a stable configuration with a bond length of 2.107 Å. The average angle of N-H is 109.782°, compared with 107° when not adsorbed. The angle increases due to the electron transfer from NH₃ to the LaMnO₃ surface, as the Mulliken charge of NH₃ is 0.327, meaning there are electrons transferred from NH₃ to the surface. It can be inferred that the lone pair electrons of NH₃ could be contained by the empty orbits of LaMnO₃, and this property leads to electron migration. The same activity has also been observed on the surfaces of other Mn based catalysts [32].

In order to explore the adsorption mechanism, the partial density of state (PDOS) was assessed and the results are shown in Figure 3. In the figure, various orbitals of the NH₃ molecule and Mn ion are shown. A strong formant peak is visible at about −4 eV between the p orbit of NH₃ and the p and d orbits of the Mn cation, which means that there a strong bond has been formed. Furthermore, there is also a relatively weak peak at −5.5 eV between the p orbit of NH₃ and the s orbit of the Mn cation, indicating the formation of a bond between the two. These results show that the NH₃ molecule could absorbed on the Mn cation as the main active site in the SCR process, fully demonstrating that the LaMnO₃ catalyst is sufficient active. The adsorption behavior has an important influence on the subsequent reactions.

There are full experimental results that show that NH₃ undergoes dehydrogenation after adsorption on different catalyst surfaces, whether with the L-H mechanism or E-R mechanism [40]. By comparing and analyzing the various possibilities after adsorption of NH₃, it can be concluded that the dehydrogenation reaction can also still happen. Figure 4 shows the diagram of potential energy and the optimized structures of the NH₃ dehydrogenation reaction. It can be seen that the dehydrogenation process is divided into two stages. In the first step, one of the H atoms is attracted by lattice oxygen on the surface and NH₂ is formed. The active energy barrier is 22.5 kcal/mol, this value is close to the surface of γFe₂O₃ which is 22.010 kcal/mol [37], but lower than those of CeO₂/TiO₂ and V-based catalysts, 37.77 kcal/mol and 63.6 kcal/mol, respectively [41,42]. This may be due to the similar chemical properties of the Fe cation and Mn cation. The reaction heat of the step is 12.4 kJ/mol, which means that it is an endothermic reaction. This is because the breaking of the NH bond absorbs energy, and the energy released by the combination of lattice oxygen and H is not enough to compensate for it. It is the same case on the surface of Fe₂O₃, but the reaction heat is lower [37] and that means the process is easier to carry out. The next step is that the NH₂ continues to lose one H atom to the lattice oxygen and forms an NH segment. The active energy barrier is 31.8 kcal/mol and the reaction heat is 19.6 kcal/mol. The barrier is higher than in the previous step, which means the process is more difficult to carry out, so NH is less likely to occur. This may be because there are three stable chemical bonds around the N, making NH₂ on the Mn site a relatively stable configuration so that the H is difficult to remove, which is the case for other catalyst surfaces [32,37]. The reaction heat has a positive value which means the reaction is also an endothermic reaction. Therefore, NH₂ is the main product after the adsorption of NH₃, with a small amount of NH.

According to the E-R mechanism, NH₂ is the main intermediate product in the SCR process, which can form NH₂NO and then decompose [38]. By analyzing the reaction pathway on the LaMnO₃ catalyst surface, it has been observed that such a reaction path exists, as shown in Figure 5. The reaction pathway in the figure is divided into the formation and decomposition of NH₂NO. The active energy barrier and reaction heat in the first step are 0.1 kcal/mol and −2.7 kcal/mol. The reaction is an exothermic reaction due to the reaction heat being a negative value because energy is released during the N-N bonding process. The energy barrier is so small as to be negligible, which means the NH₂NO forms very easily because the movement of NO to the surface encounters only minimal resistance.

The next step reaction is the composition of NH₂NO, and is also divided into two steps. The first is removal of one H atom from NH₂NO under the action of lattice oxygen, where the active energy barrier is 12.8 kcal/mol. The barrier is much lower compared with previous dehydrogenation processes, because the formation of the N-N bond means N has a high coordination number so the H atom link is not very tight, making it easy to remove. The reaction heat was reported as −17.4 kcal/mol, which means it is also an exothermic reaction. This is because H needs less energy in the process of removal and releases energy in the process of combining with lattice oxygen. The second step is the NHNO decomposition to N₂ and H₂O. The active energy barrier is 55.9 kcal/mol and the reaction heat is −29.9 kcal/mol. In the process, the N-O breaks and the O atom is combined with two H atoms which had been removed in previous steps, forming a H₂O molecule. In addition, the last H atom separates from the N atom and bonds to lattice oxygen and is left on the surface at the end. At the same time, a triple bond is formed between two N atoms to form N₂ as a stable gas as the final product. Compared with previous steps, the energy barrier is higher because the breaking of N-O bond requires considerable energy, and the phenomenon is the same as on the surface of Mn-based catalyst [43,44]. The reaction is also an exothermic reaction, because of the amount of energy released in the N₂ formation process.

Briefly, the SCR process includes the dehydrogenation process, the formation of NH₂NO, and the decomposition of NH₂NO. The whole process is similar those on the Fe-based, V-based, and Mn-based catalyst surfaces [37,42,43]. In the reaction process shown in Figure 4 and Figure 5, the determining step is the decomposition of NHNO because of it has the highest energy barrier. There is no O₂ involved in this reaction process, which means that the SCR process could be carried out in an anaerobic environment and that the lattice oxygen plays an important role. One H atom remains left at the end and needs to be oxidized, so O₂ is necessary in the SCR process.

When O₂ exists in the atmosphere, it can be absorbed on the surface. The absorption configuration is shown in Figure 6, and the corresponding parameters are listed in

Table 2. The parameters of O₂ adsorption configuration.

Mn–O Bond Length (Å)	O–O Bond Length (Å)	Mulliken Charge of O2 (a.u)	Adsorption Energy (kJ/mol)
2.015	1.268	-0.129	−47.9

. The data show that the O₂ can be absorbed on the Mn site, and one O atom combines with the Mn cation to form a stable chemical bond with a bond length of 2.015 Å. In the process, the adsorption energy is −47.9 kJ/mol, referring to the amount of energy released because of the Mn-O bond formed. There are many electrons transferred from base to O₂ molecule, indicated by the Mulliken charge of −0.129 a.u. reflecting the strong oxidation of O₂. The O-O bond length after adsorption was shown to be 1.268 Å, compared with 1.208 Å before adsorption. This may be caused by the increase of charge density around the O₂, caused by electron immigration.

The partial state density is analyzed in Figure 7. An obvious peak in Fermi energy level can be seen that at 0 eV between the p orbit of the O atom and d orbit of the Mn cation. This demonstrates that the two formed a strong resonance and formed a chemical bond. Furthermore, there are also two peaks at about −6 eV and −7 eV between O and Mn, corresponding mainly with the p orbital of O and the d orbital of Mn. Therefore, a stable resonance hybrid peak was formed between O and Mn, which should belong to a stable chemical adsorption process.

After the adsorption, the O₂ can decompose into two active O atoms on the surface which is shown in Figure 8. In the process, the chemical bond of the O₂ molecule is broken and divided. The originally free O atom is attracted by another Mn active site. Thus, two active O atoms are on the Mn site. The active O can oxidize the H atom left previously and form H₂O so as to leave the surface activity of the catalyst unaffected. Furthermore, the active O can also participate in the dehydrogenation of NH₃. The reaction heat was −3.9 kcal/mol, which means it is an exothermic reaction, because the Mn-O formation can release energy to compensate for the break of O₂ bond.

The dehydrogenation of NH₃ under active oxygen is shown in Figure 9. The active energy barrier is 77.3 kcal/mol and the reaction heat is 10.1 kcal/mol. The energy barrier is much greater than 22.5 kcal/mol because of the effect of lattice oxygen. This maybe because the lattice oxygen is closer to the H atom than the active O. Thus, it can be inferred that the dehydrogenation of NH₃ mainly occurs under the action of lattice oxygen. This is very different from on the Fe₂O₃ catalyst surface. As previously, the reaction is again an endothermic reaction.

Whether NH₂ can continue to be oxidized and dehydrogenated under the action of active O was also addressed, and the process is indicated in Figure 10. The active energy barrier was found to be 100.5 kcal/mol and the reaction heat was 10.4 kcal/mol. It can be seen that the active energy was much higher than in the previous example of dehydrogenation, which means that NH is not easy to generate in this state.

Under aerobic conditions, there are NO₂ mechanisms that speed up the SCR process. The formation of NO₂ is shown in Figure 11. In this step, the active energy barrier is 0.1 kcal/mol which is as low as negligible. The reaction heat is −14.9 kcal/mol, meaning it is an exothermic reaction. The NO molecule in the atmosphere is located near active O on the catalyst surface, and N atoms are linked with O to form NO₂ and release energy.

The NO₃⁻ is the main product of the fast SCR mechanism, and the formation of NO₃⁻ is illustrated in Figure 12. The formed NO₂ overcomes a small potential barrier and approaches another O, so that N atoms combine with it to form NO₃⁻. The active energy barrier is 0.6 kcal/mol and the reaction heat is −18.9 kcal/mol, which means it is also an exothermic reaction because the formation of the N-O bond releases energy.

It can be seen that the energy barriers formed by NO₂ and NO₃⁻ are sufficiently low so NO₃⁻ is easily formed. When there are NH₄⁺ ions on the surface, they easily combine to form NH₄NO₃, and can then participate in the rapid SCR process as previously described [45,46]:

$${\mathrm{NH}}_4{\mathrm{NO}}_3+\mathrm{NO}\leftrightarrow {\mathrm{NH}}_4{\mathrm{NO}}_2+{\mathrm{NO}}_2$$

$${\mathrm{NH}}_4{\mathrm{NO}}_2\to {\mathrm{N}}_2{+2\mathrm{H}}_2\mathrm{O}$$

The NH₄NO₃ can react with NO and be reduced to NH₄NO₂ at 170 °C, and NH₄NO₂ can decompose to N₂ and H₂O at 100 °C. This reaction is easy to carry out, so it is known as a rapid reaction process and referred to as the L-H mechanism. On the LaMnO₃ surface, the easy formation of NO₃⁻ means that it is easy to achieve a rapid SCR reaction process, demonstrating that the LaMnO₃ catalyst has excellent low-temperature catalytic activity.

In summary, the O₂ can be absorbed and decomposed on the LaMnO₃ catalyst surface, and active O oxidation to NH₃ is not obvious. The main influence of oxygen is to oxidize NO to form NO₂ and NO₃⁻, thus participating in the rapid SCR reaction process.

3.2. Reaction Process on B Acid

Following the previous steps, it can be seen there are H ions remaining on the surface that form a B acid. The NH₃ molecule can be absorbed on the B acid to form NH₄⁺. Due to different forms of O on the catalyst surface, different hydroxyl forms may be present, and ammonium NH₄⁺ ions exist in different forms, as shown in Figure 13.

In Figure 13a, there are two active O atoms on the surface, and two H ions bond to the two ions, while there is only one active O on the (b) configuration and none on the (c). The NH₄⁺ ion connect with the active O in (b) and meanwhile act with the lattice oxygen, while NH₄⁺ ions act only with surface lattice oxygen in (c). In addition to participating in the rapid SCR reaction with NO₃⁻, surface NH₄⁺ ions also react with NO to generate NH₂NO and decompose. The reaction process is shown in Figure 14, Figure 15 and Figure 16.

Figure 14 shows the reaction process of the NH₄⁺ ion in the Figure 13a configuration over the catalyst. The first step is that the NO moves to the surface and reacts with NH₄⁺ to form NH₂NO. In this step, two H atoms that were connected to active O in the form of hydrogen bonds break off. The active energy barrier is 25.7 kcal/mol and the reaction heat is −13.1 kcal/mol. Compared with the L acid site, the energy barrier is a little higher and the reaction is an exothermic reaction. After the formation of NH₂NO, the surface ON fragment twists under the action of the surface, and the O atom is inclined to the side near the surface hydroxyl group. The NH₂NO species move slightly towards the surface, one H atom close to the lattice oxygen and another H atom close to the hydroxyl. The energy barrier is 9.4 kcal/mol, which means the reaction is easy to carry out, and the reaction heat is 0.1 kcal/mol. In this step, one H atom moves close to the hydroxyl and can react in the next step to form H₂O which is absorbed by the Fe cation. This represents the next process of NHNO formation, where the energy barrier is 3.4 kcal/mol and the reaction heat is −6.3 kcal/mol, indicating that the step is easy to achieve. On the NHNO configuration, the left H atom is close to the O atom so it easily reacts with it, and after the formation of NHNO, it can easily decompose. It can be seen that the H and O atoms can break away and bond, the H atom belonging to the hydroxyl also bonding to the O and forming a H₂O atom and also a N₂ molecule as the final product. The energy barrier is 8.1 kcal/mol and the reaction heat is −42.5 kcal/mol. The energy barrier is low and considerable energy is released during the process of N₂ and H₂O formation. The highest energy barrier in the whole process is the formation of NH₂NO which is as high as 25.7 kcal/mol, the others being sufficiently low, so the first step is the rate-determining step.

Figure 15 shows the reaction process of the NH₄⁺ ion in the Figure 13b configuration over the catalyst. There are four steps in the process. The first step is again the formation of NH₂NO. The active energy barrier was 11.1 kcal/mol and the reaction heat was 9.7 kcal/mol. Unlike in the previous pathway, this step was an endothermic reaction, which may be because of the breaking of two N-H bonds that need to absorb energy. After the formation of NH₂NO, one H atom was lost to produce NHNO, the energy barrier was 17.9 kcal/mol and the reaction heat −1.2 kcal/mol. This reaction is also easy to produce. Next, the O atom on NHNO can twist and bond to two H atoms on the NHNO and lattice oxygen to produce H₂O. Thus, the N₂ atom and one H₂O atom are formed in this step. The energy barrier is 5.0 kcal/mol and the reaction heat −72.3 kcal/mol. Significant energy is released because of the formation of N₂ and H₂O. There are also a hydroxyl and a H atom remaining, which could react to form H₂O. The energy barrier and reaction heat were respectively 28.3 kcal/mol and 6.7 kcal/mol. This represents another exothermic reaction. The highest energy barrier in the whole process is 28.3 kcal/mol and the reaction is a rate-determining step. For all that, relative values were not very high, so the pathway is easy to produce.

Figure 16 shows the reaction process of NH₄⁺ ion in the Figure 13c configuration over the catalyst. There are three steps in the process, respectively the formation of NH₂NO, the formation of NHNO, and the decomposition of NHNO. The active energy barriers were 96.9 kcal/mol, 72.3 kcal/mol, and 66.7 kcal/mol. Compared with the highest barriers of 25.7 kcal/mol on Figure 14 and 28.3 kcal/mol on Figure 15, every step barrier was much higher, especially the first step which reached 96.9 kcal/mol. This was caused by the fact that active O on the surface has stronger activity than lattice oxygen. This also shows the role of O2 in promoting the SCR reaction. Due to the existence of active O, the activation energy of each step of the subsequent NH₄⁺ reaction is greatly reduced. The respective reaction heats of each step are 6.8 kcal/mol, −20.1 kcal/mol, and −40.0 kcal/mol. The first step is an endothermic reaction and the last two are exothermic reactions. Because the activation energy of each reaction in this path is very high, this step is not easy to carry out compared with other reactions.

3.3. The Main Reaction Pathway Analysis

Figure 17 shows the reaction process analysis for the whole reaction. Under anaerobic conditions, the SCR process could produce the reaction $\mathsf{\sigma }-{\mathrm{NH}}_2+\mathrm{NO}\left(\mathrm{g}\right)\to \mathsf{\sigma }-{\mathrm{NH}}_2\mathrm{NO}\to \mathsf{\sigma }-{\mathrm{N}}_2+\mathsf{\sigma }-{\mathrm{H}}_2\mathrm{O}$, known as the E-R mechanism. The rate-determination step is the decomposition process of NHNO, and the active energy barrier can reach 55.9 kcal/mol. The same phenomenon also occurs on MnO_x/SiO₂ catalyst surfaces, where the energy barrier is about 66.89 kcal/mol [43]. There is little difference between the two, which also shows the correctness of the calculation. The lattice oxygen plays a major role in the dehydrogenation of NH₃, but there remains H left to form hydroxyl that can be oxidized by oxygen to keep the surface active. When O₂ is present, it can be absorbed and decomposed to active O on the Mn site. The NO can be oxidized to NO₂ and NO₃⁻, and then participate in the rapid SCR process, which is known as the L-H mechanism. NH₄⁺ can also appear because of the existence of hydroxyl, and can also react with NO to form NH₂NO and then decompose. This process is similar to that observed for V-based catalysts [24]. There are three ways to realize this process, according to the different types of surface hydroxyl groups. When active O exists on the surface, the process can be promoted because the energy barrier of every step is reduced greatly, and this also shows that O₂ plays an important role in the SCR reaction. In conclusion, both the E-R mechanism and the L-H mechanism exist on the LaMnO₃ catalyst surface.

4. Conclusions

The NH₃ molecule can first be absorbed on the LaMnO₃ catalyst surface, then it can be oxidized by lattice oxygen to form NH₂. The NH₃ can react with NO to form NH₂NO and then decompose to N₂ and H₂O. Within the process, NHNO is an intermediate and its decomposition is the rate-determining step. When O₂ exists in the atmosphere, it can be absorbed and decomposed into two active O atoms. NO can be oxidized to NO₃⁻ and then participate in the rapid SCR process. NH₄⁺ can also appear because of the existence of hydroxyl, and can also react with NO to form NH₂NO and then decompose. There are three ways to realize this process, according to the different types of surface hydroxyl groups. When active O exists on the surface, the process can be promoted greatly as the energy barrier of every step is significantly reduced.