The Study of SCR Mechanism on LaMn_1−xFe_xO₃ Catalyst Surface Based DFT

Abstract

Perovskite SCR catalysts have become a hot research topic in the field of de-NO_x catalyst development. This article selects LaMnO₃ with high performance as the research object, modifies the catalyst by doping some iron elements instead of manganese elements, and applies density functional theory to study its reaction mechanism, providing theoretical reference for further research on perovskite. Research has found that several main reactants such as NH₃, NO, and O₂ can form stable adsorption at the active site, with NO more inclined to adsorb at the nitrogen atom end at the active site. The oxidation of O₂ molecules after adsorption is greater than that of the active site. The adsorption capacity of the Mn active site of the catalyst before modification on the above molecules is weaker than that of the Fe active site introduced after modification. Under both anaerobic and aerobic conditions in the SCR reaction process, NH₃ molecules are first adsorbed at the active site, and then influenced by lattice oxygen under anaerobic conditions. Under aerobic conditions, they are gradually dehydrogenated and produce NH₂ and NH radicals. These two radicals react with NO molecules to form intermediate products in the form of NH₂NO and NHNO molecules. Due to the instability of the intermediate products, they ultimately decompose into N₂ and H₂O molecules. The introduction of Fe active sites can increase the generation of NH₂ and NH radicals during the reaction process and simplify the reaction process between NH₂ radicals and NO molecules, which will be conducive to the completion of the reaction.

1. Introduction

NO_x is the main atmospheric pollutant and one of the common major polluting gases in industry today. SCR is currently one of the most widely used technologies in NO_x control, with a de-NO_x efficiency of over 90% [1]. SCR (selective catalytic reduction) technology uses NH₃ generated from urea, ammonia water, or liquid ammonia as a reducing agent, which adsorbs onto the catalyst surface and selectively reacts with NO_x to generate nitrogen, water, and other products, thereby achieving the goal of removing NO_x [2].

The core of SCR technology lies in catalysts which have excellent performance and are characterized by good catalytic activity, high selectivity, high mechanical strength, and stable operation [3]. Therefore, selecting appropriate catalysts has become the focus of research by many scholars. Platinum group elements have been widely used in the treatment of automotive exhaust [4]. Vanadium–titanium-based catalysts have also matured in recent decades of application. However, the reaction temperature of vanadium –titanium-based catalysts is relatively high, ranging from 300 to 400 °C, with an optimal activity window temperature of 340 to 380 °C [5]. However, due to the high sulfur content of the fuel used, there are also by-products such as sulfur oxides (SO_x) in the tail gas [6]. To prevent the poisoning and deactivation of denitrification catalysts, the de-NO_x device is usually placed behind the electrostatic precipitator and desulfurization device, which leads directly to a significant decrease in the temperature of the tail gas processed by the denitrification device. In order to avoid fuel loss caused by the demand for exhaust gas reheating, it is urgent to develop reasonable, effective, low-temperature, and high-performance denitrification catalysts to meet industrial needs [7].

“Perovskite oxides” is a general term for oxides with an ABO₃ structural form, with an ideal cubic crystal system and a microscopic symmetry group belonging to Pm3m. Perovskite oxides have many advantages, such as strong redox ability, thermal stability, and variety diversity, which have attracted the attention and research of many scholars. LaMnO₃ perovskite oxides exhibit robust stability and excellent selective catalytic reduction (SCR) activity, making them promising candidates for low-temperature catalysts [8]. Scholars have shown a keen interest in their potential [9]. Zhang’s investigation of LaMnO₃ catalysts revealed impressive SCR performance, achieving a notable 78% NO conversion at 250 °C, with increasing conversion as the temperature ranged from 100 °C to 300 °C [10]. Guo et al. also examined LaMnO₃ catalysts, demonstrating remarkable results with over 90% NO_x conversion at 135 °C and within the 135–260 °C temperature range [11]. Wang et al. extended these findings by utilizing a LaMnO₃-supported iron ore catalyst, achieving an exceptional NO_x conversion efficiency of up to 98% at 180 °C [12]. These findings collectively underscore the catalyst’s exceptional SCR capabilities. Nevertheless, the precise mechanism governing NO_x conversion over LaMnO₃ catalysts remains unclear.

However, the catalytic activity and N₂ selectivity of pure ABO₃ perovskite catalysts are not ideal, which limits their application. It is generally believed that there are two types of SCR mechanisms; one is the E–R mechanism, where NH₃ adsorbs onto the catalyst surface and reacts with weakly adsorbed or non-adsorbed NO to generate N₂ and H₂O. The other mechanism is the L–H mechanism, where NH₃ and NO simultaneously adsorb onto the catalyst surface and react to generate N₂ and H₂O. Zhang et al. [13]. prepared perovskite materials doped with Sr and Ce at the A site and Fe and V at the B site based on LaMnO₃ perovskite. They found that when doped with Fe at the B site, the denitrification efficiency at the medium- to high-temperature end was significantly improved, reaching 80% at 200 °C, maintaining over 90% efficiency at 225–350 °C, and reaching 100% at 250–300 °C. At the same time, the N₂ selectivity was also slightly improved. Other literature shows that LaMn_xFe_1−xO₃ perovskite catalysts can be used as an effective SCR denitrification catalyst for development and application [14]. However, there is currently no research on the denitrification reaction mechanism of this catalyst. Although many researchers have studied the mechanisms of Mn-based and Fe-based catalysts, there are currently no reports on the co-acting catalysts of Mn and Fe, and there is no relevant research on the synergistic mechanism between Mn and Fe. Therefore, this article applies density functional theory (DFT) to study the mechanism of LaMn_xFe_1−xO₃ type perovskite catalysts.

DFT is an effective approach in the material design and development stage and also has a wide range of applications in de-NO_x field. Yao et al. [15] used density functional theory to study the reaction mechanism of NO reduction by NH₃ on a V₂O₅ (001) surface and proposed a new reaction mechanism via calculation. The mechanism consists of four steps: the first step is to convert surface L-acid to B-acid, the second step is to generate double B-acid, the third step is to generate a vacancy, O, on the V atom, and the fourth step is to introduce O₂ to regenerate the catalyst. Yang et al. [16] used density functional theory to study the surface SCR reaction process of CeO₂/TiO₂ catalysts. The calculations showed that NH₃ was first adsorbed onto the surface L-acid site in molecular form, then activated and dehydrogenated to generate NH₂, and then reacted with NO to generate N₂ and H₂O. This reaction follows the E–R mechanism. Via this calculation, the energy barrier and reaction heat of each elementary reaction step in the entire SCR reaction can be obtained, and its possible paths can be analyzed. Song et al. [17] used density functional theory to investigate the behavior of the acatalytic reduction of NO_x by H₂O₂ on the surface of an α-Fe₂O₃ catalyst, and the surface reaction path was determined based on this. The results showed that both NO_x and H₂O₂ would react on the surface of an α-Fe₂O₃ catalyst undergoing adsorption, and H₂O₂ undergoes dissociation under the action of the catalyst, thereby reacting with the adsorbed NO to generate N₂ and H₂O. At the same time, some scholars have also studied the surface catalytic reaction of perovskite-type catalysts using DFT. Zhang et al. conducted DFT calculations to determine atomic charges for BiMnO₃ and LaMnO₃, highlighting the relationship between smaller O charges, higher electronegativity, increased acidity, and enhanced NH₃ adsorption and activation, thus boosting low-temperature NH₃-SCR activity [13]. Yan et al. examined the CO-SCR reaction process on LaMnO₃ catalyst surfaces, defining key steps, activation energy barriers, and reaction paths [18]. Ren et al. used the DFT method to study the SCR reaction process on the surface of LaMnO₃ and obtained the surface SCR reaction pathway, determining the E–R mechanism of the surface reaction [19]. Araujo et al. performed functional correction and conducted higher-level calculations on small metal clusters to improve the adsorption energy and potential barrier of periodic band structures and compared them with experimental results. This functional can be widely used for surface science evaluation and testing [20]. However, DFT research on NH₃-SCR surface reactions with LaMnO₃ catalysts remains unexplored.

In summary, the perovskite-structured LaMn_xFe_1−xO₃ catalyst exhibits strong potential as a de-NO_x catalyst. However, its mechanism requires further investigation. Employing the mature and reliable DFT method can uncover surface reaction mechanisms, providing valuable guidance for the development and application of low-temperature de-NO_x catalysts. This paper leverages DFT to study surface reaction mechanisms, capturing parameters such as molecular adsorption, elementary reaction steps, and activation energies, offering insights into adsorption behavior and reaction processes while identifying the most probable reaction pathway on the catalyst surface.

2. Model and Method

Figure 1 shows the structure of LaMn_1−xFe_xO₃ catalyst, which is established based on LaMnO₃. LaMnO₃ perovskite belongs to the category of rhombic structures; the space group is Pnma; and the cell parameters are as follows: a = 5.724, b = 7.690, and c = 5.534. Figure 1 shows the LaMnO₃ crystal model after doping Fe elements, in which two perspectives are selected for representation. The crystal model structure consists of twelve oxygen atoms, four lanthanum atoms, three manganese atoms, and one iron atom, in which the doping amount of iron element is 0.25. The XRD characterization results of iron-doped LaMnO₃ crystals show that the XRD spectra of iron-doped LaMnO₃ crystals have not changed, indicating that the main composition of iron-doped LaMnO₃ crystals is still LaMnO₃ [21]. Therefore, when the model in this paper was built, based on LaMnO₃ crystal, some iron elements were doped on the manganese site so as to modify the LaMnO₃ crystal.

In this paper, surface 001 in the LaMnO₃ crystal was selected as the reaction active surface for study, and a periodic plate model was established on this basis. At the same time, the 2 × 1 surface of the LaMnO₃ crystal was selected as the size of reactive active surface. On the surface of the reactive active surface, this paper uses the model to add a 30 Å vacuum layer to prevent interaction between adjacent layers. In addition, in the whole calculation process, in the whole calculation process, the bottom atoms are fixed and other atoms were set to be able to relax [19].

During the computational process, calculations were conducted using the DMol3 package, employing a spin-polarized DFT model [22]. The GGA method was utilized for the electron exchange correlation function, along with the application of the effective core potentials (ECP) method for Fe and Mn atom core electrons. To ensure accurate calculations, double-numeric quality plus polarization (DNP) functions were applied to all atoms, with a selected real space cutoff radius of 5.0 Å [23]. Brillouin zone integration was performed using 5 × 5 × 1 Monkhorst–Pack grids for the adsorption and reaction system [24].

To assess the adsorption properties of the catalysts, the adsorption energy was thoroughly evaluated, which was defined as follows, according to [25]:

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

(1)

where $E_{ads}$ means the adsorption energy, $E_{slab+molecule}$ means the energy of slab and molecule after adsorption, and $E_{slab}$ and $E_{molecule}$ are the energy of the catalyst slab before adsorption and that of NH₃ and NO molecule before adsorption, respectively.

The NH₃-SCR process on the Ce-doped γ-Fe₂O₃ catalyst displayed distinct stages, each with unique intermediate products (IM), transition states (TS), and final states (FS). In the computational procedure, the LST–QST method was applied to locate the transition states (TS), and a careful vibration frequency analysis was conducted to verify the presence of only a single virtual frequency [26].

The activation energy barrier (E_a) is defined as the following equation [27,28]:

E_a = E_TS − E_IM

(2)

where E_TS is intermediate energy and E_IM represent transition state energy.

3. Results and Discussion

3.1. Fe Doping on LaMnO₃ Crystal Surface

The active reaction surface of the modified LaMnO₃ crystal model is shown in Figure 2. The crystal model shown in the figure is divided into upper and lower layers, in which iron is doped in the upper layer and the lower layer is a fixed bottom layer.

Table 1. Crystal parameters at sites before and after doping.

(a)	O-Mn Bond Length (Å)	(b)	O-Fe Bond Length (Å)
O1	1.927	O6	1.917
O2	1.929	O7	1.922
O3	1.974	O8	1.977
O3	1.991	O9	1.975
O5	2.001	O10	2.013

shows the parameter changes of the crystal at the site before and after doping. It can be seen that doping iron will lead to lattice deformation, which will affect the performance of the catalyst in the SCR reaction process. At the same time, the Mulliken charge of Mn before doping is 0.658, while after Fe doping, the Mulliken charge is 0.701. The reason for the increase of the charge is that the electronegativity of iron is greater than that of manganese, and electrons are attracted by Fe ions. Therefore, doping with Fe will bring more valence electrons and increase the free electrons on the catalyst surface, thus changing the redox ability of the catalyst surface. Chen [29] and others found that the NO molecules adsorbed onto the surface of the perovskite may react with the perovskite lattice oxygen, but this process has a weak impact. Yan [18] and others pointed out that the active sites in the LaMnO₃ catalyst are mainly Mn sites. Considering the influence of doping Fe on the catalyst, Mn and Fe ion sites in the upper layer of Figure 2b were selected as the active sites for this study. The influence of some molecules adsorbed by lattice oxygen is not considered.

3.2. Adsorption of Reactant Molecules on Modified LaMnO₃ (001) Surface during NH₃-SCR Process

There are L–H mechanisms and E–R mechanisms in the NH₃-SCR de-NO_x process involving catalysts. “L–H mechanism” refers to the heterogeneous catalytic mechanism in which two reactant molecules first adsorb onto the catalyst surface and then desorb after reaction to produce products, while “E–R mechanism” refers to the mechanism in which one reactant molecule adsorbs onto another to produce products. Many scholars have no consensus on this, but both reaction mechanisms clearly indicate that NH₃ will be adsorbed onto the catalyst surface first. As the reactant NH₃ molecule, its adsorption capacity on the catalyst surface has a great impact on NO_x removal [30], so this paper will first study the adsorption capacity of NH₃ molecule. Figure 3 shows the schematic diagram of NH₃ molecules adsorbed onto Mn and Fe active sites, while

Table 2. Adsorption Parameters of NH₃ Molecule at Mn and Fe Active Sites.

Active Sites	NH3 Bond Angle (°)	N-Me Bond Length (Å)	Adsorption Energy (kJ/mol)	Mulliken Charge (a.u)
Mn	110.033	2.075	−125.707	0.349
Fe	109.864	1.999	−152.709	0.367

shows the bond angle, bond length, released adsorption energy, and Mulliken charge of adsorbed molecules after NH₃ molecules adsorbed onto Mn and Fe active sites.

It can be seen in this situation that NH₃ can be absorbed onto the Mn and Fe sites with the N atom end, and a new chemical bond is formed. The NH₃ bond angle adsorbed onto Mn active sites is greater than that of NH₃ molecules adsorbed onto Fe active sites. This is because the number of electrons lost by the H atoms of NH₃ molecules adsorbed onto Mn active sites is greater than that adsorbed onto Fe, which leads to the weaker repulsion of H atoms to hydrogen atoms, leads to a larger bond angle. The N-Mn bond length of the NH₃ molecule adsorbed onto the Mn active site is longer than that adsorbed onto the Fe active site because the radius of the Mn ion is larger than that of the Fe ion. The adsorption energy released after adsorption and the energy released by NH₃ adsorbed onto the Mn active site is less than that released by NH₃ molecule adsorbed onto Fe active site, indicating that the NH₃ molecule adsorbed onto the Fe active site is conducive to reaction. In addition, the Mulliken charges are all positive, indicating that electrons migrate from the NH₃ molecule to the catalyst surface, which is due to the existence of lone pair electrons in the NH₃ molecule, and the existence of a 3d empty orbital on the catalyst surface is sufficient to accept excess electrons. Moreover, the charge amount of the NH₃ molecule adsorbed by the Mn active site is less than that of the Fe active site, because the electronegativity of Fe is greater than that of Mn. Therefore, in both adsorption processes, a large amount of energy is released, and a large number of electrons are transferred from NH₃ molecules to the catalyst. Since the electronegativity of Fe ions is greater than that of Mn ions, there is more electron migration at the active site of Fe, the interaction between them is stronger, and more energy is released.

As one of the main reactants in SCR de-NO_x reaction, and under normal combustion temperature, the NO_x contained in the flue gas generated after the combustion of coal and other fuels is mainly NO and NO₂, of which NO accounts for more than 90% of the total NO_x, while NO₂ only accounts for 5–10%, so it is necessary to study its adsorption capacity on the catalyst surface [31].

It has been shown that NO will approach the active surface sites in the form of N atoms and O atoms, respectively, to form a stable adsorption configuration. The corresponding parameters are list in

Table 3. Adsorption Parameters of NO Molecule at Mn and Fe Active Sites.

Absorption Sites	N/O-Me Bond Length (Å)	Adsorption Energy (kJ/mol)	Mulliken Charge (a.u)
A	1.737	−67.793	0.254
B	2.452	−18.097	0.063
C	1.765	−82.414	0.244
D	2.132	−28.947	0.086

.

There are two kinds of adsorption of NO reactants in Figure 4 on Mn and Fe active sites. It can be seen from Table 3 that when NO is adsorbed onto the catalyst surface as a N atom end, its bond length is shorter than that in the form of O atom end proximity, and the adsorption energy in the case of N proximity is far greater than that in the case of O proximity. Therefore, the adsorption of NO molecules should mostly occur on Mn and Fe ions on the catalyst surface as a N atom end. During the adsorption process, no matter whether the NO molecule on Mn active site is adsorbed by a nitrogen atom or an oxygen atom, the energy released after adsorption is less than the energy released by the corresponding end of the NO molecule adsorbed onto the Fe active site, which also indicates that the NO molecule is more easily adsorbed onto the Fe active site. This is likely due to the fact that the electronegativity of Fe ions is greater than that of Mn ions, and their oxidation is stronger, which is also true of NH₃ molecules. This also shows that the doping of Fe is conducive to promoting the adsorption of NO. When the nitrogen atom is close, the bond length is shorter and more electrons are transferred, whereas when the oxygen atom is close, the bond length is longer and fewer electrons are transferred.

In the process of the NH₃-SCR de-NO_x reaction, O₂ plays an important role as a reactant, which is helpful to the smooth progress of the reaction. Figure 5 shows the schematic diagram of O₂ molecules adsorbed onto Mn and Fe active sites, and

Table 4. Adsorption Parameters of O₂ Molecule at Mn and Fe Active Sites.

Absorption Sites	O-Me Bond Length (Å)	Adsorption Energy (kJ/mol)	Mulliken Charge (a.u)
Mn	1.988	−57.211	−0.105
Fe	1.916	−72.050	−0.073

shows the corresponding adsorption parameters. Table 4 shows that the N-Mn bond length of O₂ adsorbed onto Mn active site is longer than that on Fe active site. During the adsorption process, the energy released by O₂ adsorption on the Mn active site is less than that adsorbed onto Fe active site, indicating that O₂ is more easily adsorbed onto Fe. Since the oxidizability of oxygen atom in O₂ is stronger than that of Mn and Fe active sites, O₂ molecules obtain electrons during adsorption. Since the electronegativity of Fe ions is greater than that of Mn ions, the charge amount of O₂ molecule adsorbed by Mn active site is greater than that of O₂ molecules adsorbed by an Fe active site.

3.3. Study on Surface Reaction Process of Modified LaMnO₃ (001) Surface

A large number of results show that NH₃ will first be adsorbed onto the catalyst surface, regardless of the E–R mechanism or L–H mechanism, and will decompose to produce NH₂, NH, and other intermediates under the action of lattice oxygen. Hence, it is necessary to study the process of NH₃ decomposition, which is shown in Figure 6.

Figure 6 shows the process of dehydrogenation of NH₃ molecules into NH₂, NH, and N after adsorption at the Mn and Fe active sites in the absence of lattice oxygen. The NH₃ molecule is adsorbed onto the Mn active site. At this time, the N-Mn bond length is 2.075 Å, and the distances between the three hydrogen atoms of the NH₃ molecule and the nearest lattice oxygen are 2.738 Å, 2.747 Å, and 2.822 Å, respectively. Then, under the action of lattice oxygen, the hydrogen atom nearest to the lattice oxygen desorbs and combines with the lattice oxygen to form hydroxyl. The energy barrier and reaction heat of this process are 87.034 kJ/mol and 71.445 kJ/mol, respectively, indicating that this process is an endothermic process. After the formation of NH₂ radicals on the catalyst surface, the dehydrogenation process will continue under the action of the lattice oxygen. The energy barrier and reaction heat of the process are 148.695 kJ/mol and 72.741 kJ/mol, respectively, indicating that the process is also an endothermic process. After the formation of NH radicals on the catalyst surface, the dehydrogenation process will continue under the action of the lattice oxygen. The energy barrier and reaction heat of the process are 86.322 kJ/mol and 5.556 kJ/mol, respectively. While the NH₃ molecule removes three hydrogen atoms in turn, the energy barrier of NH₂ radical removing hydrogen atom is the highest, indicating that the occurrence of this process is difficult, which leads to less NH radical generation, and fewer NH radical sources also lead to fewer nitrogen ions finally formed. The same process will occur at the Fe site. The NH₃ molecule is adsorbed onto the Fe active site, which is an exothermic process. At this time, the N-Fe bond length is 1.999 Å, and the distances between the three hydrogen atoms of the NH₃ molecule and the nearest lattice oxygen are 2.620 Å, 2.659 Å, and 2.848 Å, respectively. It can be seen that the average distance between hydrogen atom and lattice oxygen is shorter than that adsorbed onto the Mn ion at this time. This is because the electronegativity of Fe ions is stronger than that of Mn ions. As a result, after the NH₃ is adsorbed onto the active site, the N atom adsorbed onto the Fe ion obtains more electrons, which weakens the repulsion of hydrogen atoms in NH₃ molecules, thus reducing the distance between hydrogen atom and lattice oxygen. Subsequently, under the action of the lattice oxygen, the hydrogen atom nearest to the lattice oxygen desorbs and combines with lattice oxygen to form hydroxyl. The energy barrier and reaction heat of this process are 98.583 kJ/mol and 53.631 kJ/mol, respectively. NH₂ radicals continue to dehydrogenate, and the energy barrier and reaction heat of this process are 136.380 kJ/mol and 80.549 kJ/mol, respectively. However, the NH radical continues the dehydrogenation process, and the energy barrier and reaction heat of the process are 119.181 kJ/mol and 60.320 kJ/mol, respectively. In comparison with the above analysis, the dehydrogenation process of NH₃ molecule at Mn and Fe active sites is basically the same, and the energy barrier of NH₂ radical removal of hydrogen atom is the highest. Therefore, NH₂ should be the main dehydrogenation product, which is the same as that of other catalysts [32].

Figure 7 shows the decomposition process of NH₂NO. On the catalyst surface, after the formation of NH₂, it will react with gaseous NO to generate NH₂NO. In this step, the reaction potential barrier can be ignored, and the reaction is very rapid, and then NH₂NO will decompose. At Mn active site, NH₂NO molecule removes a hydrogen atom to the lattice oxygen to form the NHNO molecular structure, which is a linear structure. The energy barrier and reaction heat of the process are 28.128 kJ/mol and 24.510 kJ/mol, respectively, indicating that the process is endothermic. The generated NHNO molecule is unstable, which will further undergo dehydrogenation and finally decompose into N₂ and hydroxyl. The energy barrier and reaction heat of this process are 204.640 kJ/mol and −206.557 kJ/mol, respectively, indicating that this process is an exothermic process. Since the product N₂ has been generated, the reaction is complete, and the generated hydroxyl group will combine with the free hydrogen atom to generate the product H₂O. At the Fe active site, the NH₂NO molecule will also break an N-H bond and adsorb the desorbed hydrogen atom onto the oxygen ion in the NH₂NO molecule to form an O-H bond. The energy barrier and reaction heat of the process are 155.849 kJ/mol and −39.559 kJ/mol, respectively, indicating that the process is exothermic. The NHNO generated is unstable, so dehydrogenation will occur further and eventually produce N₂ and H₂O. The energy barrier and reaction heat of the process are 58.345 kJ/mol and −201.643 kJ/mol respectively, which also indicates that the process is exothermic. Compared with the above analysis, the reaction process of NH₂ free radicals at Mn and Fe active sites is the same, and there is a higher energy barrier step in both reaction processes, but the reaction will eventually release a lot of energy. Combined with the change in reaction energy, the occurrence of the reaction will gradually be easy to realize as the reaction proceeds.

Yang et al. pointed out that during the process of SCR, the surface oxygen on the catalyst surface has a high mobility, and its oxidation is stronger than that of lattice oxygen. Therefore, the decomposition process of O₂ molecules on the catalyst surface was studied in this paper [16]. The corresponding process is shown in Figure 8.

Figure 8 shows the decomposition process of O₂ after adsorption on Mn and Fe active sites. The O₂ molecule is adsorbed onto the active site, and then under the influence of the active site, O₂ decomposes and adsorbs an oxygen atom on the active site respectively. At the Mn site, the energy barrier and reaction heat of this process are 220.707 kJ/mol and −2.854 kJ/mol, respectively, and at the Fe site, they are 254.764 kJ/mol and 12.224 kJ/mol, respectively. By comparing the above data, it can be seen that the energy barrier of the decomposition process of O₂ molecule on the active site is relatively high, among which the energy barrier of the decomposition process of O₂ molecule adsorbed onto the Fe active site is higher, which indicates that the breaking of O=O bond requires more energy, and also indicates that the decomposition process of O₂ molecule on Mn and Fe active site is difficult to occur. This result is similar to the results on the surface of iron oxide [33]. The decomposition process of O₂ molecule adsorbed onto Mn active site is exothermic, while the decomposition process of O₂ molecule adsorbed onto Fe active site is endothermic process, indicating that O₂ molecule adsorbed onto Mn active site is easier to decompose.

Figure 9 shows the dehydrogenation process of NH₃ molecules at Mn and Fe active sites. During this process, NH₃ adsorbed onto Mn and Fe active sites undergoes dehydrogenation by oxygen ions generated by the decomposition of O₂ molecules, producing NH₂ radicals and hydroxyl groups. The hydroxyl groups combine with free hydrogen atoms on the catalyst surface to form the product H₂O. The energy barriers of this process are 50.761 kJ/mol and 63.411 kJ/mol, respectively, and the reaction heats are 25.879 kJ/mol and 46.599 kJ/mol, respectively, indicating that this process is endothermic. Comparing the above process with the process of removing one hydrogen atom from NH₃ molecules under anaerobic conditions, it can be found that under the influence of surface oxygen, the energy barrier for NH₃ molecule dehydrogenation is lower, and the reaction heat absorption is also lower. This indicates that NH₃ molecules are more inclined to react with surface oxygen and undergo dehydrogenation when surface oxygen is present. This is because surface oxygen has more coordination numbers to accommodate more hydrogen atoms. The NH₂ free radicals generated by the reaction will react with NO molecules in the air to generate NH₂NO molecules, which will then decompose and ultimately generate N₂ and H₂O products, as shown in Figure 7. In addition, the generated NH₂ radicals will continue to react with surface oxygen and undergo dehydrogenation.

Figure 10 depicts the dehydrogenation process of NH₂ radicals at Mn and Fe active sites. During this process, NH₃ adsorbed onto Mn and Fe active sites undergoes dehydrogenation by oxygen ions generated by the decomposition of O₂ molecules, resulting in the formation of NH₂ radicals and hydroxyl groups, which combine with free hydrogen atoms on the catalyst surface to form the product H₂O. The energy barriers of this process at Mn and Fe sites are 374.292 kJ/mol and 101.089 kJ/mol, respectively, and the reaction heats are 27.39 kJ/mol and 50.537 kJ/mol, respectively, indicating that this process is endothermic.

By comparing the above processes, it can be seen that the energy barrier for the dehydrogenation process of NH₂ radicals adsorbed onto Fe active sites is significantly lower than that of NH₂ radicals adsorbed onto Mn active sites. This indicates that increasing Fe active sites by doping iron is beneficial for the dehydrogenation process of NH₂ radicals adsorbed onto active sites under aerobic conditions. The generated NH radicals will continue to react with surface oxygen and undergo a dehydrogenation process to generate nitrogen ions. However, based on the previous analysis, it can be seen that the generation of NH radicals and oxygen ions is relatively small. Therefore, this article does not consider the reaction between NH radicals and surface oxygen and the dehydrogenation process.

The generated NH radicals will react with NO molecules in the air to generate NHNO molecules, as shown in Figure 11, which shows the formation process of NHNO on the catalyst surface.

Figure 11 shows that NH radicals adsorbed onto Mn and Fe active sites combine with NO molecules in the air to form NHNO molecules. In this structure, N-Mn and N-Fe bonds still exist, while oxygen ions in NHNO molecules do not form stable chemical bonds with the active sites. The energy barriers of this process are 28.537 kJ/mol and 30.629 kJ/mol, respectively, and the reaction heats are −150.812 kJ/mol and −132.462 kJ/mol, respectively, indicating that this process is an exothermic process.

By comparing the above processes, it can be found that the reaction process between NH radicals adsorbed onto Mn and Fe active sites and NO molecules in the air is different, and the reaction energy barrier and reaction heat also differ significantly. The reaction energy barrier between NH radicals adsorbed onto Mn active sites and NO molecules in the air is lower than that between NH radicals adsorbed onto Fe active sites and NO molecules in the air. The reaction heat is that the energy released by the reaction between NH radicals adsorbed onto Mn active sites and NO molecules in the air is higher than that released by the reaction between NH radicals adsorbed onto Fe active sites and NO molecules in the air. The above data indicates that NO molecules are more inclined to react with NH radicals adsorbed onto Mn active sites. However, since both reaction processes are exothermic, the occurrence of the reaction will gradually become easier to achieve as the reaction progresses.

From the analysis of the adsorption of NO₂ molecules on Mn and Fe active sites mentioned earlier, it can be concluded that the adsorption of NO₂ molecules near the nitrogen atom end on the active site or the adsorption of NO₂ molecules on the Fe active site is beneficial for the reaction, and when both conditions are met simultaneously, the reaction is most favorable. Therefore, this article studied the generation process of NO₂ at Mn and Fe active sites using this approach, as shown in Figure 12.

Figure 12 shows the formation process of NO₂ molecules at Mn and Fe active sites, with energy barriers of 199.664 kJ/mol and 205.752 kJ/mol, respectively, and reaction heats of −18.586 kJ/mol and −8.590 kJ/mol, respectively, indicating that this process is exothermic.

By comparing the above processes, it can be found that the energy barrier of NO₂ generation at both Mn and Fe active sites is relatively high. The energy barrier of NO₂ generation at Mn active sites is slightly higher than that of NO₂ generation at Fe active sites. At the same time, the reaction heat released by NO₂ generation at Mn active sites is greater than that released by NO₂ generation at Fe active sites. The results also indicate that NO₂ molecules are more easily generated at Mn active sites. However, since both reaction processes are exothermic, the occurrence of the reaction will gradually become easier to achieve as the reaction progresses. There is literature indicating that when NO₂ is present, nitric acid will be generated to further generate NH₄NO₃. Under the action of surface acidic sites, NH₄NO₃ will be reduced by NO to NH₄NO₂, which is extremely unstable and can decompose into N₂ and H₂O at 100 °C, which is a rapid SCR reaction [34,35,36].

3.4. Analysis of NH₃-SCR De-NO_x Reaction Mechanism

The NH₃-SCR denitrification reaction process on the surface of modified LaMnO₃ (001) catalysts can be divided into three processes: adsorption, reaction, and desorption. The adsorption of NH₃ molecules on the catalyst surface can be divided into adsorption at Mn active sites and adsorption at Fe active sites, with NH₃ molecules having the highest adsorption energy at Fe active sites. The adsorption of NO molecules on the catalyst surface can be divided into two types: adsorption on the Mn active site, adsorption on the Fe active site, or adsorption on the active site near the nitrogen atom end of NO molecules; and adsorption on the active site near the oxygen atom end of NO molecules. Among them, the adsorption energy of NO molecules on the Fe active site near the nitrogen atom end is the highest.

The reaction process is shown in Figure 13, listing the relationships between the various elemental reactions. Under anaerobic conditions, the adsorbed NH₃ molecules will undergo dehydrogenation with lattice oxygen on the catalyst surface to generate NH₂ radicals. Subsequently, NH₂ radicals will continue to dehydrogenate with lattice oxygen to form NH radicals. Figure 13a shows that the energy barrier of this step is 148.695 kJ/mol, while Figure 13b shows that the energy barrier of this step is 136.380 kJ/mol, indicating that the reaction energy barrier of this step is the highest under anaerobic conditions. Therefore, the generation of NH radicals is relatively low, leading to a very low generation of nitrogen ions. Under aerobic conditions, NH₃ molecules will also react and dehydrogenate with oxygen ions adsorbed onto the catalyst surface to generate NH₂ radicals. NH₂ radicals will also continue to dehydrogenate with surface oxygen to form NH radicals. The NH₂ radicals generated under anaerobic and aerobic conditions will react with NO in the air and generate NH₂NO molecules. Due to the instability of NH₂NO molecules, they will subsequently decompose and ultimately produce N₂ and H₂O products. The generated NH radicals will also react with NO molecules in the air and generate NHNO molecules. Similarly, due to the instability of NHNO molecules, they will also undergo decomposition and ultimately produce N₂ and H₂O products. After N₂ and H₂O molecules detach from the catalyst surface, the SCR reaction is completed. Comparing the generation of NH₂ and NH radicals under anaerobic and aerobic conditions, it can be found that the reaction proceeds more easily under aerobic conditions. By comparing the reactions between Mn active sites and Fe active sites, it can be seen that the reactions involving NH₃ molecules, NH₂ radicals, and NH radicals located at Fe active sites are more easily carried out. In addition, under aerobic conditions, the participation of O₂ will cause the oxidation of NO molecules into NO₂ molecules, and the participation of NO₂ molecules will undergo a “rapid SCR reaction”, accelerating the progress of the SCR reaction.

4. Conclusions

On the active site, NH₃, NO, and O₂ molecules can all adsorb, with NH₃ molecules as one of the main reactants releasing the most energy and losing the most charge after adsorption. NO molecules undergo chemical adsorption at the nitrogen atom end, while adsorption at the oxygen atom end is only physical adsorption. O₂ molecules can also adsorb well onto the surface of the catalyst.

In the SCR reaction process, under anaerobic conditions, NH₃ molecules adsorbed onto the active site are gradually dehydrogenated and generate NH₂, NH, and N. The generated NH₂ radicals will react with NO molecules to form the intermediate product NH₂NO, but due to the instability of the intermediate product, it will further react and ultimately decompose into N₂ and H₂O molecules. Under aerobic conditions in the presence of O₂, NH₂ radicals will also undergo dehydrogenation via surface oxygen and generate NH radicals. NH radicals also react with NO molecules to generate the intermediate product NHNO, which is unstable and ultimately decomposes into N₂ and H₂O molecules. In the SCR reaction at Mn active sites, the NH₂ radical dehydrogenation process has the highest energy barrier under aerobic conditions, which is the rate control step of Mn active sites. In the SCR reaction at Fe active sites, the NH₂ radical dehydrogenation process in the absence of oxygen has the highest energy barrier, indicating that this process is the rate control step of Fe active sites. Comparing the SCR reaction process between Mn and Fe active sites, it can be found that the introduction of Fe active sites doped with some iron elements can increase the generation of NH₂ and NH radicals and simplify the reaction process between NH₂ radicals and NO molecules, which will be conducive to the completion of the reaction.