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. 2020 Oct 12;5(42):26978–26985. doi: 10.1021/acsomega.0c00018

Theoretical Study of the Catalytic Activity and Anti-SO2 Poisoning of a MoO3/V2O5 Selective Catalytic Reduction Catalyst

Yanxiao Chai 1, Guizhen Zhang 1, Hong He 1,*, Shaorui Sun 1,*
PMCID: PMC7593995  PMID: 33134658

Abstract

graphic file with name ao0c00018_0010.jpg

In this paper, density functional theory has been applied to study the mechanism of anti-SO2 poisoning and selective catalytic reduction (SCR) reaction on a MoO3/V2O5 surface. According to the calculation results, the SO2 molecule can be converted into SO3 on V2O5(010) and further transformed into NH4HSO4, which poisons V2O5. If V2O5 and MoO3 are combined with each other, charge separation of V2O5 and MoO3, which are negatively and positively charged, respectively, occurs at the interface. In ammonium bisulfate liquid droplets on the MoO3/V2O5 surface, NH4+ tends to adhere to the V2O5(010) surface and can be removed through the SCR reaction and HSO4– tends to adhere to the MoO3(100) surface and can be resolved into SO3 and H2O, which can be released into the gas phase. Thus, MoO3/V2O5 materials are resistant to SO2 poisoning. In the MoO3/V2O5 material, Brønsted acid sites are easily formed on the negatively charged V2O5(010) surface; this reduces the energy barrier of the NH3 dissociation step in the NH3-SCR process and further improves the catalytic activity.

1. Introduction

NOx is one of the main air pollutants contributing to the formation of complex pollutants such as acid rain and town smog. Selective catalytic reduction (SCR) in which NH3, as a reducing agent, reacts with NOx in the presence of O2 to produce N2 and H2O is the technique most widely used to reduce NOx emitted from stationary and mobile sources.1−4

At present, commercial NH3-SCR catalysts employ V2O5-based catalysts supported on the anatase phase of TiO2. It is reported that the anatase phase of TiO2 has poor mechanical strength and that high loading of V2O5 promotes the transformation of anatase to the rutile phase, which increases the material’s catalytic activity.5 To overcome this shortcoming, SCR catalysts with high activity can be obtained by doping V2O5-based catalysts with other metal oxides; for example, the addition of WO3 and MoO3 can improve the stability and the catalytic activity of the catalysts.6−9 However, V2O5–WoO3–Ti catalysts lose their activity quickly when exposed to waste gases containing arsenic, whereas V2O5–MoO3–TiO2 catalysts can maintain their stability in the presence of arsenic.10−12 Xu et al.13 showed that the addition of MoO3 improved the sulfur resistance of the catalyst. The mechanism through which the addition of MoO3 increases the activity of V2O5–TiO2 has been investigated. Lietti et al.6,8,9 suggest that the interaction between vanadium species and the metal oxide after the addition of MoO3 improves the catalytic activity. Qiu et al.14 consider that the introduction of MoO3 enhances the adhesion between vanadium species and titanium dioxide.

The mechanism of the SCR reaction on vanadium-based catalysts supported by titanium dioxide has been widely studied. The first step in the commonly accepted SCR reaction pathway is the adsorption of ammonia on the catalytic surface. The adsorbed ammonia then reacts either with the adsorbed nitric oxide (Langmuir–Hinshelwood-type mechanism) or directly with the gaseous nitric oxide (Eley–Ridel-type mechanism) to form surface reaction intermediates, which are decomposed into nitrogen and hydrogen; the vanadium oxide sites are reduced. The reduced vanadium oxide surface is re-oxidized by oxygen molecules in the gas phase, returning the surface to its original state.15−18 However, it is still controversial whether the Lewis acid site or the Brønsted acid site is the active site of the reaction.21−23 Topsoe et al.19,20 determined the active sites on the surface of vanadium-based catalysts through in situ on-line Fourier transform infrared19,20 and found a direct relationship between the B acid site and the catalytic activity. It was considered that the catalytic activity was related to the ammonia species adsorbed on the B acid site. Vittadini et al.21 studied the process of catalysis of NH3 and NO by vanadium species on a titanium dioxide surface by density functional theory (DFT). It was found that NH3 participated in the SCR reaction and that the SCR reaction occurred even in the absence of a B acid site. Yao et al.22 also studied the SCR reaction process on a V2O5(001) surface using the DFT method. It is believed that NH4+ is more conducive to reducing NO than NH3. NH4+ participates directly in the formation of the active intermediate V2O5HH, but the formation of V2O5HH by V2O5H is very difficult. Zhu et al.23 investigated the reactivity of surface ammonia species at Lewis and Brønsted acid sites by time-resolved in situ infrared spectroscopy. It was found that although ammonia species at both the Brønsted and the Lewis acid sites participated in the SCR reaction and that surface NH4+ was more abundant, a small amount of NH3 showed a higher SCR catalyst specific activity.

In the SCR reaction process, SO2 in flue gas is transformed into SO3, which can react with NH3 to generate ammonium bisulfate adhering to the surface of the catalyst, resulting in poisoning of the catalyst. The addition of MoO3 to the V2O5/TiO2 catalyst not only improves its catalytic activity but also delays the conversion of SO2 to SO3 and improves the sulfur resistance of the catalyst.8,9,24 For the SO2 oxidation process, Dunn et al.25 concluded that the conversion of SO2 was independent of the coverage of titanium dioxide by vanadium species. A subsequent study by Ji et al.26 showed that the Ti–O–V and V–O–V bonds in the catalyst did not play a key role in the oxidation of SO2. It was speculated that only one surface vanadium site was needed for SO2 oxidation and that the terminal V=O bond participated in SO2 oxidation.

The following uncertainties about the V2O5–MoO3/TiO2 catalyst remain: (1) although it is known that the addition of MoO3 to the V2O5/TiO2 catalyst improves its catalytic activity, the mechanism of this effect is not completely clear; (2) the involvement of Brønsted and Lewis acid sites in the SCR reaction is still controversial; (3) the idea that the oxidation process of SO2 to SO3 is related only to the terminal V=O bond of the V2O5 surface still lacks supporting evidence; and (4) the mechanism that results in sulfur resistance of the V–MoO3/TiO2 catalyst has not been thoroughly studied. In the current work, we studied the mechanism through which the addition of MoO3 improves the catalytic activity and sulfur resistance of V/Ti catalysts using the DFT method.

2. Results and Discussion

2.1. Formation of NH4HSO4

2.1.1. SO2 is Oxidized to SO3 on the Surface of V2O5(010)

The adsorption of SO2 on three different sites of V2O5(010) (OI, OII, and OIII), as shown in Figure 1, was theoretically explored. The three oxygen sites on the surface of V2O5(010) can be classified into three types according to the number of connections with vanadium atoms: (1) OI connected to a vanadium atom; (2) OII with two vanadium atoms; and (3) OIII with three vanadium atoms.

Figure 1.

Figure 1

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V2O5(010) model and OI, OII, and OIII oxygen sites used in the calculation. (a) Top view of the V2O5(010) model and (b) side view of the V2O5(010) model. The purple and red circles represent V and O atoms, respectively.

The oxidation of SO2 to SO3 at the three oxygen sites was simulated. At the beginning, SO2 is adsorbed by an oxygen atom on the V2O5(010) surface. The SO2 then combines with the oxygen atom and is transformed into SO3. SO3 is desorbed into the gas phase and forms a surface with oxygen vacancies. Oxygen molecules in the gas phase occupy the oxygen vacancies and combine with another SO2 molecule. O2 dissociates to form SO3; at the same time, the remaining oxygen atom fills the vacancy on the V2O5(010) surface. The adsorption and dissociation energies of each step are presented in Figure 2.

Figure 2.

Figure 2

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Reaction mechanism for the formation of SO3 from SO2 on the OI, OII, and OIII sites of the V2O5(010) surface. The purple, red, and yellow circles represent V, O, and S atoms, respectively.

We found that SO2 was more easily adsorbed on the OI site; the adsorption energy at that site was −0.194 eV. The energy barrier of this process was 0.54 eV. The adsorption energies at the OII and OIII sites were 1.18 and 0.33 eV, respectively. In the first SO3 dissociation process, the energy change at the OI site is −0.045 eV; the energy changes at the OII and OIII positions are 0.363 and 1.523 eV, respectively. Thus, SO3 is more easily dissociated at OI (Table 1).

Table 1. Adsorption and Desorption Energies of Each Step in the Oxidation of SO2 to SO3 at Three Different Oxygen Sites (OI, OII, and OIII) on the V2O5(010) Surfacea.
  ΔE/eV
sites SO2ads(1) SO3des(1) O2ads SO2ads(2) SO3ads(2)
1V–O –0.194 –0.045 0.064 –2.183 0.326
2V–O 1.181 0.363 –0.843 –2.981 0.248
3V–O 0.306 1.523 –1.793 –2.500 0.432
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a

SO2ads(1) and SO3des(1) are the initial processes of SO2 adsorption and SO3 desorption, respectively. SO2ads(2) and SO3des(2) are the second SO2 adsorption process and the second SO3 desorption process, respectively.

The oxidation of SO2 in the gas phase was also simulated; the barrier to the SO2 reaction with oxygen in the gas phase is 2.99 eV (see Figure 3). The rate-controlling step barrier of SO2 oxidation to SO3 on the V2O5(010) surface is 0.54 eV. It can be concluded that SO3, which participates in the formation of NH4HSO4 and causes catalyst poisoning, is produced on the surface of V2O5.

Figure 3.

Figure 3

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Reaction barrier for the formation of SO3 from SO2 in the gas phase. The red and yellow circles represent O and S atoms, respectively.

2.1.2. Formation of NH4HSO4 on the V2O5(010) Surface

The process through which ammonium bisulfate is formed on the V2O5(010) surface is shown in Figure 4. The H2O and NH3 in the gas phase are first adsorbed on the surface of the catalyst. The H2O is adsorbed on the Lewis site (on the top of the V site), and the NH3 is adsorbed on the OIII site. The energy change in this process is −9.69 eV. Subsequently, the adsorbed *OH2 dissociates on the surface of V2O5 to form *NH4 and *OH groups. The energy barrier of the abovementioned process is −1.49 eV. When SO3 exists in the gas phase, the NH4–O and VOH groups can further combine with SO3 to form ammonium bisulfate.

Figure 4.

Figure 4

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(a) Energy profile for the formation of ammonium bisulfate on the V2O5(010) surface; (b) optimized geometries of the reactant, transition states, intermediate, and product for all elementary steps in the formation of ammonium bisulfate on the V2O5(010) surface. The purple, red, pink, blue, and yellow circles represent V, O, H, N, and S atoms, respectively.

From the abovementioned data, it is obvious that the reaction in which SO2 is converted to SO3 is catalyzed by V2O5 and that NH4HSO4 can be produced on the V2O5(010) surface. NH4HSO4 and H2O mix with each other to form small liquid droplets that further cover the V2O5 surface; this is the main reason for SCR catalyst poisoning.

2.2. Charge Transfer at the Interface between MoO3 and V2O5

Work function is an important physical quantity reflecting the energy of electron transmission. In solid-state physics, it is defined as the minimum energy needed to transfer an electron from the interior of a solid to the surface of the object.27 The specific value of the work function is the difference between the vacuum level and the Fermi level on a solid surface. It is calculated as follows

2.2. 1

We calculated the work functions of V2O5(010) and MoO3(100) surfaces, as shown in Figure 5.

Figure 5.

Figure 5

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(a) Schematic diagram of Fermi energy levels on V2O5 and MoO3 surfaces; (b) schematic diagram of NH4+ and HSO4– adhesion on the catalyst surface; and (c) work functions of the V2O5(010) and MoO3(100) surfaces.

The work function of the V2O5(010) surface is 7.81 eV and that of the MoO3(100) surface is 6.40 eV. The value of the surface work function of V2O5(010) is 1.14 eV larger than that of MoO3(100). When V2O5 is in contact with MoO3, the electrons on the MoO3 surface spill over and flow from the MoO3 surface to the V2O5 surface, resulting in the presence of negative charges on V2O5 and positive charges on MoO3 close to the interface. Combining the charge separation, an internal electric field is set up at the same time.

When an NH4HSO4 liquid droplet covers the MoO3/V2O5 catalyst, NH4+ ions preferably move to the negatively charged V2O5 surface and HSO4– moves to the positively charged MoO3 surface under the internal electric field.

2.3. SCR Reaction from NH4+ on the V2O5(010) Surface

The energy profile and optimized geometries for all elementary steps of the SCR process from NH4+ are shown in Figure 6. When the adsorbed NH4+ occupies the OI site of the charged V2O5(010) surface, it dissociates to adsorbed NH3 and a H atom (ii–iii). In this step, NH3 is adsorbed on the Lewis acid site and H is at an OI site. The adsorbed NH3 is activated to form NH2 intermediates, releasing a hydrogen atom, and the released H atom is combined with the OH group at the OI site to form adsorbed *OH2 (iii–iv); the energy change in this step is 1.17 eV. The energy barrier from ii to iv is 1.49 eV. The intermediate NH2 binds to NO in the gas phase to form a nitrosamine (NH2NO) intermediate (iv–v). Subsequently, NH2NO is decomposed into N2 and H2O (v–vii). The remaining surfaces with oxygen vacancies can be oxidized, restoring the surface to its original state (vii–i). This completes a catalytic cycle.

Figure 6.

Figure 6

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Mechanism of the SCR process from NH4+. (a) Mechanism of the standard NH3-SCR reaction from NH4+ and (b) structures present at each state; the purple, red, pink, and blue circles represent V, O, H, and N atoms, respectively. (c) Energy profile of each step in the standard NH3-SCR reaction (the energy barrier in the rate-determining step is marked in red).

For comparison, the energy profile and the optimized geometries for all elementary steps of the SCR process from NH3 on the V2O5(010) surface are shown in Figure 7. The NH3 adsorbed on the L-acid site first dissociates across the energy barrier of 2.27 eV to form NH2 and OH groups (ii–iii). Subsequently, NH2 binds to NO in the gas phase to form a nitrosamine intermediate (NH2NO), which is easily decomposed into N2 and H2O (iii–vi). When a V–OH group is formed, the energy of NH3 dissociation to form NH2 and H at the Lewis site changes to −1.09 eV. However, the adsorption energy of NH3 at the Lewis acid site changes to 1.80 eV (vi–vii); this represents a great change compared with the energy required during the initial NH3 adsorption process (i–ii), and this step can thus be regarded as another rate-determining process. The H atom derived from the decomposition of NH3 can then combine with VOH to form VOH2. Hydrogen dioxide is desorbed into the gas phase (viii–ix). The NH2 intermediate binds to NO in the gas phase to form the NH2NO intermediate, which can further form N2 and H2O (ix–xi). The remaining surfaces with oxygen vacancies are oxidized to complete surfaces with oxygen in the gas phase (xi–i).

Figure 7.

Figure 7

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Mechanism of the SCR process from NH3. (a) Mechanism of the standard NH3-SCR reaction from NH3 and (b) structures present at each state; the purple, red, pink, and blue circles represent V, O, H, and N atoms, respectively. (c) Energy profile of each step in the standard NH3-SCR reaction (the energy barrier at the rate-determining step is shown in red).

For the SCR from NH3 on the pure V2O5(010) surface, the energy barrier of the reaction-controlling step is 2.27 eV. For NH4+ on the charged V2O5(010) surface (composited with MoO3), the energy barrier is 1.49 eV. The latter value is obviously smaller than the former, suggesting that the NH4+ ions could be eliminated.

NH4+ and protons also prefer to occupy the OI site, which is the catalytic site for oxidizing SO2 on the V2O5 surface. These two processes hinder the transformation of SO2 to SO3 and reduce SO2 poisoning.

The results of the calculation also show that the energy barrier of the SCR reaction is greatly reduced when the V4+=OH group is present. According to transition state theory, the reduction of the energy barrier will increase the reaction rate.28 The presence of NH4+ and V4+=O groups enhanced the reaction activity, making it equivalent to the activity that occurs on the SCR catalyst.

2.4. Decomposition of HSO4– on the MoO3(100) Surface

The perfect MoO3(100) surface does not easily adsorb HSO4– ions. However, oxygen vacancies can be produced on the surface, and the energy increment is approximately 0.99 eV. If oxygen vacancies exist, HSO4– can easily be adsorbed on the MoO3 surface. When HSO4– is adsorbed on the oxygen vacancy positions of MoO3, it can be further decomposed into SO3 and an adsorbed OH* group on the surface (iii–iv). The proton in the OH* group migrates on the surface with an energy barrier of 0.98 eV; when an oxygen atom combines with two protons, H2O with an oxygen vacancy forms on the surface (vi–vii). Through this process, which is shown in Figure 8, HSO4– ions on the MoO3(100) surface can be eliminated.

Figure 8.

Figure 8

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Reaction mechanism of HSO4– decomposition into SO3 on the MoO3(100) surface. (a) Optimized geometries of the reactants, transition states, intermediates, and products for all elementary steps in the reaction mechanism of HSO4– decomposition into SO3 on the MoO3(100) surface; the red, gray, pink, and yellow circles represent O, Mo, H, and S atoms, respectively. (b) Energy profile of each step in the reaction process.

Figure 9 presents the catalytic process for SCR and the mechanism of anti-SO2 poisoning. First, charge separation occurs on the MoO3/V2O5 interface because of the difference in work functions; V2O5 is negatively charged and MoO3 is positively charged around the interface. Second, NH4+ from ammonium bisulfate is preferably adsorbed on charged V2O5 and HSO4– is preferably adsorbed on MoO3. Third, in the SCR catalytic reaction, NH4+ and NO are transformed into H2O and N2 and HSO4– is decomposed into SO3 and H2O.

Figure 9.

Figure 9

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Sketch showing the SCR catalytic process and the mechanism of anti-SO2 poisoning. (a) Charge separation process on the MoO3/V2O5 interface; (b) ionization process of ammonium bisulfate; and (c) SCR catalytic process.

3. Conclusions

Through DFT calculations, it has been clearly shown that SO2 molecules can be catalyzed into SO3 and, in the presence of NH3 and H2O, further transformed into NH4HSO4, which is the cause of SO2 poisoning. When V2O5 is combined with MoO3, some electrons transfer from MoO3 to V2O5, creating a negatively charged V2O5 surface and positively charged MO3. If an ammonium bisulfate liquid droplet adsorbs on the MoO3/V2O5 surface, NH4+ adheres to the terminal oxygen position on the V2O5(010) surface and can be eliminated in the subsequent SCR reaction. HSO4– adheres to the MoO3(100) surface and can be transformed into SO3 and H2O, which can be released from the MoO3 surface. In this way, NH4HSO4 on the MoO3/V2O5 surface can be continuously decomposed. In the MoO3/V2O5 material, the negatively charged V2O5(010) surface promotes the formation of Brønsted acid sites. The presence of Brønsted acid sites reduces the energy barrier of the NH3 dissociation step in the NH3-SCR process and thus improves the catalytic activity.

4. Computational Methodology

The theoretical calculations are performed using the Vienna Ab initio Simulation Package (VASP).29 The projector augmented wave30 method was used to treat the ion–electron interaction. The electron exchange–correlation function of Perdew–Burke–Ernzerhof31 was adopted. For the V-3d states and Mo-4d states, the strong correlation energy was calculated with the LDA + U method,32 and the values of effective interactions (U–J) were set as 4.0 and 6.3 eV, respectively.

The V2O5(010) surface is the most thermodynamically stable single-crystal surface of V2O5,33 and its electronic properties are very similar to those of the bulk material.34 The V2O5(010) surface was constructed on the bulk crystal with the Pmn21 symmetry, a 2 × 1 two-dimensional supercell (a = 7.188 Å, b = 11.468 Å) containing five layers was constructed, and the width of the vacuum layer was more than 20.0 Å. k-Point sampling used in the V2O5(010) model was 2 × 1 × 1. The MoO3(100) structure was constructed on the bulk crystal with Pnma symmetry, a 2 × 2 two-dimensional supercell (a = 7.503 Å; b = 7.761 Å) containing five layers was constructed, and the width of the vacuum layer was more than 20.0 Å. k-Point sampling used in the MoO3(100) model was 2 × 2 × 1.

The adsorption energy, Eads, was calculated according to the expression

4. 2

where Eads is the adsorption energy, Eadsorbed-species+surface is the energy of the given geometry containing the V2O5 surface and the adsorbing molecule, Eadsorbed-species is the energy of the gas-phase mercury-containing species, and Esurface is the energy of the V2O5 surface or the MoO3 surface.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC nos. 21676004 and 21777004).

The authors declare no competing financial interest.

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