Investigation of the Effect of SO2 and H2O on VPO-Cr-PEG/TiO2 for the Low-Temperature SCR de-NOx

A catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti for low-temperature SCR de-NO_x was developed and the effects of SO₂ and water vapor on its catalytic activity were investigated. Cr doping increases the molar ratio of V⁵⁺ to V⁴⁺ on the VPO catalyst, which promote oxidation of NO to NO₂ and catalytic activity of the VPO catalyst. An appropriate amount of redox-coupled V⁵⁺/V⁴⁺ also promotes catalytic activity of the VPO catalyst. The denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was above 98% at 150–350°C. Cr doping also promotes the generation of Lewis acid around V⁵⁺ center and Brønsted acid (P-OH) on the VPO catalysts. The strong surface acidity of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti could restrain the adsorption of and oxidation of SO₂. Characterization (FT-IR, TG) results indicate that nearly no sulfate was deposited on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti after activity test. The competitive adsorption of water molecules and nitric oxide on the catalyst surface decreases the catalytic activity of the VPO catalyst. The addition of Cr and PEG could increase the surface area and the exposure of active site of VPO catalyst, which also increase the unoccupied active sites of VPO catalysts in the presence of water vapor. The catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti for low-temperature SCR de-NO_x shows high catalytic activity and exhibits high resistance to SO₂ and water vapor.

Introduction

In addition to coal-fired flue gas, industrial waste gases such as coke oven flue gas, sintering flue gas, and garbage incineration flue gas are other major sources of NO_x (Chen et al., 2015; Gamrat et al., 2016; Li et al., 2018). Among these emission sources, NO_x is discharged mainly in the form of nitric oxide (NO). The selective catalytic reduction (SCR) process is currently considered to be an efficient technology for controlling NO_x emissions (Niu et al., 2016; You et al., 2017). Although the V₂O₅-WO₃(MoO₃)/TiO₂ catalyst has been widely adopted as a superior catalyst for controlling NO_x in coal-fired flue gas, the highest reactive activity of V₂O₅-WO₃(MoO₃)/TiO₂ is usually at the temperature range of 300–400°C (Niu et al., 2016; You et al., 2017). However, the temperature of coke oven flue gas, sintering flue gas, and garbage incineration flue gas is below 250°C (Chen et al., 2015; Gamrat et al., 2016). The development of catalyst for low-temperature SCR denitration is extremely urgent for the control of NO_x emission from industrial flue gas.

Much effort has focused on developing catalysts for low-temperature denitration. Different metal oxides, supporters, and polymetallic catalysts have been investigated for low-temperature denitration (Andreoli et al., 2015; Stakheev et al., 2015; Cha et al., 2016; Chen et al., 2016; Wang et al., 2016). The denitration efficiency of some of these catalysts could reach more than 95% at temperature of 150°C. However, catalyst activities were generally inhibited by SO₂ and water vapor because of deposition of (NH₄)₂SO₄ and metal sulfates on the surface of catalyst (Cha et al., 2016; Wang et al., 2016). Accordingly, the development of low-temperature denitration catalysts with excellent SO₂ resistance and water resistance is urgent.

The surface acidity of a catalyst plays an important role in the SCR denitration process (Cha et al., 2016). The Eley-Rideal mechanism indicates that NH₃ is adsorbed on the Brønsted acid site and transforms into ${NH}_4^{+}$ ions (Yu et al., 2016). This species reacts with NO to form an activated complex and then decomposes into N₂ and H₂O. SO₂ is an acidic gas, the adsorption of SO₂ on the surface of the catalyst would be restrained by increasing the surface acidity of the catalyst, which could promote the sulfation resistance of SCR catalyst. Vanadium is a widely used as a catalyst component because of its superior redox properties (Busca et al., 1998; Phil et al., 2008; Zhao et al., 2015). Vanadium phosphorus oxides (VPO), which is prepared by reacting vanadium oxides and phosphoric acid, is a type of heteropolyacid. Busca et al. investigated the oxidation of alkanes with VPO and the results show that there are abundant Brønsted acid sites (V-OH, P-OH) on the surface of VPO catalyst (Busca et al., 1986; Bond, 1991; Feng et al., 2015). Moreover, the polarization effect of the V-(O-P) bond could promote the formation of Lewis acid sites on the V⁴⁺ center due to the high electronegativity of P (Bond, 1991; Benziger et al., 1997).

Water vapor is another factor which degrades the catalytic performance of low-temperature denitration catalysts. Zhang et al. proposed that the competitive adsorption of water molecules and nitric oxide on the catalyst surface is responsible for the degraded catalytic performance (Zhang et al., 2014). Adsorption and desorption equilibrium of water vapor exist on catalyst surface and the proportion of active sites occupied by water molecules is constant at a certain temperature (Melánová et al., 1999). The amount of unoccupied active sites on the surface of catalysts increases as the specific surface area of the catalyst increases. Accordingly, optimizing the structure and specific surface area of the catalyst could increase its catalytic activity in the presence of water vapor.

The surface acidity and structural properties of a catalyst are closely related to its resistance to SO₂ and water vapor (Cha et al., 2016). Vanadium phosphorus oxides (VPO) have been investigated for the oxidation of alkane by several researchers because of their excellent redox properties and surface acidities (Busca et al., 1986; Bond, 1991; Benziger et al., 1997; Melánová et al., 1999; Feng et al., 2015). However, VPO catalyst has been rarely investigated for SCR denitration and the specific surface area of VPO is usually below 20 m²/g. Bagnasco et al. reported that the layered crystalline VPO would be formed by isomorphous substitution with some trivalent metals (Al, Cr, Fe, etc.) and the surface morphology and structural properties of VPO could also be ameliorated by trivalent metals (Bagnasco et al., 1990). In this study, Cr modified VPO catalysts with titanium dioxide as the support material were prepared and polyethylene glycol (PEG) was used to improve the dispersion of active components. The low-temperature SCR denitration performance of VPO-Cr-PEG/TiO₂ and the effect of SO₂ and water vapor on catalytic activity of VPO-Cr-PEG/TiO₂ were investigated. Combining with activity test and characterization, the physicochemical properties of the VPO-Cr-PEG/TiO₂ were also investigated.

Experimental

Catalyst Preparation

VPO-Cr was prepared according to a liquid-phase synthesis method. Ammonium metavanadate and chromic nitrate were added to an oxalic acid solution according to a certain mole ratio and the solution was stirred for 1 h. Subsequently, the mixture was stirred for a further 2 h after adding a certain amount of phosphoric acid (85 wt.%), HCl solution(36–38 wt.%), and PEG. Thereafter, the mixture was successively evaporated at 90°C and it was transformed into wet gel. The wet gel was dried at 105°C for 4 h. Afterwards, the product was calcined at 350°C for 3 h, and the active ingredient VPO-Cr-PEG was obtained.

Using commercially available TiO₂ (specific surface area is 322 m²/g) as support material, the VPO-Cr-PEG/TiO₂ catalyst was prepared by impregnation method. First of all, appropriate quantities of TiO₂ and VPO-Cr-PEG were added into distilled water and the mixture was stirred for 2 h. Then the mixture was evaporated in a water bath at 70°C and the solid obtained was calcined at 350°C for 3 h. The catalyst prepared at different conditions such as P/V (molar ratio of P to V, x), Cr/V (molar ratio of Cr to V, y), PEG/V (molar ratio of PEG to V, z) and the weight percentage of active ingredient (w) were denoted as wVP(x)O-Cr(y)-PEG(z)/Ti.

Catalyst Characterization

The specific surface area and average pore size were measured by a surface area analyzer (V-sorbet 2008S). The morphologies of the catalysts were examined with a scanning electron microscope (SEM) operated at 30 kV (JEOLJSM-6380LV). The chemical composition of the catalysts was analyzed by X-ray photoelectron spectroscopy (ESCALAB 250). The redox performance and NO adsorption properties of the catalysts were measured with a Chembet Pulsar TPR/TPD. The crystal structure was determined by X-ray diffraction (XD-3). Infrared spectroscopy measurements were performed with an FT-IR spectrometer (Bomen MB154S).

Activity Test

The SCR denitration activities of the VPO-Cr-PEG/TiO₂ catalysts were measured in a conventional fixed-bed reactor (inner diameter = 8 mm). The diagram of the experimental system is present in Figure 1.

FIGURE 1

Figure 1. Diagram of the fixed-bed reactor (1-mass flowmeter, 2-preheater, 3-reactor, 4-catalyst, 5-thermocouple, 6-condensator, 7-flue gas analyzer).

Flue gas was simulated by mixing 0.05 vol.% NO, 0.05 vol.% NH₃, 6 vol.% O₂, 0–0.1 vol.% SO₂, and 0–16 vol.% water vapor. The mixture was balanced with N₂. The flow rate of each gas was controlled by mass flowmeter. The total gas flow rate was set to 100 mL/min and the corresponding gas hourly space velocity was about 15,000 h⁻¹. Liquid water was injected into the gas pipeline located at the center of a preheater (120°C) and it was evaporated to form water vapor. The NO_x concentration at the inlet and outlet of the fixed-bed reactor were measured with a gas analyzer (MRU Varioplus). The denitration efficiency η is expressed as:

$\begin{array}{ll}\eta =\left(C_{in}-C_{\mathrm{out}}\right)/C_{in}\times 100& (1)\end{array}$

Results and Discussion

SCR Denitration Activity

The NH₃-SCR deNO_x activity of the catalysts were tested at 500 ppm of NH₃, 500 ppm of NO, 6 vol.% of O₂, and 15,000 h⁻¹ GHSV. The denitration efficiency of catalysts at different temperature are shown in Figure 2.

FIGURE 2

Figure 2. Denitration efficiency of the supported VPO catalysts.

Obviously, the denitration efficiencies of the catalysts increase as temperature increases. The denitration efficiency of 0.1VP(0.2)O/Ti is 93% at 150°C and reaches 99.3% at temperatures above 200°C. The denitration efficiencies of Cr-PEG-modified VPO/Ti catalysts are above 98% at temperatures in the range 150–350°C. Obviously, the addition of Cr and PEG could improve the catalytic activity of VPO/Ti, especially at low reaction temperature.

Influence of SO₂ on Catalytic Activity

Selectivity of catalyst is an important parameter for SCR denitration performance. The feed gas consisted of 0.05 vol.% NO, 0.08 vol.% SO₂, 6 vol.% O₂ and balanced with N₂ was passed through catalyst, and the concentration of SO₂ and NO_x in the outlet gas were measured. Corresponding results are presented in Figures 3, 4.

FIGURE 3

Figure 3. Outlet concentration of NO_x and SO₂ vs. time over 0.1VP(0.2)O/Ti catalyst.

FIGURE 4

Figure 4. Outlet concentration of NO_x and SO₂ vs. time over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst.

Figure 3 shows the outlet concentration of SO₂ and NO_x vs. time when the feed gas passed through the 0.1VP(0.2)O/Ti catalyst. The outlet concentration of SO₂ increases as time increases and it reached equilibrium and was almost the same as the inlet concentration after 40 min. Results of Figure 3 also indicate that the outlet concentration of NO_x reached equilibrium at about 90 min.

Figure 4 shows the outlet concentration of SO₂ and NO_x vs. time when the feed gas passed through the 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst. The outlet concentration of NO_x and SO₂ reached equilibrium a little faster than that of the feed gas passed through 0.1VP(0.2)O/Ti. Both results in Figures 3, 4 also indicate that almost no SO₂ oxidized when the feed gas passed through the VPO catalysts.

The feed gases with SO₂ mentioned above were cut off after they were in contact with catalysts for 6 h. 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti were then separately flushed with nitrogen (200 mL/min) for 1 h in order to remove the physically adsorbed substance. Thereafter, the NH₃-SCR deNO_x activities of the 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti were tested, and experimental results are shown in Figure 5. The experimental conditions were the same as described in Section SCR denitration activity. Compared with the results in Figure 2, the catalytic activity for SCR-deNO_x of the 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti in Figure 4 did not decrease after exposure to SO₂. It indicated that the active components of 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti are not sulfated.

FIGURE 5

Figure 5. Denitration efficiency of catalysts after SO₂ treatment.

Influence of Water Vapor on Catalytic Activity

The NH₃-SCR deNO_x activity of catalysts were tested in a feed gas containing 0.05 vol.% NH₃, 0.05 vol.% NO, 6 vol.% O₂, and 4–8 vol.% water vapor. The effect of water vapor concentration on the NH₃-SCR deNO_x catalytic activity of catalysts is shown in Figure 6.

FIGURE 6

Figure 6. Effect of water vapor on denitration efficiency.

One can see from Figure 6 that the denitration efficiency of 0.1VP(0.2)O/Ti decreased from 99.8 to 99.3% as water vapor concentration increased from 4 to 6 vol.%. A similar trend was also observed for 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti. One possible reason is that the number of active sites occupied by water molecules increases as the concentration of water vapor increases. As can be seen from Figure 6, the denitration efficiency of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was slightly higher than that of 0.1VP(0.2)O/Ti when the water vapor concentration was above 12 vol.%. The reason may be that Cr doping ameliorate the surface and structural properties of VPO and PEG improve the dispersion of active components. There are more unoccupied active sites on surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti than that of 0.1VP(0.2)O/Ti in the presence of water vapor in feed gas.

Influence of Water Vapor and SO₂ on Catalytic Activity

The feed gas containing 0.05 vol.% NH₃, 0.05 vol.% NO, 6 vol.% O₂, 8 vol.% water vapor, and 0.02–0.1 vol.% SO₂ was passed through the catalysts. The effects of SO₂ and water vapor on the denitration efficiency over the 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti were investigated and results are presented in Figure 7. It can be seen from Figure 7 that the catalytic activity did not decrease when the SO₂ concentration was below 400 ppm. As SO₂ concentration increased from 400 to 1,000 ppm, the denitration efficiency over 0.1VP(0.2)O/Ti decreased from 99.4 to 64.5% and the denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti descreased from 99.7 to 81%. Obviously, the Cr-PEG-modified VPO/Ti catalyst has higher catalytic activity than the VPO/Ti catalyst in the presence of SO₂ and water vapor.

FIGURE 7

Figure 7. Effect of SO₂ and water vapor on the denitration efficiency of the catalysts (200°C).

The sulfation of active components and the surface deposition of ammonium sulfate of catalysts are the primary reason for the decline of the NH₃-SCR deNO_x catalytic activity in the presence of SO₂ and water vapor (Cha et al., 2016; Chen et al., 2016; Wang et al., 2016).

SO₂ and water vapor were removed from the feed gas and the denitration efficiency was measured over time. These corresponding results are shown in Figure 8. Obviously, the denitration efficiency of both the 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalysts gradually recovered to the level observed when SO₂ was absent from the feed gas after 40 min. It could be concluded that the decreased catalytic activity of 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was not due to the sulfation of active components and the surface deposition of ammonium sulfate, because the sulfation of active components is irreversible and the decomposition temperature of ammonium sulfate is about 280°C.

FIGURE 8

Figure 8. Denitration efficiency of catalyst after remove SO₂ and water vapor in feed gas.

The reason for these experimental results may be that parts of ammonia react with SO₂ and water vapor in the stainless steel tube located between preheater and reactor. Accordingly, the amount of ammonia reacted with NO_x in the reactor is insufficient, which lead to the decrease of denitration efficiency. If this is the case, the denitration efficiency would increase as the ammonia concentration increases. Keeping the concentration of other gas constant as the situation presented in Figure 7, the denitration efficiency vs. NH₃ concentration was tested with SO₂ and water vapor in the feed gas, and corresponding results are present in Figure 9. Obviously, the denitration efficiency increases as NH₃ concentration increases with SO₂ and water vapor in the feed gas. When the concentration of NH₃ was above 700 ppm, the denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti is almost the same as that without SO₂ and water vapor in the feed gas. The denitration efficiency of 0.1VP(0.2)O/Ti reaches 98.3% when the NH₃ concentration is above 0.08 vol.%.

FIGURE 9

Figure 9. Effect of NH₃ concentration on the denitration efficiency in the presence of SO₂ and water vapor.

In order to further confirm the above inference, the stainless steel tube between the preheater and the reactor was heated to 120°C by an electric heating belt, and the NH₃-SCR deNO_x activity of catalysts was tested in the presence of SO₂ and water vapor. The generation of ammonium sulfite in the stainless steel tube could be restrained by electric heating belt because the decomposition temperature of ammonium sulfite is about 60°C. The experimental results in Figure 10 show that the denitration efficiency of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti is almost the same as that without SO₂ and water vapor in the feed gas. Moreover, the denitration efficiency did not decrease after 20 h of testing.

FIGURE 10

Figure 10. Effects of SO₂ and water vapor on denitration efficiency of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst.

BET Surface Area of Catalysts

The BET surface areas of the VPO catalysts are displayed in Table 1. It can be seen from Table 1 that Cr doping and PEG could increase the surface area of the VPO/Ti catalyst. The surface area of 0.1VP(0.2)O/Ti is about 171 m²/g, while that of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti is about 217 m²/g. A similar tendency was observed by Wang and Balaganapathi (Wang et al., 2003; Balaganapathi et al., 2016). PEG could reduce the surface energy during the nucleation process of the active component, which is favorable for uniform nucleation and inhibits the growth of the crystal particle. Meanwhile, PEG decomposes during the calcination of catalysts, and micropores are formed in the catalysts. Accordingly, the surface area of VPO/Ti increases by adding PEG during preparation.

TABLE 1

Table 1. BET surface area of catalysts.

Competitive adsorption of water molecules and NO_x on the catalyst decreases the denitration efficiency. The adsorption-desorption balance of water vapor exists on the surface of the catalysts at any temperature. Parts of active sites on the catalyst are occupied by water vapor, which inhibits the adsorption of other reactants. Cr doping and PEG increase the surface area and the number of unoccupied active sites on the VPO catalysts in the presence of water vapor, which enhances the water vapor resistance of the VPO catalysts. The BET results are in agreement with the activity tests in Section Influence of water vapor and SO₂ on catalytic activity.

XPS Analysis

The chemical state and atomic concentration of elements on the surface of the catalysts were investigated by XPS. The characterization results are shown in Figure 11 and Table 2. Figure 11A shows that the Ti2p spectra of both 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti could be fitted by two peaks corresponding to Ti 2p_1/2 at 464.5 eV and Ti 2p_3/2 at 458.8 eV (Reddy et al., 2006). The binding energy centered at 464.5 and 458.8 eV correspond to Ti⁴⁺. Obviously, Cr doping and PEG have little effect on the binding energy of Ti 2p_1/2 and Ti 2p_3/2 in the 0.1VP(0.2)O/Ti catalyst.

FIGURE 11

Figure 11. XPS spectra of catalysts (A) Ti 2p (B) O 1s (C) V 2p (D) Cr 2P.

TABLE 2

Table 2. Element content of catalysts.

Figure 11B indicates that the overlapped O 1s peak could be fitted by two peaks at 530.02 and 531.2 eV, which represent lattice oxygen O_β and surface chemisorbed oxygen O_α, respectively (Larachi et al., 2002). Obviously, the binding energies of O_β and O_α on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti is almost the same as that of 0.1VP(0.2)O/Ti. Moreover, the relative fractions of O_α in 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti are about 0.37.

Figure 11C shows the V 2p XPS spectra of 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti. The binding energies centered at 517.25 and 516.25 eV correspond to V⁵⁺ and V⁴⁺, respectively (Abon and Volta, 1997; Bayati et al., 2010). Although the binding energy of 0.1VP(0.2)O/Ti showed almost no shift after Cr doping, the molar ratio of V⁵⁺ to V⁴⁺ in 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti is higher than that of 0.1VP(0.2)O/Ti. The relative percentage of V⁵⁺ is 0.69 in 0.1VP(0.2)O/Ti and 0.73 in 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti. Cr doping promotes the formation of V⁵⁺ (VOPO₄) and the oxidation of NO to NO₂, which is favorable for low-temperature SCR activity (Volta, 2001; Taufiq-Yap et al., 2010). An appropriate amount of redox-coupled V⁵⁺ and V⁴⁺ also promotes the catalytic activity of the VPO catalyst (Centi, 1993; Stakheev et al., 2015; Ren et al., 2016; Salazar et al., 2016).

Figure 11D shows the Cr 2p XPS spectra of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti. The overlapped peaks at 576–580 eV could be assigned to Cr (Ren et al., 2016). The mixed peaks in Figure 11D are difficult to deconvolute, as the Cr content is relatively low.

NH₃-IR Analysis

FT-IR spectroscopy was used to analyze the surface acidity of 0.1VP(0.2)O/Ti and 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti due to ammonia adsorption. Figure 12 shows the spectrum of the adsorbed NH₃ species on the surface of the two catalysts. The band centered at 1,401 cm⁻¹ is ascribed to N-H bending vibrations in chemisorbed NH₃ on Brønsted acid sites, while the bands centered at 1,619 cm⁻¹ are assigned to ${NH}_4^{+}$ on Lewis acid sites.

FIGURE 12

Figure 12. NH₃-IR spectra of catalysts.

As Figure 12 shows, the addition of PEG and Cr to VPO/Ti could strengthen the intensity of the Brønsted acid sites and Lewis acid sites. This could be explained as follows: as the surface area of 0.1VP(0.2)O/Ti increases by adding PEG and Cr, the number of exposed surface acid sites also increases. Meanwhile, Cr doping leads to phosphorus deposition on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti, which could promote the formation of a Brønsted acid (P-OH). Moreover, the addition of Cr in VPO/Ti could increase the amount of V⁵⁺ and Lewis acid sites. A similar tendency was also observed by Pierini (Pierini and Lombardo, 2005a,b).

Brønsted acid sites on the catalyst are widely recognized as active sites for NH₃-SCR deNO_x. The NH₃-IR results in Figure 12 are in good agreement with the activity tests in Figure 2. Furthermore, 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti with strong surface acidity could restrain the surface adsorption of SO₂. The NH₃-IR spectrum further confirms the experimental results in Section Influence of SO₂ on catalytic activity and Section Influence of water vapor and SO₂ on catalytic activity.

FT-IR Spectra

Figure 13 shows the FTIR spectra of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti with/without SO₂ and water vapor in the feed gas after the activity test. The characteristic peak centered at 502 cm⁻¹ is attributed to symmetric stretching vibrations in TiO₂. The band at around 1,053 cm⁻¹ is ascribed to asymmetric stretching vibrations of V⁵⁺ = O. The band at 1,382 cm⁻¹ is attributed to nitrate. The bands around 1,450, 1,631, and 3,432 cm⁻¹ could be attributed to NH₃, an -OH group, and H₂O, respectively. The bands centered at 2,852 and 2,923 cm⁻¹ are ascribed to symmetric stretching and asymmetric stretching vibrations in methylene, respectively. Obviously, nearly no difference was observed in the FT-IR spectra for 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti after activity test with/without SO₂ and water vapor in feed gas. Moreover, the FT-IR spectra also indicate that no sulfate formed on the surface of catalyst after the activity test. The 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti shows excellent resistance to SO₂ and water vapor (Figure 10), which is further confirmed by the FT-IR spectra.

FIGURE 13

Figure 13. FT-IR spectra of catalyst.

Thermogravimetric Analysis (TG)

The TG curves of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti before and after the activity test with SO₂ and water vapor in the feed gas were present in Figure 14. Obviously, water is lost in the temperature range 25–90°C before and after the activity test. Figure 14 The bound water linked to phosphate groups is lost in the temperature range 70–130°C, and that linked to Cr³⁺ and VO³⁺ is lost between 150 and 250°C. Furthermore, mass was lost in the 300–620°C temperature range for both catalysts. This loss could be ascribed to water being coordinated to Cr in the catalysts.

FIGURE 14

Figure 14. TG curve of catalyst before and after test.

Compared to the 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst before the activity test, no accelerated mass loss in the range 200–400°C was observed in the TG curves for the 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst after the activity test. Ammonium sulfate decomposes at a temperature of 230°C, and ammonium bisulfate decomposes at a temperature of 350°C. The TG results further confirmed that there are no ammonium sulfate and ammonium bisulfate deposited on the 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti, which is consistent with the FTIR results shown in Figure 10. This could be explained as follows: SO₂ adsorption on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was restrained because of the strong surface acidity of catalysts. Meanwhile, SO₂ is hardly to be oxidize to SO₃ in the gas phase without catalyst. The ammonium sulfite generated by reaction of SO₂ with NH₃ is thermally unstable at a reaction temperature of 200°C.

Conclusion

A low-temperature SCR de-NO_x catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was developed and the effects of SO₂ and water vapor on the catalytic activity of catalyst were investigated. The activity test results show that the denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti was above 98% at temperatures in the range 150–350°C. Cr doping increases the molar ratio of V⁵⁺ to V⁴⁺ on the VPO catalyst, which promote oxidation of NO to NO₂ and catalytic activity of the VPO catalyst. An appropriate amount of redox-coupled V⁵⁺/V⁴⁺ promotes the catalytic activity of the VPO catalyst. The addition of Cr increases Brønsted acid (P-OH) and Lewis acid sites around V⁵⁺ on the VPO catalysts. The NH₃-IR spectra show that the intensity of the Brønsted acid (P-OH, V-OH) and Lewis acid of the VPO catalyst increases after Cr doping. 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti catalyst with strong surface acidity could restrain the SO₂ adsorption and the oxidation of SO₂ on the catalyst. SO₂ had little effect on the catalytic activity of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti. Characterization (FTIR, TG) results indicate that there was no ammonium sulfate deposited on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti and the active components of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti were not sulfated in the presence of SO₂ in feed gas. Generally, competitive adsorption of water molecules and reactant on the active site decreases the catalytic activity of low temperature NH₃-SCR denitration catalyst. The addition of Cr and PEG increase the surface area and the unoccupied active sites in the VPO catalysts in the presence of water vapor, which enhance the water vapor resistance of the VPO catalysts. The low-temperature SCR deNO_x catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10⁻⁴)/Ti shows high catalytic activity and exhibits good resistance to SO₂ and water vapor.

Data Availability Statement

All datasets generated for this study are included in the article/supplementary material.

Author Contributions

YJ, JJ, and JY was mainly responsible for the preparation and testing of catalysts. LG is mainly responsible for the characterization of catalysts. MG, YZ, and YC put forward their opinions on the grammar and structure of the whole paper.

Funding

This work was financially supported by the Major National R&D projects of China (2017YFB0601805) and Natural Science Foundation for the Higher Education Institutions of Anhui Province of China (KJ2019A0079).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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