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. 2022 Mar 29;7(14):12098–12110. doi: 10.1021/acsomega.2c00350

Effects of Chlorine Addition on Nitrogen Oxide Reduction and Mercury Oxidation over Selective Catalytic Reduction Catalysts

Mingxuan Ji †, Honghu Li ‡, Kang Hu †, Jiangjun Hu †,*
PMCID: PMC9016876  PMID: 35449900

Abstract

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The effect of chlorine on mercury oxidation and nitrogen oxides (NOx) reduction over selective catalytic reduction (SCR) catalysts was investigated in this study. Commercial SCR catalysts achieved a high Hg0 oxidation efficiency when Cl2 was sprayed into the flue gas. Results indicated that an appropriate concentration of Cl2 was found to promote NOx reduction and Hg0 oxidation significantly. An optimal concentration of Cl2 (25 ppm) was found to significantly promote NOx reduction and Hg0 oxidation. Moreover, we studied the effects of Cl2 on NOx reduction and Hg0 oxidation over SCR catalysts under different concentrations of SO2. The SO2 poisoning effect was decreased by Cl2 when the SO2 concentration was low (below 1500 ppm). However, sulfate gradually covered the catalyst surface over time during the reaction, which limited the impact of Cl2. Finally, different sulfur-poisoned catalysts were examined in the presence of Cl2. The NOx reduction and Hg0 oxidation performances of sulfate-poisoned catalysts improved when Cl2 was added to the flue gas. Mechanisms for NOx reduction and Hg0 oxidation over fresh catalysts and sulfate-poisoned catalysts in the presence of Cl2 were proposed in this study. The mechanism of Cl2-influenced NOx reduction was similar to that for the NH3-SCR process. With Cl2 in the flue gas, the number of Brønsted active sites increased, which improved catalytic activity. Furthermore, Cl2 reoxidized V4+–OH to V5+=O and caused the NH3-SCR process to operate continuously. The Langmuir–Hinshelwood mechanism was followed for Hg0 oxidation by SCR catalysts when Cl2 was in the flue gas. Cl2 increased the number of Lewis active sites, and catalytic activity increased. Hg0 adsorbed on the surface of the catalysts and was then oxidized to HgCl2. Adding Cl2 to the flue gas increased the strength and number of acid sites on sulfate-poisoned catalysts.

1. Introduction

Mercury is highly toxic, volatile, persistent in the environment, and can bioaccumulate in living organisms.1,2 Mercury pollution has significant impacts on human health; it leads to the loss of sensory or cognitive ability and can cause tremors, inability to walk, convulsions, and death.3 Coal-fired power plants are the major anthropogenic source of mercury emissions into the atmosphere.4,5 Once emitted into the atmosphere, elemental mercury can persist for weeks and travel long distances until it is oxidized, whereupon it can deposit in water bodies and enter the food chain.4 Mercury exists in three forms in the flue gas produced by coal combustion: elemental (Hg0), oxidized (Hg2+), and particulate-bound species (HgP). The oxidized mercury (Hg2+) and particle mercury (Hgp) can be easily removed using existing pollution control devices, such as in wet flue gas desulfurization and dedusting equipment, but Hg0 is difficult to capture because it is water-insoluble and has high volatility.6

The primary methods for capturing Hg0 are adsorbent- or oxidation-based methods.7 Activated carbon and modified activated carbon are commonly used mercury-removal adsorbents due to their large specific surface areas, high surface reactivities, and favorable pore sizes. Activated carbon adsorbs Hg0 on its surface and is usually injected upstream of a particulate control device in the removal process. However, the application of activated-carbon-based adsorbents is limited by the high cost of operation and the difficulty of reclaiming fly ash.7,8 Oxidation-based removal methods convert Hg0 into Hg2+. For example, Fenton and Fenton-like reagents were prepared by introducing Fe2+, Cu2+, and Mn2+ into hydrogen peroxide. Additional •OH ions were generated when the introduced metal ions increased H2O2 decomposition, resulting in the oxidization of Hg0 to Hg2+.7 Acidic potassium permanganate solution, commonly used as an absorbent for the Ontario Hydro method, readily oxidizes Hg0.9 However, higher costs are incurred when capturing Hg0 with Fenton reagent, Fenton-like reagent, or acidic potassium permanganate solution.

One cost-effective process for oxidizing and removing Hg0 is selective catalytic reduction (SCR).10 Coal-fired power plants widely use the SCR method for NOx reduction. NH3 reduces NOx in the flue gas over the surface of an SCR catalyst to produce N2 and H2O; Hg0 is oxidized to Hg2+ when the flue gas passes through the SCR catalyst. However, Hg0 oxidation using commercial SCR catalysts (V2O5-WO3/TiO2) typically exhibits a low efficiency.11 Therefore, two methods were proposed in previous studies to increase the efficiency of Hg0 oxidation in flue gas: (1) synthesis of effective transition-metal-oxide catalysts, such as Mn-based catalysts,12 Cu-based catalysts,13 or Co-based catalysts;14 (2) addition of halogen compounds such as hydrogen chloride (HCl) or chlorine (Cl2) into coal-fired power plant flue gas.15,16 Previous studies have investigated the effect of HCl on Hg0 oxidation over SCR catalysts, but there is limited research on the effect of Cl2 addition on this process.11,17 Furthermore, there has not been sufficient research on the effects of Cl2 addition on simultaneous NOx conversion and Hg0 oxidation over SCR catalysts.

We designed three sets of experiments in this study. In set 1, Cl2 was added to flue gas in various concentrations to investigate the effects on NOx conversion and Hg0 oxidation over the surface of fresh SCR catalysts. The mechanism of how Cl2 impacts NOx reduction and Hg0 oxidation is discussed below. In set 2, we added different concentrations of SO2 to flue gas, and the effects of Cl2 on NOx conversion and Hg0 oxidation over fresh catalysts under these conditions were investigated. Finally, in set 3, we studied the effects of chlorine addition on NOx conversion and Hg0 oxidation over different sulfur-poisoned catalysts. A proposed mechanism for the influence of Cl2 on NOx reduction and Hg0 oxidation over different sulfur-poisoned catalysts was also described.

2. Materials and Methods

2.1. Catalyst Preparation

The fresh SCR catalysts used in this study are honeycomb commercial catalysts purchased from Yigang Environmental Engineering Materials Co. Ltd., China. The honeycomb catalysts were first ground to powder and then sieved with a sifter. A powder with a particle size of <60 mesh was used in the experiments as a fresh catalyst.

The SCR catalysts poisoned by sulfate were synthesized using the fresh SCR catalysts. Fresh SCR catalysts were immersed in different sulfate solutions including 0, 0.1, 0.3, and 0.5 mol/L NH4HSO4 solution, 0.1 mol/L CaSO4 solution, and 0.1 mol/L MgSO4 solution. First, 5 g of fresh SCR catalyst was poured into 300 mL of each sulfate solution and stirred for 4 h. Then, each sulfate solution was filtered and the precipitate was dried in a desiccator for 24 h. Next, the precipitates were ground to powder and sieved with a sifter. A powder with a particle size of <60 mesh was used in the experiments as a sulfate-poisoned catalyst.

2.2. Catalyst Evaluation

The catalytic activity for NOx conversion and Hg0 oxidation was carried out in a fixed-bed flow reactor. The schematic diagram of the experimental setup is shown in Figure 1. The experimental apparatus contained four parts: a simulation flow gas generating system, a fixed-bed reactor, a flue gas test system, and a mercury gas test system.

Figure 1.

Figure 1

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Schematic diagram of the experimental system.

The simulation flow gas generating system included a gas-component-generating device and a mercury-vapor-generating device. The components of the flue gas were from cylinder gases and were precisely controlled by mass flow controllers, with a total gas flow rate of 1000 mL/min. Mercury vapor from a mercury permeation tube (HE-SR, VICI Metronics) placed in a water bath was the source of elemental mercury. The concentration of Hg0 was approximately 65 μg/m3.18 Before entering the reactor, mercury vapor was carried by N2 and mixed thoroughly with other gases in a buffer tank.

The fixed-bed reactor included a quartz tube and a tubular electric furnace. The quartz tube with an inner diameter of 10 mm and a length of 1000 mm was loaded with 0.5 g of catalyst, with the aid of quartz wool to keep the catalyst fixed in the quartz tube. The quartz tube was placed in a tubular electric furnace to maintain the reaction temperature. The NOx conversion and mercury oxidation reactions were performed at a temperature of 350 °C, which was the optimal temperature for the catalysts.

The NOx concentrations in the inlet and outlet gas were measured by a gas analyzer (Ecom-J2KN, Germany). The mercury in the simulated gas was sampled simultaneously using the Ontario Hydro method (OHM).9 Hg2+ was absorbed in the 1 mol/L KCl solution, Hg0 was absorbed in the 5% HNO3–10% H2O2 and 4% KMnO4–10% H2SO4 solution. The concentrations of Hg2+ and Hg0 in the absorbent solution were detected by inductively coupled plasma mass spectrometry (PQ-MS, Analytik Jena AG, Germany) after recovery and digestion.

In each test, it took at least 30 min to obtain steady Hgin0 and NOxin concentrations. Saturated Hg0 was adsorbed on the catalysts. The test data were recorded after 60 min of reaction. The NOx conversion and Hg0 oxidation efficiency were calculated using eqs 1 and 2

2.2. 1
2.2. 2

The three sets of experiments designed in this study are summarized in Table 1. Set 1 was conducted to confirm that Cl2 can improve NOx reduction and Hg0 oxidation efficiency over SCR catalysts and to determine the optimal Cl2 concentration for enhanced NOx reduction and Hg0 oxidation. Set 2 was conducted to show that Cl2 can improve NOx reduction and Hg0 oxidation under different SO2 concentrations. Set 3 was conducted to show that Cl2 can improve NOx reduction and Hg0 oxidation over different sulfur-poisoned catalysts.

Table 1. Design of Three Sets of Experiments.

    gas components
set catalysts SO2 (ppm) Cl2 (ppm)
Set 1 SCR 500 0–30
Set 2 SCR 0–2500 25
Set 2 SCR 0–2500 0
Set 3 NH4HSO4-SCR 500 25
Set 3 NH4HSO4-SCR 500 0
Set 3 CaSO4- SCR 500 25
Set 3 CaSO4- SCR 500 0
Set 3 MgSO4- SCR 500 25
Set 3 MgSO4- SCR 500 0
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All experiments mentioned in the table were carried out at 350 °C. The space velocity of the experiments was 3 × 104 h–1. The concentration of Hg0 was approximately 65 μg/m3.18 The concentrations of NO and NH3 were 500 ppm, and the concentrations of O2 and H2O were 5%. The concentration units of NO, NH3, SO2, and Cl2 were ppm, and the concentrations of O2 and H2O were volume fractions of the total gas flow.

2.3. Catalyst Characterization

The chemical components of the catalysts were determined using an X-ray fluorescence spectrometer (XRF). The specific surface area and pore volume were determined using the Brunauer–Emmett–Teller (BET) method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to examine the microstructure of the catalysts. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were used to analyze the reactions of the SCR process. Temperature-programmed reduction (H2-TPR) was used to find the most efficient reduction conditions of the catalysts. Temperature-programmed desorption of ammonia (NH3-TPD) was used to analyze the active sites on the catalysts. The instruments and detailed instructions are described as follows:

  • (1)

    XRF was performed using an ARL PERFORM’X sequential X-ray fluorescence spectrometer, and the results were used to determine the chemical components of the catalysts.

  • (2)

    BET: The specific surface area and pore volume were determined by N2 adsorption isotherms at 77 K using a Micromeritics ASAP 2460 analyzer.

  • (3)

    XRD patterns of the catalysts were carried out using a diffractometer and obtained in the 2θ range from 10 to 80 with Cu Kα radiation (λ = 1.54 Å).

  • (4)

    TEM was performed using a JEOL JEM-2100 instrument and was used to examine the morphology of the SCR catalysts.

  • (5)

    XPS experiments were performed using an ESCALAB 250Xi high-performance electron spectrometer, using Al Kα (1486.6eV) as the excitation source (12.5 kV, 16 mA). The sample charging effects were compensated for by calibrating all binding energies (BEs) with the adventitious C 1s peak at 284.8 eV.

  • (6)

    FTIR spectroscopy was conducted using an FTIR 5700 to analyze the chemical functional groups on the SCR catalyst surface. The SCR catalyst sample powders were mixed with potassium bromide (KBr), ground, and pressed into self-supporting disks. The ratio of the weight of the SCR catalyst sample to that of KBr was 1:100. The skeletal spectra ranged from 4000 to 400 cm–1 with a resolution of 4 cm–1.

  • (7)

    H2-TPR experiments were performed using an AUTO CHEM 2920. Prior to the H2-TPR test, 100 mg of sample was heated from room temperature to 300 °C in He gas flow at the rate of 10 °C/min for pretreatment and purged at 300 °C for 1 h in He gas flow, then cooled to 50 °C. H2 adsorption was carried out at 50 °C using a 10% H2/He gas mixture (30–50 mL/min) until saturation of the sample. Then, the sample was exposed to a flow of He (30 mL/h) to remove the weakly absorbed H2 at 50 °C. Finally, the catalyst was heated from 50 to 800 °C in He gas flow at the rate of 10 °C/min; the outlet gas was detected with TCD.

  • (8)

    NH3-TPD experiments were performed using an AUTO CHEM 2920. Prior to the NH3-TPD test, 100 mg of sample was heated from room temperature to 300 °C in He gas flow at the rate of 10 °C/min for pretreatment and purged at 300 °C for 1 h in He gas flow, then cooled to 50 °C. NH3 adsorption was carried out at 50 °C using a 10% NH3/He gas mixture (30–50 mL/min) until saturation of the sample. Then, the sample was exposed to a flow of He (30 mL/h) to remove the weakly absorbed NH3 at 50 °C. Finally, the catalyst was heated from 50 to 800 °C in He gas flow at the rate of 10 °C/min and the outlet gas was detected with TCD.

3. Results and Discussion

3.1. Effect of Cl2 Addition on NOx Reduction and Hg0 Oxidation by Fresh Catalysts

Cl2 is known to influence NOx reduction and Hg0 oxidation over SCR catalysts;11,19 thus, it is necessary to investigate these effects. The conditions of the experiments are listed in Table 1 Set 1.

3.1.1. Effect of Cl2 on NOx Conversion over Fresh Catalysts

The SCR process is typically used to reduce NOx in flue gas.20 Cl2 sprayed into the flue gas may affect NOx reduction over SCR catalysts. In this study, the impact of Cl2 on NOx reduction over SCR catalysts was studied for a series of Cl2 concentrations. As displayed in Figure 2a, the NOx conversion rate was approximately 86% on SCR catalysts in the absence of Cl2. The Eley–Rideal mechanism, wherein NH3 is adsorbed on the catalysts and reacts with NOx in the gas phase, describes NOx reduction on SCR catalysts. O2 plays a substantial role in the SCR process;21 it oxidizes the active sites on the catalyst surface and keeps the SCR process operating continuously. When 5 and 10 ppm of Cl2 were added to the gas, the NOx conversion efficiency increased slightly. When the Cl2 content was further increased from 15 to 25 ppm, the efficiency increased from 90% to approximately 93%. At higher temperatures, Cl2 becomes an even stronger oxidizing agent.22,23 In addition to O2, Cl2 can oxidize the active sites on the SCR catalysts and ensure that the process continues. In other words, Cl2 plays the same role as that of oxygen in the reaction and improves the NOx reduction reaction efficiency at 350 °C. However, when the Cl2 content increased to 30 ppm, NOx reduction reaction efficiency decreased slightly. This may correspond to poor catalyst redox properties and a decrease in the number of active sites on the catalyst surface.

Figure 2.

Figure 2

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Effect of different concentrations of Cl2 on (a) NOx conversion and (b) Hg0 oxidation efficiency over SCR catalysts.

3.1.2. Effect of Cl2 on Hg0 Oxidation over Fresh Catalysts

Gas-phase Hg0 can react with several gas-phase Cl2 compounds, such as Cl2,15,16 HCl,15 and chlorine radicals.24 Although the homogeneous reaction between Hg0 and Cl2 is too slow to cause significant Hg0 conversion, Cl2 can promote Hg0 oxidation efficiency over SCR catalysts.25 The impact of Cl2 on Hg0 removal was studied over a series of Cl2 concentrations. As presented in Figure 2b, only approximately 28% of the Hg0 was oxidized over SCR catalysts in the absence of Cl2. When the flue gas contained only 5% O2 without other Cl2 compounds, the Hg0 oxidation efficiency was very low.11,26 Thus, it is necessary to add some Cl2 compounds to improve the oxidation rate. The SCR catalysts first showed an increasing trend and then a decreasing trend in Hg0 oxidation efficiency as the Cl2 concentration increased from 5 to 30 ppm. As the Cl2 concentration increased from 5 to 25 ppm, the Hg0 oxidation efficiency increased from 48 to 89%. These results suggest that Cl2 in flue gas can promote the Hg0 oxidation process.27 As the Cl2 concentration increased from 25 to 30 ppm, the Hg0 oxidation efficiency decreased to 78%. At the highest concentrations of Cl2, SO2 may be oxidizing into high-valency sulfur compounds, such as sulfuryl chloride (SO2Cl2), sulfur trioxide (SO3), or sulfuric acid (H2SO4) (eqs 3–5), which react with ammonia and vanadium pentoxide to generate ammonium sulfates28 or vanadyl sulfate (VOSO4)29 (eqs 6–10). Figure 3 displays the FTIR spectra of the catalysts. The bands at 3724 and 3660 cm–1 can be attributed to the hydroxyl groups on TiO2.30,31 Infrared spectra of VOSO4 showed two broad bands at 1630 and 1410 cm–1 and S=O shows a band at 1410 cm–1.30,32,33 The bands at 1450 and 1703 cm–1 were assigned to the asymmetric bending vibration of NH4+ and symmetric bending of NH4+, respectively. The bands from 3000 to 2600 cm–1 caused by protonated ammonia species were also observed (2972, 2924, and 2854 cm–1).30,34,35 The FTIR spectra of the catalysts showed that (NH4)2SO4 and VOSO4 were generated on the catalysts, forming metal sulfates and ammonium sulfates that occupied the active sites on the surface of the catalysts and gradually deactivated the catalysts throughout the reaction. Thus, 30 ppm Cl2 reduced the Hg0 oxidation efficiency.

Figure 3.

Figure 3

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Infrared spectra under different conditions in the region of 4000–1200 cm–1.

As discussed in Sections 3.1.1 and 3.1.2, 30 ppm Cl2 reduced the NOx reduction reaction efficiency and the Hg0 oxidation rate. Hence, 25 ppm Cl2 was considered optimal and was used in the experiments.

3.1.2. 3
3.1.2. 4
3.1.2. 5
3.1.2. 6
3.1.2. 7
3.1.2. 8
3.1.2. 9
3.1.2. 10
3.1.2. 11
3.1.2. 12
3.1.2. 13

3.2. Effect of SO2 on NOx Reduction and Hg0 Oxidation over Fresh Catalysts in the Presence of Cl2

SO2 is known to poison SCR catalysts, destroy the catalyst structures, and reduce the catalyst activities. Thus, the influence of SO2 on the performance of fresh catalysts for NOx conversion and Hg0 oxidation efficiency requires further research. The conditions of the experiments are listed in Table 1 Set 2. One set of experiments labeled the test group was operated at 25 ppm Cl2, while the other set of experiments was run at 0 ppm Cl2 as a control.

3.2.1. Effect of SO2 on NOx Conversion in the Presence of Cl2

The activity of SCR catalysts decreased in the presence of SO2 in the flue gas, but the presence of Cl2 diminished the degree of SO2 influence. As shown in Figure 4a, as the SO2 concentration increased from 0 to 2500 ppm, the NOx conversion efficiency decreased with and without Cl2. Therefore, the existence of SO2 in the flue gas reduces the NOx conversion efficiency. However, at 500 ppm SO2 and 25 ppm Cl2, the NOx conversion efficiency was approximately 92%, which was approximately 8% higher than that without Cl2. At 1000 ppm SO2 and 25 ppm Cl2, the NOx conversion efficiency was approximately 82%, which was approximately 16% higher than that without Cl2. When the SO2 concentration increased to 1500 ppm, the results were similar, but when it was increased to 2000 or 2500 ppm, the NOx conversion efficiency was approximately the same with or without Cl2. For SO2 concentrations below 1500 ppm, Cl2 diminished the effect of SO2 on NOx conversion efficiency, whereas for SO2 concentrations above 1500 ppm, the NOx conversion efficiency showed no difference.

Figure 4.

Figure 4

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Effect of different concentrations of SO2 on (a) NOx conversion and (b) Hg0 oxidation efficiency over SCR catalysts in the presence of Cl2.

3.2.2. Effect of SO2 on Hg0 Oxidation in the Presence of Cl2

As shown in Figure 4b, as the concentration of SO2 increased from 0 to 2500 ppm, Hg0 oxidation efficiency first increased and then decreased both in the presence and absence of Cl2 in the flue gas. When SO2 concentrations increased to 500 ppm, the Hg0 oxidation efficiency reached the maximum value. With the aid of O2, low concentrations of SO2 promoted Hg0 oxidation, while high concentrations of SO2 deteriorated Hg0 oxidation.36,37 Metal-oxide-based SCR catalysts are known to oxidize SO2 to form SO3.38,39 During the Hg0 oxidation process, SO3 facilitates Hg0 oxidation and mercury sulfate (HgSO4) forms (eqs 11–13). As SO2 concentrations increase to higher levels, the inhibitive influence of SO2 prevails over its promotional effect over SCR catalysts.37 Sulfur oxides (SOx) react with ammonia, generating ammonium sulfate or ammonium bisulfate, which occupies the activity sites of SCR catalysts,28,40 or reacts with vanadium oxide, generating vanadyl sulfate (VOSO4), which destroys the structure of the catalysts.29 Thus, low SO2 concentrations facilitate Hg0 oxidation, but high concentrations lead to its inhibition. Figure 5 displays the FTIR spectra of the catalysts. The bands at 3724 and 3660 cm–1 can be attributed to the hydroxyl groups on TiO2.30,31 The FTIR spectra of VOSO4 shows two broad bands at 1630 and 1380 cm–1 and S=O shows a band at 1410 cm–1.30,32,33 The bands around 3000–2600 cm–1 caused by protonated ammonia species were also observed (2955, 2924, and 2854 cm–1).30,34,35 The FTIR spectra of the catalysts illustrated that (NH4)2SO4 and VOSO4 were generated on the catalysts. This formed metal sulfates and ammonium sulfates that occupied the active sites on the surface of the catalysts and gradually deactivated the catalysts throughout the reaction.

Figure 5.

Figure 5

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Infrared spectra of different conditions in the region of 4000–1200 cm–1.

The addition of Cl2 to the flue gas diminished the effect of SO2 on Hg0 catalytic oxidation.36,41 As shown in Figure 4b, as the concentration of SO2 ranged from 0 to 2500 ppm, the Hg0 oxidation efficiency in the presence of Cl2 was much higher than that in the absence of Cl2. The results showed that Cl2 and SO2 exhibited competition for the activity sites on the catalysts, and the addition of Cl2 into the flue gas prevented SO2 from occupying the activity sites.41 Cl2 was activated over the activity sites and reacted with Hg0 preferentially. Therefore, Cl2 diminished the effect of SO2 on Hg0 catalytic oxidation.

3.3. Effect of Cl2 on NOx Reduction and Hg0 Oxidation over Sulfate-Poisoned Catalysts

After extended operating times, different types of sulfates may cover the surface of the SCR catalysts. According to the results of the experiments described in Sections 3.1.1 and 3.1.2, Cl2 is known to improve the performance of the poisoned catalysts for NOx reduction and Hg0 oxidation. Therefore, it is necessary to thoroughly investigate the effect of Cl2 addition on NOx conversion and Hg0 oxidation by sulfate-poisoned catalysts. The preparation of sulfate-poisoned catalysts is presented in Section 2.1, and the experimental conditions are listed in Table 1 Set 3. Note that the 0 mol/L NH4HSO4 experiment was the control group.

3.3.1. Effect of Cl2 Addition on NOx Conversion by Sulfate-Poisoned Catalysts

Figure 6a presents the NOx conversion efficiency over different sulfate-poisoned catalysts in the presence and absence of Cl2. Different types of sulfates inhibited the NOx conversion efficiency by different amounts. As the concentration of NH4HSO4 increased, the inhibitory influence increased significantly. The NOx conversion efficiency of 0.5 mol/L NH4HSO4 poisoned catalysts was lower than 50%. Previous studies showed that NO can be reduced by ammonia over CaSO4,42,43 which decreases the inhibitory effect on NOx conversion efficiency. When Cl2 was added to the flue gas, the NOx reduction ability of sulfate-poisoned catalysts improved. The increase in efficiency was more than 10% over 0.3 and 0.5 mol/L NH4HSO4-poisoned catalysts. The NOx conversion efficiency over MgSO4-poisoned catalysts likewise showed a substantial increase of more than 10%. These results suggest that Cl2 can improve NOx conversion efficiency over different sulfate-poisoned catalysts.

Figure 6.

Figure 6

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Effect of Cl2 on (a) NOx conversion and (b) Hg0 oxidation efficiency over different sulfate-poisoned catalysts.

3.3.2. Effect of Cl2 on Hg0 Oxidation over Sulfate-Poisoned Catalysts

According to the results of the experiment in Section 3.2.2, the addition of Cl2 to flue gas increases the Hg0 oxidation rate. As shown in Figure 6b, the Hg0 oxidation efficiency is lower than 30% in the absence of Cl2. The effects of different sulfate-poisoned catalysts on Hg0 oxidation were not evident because SCR catalysts have a weaker ability to oxidize Hg0 when no Cl2 exists in the flue gas.11 When Cl2 was added to the flue gas, Hg0 oxidation efficiency increased to over 75%. Although sulfates had a slight inhibitory effect on the catalysts, Cl2 significantly promoted the Hg0 oxidation efficiency.

3.4. Mechanisms of Cl2 Affecting NOx Reduction and Hg0 Oxidation by V2O5-WO3/TiO2 Catalysts

3.4.1. Mechanisms of Cl2 Affecting NOx Reduction

The reduction behavior of the active components in the catalysts was studied using H2-TPR. For the H2-TPR profile under 0 ppm Cl2 (Figure 7, profile (a)), there are two reduction peaks at approximately 373 and 473 °C, which were ascribed to the reduction of V5+ to V3+ by highly dispersed, polymeric, vanadium species.44 Monomeric vanadium species are reduced at a lower temperature than polymeric vanadium species, as previously reported.44,45 Thus, the reduction peaks below 400 °C correspond to monomeric vanadium species, while the peaks at approximately 400–500 °C correspond to polymeric vanadium species. The profile under 25 ppm Cl2 (b) had an additional reduction peak at approximately from 300 to 500 °C, which was ascribed to the reduction of V5+ to V4+.46 The reduction peaks of polymeric vanadium shifted to higher values, indicating that more polymeric vanadium species might have been formed. The two peaks of the profile under 30 ppm Cl2 (c) were much weaker, suggesting that the sample exhibited poor redox characteristics after exposure to this level of Cl2.

Figure 7.

Figure 7

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H2-TPR profiles of catalysts exposed to different Cl2 concentrations. Flue gas conditions: 500 ppm NO, 500 ppm NH3, 500 ppm SO2, and 5% O2; (a) 0 ppm Cl2, (b) 25 ppm Cl2, and (c) 30 ppm Cl2.

Figure 8 contains the H2-TPR profiles of samples after exposure to different concentrations of SO2. The profile under 0 ppm SO2 (a) and the profile under 500 ppm SO2 (b) have three peaks corresponding to the reductions V2O5 → V2O4 → V2O3.47 However, the reduction peaks of the profile under 2500 ppm SO2 (c) shifted to higher values. It was concluded that high concentrations of SO2 and the sulfate formed on the catalyst surface inhibited the reduction of vanadium.

Figure 8.

Figure 8

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H2-TPR profiles of catalysts exposed to different SO2 concentrations. Flue gas conditions: 500 ppm NO, 500 ppm NH3, 25 ppm Cl2, and 5% O2; (a) 0 ppm SO2, (b) 500 ppm SO2, and (c) 2500 ppm SO2.

The number and strengths of the acid sites in the SCR catalysts after exposure to different concentrations of Cl2 were determined by NH3-TPD, as shown in Figure 9. The peaks at approximately 400 °C were attributed to Brønsted acid sites, and the peaks at approximately 600 °C were attributed to Lewis acid sites.48,49

Figure 9.

Figure 9

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NH3-TPD profiles of catalysts exposed to different Cl2 concentrations. Flue gas conditions: 500 ppm NO, 500 ppm NH3, 500 ppm SO2, and 5% O2; (a) 0 ppm Cl2, (b) 25 ppm Cl2, and (c) 30 ppm Cl2.

As presented in Figure 9, the desorption peak of the profile under 25 ppm Cl2 (b) was larger than that of the profile under 0 ppm Cl2 (a). This means that 25 ppm Cl2 increased the number of Brønsted active sites and improved the catalytic activity. More NH3 adsorbed on the sample after exposure to 25 ppm Cl2. However, the desorption peak at approximately 400 °C for the profile under 30 ppm Cl2 (c) was not readily apparent. The amount of Brønsted active sites decreased after exposure to 30 ppm Cl2 and the adsorption of NH3 was greatly inhibited. NOx reduction efficiency also decreased when 30 ppm Cl2 was added to the flue gas, as shown in Figure 2a.

Figure 10 includes the NH3-TPD profiles for the SCR catalysts after exposure to different concentrations of SO2. The desorption peak at approximately 400 °C of the profile under 2500 ppm SO2 (c) was weaker than those of the profiles under 0 ppm SO2 (a) and 500 ppm SO2 (b). This result shows that higher concentrations of SO2 reduced the number of Brønsted active sites and suppressed the catalytic activity. The adsorption of NH3 was greatly inhibited and the NOx reduction efficiency significantly decreased, as shown in Figure 4a.

Figure 10.

Figure 10

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NH3-TPD profiles of catalysts exposed to different SO2 concentrations. Flue gas conditions: 500 ppm NO, 500 ppm NH3, 25 ppm Cl2, and 5% O2; (a) 0 ppm SO2, (b) 500 ppm SO2, and (c) 2500 ppm SO2.

When there is no Cl2 in flue gas, the NH3-SCR process can be used to explain the NOx reduction by SCR catalysts.21,50−52 In this study, NH3 in the gas phase was first adsorbed on the catalyst surface (eq 14); a majority of the ammonia was adsorbed on Brønsted acid sites (V5+–OH) because Brønsted acid sites are active centers for SCR reactions. Then, NH3 was activated by V5+=O (eq 15). The activated NH3 converted NOx in the gas phase, producing N2 and H2O. In the process of NOx reduction, V5+=O was transformed to V4+–OH (eq 16). Since V5+ was transformed to V4+, the active sites were unable to adsorb ammonia or reduce NOx. If V4+–OH is not reoxidized to V5+=O, the SCR process may stop gradually; thus, it is necessary to regenerate the active sites. O2 in the gas phase reoxidized V4+–OH to V5+=O (eq 1721,51), and then the catalysts continued to adsorb ammonia or reduce NOx. Therefore, O2 is essential for regenerating the active sites. Figure 11 displays the NOx conversion by SCR catalysts under different gas conditions. When there was no O2 in the flue gas, the SCR process could not operate continuously, and the NOx conversion efficiency was minimized.

3.4.1. 14
3.4.1. 15
3.4.1. 16
3.4.1. 17

As Cl2 was added to the flue gas, it increased the catalytic activity and oxidated V4+ to V5+. The first few steps of the reaction were the same as those of the NH3-SCR reaction. NH3 in the gas phase was adsorbed on the catalyst surface and activated by V5+=O, which reduced NOx in the gas phase to create N2 and H2O. In the process, V5+=O was converted to V4+-OH, but Cl2 could reoxidize V4+–OH to V5+=O. First, Cl2 in the gas phase was converted to an active chlorine atom (Cl) on the catalyst surface (eq 1811). Then, the active chlorine atom oxidized V4+–OH to Cl–V5+–OH (eq 1911). Subsequently, Cl–V5+–OH reacted with ammonia generating V5+=O, and the intermediate substance HCl was converted to Cl2 via the Deacon process (eqs 20 and 21).11,53 The active sites were recreated, and they continued to adsorb ammonia or react with NOx. In this process, O2 was not essential. As shown in Figure 11, when the flue gas contained Cl2 but no O2, the NOx conversion efficiency was approximately 87%, which is very close to that of the O2-only condition. When the flue gas contained both Cl2 and O2, the NOx conversion efficiency was approximately 93%. These results suggest that Cl2 can enhance the oxidation by converting V4+–OH into V5+=O, promote the NH3-SCR process, and increase the NOx conversion efficiency.

3.4.1. 18
3.4.1. 19
3.4.1. 20
3.4.1. 21
Figure 11.

Figure 11

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SCR catalyst NOx conversion under different gas conditions.

3.4.2. Mechanisms of Cl2 Affecting Hg0 Oxidation

As presented in Figure 9, the peak at approximately 600 °C corresponded to Lewis acid sites, which are associated with Hg0 oxidation. As the Cl2 concentration of the samples varied from 0 to 25 ppm, the desorption peak area and signal gradually increased. This implies that the number of Lewis active sites increased, and more Hg0 can absorb on the catalyst surface and be oxidized with active chlorine on the active sites. The Hg0 oxidation efficiency may have improved because the catalytic activity improved. However, the profile under 30 ppm Cl2 (c) in Figure 9 had only one peak. The amount of Lewis acid sites decreased and Hg0 oxidation efficiency decreased to 78%, as presented in Figure 2b.

However, as the SO2 concentration of the samples varied from 0 to 2500 ppm, the desorption peak area and signal decreased, as shown in Figure 10. Higher concentrations of SO2 decreased the number of Lewis active sites and suppressed the catalytic activity. The adsorption of Hg0 on Lewis acid sites was greatly inhibited and the Hg0 oxidation efficiency decreased.

When there was no Cl2 in the flue gas, Hg0 oxidation by SCR catalysts followed the Mars–Maessen mechanism, and O2 participated in the reaction as the oxidant.25,50 First, Hg0 collided on the catalyst surface and was captured by the catalyst, resulting in Hg0 adsorption (eq 22). Then, Hg0 reacted with lattice oxygen of vanadium pentoxide, generating adsorbed mercuric oxide (eq 23). Next, vanadium oxide was reoxidized by gas-phase oxygen and mercuric oxide desorbed from the catalyst surface (eqs 24 and 25). The mechanism of enhanced Hg0 oxidation can be explained by the following reactions

3.4.2. 22
3.4.2. 23
3.4.2. 24
3.4.2. 25

When Cl2 existed in the flue gas, Hg0 oxidation by SCR catalysts followed the Langmuir–Hinshelwood mechanism. Gaseous Hg0 was adsorbed on the active sites, and V5+–OH became V4+–O···Hg(ads) (eq 26).11,54 Then, Cl2 participated in the reaction transforming Hg0 to Hg2+. To understand the role of vanadium sites in Cl2 adsorption, the SCR catalysts were analyzed via XPS. The V 2p and O 1s peaks were used to analyze the chemical states, and the results are shown in Figure 12a,b, where (1) is for fresh SCR catalysts, (2) is for SCR catalysts with 25 ppm Cl2, and (3) is for SCR catalysts with 25 ppm Cl2 and 500 ppm SO2. In Figure 12a, the binding energy value of V 2p for fresh SCR catalysts (517.2 eV) is slightly lower than the values reported in other studies for bulk vanadium (517.7 eV).55 This result is consistent with the fact that vanadium well dispersed the support.11 Two peaks corresponding to 517.2 and 516.1 eV were assigned to the oxidation states of V5+ and V4+, respectively, via peak deconvolution. When Cl2 was added into the flue gas, the peaks of V5+ in spectra (2) and (3) were higher than that in (1). The increased peaks of V5+ are considered to represent the shift of the valence from V4+ to V5+ due to the electronegativity of Cl–. The reaction is described by eq 27. Next, Cl–V5+–OH reacted with adsorbed Hg0 to form HgCl2 (eq 28). Then, the reoxidation of V4+–OH by O2 and Cl2 formed V5+=O (eqs 29 and 30). The XPS analysis of O 1s is shown in Figure 12b. Compared with spectra (1), (2), and (3), when Cl2 was added into the flue gas, the peak of lattice oxygen (O2–) decreased, and the peak of H2O increased. Adsorbed Hg0 was oxidized, and the sites of V4+–OH species were reoxidized.11 The mercury oxidation process on the vanadium-based SCR catalysts can be summarized by the following reactions

3.4.2. 26
3.4.2. 27
3.4.2. 28
3.4.2. 29
3.4.2. 30
3.4.2. 31
Figure 12.

Figure 12

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XPS spectra of the catalysts: (a) V 2p and (b) O 1s; (1) fresh SCR, (2) with 25 ppm Cl2, and (3) with 25 ppm Cl2 and 500 ppm SO2.

3.4.3. Mechanism of Cl2 Affecting NOx Reduction and Hg0 Oxidation over Sulfate-Poisoned Catalysts

When the SCR process was operated for a long time, NH4HSO4 was deposited on the catalyst surface,56 causing pore plugging, decreasing the surface area, and inactivating the active sites.57,58 According to the experimental results reported in Section 3.3.1, the addition of Cl2 into the flue gas promoted both NOx reduction and Hg0 oxidation over NH4HSO4 poisoned catalysts. Cl2 reoxidized V4+ into V5+, and the growth in the number of active sites enhanced the catalytic activity. Figure 13a shows the NH3-TPD curves of the NH4HSO4 poisoned catalysts pretreated and not pretreated by Cl2 at temperatures ranging from 50 to 700 °C. There are two types of NH3 desorption peaks: one near 400 °C and the other near 600 °C. When the catalysts were pretreated with Cl2, the elevation of two types of NH3 desorption peaks increased, which suggests that Cl2 increased the number of active sites and improved the catalytic activity.

Figure 13.

Figure 13

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NH3-TPD on the sulfate-poisoned catalysts with and without Cl2: (a) NH4HSO4-poisoned catalysts, (b) CaSO4-poisoned catalysts, and (c) MgSO4-poisoned catalysts.

Alkali metals (Ca/Mg) also affect the SCR catalysts as follows: (1) alkali metals decrease the strength and number of acid sites on V2O5-WO3/TiO2 catalysts,59,60 (2) they affect the surface chemisorbed oxygen and the reducibility of surface species,51,52,59,61 and (3) these metals interact with tungsten species and decrease the SCR activity.59 CaSO4 has a higher degree of poisoning effect than MgSO4. However, NOx can be reduced by NH3 over CaSO4, thereby reducing the inhibition effect of CaSO4 on NOx conversion efficiency.42,43 When Cl2 is added to the flue gas, it increases the strength and number of acid sites. Figure 13b,c shows NH3-TPD curves of CaSO4- and MgSO4-poisoned catalysts with and without Cl2 pretreatment at temperatures ranging from 50 to 700 °C. Since acid sites are the reaction center of NOx reduction and Hg0 oxidation, the increase in the number of acid sites promotes NOx conversion and Hg0 oxidation. Cl2 in the flue gas oxidizes V4+–OH to V5+=O (eq 30), making up the reduction of surface chemisorbed oxygen. Therefore, Cl2 promotes NOx conversion and Hg0 oxidation over CaSO4- and MgSO4-poisoned catalysts.

4. Conclusions

In this study, the effects of Cl2 on NOx reduction and Hg0 oxidation over SCR catalysts were investigated by spraying Cl2 into flue gas. The efficiency of NOx conversion and Hg0 oxidation increased when the concentration of Cl2 was less than 25 ppm. The optimal Cl2 concentration was determined to be 25 ppm.

Then, the effect of SO2 on NOx conversion and Hg0 oxidation in the presence of Cl2 was also investigated. Cl2 decreased sulfate-toxicity inhibition on fresh SCR catalysts at low SO2 concentrations (below 1500 ppm). Cl2 and SO2 competed for active sites on the catalysts; thus, the addition of Cl2 into the flue gas decreased the number of SO2-occupied active sites compared to that when there was no Cl2. At high SO2 concentrations (above 1500 ppm), SO2 reacted with V2O5 and NH3 to generate sulfate, which covered the catalyst surface and minimized the effect of Cl2.

The effects of Cl2 on different sulfur-poisoned catalysts were also studied. The number of active sites on the surface of sulfate-poisoned catalysts increased when Cl2 was added. The presence of Cl2 also significantly increased NOx conversion and Hg0 oxidation over sulfate-poisoned catalysts.

Chlorine from cylinders was used in the experiments. In future studies, we will use industrial production methods to provide a continuous and stable chlorine supply. Wastewater from the limestone-gypsum wet flue gas desulfurization process contains a high concentration of chloride ions. Thus, in these future experiments, we will electrolyze desulfurization wastewater to generate chlorine and spray it into flue gas.

Acknowledgments

This work was financially supported by the program of study on electrolysis treatment of high-chlorine desulfurization wastewater and coupled denigration and mercury oxidation technology, the program is funded by Huadian Hubei Power Generation Company Limited, Huangshi Thermal Power Branch. The authors thank the support from the Test Center of Wuhan University. They also thank the staff of Shiyanjia Lab (www.shiyanjia.com) for BET, XPS, NH3-TPD, and H2-TPR analyses and language editing.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c00350.

  • XRD diagram, TEM diagram, and composition analysis of commercial SCR catalyst (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c00350_si_001.pdf (762.1KB, pdf)

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