Byproduct Analysis of SO₂ Poisoning on NH₃-SCR over MnFe/TiO₂ Catalysts at Medium to Low Temperatures

Abstract

The byproducts of ammonia-selective catalytic reduction (NH₃-SCR) process over MnFe/TiO₂ catalysts under the conditions of both with and without SO₂ poisoning were analyzed. In addition to the NH₃-SCR reaction, the NH₃ oxidation and the NO oxidation reactions were also evaluated at temperatures of 100–300 °C to clarify the reactions occurred during the SCR process. The results indicated that major byproducts for the NH₃ oxidation and NO oxidation tests were N₂O and NO₂, respectively, and their concentrations increased as the reaction temperature increased. For the NH₃-SCR test without the presence of SO₂, it revealed that N₂O was majorly from the NH₃-SCR reaction instead of from NH₃ oxidation reaction. The byproducts of N₂O and NO₂ for the NH₃-SCR reaction also increased after increasing the reaction temperature, which caused the decreasing of N₂-selectivity and NO consumption. For the NH₃-SCR test with SO₂ at 150 °C, there were two decay stages during SO₂ poisoning. The first decay was due to a certain amount of NH₃ preferably reacted with SO₂ instead of with NO or O₂. Then the catalysts were accumulated with metal sulfates and ammonium salts, which caused the second decay of NO conversion. The effluent N₂O increased as poisoning time increased, which was majorly from oxidation of unreacted NH₃. On the other hand, for the NH₃-SCR test with SO₂ at 300 °C, the NO conversion was not decreased after increasing the poisoning time, but the N₂O byproduct concentration was high. However, the SO₂ led to the formation of metal sulfates, which might inhibit NO oxidation reactions and cause the concentration of N₂O gradually decreased as well as the N₂-selectivity increased.

1. Introduction

Nitrogen oxides (NO_x, NO and NO₂) produced from stationary sources are major air pollutants that lead to environmental concerns such as photochemical smog and acid rain [1]. The most effective technology for the removal of NO_x emission from coal-fired power plants is ammonia-selective catalytic reduction (NH₃-SCR; SCR hereafter) [2]. The traditional SCR catalysts are active within the temperature window of 300–400 °C [3,4]. Even though some of the traditional catalyst compositions such as V₂O₅–WO₃/ TiO₂ or Fe-zeolite-based catalysts can lower down their working temperature window to be as low as 250 or even 200 °C [5,6,7,8,9,10], there is still a strong demand in developing SCR catalysts to be active at less than 200 °C and placing them downstream of the electrostatic precipitator and desulfurizer [11,12,13].

Literature data showed that Mn-based catalysts have good activity for low-temperature SCR [14,15,16,17]. Moreover, in iron containing SCR catalysts, the introduction of Mn could obviously enhance the low-temperature activity, probably due to the fact that synergistic effect between iron and manganese species [18]. It was reported that the MnFe/TiO₂ could improve the activity, stability and SO₂ durability of the SCR catalysts using NH₃ as the reducing agent [19,20,21,22,23].

In the past, there have been extensive studies usingFourier-transform infrared spectroscopy (FTIR) for understanding the mechanism of SCR reaction on the surface of MnFe catalysts [24,25,26,27,28]. In addition, several different types of reaction mechanisms have been proposed including the typical Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism. For the Eley–Rideal mechanism, it is assumed that the gaseous NO directly reacts with an activated ammonia surface complex [29]. On the other hand, the Langmuir–Hinshelwood reaction mechanism involves that a surface NO complex reacts with an activated ammonia [3,30]. Moreover, Yang et al. [31] used in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS-FTIR) to reveal the mechanism of low-temperature SCR reaction over the MnFe spinel. The results indicated that the contribution of Eley–Rideal mechanism to NO conversion increased after increasing the reaction temperature.

Although many studies have been done on the SCR mechanism, to the authors’ best knowledge, there has been no work on clarifying reactions occurred in the medium to low-temperature SCR system with and without the presence of SO₂. Therefore, this study employed MnFe/TiO₂ catalyst to study the oxidation of NH₃, the oxidation of NO and the NH₃-SCR reaction with and without the presence of SO₂ at 100–300 °C. The reactants and byproducts of gaseous NO, NH₃, N₂O, and NO₂ in the effluent streams as well as solid byproducts of ammonium salts and metal sulfates on the catalysts were analyzed. The results can offer useful information to understand the reaction pathway at different operation conditions for the application of medium to low-temperature SCR catalysts.

2. Results and Discussion

2.1. Oxidation Reactions of NH₃ and NO without SO₂

To understand the products and byproducts of oxidation reactions, the NH₃ and NO oxidation reactions over the MnFe/TiO₂ catalyst were studied within the temperature range of 100–300 °C, which was the most active temperature region for MnFe/TiO₂ catalyst in SCR reaction [32]. In addition, the N-balance of NH₃ and NO oxidation tests were calculated by Equations (1) and (2), respectively.

$$\text{N-balance }\mathrm{in}{\mathrm{NH}}_3\mathrm{oxidation}=\frac{{[{\mathrm{NH}}_3]}_{\mathrm{out}}+{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}+{[\mathrm{NO}]}_{\mathrm{out}}+{[{\mathrm{NO}}_2]}_{\mathrm{out}}}{{[{\mathrm{NH}}_3]}_{\mathrm{in}}} \times 100\%$$

(1)

$$\text{N-balance }\mathrm{in}\mathrm{NO}\mathrm{oxidation}=\frac{{[\mathrm{NO}]}_{\mathrm{out}}+{[{\mathrm{NO}}_2]}_{\mathrm{out}}}{{[\mathrm{NO}]}_{\mathrm{in}}} \times 100\%$$

(2)

The results of N-balance and outlet gas concentrations in the NH₃ oxidation reaction without SO₂ over MnFe/TiO₂ catalyst at different reaction temperatures are shown in Figure 1. It can be seen that the major oxidation product of NH₃ was N₂O (i.e., Equation (3) shown later in the Materials and Method section) instead of NO (Equation (4)). In addition, N₂O increased from 31 ppm to 219 ppm after increasing the reaction temperature from 100 °C to 300 °C. On the other hand, the effluent NO concentration was only 36 ppm at most, which occurred at reaction temperature of 300 °C; but no NO₂ was found in the effluent at all tested temperatures.

The results of N-balance and outlet gas concentrations in the NO oxidation reaction over MnFe/TiO₂ catalyst at different reaction temperatures are shown in Figure 2, it can be seen that NO could be oxidized to NO₂, and the concentration of NO₂ was significantly increased from 10 to 309 ppmv for reaction temperatures from 100 to 300 °C.

The above results indicated that NH₃ and NO oxidations (Equations (3)–(5)) increased after increasing the reaction temperature, and the major products of NH₃ oxidation and NO oxidation were N₂O and NO₂, respectively. In addition, it can be observed that both the N-balance results shown in Figure 1 and Figure 2 were very high at 97~100% in the 100–300 °C range. This indicates that we can detect almost all the reaction species.

2.2. SCR Reactions without SO₂

Figure 3 shows the NO consumption (in ppmv) and outlet gas concentrations of N₂O, NO₂ and NH₃ over the MnFe/TiO₂ catalyst for reaction temperatures of 100–300 °C. It can be seen that the MnFe/TiO₂ catalyst maintained high NO consumption in the SCR reaction. When raising reaction temperature from 100 to 200 °C, the NO consumption in the SCR reaction increased slightly from 450 ppm to 495 ppm, and the outlet concentration of NH₃ was slightly decreased from 63 to 0 ppm.

At the reaction temperature of 200 °C, the concentrations of N₂O and NO₂ were 175 and 0 ppm, respectively, with 98% of NO consumption in the SCR reactions (500 ppm NO reacted with 500 ppm NH₃). On the other hand, the N₂O from NH₃ oxidation (500 ppm NH₃ reacted with O₂) at the reaction temperature of 200 °C was 156 ppm (as indicated by Figure 1). Therefore, it can be seen that in the SCR system, lower N₂O percentage ($\frac{175\mathrm{ppm}{\mathrm{N}}_2\mathrm{O}}{500\mathrm{ppm}\mathrm{NO}+500\mathrm{ppm}{\mathrm{NH}}_3}$ = 17.5%) was being produced as compared to that during only NH₃ oxidation ($\frac{156\mathrm{ppm}{\mathrm{N}}_2\mathrm{O}}{500\mathrm{ppm}{\mathrm{NH}}_3}$ = 31.2%).

The “fast SCR” (Equation (8)), first proposed in 1986 [33], proceeds at a much higher reaction rate than “standard SCR” reactions (Equations (6) and (7)) was developed to improve deNO_x efficiency at lower temperatures [34,35]. As indicated by Figure 2 for the NO oxidation reaction, NO could be oxidized to NO₂ at temperatures higher than 150 °C, but NO₂ was not detected at temperature below 200 °C in the SCR test as seen in Figure 3. This might be due to the fact that NO₂ would react with NO and NH₃ according to the fast SCR reaction (Equation (8)).

After further raising the reaction temperature from 200 to 300 °C, NO consumption in the SCR reaction was slightly decreased from 495 ppm to 455 ppm, and concentrations of N₂O and NO₂ increased from 175 to 270 ppm and 0 to 36 ppm, respectively. When reaction temperature increased, oxidation reactions occurred more quickly as indicated by Figure 1 and Figure 2. Hence the reason for slight decreases in the NO consumption at above 200 °C might be due to the fact that NO was oxidized (Equation (5)) as also demonstrated in the literature [26,36,37,38]. Moreover, the results of Figure 3 also indicated that products of the SCR reaction gradually changed from N₂ to N₂O when raising the temperature from 100 to 300 °C. Therefore, the N₂-selectivity of SCR reaction was gradually decreased from 93% to 41% after increasing the reaction temperature from 100 °C to 300 °C.

2.3. SCR Reactions with SO₂

To clarify the byproducts of SCR reactions with SO₂, NO consumption in the SCR reaction and outlet gas concentrations in the NH₃-SCR reaction over MnFe/TiO₂ catalyst at 150 and 300 °C are shown in Figure 4a,b, respectively. It can be seen in Figure 4a that there were four stages during the SCR test period. At stage I where no SO₂ was introduced, the NO consumption in the SCR reaction was very high (495 ppm out of 500 ppm). After 150 ppm SO₂ was introduced, the NO consumption in the SCR reaction decreased rapidly within 60 min SO₂ poisoning (stage II), and they remained roughly stable for another 60 min (stage III), then decreased gradually with time again (stage IV).

In our previous work [32], it was found that the gradual decrease (stage II) of NO consumption in the SCR reaction was probably due to the fact that SO₂ competed with NO to react with NH₃ and form ammonium salts. However, the MnFe/TiO₂ catalyst still had good activity at stage III. Thus, during this stage, there was no sufficient NH₃ to react with NO, which caused NO consumption remained relatively lower but stable at around 425 ppm from 60 to 120 min SO₂ poisoning time as compared to stages I and II. At stage IV, NO consumption in the SCR reaction decreased significantly from 420 ppm to 153 ppm for poisoning time from 120 to 360 min, which was due to accumulation of metal sulfates and ammonium salts which blocked the active sites of catalyst.

On the other hand, no effluent NH₃ was detected from stage I to stage III. Then NH₃ slip occurred at stage IV as seen in Figure 4a, which was due to less active sites and decreasing NO consumption in the SCR reaction. One can also see that concentrations of N₂O slightly decreased from 76 to 63 ppm within 120 min SO₂ poisoning time (stage II and III), then increased gradually to 86 ppm after 360 min SO₂ poisoning time (stage IV). This result indicated that SO₂ would react with NH₃ and form ammonium salts, which cause concentrations of N₂O slightly decreased at stages II and III. Then NH₃ slip occurred at stage IV, which caused increasing N₂O concentrations.

For the SCR test with SO₂ at 300 °C as shown in Figure 4b, it can be seen that the SO₂ poisoning effect was negligible as compared to the poisoning test at low temperature. When the reaction temperature was at 300 °C, the NO consumption in the SCR reaction remained around 460 ppm, which was almost the same as those without SO₂ poisoning. Moreover, during the SCR activity tests no SO₂ concentration was detected, which indicated that all the gas phase SO₂ molecules might be adsorbed and/or reacted with metal catalysts.

The main reason for the inhibition of SO₂ poisoning might be attributed to different SCR reaction mechanisms at different temperatures. Literature results showed that at lower temperatures (<200 °C), SCR reactions would follow the Langmuir−Hinshelwood mechanism. In the Langmuir−Hinshelwood mechanism, SO₂ would compete with NO to be adsorbed on the active sites, which cause the decreases of SCR efficiencies. However, when reaction temperature increased, the SCR reaction mechanism would transform from Langmuir−Hinshelwood mechanism to Eley–Rideal mechanism. In the high temperature range (>200 °C), the SCR reaction mainly followed the Eley–Rideal mechanism, over which the gaseous NO could directly react with an activated ammonia [30,39,40].

One can also see from Figure 4b that at reaction temperature of 300 °C, the SCR system did not have NH₃ slip. The concentrations of NO₂ and N₂O decreased from 30 to 0 ppm and from 270 to 172 ppm, respectively; and the N₂-selectivity increased from 41% to 62% for the SCR reaction from stage II to stage IV. The result indicated that by increasing the reaction temperature to 300 °C the SO₂ poisoning effect could be inhibited, and the N₂ selectivity can be enhanced due to the decreased N₂O concentrations.

The amount of sulfate species on the catalysts was estimated by thermo-gravimetric analysis (TGA) analysis and the results are shown in Figure 5 in terms of differential thermogram (DTG) spectra. The weight loss profiles of all samples showed three distinct decomposition steps: (1). the weight loss at low temperature (<200 °C) was assigned to the water desorption on the catalyst surface. (2). the weight loss at 200–400 °C could be attributed to decomposition of ammonium salts [41,42,43]. (3). the weight loss at high temperature (>670 °C) was originated from metal sulfates [44,45].

It can be seen from Figure 5 that the fresh catalyst had only one major weight loss peak, which appeared at 50–150 °C and corresponded to H₂O desorption on the catalyst surface. On the other hand, decomposition peaks of both ammonium salts (200–400 °C) and metal sulfates (670–900 °C) were observed on all MnFe/TiO₂ catalysts poisoned at temperature ranges of 100–200 °C. When reaction temperatures were 250 °C and 300 °C, there were no decomposition peak of ammonium salts. This reveals that ammonium salts were not formed on the catalyst surface at temperatures above 250 °C. As also noted in Figure 5, there was another peak in the range of temperature of 400–600 °C for the fresh catalyst and poisoned catalyst at 100 °C. However, since we cannot find related literature discussed on this, so the reason for this peak is not clear.

The amounts of sulfate species of fresh and poisoned catalysts are listed in

Table 1. Amounts of ammonium salts and metal sulfates deposited on the catalyst surfaces.

MnFe/TiO2 Poisoned with Different Reaction Temperatures	Ammonium Salts a (by weight) %	Metal Sulfates b (by Weight) %
			Fresh	0.0	0.0
100 °C	2.3	4.21 c
150 °C	1.8	4.11 c
200 °C	1.1	4.03 c
250 °C	0.1	4.08 c
300 °C	0(−0.1)	4.52 c

^a The amount of ammonium salts was calculated by weight difference between the fresh and poisoned catalysts from the TGA spectrum of 200–400 °C. ^b The amount of metal sulfates was calculated by weight difference between the fresh and poisoned catalysts from the TGA spectrum of 670–900 °C. ^c Rigid quantification of the amounts of metal sulfates is not possible via the DTG data because it was possible that some of the ammonium salts could transform into metal sulfates during the heating process of TGA.

. It is observed that the amounts of ammonium salts on the catalysts decreased from 2.3 wt.% to negligible amounts by increasing the SCR reaction temperature from 100 to 300 °C. This indicated that the reaction temperature would directly affect the formation of ammonium salts. It is noted that rigid quantification of metal sulfates accumulated on the poisoned catalyst was not possible via the DTG data because some ammonium salts could also be transformed into metal sulfates during the continuous TGA heating process [46]. Hence we can only confirm that all the poisoned catalysts had roughly similar amounts of metal sulfates during the temperature from 100 °C to 250 °C (within experimental error of TGA instrument, ±0.3 wt.%), except for the case at 300 °C.

At high temperature of 300 °C, the NO consumption was not affected by SO₂. Thus, it is easy to predict that adding an excessive amount of NH₃ tends to be oxidized and forming N₂O and NO as observed in Figure 1. However, at low temperature of 150 °C, the NO consumption was significantly affected by SO₂. Thus, to ensure the gradual decrease of NO consumption at stage II of Figure 4a was due to the competition between SO₂ and NO to react with NH₃, different inlet amounts of NH₃ were tested to study the SO₂ poisoning mechanism at temperature of 150 °C. The results are shown in Figure 6. One can see that adding different amounts of NH₃ had a similar effect on NO consumption at stages I and IV; but it had different NO consumptions at stages II and III. On the other hand, the NH₃ outlet concentrations were different at all stages. At stage I, NH₃ only reacted with a certain amount of NO and thus extra NH₃ slip was detected in the outlet gas. When SO₂ was introduced, NH₃ not only reacted with NO but also reacted with SO₂. Therefore, both the outlet concentrations of NH₃ and NO consumption decreased at stages II and III. Besides, when increasing NH₃ amount to above 550 ppm (i.e., NH₃/NO molar ratio of 1.1), the first decay of NO consumption at stage II could be inhibited. This indicated that NH₃ concentration was sufficient to react with NO and SO₂. The result shown in Figure 6 indicated that if a sufficient amount of NH₃ can be provided to react with both SO₂ and NO, then the first decay of NO consumption could be inhibited at early time of stage II. However, after 120 min of SO₂ poisoning (stage IV), the NO consumption could not be affected by different amounts of NH₃. Thus, the more injection amount of NH₃ led to eventually the more amount of NH₃ slip to the atmosphere.

2.4. Product and Byproduct Analysis

Since different reactions occurred during the SCR process, i.e., Equations (3)–(12), were considered both with and without SO₂, thus the percentages of product and byproducts can be calculated.

Table 2. Formulas for calculating the N-containing product and byproduct percentages during the SCR process.

Product and Byproduct	Reaction Equation	Percentage Calculation Equation
NO2	NO oxidation to NO2 (Equation (5)): 2NO + O2 → 2NO2	The percentage of NO2 from NO oxidation $[{\mathrm{NO}}_2{]}_{\mathrm{NO}}(\%)=[\frac{{[{\mathrm{NO}}_2]}_{\mathrm{out}}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}} ] \times 100\%$
Salts (NH4)2SO4 NH4HSO4	Ammonium salts Equations (10) and (11): SO3 + 2NH3 + H2O (NH4)2SO4 SO3 + NH3 + H2O → NH4HSO4	The percentage of salts $[\mathrm{Salt}](\%)=[\frac{{[{\mathrm{NH}}_3]}_{\mathrm{Salt}}{}^{\#}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}} ] \times 100\%$ #the NH3 consumption of salts reaction (${[{\mathrm{NH}}_3]}_{\mathrm{Salt}})$ were indicated by result of Figure 6
N2O	NH3 slip oxidation to form N2O [N2O] slip (%)	The percentage of N2O from NH3 slip oxidation $[{\mathrm{N}}_2\mathrm{O}]{}_{\mathrm{slip}}(\%)=[\frac{{[{\mathrm{NH}}_3]}_{\mathrm{out}}\times {\frac{[{\mathrm{N}}_2\mathrm{O}]}{[{\mathrm{NH}}_3]}}^{*}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}} ] \times 100\%$ *$\frac{[{\mathrm{N}}_2\mathrm{O}]}{[{\mathrm{NH}}_3]}$: from results of NH3 oxidation test (Figure 1)
	NH3 oxidation to form N2O Equation (3): 2NH3 + 2O2 → N2O + 3H2O	The percentage of N2O from NH3 oxidation $[{\mathrm{N}}_2\mathrm{O}]{\mathrm{NH}}_3(\%{)}_{}= 1-[\frac{2{[\mathrm{NO}]}_{\mathrm{SCR}}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}}]\times 100\%-{[{\mathrm{NO}}_2]}_{\mathrm{NO}}(\%)-[\mathrm{Salt}](\%)+{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{Slip}}(\%)$
	SCR reaction to form N2O Equation (7): 4NO + 4NH3 + 3O2 → 4N2O + 6H2O	The percentage of N2O from SCR reaction $[{\mathrm{N}}_2\mathrm{O}]{\mathrm{SCR}}_{}(\%)=[\frac{2{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}}] \times 100\%-{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{NH}3}(\%)$
N2	SCR reaction to form N2 Equation (6): 4NO + 4NH3 + O2 → 4N2 + 6H2O	The percentage of N2 from SCR reaction $[{\mathrm{N}}_2]{\mathrm{SCR}}_{}(\%)=[\frac{2{[\mathrm{NO}]}_{\mathrm{SCR}}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}}}] \times 100\%-{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{SCR}}(\%)$

lists formulas for calculating the percentages of N-containing product (N₂) as well as gaseous and solid byproducts (NO, N₂O and salts) formed during the SCR process. The byproducts could be formed by the SCR reaction as well as by the NH₃ oxidation or the NO oxidation reactions. Because it is difficult to clarify that the outlet NO was from NH₃ oxidation or from the unreacted NO, therefore the NO formation from NH₃ oxidation (Equation (4)) is neglected and all the effluent NO was assumed to be only from the unreacted NO. This is an acceptable assumption since the formation of NO from NH₃ oxidation was very minor (0~7%) at reaction temperatures of 100~300 °C as observed from Figure 1. Besides, it was assumed that the fast SCR reaction (Equation (8)) only served as the intermediate reaction at the low temperature SCR process [35]. Thus, the fast SCR reaction was not considered in the calculation of the percentages of all N-containing product and byproducts.

Based on formulas shown in

Table 3. Operation conditions of experimental tests in this study.

	NH3 (ppmv)	NO (ppmv)	SO2 (ppmv)	O2 (%)	Temperature (°C)	GHSV (hr−1)
NH3 oxidation test	500	0	0	10%	100–300	50,000
NO oxidation test	0	500	0	10%	100–300	50,000
SCR test without SO2	500	500	0	10%	100–300	50,000
SCR test with SO2	500	500	150	10%	150 & 300	50,000
SCR test with SO2 at different NH3 amounts	500–600	500	150	10%	150	50,000

, the results on percentages of all N-containing species during the NH₃-SCR process tested at 150 °C are shown in Figure 7a; and those tested at 300 °C are shown in Figure 7b. It can be seen from Figure 7a that at SCR operation temperature of 150 °C and without the presence of SO₂, the major product of SCR process appeared to be N₂. This indicated that MnFe/TiO₂ catalyst can serve as a good catalyst and achieve high N₂ selectivity at low temperature of 150 °C when SO₂ was not presented in the system. The minor presence of N₂O byproduct in the exhaust was majorly from the SCR reaction rather than from the NH₃ oxidation reaction.

On the other hand, when SO₂ was introduced at 150 °C, the percentages of N₂ and N₂O decreased after increasing the poisoning time as observed in Figure 7a. Because NH₃ could not be reacted with NO due to decreased availability of active sites, it tends to increase the NO and NH₃ slip to the atmosphere. In addition, the portion of N₂O from NH₃ oxidation was also gradually increased as increasing the SO₂ poisoning time. After 360 min of SO₂ poisoning, the exhaust N₂O from NH₃ oxidation was more than from SCR reaction. One can also see that percentages of ammonium salts were similar at different SO₂ poisoning times. This indicated that certain amounts of NH₃ would preferentially be reacted with SO₂. In addition, the remaining NH₃ would then be reacted with NO or O₂. Moreover, it can be seen that percentages of NO slip were higher than percentages of NH₃ slip at different SO₂ poisoning times, which is due to the fact that NH₃ not only reacted with NO but also reacted with SO₂ and O₂.

For results at 300 °C as seen in Figure 7b, it is observed that NO₂ from NO oxidation disappeared during the SO₂ poisoning. However, the unreacted NO (NO slip) appeared during the SO₂ poisoning. This indicated that the formation of metal sulfates at 300 °C might inhibit the NO oxidation reaction. At high temperature, the SCR reaction mainly followed the Eley–Rideal mechanism, so the continuous decreasing of N₂O concentration as poisoning time increases and no SO₂ concentration in the outlet gases might be related to the reaction between SO₂ and activated NH₃ instead of the NH₃ oxidation reaction at 300 °C. Moreover, the total percentages of N₂ and N₂O from SCR reactions remained almost the same no matter SO₂ was presented in the system or not. The low N₂ selectivity revealed that MnFe/TiO₂ catalyst may not be a good candidate for SCR process at 300 °C unless the space velocity can be reduced to further enhance the more complete reduction of NO to N₂ instead of forming the N₂O byproduct. However, it is interested to note that the N₂ selectivity gradually increased and the percentages of N₂O from SCR reaction gradually decreased after increasing SO₂ poisoning time. This indicated that SO₂ promotion phenomenon might exist at 300 °C, which was attributed to the formation of SO₄²⁻ on the catalyst surface. This increased NH₃ adsorption and promoted NH₃ reaction with NO via Eley−Rideal mechanism.

2.5. Reaction Pathways

From the above results, one could surmise the reaction pathway in the NH₃-SCR system with/without SO₂ at 150 °C and 300 °C as shown in Figure 8. When the SCR system was at 150 °C without SO₂, it can be seen from the top left plot that a fraction of NH₃ and NO would be oxidized to N₂O and NO₂, respectively. In addition, the major reaction product of SCR reaction was N₂ instead of N₂O. On the other hand, when increasing temperature to 300 °C, it can be seen from the bottom left plot that NH₃ would be oxidized to both N₂O and NO. Moreover, the major reaction product of SCR reaction was N₂O instead of N₂, which was revealed by the low N₂-selectivity as seen in Figure 7b.

When SO₂ was introduced at 150 °C, it can be seen from the top right plot of Figure 8 that SO₂ would be reacted with both NH₃ and metal catalyst, which resulted in the formation of ammonium salts and metal sulfates, respectively. In addition, because NO could not be adsorbed on metal sulfates, therefore it could not be reacted with NH₃ at low temperature. As a result, unreacted NH₃ (NH₃ slip) turned out to be the major N-containing species in addition to the unreacted NO.

When increasing temperature to 300 °C, it can be seen from the bottom right plot that ammonium salts would not be formed in the presence of SO₂, but SO₂ would react with metal catalyst to form metal sulfates. In addition, gaseous NO could directly react with adsorbed ammonia via Eley−Rideal mechanism [29]. The major reaction product of SCR reaction gradually changed from N₂O to N₂ after increasing poisoning time.

3. Materials and Method

3.1. Reactions in SCR System

In the SCR system, it may contain oxidation reactions, SCR reactions and SO₂ poisoning reactions [46,47,48,49,50,51,52].

NH₃ oxidation:

2NH₃ + 2O₂ → N₂O + 3H₂O

(3)

4NH₃ + 5O₂ → 4NO + 6H₂O

(4)

NO oxidation:

2NO + O₂ → 2NO₂

(5)

SCR reactions:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

(6)

4NO + 4NH₃ + 3O₂ → 4N₂O + 6H₂O

(7)

NO + NO₂+ 2NH₃ → 2N₂ + 3H₂O (fast SCR)

(8)

SO₂ oxidation and poisoning:

SO₂ + ½ O₂ → SO₃

(9)

SO₃ + 2NH₃ + H₂O → (NH₄)₂SO₄

(10)

SO₃ + NH₃ + H₂O → NH₄HSO₄

(11)

SO₂ + metal → metal sulfates (e.g., MnSO₄ and Ti(SO₄)₂)

(12)

In this study, experimental tests were designed to clarify the products and byproducts of the above reactions.

3.2. Synthesis of MnFe/TiO₂ Catalysts

Mn and Fe metal oxides were supported on TiO₂ (in the form of TiO(OH)₂) by the co-precipitation method. In a typical procedure, 8 g of TiO₂ (China Steel Corp., Kaohsiung, Taiwan), 11.57 g of ferric nitrate 9-hydrate (99%,J.T. Baker, Radnor, PA, USA), 7.13 g of manganese (II) acetate tetrahydrate (99%, Merck, Kenilworth, NJ, USA) and D.I. water (76 g) were mixed then adjusted to pH = 10 with 25 wt.% ammonia solution to form a precipitate. It was filtered and washed thoroughly with D.I. water, then dried at 120 °C for 12 h. Finally, the material was calcined at 350 °C for 6 h in air.

3.3. Catalyst Reaction

The NH₃ oxidation, NO oxidation and SCR activity tests were carried out at atmospheric pressure in a fixed-bed reactor loaded with sieved pelletized (16–30 mesh) catalysts. The operation conditions of inlet gas concentrations, reaction temperatures, and gas hourly space velocity for different tests are shown in Table 3. Under typical NH₃ oxidation, NO oxidation, and SCR reaction tests, the concentrations of NH₃ and/or NO were the same at 500 ppmv, and the SO₂ concentration was 150 ppmv if SO₂ poisoning effect was considered. The feed gases were mixed in a gas mixer. Then the catalysts were preheated in the reactor for 30 min to ensure that an isothermal reaction temperature was reached. During the oxidation and SCR test, the NO and SO₂ concentrations at the inlet and outlet of the reactor were monitored by a NO/SO₂ analyzer (Ultramat 23, SIEMENS, Munich, Germany). In addition, the concentrations of NH₃, N₂O, and NO₂ at the inlet and outlet of the reactor were monitored by a FTIR Spectrophotometer (Bomem MB 104,San Jose, CA, USA and ITRI, Hsinchu, Taiwan).

The NO consumption due to SCR reactions (Equations (6)–(8)) must be subtracted by the NO oxidation to NO₂ (Equation (5)). Thus, the NO consumption of SCR, [NO]_SCR is defined by:

[NO]_SCR = [NO]_in − ([NO]_out − [NO₂]_out)

(13)

Since the N-containing product and byproduct of SCR reaction are N₂ and N₂O from Equations (6)–(8), thus the N₂-selectivity of SCR reactions was calculated by

$$\begin{gathered} {\mathrm{N}}_2\text{ selectivity of SCR reaction }= \\ [1-\frac{{[\mathrm{NO}]}_{\mathrm{out}}+ {[{\mathrm{NH}}_3]}_{\mathrm{out}}+{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}+{[{\mathrm{NO}}_2]}_{\mathrm{out}}}{{[\mathrm{NO}]}_{\mathrm{in}}+{[{\mathrm{NH}}_3]}_{\mathrm{in}} } ]\times 100\% \end{gathered}$$

(14)

The TGA was conducted to determine the sulfates species forming on the surface of the catalysts with a NETZSCH TG 209 F1 apparatus. The heating program was carried out under airflow of 10 mL/min with a heating rate of 10 °C/min from room temperature to 900 °C.

4. Conclusions

This study employed MnFe/TiO₂ catalyst to study the product/byproducts for the oxidation of NH₃, the oxidation of NO and the NH₃-SCR reaction with/without SO₂ to understand the reaction pathway of medium to low-temperature SCR process. For SCR operation temperature of 150 °C without the presence of SO₂, the major product of SCR process appeared to be N₂. The minor presence of N₂O byproduct was majorly from the SCR reaction instead of from the NH₃ oxidation reaction. Moreover, the result indicated that products of the SCR reaction gradually changed from N₂ to N₂O when raising the temperature from 100 to 300 °C. Therefore, the N₂-selectivity of SCR reaction was gradually decreased. One the other hand when SO₂ was introduced at 150 °C, the percentages of N₂ and N₂O decreased after increased poisoning time. However, when increasing temperature to 300 °C, the percentages of N₂ increased while that of N₂O decreased after increasing the poisoning time. This indicated the existence of SO₂ promotion effect on the NH₃-SCR at 300 °C.