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Article

Poisoning Effect of SO2 on Honeycomb Cordierite-Based Mn–Ce/Al2O3Catalysts for NO Reduction with NH3 at Low Temperature

by
Chengzhi Wang
1,
Cheng Zhang
1,
Yonggang Zhao
1,
Xin Yan
2 and
Peng Cao
1,*
1
Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China
2
Tianfu South Thermoelectric Co., Ltd., Shihezi 832000, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2018, 8(1), 95; https://doi.org/10.3390/app8010095
Submission received: 30 November 2017 / Revised: 26 December 2017 / Accepted: 8 January 2018 / Published: 11 January 2018
(This article belongs to the Section Chemical and Molecular Sciences)
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Abstract

:
Honeycomb cordierite-based Mn–Ce/Al2O3 catalysts were prepared by the impregnation method and used for low-temperature selective catalytic reduction (SCR) of NOx with NH3, with and without SO2 and/or H2O in a homemade fixed-bed tubular reactor. The catalyst reached nearly 80% NOx conversion at 100 °C in the absence of SO2. However, SO2 reduces the catalytic activity (80% to 72%) of the honeycomb cordierite-based Mn–Ce/Al2O3 catalysts under identical conditions. This finding demonstrated that the catalyst exhibited high activity at low temperature and excellent SO2 resistance in the presence of 50 ppm SO2. The fresh and sulfated honeycomb cordierite-based Mn–Ce/Al2O3 catalysts were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), N2 adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry and differential thermal analysis (TG-DTA), and Fourier transform infrared (FT-IR) spectroscopy. Characterization results indicated that the deactivation by SO2 was primarily the result of the deposition of ammonium hydrogen sulfate and sulfated CeO2 on the catalyst surface during the SCR process. The formed sulfates depressed the catalytic activity via the blocking of pores and the occupation of active sites. Additionally, the competitive adsorption between SO2 and NH3 always decreased the catalytic activity.

Graphical Abstract

1. Introduction

Nitrogen oxides (NOx) are global air pollutants, resulting from high-temperature combustion processes and adverse poisons in the atmosphere [1,2,3], including nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), and their derivatives, which have a wide range of health and environmental impacts. Selective catalytic reduction (SCR) of NOx with NH3 is an advanced, economic, and fuel-efficient technology that allows the removal of NOx from stationary sources [4]. SCR can not only alleviate NOx emissions by more than 90%, but can also concurrently reduce carbon monoxide emissions by 50–90%, and particulate matter emissions by 30–50%. V2O5–WO3/TiO2 and V2O5–MoO3/TiO2 have been widely used as commercial SCR catalysts [5,6]. However, their operating temperature (300–400 °C) limits the location of the SCR reactor to upstream from the electrostatic precipitator and desulfurizer [7,8]. This limitation causes some problems, such as the tendency to catalyse the rapid oxidation of SO2 to SO3, the corrosion of the downstream devices, and high concentrations of particulates and SO2 in the flue gases, which shorten the life of the catalyst [9]. Toavoid these adverse effects, it has been suggested that the SCR reactor could be located downstream of the particle control and desulfurizer devices where the flue gas temperature is typically below 150 °C. In particular, after the wet desulfurization process, the flue gas will be cool, even below 100 °C [10,11]. Alow-temperature SCR unit could be located downstream of the particulate control device and desulfurizer without heating, which results in low system energy consumption and ease of retrofitting into the boiler system; this would eliminate the need to reheat the stack gas,thus increasing the operational lifetime of the catalyst. Therefore, it is urgent to develop low-temperature SCR catalysts with high activity, high stability, a broad range of operating temperatures, and good SO2 resistance.
Manganese oxides (MnOx) have been extensively studied and proven to be highly active for low-temperature SCR of NO with NH3 since they contain various types of labile oxygen species, which are essential for completing the catalytic cycle [12,13,14,15,16]. Researchers have also developed various Mn-based catalysts, such as MnOx/Al2O3 [15], MnOx/TiO2 [16,17] andMn–Ce–SnO2 [18]. In addition, small concentrations of SO2 remain in the flue gases after the desulfurizer, thus requiring consideration of the effect of SO2 on activity. Many researchers [19,20,21] have shown that SO2 has an apparent passivation effect on the catalytic activity of manganese oxide at low temperatures. Jin et al. [20,21] used the addition of CeO2 to modified Mn/TiO2 to significantly enhance the SO2 poisoning resistance because CeO2 can generate highly thermally stable Ce2(SO4)3 and Ce(SO4)2, which prevent the formation of Mn(SO4)x and inhibit the deposition of NH4HSO4 and (NH4)2SO3. However, the majority of catalysts for the preparation are in granular or powder form, which are unable to meet the needs of industrial applications due to the large bed pressure decrease and unequal surface distribution of the reactants in the catalyst. Limited research has focused on studying the mechanism of the effect, at pilot-scale, of alow concentration of SO2 on honeycomb cordierite-based Mn–Ce/Al2O3 catalysts at 100 °C [3,7]. To realize the industrial application of an SCR unit located downstream of the particulate control device and desulfurizer without heating, it is advantageous to study the SO2 resistance of the honeycomb catalyst at 100 °C. In addition, in order to be closer to the real conditions of industrial application, an industrial experiment platform device was built to test the effect of the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst in Shihezi Tianfunan Thermoelectric Co., Ltd. (Xin Jiang, China). Before theindustrial experiment platform test, the experiment was conducted in the laboratory according to the experimental conditions of the industrial experiment platform test. This experiment adopts an industrial-grade raw material preparation of the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst in the lab asan enlargement of the pilot-scale experiment, for exploring the catalyst performance under the condition of simulating actual flue gas and the effects of SO2 on SCR activities at low temperature. The physical and chemical properties were characterized in order to explore the reaction mechanism and provide theoretical support for the denitration catalyst at low temperatures in industrial applications.

2. Materials and Methods

2.1. Catalyst Preparation

The Mn–Ce/Al2O3 catalyst powder was prepared by the coprecipitation method (Mn/Ce/Al molar ratioof 2:3:1) using manganese nitrate, cerium nitrate, and aluminum nitrateas precursors. First, a fixed quantity of Al(NO3)3·9H2O (Aladdin, Shanghai, China) was dissolved in distilled water with stirring. Then, prescribed amounts of Ce(NO3)3·6H2O (Aladdin, Shanghai, China) and Mn(NO3)2·4H2O (50%, Aladdin, Shanghai, China) were added to the above solution. The mixed solution was vigorously stirred for 2 h, and then slowly added into excess ammonia with stirring until the solution reached pH10 and aged for 24 h, followed by washing until the pH remained stable. The sample was then dried at 110 °C for 12 h and calcined at 550 °C for 5 h. The catalyst was crushed and sieved to 150 mesh. Finally, impregnation was used to load the catalyst powder in the cordierite honeycomb carrier (Zhonghui Honeycomb Ceramics Ltd., Taicang, Jiangsu, China). Poly vinyl alcohol (GRACE, Tianjin, China), emulsifier (OP-10) (Aladdin, Shanghai, China), and sodium silicate (Aladdin, Shanghai, China) were dissolved in distilled water to form a solution and stirred for 1 h, then slowly added into the catalyst powder with stirring for 0.5 h. Then, the support and mixed solutions were stirred for a further 10 min. After impregnation, the resulting material was dried at 110 °C for 6 h, followed by calcination at 500 °C for 3 h in air. The loading amount of Mn–Ce/Al2O3 was 15 wt %. For convenience, the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst before the reaction was denoted F-Mn–Ce/Al2O3, while the catalyst after reaction without SO2 and with SO2 (50 ppm) for 24 h were denoted as R-Mn–Ce/Al2O3 and S-Mn–Ce/Al2O3, respectively.

2.2. Catalytic Activity Evaluation

The catalyst reaction was performed in a homemade fixed-bed tubular reactor (inner diameter 150 mm) at 100 °C containing 4 pieces of catalyst(catalyst specification: 100 mm (length) × 100 mm (width) × 150 mm (height), a honeycomb support with a channel density of 160 cells per square inch and a wall thickness of 0.5 mm). The premixed simulated gases (200 ppm NO, 200 ppm NH3, 50 ppm SO2 (when used) and 5 vol % H2O (when used) with the balance air) were introduced into the reactor with a gas flow rate of 167 L/min, corresponding to a gas hour space velocity (GHSV) of 1667 h−1. The concentration of NO was measured using a flue gas analyser (Testo 350, Lenzkirch, Germany).

2.3. Catalyst Characterization

Scanning electron microscopy (SEM) was used to observe the surface microstructure (XL-30ESEM, Phillips, Amsterdam, The Netherlands). Energy dispersive spectroscopy (EDS) was carried out on an X-MAXN 80TLE windowless EDS (Oxford Instruments, Oxford, UK) toget the dispersion of the metal on the catalyst surface by EDS elemental mapping. Specific surface areas, pore volume, and pore size distribution of the catalyst were measured by nitrogen absorption at 77 K (Micrometrics ASAP-2020 Analyzer, Micro-meritics, Norcross, GA, USA) using the Brunauer–Emmet–Teller (BET) method. Before each adsorption measurement, approximately 0.3 g of the sample was degassed in N2 at 300 °C for 4 h. X-ray powder diffraction (XRD) was performed to elucidate the crystal phase structure (Rigaku D/MAX RA) with Cu Kαradiation operated at 40 kV and 100 mA. The intensity data were collected in the range of 10–80°. Photoelectron spectroscopy (XPS) was used to analyse the surface atomic state of the catalyst with Al Kα X-rays (Thermal ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA). The sample charging effects were compensated by calibrating all binding energies (BE) with the adventitious C1s line at 284.6 eV. The concentrations of Mn, Ce, O, and S on the catalyst surface were calculated from the peak intensity. Thermogravimetry differential thermal analysis (TG-DTA) of the samples was performed (NetzschThermoAnalyzer STA 449C, Netzsch, Waldkraiburg, Bavarian State, Germany) with a heating rate of 10 °C·min−1 in flowing air. Fourier transform infrared (FT-IR) spectroscopy was performed using a Bruker Vector Fourier transform infrared spectrometer. The FT-IR spectra were recorded over the 800–4000 cm−1 range to determine the sulfate and ammonium groups on the catalyst surface.

3. Results and Discussion

3.1. NH3-SCR Activity Results

The effects of temperature on the SCR activities of the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst are illustrated in Figure 1. The activities of the catalyst achieved more than 80% NO conversion at 100 °C, which is maintained at almost 100% over the entire 150–180 °C range. Furthermore, the catalyst exhibited excellent low-temperatureNH3-SCR activity which enlarged the active temperature window of the catalyst. Considering the energy consumption of flue gas reheating and catalyst efficiency in practical applications, the effects of SO2 and H2O were studied at 100 °C.

3.2. Effects of SO2 and H2O on NO Removal Activity

In real applicationsincoal-fired power plants, small concentrations of H2O and SO2 still remain in the flue gas after the electrostatic precipitator and desulphurization devices. Therefore, the combined effect of SO2 and H2O should be considered for the SCR reaction. To explore the SO2 and H2O effects on the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst in terms of NO removal efficiency, transient response experiments were performed at 100 °C. Figure 2a shows the results of the catalytic activity in the presence of SO2 (50 ppm) over Mn–Ce/Al2O3. In the absence of SO2, the steady-state NO conversion over the Mn–Ce/Al2O3 was approximately 80%, indicating that the catalyst has high activity at low temperature. AfterSO2 (50 ppm) was introduced, NO conversion decreased rapidly to 72% at 24 h. After SO2 was shut off, NO conversion increased slightly but could not be completely recovered. During the NO conversion process in the presence of SO2, irreversible deactivation of the catalyst occurred. This result is attributed to the SO2 transformation into ammonium sulfates with NH3, blocking the active sites on the surface of the catalyst, and competition with NO for adsorption onto the active sites [21]. Furthermore, Figure 2b shows the results of the catalytic activity in the presence of H2O (5 vol %) over Mn–Ce/Al2O3; when H2O addition began, the NO conversions of Mn–Ce/Al2O3 decreased to ca. 13%. Over the next 23 h, the NO conversion was nearly unchanged and almost recovered to the initial activity after H2O was removed, which resulted from the competitive adsorption between H2O and NH3/NOx.
In addition, the stability of the Mn–Ce/Al2O3 catalyst was also tested at the same temperature and the results are presented in Figure 2c, indicating thatthere is no deactivation during the successive testing performed over the 168 h period at 100 °C in the absence of SO2 and H2O. Thus, the Mn–Ce/Al2O3 catalyst appears to be highly stable for NO elimination at low temperatures under the examined period of operation. Nevertheless, NO conversion by Mn–Ce/Al2O3 decreased quickly to 58% when SO2 and H2O were introduced into the gas flow simultaneously and the NO conversion was nearly stable over the next 124 h. The NO conversion then increased quickly to 70%, after the SO2 and H2O were removed. Meanwhile, the passivation effect was noticeably enhanced in the co-presence of SO2 and H2O because in the atmosphere with O2 and H2O, SO2 reacts with NH3 to form (NH4)2SO4 and NH4HSO4, which are deposited on the catalyst surface, resulting in its deactivation [14]. According to the above results, it can be concluded that the Mn–Ce/Al2O3 catalyst exhibits some degree of SO2 and H2O resistance and that SO2 has a more significant inhibiting effect on the NH3-SCR catalytic activity than H2O [22,23]. Combined with the NH3-SCReffect of sulfation over this catalyst, we attempted to understand the interaction mechanism between SO2 and the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst.

3.3. Physico-Chemical Characterization of Honeycomb Cordierite-Based Mn–Ce/Al2O3 Catalysts

3.3.1. BET, SEM-EDS Mapping

The surface area data of the fresh and sulfated samples are illustrated in Figure 3. Compared with that of the fresh Mn–Ce/Al2O3 catalyst, the BET surface area of the sulphated Mn–Ce/Al2O3 catalyst is decreased by 53.1%. Compared to those of S-Mn–Ce/Al2O3, the pore size and pore volume of F-Mn–Ce/Al2O3 are decreased by 27.6% and 37.1%, respectively. These results indicate that NO removal in the presence of SO2 leads to a decrease in the BET surface area, pore size, and pore volume, which can be correlated with the formation of some species on the surface of the catalyst, which leads to the clogging of some of the catalyst pores.
SEM studies were conducted to investigate the change in morphology of the catalysts after the SCR reaction. The SEM images in Figure 4 demonstrate that the surface of the F-Mn–Ce/Al2O3 was smoother and more compact than those of R-Mn–Ce/Al2O3 and S-Mn–Ce/Al2O3. As shown in Figure 4, the surface of the F-Mn–Ce/Al2O3 catalyst is almost flat. However, after SO2 (50 ppm) poisoning, a high degree of aggregation on the surface was observed (Figure 4a,c). Additionally, compared with F-Mn–Ce/Al2O3 (Figure 4a), there was no apparent change on the surface of the R-Mn–Ce/Al2O3 catalyst (Figure 4b), but a few depositions could be seen on S-Mn–Ce/Al2O3 (Figure 4c). This finding can be interpreted as a cause for the decrease in the surface area and pore volume (Figure 3). In addition, EDS displayed the Mn, Ce, Al, C, and O signals, further proving that mixed metal oxide catalysts have been fabricated and supported on the honeycomb cordierite (Figure 4d). To further investigate the dispersion degree of the catalyst powder on the surface of the carrier, the SEM-EDS mapping of the F-Mn–Ce/Al2O3 is displayed in Figure 4e–h. The existence of Mn, Ce, Al, and O elements is clear, and high dispersion is shown (Figure 4e–g), indicating a well-mixed state of the catalyst. The above characteristics are also consistent with the analysis results of EDS.
Figure 5 shows the N2 adsorption–desorption isotherms and pore size distribution of the fresh and poisoned catalysts, in which the isotherms are classical type IV [24]. The H2-type hysteresis loop was observed, which is typical for a wormhole-like mesostructure [18]. In addition, the beginning and end of the adsorption volume shifted in the direction of a large p/p0, which shows that there was some smaller pore clogging of the catalyst after poisoning by SO2.
To further understand the microstructure of the poisoned Mn–Ce/Al2O3, XRD analysis was performed. The XRD patterns of the honeycomb cordierite-based Mn–Ce/Al2O3 catalysts before and after the reaction are depicted in Figure 6. Several diffraction peaks observed at 28.95°, 33.29°, 47.49°, 57.16°, and 76.71° could be assigned to a cubic CeO2 crystallite (PDF#34-0394). The peaks attributed to CeO2 could also be seen before and after the SCR reaction [25]. However, no manganese oxide phases could be detected, indicating that the manganese oxides formed as primary crystals of less than approximately 4 nm, which was consistent with the report by Machida et al. [26]. After the reaction, there were no new peaks that emerged on the S-Mn–Ce/Al2O3XRD curve, which indicated that no crystal sulfate phase was formed on Mn–Ce/Al2O3 after the reaction or that the surface sulphate amount was below the detection limit [27].

3.3.2. XPS

XPS experiments were performed to investigate the surface component and chemical state change of elements over the fresh and sulfated catalysts. The surface atomic concentrations of Mn, Ce, O, and S are summarized in Table 1, and the photoelectron spectra of the Mn 2p, Ce 3d, O 1s, and S 2p levels are displayed in Figure 7.
The Mn 2p XPS spectra of the fresh and sulphated Mn–Ce/Al2O3 samples are shown in Figure 7a. Two peaks, due to Mn 2p1/2 and Mn 2p3/2, were observed from 637.0 eV to 662.5 eV, which are characteristic of a mixed-manganese system (Mn2+, Mn3+, and Mn4+) [28]. The Mn 2p XPS spectra of the F-Mn–Ce/Al2O3 could be classified into six subpeaks: Mn2+ (641.6 and 652.1 eV), Mn3+ (643.1 and 653.9 eV), and Mn4+ (644.2 and 655.1 eV) [29], indicating that Mn2+, Mn3+, and Mn4+ co-exist on the surface of the catalysts. Compared with F-Mn–Ce/Al2O3, the Mn 2p peak for the poisoned S-Mn–Ce/Al2O3 slightly shifted to a higher binding energy (0.2–0.5 eV higher), which may be attributed to the formation of species other than manganese oxides for Mn–Ce/Al2O3, which could occur when catalysts are sulfated, shifting them to a higher binding energy [30]. Here, the binding energies were shifted higher during the SCR reaction in the presence of SO2, which means that the Mn species on the surface of the Mn–Ce/Al2O3 catalyst were less active, thus resulting in performance degradation. In addition, Kapteijn et al. [31] demonstrated that Mn4+ has a better redox ability than Mn2+and Mn3+ over the spectrum of manganese-based catalysts. For the two samples, the ratio Mn4+/(Mn2++Mn3++Mn4+) can be calculated from the characteristic peaks intensities of Mn2+, Mn3+, and Mn4+. The ratio of Mn4+/(Mn2++Mn3++Mn4+) decreased by 8.86% for F-Mn–Ce/Al2O3 and S-Mn–Ce/Al2O3. Therefore, the decreased concentration of Mn4+ can also reflect the reason for the activity decrease.
The complicated Ce 3d XPS spectra of the fresh and sulphated Mn–Ce/Al2O3 catalyst are presented in Figure 7b. The bands labelled u and v are representative of the 3d3/2 spin-orbit states and 3d5/2 states, respectively. The peaks assigned u′ and v′ are representative of the 3d104f1 electronic state of Ce3+ ions, while u, u″, u′″ and v, v″, v′″ are attributed to the 3d104f0 state of the Ce4+ species [32,33]. The intensities of the Ce3+ peaks slightly increased after the SCR reaction in the presence of SO2. The concentration of Ce3+ was used to evaluate the number of formed oxygen vacancies. The oxygen vacancies were driven by the transition fromCe4+to Ce3+, which accelerated the transport of active oxygen species [18] and facilitated the reaction. The ratio of Ce3+/(Ce3++Ce4+) over the used sample(12.7%) was slightly greater than that of the fresh sample (11.6%) (Table 1), due to the following reaction [34,35]: 2CeO2 + 3SO2 + O2→Ce2(SO4)3. Since the generated Ce2(SO4)3 on the surface of the Mn–Ce/Al2O3 catalyst does not decompose at 100 °C, the Ce2(SO4)3 would lead to a reduction of the NO conversion (Figure 2). Figure 7c shows the O1s XPS spectra of the fresh and used Mn–Ce/Al2O3 samples. The spectra could be fitted into two sub-bands, the first sub-band at 531.3–532.2 eV corresponding to the surface chemisorbed oxygen (O′), similar to O22− pertaining to the defect-oxide or hydroxyl-like groups [36], and the second sub-band at 528.5–530.5 eV corresponding to the lattice oxygen O2− (O″) [37]. After sulfation, there was a significant increase in the ratio of O′/(O′ + O″) in S-Mn–Ce/Al2O3, which was due to the oxygen of the sulfate or the hydroxyl on the surface. Moreover, the accumulation of O′ might result in a suppressed effect in the adsorption and desorption of NO [38]; the adsorption, transformation, and desorption of NO were the controlling steps of the SCR reaction. Consequently, the catalyst activity significantly decreased after SO2 poisoning. The S 2p XPS of the sulphated Mn–Ce/Al2O3 catalyst (Figure 7d) had two sub-peaks at 168.7 eV and 169.7 eV, assigned to the SO32- and SO42− species, respectively [39,40]. This result confirmed that sulfate species were formed over the sulphated Mn–Ce/Al2O3.

3.3.3. TG-DTA

To distinguish the different sulfate species formed during the sulfation process, the fresh and used catalysts were examined using TG-DTA.
Figure 8a shows the TG and Differential Scanning Calorimetry (DSC) curves of F-Mn–Ce/Al2O3, in which the TG curve presented one primary weight loss at approximately 80 °C, with the DSC curve revealing a corresponding valley. The weight loss was attributed to the desorption of adsorbed water on the catalyst [41]. In the TG-DTA curves of S-Mn–Ce/Al2O3 (Figure 8b), there are three main weight losses at approximately 80 °C, 370 °C, and 790 °C. The weight loss at 370 °C was attributed to the decomposition of NH4HSO4 into (NH4)2S2O7 and H2O, and the (NH4)2S2O7 decomposition generated NH3, SO2, H2O, and N2, as reported in the literature [42,43]. According to the report, the decomposition temperature of Ce2(SO4)3 or Ce(SO4)2 was greater than 700 °C [34]. Therefore, sulfated CeO2 decomposition or desorption can explain the weight loss peak at 790 °C. Upon completion of the TG-DTA analysis, it can be concluded that ammonium hydrogen sulfate and sulfated CeO2 were formed during the reaction process in the presence of SO2.

3.3.4. FT-IR Analysis

Further confirmation of the TG-DTA data was obtained by FT-IR analysis to investigate the sulfate and ammonium groups on the catalyst surface. Figure 9 shows the FT-IR spectra of fresh and used catalysts.
The freshand used catalyst both exhibited bands at 910 and 1635 cm−1 and featured different metal oxides. In comparison with the fresh Mn–Ce/Al2O3, the used Mn–Ce/Al2O3 revealed some new bands at 1025, 1142, 1422, and 3260 cm−1. According to literature [44,45,46], the bands at 1025 and 1142 cm−1 were assigned to the ant symmetric stretching vibration of the SO42− group [47]. The peak at 1422 cm−1 indicated the presence of NH4+ species, adsorbed on the Brønsted acid sites [46,48]. Additionally, the broad peak at 3260 cm−1 was attributed to the N–H stretching vibration of the NH4+ group [49]. Combining the TG-DTA result with the FT-IR result, the conclusion further confirms that the formation of sulfate species on the surface of the catalyst during the SCR reaction in the presence of SO2 can lead to pore plugging, a decrease in the surface area, and occupation of the active sites, which explain the decline in the SCR activity of the catalyst [18].

4. Conclusions

In the present work, the poisoning effect of SO2 on the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst performance and the physicochemical properties were studied. The honeycomb cordierite-based Mn–Ce/Al2O3 catalyst showed satisfactory catalytic activity at 100 °C in the presence of SO2. However, the catalytic activity decreased to 72% after a 24 h reaction. The physiochemical characterization results indicated that the ammonium hydrogen sulfate and sulfated CeO2 were formed and were deposited on the surface of the honeycomb cordierite-based Mn–Ce/Al2O3 catalysts during the NH3-SCR process in the presence of SO2. This process could result in blocking the pores of the catalyst and decreasing the surface area, thus decreasing the SCR activity. In addition, larger losses of Mn and Ce occur on the surface of sulphated Mn–Ce/Al2O3 than on that of fresh Mn–Ce/Al2O3, directly leading to the decreased NOx removal ability.

Acknowledgments

This work was financially supported by the National High Technology Research and Development Program of China (2015AA03A401).

Author Contributions

C.Z. and P.C. conceived and designed the experiments; C.W. performed the experiments; Y.Z. and X.Y. analyzed the data; C.W. wrote the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. NO conversion byMn–Ce/Al2O3 catalyst as a function of temperature. Reaction conditions: 200 ppm NO, 200 ppm NH3, air balance, gas hour space velocity (GHSV) = 1667 h−1.
Figure 1. NO conversion byMn–Ce/Al2O3 catalyst as a function of temperature. Reaction conditions: 200 ppm NO, 200 ppm NH3, air balance, gas hour space velocity (GHSV) = 1667 h−1.
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Figure 2. Effect of SO2 and/or H2O on NO removal activities of the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst at 100 °C. (a) 50 ppm SO2 resistance tests; (b) 5 vol % H2O resistance tests; (c) 50 ppm SO2 and 5 vol % H2O resistance tests.
Figure 2. Effect of SO2 and/or H2O on NO removal activities of the honeycomb cordierite-based Mn–Ce/Al2O3 catalyst at 100 °C. (a) 50 ppm SO2 resistance tests; (b) 5 vol % H2O resistance tests; (c) 50 ppm SO2 and 5 vol % H2O resistance tests.
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Figure 3. Brunauer–Emmet–Teller (BET) Surface Area, pore size, and pore volume of the fresh (F) and sulfated (S) honeycomb cordierite-based Mn–Ce/Al2O3 catalysts (∆A = (AF − AS/AF) × 100%).
Figure 3. Brunauer–Emmet–Teller (BET) Surface Area, pore size, and pore volume of the fresh (F) and sulfated (S) honeycomb cordierite-based Mn–Ce/Al2O3 catalysts (∆A = (AF − AS/AF) × 100%).
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Figure 4. (a–c) Scanning electron microscopy (SEM) images of fresh and SO2 poisoned catalysts and (d–h) SEM-Energy dispersive spectroscopy (EDS) element mapping of fresh catalyst. (a) F-Mn–Ce/Al2O3; (b) R-Mn–Ce/Al2O3; (c) S-Mn–Ce/Al2O3; and (d) EDS spectra of the Mn–Ce/Al2O3 particles in yellow panel (a); (e) Mn; (f) Ce; (g) Al, (h) O.
Figure 4. (a–c) Scanning electron microscopy (SEM) images of fresh and SO2 poisoned catalysts and (d–h) SEM-Energy dispersive spectroscopy (EDS) element mapping of fresh catalyst. (a) F-Mn–Ce/Al2O3; (b) R-Mn–Ce/Al2O3; (c) S-Mn–Ce/Al2O3; and (d) EDS spectra of the Mn–Ce/Al2O3 particles in yellow panel (a); (e) Mn; (f) Ce; (g) Al, (h) O.
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Figure 5. N2 adsorption–desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution of Mn–Ce/Al2O3 before (F) and after (S) poisoning by SO2.
Figure 5. N2 adsorption–desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution of Mn–Ce/Al2O3 before (F) and after (S) poisoning by SO2.
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Figure 6. X-ray diffraction (XRD) patterns of Mn–Ce/Al2O3 before (F) and after (S) poisoning by SO2.
Figure 6. X-ray diffraction (XRD) patterns of Mn–Ce/Al2O3 before (F) and after (S) poisoning by SO2.
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Figure 7. X-ray photoelectron spectroscopy (XPS) spectra of the catalysts before and after 24 h of an SO2-containing reaction at 100 °C. (a) Mn 2p; (b) Ce 3d; (c) O 1s; and (d) S 2p.
Figure 7. X-ray photoelectron spectroscopy (XPS) spectra of the catalysts before and after 24 h of an SO2-containing reaction at 100 °C. (a) Mn 2p; (b) Ce 3d; (c) O 1s; and (d) S 2p.
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Figure 8. Thermogravimetry and Differential Thermal Analysis (TG-DTA) curves of fresh and used catalysts: (a) F-Mn–Ce/Al2O3; (b) S-Mn–Ce/Al2O3.
Figure 8. Thermogravimetry and Differential Thermal Analysis (TG-DTA) curves of fresh and used catalysts: (a) F-Mn–Ce/Al2O3; (b) S-Mn–Ce/Al2O3.
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Figure 9. Fourier transform infrared (FT-IR) spectra of fresh (F) and used (S) Mn–Ce/Al2O3 catalyst.
Figure 9. Fourier transform infrared (FT-IR) spectra of fresh (F) and used (S) Mn–Ce/Al2O3 catalyst.
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Table
Table 1. Surface atomic distributions of the catalysts by X-ray photoelectron spectroscopy (XPS).
Table 1. Surface atomic distributions of the catalysts by X-ray photoelectron spectroscopy (XPS).
SamplesAtomic Concentration (at %)Surface Atomic Ratio (%)
MnCeOSCe3+/(Ce3+ + Ce4+)Mn4+/(Mn2+ + Mn3+ + Mn4+)O′/O′ + O′′
F-Mn–Ce/Al2O37.312.4955.21011.635.615.8
S-Mn–Ce/Al2O34.051.9966.753.0112.726.7430.5

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Wang, C.; Zhang, C.; Zhao, Y.; Yan, X.; Cao, P. Poisoning Effect of SO2 on Honeycomb Cordierite-Based Mn–Ce/Al2O3Catalysts for NO Reduction with NH3 at Low Temperature. Appl. Sci. 2018, 8, 95. https://doi.org/10.3390/app8010095

AMA Style

Wang C, Zhang C, Zhao Y, Yan X, Cao P. Poisoning Effect of SO2 on Honeycomb Cordierite-Based Mn–Ce/Al2O3Catalysts for NO Reduction with NH3 at Low Temperature. Applied Sciences. 2018; 8(1):95. https://doi.org/10.3390/app8010095

Chicago/Turabian Style

Wang, Chengzhi, Cheng Zhang, Yonggang Zhao, Xin Yan, and Peng Cao. 2018. "Poisoning Effect of SO2 on Honeycomb Cordierite-Based Mn–Ce/Al2O3Catalysts for NO Reduction with NH3 at Low Temperature" Applied Sciences 8, no. 1: 95. https://doi.org/10.3390/app8010095

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