- Email: [email protected]
SCR catalyst
Applied Surface Science 507 (2020) 145153
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
The effect of SO2 on NH3-SCO and SCR properties over Cu/SCR catalyst ⁎
⁎
T
Chuanmin Chen , Yue Cao, Songtao Liu , Wenbo Jia The Key Laboratory of Resources and Environmental Systems Optimization, Ministry of Education, Beijing 102206, China School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SCR catalyst Selective catalytic oxidation (SCO) of NH3 Slip ammonia SO2 resistance
The sulfur resistance in NH3-SCO and NH3-SCR reactions of 1%Cu/SCR catalyst prepared by improved wet impregnation method was tested at 350 °C. In the presence of 500 ppm SO2, 1%Cu/SCR maintained good SCR performance, but its NH3 oxidation efficiency was visibly decreased. The NH3 conversion was promoted to about 90% after adding NO since SCR reactions occurred. As observed in ionic chromatography spectra, no more SO3 were generated over the Cu doped catalyst. NH3-TPD, H2-TPR results showed that some sulfite and sulfate species on the catalyst surface significantly increased the amount of weak acid sites and decreased the reducibility of copper species. In addition, the influence mechanism of SO2 on NH3 and NO adsorption and conversion over 1%Cu/SCR catalyst were revealed by using in situ DRIFT methods. The suppression of NH2 → NOx conversion could account for the SO2 deactivation in NH3 oxidation. Gaseous NO can directly react with NH2 through iSCR mechanism.
1. Introduction In June 2014, the General Office of the State Council of the People’s Republic of China firstly put forward that the emission of air pollutants from new coal-fired generating units should be close to that of gas-fired units, thus opening the prelude to the ultra-low emission of coal-fired power plants in China [1]. Nitrogen oxide (NOx) is one of the major atmospheric pollutants emitted from coal-fired power plants. According to the “Coal Energy Saving and Emission Reduction and Action Plan (2014–2020)”, the NOx emitted from them should be less than 50 mg/N m3 [2], which poses new challenge for flue gas denitration equipment. Up to date, NH3-selective catalytic reduction (SCR) is still the most commonly used flue gas denitration technology for coal-fired power plants [3,4]. In this method, NH3 is added as the reducing agent to selectively reduce NOx to N2, and V2O5-WO3/TiO2 is usually used as the commercial SCR catalyst with a deNOx efficiency of over 90% [5]. In theory, adding more NH3 would lead to higher NOx removal efficiency. However, excessive pursuit of NOx emission reduction is likely to cause the problem of ammonia slip [6]. As a typical alkaline gas, NH3 would easily react with acidic flue gas components under specific conditions to form a series of ammonium sulfate and ammonium nitrate, which can increase the emission of PM2.5 from coal-fired power plants and do harm to human health [7]. What’s more, the slip ammonia will easily interact with SO3 and H2O in the flue gas to generate the ammonium
bisulfate (ABS) with high viscosity, resulting in catalyst deactivation, blockage and corrosion of the channel, and decrease of conversion efficiency of downstream air preheater when the temperature is lower than 300 °C [8]. All these would seriously affect the stable operation of flue gas system. Therefore, the slip ammonia is strictly limited in China, the United States and the European countries, the threshold limit of which was set as 2–10 ppm [9–11]. Considering these problems, the NH3/NO ratio of 0.9–0.95 is usually used to avoid the generation of ammonia slip [12]. But the NOx removal performance may be difficult to reach the requirement in “ultra-low emissions standards”. And even in this case, a part of NH3 may still not be consumed in SCR reactions when operating conditions change, such as the stratification of the gas stream, the improper distribution of ammonia or the reduced catalytic efficiency caused by long-time used SCR catalysts. At present, selective catalytic oxidation of NH3 (NH3-SCO) is considered as one of the promising methods for removing NH3 from the exhaust. NH3 could react with O2 over suitable catalysts to form harmless N2 and H2O [13,14]. Some noble metals [15,16], transition metal oxides [12,17], mixed rare earth and transition metal oxides [1820] and noble metals/transition metal oxides modified zeolites [21,22] possess NH3 oxidation ability under specific reaction conditions in varying degrees. Considering NH3 conversion, N2 selectivity and the cost, copper containing materials may be one of the most promising NH3-SCO catalysts [12]. Under the respective suitable reaction
⁎ Corresponding authors at: The Key Laboratory of Resources and Environmental Systems Optimization, Ministry of Education, Beijing 102206, China. School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China. E-mail addresses: [email protected] (C. Chen), [email protected] (S. Liu).
https://doi.org/10.1016/j.apsusc.2019.145153 Received 5 September 2019; Received in revised form 18 December 2019; Accepted 21 December 2019 Available online 23 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
2.2. Catalytic activity measurement
conditions, 10wt.%Cu/TiO2 [17], 10wt.%Cu/Al2O3 [23], 4.4wt.%Cu/ ZSM-5 [21], 3wt.%Cu/Beta [24], 3.7wt.%Cu/Y treated with NaOH [25], etc. could all reach excellent NH3 oxidation efficiency and N2 selectivity of over 90%. The 1%Cu/SCR catalyst, which was made by adding only 1 wt% copper species to the commercial SCR catalyst, has also been proved to possess good NH3-SCO performance in our previous research [26]. Even at a high GHSV of 3 × 105 h−1, it oxidized 89% of NH3 with a N2 selectivity of 86% in the presence of 4%O2 at 350 °C. When the NH3/NO ratio was 1:1, its NH3 and NOx removal efficiency came to 92% and 93%, respectively, and the N2 selectivity reached 91%. This suggests that it may simultaneously remove residual NO and NH3 from the flue gas. However, since the SCR equipment is usually installed before desulfurization system, the flue gas entering SCR unit may contain a large amount of SO2. It has been proved to have various effects on the catalytic performance of different catalyst systems [27], such as reacting with the reactants [28], competing with the reactants for the active site over the catalyst surface [29,30] or even reacting with the active component to form sulfates [31]. In addition, due to good oxidation properties, some catalysts may also catalyze the SO2 oxidation to form SO3 as a side reaction. This would promote the formation of ABS to block the catalyst pores below 280 °C [32,33]. All these may cause a deactivation of the catalysts. Therefore, it is of great importance to reveal the effect of SO2 on the catalytic performance in the development of novel NH3-SCO or SCR catalysts. Until now, the influence mechanism of SO2 on the catalytic properties of 1%Cu/SCR catalyst has not been studied yet. And the SO3 formation performance over the catalyst is still unclear. Since the catalyst is expected to be used at a typical SCR temperature, the effect shown at around 350 °C of SO2 on the catalytic activity may be mainly caused by changing the active components of the catalyst or affecting the adsorption and conversion of the reactants. But the amount of SO3 production is still an important parameter to be investigated to avoid its adverse effects on the catalyst and downstream flue gas equipment when the temperature is lowered in practical applications. Therefore, in this study, the sulfur resistance in NH3-SCO and SCR reactions and the SO2 oxidation performance of 1%Cu/SCR were further investigated. NH3 temperature programmed desorption (NH3TPD), H2 temperature program reduction (H2-TPR) were adopted to reveal the effect of SO2 on acid sites and reducibility of the catalyst. And in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) methods were used to explore the influence mechanism of SO2 on NH3 and NO adsorption and conversion over 1%Cu/SCR catalyst. This study would provide an important reference for the control of residual NOx and slip ammonia in sulfur-containing flue gas from coalfired power plants.
The catalytic activity of 1%Cu/SCR catalyst was investigated on a vertical fixed bed reaction system. 0.3 g catalyst was placed in the quartz reactor with 10 mm i.d. and was then heated to 350 °C in each test. The flue gases containing 30 ppm NH3, 30 ppm NO (if used), 500 ppm SO2 (if used) and 4%O2 with N2 as the balance were adjusted by corresponding mass flow controllers to maintain a total flow rate of around 1.5 L/min. A DX4000 Fourier transform infrared spectrometer (Gasmet, Finland) was used for online detecting the concentrations of NH3, N2O, NO, and NO2 in inlet and outlet flue gases. The removal efficiencies of NH3 and NOx as well as the N2 selectivities were calculated by using Eqs. (1)–(4) [26]. The N2 selectivity with superscript a and b respectively represent the proportion of N2 in nitrogen-containing products in the absence and presence of NO in the feed gas.
NH3 removal efficiency =
[NH3]inlet − [NH3]outlet × 100% [NH3]inlet
[NO]outlet + [NO2 ]outlet ⎤ × 100% NOx removal efficiency = ⎡1 − ⎢ ⎥ [NO]inlet ⎣ ⎦
(1)
(2)
[NO]outlet + [NO2 ]outlet + 2[N2 O]outlet ⎤ × 100% N2 selectivity a = ⎡1 − ⎥ ⎢ [NH3 ]inlet − [NH3 ]outlet ⎦ ⎣ (3)
N2 selectivity b [NO2 ]outlet + 2[N2 O]outlet ⎤ × 100% = ⎡1 − ⎢ [NH3 ]inlet + [NO]inlet − [NH3 ]outlet − [NO]outlet ⎥ ⎦ ⎣
(4)
Besides, the SO2 oxidation efficiency of 1%Cu/SCR catalysts was also investigated. The inlet flue gas contains 2000 ppm SO2, 4%O2 and N2. According to EPA method 8 [35,36], the SO3 at the outlet of the flue gas was absorbed by an 80% isopropyl alcohol (IPA) aqueous solution for 3.5 h, and the SO42- content in the solution was measured by a 792 Basic ionic chromatography (Metrohm AG,) equipped with a Metrosep A Supp 4 Anion chromatographic column. The SO3 concentration in the gas across SCR catalyst and the blank reactor was also tested for comparison. 2.3. NH3-TPD The NH3-TPD curves were acquired using a PCA-1200 chemisorption analyzer (Biaode, China). Before each test, 0.2 g catalyst (20–40 mesh) was heated in pure He atmosphere to 350 °C and kept for 30 min to remove the adsorbed substances. The sample was then cooled down to 100 °C, kept at this temperature to adsorb pure NH3 (30 mL/min) for 15 min, and then swept by He stream for 60 min to remove physically adsorbed NH3 on catalyst surface. NH3 desorption was performed from 50 °C to 600 °C with a temperature ramp of 10 °C/min in 30 mL/min He flow. The signal of desorbed NH3 was recorded online using a thermal conductivity detector (TCD).
2. Experimental 2.1. Catalysts preparation
2.4. H2-TPR
The 1%Cu/SCR catalyst used in this study was prepared by an improved impregnation method [34]. First, the commercial SCR catalyst (contains 0.7%V2O5, 4.8%WO3, TiO2 supported) was ground into powder. Cu(NO3)2⋅3H2O was dissolved in deionized water, and a certain amount of SCR catalyst was then added into the solution to provide a mass ratio of 1:99 for Cu and SCR catalyst. After magnetically stirring for 2 h at 60 °C, the solution was dried at 105 °C overnight to remove the water. The obtained solid was then transferred to the muffle furnace and underwent calcination at 400 °C in air for 4 h. At last, the prepared catalysts were ground to 20–40 mesh for NH3 oxidation activity test, H2-TPR and NH3-TPD experiment, and the powder catalysts were used for in situ DRIFT studies.
In H2 temperature program reduction (H2-TPR) experiment, the test instrument, catalyst dosage and pretreatment method were the same as that in NH3-TPD test. After purged in He at 350 °C, the catalyst was cooled down to room temperature and 10% H2/Ar reducing gas was introduced at a flow rate of 30 mL/min. After the baseline was stabilized, the temperature was raised to 800 °C at a ramping rate of 10 °C/ min, and the H2 signal was recorded. 2.5. In situ DRIFT studies The in situ DRIFT spectroscopy was performed using a diffuse reflectance accessory connected to a Nicolet iZ10 DRIFTS spectrometer 2
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
gas may be simultaneously oxidized to the more harmful SO3 over the catalysts. In order to prevent the damage of excessive SO3 to the flue gas system and the environment, the SO2 oxidation performance of 1%Cu/ SCR was investigated. Fig. S1 showed the ionic chromatography (IC) spectra of 80% IPA aqueous solution collected under different conditions. The peak appearing at about 13.18 min was identified as the peak of sulfate. It can be been that no significant difference appeared in the peak area for the three sets of experiments with no catalyst (blank), SCR and 1%Cu/SCR catalyst placed in the reactor. Among them, the SO42existed in the blank group may be derived from the oxidation of a small amount of SO32- by O2 in the IPA solution in the sampling process [36]. Similar amount of SO3 was also generated for the two catalysts, indicating that the SO2 oxidation performance of them was negligible, and the addition of 1%Cu did not significantly affect the SO2 oxidation of SCR catalyst. Accordingly, no excessive SO3 will be generated when a small amount of 1%Cu/SCR catalyst is placed in the tail of the SCR system to catalytic oxidize the slip ammonia from coal-fired power plants.
Fig. 1. NH3 and NO removal efficiency and N2 selectivity of 1%Cu/SCR catalyst at 350 °C.
(Thermo, USA), which was equipped with a mercury cadmium telluride (MCT) detector. The spectra were recorded at a spectral resolution of 4 cm−1 with 32 scans. Before all experiments, the powder catalysts were flushed with N2 at 400 °C for 1 h, then cooled down to 350 °C and exposed to different treatment conditions (described in the following sections) for the test. If used, SO2, NH3 and NO in the treatment gas was all applied at the concentration of 500 ppm, and O2 was set as 4%. Besides, N2 was used as the balance gas to provide a total flow rate of 50 mL/min, which was 1/30 of that in activity test, to explore the reaction process in detail.
3.3. H2-TPR results Previous studies showed that SO2 is likely to influence the redox properties of catalysts [12]. Therefore, the H2-TPR profiles of fresh 1%Cu/SCR catalyst and the catalyst after treated by 500 ppm SO2 and 4%O2 for 100 min were compared in Fig. 2. For the fresh one, the reduction peaks of H2 presented in a wide temperature range. The overlapping peaks at 253 °C and 265 °C and the one at 468 °C can be assigned to the reduction of copper species [38–40] and vanadium species dispersed on the catalyst surface, respectively. The peaks appeared between 339 and 422 °C may be Cu–O–V [38] or Cu–O–Ti species [41]. And the two at 578 and 636 °C represented tungsten and titanium species [26]. After sulfurization, the reduction peaks of copper species appeared at 346 and 363 °C, which were moved to high-temperature region. The change could be attributed to the transformation of CuO to CuSO4 [42]. However, the reduction temperature of other species was basically unchanged. This indicated that the sulfated treatment would visibly reduce the reducibility of copper species, but has little effect on that of other species. According to the research of Neumann, after treated with SO2 and air, V2O5 would probably not convert into sulfate, a small amount of TiO2 may transform into TiOSO4, while much CuO tend to form CuSO4 when the temperature was below 650 °C [43]. Their result also verifies our inference from this test. Meanwhile, the H2 consumption slightly increased after sulfurization, suggesting the amount of lattice oxygen reacted with H2 was probably increased by some formed sulfur species on catalyst surface. These phenomena were basically in line with the influence of SO2 on reducibility of Cu/Al2O3
3. Results and discussion 3.1. Catalytic activity Ammonia oxidation and NOx removal properties of 1%Cu/SCR catalyst were investigated in the presence and absence of SO2 at 350 °C. As shown in Fig. 1, the catalyst exhibited a good NH3-SCO performance. At the high gas hourly space velocity (GHSV) of 3 × 105 h−1, the NH3 oxidation efficiency and N2 selectivity reached about 88% and 86%, respectively. However, only 63% of NH3 was removed after 500 ppm SO2 was introduced into the flue gas, indicating that SO2 would visibly inhibit the catalytic oxidation of NH3. When NO was added to provide an NH3/NO ratio of 1:1, the experimental phenomenon was somewhat different. NH3 removal efficiency and N2 selectivity increased slightly to 93% and 91%, respectively. This suggested that NO would react with NH3 and thus promote the selective conversion of NH3. Meanwhile, the deNOx efficiency of 1%Cu/SCR catalyst reached 92%. More importantly, although SO2 was added, the removal efficiency of NH3 and NO was still maintained at 89% and 92%, respectively. It showed that the catalyst possesses good sulfur resistance in the presence of NO and could remove NH3 and NO simultaneously. In addition, the selectivity to N2 was slightly increased by the addition of SO2 regardless of whether the flue gas contained NO or not, which was 95% and 90%, respectively. This positive effect, probably resulting from the generated surface sulphates, was also found for other copper-containing catalysts [12]. The oxide anions in CuO lattice were reported to be more stable for these sulfated catalysts, which was not conducive to the formation of nitrogen oxides [37]. Since the added SO2 inhibited the NH3 conversion to significantly different extend in the presence and absence of NO, the reaction mechanism involving SO2 must be clarified to rationally reveal the NH3 and NO removing process over 1%Cu/SCR catalyst in coal-fired flue gas. 3.2. SO2 oxidation performance
Fig. 2. H2-TPR curves of sulfated and fresh 1%Cu/SCR catalyst.
As the NH3-SCO or SCR reaction occurs, SO2 in the coal-fired flue 3
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 4. In situ DRIFT spectra of NH3 adsorption on fresh and sulfated 1%Cu/ SCR catalyst at 350 °C.
Fig. 3. NH3-TPD curves of sulfated and fresh 1%Cu/SCR catalyst.
catalyst [44].
over the catalyst.
3.4. NH3-TPD results
3.6. DRIFT of NH3 adsorption
The NH3-TPD method was used to examine the influence of SO2 on the acid sites of 1%Cu/SCR catalyst. It can be seen that the sulfurization dramatically changed the NH3 desorption curve of the catalyst (Fig. 3). Before treated with SO2, a serious of peaks with similar intensities were evenly distributed in the range of 100–450 °C for the catalyst. Among them, the peaks appearing below 350 °C were identified as the weakly adsorbed NH3 species on Lewis and Brønsted acid sites, while the other two at about 358 and 405 °C were the adsorbed NH3 on Lewis acid sites or strong Brønsted acid sites [26]. In contrast, the peaks below 200 °C were weakened for the sulfated catalyst. One remarkable reduction peak was seen at 202 °C, which indicated the presence of abundant weak acid sites. Two small peaks of strong acid sites were also noted at 358 and 419 °C, of which the latter one showed a slight increase compared with the fresh catalyst. In general, the introduction of SO2 improved the total amount of acid sites of the catalyst, especially the weak acid sites. Therefore, the sulfated catalyst exhibited a more superior NH3 adsorption capacity at the reaction temperature (350 °C).
Ammonia adsorption is usually regarded as an important step in both NH3-SCR and NH3-SCO reactions. As shown in Fig. 4, after treating by 500 ppm NH3 for 30 min, several bands were observed at 1697–1700, 1606–1610, 1419–1435, 1325–1327 and 1226–1255 cm−1 for both the fresh and sulfated 1%Cu/SCR catalyst. The bands at 1697–1700 and 1419–1435 cm−1 could correspond to the NH4+ species coordinated on Brønsted acid sites [16,50], and the two at 1606–1610 and 1226–1255 cm−1 should be assigned to the NH3 strongly adsorbed on Lewis acid sites [16,50]. Besides, the band appeared at 1325–1327 cm−1 can be attributed to the wagging deformation modes of amide species (−NH2), which is commonly regarded as the active intermediate species in NH3-SCR and NH3-SCO reactions [16,51]. For the fresh catalyst, the bands at 1435 and 1697 cm−1 were very weak, suggesting the NH3 species mainly adsorbed on Lewis acid sites. Compared with the spectrum of fresh catalyst, the band intensities for sulfated 1%Cu/SCR were significantly higher, especially the two at 1419 and 1255 cm−1. This indicated that the sulfurization increased both the Brønsted and Lewis acid sites on the catalyst surface and promoted the adsorption of NH3 species on them.
3.5. DRIFT of SOx adsorption As stated above, the effect of SO2 on NH3 removal performance over 1%Cu/SCR catalyst was very different when NO was present or not. In order to reveal the influence mechanism of SO2 on NH3 conversion, the interaction of the reactants with fresh and sulfated catalyst needs to be separately elucidated. Before this, the generated sulfate species over sulfated 1%Cu/SCR catalyst was firstly analyzed by DRIFT method. Fig. S2 displayed the spectrum of the catalyst treated in a flow of 500 ppm SO2 + 4%O2 at 350 °C for 30 min. It can be seen that several bands at 1418, 1362, 1300, 1190, 1034 and 839 cm−1 as well as a negative bands at 3659 cm−1 appeared. According to related literature, the negative bands belonged to the vibration of the consumed surface hydroxyl groups which interacted with SO2, indicating the generation of hydrogen-bonded sulfite species [28,45]. The weak band observed at 1418 cm−1 was attributed to a small amount of SO3 like species [27,46,47]. The strong bands at 1362 and 1300 cm−1 in spectrum belonged to the surface sulfate species bonded on different sites [28,48], while the one at 1190 cm−1 was ascribed to the bulk sulfate species [16,47], which was most likely to be CuSO4 according to the previous study [49] and our H2-TPR result. In addition, the presence of the band at 1034 and 839 cm−1 confirmed the formation of sulfite species [46,47]. The presence of these sulfur species may affect the adsorption and conversion of reactants in NH3-SCO and SCR reactions
3.7. Interaction of O2 with in situ formed NH3 The 1%Cu/SCR catalyst exhibited good activity in promoting the reaction between NH3 and O2. In order to reveal the NH3 oxidation mechanism, the gas containing 4% O2 was passed through the NH3 pretreated catalysts and the spectra were recorded. The gas flow was set as 20 mL/min, which was much slower than the 1 L/min in catalytic activity tests, to make it easier for observing the reaction process. As shown in Fig. 5(a), the spectra of fresh 1%Cu/SCR catalyst changed significantly with the introduction of O2. All the bands weakened slowly and vanished in 40 min, which suggested that all the NH3, NH4+ and NH2 species were consumed by O2. Two new bands appeared at 1363 and 1288 cm−1, indicating the generation of nitro compounds (−NO2) and monodentate nitrate (NO3) species on the catalyst surface [52-55]. However, no NO and NO2 were detected as the product of NH3 oxidation at 350 °C in our experiment. A possible explanation was that the −NO2 and NO3 species existed as the adsorbed intermediates in NH3 oxidation. Previous researches have proposed an internal SCR (iSCR) mechanism to describe a possible NH3 oxidation process in which NH3 tend to react with O2 to form NOx firstly, and the generated NOx could rapidly react with remaining NH3 to form harmless N2 and 4
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 5. In situ DRIFT spectra of the NH3 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 4% O2 at 350 °C.
H2O [12,56]. The appeared −NO2 and NO3 species in these spectra could be ascribed to the absorption species of generated NOx by NH3 oxidation. Since the NH3 species had already been oxidized, the NOx could not be removed without reductant and thus accumulated on the catalyst surface. Although more NH3 and NH4+ species were adsorbed on the surface of sulfated 1%Cu/SCR catalyst, no more NH3 was converted by it (Fig. 5(b)). Conversely, the NH3 oxidation efficiency was significantly reduced. The band at 1327 cm−1 was still clearly observed in the spectra, indicating that the sulfurization did not significantly inhibit the formation of intermediate NH2. When 4% O2 was introduced, the bands on sulfated catalyst only showed a slight decrease, and no bands of NOx species appeared even after 60 min. This suggested that only a small amount of NH3 and −NH2 species over sulfated catalyst reacted with O2, which was consistent with the inhibition of SO2 to NH3 oxidation in experimental results.
Fig. 6. In situ DRIFT spectra of the NH3 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 4% O2 + 500 ppm NO at 350 °C.
subsequently appeared at 1340 cm−1 and 1287 cm−1, respectively [52–55], and the spectra remained unchanged in 60 min. These species may be attributed to the product of NH3 transformation or NO adsorption. In sharp contrast to the introduction of only 4%O2, promoted by NO, the bands for sulfated catalyst were almost completely consumed in 13 min despite the large amount of adsorbed NH3 species, and then −NO2 and NO3 species generated. This result showed that NO played an important role in NH3 conversion over the sulfated catalyst. Although the adsorbed sulfur species significantly inhibited the reaction between NH3 and O2 over 1%Cu/SCR catalyst, the introduction of NO could significantly weaken this inhibitory effect to maintain good NH3 removal performance. However, compared with the fresh catalyst, the existing surface sulfur species still slightly inhibited the removal rate of NH3 species even in the presence of NO.
3.8. Interaction of NO and O2 with in situ formed NH3 After 500 ppm NO and 4% O2 were simultaneously added as the treatment gas, the adsorbed NH3 species were consumed rapidly for both the fresh and sulfated 1%Cu/SCR catalyst (Fig. 6). It can be observed that the original bands for the fresh catalyst vanished in 5 min, which was much quicker than that when only O2 was added, and thus verified the promoting effect of NO on NH3 conversion. Similarly, the bands of nitro compounds (−NO2) and monodentate nitrate (NO3)
3.9. DRIFT of NOx adsorption In order to reveal the reaction occurring over the fresh and sulfated catalysts in the presence of NO, the adsorption of NOx on the catalyst surface and the interaction between NH3 and adsorbed NOx species 5
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 7. In situ DRIFT spectra of NO + O2 adsorption on fresh and sulfated 1%Cu/SCR catalyst.
were also investigated. As shown in Fig. 7, after treating with 500 ppm NO + 4%O2 for 30 min and followed by N2 purging, several bands appeared for the fresh and sulfated 1%Cu/SCR catalysts, including the bands at 1697–1699, 1522–1541, 1362–1375, 1288–1298 and 1163–1167 cm−1. According to previous studies, the bands at 1522–1541 and 1288–1298 cm−1 belong to the monodentate nitrate (NO3) species [52–55]. The bands at 1362–1375 cm−1 can be attributed to the adsorbed nitro compounds (–NO2) on the catalyst surface [53,57,58]. And the weak one appeared at 1163–1167 cm−1 could be assigned to the nitrosyl anion (NO–) species [59,60]. Additionally, the bands appearing at 1697–1699 cm−1 can be ascribed to the contaminants on the TiO2 surface [52]. It can be clearly seen that the bands of sulfated 1%Cu/SCR catalyst were much weaker than the fresh one, indicating that the sulfurization significantly inhibited the adsorption of NO on the catalyst surface. 3.10. Interaction of NH3 with in situ formed NO Fig. 8(a) displayed the spectra of interaction between NH3 and the NO formed on the fresh 1%Cu/SCR catalyst at 350 °C. After adding NH3, the bands for NOx species weakened gradually, suggesting that the NOx on the catalyst surface reacted with NH3. When the NOx adsorbed consumed completely, NH3 species began to accumulate on the catalyst surface. New bands at 1616, 1318 and 1230 cm−1 appeared in the 10th min. All of them became stronger as the time increased, and basically kept unchanged after 20 min. The two bands at 1616 and 1230 cm−1 represented the NH3 adsorbed on Lewis acid sites and the one at 1318 cm−1 demonstrated the presence of –NH2 species. This also proved that NH3 species were more inclined to adsorb on Lewis acid sites on fresh 1%Cu/SCR catalyst. As depicted in Fig. 8(b), the NO adsorption amount was significantly lower for sulfated 1%Cu/SCR catalyst, and the bands corresponding to NOx disappeared more rapidly after passing through NH3. In the 5th min, new vibrational bands were found at 1424, 1323 and 1275 cm−1. And the band at 1613 appeared in the 10th min. The NH3 species on Brønsted and Lewis acid sites, together with the NH2 species were all observed. These bands increased rapidly over time, which was much stronger than the corresponding ones for the fresh catalyst. This result was consistent with the spectrum of NH3 adsorption in Section 3.4, which confirmed that the sulfated catalyst possessed stronger NH3 adsorption capacity.
Fig. 8. In situ DRIFT spectra of the NO + O2 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 500 ppm NH3 at 350 °C.
Fig. 9. Schematic diagram of NH3-SCO and NH3-SCR reactions over 1%Cu/SCR catalyst.
follows the iSCR mechanism (Fig. 9). NH3 would adsorb on the catalyst surface and be activated to form NH2*, which is then oxidized to NO2* and NO3 when O2 is introduced. These NOx species could rapidly react with NH3 to form N2 and H2O. The sulfurization would increase the amount of adsorbed NH3. However, it may also inhibit the conversion of NH2 to NOx. Without the formation of NOx, the adsorbed NH3 cannot be consumed by iSCR mechanism, which may be the reason for the poor NH3 oxidation performance exhibited by the sulfated catalyst. When NO is introduced, gaseous NO or adsorbed NOx can directly
3.11. The reaction mechanism The DRIFT results found that the NH3-SCO over 1%Cu/SCR catalyst 6
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
react with NH2* through the SCR reaction, which is also an important step in iSCR reaction. That is to say, NH3-SCO and NH3-SCR reactions would occur simultaneously over the fresh catalyst surface, and thus the addition of NO accelerates the consumption of NH3. Since the NH2 → NOx process is inhibited, almost only the NH3-SCR reaction could carry out over the sulfated catalyst. Therefore, the NH3 removal rate is slightly lower than the fresh catalyst. It is worth noting that SO2 significantly would reduce the amount of NO adsorbed on the catalyst surface. But the catalyst still maintains a good deNOx performance, indicating that for the sulfated catalyst, the NO mainly participates in the reaction in a gaseous form.
[7] K. Na, C. Song, C. Switzer, D. Cocker, Effect of ammonia on secondary organic aerosol formation from α-pinene ozonolysis in dry and humid conditions, Environ. Sci. Technol. 41(2007) 6096–6102. https://doi.org/ 10.1021/es061956y. [8] G. Qi, R.T. Yang, Low-temperature selective catalytic reduction of NO with NH3 over iron and manganese oxides supported on titania, Appl. Catal. B Environ. 44 (2003) 217–225, https://doi.org/10.1016/S0926-3373(03)00100-0. [9] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, Review on the latest developments in modified vanadium-titanium-based SCR catalysts, Chinese J. Catal. 39 (2018) 1347–1365, https://doi.org/10.1016/S1872-2067(18)63090-6. [10] Regulation (EC) no. 595/2009 of the European Parliament and of the Council of 18 June 2009. [11] Air Pollution Control Technology Fact Sheet Online, http://www.epa.gov/ttncatc1/ dir1/fscr.pdf (accessed January 2015). [12] M. Jabłońska, R. Palkovits, Copper based catalysts for the selective ammonia oxidation into nitrogen and water vapour—recent trends and open challenges, Appl. Catal. B Environ. 181 (2016) 332–351, https://doi.org/10.1016/j.apcatb.2015.07. 017. [13] X. Tang, J. Li, H. Yi, Q. Yu, F. Gao, R. Zhang, C. Li, C. Chu, An efficient two-step method for NH3 removal at low temperature using CoOx-CuOx/TiO2 as SCO catalyst followed by NiMn2O4 as SCR catalyst, Energ. Fuel. 31 (2017) 8580–8593, https:// doi.org/10.1021/acs.energyfuels.7b01329. [14] J. Leppälahti, T. Koljonen, M. Hupa, P. Kilpinen, Selective catalytic oxidation of NH3 in gasification gas. 2. Oxidation on aluminum oxides and aluminum silicates, Energ. Fuel. 11 (1997) 39–45. https://doi.org/10.1021/ef9502665. [15] W. Chen, Q. Zan, W. Huang, X. Hu, N. Yan, Novel effect of SO2 on selective catalytic oxidation of slip ammonia from coal-fired flue gas over IrO2 modified Ce-Zr solid solution and the mechanism investigation, Fuel 166 (2016) 179–187, https://doi. org/10.1016/j.fuel.2015.10.116. [16] W. Chen, Y. Ma, Z. Qu, Q. Liu, W. Huang, X. Hu, N. Yan, Mechanism of the selective catalytic oxidation of slip ammonia over Ru-modified Ce-Zr complexes determined by in-situ DRIFT, Environ. Sci. Technol. 48 (2014) 12199–12205, https://doi.org/ 10.1021/es502369f. [17] M. Jabłońska, Selective catalytic oxidation of ammonia into nitrogen and water vapour over transition metals modified Al2O3, TiO2 and ZrO2, Chem. Pap. 69 (2015) 1141–1155, https://doi.org/10.1515/chempap-2015-0120. [18] C. Hung, Selective catalytic oxidation of ammonia to nitrogen on CuO-CeO2 bimetallic oxide catalysts, Aerosol Air Qual. Res. 6 (2006) 150–169, https://doi.org/ 10.4209/aaqr.2006.06.0004. [19] J. Lou, C. Hung, S. Yang, Selective catalytic oxidation of ammonia over coppercerium composite catalyst, J. Air Waste Manage. Assoc. 54 (2004) 68–76, https:// doi.org/10.1080/10473289.2004.10470881. [20] M.L. Sang, S.C. Hong, Promotional effect of vanadium on the selective catalytic oxidation of NH3 to N2 over Ce/V/TiO2 catalyst, Appl. Catal. B Environ. 163 (2015) 30–39, https://doi.org/10.1016/j.apcatb.2014.07.043. [21] R.Q. Long, R.T. Yang, Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen, Chem. Commun. 17 (2000) 1651–1652, https:// doi.org/10.1039/b004957n. [22] T. Curtin, S. Lenihan, Copper exchanged beta zeolites for the catalytic oxidation of ammonia, Chem. Commun. 9 (2003) 1280–1281, https://doi.org/10.1039/ b301894f. [23] C. Liang, X. Li, Z. Qu, M. Tade, S. Liu, The role of copper species on Cu/γ-Al2O3 catalysts for NH3–SCO reaction, Appl. Surf. Sci. 258 (2012) 3738–3743, https://doi. org/10.1016/j.apsusc.2011.12.017. [24] S. Lenihan, T. Curtin, The selective oxidation of ammonia using copper-based catalysts: The effects of water, Catal. Today 145 (2009) 85–89, https://doi.org/10. 1016/j.cattod.2008.06.017. [25] L. Gang, B.G. Anderson, J. van Grondelle, R.A. van Santen, NH3 oxidation to nitrogen and water at low temperatures using supported transition metal catalysts, Catal. Today 61 (2000) 179–185, https://doi.org/10.1016/S0920-5861(00) 00375-8. [26] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, SCR catalyst doped with copper for synergistic removal of slip ammonia and elemental mercury, Fuel Process. Technol. 181 (2018) 268–278, https://doi.org/10.1016/j.fuproc.2018.09.025. [27] B.Q. Jiang, Z.B. Wu, Y. Liu, S.C. Lee, W.K. Ho, DRIFT study of the SO2 effect on lowtemperature SCR reaction over Fe−Mn/TiO2, J. Phys. Chem. C. 114 (2010) 4961–4965, https://doi.org/10.1021/jp907783g. [28] W. Xu, H. He, Y. Yu, Deactivation of a Ce/TiO2 catalyst by SO2 in the selective catalytic reduction of NO by NH3, J. Phys. Chem. C. 113 (2009) 4426–4432, https://doi.org/10.1021/jp8088148. [29] F. Liu, H. He, C. Zhang, Novel iron titanate catalyst for the selective catalytic reduction of NO with NH3 in the medium temperature range, Chem. Commun. 164 (2008) 2043–2045, https://doi.org/10.1039/b800143j. [30] W. Xu, Y. Yu, C. Zhang, H. He, Selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst, Catal. Commun. 9 (2008) 1453–1457, https://doi.org/10.1016/j. catcom.2007.12.012. [31] W. Sjoerd Kijlstra, M. Biervliet, E. K. Poels, A. Bliek, Deactivation by SO2 of MnOx/ Al2O3 catalysts used for the selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal. B Environ. 16 (1998) 327–337. https://doi.org/10. 1016/S0926-3373(97)00089-1. [32] G. Ramis, L. Yi, G. Busca, M. Turco, E. Kotur, R.J. Willey, Adsorption, activation, and oxidation of ammonia over SCR catalysts, J. Catal. 157 (1995) 523–535, https://doi.org/10.1006/jcat.1995.1316. [33] S.T. Choo, S.D. Yim, I. Nam, S. Ham, J. Lee, Effect of promoters including WO3 and BaO on the activity and durability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3, Appl. Catal. B Environ. 44 (2003) 237–252, https://doi.org/10.1016/ S0926-3373(03)00073-0.
4. Conclusions The 1%Cu/SCR catalyst possessed good NH3-SCO and NH3-SCR properties. It exhibited good sulfur resistance in SCR reaction, while the oxidation of NH3 was somewhat inhibited by the added SO2. Fortunately, a small amount of NO existed in the flue gas increased the NH3 conversion, which maintained about 90% even in the presence of 500 ppm SO2. Compared with the SCR catalyst, the added 1% copper species did not significantly improve the formation of SO3. SO2 would form some sulfite and sulfate species on the catalyst surface, which promoted the adsorption of NH3. The sulfated treatment slightly increased the H2 consumption and introduced abundant weak acid sites, but reduced the reducibility of copper species. The suppression of NH2 → NOx process may be the main reason why the NH3 oxidation over 1% Cu/SCR catalyst was inhibited in the presence of SO2. Although the presence of SO2 was not conducive to the adsorption of NO, gaseous NO can also directly react with NH2 through SCR reaction, thereby achieving good removal performance of both NH3 and NO. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51976060), the Science and Technology Research and Development Program of Baoding City of China (18FH04) and the Fundamental Research Funds for the Central Universities (2018MS118). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145153. References [1] No 31/2014 of the General Office of the State Council, Energy development strategic action plan (2014–2020). [2] No 2093/2014 of National Development and Reform Commission, Coal-fired energy-saving emission reduction upgrade and transformation action plan (2014–2020). [3] Z. Lei, C. Wen, B. Chen, Optimization of internals for selective catalytic reduction (SCR) for NO removal, Environ. Sci. Technol. 45 (2011) 3437–3444, https://doi. org/10.1021/es104156j. [4] Z. Zhu, Z. Liu, S. Liu, H. Niu, T. Hu, T. Liu, Y. Xie, NO reduction with NH3 over an activated carbon-supported copper oxide catalysts at low temperatures, Appl. Catal. B Environ. 26 (2000) 25–35, https://doi.org/10.1016/S0926-3373(99)00144-7. [5] S.M. Jeong, S.H. Jung, K.S. Yoo, S.D. Kim, Selective catalytic reduction of NO by NH3 over a bulk sulfated CuO/γ-Al2O3 catalyst, Ind. Eng. Chem. Res. 38 (1999) 2210–2215, https://doi.org/10.1021/ie9807147. [6] L. Wen, J. Chen, L. Yang, X. Wang, C. Xu, X. Sui, L. Yao, Y. Zhu, J. Zhang, T. Zhu, Enhanced formation of fine particulate nitrate at a rural site on the North China Plain in summer: the important roles of ammonia and ozone, Atmos. Environ. 101 (2015) 294–302, https://doi.org/10.1016/j.atmosenv.2014.11.037.
7
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
[47] M. Waqif, P. Bazin, O. Saur, J.C. Lavalley, G. Blanchard, O. Touret, Study of ceria sulfation, Appl. Catal. B Environ. 11 (1997) 193–205, https://doi.org/10.1016/ S0926-3373(96)00040-9. [48] T. Luo, R.J. Gorte, Characterization of SO2-poisoned ceria-zirconia mixed oxides, Appl. Catal. B Environ. 53 (2004) 77–85, https://doi.org/10.1016/j.apcatb.2004. 04.020. [49] G. Centi, N. Passarini, S. Perathoner, A. Riva, Combined deSOx/deNOx reactions on a copper on alumina sorbent-catalyst. I. Mechanism of SO2 oxidation–adsorption, Ind. Eng. Chem. Res. 31(1992) 1947–1955. https://doi.org/10.1021/ie00008a016. [50] L. Chen, J. Li, M. Ge, DRIFT study on cerium-tungsten/titania catalyst for selective catalytic reduction of NOx with NH3, Envrion. Sci. Technol. 44 (2010) 9590–9596, https://doi.org/10.1021/es102692b. [51] S. He, C. Zhang, M. Yang, Y. Zhang, W. Xu, N. Cao, H. He, Selective catalytic oxidation of ammonia from MAP decomposition, Sep. Purif. Technol. 58 (2007) 173–178, https://doi.org/10.1016/j.seppur.2007.07.015. [52] J.C.S. Wu, Y. Cheng, In situ FTIR study of photocatalytic NO reaction on photocatalysts under UV irradiation, J. Catal. 237 (2006) 393–404, https://doi.org/10. 1016/j.jcat.2005.11.023. [53] M. Kantcheva, Identification, stability, and reactivity of NOx species adsorbed on titania-supported manganese catalysts, J. Catal. 204 (2001) 479–494, https://doi. org/10.1006/jcat.2001.3413. [54] K. Hadjiivanov, V. Bushev, M. Kantcheva, D. Klissurski, Infrared spectroscopy study of the species arising during nitrogen dioxide adsorption on titania (anatase), Langmuir 10 (1994) 464–471, https://doi.org/10.1021/la00014a021. [55] G. Ramis, G. Busca, V. Lorenzelli, P. Forzatti, Fourier transform infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on TiO2 anatase, Appl. Catal. 64 (1990) 243–257, https://doi.org/10.1016/S0166-9834(00) 81564-X. [56] R.Q. Long, R.T. Yang, Selective catalytic oxidation of ammonia to nitrogen over Fe2O3-TiO2 prepared with a sol-gel method, J. Catal. 207 (2002) 158–165, https:// doi.org/10.1006/jcat.2002.3545. [57] J. Valyon, W.K. Hall, Effects of reduction and reoxidation on the infrared spectra from Cu-Y and Cu-ZSM-5 zeolites, J. Phys. Chem. 97 (1993) 7054–7060, https:// doi.org/10.1021/j100129a021. [58] F. Cao, J. Xiang, S. Su, P. Wang, S. Hu, L. Sun, Ag modified Mn-Ce/γ-Al2O3 catalyst for selective catalytic reduction of NO with NH3 at low-temperature, Fuel Process. Technol. 135 (2015) 66–72, https://doi.org/10.1016/j.fuproc.2014.10.021. [59] K. Hadjiivanov, Identification of neutral and charged NxOy surface species by IR spectroscopy, Catal. Rev. 42 (2000) 71–144, https://doi.org/10.1081/CR100100260. [60] B. Klingenberg, M.A. Vannice, NO adsorption and decomposition on La2O3 studied by DRIFTS, Appl. Catal. B Environ. 21 (1999) 19–33, https://doi.org/10.1016/ S0926-3373(99)00003-X.
[34] C. Chen, W. Jia, S. Liu, Y. Cao, The enhancement of CuO modified V2O5-WO3/TiO2 based SCR catalyst for Hg0 oxidation in simulated flue gas, Appl. Surf. Sci. 436 (2018) 1022–1029, https://doi.org/10.1016/j.apsusc.2017.12.123. [35] Y. Cao, H. Zhou, W. Jiang, C. Chen, W. Pan, Studies of the fate of sulfur trioxide in coal-fired utility boilers based on modified selected condensation methods, Environ. Sci. Technol. 44(2010) 3429–3434. https://doi.org/ 10.1021/es903661b. [36] N. Yan, W. Chen, J. Chen, Z. Qu, Y. Guo, S. Yang, J. Jia, Significance of RuO2 modified SCR catalyst for elemental mercury oxidation in coal-fired flue gas, Environ. Sci. Technol. 45 (2011) 5725–5730, https://doi.org/10.1021/es200223x. [37] L. Chmielarz, P. Kuśtrowski, A. Rafalska-łasocha, R. Dziembaj, Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides, Appl. Catal. B Environ. 58 (2005) 235–244, https://doi.org/10.1016/j.apcatb.2004.12. 009. [38] L. Dong, L. Zhang, C. Sun, W. Yu, J. Zhu, L. Liu, B. Liu, Y. Hu, F. Gao, L. Dong, Study of the properties of CuO/VOx/Ti0.5Sn0.5O2 catalysts and their activities in NO + CO reaction, ACS Catal. 1 (2011) 468–480. https://doi.org/10.1021/cs200045f. [39] B. Chen, R. Xu, R. Zhang, N. Liu, Economical way to synthesize SSZ-13 with abundant ion-exchanged Cu+ for an extraordinary performance in selective catalytic reduction (SCR) of NOx by ammonia, Environ. Sci. Technol. 48 (2014) 13909–13916, https://doi.org/10.1021/es503707c. [40] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, The catalytic properties of Cu modified attapulgite in NH3–SCO and NH3–SCR reactions, Appl. Surf. Sci. 480 (2019) 537–547, https://doi.org/10.1016/j.apsusc.2019.03.024. [41] L. Yu, Q. Zhong, S. Zhang, W. Zhao, R. Zhang, A CuO-V2O5/TiO2 catalyst for the selective catalytic reduction of NO with NH3, Combust. Sci. Technol. 187 (2015) 925–936, https://doi.org/10.1080/00102202.2014.993028. [42] G. Xie, Z. Liu, Z. Zhu, Q. Liu, J. Ge, Z. Huang, Simultaneous removal of SO2 and NOx from flue gas using a CuO/Al2O3 catalyst sorbent: I. Deactivation of SCR activity by SO2 at low temperatures, J. Catal. 224(2004) 36–41. https://doi.org/10. 1016/j.jcat.2004.02.015. [43] P.S. Lowell, K. Schwitzgebel, T.B. Parsons, K.J. Sladek, Selection of metal oxides for removing SO2 from flue gas, Ind. Eng. Chem. Proc. Des. Dev. 10 (1971) 384–390, https://doi.org/10.1021/i260039a018. [44] T. Curtin, F. O' Regan, C. Deconinck, N. Knüttle, B.K. Hodnett, The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts, Catal. Today 55(2000) 189–195. https://doi.org/10.1016/S0920-5861(99)00238-2. [45] D. Pietrogiacomi, D. Sannino, A. Magliano, P. Ciambelli, S. Tuti, V. Indovina, The catalytic activity of CuSO4/ZrO2 for the selective catalytic reduction of NOx with NH3 in the presence of excess O2, Appl. Catal. B Environ. 36 (2002) 217–230, https://doi.org/10.1016/S0926-3373(01)00310-1. [46] R. Zhang, H. Alamdari, S. Kaliaguine, SO2 poisoning of LaFe0.8Cu0.2O3 perovskite prepared by reactive grinding during NO reduction by C3H6, Appl. Catal. A Gen. 340(2008) 140–151. https://doi.org/10.1016/j.apcata.2008.02.028.
8
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
The effect of SO2 on NH3-SCO and SCR properties over Cu/SCR catalyst ⁎
⁎
T
Chuanmin Chen , Yue Cao, Songtao Liu , Wenbo Jia The Key Laboratory of Resources and Environmental Systems Optimization, Ministry of Education, Beijing 102206, China School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SCR catalyst Selective catalytic oxidation (SCO) of NH3 Slip ammonia SO2 resistance
The sulfur resistance in NH3-SCO and NH3-SCR reactions of 1%Cu/SCR catalyst prepared by improved wet impregnation method was tested at 350 °C. In the presence of 500 ppm SO2, 1%Cu/SCR maintained good SCR performance, but its NH3 oxidation efficiency was visibly decreased. The NH3 conversion was promoted to about 90% after adding NO since SCR reactions occurred. As observed in ionic chromatography spectra, no more SO3 were generated over the Cu doped catalyst. NH3-TPD, H2-TPR results showed that some sulfite and sulfate species on the catalyst surface significantly increased the amount of weak acid sites and decreased the reducibility of copper species. In addition, the influence mechanism of SO2 on NH3 and NO adsorption and conversion over 1%Cu/SCR catalyst were revealed by using in situ DRIFT methods. The suppression of NH2 → NOx conversion could account for the SO2 deactivation in NH3 oxidation. Gaseous NO can directly react with NH2 through iSCR mechanism.
1. Introduction In June 2014, the General Office of the State Council of the People’s Republic of China firstly put forward that the emission of air pollutants from new coal-fired generating units should be close to that of gas-fired units, thus opening the prelude to the ultra-low emission of coal-fired power plants in China [1]. Nitrogen oxide (NOx) is one of the major atmospheric pollutants emitted from coal-fired power plants. According to the “Coal Energy Saving and Emission Reduction and Action Plan (2014–2020)”, the NOx emitted from them should be less than 50 mg/N m3 [2], which poses new challenge for flue gas denitration equipment. Up to date, NH3-selective catalytic reduction (SCR) is still the most commonly used flue gas denitration technology for coal-fired power plants [3,4]. In this method, NH3 is added as the reducing agent to selectively reduce NOx to N2, and V2O5-WO3/TiO2 is usually used as the commercial SCR catalyst with a deNOx efficiency of over 90% [5]. In theory, adding more NH3 would lead to higher NOx removal efficiency. However, excessive pursuit of NOx emission reduction is likely to cause the problem of ammonia slip [6]. As a typical alkaline gas, NH3 would easily react with acidic flue gas components under specific conditions to form a series of ammonium sulfate and ammonium nitrate, which can increase the emission of PM2.5 from coal-fired power plants and do harm to human health [7]. What’s more, the slip ammonia will easily interact with SO3 and H2O in the flue gas to generate the ammonium
bisulfate (ABS) with high viscosity, resulting in catalyst deactivation, blockage and corrosion of the channel, and decrease of conversion efficiency of downstream air preheater when the temperature is lower than 300 °C [8]. All these would seriously affect the stable operation of flue gas system. Therefore, the slip ammonia is strictly limited in China, the United States and the European countries, the threshold limit of which was set as 2–10 ppm [9–11]. Considering these problems, the NH3/NO ratio of 0.9–0.95 is usually used to avoid the generation of ammonia slip [12]. But the NOx removal performance may be difficult to reach the requirement in “ultra-low emissions standards”. And even in this case, a part of NH3 may still not be consumed in SCR reactions when operating conditions change, such as the stratification of the gas stream, the improper distribution of ammonia or the reduced catalytic efficiency caused by long-time used SCR catalysts. At present, selective catalytic oxidation of NH3 (NH3-SCO) is considered as one of the promising methods for removing NH3 from the exhaust. NH3 could react with O2 over suitable catalysts to form harmless N2 and H2O [13,14]. Some noble metals [15,16], transition metal oxides [12,17], mixed rare earth and transition metal oxides [1820] and noble metals/transition metal oxides modified zeolites [21,22] possess NH3 oxidation ability under specific reaction conditions in varying degrees. Considering NH3 conversion, N2 selectivity and the cost, copper containing materials may be one of the most promising NH3-SCO catalysts [12]. Under the respective suitable reaction
⁎ Corresponding authors at: The Key Laboratory of Resources and Environmental Systems Optimization, Ministry of Education, Beijing 102206, China. School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China. E-mail addresses: [email protected] (C. Chen), [email protected] (S. Liu).
https://doi.org/10.1016/j.apsusc.2019.145153 Received 5 September 2019; Received in revised form 18 December 2019; Accepted 21 December 2019 Available online 23 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
2.2. Catalytic activity measurement
conditions, 10wt.%Cu/TiO2 [17], 10wt.%Cu/Al2O3 [23], 4.4wt.%Cu/ ZSM-5 [21], 3wt.%Cu/Beta [24], 3.7wt.%Cu/Y treated with NaOH [25], etc. could all reach excellent NH3 oxidation efficiency and N2 selectivity of over 90%. The 1%Cu/SCR catalyst, which was made by adding only 1 wt% copper species to the commercial SCR catalyst, has also been proved to possess good NH3-SCO performance in our previous research [26]. Even at a high GHSV of 3 × 105 h−1, it oxidized 89% of NH3 with a N2 selectivity of 86% in the presence of 4%O2 at 350 °C. When the NH3/NO ratio was 1:1, its NH3 and NOx removal efficiency came to 92% and 93%, respectively, and the N2 selectivity reached 91%. This suggests that it may simultaneously remove residual NO and NH3 from the flue gas. However, since the SCR equipment is usually installed before desulfurization system, the flue gas entering SCR unit may contain a large amount of SO2. It has been proved to have various effects on the catalytic performance of different catalyst systems [27], such as reacting with the reactants [28], competing with the reactants for the active site over the catalyst surface [29,30] or even reacting with the active component to form sulfates [31]. In addition, due to good oxidation properties, some catalysts may also catalyze the SO2 oxidation to form SO3 as a side reaction. This would promote the formation of ABS to block the catalyst pores below 280 °C [32,33]. All these may cause a deactivation of the catalysts. Therefore, it is of great importance to reveal the effect of SO2 on the catalytic performance in the development of novel NH3-SCO or SCR catalysts. Until now, the influence mechanism of SO2 on the catalytic properties of 1%Cu/SCR catalyst has not been studied yet. And the SO3 formation performance over the catalyst is still unclear. Since the catalyst is expected to be used at a typical SCR temperature, the effect shown at around 350 °C of SO2 on the catalytic activity may be mainly caused by changing the active components of the catalyst or affecting the adsorption and conversion of the reactants. But the amount of SO3 production is still an important parameter to be investigated to avoid its adverse effects on the catalyst and downstream flue gas equipment when the temperature is lowered in practical applications. Therefore, in this study, the sulfur resistance in NH3-SCO and SCR reactions and the SO2 oxidation performance of 1%Cu/SCR were further investigated. NH3 temperature programmed desorption (NH3TPD), H2 temperature program reduction (H2-TPR) were adopted to reveal the effect of SO2 on acid sites and reducibility of the catalyst. And in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) methods were used to explore the influence mechanism of SO2 on NH3 and NO adsorption and conversion over 1%Cu/SCR catalyst. This study would provide an important reference for the control of residual NOx and slip ammonia in sulfur-containing flue gas from coalfired power plants.
The catalytic activity of 1%Cu/SCR catalyst was investigated on a vertical fixed bed reaction system. 0.3 g catalyst was placed in the quartz reactor with 10 mm i.d. and was then heated to 350 °C in each test. The flue gases containing 30 ppm NH3, 30 ppm NO (if used), 500 ppm SO2 (if used) and 4%O2 with N2 as the balance were adjusted by corresponding mass flow controllers to maintain a total flow rate of around 1.5 L/min. A DX4000 Fourier transform infrared spectrometer (Gasmet, Finland) was used for online detecting the concentrations of NH3, N2O, NO, and NO2 in inlet and outlet flue gases. The removal efficiencies of NH3 and NOx as well as the N2 selectivities were calculated by using Eqs. (1)–(4) [26]. The N2 selectivity with superscript a and b respectively represent the proportion of N2 in nitrogen-containing products in the absence and presence of NO in the feed gas.
NH3 removal efficiency =
[NH3]inlet − [NH3]outlet × 100% [NH3]inlet
[NO]outlet + [NO2 ]outlet ⎤ × 100% NOx removal efficiency = ⎡1 − ⎢ ⎥ [NO]inlet ⎣ ⎦
(1)
(2)
[NO]outlet + [NO2 ]outlet + 2[N2 O]outlet ⎤ × 100% N2 selectivity a = ⎡1 − ⎥ ⎢ [NH3 ]inlet − [NH3 ]outlet ⎦ ⎣ (3)
N2 selectivity b [NO2 ]outlet + 2[N2 O]outlet ⎤ × 100% = ⎡1 − ⎢ [NH3 ]inlet + [NO]inlet − [NH3 ]outlet − [NO]outlet ⎥ ⎦ ⎣
(4)
Besides, the SO2 oxidation efficiency of 1%Cu/SCR catalysts was also investigated. The inlet flue gas contains 2000 ppm SO2, 4%O2 and N2. According to EPA method 8 [35,36], the SO3 at the outlet of the flue gas was absorbed by an 80% isopropyl alcohol (IPA) aqueous solution for 3.5 h, and the SO42- content in the solution was measured by a 792 Basic ionic chromatography (Metrohm AG,) equipped with a Metrosep A Supp 4 Anion chromatographic column. The SO3 concentration in the gas across SCR catalyst and the blank reactor was also tested for comparison. 2.3. NH3-TPD The NH3-TPD curves were acquired using a PCA-1200 chemisorption analyzer (Biaode, China). Before each test, 0.2 g catalyst (20–40 mesh) was heated in pure He atmosphere to 350 °C and kept for 30 min to remove the adsorbed substances. The sample was then cooled down to 100 °C, kept at this temperature to adsorb pure NH3 (30 mL/min) for 15 min, and then swept by He stream for 60 min to remove physically adsorbed NH3 on catalyst surface. NH3 desorption was performed from 50 °C to 600 °C with a temperature ramp of 10 °C/min in 30 mL/min He flow. The signal of desorbed NH3 was recorded online using a thermal conductivity detector (TCD).
2. Experimental 2.1. Catalysts preparation
2.4. H2-TPR
The 1%Cu/SCR catalyst used in this study was prepared by an improved impregnation method [34]. First, the commercial SCR catalyst (contains 0.7%V2O5, 4.8%WO3, TiO2 supported) was ground into powder. Cu(NO3)2⋅3H2O was dissolved in deionized water, and a certain amount of SCR catalyst was then added into the solution to provide a mass ratio of 1:99 for Cu and SCR catalyst. After magnetically stirring for 2 h at 60 °C, the solution was dried at 105 °C overnight to remove the water. The obtained solid was then transferred to the muffle furnace and underwent calcination at 400 °C in air for 4 h. At last, the prepared catalysts were ground to 20–40 mesh for NH3 oxidation activity test, H2-TPR and NH3-TPD experiment, and the powder catalysts were used for in situ DRIFT studies.
In H2 temperature program reduction (H2-TPR) experiment, the test instrument, catalyst dosage and pretreatment method were the same as that in NH3-TPD test. After purged in He at 350 °C, the catalyst was cooled down to room temperature and 10% H2/Ar reducing gas was introduced at a flow rate of 30 mL/min. After the baseline was stabilized, the temperature was raised to 800 °C at a ramping rate of 10 °C/ min, and the H2 signal was recorded. 2.5. In situ DRIFT studies The in situ DRIFT spectroscopy was performed using a diffuse reflectance accessory connected to a Nicolet iZ10 DRIFTS spectrometer 2
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
gas may be simultaneously oxidized to the more harmful SO3 over the catalysts. In order to prevent the damage of excessive SO3 to the flue gas system and the environment, the SO2 oxidation performance of 1%Cu/ SCR was investigated. Fig. S1 showed the ionic chromatography (IC) spectra of 80% IPA aqueous solution collected under different conditions. The peak appearing at about 13.18 min was identified as the peak of sulfate. It can be been that no significant difference appeared in the peak area for the three sets of experiments with no catalyst (blank), SCR and 1%Cu/SCR catalyst placed in the reactor. Among them, the SO42existed in the blank group may be derived from the oxidation of a small amount of SO32- by O2 in the IPA solution in the sampling process [36]. Similar amount of SO3 was also generated for the two catalysts, indicating that the SO2 oxidation performance of them was negligible, and the addition of 1%Cu did not significantly affect the SO2 oxidation of SCR catalyst. Accordingly, no excessive SO3 will be generated when a small amount of 1%Cu/SCR catalyst is placed in the tail of the SCR system to catalytic oxidize the slip ammonia from coal-fired power plants.
Fig. 1. NH3 and NO removal efficiency and N2 selectivity of 1%Cu/SCR catalyst at 350 °C.
(Thermo, USA), which was equipped with a mercury cadmium telluride (MCT) detector. The spectra were recorded at a spectral resolution of 4 cm−1 with 32 scans. Before all experiments, the powder catalysts were flushed with N2 at 400 °C for 1 h, then cooled down to 350 °C and exposed to different treatment conditions (described in the following sections) for the test. If used, SO2, NH3 and NO in the treatment gas was all applied at the concentration of 500 ppm, and O2 was set as 4%. Besides, N2 was used as the balance gas to provide a total flow rate of 50 mL/min, which was 1/30 of that in activity test, to explore the reaction process in detail.
3.3. H2-TPR results Previous studies showed that SO2 is likely to influence the redox properties of catalysts [12]. Therefore, the H2-TPR profiles of fresh 1%Cu/SCR catalyst and the catalyst after treated by 500 ppm SO2 and 4%O2 for 100 min were compared in Fig. 2. For the fresh one, the reduction peaks of H2 presented in a wide temperature range. The overlapping peaks at 253 °C and 265 °C and the one at 468 °C can be assigned to the reduction of copper species [38–40] and vanadium species dispersed on the catalyst surface, respectively. The peaks appeared between 339 and 422 °C may be Cu–O–V [38] or Cu–O–Ti species [41]. And the two at 578 and 636 °C represented tungsten and titanium species [26]. After sulfurization, the reduction peaks of copper species appeared at 346 and 363 °C, which were moved to high-temperature region. The change could be attributed to the transformation of CuO to CuSO4 [42]. However, the reduction temperature of other species was basically unchanged. This indicated that the sulfated treatment would visibly reduce the reducibility of copper species, but has little effect on that of other species. According to the research of Neumann, after treated with SO2 and air, V2O5 would probably not convert into sulfate, a small amount of TiO2 may transform into TiOSO4, while much CuO tend to form CuSO4 when the temperature was below 650 °C [43]. Their result also verifies our inference from this test. Meanwhile, the H2 consumption slightly increased after sulfurization, suggesting the amount of lattice oxygen reacted with H2 was probably increased by some formed sulfur species on catalyst surface. These phenomena were basically in line with the influence of SO2 on reducibility of Cu/Al2O3
3. Results and discussion 3.1. Catalytic activity Ammonia oxidation and NOx removal properties of 1%Cu/SCR catalyst were investigated in the presence and absence of SO2 at 350 °C. As shown in Fig. 1, the catalyst exhibited a good NH3-SCO performance. At the high gas hourly space velocity (GHSV) of 3 × 105 h−1, the NH3 oxidation efficiency and N2 selectivity reached about 88% and 86%, respectively. However, only 63% of NH3 was removed after 500 ppm SO2 was introduced into the flue gas, indicating that SO2 would visibly inhibit the catalytic oxidation of NH3. When NO was added to provide an NH3/NO ratio of 1:1, the experimental phenomenon was somewhat different. NH3 removal efficiency and N2 selectivity increased slightly to 93% and 91%, respectively. This suggested that NO would react with NH3 and thus promote the selective conversion of NH3. Meanwhile, the deNOx efficiency of 1%Cu/SCR catalyst reached 92%. More importantly, although SO2 was added, the removal efficiency of NH3 and NO was still maintained at 89% and 92%, respectively. It showed that the catalyst possesses good sulfur resistance in the presence of NO and could remove NH3 and NO simultaneously. In addition, the selectivity to N2 was slightly increased by the addition of SO2 regardless of whether the flue gas contained NO or not, which was 95% and 90%, respectively. This positive effect, probably resulting from the generated surface sulphates, was also found for other copper-containing catalysts [12]. The oxide anions in CuO lattice were reported to be more stable for these sulfated catalysts, which was not conducive to the formation of nitrogen oxides [37]. Since the added SO2 inhibited the NH3 conversion to significantly different extend in the presence and absence of NO, the reaction mechanism involving SO2 must be clarified to rationally reveal the NH3 and NO removing process over 1%Cu/SCR catalyst in coal-fired flue gas. 3.2. SO2 oxidation performance
Fig. 2. H2-TPR curves of sulfated and fresh 1%Cu/SCR catalyst.
As the NH3-SCO or SCR reaction occurs, SO2 in the coal-fired flue 3
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 4. In situ DRIFT spectra of NH3 adsorption on fresh and sulfated 1%Cu/ SCR catalyst at 350 °C.
Fig. 3. NH3-TPD curves of sulfated and fresh 1%Cu/SCR catalyst.
catalyst [44].
over the catalyst.
3.4. NH3-TPD results
3.6. DRIFT of NH3 adsorption
The NH3-TPD method was used to examine the influence of SO2 on the acid sites of 1%Cu/SCR catalyst. It can be seen that the sulfurization dramatically changed the NH3 desorption curve of the catalyst (Fig. 3). Before treated with SO2, a serious of peaks with similar intensities were evenly distributed in the range of 100–450 °C for the catalyst. Among them, the peaks appearing below 350 °C were identified as the weakly adsorbed NH3 species on Lewis and Brønsted acid sites, while the other two at about 358 and 405 °C were the adsorbed NH3 on Lewis acid sites or strong Brønsted acid sites [26]. In contrast, the peaks below 200 °C were weakened for the sulfated catalyst. One remarkable reduction peak was seen at 202 °C, which indicated the presence of abundant weak acid sites. Two small peaks of strong acid sites were also noted at 358 and 419 °C, of which the latter one showed a slight increase compared with the fresh catalyst. In general, the introduction of SO2 improved the total amount of acid sites of the catalyst, especially the weak acid sites. Therefore, the sulfated catalyst exhibited a more superior NH3 adsorption capacity at the reaction temperature (350 °C).
Ammonia adsorption is usually regarded as an important step in both NH3-SCR and NH3-SCO reactions. As shown in Fig. 4, after treating by 500 ppm NH3 for 30 min, several bands were observed at 1697–1700, 1606–1610, 1419–1435, 1325–1327 and 1226–1255 cm−1 for both the fresh and sulfated 1%Cu/SCR catalyst. The bands at 1697–1700 and 1419–1435 cm−1 could correspond to the NH4+ species coordinated on Brønsted acid sites [16,50], and the two at 1606–1610 and 1226–1255 cm−1 should be assigned to the NH3 strongly adsorbed on Lewis acid sites [16,50]. Besides, the band appeared at 1325–1327 cm−1 can be attributed to the wagging deformation modes of amide species (−NH2), which is commonly regarded as the active intermediate species in NH3-SCR and NH3-SCO reactions [16,51]. For the fresh catalyst, the bands at 1435 and 1697 cm−1 were very weak, suggesting the NH3 species mainly adsorbed on Lewis acid sites. Compared with the spectrum of fresh catalyst, the band intensities for sulfated 1%Cu/SCR were significantly higher, especially the two at 1419 and 1255 cm−1. This indicated that the sulfurization increased both the Brønsted and Lewis acid sites on the catalyst surface and promoted the adsorption of NH3 species on them.
3.5. DRIFT of SOx adsorption As stated above, the effect of SO2 on NH3 removal performance over 1%Cu/SCR catalyst was very different when NO was present or not. In order to reveal the influence mechanism of SO2 on NH3 conversion, the interaction of the reactants with fresh and sulfated catalyst needs to be separately elucidated. Before this, the generated sulfate species over sulfated 1%Cu/SCR catalyst was firstly analyzed by DRIFT method. Fig. S2 displayed the spectrum of the catalyst treated in a flow of 500 ppm SO2 + 4%O2 at 350 °C for 30 min. It can be seen that several bands at 1418, 1362, 1300, 1190, 1034 and 839 cm−1 as well as a negative bands at 3659 cm−1 appeared. According to related literature, the negative bands belonged to the vibration of the consumed surface hydroxyl groups which interacted with SO2, indicating the generation of hydrogen-bonded sulfite species [28,45]. The weak band observed at 1418 cm−1 was attributed to a small amount of SO3 like species [27,46,47]. The strong bands at 1362 and 1300 cm−1 in spectrum belonged to the surface sulfate species bonded on different sites [28,48], while the one at 1190 cm−1 was ascribed to the bulk sulfate species [16,47], which was most likely to be CuSO4 according to the previous study [49] and our H2-TPR result. In addition, the presence of the band at 1034 and 839 cm−1 confirmed the formation of sulfite species [46,47]. The presence of these sulfur species may affect the adsorption and conversion of reactants in NH3-SCO and SCR reactions
3.7. Interaction of O2 with in situ formed NH3 The 1%Cu/SCR catalyst exhibited good activity in promoting the reaction between NH3 and O2. In order to reveal the NH3 oxidation mechanism, the gas containing 4% O2 was passed through the NH3 pretreated catalysts and the spectra were recorded. The gas flow was set as 20 mL/min, which was much slower than the 1 L/min in catalytic activity tests, to make it easier for observing the reaction process. As shown in Fig. 5(a), the spectra of fresh 1%Cu/SCR catalyst changed significantly with the introduction of O2. All the bands weakened slowly and vanished in 40 min, which suggested that all the NH3, NH4+ and NH2 species were consumed by O2. Two new bands appeared at 1363 and 1288 cm−1, indicating the generation of nitro compounds (−NO2) and monodentate nitrate (NO3) species on the catalyst surface [52-55]. However, no NO and NO2 were detected as the product of NH3 oxidation at 350 °C in our experiment. A possible explanation was that the −NO2 and NO3 species existed as the adsorbed intermediates in NH3 oxidation. Previous researches have proposed an internal SCR (iSCR) mechanism to describe a possible NH3 oxidation process in which NH3 tend to react with O2 to form NOx firstly, and the generated NOx could rapidly react with remaining NH3 to form harmless N2 and 4
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 5. In situ DRIFT spectra of the NH3 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 4% O2 at 350 °C.
H2O [12,56]. The appeared −NO2 and NO3 species in these spectra could be ascribed to the absorption species of generated NOx by NH3 oxidation. Since the NH3 species had already been oxidized, the NOx could not be removed without reductant and thus accumulated on the catalyst surface. Although more NH3 and NH4+ species were adsorbed on the surface of sulfated 1%Cu/SCR catalyst, no more NH3 was converted by it (Fig. 5(b)). Conversely, the NH3 oxidation efficiency was significantly reduced. The band at 1327 cm−1 was still clearly observed in the spectra, indicating that the sulfurization did not significantly inhibit the formation of intermediate NH2. When 4% O2 was introduced, the bands on sulfated catalyst only showed a slight decrease, and no bands of NOx species appeared even after 60 min. This suggested that only a small amount of NH3 and −NH2 species over sulfated catalyst reacted with O2, which was consistent with the inhibition of SO2 to NH3 oxidation in experimental results.
Fig. 6. In situ DRIFT spectra of the NH3 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 4% O2 + 500 ppm NO at 350 °C.
subsequently appeared at 1340 cm−1 and 1287 cm−1, respectively [52–55], and the spectra remained unchanged in 60 min. These species may be attributed to the product of NH3 transformation or NO adsorption. In sharp contrast to the introduction of only 4%O2, promoted by NO, the bands for sulfated catalyst were almost completely consumed in 13 min despite the large amount of adsorbed NH3 species, and then −NO2 and NO3 species generated. This result showed that NO played an important role in NH3 conversion over the sulfated catalyst. Although the adsorbed sulfur species significantly inhibited the reaction between NH3 and O2 over 1%Cu/SCR catalyst, the introduction of NO could significantly weaken this inhibitory effect to maintain good NH3 removal performance. However, compared with the fresh catalyst, the existing surface sulfur species still slightly inhibited the removal rate of NH3 species even in the presence of NO.
3.8. Interaction of NO and O2 with in situ formed NH3 After 500 ppm NO and 4% O2 were simultaneously added as the treatment gas, the adsorbed NH3 species were consumed rapidly for both the fresh and sulfated 1%Cu/SCR catalyst (Fig. 6). It can be observed that the original bands for the fresh catalyst vanished in 5 min, which was much quicker than that when only O2 was added, and thus verified the promoting effect of NO on NH3 conversion. Similarly, the bands of nitro compounds (−NO2) and monodentate nitrate (NO3)
3.9. DRIFT of NOx adsorption In order to reveal the reaction occurring over the fresh and sulfated catalysts in the presence of NO, the adsorption of NOx on the catalyst surface and the interaction between NH3 and adsorbed NOx species 5
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
Fig. 7. In situ DRIFT spectra of NO + O2 adsorption on fresh and sulfated 1%Cu/SCR catalyst.
were also investigated. As shown in Fig. 7, after treating with 500 ppm NO + 4%O2 for 30 min and followed by N2 purging, several bands appeared for the fresh and sulfated 1%Cu/SCR catalysts, including the bands at 1697–1699, 1522–1541, 1362–1375, 1288–1298 and 1163–1167 cm−1. According to previous studies, the bands at 1522–1541 and 1288–1298 cm−1 belong to the monodentate nitrate (NO3) species [52–55]. The bands at 1362–1375 cm−1 can be attributed to the adsorbed nitro compounds (–NO2) on the catalyst surface [53,57,58]. And the weak one appeared at 1163–1167 cm−1 could be assigned to the nitrosyl anion (NO–) species [59,60]. Additionally, the bands appearing at 1697–1699 cm−1 can be ascribed to the contaminants on the TiO2 surface [52]. It can be clearly seen that the bands of sulfated 1%Cu/SCR catalyst were much weaker than the fresh one, indicating that the sulfurization significantly inhibited the adsorption of NO on the catalyst surface. 3.10. Interaction of NH3 with in situ formed NO Fig. 8(a) displayed the spectra of interaction between NH3 and the NO formed on the fresh 1%Cu/SCR catalyst at 350 °C. After adding NH3, the bands for NOx species weakened gradually, suggesting that the NOx on the catalyst surface reacted with NH3. When the NOx adsorbed consumed completely, NH3 species began to accumulate on the catalyst surface. New bands at 1616, 1318 and 1230 cm−1 appeared in the 10th min. All of them became stronger as the time increased, and basically kept unchanged after 20 min. The two bands at 1616 and 1230 cm−1 represented the NH3 adsorbed on Lewis acid sites and the one at 1318 cm−1 demonstrated the presence of –NH2 species. This also proved that NH3 species were more inclined to adsorb on Lewis acid sites on fresh 1%Cu/SCR catalyst. As depicted in Fig. 8(b), the NO adsorption amount was significantly lower for sulfated 1%Cu/SCR catalyst, and the bands corresponding to NOx disappeared more rapidly after passing through NH3. In the 5th min, new vibrational bands were found at 1424, 1323 and 1275 cm−1. And the band at 1613 appeared in the 10th min. The NH3 species on Brønsted and Lewis acid sites, together with the NH2 species were all observed. These bands increased rapidly over time, which was much stronger than the corresponding ones for the fresh catalyst. This result was consistent with the spectrum of NH3 adsorption in Section 3.4, which confirmed that the sulfated catalyst possessed stronger NH3 adsorption capacity.
Fig. 8. In situ DRIFT spectra of the NO + O2 pretreated (a) 1%Cu/SCR and (b) sulfated 1%Cu/SCR catalyst, followed by treated with 500 ppm NH3 at 350 °C.
Fig. 9. Schematic diagram of NH3-SCO and NH3-SCR reactions over 1%Cu/SCR catalyst.
follows the iSCR mechanism (Fig. 9). NH3 would adsorb on the catalyst surface and be activated to form NH2*, which is then oxidized to NO2* and NO3 when O2 is introduced. These NOx species could rapidly react with NH3 to form N2 and H2O. The sulfurization would increase the amount of adsorbed NH3. However, it may also inhibit the conversion of NH2 to NOx. Without the formation of NOx, the adsorbed NH3 cannot be consumed by iSCR mechanism, which may be the reason for the poor NH3 oxidation performance exhibited by the sulfated catalyst. When NO is introduced, gaseous NO or adsorbed NOx can directly
3.11. The reaction mechanism The DRIFT results found that the NH3-SCO over 1%Cu/SCR catalyst 6
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
react with NH2* through the SCR reaction, which is also an important step in iSCR reaction. That is to say, NH3-SCO and NH3-SCR reactions would occur simultaneously over the fresh catalyst surface, and thus the addition of NO accelerates the consumption of NH3. Since the NH2 → NOx process is inhibited, almost only the NH3-SCR reaction could carry out over the sulfated catalyst. Therefore, the NH3 removal rate is slightly lower than the fresh catalyst. It is worth noting that SO2 significantly would reduce the amount of NO adsorbed on the catalyst surface. But the catalyst still maintains a good deNOx performance, indicating that for the sulfated catalyst, the NO mainly participates in the reaction in a gaseous form.
[7] K. Na, C. Song, C. Switzer, D. Cocker, Effect of ammonia on secondary organic aerosol formation from α-pinene ozonolysis in dry and humid conditions, Environ. Sci. Technol. 41(2007) 6096–6102. https://doi.org/ 10.1021/es061956y. [8] G. Qi, R.T. Yang, Low-temperature selective catalytic reduction of NO with NH3 over iron and manganese oxides supported on titania, Appl. Catal. B Environ. 44 (2003) 217–225, https://doi.org/10.1016/S0926-3373(03)00100-0. [9] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, Review on the latest developments in modified vanadium-titanium-based SCR catalysts, Chinese J. Catal. 39 (2018) 1347–1365, https://doi.org/10.1016/S1872-2067(18)63090-6. [10] Regulation (EC) no. 595/2009 of the European Parliament and of the Council of 18 June 2009. [11] Air Pollution Control Technology Fact Sheet Online, http://www.epa.gov/ttncatc1/ dir1/fscr.pdf (accessed January 2015). [12] M. Jabłońska, R. Palkovits, Copper based catalysts for the selective ammonia oxidation into nitrogen and water vapour—recent trends and open challenges, Appl. Catal. B Environ. 181 (2016) 332–351, https://doi.org/10.1016/j.apcatb.2015.07. 017. [13] X. Tang, J. Li, H. Yi, Q. Yu, F. Gao, R. Zhang, C. Li, C. Chu, An efficient two-step method for NH3 removal at low temperature using CoOx-CuOx/TiO2 as SCO catalyst followed by NiMn2O4 as SCR catalyst, Energ. Fuel. 31 (2017) 8580–8593, https:// doi.org/10.1021/acs.energyfuels.7b01329. [14] J. Leppälahti, T. Koljonen, M. Hupa, P. Kilpinen, Selective catalytic oxidation of NH3 in gasification gas. 2. Oxidation on aluminum oxides and aluminum silicates, Energ. Fuel. 11 (1997) 39–45. https://doi.org/10.1021/ef9502665. [15] W. Chen, Q. Zan, W. Huang, X. Hu, N. Yan, Novel effect of SO2 on selective catalytic oxidation of slip ammonia from coal-fired flue gas over IrO2 modified Ce-Zr solid solution and the mechanism investigation, Fuel 166 (2016) 179–187, https://doi. org/10.1016/j.fuel.2015.10.116. [16] W. Chen, Y. Ma, Z. Qu, Q. Liu, W. Huang, X. Hu, N. Yan, Mechanism of the selective catalytic oxidation of slip ammonia over Ru-modified Ce-Zr complexes determined by in-situ DRIFT, Environ. Sci. Technol. 48 (2014) 12199–12205, https://doi.org/ 10.1021/es502369f. [17] M. Jabłońska, Selective catalytic oxidation of ammonia into nitrogen and water vapour over transition metals modified Al2O3, TiO2 and ZrO2, Chem. Pap. 69 (2015) 1141–1155, https://doi.org/10.1515/chempap-2015-0120. [18] C. Hung, Selective catalytic oxidation of ammonia to nitrogen on CuO-CeO2 bimetallic oxide catalysts, Aerosol Air Qual. Res. 6 (2006) 150–169, https://doi.org/ 10.4209/aaqr.2006.06.0004. [19] J. Lou, C. Hung, S. Yang, Selective catalytic oxidation of ammonia over coppercerium composite catalyst, J. Air Waste Manage. Assoc. 54 (2004) 68–76, https:// doi.org/10.1080/10473289.2004.10470881. [20] M.L. Sang, S.C. Hong, Promotional effect of vanadium on the selective catalytic oxidation of NH3 to N2 over Ce/V/TiO2 catalyst, Appl. Catal. B Environ. 163 (2015) 30–39, https://doi.org/10.1016/j.apcatb.2014.07.043. [21] R.Q. Long, R.T. Yang, Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen, Chem. Commun. 17 (2000) 1651–1652, https:// doi.org/10.1039/b004957n. [22] T. Curtin, S. Lenihan, Copper exchanged beta zeolites for the catalytic oxidation of ammonia, Chem. Commun. 9 (2003) 1280–1281, https://doi.org/10.1039/ b301894f. [23] C. Liang, X. Li, Z. Qu, M. Tade, S. Liu, The role of copper species on Cu/γ-Al2O3 catalysts for NH3–SCO reaction, Appl. Surf. Sci. 258 (2012) 3738–3743, https://doi. org/10.1016/j.apsusc.2011.12.017. [24] S. Lenihan, T. Curtin, The selective oxidation of ammonia using copper-based catalysts: The effects of water, Catal. Today 145 (2009) 85–89, https://doi.org/10. 1016/j.cattod.2008.06.017. [25] L. Gang, B.G. Anderson, J. van Grondelle, R.A. van Santen, NH3 oxidation to nitrogen and water at low temperatures using supported transition metal catalysts, Catal. Today 61 (2000) 179–185, https://doi.org/10.1016/S0920-5861(00) 00375-8. [26] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, SCR catalyst doped with copper for synergistic removal of slip ammonia and elemental mercury, Fuel Process. Technol. 181 (2018) 268–278, https://doi.org/10.1016/j.fuproc.2018.09.025. [27] B.Q. Jiang, Z.B. Wu, Y. Liu, S.C. Lee, W.K. Ho, DRIFT study of the SO2 effect on lowtemperature SCR reaction over Fe−Mn/TiO2, J. Phys. Chem. C. 114 (2010) 4961–4965, https://doi.org/10.1021/jp907783g. [28] W. Xu, H. He, Y. Yu, Deactivation of a Ce/TiO2 catalyst by SO2 in the selective catalytic reduction of NO by NH3, J. Phys. Chem. C. 113 (2009) 4426–4432, https://doi.org/10.1021/jp8088148. [29] F. Liu, H. He, C. Zhang, Novel iron titanate catalyst for the selective catalytic reduction of NO with NH3 in the medium temperature range, Chem. Commun. 164 (2008) 2043–2045, https://doi.org/10.1039/b800143j. [30] W. Xu, Y. Yu, C. Zhang, H. He, Selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst, Catal. Commun. 9 (2008) 1453–1457, https://doi.org/10.1016/j. catcom.2007.12.012. [31] W. Sjoerd Kijlstra, M. Biervliet, E. K. Poels, A. Bliek, Deactivation by SO2 of MnOx/ Al2O3 catalysts used for the selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal. B Environ. 16 (1998) 327–337. https://doi.org/10. 1016/S0926-3373(97)00089-1. [32] G. Ramis, L. Yi, G. Busca, M. Turco, E. Kotur, R.J. Willey, Adsorption, activation, and oxidation of ammonia over SCR catalysts, J. Catal. 157 (1995) 523–535, https://doi.org/10.1006/jcat.1995.1316. [33] S.T. Choo, S.D. Yim, I. Nam, S. Ham, J. Lee, Effect of promoters including WO3 and BaO on the activity and durability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3, Appl. Catal. B Environ. 44 (2003) 237–252, https://doi.org/10.1016/ S0926-3373(03)00073-0.
4. Conclusions The 1%Cu/SCR catalyst possessed good NH3-SCO and NH3-SCR properties. It exhibited good sulfur resistance in SCR reaction, while the oxidation of NH3 was somewhat inhibited by the added SO2. Fortunately, a small amount of NO existed in the flue gas increased the NH3 conversion, which maintained about 90% even in the presence of 500 ppm SO2. Compared with the SCR catalyst, the added 1% copper species did not significantly improve the formation of SO3. SO2 would form some sulfite and sulfate species on the catalyst surface, which promoted the adsorption of NH3. The sulfated treatment slightly increased the H2 consumption and introduced abundant weak acid sites, but reduced the reducibility of copper species. The suppression of NH2 → NOx process may be the main reason why the NH3 oxidation over 1% Cu/SCR catalyst was inhibited in the presence of SO2. Although the presence of SO2 was not conducive to the adsorption of NO, gaseous NO can also directly react with NH2 through SCR reaction, thereby achieving good removal performance of both NH3 and NO. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51976060), the Science and Technology Research and Development Program of Baoding City of China (18FH04) and the Fundamental Research Funds for the Central Universities (2018MS118). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145153. References [1] No 31/2014 of the General Office of the State Council, Energy development strategic action plan (2014–2020). [2] No 2093/2014 of National Development and Reform Commission, Coal-fired energy-saving emission reduction upgrade and transformation action plan (2014–2020). [3] Z. Lei, C. Wen, B. Chen, Optimization of internals for selective catalytic reduction (SCR) for NO removal, Environ. Sci. Technol. 45 (2011) 3437–3444, https://doi. org/10.1021/es104156j. [4] Z. Zhu, Z. Liu, S. Liu, H. Niu, T. Hu, T. Liu, Y. Xie, NO reduction with NH3 over an activated carbon-supported copper oxide catalysts at low temperatures, Appl. Catal. B Environ. 26 (2000) 25–35, https://doi.org/10.1016/S0926-3373(99)00144-7. [5] S.M. Jeong, S.H. Jung, K.S. Yoo, S.D. Kim, Selective catalytic reduction of NO by NH3 over a bulk sulfated CuO/γ-Al2O3 catalyst, Ind. Eng. Chem. Res. 38 (1999) 2210–2215, https://doi.org/10.1021/ie9807147. [6] L. Wen, J. Chen, L. Yang, X. Wang, C. Xu, X. Sui, L. Yao, Y. Zhu, J. Zhang, T. Zhu, Enhanced formation of fine particulate nitrate at a rural site on the North China Plain in summer: the important roles of ammonia and ozone, Atmos. Environ. 101 (2015) 294–302, https://doi.org/10.1016/j.atmosenv.2014.11.037.
7
Applied Surface Science 507 (2020) 145153
C. Chen, et al.
[47] M. Waqif, P. Bazin, O. Saur, J.C. Lavalley, G. Blanchard, O. Touret, Study of ceria sulfation, Appl. Catal. B Environ. 11 (1997) 193–205, https://doi.org/10.1016/ S0926-3373(96)00040-9. [48] T. Luo, R.J. Gorte, Characterization of SO2-poisoned ceria-zirconia mixed oxides, Appl. Catal. B Environ. 53 (2004) 77–85, https://doi.org/10.1016/j.apcatb.2004. 04.020. [49] G. Centi, N. Passarini, S. Perathoner, A. Riva, Combined deSOx/deNOx reactions on a copper on alumina sorbent-catalyst. I. Mechanism of SO2 oxidation–adsorption, Ind. Eng. Chem. Res. 31(1992) 1947–1955. https://doi.org/10.1021/ie00008a016. [50] L. Chen, J. Li, M. Ge, DRIFT study on cerium-tungsten/titania catalyst for selective catalytic reduction of NOx with NH3, Envrion. Sci. Technol. 44 (2010) 9590–9596, https://doi.org/10.1021/es102692b. [51] S. He, C. Zhang, M. Yang, Y. Zhang, W. Xu, N. Cao, H. He, Selective catalytic oxidation of ammonia from MAP decomposition, Sep. Purif. Technol. 58 (2007) 173–178, https://doi.org/10.1016/j.seppur.2007.07.015. [52] J.C.S. Wu, Y. Cheng, In situ FTIR study of photocatalytic NO reaction on photocatalysts under UV irradiation, J. Catal. 237 (2006) 393–404, https://doi.org/10. 1016/j.jcat.2005.11.023. [53] M. Kantcheva, Identification, stability, and reactivity of NOx species adsorbed on titania-supported manganese catalysts, J. Catal. 204 (2001) 479–494, https://doi. org/10.1006/jcat.2001.3413. [54] K. Hadjiivanov, V. Bushev, M. Kantcheva, D. Klissurski, Infrared spectroscopy study of the species arising during nitrogen dioxide adsorption on titania (anatase), Langmuir 10 (1994) 464–471, https://doi.org/10.1021/la00014a021. [55] G. Ramis, G. Busca, V. Lorenzelli, P. Forzatti, Fourier transform infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on TiO2 anatase, Appl. Catal. 64 (1990) 243–257, https://doi.org/10.1016/S0166-9834(00) 81564-X. [56] R.Q. Long, R.T. Yang, Selective catalytic oxidation of ammonia to nitrogen over Fe2O3-TiO2 prepared with a sol-gel method, J. Catal. 207 (2002) 158–165, https:// doi.org/10.1006/jcat.2002.3545. [57] J. Valyon, W.K. Hall, Effects of reduction and reoxidation on the infrared spectra from Cu-Y and Cu-ZSM-5 zeolites, J. Phys. Chem. 97 (1993) 7054–7060, https:// doi.org/10.1021/j100129a021. [58] F. Cao, J. Xiang, S. Su, P. Wang, S. Hu, L. Sun, Ag modified Mn-Ce/γ-Al2O3 catalyst for selective catalytic reduction of NO with NH3 at low-temperature, Fuel Process. Technol. 135 (2015) 66–72, https://doi.org/10.1016/j.fuproc.2014.10.021. [59] K. Hadjiivanov, Identification of neutral and charged NxOy surface species by IR spectroscopy, Catal. Rev. 42 (2000) 71–144, https://doi.org/10.1081/CR100100260. [60] B. Klingenberg, M.A. Vannice, NO adsorption and decomposition on La2O3 studied by DRIFTS, Appl. Catal. B Environ. 21 (1999) 19–33, https://doi.org/10.1016/ S0926-3373(99)00003-X.
[34] C. Chen, W. Jia, S. Liu, Y. Cao, The enhancement of CuO modified V2O5-WO3/TiO2 based SCR catalyst for Hg0 oxidation in simulated flue gas, Appl. Surf. Sci. 436 (2018) 1022–1029, https://doi.org/10.1016/j.apsusc.2017.12.123. [35] Y. Cao, H. Zhou, W. Jiang, C. Chen, W. Pan, Studies of the fate of sulfur trioxide in coal-fired utility boilers based on modified selected condensation methods, Environ. Sci. Technol. 44(2010) 3429–3434. https://doi.org/ 10.1021/es903661b. [36] N. Yan, W. Chen, J. Chen, Z. Qu, Y. Guo, S. Yang, J. Jia, Significance of RuO2 modified SCR catalyst for elemental mercury oxidation in coal-fired flue gas, Environ. Sci. Technol. 45 (2011) 5725–5730, https://doi.org/10.1021/es200223x. [37] L. Chmielarz, P. Kuśtrowski, A. Rafalska-łasocha, R. Dziembaj, Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides, Appl. Catal. B Environ. 58 (2005) 235–244, https://doi.org/10.1016/j.apcatb.2004.12. 009. [38] L. Dong, L. Zhang, C. Sun, W. Yu, J. Zhu, L. Liu, B. Liu, Y. Hu, F. Gao, L. Dong, Study of the properties of CuO/VOx/Ti0.5Sn0.5O2 catalysts and their activities in NO + CO reaction, ACS Catal. 1 (2011) 468–480. https://doi.org/10.1021/cs200045f. [39] B. Chen, R. Xu, R. Zhang, N. Liu, Economical way to synthesize SSZ-13 with abundant ion-exchanged Cu+ for an extraordinary performance in selective catalytic reduction (SCR) of NOx by ammonia, Environ. Sci. Technol. 48 (2014) 13909–13916, https://doi.org/10.1021/es503707c. [40] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, The catalytic properties of Cu modified attapulgite in NH3–SCO and NH3–SCR reactions, Appl. Surf. Sci. 480 (2019) 537–547, https://doi.org/10.1016/j.apsusc.2019.03.024. [41] L. Yu, Q. Zhong, S. Zhang, W. Zhao, R. Zhang, A CuO-V2O5/TiO2 catalyst for the selective catalytic reduction of NO with NH3, Combust. Sci. Technol. 187 (2015) 925–936, https://doi.org/10.1080/00102202.2014.993028. [42] G. Xie, Z. Liu, Z. Zhu, Q. Liu, J. Ge, Z. Huang, Simultaneous removal of SO2 and NOx from flue gas using a CuO/Al2O3 catalyst sorbent: I. Deactivation of SCR activity by SO2 at low temperatures, J. Catal. 224(2004) 36–41. https://doi.org/10. 1016/j.jcat.2004.02.015. [43] P.S. Lowell, K. Schwitzgebel, T.B. Parsons, K.J. Sladek, Selection of metal oxides for removing SO2 from flue gas, Ind. Eng. Chem. Proc. Des. Dev. 10 (1971) 384–390, https://doi.org/10.1021/i260039a018. [44] T. Curtin, F. O' Regan, C. Deconinck, N. Knüttle, B.K. Hodnett, The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts, Catal. Today 55(2000) 189–195. https://doi.org/10.1016/S0920-5861(99)00238-2. [45] D. Pietrogiacomi, D. Sannino, A. Magliano, P. Ciambelli, S. Tuti, V. Indovina, The catalytic activity of CuSO4/ZrO2 for the selective catalytic reduction of NOx with NH3 in the presence of excess O2, Appl. Catal. B Environ. 36 (2002) 217–230, https://doi.org/10.1016/S0926-3373(01)00310-1. [46] R. Zhang, H. Alamdari, S. Kaliaguine, SO2 poisoning of LaFe0.8Cu0.2O3 perovskite prepared by reactive grinding during NO reduction by C3H6, Appl. Catal. A Gen. 340(2008) 140–151. https://doi.org/10.1016/j.apcata.2008.02.028.
8