TiO2 catalyst for the SCR reaction: Comparison of different forms of calcium

Molecular Catalysis 434 (2017) 16–24

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The poisoning effects of calcium on V2 O5 -WO3 /TiO2 catalyst for the SCR reaction: Comparison of different forms of calcium Xiang Li a,b , Xiansheng Li a , Ralph T. Yang b , Jiansong Mo c , Junhua Li a,∗ , Jiming Hao a a b c

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA Zhejiang Tianlan Environmental Protection Technology Co., LTD, PR China

a r t i c l e

i n f o

Article history: Received 16 November 2016 Received in revised form 5 January 2017 Accepted 6 January 2017 Keywords: NH3 -SCR Ca poisoning Reducibility SO4 2− Acid sites

a b s t r a c t The deactivation of the V2 O5 -WO3 /TiO2 catalyst for selective catalytic reduction of NOx with NH3 (NH3 SCR) by calcium was investigated via doping of CaO, CaSO4 and CaCO3 . It is found that the degree of the poisoning for the three kinds of deactivated catalysts follows the order of CaCO3 > CaO > CaSO4 . For a detailed understanding of the deactivation mechanism, the fresh and poisoned catalysts were analyzed and compared by using XRD, Raman, XPS, H2 -TPR, NH3 -TPD and in situ DRIFTS. The results manifest that the introduction of Ca species not only decreases the BET surface area but also leads to CaWO4 formation on the surface. CaO and CaCO3 significantly decrease the surface chemisorbed oxygen, the reducibility of vanadium species, the NH3 chemisorbed amount, and the intensity of surface acid sites, all important factors leading to lower SCR activity. In contrast, the sulfate species can enhance the concentrations of the surface active oxygen and the Brönsted acid sites, thus partially offsets the negative effect in the loss of active sites by CaO. However, due to its higher reducibility, the surface SO4 2− species also leads to more N2 O production. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Large amounts of NOx emission from stationary sources like coal-fired power plants, industrial boilers and cement production process are known as important and hazardous air pollutants [1–3]. Among the technologies for NOx abatement, selective catalytic reduction of nitrogen oxides with NH3 (NH3 -SCR) has been proved as the most effective technology [4–8]. V2 O5 -WO3 /TiO2 is the core of current commercial SCR catalysts for stationary source applications. Generally, vanadia and titania are considered as the main active ingredient and support, respectively, over the catalyst [9]. Tungsten (or molybdenum) is known as the best catalyst promoter for improving the surface acidity, support stability and the vanadia dispersion on the catalyst surface [10]. On the one hand, low-rank coals (i.e., lignites and subbituminous coals) generally have high calcium contents and much of these coals are being used today for power generation in the world. The calcium exists in the form of high calcium fly ash that forms deposits in the combustion system including the SCR reactor. Thus high content calcium oxide in the flue gas is always a

∗ Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.mcat.2017.01.010 2468-8231/© 2017 Elsevier B.V. All rights reserved.

serious poison causing SCR catalyst deactivation due to its basicity [11,12]. In coexistence with CaO, there are CO2 , H2 O and SO2 in the real flue gases. Calcium sulfate may be another noteworthy problem, since it tends to “cake” and accumulate and is difficult to remove from the surface of the catalyst. The concentration of Ca-based particles (mainly CaO and CaCO3 ) in the pre-calciner can reach as high as 1 kg/Nm3 in cement kilns and circulating fluidized bed boilers (CFBBs) [13]. Thus, the influence of CaO, CaSO4 and CaCO3 on vanadium-based catalyst must be considered when SCR deNOx process is applied in coal-fired power plants, cement kilns and other coal-based combustion systems. Several studies have been undertaken to investigate the effect of calcium-based substances on NO reduction by NH3 . Lin et al. found that CaO inhibited the reaction process and narrowed the temperature window when NH3 is used as reductant [14]. Zijlma et al. studied the NH3 oxidation reaction over CaO catalyst and believed that CaO could firstly react with NH3 to produce CaN and then was further oxidized by O2 to form NO [15]. Yang et al. investigated the adsorption and transformation of ammonia over CaO and sulfated CaO by DRIFTS. They confirmed that NH3 could be dissociated into NH2 on both the CaO and sulfated CaO surfaces. For CaO catalyst the intermediate NH2 could react with surface oxygen to produce NO, while it would be apt to reduce NO for the CaSO4 catalyst [16]. Fu et al. performed a study on the adsorption and reactions of NH3 ,

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Fig. 1. (a) NO conversion and (b) N2 O production of fresh and Ca poisoned catalysts. Reaction condition: [NO] = [NH3 ] = 500 ppm, [O2 ] = 3%, N2 balance, total flow rate = 200 mL/min, GHSV = 240000 mL/(g·h).

Table 1 Related structure and surface acidities results of fresh and Ca poisoned catalysts. Sample

BET (m2 /g)

Pore volume (cm3 /g)

W/Ti(%)

Ca/Ti(%)

O␣ ratio (%)

Total acidity(␮mol/m2 )

Fresh CaO CaSO4 CaCO3

46 44 41.9 40.9

0.39 0.3 0.31 0.31

15.1 17.5 17.6 16.5

– 4 7.5 5

27.8 24 30.6 17.7

6.4 3.8 5.8 2.5

Calculated by XPS spectra.

Fig. 2. XRD patterns of fresh and Ca poisoned catalysts.

NO, and O2 on the CaO surface. They found that the intermediate NH2 was primarily oxidized by O2 to NO while the other reacted with NO to form N2 [16] . However, their work mainly focused on the SNCR technology, and few studies were concentrated on the CaCO3 effect. More recently, Chen et al. systematically investigated the poisoning effect of alkali metals over V2 O5 –WO3 /TiO2 catalysts. They considered that the degree of the Ca poisoning effect was much less than that of K and Na, and when the concentration of CaO was over 2 wt%, obvious activity loss could be found [17]. But the relationship between structural change and activity loss of the CaO-doped catalyst is still unclear, and no studies have been done on the CaSO4 and CaCO3 poisoned catalysts. To investigate the deactivation effects on vanadium-based catalysts by the different Ca species, fresh V2 O5 -WO3 /TiO2 catalyst and that doped with CaO, CaSO4 and CaCO3 were prepared and compared on the basis of the textural characteristics, redox properties,

Fig. 3. Raman spectra of fresh and Ca poisoned catalysts.

surface acidity and the SCR deactivation mechanism. Moreover, the relationship between structure change and activity loss is also described and discussed in the study. 2. Experiments 2.1. Catalysts preparation and poisoning The V2 O5 -WO3 /TiO2 catalyst with V2 O5 (1 wt%) and WO3 (9 wt%) loadings and commercial titania P25 as support was prepared by impregnation method. Typically, a certain amount of ammonium metavanadate, ammonium paratungstate and oxalate were dissolved firstly in the solution of desired proportions. Then a given mass of P25 powder was impregnated in this solution with strong stirring for 10 h. The obtained samples were dried overnight at 120 ◦ C, followed by calcination at 500 ◦ C for 4 h in air. The CaO, CaSO4 and CaCO3 doped V2 O5 –WO3 /TiO2 catalysts (denoted as

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CaO, CaSO4 and CaCO3 for short) were also prepared by impregnation method with the same weight loadings of CaO (2 wt%). Briefly, equal amount of fresh V2 O5 –WO3 /TiO2 catalysts were impregnated separately (or individually) with the aqueous solutions of Ca(OH)2 for CaO poisoned catalyst, Ca(OH)2 and (NH4 )2 SO4 for CaSO4 poisoned catalyst and Ca(OH)2 and (NH4 )2 CO3 for CaCO3 poisoned catalyst. After stirring for 4 h, the poisoned catalyst were dried at 120 ◦ C for 12 h and then calcined at 500 ◦ C for 4 h. All the final samples were ground and sieved to 40–60 mesh for activity evaluation. 2.2. Catalysts activity 100 mg of each sample with 40–60 mesh was used to measure the SCR activity in a fixed−bed quartz reactor (I. D. = 6 mm) at atmospheric pressure. The mixed feed gases with a total flow rate of 200 mL/min consists of 500 ppm of NO, 500 ppm of NH3 , 3% of O2 and N2 as balance gas. The corresponding gas hourly space velocity was 240000 cm3 ·(g·h)−1 . The outlet concentration of NO, NO2 NH3 and N2 O at were monitored using an FTIR spectrometer (Gasmet Dx-4000 Analyzer). The activity data were recorded after about 1 h at each temperature. 2.3. Catalysts characterization The X-ray powder diffraction was performed on an X-ray diffractometer (Rigaku, D/max-2200/PC) between 10◦ and 65◦ at a step rate of 5◦ min−1 using Cu K␣ (␭ = 0.15405 nm) radiation. The Raman spectra of catalysts were carried out with visible 532 nm excitation by a Raman microscope (Renishaw, InVia Reflex, UK) equipped with a charge-coupled device (CCD) array detector and a highgrade Leica microscope. The BET surface areas of the samples were measured by nitrogen adsorption at 77 K by surface area analyzer (Micromeritics, ASAP 2020, US). X-ray photoelectron spectroscopy (XPS) measurement was obtained on an XPS electron spectrometer (VG Scientific, Esca lab 220i-XL, UK) using 300 W Al K␣ radiations. The C 1 s line at 284.8 eV from the carbon is used as a reference for binding energies calibration. Temperature programmed desorption of NH3 (NH3 -TPD) and temperature programmed reduction with H2 (H2 -TPR) experiments were performed on a fixed bed reaction system with the detectors of FTIR spectrometer (MKS, MultiGas 2030HS, US) and chemisorption analyzer (Micromeritics, ChemiSorb 2920 TPx, US), respectively. Prior to each test, the samples of 100 mg were treated under nitrogen at 350 ◦ C for 1 h.

Fig. 4. O1s XPS spectra of fresh and Ca poisoned catalysts.

In situ DRIFTS were carried out on a Fourier transform infrared spectrometer (ThermoFisher, Nicolet NEXUS 6700, US) equipped with an MCT/A detector cooled by liquid nitrogen and an in situ cell with a ZnSe window. Prior to reaction gases adsorption, the catalyst was heated at 450 ◦ C for 2 h in a flow of N2 and then cooled down to 30 ◦ C. The gas used for DRIFRS experiment consisted of 500 ppm NH3 /N2 , 500 ppm NO/N2 and 3% O2 /N2 . All the spectra were recorded by accumulating 32 scans with a resolution of 4 cm−1 .

Fig. 5. H2 -TPR profiles (a) and H2 consumption rate profiles (b) of fresh and Ca poisoned catalysts.

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Fig. 7. Loss ratios of the Lewis and Brønsted over the fresh and Ca poisoned catalysts with temperature increase.

Fig. 6. NH3 -TPD curves of the fresh and Ca poisoned catalysts.

3. Results and discussion 3.1. Catalytic performances The SCR activity and N2 O selectivity over fresh and different kinds of Ca-poisoned catalysts were tested with a high GHSV of 240,000 mL/(g·h) and the results are shown in Fig. 1. The fresh catalyst showed excellent NO conversion over the whole temperature range. Upon CaO addition to V2 O5 -WO3 /TiO2 catalyst, sharp decrease for the SCR activity and N2 O production increase within the temperature range could be observed. In particular, the highest NO conversion for CaO poisoned catalyst was less than 70% at 450 ◦ C. However, after carbonation, both the steady-state SCR activity and N2 O production showed an more obviously decrease, since the maxima of NO conversion and N2 O formation were only 55% and 60.7 ppm at 500 ◦ C, respectively. We believed that the coverage or channel plugging effect originated from calcium carbonate precipitation might be the important reason of the lowest NO conversion and N2 O formation for the CaCO3 poisoned catalyst. In contrast, in the presence of CaSO4 , Ca deactivation had a minimal impact on SCR activity, but it caused a large amount of N2 O appearance: up to 128 ppm at 500 ◦ C. Therefore, it can be deduced that the addition of sulphate could induce an obvious increase in N2 O formation because of non-selective catalytic reduction (NSCR) or NH3 selective oxidation (NSO) [18,19]. Based on these results above, the degree of the poisoning influence on SCR activity for three different kinds of Ca doped catalysts can be ordered as follows: CaCO3 > CaO > CaSO4 . Additionally, the severity of Ca doping effect on N2 O productions follows a reverse order: CaSO4 > CaO > CaCO3 . 3.2. Textural characteristics of catalysts The channel blocking effect by calcium species can be found in Table 1, since both the BET surface areas and pore volumes of poisoned catalysts decrease in comparison to fresh one. Notably, when sulfate or carbonate is doped with Ca2+ , the BET surface area is reduced more than CaO poisoned catalyst. This may be related to agglomeration effect for CaSO4 and CaCO3 on the catalyst surface, in consideration of the difference between surface and bulk Ca/Ti molar ratios (Table 1). In view of the more remarkable NO

Fig. 8. IR spectra of NH3 adsorption over the fresh and Ca poisoned catalysts at 150 ◦ C (Y-axis is absorbance.).

conversions loss for the poisoned catalysts, chemical effect rather than physical effect such as channel blocking and surface deposit may be the main factor for catalyst deactivation. XRD patterns of the catalysts are shown in Fig. 2. Fresh catalyst shows not only sharp diffraction peaks of the anatase phase (PDF-ICDD 65-5714) and rutile phase TiO2 (PDF-ICDD 21-1276) belonging to P25 support but also some small bumps attributed to crystal WO3 (PDF-ICDD 20-1324), implying that some WO3 cannot be well dispersed on the TiO2 surface [20]. When Ca species are added into V2 O5 -WO3 /TiO2 , the original diffraction peak positions of crystal WO3 shifted for CaO and CaCO3 doped samples, meantime scheelite phase CaWO4 (PDF-ICDD 41-1431) appears for all the poisoned catalyst [21]. This implies that Ca species could react with surface WOx to form CaWO4 and destroy the surface active

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sites (VOx and WOx ) dispersion that ensures the high SCR activity of V2 O5 -WO3 /TiO2 catalyst [10]. According to the intensity and amount of the scheelite phase characteristic peaks, the order for the ease of CaWO4 formation is: CaCO3 > CaO > CaSO4 , which is in accordance with the catalytic activity. Meanwhile, the surface W contents of poisoned catalysts also increased via comparison with the fresh catalyst as the W/Ti ratio calculated by the XPS results shown in Table 1. According to the XRD results, the increased W/Ti ratios for poisoned catalyst should be connected with the poor dispersion of active sites and CaWO4 formation. Therefore, these results signify that, besides surface basicity of calcium ions, the active sites (VOx and WOx) dispersion and CaWO4 formation also play important roles on the degree of the deactivation [17]. The Raman spectra of the samples are provided in Fig. 3. The three predominant bands at 409 cm−1 (Eg ), 527 cm−1 (A1g /B1g ) and 648 cm−1 (B1g ) and the weak band at 795 cm−1 can be associated with anatase structure and the peaks at 457 cm−1 with rutile structure [20]. Small quantities of crystalline WO3 particles are present by the bands at 806 cm−1 for the calcium poisoned catalyst [22]. Generally, the Raman band in the 968–997 cm−1 region caused by the symmetric stretching vibrations of the W = O bonds of the surface WOx species [23]. This peak shifts towards lower wavenumbers with intensity lessening in the order of fresh > CaSO4 > CaO > CaCO3 , which is in keeping with the order of the decrease in the SCR activity. Moreover, along with the shift, a new band at around 920 cm−1 appears gradually with increasing intensity which is indicative of Raman-active modes of CaWO4 [24]. These changes may be related to the polymerization of the surface WOx species from crystalline polytungstate species to isolated CaWO4 species, in line with the XRD results (Fig. 2). In addition, the appearance of a small peak at 1023 cm−1 indicates the presence of monodispersed Vx Oy species on P25. Therefore, SO4 2− could weaken the CaWO4 formation effect and improve the active ingredients dispersion on the surface of the support. By contrast, CO3 2− exacerbates the CaWO4 formation from interaction between CaO and surface WOx species, thus leads to the further drop in the SCR activity. 3.3. Surface analysis and redox properties The XPS results of O1s for fresh and Ca poisoned catalysts are shown in Fig. 4. All the four samples can be fitted into two primary sub-bands. The binding energies located at 529.9–530.4 eV correspond to lattice oxygen O2− (labelled as O␤ ); the ones at 530.5–530.8 eV can be assigned to the surface adsorbed oxygen (designated as O␣ ) like defect-oxide O2 2− or hydroxyl group OH− [25–27]. For CaSO4 poisoned catalyst, another small sub-band at around 533 eV can be found, indicating the existence of ionization associated with weakly adsorbed oxygen species. Because the significant increase of the electronic density around oxygen atoms after CaO and CaCO3 introduction, the shift of the binding energies towards lower energies is present as seen in the figure, which is in agreement with Dupin’s observation [27]. However, the binding energy of the CaSO4 poisoned catalyst is almost the same as the fresh one, suggesting that SO4 2− could suppress the shift changes that resulted from doping by alkaline-earth Ca. The surface chemisorbed oxygen O␣ was considered to be highly active in oxidation reaction and NH3 activation process because of its higher mobility than lattice oxygen O␤ [28]. For the first step of SCR, NH3 is adsorbed onto the catalyst surface and the adsorbed NH3 then dissociates into NH2 . Hence, the higher relative O␣ amount on the catalyst surface can be correlated with the better SCR activity. As presented in Table 1, the surface O␣ concentration decreases in the following order: CaSO4 > Fresh > CaO > CaCO3 , which is in good agreement with the evolution of N2 O production at 500 ◦ C. For CaSO4 poisoned catalyst, the enhanced amount of O␣ may promote

the further dissociation of NH2 to NH at high temperatures, which can react with gaseous NO to form N2 O. Conversely, because of the existence of CaCO3 and CaO, less surface chemisorbed oxygen can be involved in the NH3 activation, thus inhibiting both SCR and the side reactions. The reducibility and stability of the catalysts have also been investigated by H2 -TPR experiment, and the results are shown in Fig. 5. Two peaks can be observed on the fresh sample: the lowtemperature peak (480 ◦ C) can be assigned to the reduction of V5+ to V3+ , while the high-temperature peak (769 ◦ C) corresponds to the reduction of W6+ to W [29]. The thresholds of hydrogen consumption and the first peak position increase in the sequence: Fresh < CaSO4 < CaO < CaCO3 . Meanwhile, the reduction peak of W6+ to W for the three kinds of Ca-poisoned catalysts cannot be seen at below 800 ◦ C. Combined with XRD and Raman results, CaWO4 formation may play a crucial role on disrupting the interaction and fine dispersion of Vx Oy and WOx on the catalyst surface thus lessening the reduction of active V5+ and W6+ species. It is noteworthy that a distinct peak at 682 ◦ C occurs for the CaSO4 poisoned catalyst, which is supposed to be the reduction of sulfate by hydrogen [30]. Considering the results of XPS, the more surface O species can further oxidize the adsorbed NO to NO2 on the surface and then leads to the NH4 NO3 formation, thereby increasing the N2 O production of the catalyst. Additionally, the initial H2 consumption rate according to the first reduction band of each sample has been calculated to better explore the reducibility influenced by the addition of Ca. As shown in Fig. 5(b), the initial H2 consumption rate is in the sequence: Fresh> CaO > CaSO4 > CaCO3 , in accordance with the relative decreases in the BET surface area. These results suggest the physical surface coverage by Ca may be another important factor to determine the H2 consumption rate, because the reduction of catalyst is initiated from surface adsorption. 3.4. Surface acidity The surface acidity is considered as one of the most important properties for the SCR activity. Fig. 6 shows the NH3 -TPD curves over fresh and Ca poisoned catalysts, and the total acid amount normalized by BET surface area is also shown in Table 1. In the temperature range from 100 ◦ C to 700 ◦ C, the fresh samples showed two NH3 desorption peaks at 290 ◦ C and 350 ◦ C. Comparing with the in situ DRIFTS results of NH3 -TPD in Fig. S2, these two peaks can be assigned as follows: the first peak at 290 ◦ C is mainly caused by the desorption of NH4 + bound to surface hydroxyls (Brønsted acid sites); and the second peaks centered at 350 ◦ C is related to the desorption of coordinated NH3 bound to Lewis acid sites. However, only one peak is shown for the three kinds of poisoned catalysts. For CaSO4 poisoned catalyst, because NH4 + bound to surface hydroxyls are significantly enhanced by sulfate species, the first desorption peak has moved slightly to a higher temperature (321 ◦ C). Over the CaO poisoned catalyst, only the first NH3 desorption band centered at 290 ◦ C remains. This result suggests that NH3 adsorbed on the surface V = O sites over vanadia-based catalysts is more seriously affected by CaO than the Brønsted acid sites such as V OH or W OH [31,32]. Nevertheless, with the introduction of CO3 2− , the desorption peak shifts to a higher temperature at 340 ◦ C, indicating the amount of NH4 + adsorbed on surface hydroxyls decreases further due to more CaWO4 formation and carbonate species disruption. Combining the results of XRD and total acidity in Table 1, it can be deduced that more CaWO4 on the catalyst surface is mainly responsible for Brønsted acid sites loss while the surface basicity of CaO for Lewis acid sites loss. In order to investigate the thermal stability of acid sites, loss ratios of the two kinds of acid sites with temperature are quantified by integration of the corresponding peak areas shown in Fig. 7. It can be found that both the NH3 adsorption stability on the two kinds of acid sites are markedly influenced after

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Fig. 9. Sequential DRIFTS spectra of fresh (a), CaSO4 (b), CaO (c) and CaCO3 (d) poisoned catalysts recorded under reaction atmospheres: the dehydrated catalyst is first treated by NH3 , then NO + O2 is added at 300 ◦ C.

the addition of CaO and CaCO3 . It is should be noted that since we selected the maximum peak area among the datum at different temperatures as denominator to calculate the loss ratio of acid sites. For CaSO4 poisoned catalyst, the maximum peak area attributed to the Lewis acid sites were obtained at 250 ◦ C, so a quasi “volcano” profile was obtained in the figure. Thus SO4 2− can not only promote the NH3 adsorption stability on the Brønsted acid but causes partial NH4 + bound to Brønsted acid sites transform into coordinated NH3 bound to Lewis acid sites because of the dehydration effect below 250 ◦ C. And this result was also found by the Liu’s research on the influence of sulfation on iron titanate catalyst [33]. Fig. 8 shows the detailed IR spectra of NH3 adsorption on different catalysts at 150 ◦ C. For fresh catalyst, the bands at 1222 and 1600 cm−1 can be assigned to bending vibrations of the N H bonds on the Lewis acid sites, whereas the band at 1426 and 1671 cm−1 can be attributed to asymmetric and symmetric bending vibrations of NH4 + species on the Brønsted acid sites. Three small peaks on the

broad band in the range 3160–3400 cm−1 are observed in the spectrum also belong to contributions from N H and H N H vibrations of ammonia adsorbed on the Lewis acid sites [35,36]. In addition, the negative band centered at approximately 3645 cm−1 is ascribed to V OH surface hydroxyl, indicating the high dispersion of Vx Oy species on the TiO2 support; while the other peak at 3600 cm−1 may be attributed to the W OH stretching frequency from crystalline WO3 species [37,38]. For CaSO4 poisoned catalyst, as the S O has a strong inductive effect of adsorbing electrons, thereby decreasing the bond energy of the OH, which causes the two peaks of the CaSO4 poisoned catalyst to shift towards a lower wavenumber. Moreover, although the peak intensities of hydroxyl and Lewis acid sites decline comparably because of Ca addition, SO4 2− greatly favors the NH3 adsorption on the Brønsted acid site, which would partially offset the CaO deactivation effect. Whereas for the other poisoned catalysts, due to the presence of Ca, all the peak intensi-

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Fig. 10. Normalized intensity on the consumptions of different peaks for the Sequential DRIFTS spectra.

ties have been significantly weakened with the positions shifted. Because the NH3 adsorption over CaO and CaCO3 containing catalysts are much less than CaSO4 poisoned catalyst, the important first step (chemisorption of NH3 ) for the catalytic cycle model proposed by Topsøe et al. is suppressed to a great degree for these two kinds of Ca deactivation catalysts [39]. Hence, this is also an important reason for their larger activity loss.

3.5. Studies of reaction process The in situ DRIFTS experiment of the reaction between NO + O2 and pre-adsorbed NH3 species over fresh and Ca-poisoned catalysts was carried out and the detailed information is shown in Figs. 9 and 10. After being pretreated by NH3 and then N2 purge, the fresh sample shows several bands ascribed to ionic NH4 + (␦s at1670 cm−1 and ␦as at 1430 cm−1 ) and coordinated NH3 (␦as at 1600 cm−1 and ␦s at 1228 cm−1 ), similar to the results of NH3 adsorption IR spectra at 150 ◦ C shown before. The subsequent introduction of NO + O2 leads to an evident intensity weakening of these bands and then disappearance within 2.5 min, indicating that both the Brønsted and Lewis acid sites can participate in the SCR reaction. With the increasing of the reaction time, only the ␦HOH vibration peak at 1620 cm−1 appears, confirming the formation of a large amount of H2 O [40]. After the CaSO4 introduction, the reducing rates of both the adsorbed NH4 + and NH3 on the surface are greatly lowered without any other peak occurrence, which means that although NH4 + bound to Brønsted acid sites can be enhanced by sulfate species, the surface coverage effect cannot be ignored due

to its great side influence on the reaction rate. Over the CaO doped catalyst, only Lewis acid sites can take part in the SCR reaction with slower reaction rates. Unlike other Ca-doped catalysts, CaCO3 poisoned catalyst shows two nitro compounds and monodentate nitrate characteristic peak at 1321 cm−1 with the formation of H2 O [34]. As the reaction proceeds, all the three peaks weaken after 4.5 min. Since the Eley–Rideal reaction mechanism between the adsorbed NH3 species and gaseous NO is more appropriate to follow on the fresh V2 O5 -WO3 /TiO2 catalyst. The adsorbed nitrate species may suppress the NH3 adsorption and activation because of limited reaction sites on the surface, and then further lowers the SCR activity [31,41].

3.6. Deactivation comparison CaO, CaSO4 and CaCO3 were chosen to investigate the deactivation effect over the V2 O5 -WO3 /TiO2 catalyst. The degree of the activity loss for all the poisoned catalysts can be ordered as follows: CaCO3 > CaO > CaSO4 , and the results indicate the two different kinds of negative ions play the opposite effects on the basis of CaO deactivation. Texture, reducibility and surface acidity can be the main influence factors. According to the XRD and Raman results above, the original structure and dispersion of WOx on the catalyst surface can be significantly destroyed by Ca species because of more CaWO4 formation. Previous researches thought that strength and the amount of Brönsted acid sites over vanadia-based catalyst can be decreased due to the basicity of alkali-earth Ca [17]. While we find that of both the Brönsted and Lewis acid sites can be signifi-

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cantly affected by the disruption of Vx Oy and WOx dispersion on the surface with CaWO4 formation. Moreover, in the presence of CO3 2− , this effect becomes much more severe along with the formation of CaWO4 on the surface. As is well known, SO4 2− is a typical Brönsted acid, and our work also shows that NH4 + bounded to the Brönsted acid sites are enhanced significantly after CaSO4 addition. Meanwhile, the amount of surface CaWO4 is less than the other kinds of Ca poisoned catalysts. Therefore besides Ca alkalinity, CaWO4 formation, which makes structure changes in the surface Vx Oy and WOx , is another important reason for the loss of acid sites. Proper oxidability of surface species is also important for Habstraction of NH4 + or NH3 , which is a key step in the SCR cycle. Yang et al. studied ammonia activation over CaO and sulfated CaO for NO reduction by NH3 . They found that CaO could activate the ammonia adsorbed to the NH form, while ammonia adsorbed over sulfated CaO is activated mainly in the NH2 form [16]. Zijlma et al. concluded that −CaN would be formed after NH3 is adsorbed on the CaO surface, and then it would further react with NO to produce N2 O [15]. Because of higher reducibility, CaO is very apt to lead to a higher yield of the byproduct, based on their results. However, from our results, CaO has obvious negative effect on the surface chemisorbed oxygen and the oxidability of surface species over the V2 O5 -WO3 /TiO2 catalyst. Furthermore, negative ions play the opposite roles on CaO poisoning catalyst: SO4 2− increases both the surface chemisorbed oxygen concentration and H2 consumption; CO3 2− can further intensify the effect of CaO on the reducibility of catalyst. According to the mechanism proposed by Topsøe, the second step is that oxidation of the NH4 + or NH3 by adjacent V5+ = O to form NH3 + or NH2 and reduction of V5+ = O to H O V4 + [39]. The loss in surface chemisorbed oxygen and oxidability of surface vanadium species originated from CaO and CaCO3 may be another crucial reason for deactivation. Nevertheless, the more surface O oxygen species caused by surface SO4 2− introduction may lead to NH3 further dissociating to NH, which can react with NO to form N2 O.

4. Conclusions Three different kinds of Ca poisoning samples were investigated and compared with fresh catalyst based on the changes in texture, redox property, acidity and reaction process in this paper. The poisoning effect on the SCR activity decreases in the order: CaCO3 > CaO > CaSO4 , while the effect on N2 O production follows a reverse order: CaSO4 > CaO > CaCO3 . The introduction of Ca species disrupts the original WOx structure and dispersion on the catalyst surface and leads to CaWO4 formation. The surface chemisorbed oxygen and the reducibility of surface species can be significantly weakened by the presence of CaO and CaCO3 , while being enhanced by CaSO4 . On the surface acidity aspect, SO4 2− greatly favors the NH3 adsorption on Brønsted acid site and can partly offset the CaO deactivation effect on acid sites. However, both the NH3 adsorption amount and intensity on Brönsted and Lewis acid sites over V2 O5 WO3 /TiO2 catalyst are markedly influenced after the addition of CaO and CaCO3 . In situ DRIFTS experiment shows that CaSO4 can decrease the reaction rates of both the adsorbed NH4 + and NH3 on the surface, while only left Lewis acid sites over the CaO and CaCO3 doped catalysts can take part in the SCR reaction at 300 ◦ C.

Acknowledgements This work was financially supported by Public Projects Foundation of China Ministry of Environmental Protection (201509021 and 201509012) and China Postdoctoral Science Foundation (2013M530643).

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.01. 010.

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