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Role of CTAB in the improved H2O resistance for selective catalytic reduction of NO with NH3 over iron titanium catalyst
Chemical Engineering Journal 347 (2018) 313–321
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Role of CTAB in the improved H2O resistance for selective catalytic reduction of NO with NH3 over iron titanium catalyst
T
⁎
Yulin Lia,b, Xiaojin Hana, Yaqin Houa,b, , Yaoping Guoa,b, Yongjin Liua,b, Yan Cuia, ⁎ Zhanggen Huanga, a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
catalysts showed high SCR • CT-FeTi activity and good H O resistance. optimized the pore size to avoid • CTAB being excessively enlarged in the pre2
sence of H2O.
promoted the adsorption of • CTAB bridging nitrate and NH species on 3
Lewis acid sites.
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron titanium catalyst CTAB H2O-resistance NH3-SCR
Uniform mesoporous iron titanium (CT-FeTi) catalysts were synthesized by a CTAB-assisted process and exhibited good catalytic activity and H2O resistance when tested in the selective catalytic reduction (SCR) of NO with NH3 at low temperature. BET, TPD, TPSR and in situ DRIFTS were carried out to reveal the inhibition mechanism of H2O on SCR reaction and determine the role of CTAB in the improved H2O resistance during lowtemperature SCR processes. Results showed that H2O inhibited the adsorption of NO on the surface of CT-FeTi catalysts, as well as the formation of the intermediate species (–NH2), which was produced by the reaction Fe3+ + NH3 → Fe2+−NH2 + H+ . CTAB acted as a “structural” and “chemical” promoter, not only optimizing the pore size to avoid being excessively enlarged in the presence of H2O, but also enhancing the adsorption of bridging nitrate and NH3 species on Lewis acid sites, thus improving the catalytic activity and H2O resistance.
1. Introduction Nitrogen oxides (NOx) emitted from stationary sources have been a major source of atmospheric pollution, and can cause environmental problems such as acid rain, photochemical smog and ozone depletion [1]. The selective catalytic reduction of NOx with NH3 over V2O5/TiO2
⁎
or V2O5-WO3/TiO2 catalysts is a well proven technology for the removal of nitrogen oxides from stationary sources [2], however, this process must operate at temperatures higher than 350–450 °C to avoid SO2 poisoning. So the low temperature SCR process is preferable because it allows the reactor to be installed downstream of the particle controller and desulfurization devices without need to reheat the flue
Corresponding authors at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China (Y. Hou). E-mail addresses: [email protected] (Y. Hou), [email protected] (Z. Huang).
https://doi.org/10.1016/j.cej.2018.04.107 Received 13 February 2018; Received in revised form 4 April 2018; Accepted 13 April 2018 Available online 18 April 2018 1385-8947/ © 2018 Published by Elsevier B.V.
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the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the isotherms. Prior to the surface area measurements, the samples were degassed under vacuum conditions at 90 °C for 10 h The NO/NH3 temperature programmed desorption measurement was carried out with catalyst samples of 0.25 g under a total flow rate of 100 ml/min. Before experiment, the samples were pretreated in situ at 300 °C in a flow of pure N2 for 0.5 h and subsequently cooled to 90 °C. Then the samples were treated with 1000 ppm NO/N2 and 6.5 vol% O2 or 1000 ppm NH3/N2 in the presence of 5 vol% H2O for 1 h. The NO/ NH3 was purged with N2 for 2 h at 90 °C to remove the physically adsorbed species. Desorption was carried out by heating samples from 90 °C to 500 °C, and the NO/NH3 was continuously monitored using a portable Fourier Transform Infrared (FT-IR) gas analyzer. Temperature-programmed surface reaction (TPSR) was performed to study the reactivity of adsorbed NH3 with NO on the surface of the catalysts in the absence or presence of H2O. The samples that had adsorbed NH3 were exposed to a gas stream containing 250 ppm of NO/N2 and 6.5 vol% O2 in the absence or presence of 5 vol% H2O and heated from 50 to 300 °C at a rate of 10 °C/min. The temperature was kept at 300 °C until the outlet NO concentration became equal to the inlet value (250 ppm). The outlet NO concentration was continuously measured using a portable FT-IR gas analyzer. In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) were recorded from 4000 to 1000 cm−1 at a spectral resolution of 4 cm−1 on a FTIR spectrometer (Bruker Tensor 27) equipped with an in situ diffuse reflectance cell containing a MCT detector and ZnSe windows. Prior to each spectrum recording, the samples were purged at 350 °C in N2 for 30 min, and cooled down to 240 °C, while the background spectrum with N2 flow was recorded. Finally, the sample was exposed to a controlled steam flow of 100 ml/min, containing 500 ppm NH3/N2 or 500 ppm NO/N2 and 6.5 vol% O2 in the absence or presence of 5 vol% H2O. The adsorption/reaction spectra were recorded for various target temperatures or times by subtraction of the corresponding background reference.
gas. In recent years, catalysts active at low temperature, such as Mn, Fe and Ce loaded on different supports [3–5], have been studied, with a focus on their susceptibility to SO2 poisoning and proposed methods to increase SO2 resistance [6]. On the other hand, the inhibition effect of H2O on the catalytic activity at low temperature has often been neglected, although it is a critical factor. The inhibition effect of H2O on SCR has been previously studied on other catalyst systems, such as vanadium-based catalysts and activated carbon. The main conclusions reached may be summarized that H2O might cause competitive adsorption with reactant molecules or/and inhibit the reaction of NO with adsorbed NH3 [7,8]. Many studies ascribed the inhibition effect of H2O to the competitive adsorption with NO/NH3 on the active sites of the catalysts; on the contrary, Topsøe [9] found that H2O promotes NH3 adsorption on V2O5/TiO2 catalysts. Similarly, Huang et al. [10] also found that H2O does not compete with NO and NH3 for active sites on the V2O5/AC catalyst, but rather promotes NH3 adsorption. The lack of agreement on this issue, even for the commercial vanadium-based catalysts suggests the necessity to study the effect of H2O to develop novel low temperature catalysts. Recently, an interesting approach was reported for the preparation of low temperature SCR catalysts using structure directing agents such as Pluronic F127, Pluronic P123 and cetyl trimethyl ammonium bromide (CTAB). Shi et al. [11] used F127 as the structural template to synthesize hierarchically macro-mesoporous Mn/TiO2, which showed a more promising behavior compared to the non-modified catalyst. Wu et al. [12] found that the introduction of CTAB in the preparation of the FeMnTi catalyst increased its activity and the operating temperature window. Zhan et al. [13] used Pluronic P123 to synthesize mesoporous FeCeTi nanocatalysts that exhibited high SCR activity. On the other hand, the effect of H2O on the catalytic activity of catalysts prepared using structure directing agents has been rarely reported. In present work, a novel iron titanium catalyst was synthesized using CTAB and applied to the NH3-SCR process at low temperature. It was found that the addition of CTAB changed the pore structure and enhanced the catalytic performance of FeTi catalysts both in the absence and in the presence of H2O. Based on the characterization results, the mechanisms of the influence of CTAB on activity and H2O resistance were further elucidated. The main objective was to clarify the actual role of CTAB in the H2O resistance at low temperatures, thus providing a reference for designing novel H2O resistance catalysts based on structure directing agents.
2.3. Catalyst performance tests The schematic diagram of experimental apparatus for SCR measurement is displayed in Fig. 1 .Catalytic tests were conducted in a fixbed flow reactor (length 250 mm, internal diameter 12 mm) with a feed of 500 ppm NO, 500 ppm NH3, 6.5 vol% O2/N2, and a GHSV of 18,000 h−1 using 1 g of catalyst particles of 40–60 mesh, and a total flow of 400 ml/min·H2O vapour was generated by passing N2 and O2 through a heated gas-wash bottle containing deionized water. The NOx concentrations in the inlet and outlet gas were analyzed simultaneously by a flue gas analyzer (KM9106 Quintox, Kane International Limited).
2. Experimental 2.1. Catalyst preparation All chemicals used were of analytical grade and procured from Sinopharm Chemical Reagent Company. The catalyst was prepared by a sol–gel method using CTAB as template-directing regent. In a typical synthesis process, absolute ethanol (1 mol), CTAB (0.005 mol) and tetrabutyltitanate (0.05 mol) were mixed under vigorous stirring at room temperature to form a transparent yellow solution. Fe (NO3)3·9H2O (0.007 mol) was then added to the above solution. After stirring for 2 h, the mixture was aged at 60 °C for 12 h and dried at 110 °C overnight. Finally, the resulting powder was calcinated in air at a rate of 5 °C/min, from room temperature to 500 °C, and kept for 6 h. The sample was then crushed and sieved to 40–60 meshes for the catalytic activity tests. The catalyst prepared through this procedure was denoted as CT-FeTi; whereas the catalyst prepared, for comparison purposes, without adding CTAB, was denoted as FeTi.
[NOX ]out ⎞ NOX conversion = ⎛1− × 100% with [NOX ] = [NO] + [NO2] [NOX ]in ⎠ ⎝ (1) ⎜
N2 selectivity =
⎟
[NOx ]in + [NH3]in −2[N2 O]out −[NOx ]out −[NH3]out [NOx ]in + [NH3]in −[NOx ]out −[NH3]out × 100%
To facilitate the discussion, the catalysts used were labelled as follows: CT-FeTi-X represents the sample prepared by the CTAB-assisted process reacted for 8 h under the above conditions in the presence of X % H2O, FeTi-X represents the sample without CTAB addition reacted for 8 h under the above conditions in the presence of X% H2O.
2.2. Catalyst characterization
3. Results and discussion
The Brunauer-Emmett-Teller (BET) surface area was measured by N2 adsorption equilibrium isotherms at -196 °C using a Micromeritics ASAP-2020 instrument. The pore size distributions were calculated by
3.1. Effect of H2O on activity and N2 selectivity Fig. 2A and B show the SCR activity of the FeTi and CT-FeTi 314
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Fig. 1. The schematic diagram of experimental apparatus for SCR activity. 1, 2 mass flow controller; 3, 4 rotameter; 5 water bath; 6 mixing chamber; 7 preheater; 8 furnace; 9 flue gas analyzer; 10 program temperature controller; 11 reactor; 12 quartz wool; 13 catalyst.
compared in Fig. 2C. In the absence of H2O, the NOx conversion over FeTi and CT-FeTi catalysts was 70% and 94%, respectively. With the introduction of 2.5 vol% H2O, the NOx conversion over FeTi catalyst decreased rapidly from 70% to 45%, and a decrease of about 80% was observed when the H2O content was 10 vol%. While the NOx conversion of the CT-FeTi catalyst decreased slowly with increasing H2O content from 0 to 7 vol%, and remained at approximately 80% in the presence of 10 vol% H2O. These results clearly indicated that H2O inhibited the SCR activity and that the addition of CTAB during the preparation process could enhance H2O resistance of the FeTi catalyst. Fig. 2D
catalysts at different temperatures. It can be seen that the steady NOx conversion of CT-FeTi catalyst in the absence of H2O at 210 °C, 240 °C and 270 °C was respectively 78%, 94% and 98%, which was higher than those of FeTi catalyst at the same conditions. Introduction of H2O resulted in a decrease of NOx conversion. What’s more, the lower the temperature, the more effect of H2O on SCR activity. Noticeably, compared to FeTi catalyst, the decreasing rate of NOx conversion over CT-FeTi catalyst became slower. In order to further study the effect of H2O on the two samples, the steady NOx conversion (at time on stream 8 h) of the two samples at 240 °C under different H2O contents was
Fig. 2. A: SCR activity of the FeTi catalyst at different temperatures. B: SCR activity of the CT-FeTi catalyst at different temperatures. C: Effect of H2O content on the SCR activity of the catalysts at 240 °C. D: Effect of H2O content on the N2 selectivity of the catalysts at 240 °C. Reaction conditions: 500 ppm NO, 500 ppm NH3, 6.5 vol.%O2, N2 balance and different H2O contents. 315
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Table 1 Textual property of catalysts. Catalyst
SBET/m2·g−1
Pore volume/cm3·g−1
FeTi FeTi-2.5%H2O FeTi-5% H2O FeTi-7%H2O FeTi-10%H2O CT-FeTi CT-FeTi-2.5%H2O CT-FeTi-5%H2O CT-FeTi-7%H2O CT-FeTi-10%H2O
61.98 63.74 62.06 58.50 54.50 90.65 92.08 94.94 99.14 98.91
0.172 0.173 0.172 0.178 0.179 0.178 0.183 0.185 0.186 0.184
illustrates the N2 selectivity of both catalysts for different H2O contents. It can be seen that the N2 selectivity of CT-FeTi catalyst was slightly higher than that of FeTi catalyst; however, the H2O content had no obvious impact on N2 selectivity. In most cases, catalyst deactivation is caused by competitive adsorption between H2O and reactant molecules when the introduction of H2O, as well as by loss of pore surface area resulting from capillary H2O condensation [14]. In the following sections we would investigate the role of CTAB in the improved H2O resistance. 3.2. Textural characteristics Results concerning the specific surface area and pore volume of the samples are summarized in Table 1 and the pore size distributions of the samples are displayed in Fig. 3. The CT-FeTi catalyst showed a large specific surface area of 90.65 m2/g and a pore volume of 0.178 cm3/g, both of which were significantly higher than the corresponding values measured for FeTi catalyst, 61.98 m2/g and 0.172 cm3/g, respectively. The increase of surface area reflected rightly the template role of CTAB. Before calcination, micelles initially occupied the pores of the catalyst precursor. When CTAB micelles were removed by calcination, the void spaces became the pores, leading to an increase of the surface area and pore volume [15]. On the other hand, in this case, the shapes of CTAB micelles controlled the pore structures of the calcined product. Thus uniform mesoporous structure was created on the CT-FeTi catalyst, which is confirmed by the pore size distribution of CT-FeTi catalyst (shown in Fig. 3). Meanwhile, Table 1 compares the surface properties of the FeTi and CT-FeTi catalysts in different H2O contents. An obvious enlargement of pore diameter may be noticed with the increase of H2O content, which led to an initial decrease followed by an increase in the surface area of the FeTi catalyst and a continuous increase in the surface area of the CT-FeTi catalyst. The results indicated that the moderate enlargement of pore diameter in the presence of H2O over the catalysts might be a positive factor on improving the surface area. However, the combination of these results with those of the SCR activity, which showed a continuous decrease for the CT-FeTi catalyst, allows to conclude that catalytic activity in the presence of H2O was also influenced by other factors in addition to the BET surface area.
Fig. 3. BJH pore size distribution of different catalysts.
competitive adsorption of H2O with NO. The amount of NO desorbed from the CT-FeTi catalyst was higher than the corresponding value for the FeTi catalyst in the absence of H2O, although the gap was reduced in the presence of H2O. Since the two catalysts had the same composition, the result revealed that catalyst structure plays a determining role in NO adsorption, which in turn affects the SCR reaction according to the Langmuir-Hinshelwood mechanism. To investigate the influence of H2O on NH3 adsorption over FeTi and CT-FeTi catalysts, TPD measurements were also carried out over the catalyst pre-adsorbed NH3 in the presence or absence of H2O. As shown in Fig. 4(B1, B2), broad desorption peaks spanned the temperature range of 150–400 °C and 100–500 °C for FeTi and CT-FeTi catalysts, respectively. This range includes the NH3 desorption peaks from weak (60–180 °C) and medium (180–400 °C) acid sites [16,17]. The acid amount per unit BET surface of the samples was calculated based on the area of desorption peak. In the absence of H2O, the results for FeTi and CT-FeTi catalysts were 0.61 and 0.75 µmol/g·m2, respectively. Obviously, the pore structure modified by CTAB introduced more surface acid sites on the surface of the CT-FeTi catalyst. Meanwhile, it was noted that the capacity of NH3 adsorption increased over both samples in the presence of H2O, and the acid amount of FeTi and CT-FeTi catalysts increased to 0.87 and 1.17 µmol/g·m2, respectively, demonstrating that H2O promoted NH3 adsorption and that the facilitating effect was more remarkable for the CT-FeTi catalyst.
3.3. Temperature programmed desorption (TPD) Previous studies had attributed the decrease of SCR activity in the presence of H2O to the competitive adsorption of H2O with NO and/or NH3 [7]; however, different studies reported that H2O could actually promote NH3 adsorption [10]. To understand the influence of H2O on NO adsorption over FeTi and CT-FeTi catalysts, temperature programmed desorption (TPD) experiments were conducted over the samples pre-adsorbed NO + O2 in the presence or absence of H2O. The desorption behaviors of NO are illustrated in Fig. 4(A1, A2). Both samples released NO at 100 °C, and the NO desorption capability declined obviously for the catalysts in the presence of H2O, indicating a 316
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Fig. 4. TPD of NO/NH3 on FeTi (A1, B1) and CT-FeTi (A2, B2) catalysts.
the surface of the catalyst. Moreover, it could be noted that the NO concentration in the presence of H2O reached saturation faster than in the absence of H2O, implying a competitive adsorption between NO and H2O (demonstrated by Section 3.3). The total NO consumption over the FeTi and CT-FeTi catalysts in the presence and absence H2O is listed in Table 2. Fig. 5A shows the TPSR results of the FeTi catalyst with pre-adsorbed NH3 (FeTi-a-50). The NO concentration began to decrease rapidly at a temperature of about 90 °C and reached its lowest level at 270 or 265 °C, depending on whether H2O was absent or present. Then the NO concentration began increasing until it reached a stable value, equal to the inlet concentration. The same behavior was observed for the CTFeTi catalyst (CT-FeTi-a-50), starting from about 90 °C and dropping at about 275 °C (Fig. 5B). The higher NO consumption, compared with the fresh catalyst, was attributed to NO reaction with the pre-adsorbed NH3. Table 2 lists the NO consumption over both catalysts under different conditions. It is generally accepted that the stoichiometric ratio
3.4. Temperature programmed surface reaction (TPSR) To further understand the H2O effect on the reaction between the adsorbed NH3 with NO, TPSR experiments were carried out in the absence and presence of 5 vol% H2O. Fig. 5 displays the NO concentration in the reactor effluent during the TPSR process over the fresh samples and the NH3-adsorbed samples. The corresponding reaction temperatures and NO consumptions are shown in Table 2. Since there was no NH3 in the feed during TPSR experiment, the decrease in NO concentration was ascribed to NO adsorption or reaction with NH3 preadsorbed on the catalysts. Fresh catalysts without pre-absorbed NH3 were first investigated in the presence and in absence of H2O, and the results are shown in Fig. 5 as curves a and b, respectively. The addition of NO at the inlet led to a rapid increase of NO concentration at the outlet, from 0 to 250 ppm (inlet value) when the temperature increased from 50 to 250 °C. Since the fresh samples contained no pre-adsorbed NH3, the amount of NO consumed resulted only from the adsorption on
Fig. 5. TPSR with NO + O2 in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts. Reaction conditions: 250 ppm NO, 6.5 vol% O2, 5.0% H2O (when used), balance N2, space velocity of 160,000 h−1. 317
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Table 2 The reaction behavior of NO with adsorbed NH3. Samples
FeTi
CT-FeTi
FeTi-a-50*
CT-FeTi-a50*
Reaction condition
In of In of In of In of In of In of In of In of
the absence H2O the presence H2O the absence H2O the presence H2O the absence H2O the presence H2O the absence H2O the presence H2O
Reaction temperature/°C Starting
Peaking
–
–
NO consumption/ µmol/g
3.3 1.6
–
–
3.8 2.4
90
270
18.0
90
265
12.6
90
275
25.2
90
275
21.1
* FeTi-a-50 represents the FeTi catalyst adsorbed with NH3 at 50 °C, CT-FeTia-50 represents the CT-FeTi catalyst adsorbed with NH3 at 50 °C.
of NH3 to NO in the SCR reaction is 1 [18], thus the amount of NO consumed in addition to that measured on the fresh catalysts (curve a/ b), could be used to estimate the amount of NH3 adsorbed on the catalysts [20,24], which amounted to 18.0 µmol/g for FeTi-a-50 and 25.2 µmol/g for CT-FeTi-a-50 in the absence of H2O. This suggests that the catalyst prepared by the CTAB-assisted process could adsorb more NH3, indicating that the structure and surface properties might be adjusted during the preparation process. On the other hand, NO consumption in the presence of H2O was always lower than that in the absence of H2O (12.6 µmol/g in the presence of H2O and 18.0 µmol/g in the absence of H2O for FeTi-a-50; 21.1 µmol/g in the presence of H2O and 25.2 µmol/g in the absence of H2O for CT-FeTi-a-50), suggesting that the presence of H2O resulted to less amount of NO being reacted. Meanwhile, in the presence of H2O, the decrease of NO consumption for CT-FeTi-a-50 was less pronounced than the one for FeTi-a-50, indicating that H2O had a milder inhibition effect on the catalyst modified by CTAB, thereby improving the SCR performance of CT-FeTi catalyst in the presence of H2O.
Fig. 6. DRIFT spectra of NO + O2 at 240 °C in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts.
3.5.2. In situ DRIFT spectra of NH3 adsorption over FeTi and CT-Fe/Ti catalysts NH3 adsorption on the FeTi and CT-FeTi catalysts at 240 °C were carried out in the absence and in presence of H2O, and the results are presented in Fig. 7. After NH3 (1000 ppm) adsorption and N2 purging, several bands at 1645, 1602, 1565, 1437, 1192, 3650 cm−1 and a broad band in the range of 3100–3400 cm−1 were detected. The bands at 1602 and 1192 cm−1 were attributed to the asymmetric and symmetric deformation of coordinated ammonia linked to Lewis acid sites, while the bands at 1645 and 1437 cm−1 were assigned to asymmetric and symmetric bending vibrations of the NH4+ species on Brønsted acid sites [21]. In the N-H stretching region, bands at 3340/3242 cm−1 and 3137 cm−1 were attributed to N-H stretching vibration modes of coordinated NH3 and NH4+ species, respectively [22,23]. The band around 3650 cm−1 was assigned to the surface –OH group, indicating the presence of H2O [24]. In addition, the band at 1565 cm−1 implied the existence of amide species (–NH2), which was formed through the partial oxidation (activation) of NH3 in the presence of Fe3+ [25]. In the absence of H2O, the Brønsted acid sites (1645, 1437 cm−1) on CTFeTi catalyst were slightly stronger than on FeTi catalyst. In addition, the intensity of the bands at 1602 and 1192 cm−1 on FeTi catalyst were significantly lower than on CT-FeTi catalyst. In fact, the band at 1602 cm−1 on FeTi catalyst was not detected at all due to weakly adsorbed NH3. These results suggested that the catalyst prepared by a
3.5. In situ DRIFTS studies 3.5.1. In situ DRIFT spectra of NO + O2 adsorption over FeTi and CT-Fe/Ti catalysts To further understand which form of NO species is influenced by the competitive adsorption of H2O on the catalysts, the DRIFT spectra of NO + O2 were recorded both in the absence and in presence of H2O. The results are illustrated in Fig. 6. Several bands at 1625/1612, 1576 and 1240 cm−1 appeared after the two samples were treated with 1000 ppm NO + 6.5 vol% O2/N2 for 30 min and purged with N2 for further 15 min at 240 °C. The bands at 1625/1612 and 1240 cm−1 were attributed to bridging nitrates, and the band at 1576 cm−1 could be assigned to bidentate nitrates [11,19,20]. It is evident that both catalysts were predominantly covered by bridging nitrate. In the absence of H2O, the bridging nitrate on CT-FeTi was stronger than that on FeTi catalyst, indicating that the structure and properties of the catalyst prepared by the CTAB-assisted process could enhance the adsorption of this species. The addition of H2O resulted in the disappearance of bidentate nitrate and a reduction of bridging nitrate on the FeTi catalyst. However, the CT-FeTi catalyst displayed a good resistance to H2O. Although the bidentate nitrate decreased and nearly disappeared, the bridging nitrate decreased slowly and still maintained higher signal intensity compared to the one measured on FeTi, possibly favoring the SCR reaction of NH3. 318
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Fig. 7. DRIFT spectra of NH3 at 240 °C in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts.
Fig. 8. In situ DRIFT spectra of reaction in NH3 + NO + O2 at 240 °C over CTFeTi catalyst in the absence (A) and presence (B) of H2O.
CTAB-assisted process might favor the adsorption of NH3 on Lewis acid sites. With the addition of H2O, it was found that the band intensity of the –OH group increased noticeably on both catalysts. Meanwhile, the band intensity at 1602/1192 cm−1 decreased whereas the band intensity at 1437/1645 cm−1 increased, implying a decrease in Lewis acid and an increase on Brønsted acid sites. Accordingly, H2O promoted the adsorption of NH3 on Brønsted acid sites while inhibiting the adsorption of NH3 on Lewis acid sites; however, the inhibiting effect on CT-FeTi catalyst was weaker than on FeTi and a relatively strong adsorption on Lewis acid sites could still observed clearly on the former. Since Lewis acids played an important role in the SCR reaction [26], thus CT-FeTi catalyst exhibited a better catalytic performance compared to FeTi catalyst. Moreover, it was interesting to note that the band at 1565 cm−1 disappeared on both catalysts upon addition of H2O. This result indicated that partial oxidation of NH3 by the Fe3+ species on the surface of the catalysts might be inhibited, causing a decline in catalytic performance.
been caused by the overlap of bands of bridging nitrate and coordinated NH3. The band at 1565 cm−1 indicated the formation of the –NH2 species, which is an important intermediate in the reaction. A new band appeared at 1360 cm−1, which was quite different from those due to the adsorbed NOx and NH3 species mentioned above. Chen et al. [27] attributed this band to the intermediate species deriving from the combination of adsorbed NH3 and NOx. It can be seen that, in the presence of H2O, the Brønsted acids (1645 and 1437 cm−1) were strengthened, while the Lewis acids were slightly weakened. The decrease in intensity of the band at 1608 cm−1 might result from the weak adsorption of bridging nitrate and coordinated NH3. Simultaneously, it is worth noting that the peak due to the –NH2 intermediated declined rapidly. It is generally accepted that in the SCR reaction, NH3 is initially adsorbed on Brønsted or Lewis acid sites on the surface of the catalyst to form intermediate species (–NH2) that subsequently react with gaseous NO or adsorbed nitrate species, corresponding to the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms [28,29]. Based on this reaction scheme, as well as on earlier reports [27,30], a simplified reaction recycle on the CT-FeTi catalyst most probably took place, as shown in Fig. 9. NH3 adsorption was reported to be an important step to influence the SCR reaction. In the absence of H2O, the enhanced adsorption of NH3 on Lewis acids (Fig. 7) might lead to a higher catalytic activity of CT-FeTi compared to FeTi, and the high intensity of intermediate species (–NH2) demonstrated that a higher amount of NH3 was activated to participate in the reaction. The high quantity of bridging
3.5.3. Reaction mechanism over CT-FeTi catalyst in the absence and presence of H2O In order to further investigate the reaction mechanism over CT-FeTi catalyst, an in situ DRIFT experiment of the NH3-SCR reaction as a function of time was conducted and the results are shown in Fig. 8. During the whole process, the bands relative to the NH3 species (1192, 1437 and 1645 cm−1) dominated. Bands at 1608 cm−1 might have 319
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Fig. 9. Possible reaction recycle for NH3-SCR over the CT-FeTi catalysts.
nitrate species on CT-FeTi catalyst (Fig. 6) also played an important role in the improved catalytic activity according to the L-H mechanism. In the presence of H2O, a stronger adsorption of NH3 on Brønsted acids was observed on CT-FeTi catalyst, however, catalytic performance was still decreased according to the result of activity test. This might due to the decrease in the absorbed intermediate species (–NH2). Since the intermediate species (–NH2) was usually formed on Fe-based catalysts through the reaction: Fe3 + + NH3 → Fe2 +−NH2 + H+ [30], the presence of H2O obviously inhibited the process, further influencing the SCR reaction.
1044–1050. [2] D.W. Kwon, K.H. Park, S.C. Hong, Enhancement of SCR activity and SO2 resistance on VOx/TiO2 catalyst by addition of molybdenum, Chem. Eng. J. 284 (2016) 315–324. [3] C.J. Tang, H.L. Zhang, L. Dong, Ceria-based catalysts for low-temperature selective catalytic reduction of NO with NH3, Catal. Sci. Technol. 6 (2016) 1248–1264. [4] Z.M. Liu, H. Su, B.H. Chen, J.H. Li, S.I. Woo, Activity enhancement of WO3 modified Fe2O3 catalyst for the selective catalytic reduction of NOx by NH3, Chem. Eng. J. 299 (2016) 255–262. [5] J. Shi, Z.H. Zhang, M.X. Chen, Z.X. Zhang, W.F. Shangguan, Promotion effect of tungsten and iron co-addition on the catalytic performance of MnOx/TiO2 for NH3SCR of NOx, Fuel 210 (2017) 783–789. [6] F.Y. Gao, X.L. Tang, H.H. Yi, J.Y. Li, S.Z. Zhao, J.G. Wang, C. Chu, C.L. Li, Promotional mechanisms of activity and SO2 tolerance of Co- or Ni-doped MnOxCeO2 catalysts for SCR of NOx with NH3 at low temperature, Chem. Eng. J. 317 (2017) 20–31. [7] F. Giraud, J. Couble, C. Geantet, N. Guilhaume, E. Puzenat, S. Gros, L. Porcheron, M. Kanniche, D. Bianchi, Experimental microkinetic approach of De-NOx by NH3 on V2O5/WO3/TiO2 catalysts. 4. Individual heats of adsorption of adsorbed H2O species on sulfate-free and sulfated TiO2 supports, J. Phys. Chem. C 119 (2015) 16089–16105. [8] I. Nova, L. Lietti, E. Tronconi, P. Forzatti, Transient response method applied to the kinetic analysis of the DeNOx-SCR reaction, Chem. Eng. Sci. 56 (2001) 1229–1237. [9] N.Y. Topsøe, T. Slabiak, B.S. Clausen, T.Z. Srnak, J.A. Dumesic, Influence of water on the reactivity of vanadia/titania for catalytic reduction of NOx, J. Catal. 134 (1992) 742–746. [10] Z.G. Huang, Z.Y. Liu, X.L. Zhang, Q.Y. Liu, Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature, Appl. Catal. B 63 (2006) 260–265. [11] Y.N. Shi, S. Chen, H. Sun, Y. Shu, X. Quan, Low-temperature selective catalytic reduction of NOx with NH3 over hierarchically macro-mesoporous Mn/TiO2, Catal. Commun. 42 (2013) 10–13. [12] S.G. Wu, L. Zhang, X.B. Wang, W.X. Zou, Y. Cao, J.F. Sun, C.J. Tang, F. Gao, Y. Deng, L. Dong, Synthesis, characterization and catalytic performance of FeMnTiOx mixed oxides catalyst prepared by a CTAB-assisted process for mid-low temperature NH3-SCR, Appl. Catal. A 505 (2015) 235–242. [13] S.H. Zhan, D.D. Zhu, S.S. Yang, M.Y. Qiu, Y. Li, H.B. Yu, Z.Q. Shen, Sol-gel preparation of mesoporous cerium-doped FeTi nanocatalysts and its SCR activity of NOx with NH3 at low temperature, J. Sol-gel. Sci. Technol. 73 (2015) 443–451. [14] Z.G. Huang, Z.P. Zhu, Z.Y. Liu, Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures, Appl. Catal. B 39 (2002) 361–368. [15] Q. Liu, W.M. Zhang, Z.M. Cui, B. Zhang, L.J. Wan, W.G. Song, Aqueous route for mesoporous metal oxides using inorganic metal source and their applications, Microporous Mesoporous Mater. 100 (2007) 233–240. [16] L.Q. Chen, R. Li, Z.B. Li, F.L. Yuan, X.Y. Niu, Y.J. Zhu, Effect of Ni doping in NixMn1xTi10 (x 0.1–0.5) on activity and SO2 resistance for NH3-SCR of NO studied with in situ DRIFTS, Catal Sci. Technol. 7 (2017) 3243–3257. [17] T.Y. Du, H.X. Qu, Q. Liu, Q. Zhong, W.H. Ma, Synthesis, activity and hydrophobicity of Fe-ZSM-5@silicalite-1 for NH3-SCR, Chem. Eng. J. 262 (2015) 1199–1207. [18] W.C. Wong, K. Nobe, Reduction of nitric oxide with ammonia on alumina and titania-supported metal oxide catalysts, Ind. Eng. Chem. Prod. Res. 25 (1986) 179–186. [19] Z. Si, D. Weng, X. Wu, J. Li, G. Li, Structure, acidity and activity of CuOx/WOx-ZrO2 catalyst for selective catalytic reduction of NO by NH3, J. Catal. 271 (2010) 43–51. [20] J. Fan, P. Ning, Z.X. Song, X. Liu, L.Y. Wang, J. Wang, H.M. Wang, K.X. Long, Q.L. Zhang, Mechanistic aspects of NH3-SCR reaction over CeO2/TiO2-ZrO2-SO42− catalyst: in situ DRIFTS investigation, Chem. Eng. J. 334 (2018) 855–863. [21] C.L. Yu, B.C. Huang, L.F. Dong, F. Chen, X.Q. Liu, In situ FT-IR study of highly dispersed MnOx/SAPO-34 catalyst for low-temperature selective catalytic reduction
4. Conclusion CT-FeTi catalysts were synthesized using CTAB as a template-directing reagent, and applied to the selective catalytic reduction of NO with NH3. Comparing with FeTi catalysts, CT-FeTi catalysts exhibited an excellent SCR activity with over 80% of NO conversion achieved at 240 °C in a simulated flue gas containing 500 ppm NO, 500 ppm NH3 and 6.5 vol% O2. Meanwhile, CT-FeTi catalyst showed good H2O resistance when H2O was included into the flue gas, whereas the activity of FeTi catalyst decreased rapidly and continuously. BET analyses of the both catalysts indicated that the addition of CTAB during preparation process promoted the formation of uniform mesoporous structure, which was beneficial to avoid being excessively enlarged in the presence of H2O. Results of TPD and TPSR showed that H2O inhibited the NO adsorption and the reaction of NO with adsorbed NH3, however, the inhibition effect on CT-FeTi catalysts was less pronounced than that on FeTi catalysts. In addition, in situ DRIFTS revealed that the formation of the intermediate species (–NH2) during the reaction: Fe3+ + NH3 → Fe2+−NH2 + H+was inhibited when the introduction of H2O, and the more adsorption of bridging nitrate and NH3 species on Lewis acid sites on the surface of CT-FeTi catalysts might be an important factor to improve catalytic activity. All of these results showed that CTAB could efficiently modify the structure and surface properties of the catalysts, leading to the enhancement of SCR activity and H2O. Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFC0210200) and Development Program of Shanxi Province (No. 201703D111018). References [1] Z.M. Liu, Y.X. Liu, Y. Li, H. Su, L.L. Ma, WO3 promoted Mn-Zr mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, Chem. Eng. J. 283 (2016)
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of NOx by NH3, Catal. Today 281 (2017) 610–620. [22] D.K. Sun, Q.Y. Liu, Z.Y. Liu, G.Q. Gui, Z.G. Huang, Adsorption and oxidation of NH3 over V2O5/AC surface, Appl. Catal. B 92 (2009) 462–467. [23] Y.H. Li, W.Y. Song, J. Liu, Z. Zhao, M.L. Gao, Y.C. Wei, Q. Wang, J.L. Deng, The protection of CeO2 thin film on Cu-SAPO-18 catalyst for highly stable catalytic NH3SCR performance, Chem. Eng. J. 330 (2017) 926–935. [24] Z.B. Wu, B.Q. Jiang, Y. Liu, H.Q. Wang, R.B. Jin, DRIFT study of manganese/titaniabased catalysts for low-temperature selective catalytic reduction of NO with NH3, Environ. Sci. Technol. 41 (2007) 5812–5817. [25] 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. [26] Y. Li, Y.P. Li, P.F. Wang, W.P. Hu, S.G. Zhang, Q. Shi, S.H. Zhan, Low-temperature selective catalytic reduction of NOx with NH3 over MnFeOx nanorods, Chem. Eng. J.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Role of CTAB in the improved H2O resistance for selective catalytic reduction of NO with NH3 over iron titanium catalyst
T
⁎
Yulin Lia,b, Xiaojin Hana, Yaqin Houa,b, , Yaoping Guoa,b, Yongjin Liua,b, Yan Cuia, ⁎ Zhanggen Huanga, a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
catalysts showed high SCR • CT-FeTi activity and good H O resistance. optimized the pore size to avoid • CTAB being excessively enlarged in the pre2
sence of H2O.
promoted the adsorption of • CTAB bridging nitrate and NH species on 3
Lewis acid sites.
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron titanium catalyst CTAB H2O-resistance NH3-SCR
Uniform mesoporous iron titanium (CT-FeTi) catalysts were synthesized by a CTAB-assisted process and exhibited good catalytic activity and H2O resistance when tested in the selective catalytic reduction (SCR) of NO with NH3 at low temperature. BET, TPD, TPSR and in situ DRIFTS were carried out to reveal the inhibition mechanism of H2O on SCR reaction and determine the role of CTAB in the improved H2O resistance during lowtemperature SCR processes. Results showed that H2O inhibited the adsorption of NO on the surface of CT-FeTi catalysts, as well as the formation of the intermediate species (–NH2), which was produced by the reaction Fe3+ + NH3 → Fe2+−NH2 + H+ . CTAB acted as a “structural” and “chemical” promoter, not only optimizing the pore size to avoid being excessively enlarged in the presence of H2O, but also enhancing the adsorption of bridging nitrate and NH3 species on Lewis acid sites, thus improving the catalytic activity and H2O resistance.
1. Introduction Nitrogen oxides (NOx) emitted from stationary sources have been a major source of atmospheric pollution, and can cause environmental problems such as acid rain, photochemical smog and ozone depletion [1]. The selective catalytic reduction of NOx with NH3 over V2O5/TiO2
⁎
or V2O5-WO3/TiO2 catalysts is a well proven technology for the removal of nitrogen oxides from stationary sources [2], however, this process must operate at temperatures higher than 350–450 °C to avoid SO2 poisoning. So the low temperature SCR process is preferable because it allows the reactor to be installed downstream of the particle controller and desulfurization devices without need to reheat the flue
Corresponding authors at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China (Y. Hou). E-mail addresses: [email protected] (Y. Hou), [email protected] (Z. Huang).
https://doi.org/10.1016/j.cej.2018.04.107 Received 13 February 2018; Received in revised form 4 April 2018; Accepted 13 April 2018 Available online 18 April 2018 1385-8947/ © 2018 Published by Elsevier B.V.
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the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the isotherms. Prior to the surface area measurements, the samples were degassed under vacuum conditions at 90 °C for 10 h The NO/NH3 temperature programmed desorption measurement was carried out with catalyst samples of 0.25 g under a total flow rate of 100 ml/min. Before experiment, the samples were pretreated in situ at 300 °C in a flow of pure N2 for 0.5 h and subsequently cooled to 90 °C. Then the samples were treated with 1000 ppm NO/N2 and 6.5 vol% O2 or 1000 ppm NH3/N2 in the presence of 5 vol% H2O for 1 h. The NO/ NH3 was purged with N2 for 2 h at 90 °C to remove the physically adsorbed species. Desorption was carried out by heating samples from 90 °C to 500 °C, and the NO/NH3 was continuously monitored using a portable Fourier Transform Infrared (FT-IR) gas analyzer. Temperature-programmed surface reaction (TPSR) was performed to study the reactivity of adsorbed NH3 with NO on the surface of the catalysts in the absence or presence of H2O. The samples that had adsorbed NH3 were exposed to a gas stream containing 250 ppm of NO/N2 and 6.5 vol% O2 in the absence or presence of 5 vol% H2O and heated from 50 to 300 °C at a rate of 10 °C/min. The temperature was kept at 300 °C until the outlet NO concentration became equal to the inlet value (250 ppm). The outlet NO concentration was continuously measured using a portable FT-IR gas analyzer. In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) were recorded from 4000 to 1000 cm−1 at a spectral resolution of 4 cm−1 on a FTIR spectrometer (Bruker Tensor 27) equipped with an in situ diffuse reflectance cell containing a MCT detector and ZnSe windows. Prior to each spectrum recording, the samples were purged at 350 °C in N2 for 30 min, and cooled down to 240 °C, while the background spectrum with N2 flow was recorded. Finally, the sample was exposed to a controlled steam flow of 100 ml/min, containing 500 ppm NH3/N2 or 500 ppm NO/N2 and 6.5 vol% O2 in the absence or presence of 5 vol% H2O. The adsorption/reaction spectra were recorded for various target temperatures or times by subtraction of the corresponding background reference.
gas. In recent years, catalysts active at low temperature, such as Mn, Fe and Ce loaded on different supports [3–5], have been studied, with a focus on their susceptibility to SO2 poisoning and proposed methods to increase SO2 resistance [6]. On the other hand, the inhibition effect of H2O on the catalytic activity at low temperature has often been neglected, although it is a critical factor. The inhibition effect of H2O on SCR has been previously studied on other catalyst systems, such as vanadium-based catalysts and activated carbon. The main conclusions reached may be summarized that H2O might cause competitive adsorption with reactant molecules or/and inhibit the reaction of NO with adsorbed NH3 [7,8]. Many studies ascribed the inhibition effect of H2O to the competitive adsorption with NO/NH3 on the active sites of the catalysts; on the contrary, Topsøe [9] found that H2O promotes NH3 adsorption on V2O5/TiO2 catalysts. Similarly, Huang et al. [10] also found that H2O does not compete with NO and NH3 for active sites on the V2O5/AC catalyst, but rather promotes NH3 adsorption. The lack of agreement on this issue, even for the commercial vanadium-based catalysts suggests the necessity to study the effect of H2O to develop novel low temperature catalysts. Recently, an interesting approach was reported for the preparation of low temperature SCR catalysts using structure directing agents such as Pluronic F127, Pluronic P123 and cetyl trimethyl ammonium bromide (CTAB). Shi et al. [11] used F127 as the structural template to synthesize hierarchically macro-mesoporous Mn/TiO2, which showed a more promising behavior compared to the non-modified catalyst. Wu et al. [12] found that the introduction of CTAB in the preparation of the FeMnTi catalyst increased its activity and the operating temperature window. Zhan et al. [13] used Pluronic P123 to synthesize mesoporous FeCeTi nanocatalysts that exhibited high SCR activity. On the other hand, the effect of H2O on the catalytic activity of catalysts prepared using structure directing agents has been rarely reported. In present work, a novel iron titanium catalyst was synthesized using CTAB and applied to the NH3-SCR process at low temperature. It was found that the addition of CTAB changed the pore structure and enhanced the catalytic performance of FeTi catalysts both in the absence and in the presence of H2O. Based on the characterization results, the mechanisms of the influence of CTAB on activity and H2O resistance were further elucidated. The main objective was to clarify the actual role of CTAB in the H2O resistance at low temperatures, thus providing a reference for designing novel H2O resistance catalysts based on structure directing agents.
2.3. Catalyst performance tests The schematic diagram of experimental apparatus for SCR measurement is displayed in Fig. 1 .Catalytic tests were conducted in a fixbed flow reactor (length 250 mm, internal diameter 12 mm) with a feed of 500 ppm NO, 500 ppm NH3, 6.5 vol% O2/N2, and a GHSV of 18,000 h−1 using 1 g of catalyst particles of 40–60 mesh, and a total flow of 400 ml/min·H2O vapour was generated by passing N2 and O2 through a heated gas-wash bottle containing deionized water. The NOx concentrations in the inlet and outlet gas were analyzed simultaneously by a flue gas analyzer (KM9106 Quintox, Kane International Limited).
2. Experimental 2.1. Catalyst preparation All chemicals used were of analytical grade and procured from Sinopharm Chemical Reagent Company. The catalyst was prepared by a sol–gel method using CTAB as template-directing regent. In a typical synthesis process, absolute ethanol (1 mol), CTAB (0.005 mol) and tetrabutyltitanate (0.05 mol) were mixed under vigorous stirring at room temperature to form a transparent yellow solution. Fe (NO3)3·9H2O (0.007 mol) was then added to the above solution. After stirring for 2 h, the mixture was aged at 60 °C for 12 h and dried at 110 °C overnight. Finally, the resulting powder was calcinated in air at a rate of 5 °C/min, from room temperature to 500 °C, and kept for 6 h. The sample was then crushed and sieved to 40–60 meshes for the catalytic activity tests. The catalyst prepared through this procedure was denoted as CT-FeTi; whereas the catalyst prepared, for comparison purposes, without adding CTAB, was denoted as FeTi.
[NOX ]out ⎞ NOX conversion = ⎛1− × 100% with [NOX ] = [NO] + [NO2] [NOX ]in ⎠ ⎝ (1) ⎜
N2 selectivity =
⎟
[NOx ]in + [NH3]in −2[N2 O]out −[NOx ]out −[NH3]out [NOx ]in + [NH3]in −[NOx ]out −[NH3]out × 100%
To facilitate the discussion, the catalysts used were labelled as follows: CT-FeTi-X represents the sample prepared by the CTAB-assisted process reacted for 8 h under the above conditions in the presence of X % H2O, FeTi-X represents the sample without CTAB addition reacted for 8 h under the above conditions in the presence of X% H2O.
2.2. Catalyst characterization
3. Results and discussion
The Brunauer-Emmett-Teller (BET) surface area was measured by N2 adsorption equilibrium isotherms at -196 °C using a Micromeritics ASAP-2020 instrument. The pore size distributions were calculated by
3.1. Effect of H2O on activity and N2 selectivity Fig. 2A and B show the SCR activity of the FeTi and CT-FeTi 314
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Fig. 1. The schematic diagram of experimental apparatus for SCR activity. 1, 2 mass flow controller; 3, 4 rotameter; 5 water bath; 6 mixing chamber; 7 preheater; 8 furnace; 9 flue gas analyzer; 10 program temperature controller; 11 reactor; 12 quartz wool; 13 catalyst.
compared in Fig. 2C. In the absence of H2O, the NOx conversion over FeTi and CT-FeTi catalysts was 70% and 94%, respectively. With the introduction of 2.5 vol% H2O, the NOx conversion over FeTi catalyst decreased rapidly from 70% to 45%, and a decrease of about 80% was observed when the H2O content was 10 vol%. While the NOx conversion of the CT-FeTi catalyst decreased slowly with increasing H2O content from 0 to 7 vol%, and remained at approximately 80% in the presence of 10 vol% H2O. These results clearly indicated that H2O inhibited the SCR activity and that the addition of CTAB during the preparation process could enhance H2O resistance of the FeTi catalyst. Fig. 2D
catalysts at different temperatures. It can be seen that the steady NOx conversion of CT-FeTi catalyst in the absence of H2O at 210 °C, 240 °C and 270 °C was respectively 78%, 94% and 98%, which was higher than those of FeTi catalyst at the same conditions. Introduction of H2O resulted in a decrease of NOx conversion. What’s more, the lower the temperature, the more effect of H2O on SCR activity. Noticeably, compared to FeTi catalyst, the decreasing rate of NOx conversion over CT-FeTi catalyst became slower. In order to further study the effect of H2O on the two samples, the steady NOx conversion (at time on stream 8 h) of the two samples at 240 °C under different H2O contents was
Fig. 2. A: SCR activity of the FeTi catalyst at different temperatures. B: SCR activity of the CT-FeTi catalyst at different temperatures. C: Effect of H2O content on the SCR activity of the catalysts at 240 °C. D: Effect of H2O content on the N2 selectivity of the catalysts at 240 °C. Reaction conditions: 500 ppm NO, 500 ppm NH3, 6.5 vol.%O2, N2 balance and different H2O contents. 315
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Table 1 Textual property of catalysts. Catalyst
SBET/m2·g−1
Pore volume/cm3·g−1
FeTi FeTi-2.5%H2O FeTi-5% H2O FeTi-7%H2O FeTi-10%H2O CT-FeTi CT-FeTi-2.5%H2O CT-FeTi-5%H2O CT-FeTi-7%H2O CT-FeTi-10%H2O
61.98 63.74 62.06 58.50 54.50 90.65 92.08 94.94 99.14 98.91
0.172 0.173 0.172 0.178 0.179 0.178 0.183 0.185 0.186 0.184
illustrates the N2 selectivity of both catalysts for different H2O contents. It can be seen that the N2 selectivity of CT-FeTi catalyst was slightly higher than that of FeTi catalyst; however, the H2O content had no obvious impact on N2 selectivity. In most cases, catalyst deactivation is caused by competitive adsorption between H2O and reactant molecules when the introduction of H2O, as well as by loss of pore surface area resulting from capillary H2O condensation [14]. In the following sections we would investigate the role of CTAB in the improved H2O resistance. 3.2. Textural characteristics Results concerning the specific surface area and pore volume of the samples are summarized in Table 1 and the pore size distributions of the samples are displayed in Fig. 3. The CT-FeTi catalyst showed a large specific surface area of 90.65 m2/g and a pore volume of 0.178 cm3/g, both of which were significantly higher than the corresponding values measured for FeTi catalyst, 61.98 m2/g and 0.172 cm3/g, respectively. The increase of surface area reflected rightly the template role of CTAB. Before calcination, micelles initially occupied the pores of the catalyst precursor. When CTAB micelles were removed by calcination, the void spaces became the pores, leading to an increase of the surface area and pore volume [15]. On the other hand, in this case, the shapes of CTAB micelles controlled the pore structures of the calcined product. Thus uniform mesoporous structure was created on the CT-FeTi catalyst, which is confirmed by the pore size distribution of CT-FeTi catalyst (shown in Fig. 3). Meanwhile, Table 1 compares the surface properties of the FeTi and CT-FeTi catalysts in different H2O contents. An obvious enlargement of pore diameter may be noticed with the increase of H2O content, which led to an initial decrease followed by an increase in the surface area of the FeTi catalyst and a continuous increase in the surface area of the CT-FeTi catalyst. The results indicated that the moderate enlargement of pore diameter in the presence of H2O over the catalysts might be a positive factor on improving the surface area. However, the combination of these results with those of the SCR activity, which showed a continuous decrease for the CT-FeTi catalyst, allows to conclude that catalytic activity in the presence of H2O was also influenced by other factors in addition to the BET surface area.
Fig. 3. BJH pore size distribution of different catalysts.
competitive adsorption of H2O with NO. The amount of NO desorbed from the CT-FeTi catalyst was higher than the corresponding value for the FeTi catalyst in the absence of H2O, although the gap was reduced in the presence of H2O. Since the two catalysts had the same composition, the result revealed that catalyst structure plays a determining role in NO adsorption, which in turn affects the SCR reaction according to the Langmuir-Hinshelwood mechanism. To investigate the influence of H2O on NH3 adsorption over FeTi and CT-FeTi catalysts, TPD measurements were also carried out over the catalyst pre-adsorbed NH3 in the presence or absence of H2O. As shown in Fig. 4(B1, B2), broad desorption peaks spanned the temperature range of 150–400 °C and 100–500 °C for FeTi and CT-FeTi catalysts, respectively. This range includes the NH3 desorption peaks from weak (60–180 °C) and medium (180–400 °C) acid sites [16,17]. The acid amount per unit BET surface of the samples was calculated based on the area of desorption peak. In the absence of H2O, the results for FeTi and CT-FeTi catalysts were 0.61 and 0.75 µmol/g·m2, respectively. Obviously, the pore structure modified by CTAB introduced more surface acid sites on the surface of the CT-FeTi catalyst. Meanwhile, it was noted that the capacity of NH3 adsorption increased over both samples in the presence of H2O, and the acid amount of FeTi and CT-FeTi catalysts increased to 0.87 and 1.17 µmol/g·m2, respectively, demonstrating that H2O promoted NH3 adsorption and that the facilitating effect was more remarkable for the CT-FeTi catalyst.
3.3. Temperature programmed desorption (TPD) Previous studies had attributed the decrease of SCR activity in the presence of H2O to the competitive adsorption of H2O with NO and/or NH3 [7]; however, different studies reported that H2O could actually promote NH3 adsorption [10]. To understand the influence of H2O on NO adsorption over FeTi and CT-FeTi catalysts, temperature programmed desorption (TPD) experiments were conducted over the samples pre-adsorbed NO + O2 in the presence or absence of H2O. The desorption behaviors of NO are illustrated in Fig. 4(A1, A2). Both samples released NO at 100 °C, and the NO desorption capability declined obviously for the catalysts in the presence of H2O, indicating a 316
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Fig. 4. TPD of NO/NH3 on FeTi (A1, B1) and CT-FeTi (A2, B2) catalysts.
the surface of the catalyst. Moreover, it could be noted that the NO concentration in the presence of H2O reached saturation faster than in the absence of H2O, implying a competitive adsorption between NO and H2O (demonstrated by Section 3.3). The total NO consumption over the FeTi and CT-FeTi catalysts in the presence and absence H2O is listed in Table 2. Fig. 5A shows the TPSR results of the FeTi catalyst with pre-adsorbed NH3 (FeTi-a-50). The NO concentration began to decrease rapidly at a temperature of about 90 °C and reached its lowest level at 270 or 265 °C, depending on whether H2O was absent or present. Then the NO concentration began increasing until it reached a stable value, equal to the inlet concentration. The same behavior was observed for the CTFeTi catalyst (CT-FeTi-a-50), starting from about 90 °C and dropping at about 275 °C (Fig. 5B). The higher NO consumption, compared with the fresh catalyst, was attributed to NO reaction with the pre-adsorbed NH3. Table 2 lists the NO consumption over both catalysts under different conditions. It is generally accepted that the stoichiometric ratio
3.4. Temperature programmed surface reaction (TPSR) To further understand the H2O effect on the reaction between the adsorbed NH3 with NO, TPSR experiments were carried out in the absence and presence of 5 vol% H2O. Fig. 5 displays the NO concentration in the reactor effluent during the TPSR process over the fresh samples and the NH3-adsorbed samples. The corresponding reaction temperatures and NO consumptions are shown in Table 2. Since there was no NH3 in the feed during TPSR experiment, the decrease in NO concentration was ascribed to NO adsorption or reaction with NH3 preadsorbed on the catalysts. Fresh catalysts without pre-absorbed NH3 were first investigated in the presence and in absence of H2O, and the results are shown in Fig. 5 as curves a and b, respectively. The addition of NO at the inlet led to a rapid increase of NO concentration at the outlet, from 0 to 250 ppm (inlet value) when the temperature increased from 50 to 250 °C. Since the fresh samples contained no pre-adsorbed NH3, the amount of NO consumed resulted only from the adsorption on
Fig. 5. TPSR with NO + O2 in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts. Reaction conditions: 250 ppm NO, 6.5 vol% O2, 5.0% H2O (when used), balance N2, space velocity of 160,000 h−1. 317
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Table 2 The reaction behavior of NO with adsorbed NH3. Samples
FeTi
CT-FeTi
FeTi-a-50*
CT-FeTi-a50*
Reaction condition
In of In of In of In of In of In of In of In of
the absence H2O the presence H2O the absence H2O the presence H2O the absence H2O the presence H2O the absence H2O the presence H2O
Reaction temperature/°C Starting
Peaking
–
–
NO consumption/ µmol/g
3.3 1.6
–
–
3.8 2.4
90
270
18.0
90
265
12.6
90
275
25.2
90
275
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* FeTi-a-50 represents the FeTi catalyst adsorbed with NH3 at 50 °C, CT-FeTia-50 represents the CT-FeTi catalyst adsorbed with NH3 at 50 °C.
of NH3 to NO in the SCR reaction is 1 [18], thus the amount of NO consumed in addition to that measured on the fresh catalysts (curve a/ b), could be used to estimate the amount of NH3 adsorbed on the catalysts [20,24], which amounted to 18.0 µmol/g for FeTi-a-50 and 25.2 µmol/g for CT-FeTi-a-50 in the absence of H2O. This suggests that the catalyst prepared by the CTAB-assisted process could adsorb more NH3, indicating that the structure and surface properties might be adjusted during the preparation process. On the other hand, NO consumption in the presence of H2O was always lower than that in the absence of H2O (12.6 µmol/g in the presence of H2O and 18.0 µmol/g in the absence of H2O for FeTi-a-50; 21.1 µmol/g in the presence of H2O and 25.2 µmol/g in the absence of H2O for CT-FeTi-a-50), suggesting that the presence of H2O resulted to less amount of NO being reacted. Meanwhile, in the presence of H2O, the decrease of NO consumption for CT-FeTi-a-50 was less pronounced than the one for FeTi-a-50, indicating that H2O had a milder inhibition effect on the catalyst modified by CTAB, thereby improving the SCR performance of CT-FeTi catalyst in the presence of H2O.
Fig. 6. DRIFT spectra of NO + O2 at 240 °C in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts.
3.5.2. In situ DRIFT spectra of NH3 adsorption over FeTi and CT-Fe/Ti catalysts NH3 adsorption on the FeTi and CT-FeTi catalysts at 240 °C were carried out in the absence and in presence of H2O, and the results are presented in Fig. 7. After NH3 (1000 ppm) adsorption and N2 purging, several bands at 1645, 1602, 1565, 1437, 1192, 3650 cm−1 and a broad band in the range of 3100–3400 cm−1 were detected. The bands at 1602 and 1192 cm−1 were attributed to the asymmetric and symmetric deformation of coordinated ammonia linked to Lewis acid sites, while the bands at 1645 and 1437 cm−1 were assigned to asymmetric and symmetric bending vibrations of the NH4+ species on Brønsted acid sites [21]. In the N-H stretching region, bands at 3340/3242 cm−1 and 3137 cm−1 were attributed to N-H stretching vibration modes of coordinated NH3 and NH4+ species, respectively [22,23]. The band around 3650 cm−1 was assigned to the surface –OH group, indicating the presence of H2O [24]. In addition, the band at 1565 cm−1 implied the existence of amide species (–NH2), which was formed through the partial oxidation (activation) of NH3 in the presence of Fe3+ [25]. In the absence of H2O, the Brønsted acid sites (1645, 1437 cm−1) on CTFeTi catalyst were slightly stronger than on FeTi catalyst. In addition, the intensity of the bands at 1602 and 1192 cm−1 on FeTi catalyst were significantly lower than on CT-FeTi catalyst. In fact, the band at 1602 cm−1 on FeTi catalyst was not detected at all due to weakly adsorbed NH3. These results suggested that the catalyst prepared by a
3.5. In situ DRIFTS studies 3.5.1. In situ DRIFT spectra of NO + O2 adsorption over FeTi and CT-Fe/Ti catalysts To further understand which form of NO species is influenced by the competitive adsorption of H2O on the catalysts, the DRIFT spectra of NO + O2 were recorded both in the absence and in presence of H2O. The results are illustrated in Fig. 6. Several bands at 1625/1612, 1576 and 1240 cm−1 appeared after the two samples were treated with 1000 ppm NO + 6.5 vol% O2/N2 for 30 min and purged with N2 for further 15 min at 240 °C. The bands at 1625/1612 and 1240 cm−1 were attributed to bridging nitrates, and the band at 1576 cm−1 could be assigned to bidentate nitrates [11,19,20]. It is evident that both catalysts were predominantly covered by bridging nitrate. In the absence of H2O, the bridging nitrate on CT-FeTi was stronger than that on FeTi catalyst, indicating that the structure and properties of the catalyst prepared by the CTAB-assisted process could enhance the adsorption of this species. The addition of H2O resulted in the disappearance of bidentate nitrate and a reduction of bridging nitrate on the FeTi catalyst. However, the CT-FeTi catalyst displayed a good resistance to H2O. Although the bidentate nitrate decreased and nearly disappeared, the bridging nitrate decreased slowly and still maintained higher signal intensity compared to the one measured on FeTi, possibly favoring the SCR reaction of NH3. 318
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Fig. 7. DRIFT spectra of NH3 at 240 °C in the presence and absence of H2O over (A) FeTi and (B) CT-FeTi catalysts.
Fig. 8. In situ DRIFT spectra of reaction in NH3 + NO + O2 at 240 °C over CTFeTi catalyst in the absence (A) and presence (B) of H2O.
CTAB-assisted process might favor the adsorption of NH3 on Lewis acid sites. With the addition of H2O, it was found that the band intensity of the –OH group increased noticeably on both catalysts. Meanwhile, the band intensity at 1602/1192 cm−1 decreased whereas the band intensity at 1437/1645 cm−1 increased, implying a decrease in Lewis acid and an increase on Brønsted acid sites. Accordingly, H2O promoted the adsorption of NH3 on Brønsted acid sites while inhibiting the adsorption of NH3 on Lewis acid sites; however, the inhibiting effect on CT-FeTi catalyst was weaker than on FeTi and a relatively strong adsorption on Lewis acid sites could still observed clearly on the former. Since Lewis acids played an important role in the SCR reaction [26], thus CT-FeTi catalyst exhibited a better catalytic performance compared to FeTi catalyst. Moreover, it was interesting to note that the band at 1565 cm−1 disappeared on both catalysts upon addition of H2O. This result indicated that partial oxidation of NH3 by the Fe3+ species on the surface of the catalysts might be inhibited, causing a decline in catalytic performance.
been caused by the overlap of bands of bridging nitrate and coordinated NH3. The band at 1565 cm−1 indicated the formation of the –NH2 species, which is an important intermediate in the reaction. A new band appeared at 1360 cm−1, which was quite different from those due to the adsorbed NOx and NH3 species mentioned above. Chen et al. [27] attributed this band to the intermediate species deriving from the combination of adsorbed NH3 and NOx. It can be seen that, in the presence of H2O, the Brønsted acids (1645 and 1437 cm−1) were strengthened, while the Lewis acids were slightly weakened. The decrease in intensity of the band at 1608 cm−1 might result from the weak adsorption of bridging nitrate and coordinated NH3. Simultaneously, it is worth noting that the peak due to the –NH2 intermediated declined rapidly. It is generally accepted that in the SCR reaction, NH3 is initially adsorbed on Brønsted or Lewis acid sites on the surface of the catalyst to form intermediate species (–NH2) that subsequently react with gaseous NO or adsorbed nitrate species, corresponding to the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms [28,29]. Based on this reaction scheme, as well as on earlier reports [27,30], a simplified reaction recycle on the CT-FeTi catalyst most probably took place, as shown in Fig. 9. NH3 adsorption was reported to be an important step to influence the SCR reaction. In the absence of H2O, the enhanced adsorption of NH3 on Lewis acids (Fig. 7) might lead to a higher catalytic activity of CT-FeTi compared to FeTi, and the high intensity of intermediate species (–NH2) demonstrated that a higher amount of NH3 was activated to participate in the reaction. The high quantity of bridging
3.5.3. Reaction mechanism over CT-FeTi catalyst in the absence and presence of H2O In order to further investigate the reaction mechanism over CT-FeTi catalyst, an in situ DRIFT experiment of the NH3-SCR reaction as a function of time was conducted and the results are shown in Fig. 8. During the whole process, the bands relative to the NH3 species (1192, 1437 and 1645 cm−1) dominated. Bands at 1608 cm−1 might have 319
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Fig. 9. Possible reaction recycle for NH3-SCR over the CT-FeTi catalysts.
nitrate species on CT-FeTi catalyst (Fig. 6) also played an important role in the improved catalytic activity according to the L-H mechanism. In the presence of H2O, a stronger adsorption of NH3 on Brønsted acids was observed on CT-FeTi catalyst, however, catalytic performance was still decreased according to the result of activity test. This might due to the decrease in the absorbed intermediate species (–NH2). Since the intermediate species (–NH2) was usually formed on Fe-based catalysts through the reaction: Fe3 + + NH3 → Fe2 +−NH2 + H+ [30], the presence of H2O obviously inhibited the process, further influencing the SCR reaction.
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4. Conclusion CT-FeTi catalysts were synthesized using CTAB as a template-directing reagent, and applied to the selective catalytic reduction of NO with NH3. Comparing with FeTi catalysts, CT-FeTi catalysts exhibited an excellent SCR activity with over 80% of NO conversion achieved at 240 °C in a simulated flue gas containing 500 ppm NO, 500 ppm NH3 and 6.5 vol% O2. Meanwhile, CT-FeTi catalyst showed good H2O resistance when H2O was included into the flue gas, whereas the activity of FeTi catalyst decreased rapidly and continuously. BET analyses of the both catalysts indicated that the addition of CTAB during preparation process promoted the formation of uniform mesoporous structure, which was beneficial to avoid being excessively enlarged in the presence of H2O. Results of TPD and TPSR showed that H2O inhibited the NO adsorption and the reaction of NO with adsorbed NH3, however, the inhibition effect on CT-FeTi catalysts was less pronounced than that on FeTi catalysts. In addition, in situ DRIFTS revealed that the formation of the intermediate species (–NH2) during the reaction: Fe3+ + NH3 → Fe2+−NH2 + H+was inhibited when the introduction of H2O, and the more adsorption of bridging nitrate and NH3 species on Lewis acid sites on the surface of CT-FeTi catalysts might be an important factor to improve catalytic activity. All of these results showed that CTAB could efficiently modify the structure and surface properties of the catalysts, leading to the enhancement of SCR activity and H2O. Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFC0210200) and Development Program of Shanxi Province (No. 201703D111018). References [1] Z.M. Liu, Y.X. Liu, Y. Li, H. Su, L.L. Ma, WO3 promoted Mn-Zr mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, Chem. Eng. J. 283 (2016)
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