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Article Cite This: ACS Omega 2019, 4, 5088−5097
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New Findings in Hydrothermal Deactivation Research on the Vanadia-Selective Catalytic Reduction Catalyst Xingwen Li,† Dongwei Yao,*,† Feng Wu,† Xinlei Wang,‡ Lai Wei,† and Biao Liu† †
Institute of Power Machinery and Vehicular Engineering, College of Energy Engineering, Zhejiang University, Zheda Road 38, Xihu District, Hangzhou 310027, China ‡ Agricultural Engr Sciences, Bld 1304 W. Pennsylvania, Urbana, Illinois 61801, United States
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S Supporting Information *
ABSTRACT: Considering the risks of hydrothermal deterioration in vehicles, power plants, and oceangoing vessels, V2O5-WO3/TiO2 catalysts were subject to hydrothermal and thermal aging at 600, 625, 635, and 650 °C for 4−48 h. The different ratio and significant loss of active sites are main reasons for catalyst deactivation. Both Lewis and Brønsted acid sites are involved in the selective catalytic reduction reaction. Brønsted acid sites are more susceptible. High temperature plays a major role in the aging. It causes sintering, particle growth, and the anatase phase transition. Phase transformation turns out to be less important than sintering. Sintering leads to the reduction of the BET surface area, which in turn causes decrease of NH3 adsorption amount and changes of active sites. Aging time can accelerate the degree of deactivation. It also helps to change the proportion of active sites. Water vapor has no significant effect on NOX conversion rates. V2O5 had the best hydrothermal stability. Maunula et al.16 deactivated the catalyst using 10% H2O at 600, 650, 700, 750, and 800 °C for 20 h, and aged coated samples in static furnace conditions at 600, 650, 700, and 800 °C for 3 h. Li et al.17 aged SCR catalysts at 670 °C for 64 h. Beale et al.18 tested the catalyst at 650 and 700 °C for 20 h in a flow of 10% H2O. Most existing studies attribute the deactivation of hydrothermal and thermal aging to the morphological changes of vanadium and the reduction in the Brunauer, Emmett, and Teller (BET) surface area. Went et al.19 reported that lower oligomers, such as monomeric and dimeric vanadia species, had better SCR reactivity and N2 selectivity. Jin et al.20 and Reddy et al.21 stated that monomeric vanadyl species gradually converted to crystalline V2O5 during thermal aging. Similar conclusions were supported by Went et al.,22 Wachs et al.,23 Djerad et al.,24 Beale et al.,18 Lai and Wachs,25 Madia et al.,9 and other researchers. Xi et al.15 and Jin et al.20 also mentioned that the decrease in the BET surface area increased the surface density of vanadia, which may result in the formation of less active polymeric species and even crystalline V2O5. Nova et al.26 thought that loss of surface area was the main cause of catalyst deactivation. Kompio et al.27 had the same opinion. Many authors also suggested that the decrease in catalytic activity was attributed to anatase phase transition.16,18,20,28 Sintering of the TiO2 support could lead to aggregation of
1. INTRODUCTION The selective catalytic reduction (SCR) technology has evolved a lot in terms of efficiency, selectivity, and economics1 to reduce NOX in diesel vehicles, power plants, and more than 90% of oceangoing vessels.2 To meet the growing demand for SCR technology due to more stringent emission legislation, two main types of SCR catalysts occupied are vanadium and zeolite catalysts. In the past decades, the commercial V2O5WO3/TiO2 catalyst of SCR has been proven to have good catalytic activity and selectivity, excellent resistance to sulfur poisoning,3 good tolerance to lubricant poisoning,4 and lower N2O emission.5 However, poor thermal stability6 and volatilization of vanadium7 have prohibited the vanadia-catalyst from being extensively used. Thermal stability at temperatures above 600 °C in power plants8 is a problem. Meanwhile, extreme operating points9 and the regeneration of diesel particulate filter (DPF)10 can result in temperature above 650 °C. Thus, the challenge for the Vanadia-SCR catalyst is the hydrothermal durability. There are many similar studies11−13 focused on hydrothermal and thermal aging on the vanadia-SCR catalyst. Rasmussen and Abrams14 demonstrated that vanadium tungsten catalyst was stable under hydrothermal conditions at 550 °C (110 h) and the deactivation was not due to vanadium evaporation. Xi et al.15 aged the Vanadia-SCR catalyst from 525 to 700 °C for 2 h. Asako et al.10 aged vanadia catalyst for 100 h at 550 °C and found that it lost 40−60% activity. Madia et al.9 performed progressive aging experiments for 100 h at 550 °C and 30 h at 600 °C and found that 2 wt % © 2019 American Chemical Society
Received: December 20, 2018 Accepted: February 20, 2019 Published: March 8, 2019 5088
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Figure 1. NOX conversion rates evaluated in the gas flow of 450 ppm NO, 450 ppm NH3, 10% O2, 7.8% H2O, and balance N2 with a space velocity of 3 × 105 h−1 between 200 and 450 °C; (a) fresh and hydrothermal aging samples, (b) comparisons of thermal and hydrothermal aging samples deactivated at 650 °C.
isolated vanadium ions26 and could increase the polymeric vanadyl surface species.9 The rutile-supported vanadia catalyst was substantially less active than the anatase-supported vanadia catalyst.29 Despite the considerable studies on hydrothermal aging of vanadium-based catalysts as described above, there are still some controversial and insufficient fields. Although there are numbers of active sites studies,25,30,31 few studies have focused on active-site changes during hydrothermal aging. Literatures32−34 described that H2O caused a certain degree of activity reduction because of competitive adsorption of H2O and NH3. However, the role of H2O in hydrothermal aging has been neglected. Besides, ammonia storage capacity decreases as aging temperature increases15 and it is facilitated by Brønsted acid sites.17 Aging leads to a decrease in NH3 storage capacity.10 Therefore, changes of NH3 storage capacity during hydrothermal aging needs further study. Furthermore, the reduction in the BET surface area originates from anatase sintering and phase transition.20,35 Then which of the anatase sintering and phase transition is in closer contact with catalyst activity. The main disadvantage of vanadium-based SCR is the deactivation at temperatures exceeding 600−650 °C.36 Pure V2O5 has a melting point of 670 °C, which easily leads to sintering of V2O5 particles and a decrease in catalytic activity.37,38 As mentioned by Asako et al.,10 DPF regeneration raises the SCR inlet temperature to 650 °C. By estimation, this situation lasts for 50 h throughout the life of the after treatment system.10 Hence, 650 °C is a particularly noteworthy aging temperature. In short, there are many areas worth further study. Herein, the commercial V2O5-WO3/TiO2 catalyst was thermally aged at 650 °C for 4−24 h in a step size of 4 h. Hydrothermal aging experiments were carried out at 650 °C for 4, 8, 12, and 24 h, and at 600 °C for 48 h, at 625 and 635 °C for 24 h. A fixed-bed quartz reactor with standard gas to simulate diesel exhaust was used. For catalyst characterization, X-ray diffraction (XRD), in situ Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N2 adsorption isotherms, and NH3-temperature programmed desorption (TPD) experiments were used. The anatase phase transition, sintering of the carrier, and changes of BET surface area were fully studied.
Particularly, the active-site variations in species and amount were discussed. The reduction of catalytic activity and the mechanism of hydrothermal deactivation were clearly explained. Finally, the effects of temperature, aging time, and H2O during hydrothermal aging were clarified.
2. RESULTS AND DISCUSSION 2.1. Catalytic Performance. The NOX conversion rates directly reflect the catalyst activity. Overall, the DeNOX ability has dropped dramatically and the temperature window has shrunk massively during hydrothermal aging. The fresh sample has a very good DeNOX ability and a very wide temperature window, as shown in the Figure 1a. Its NOX conversion rate can reach 79.06% at 250 °C and 90.4% at 450 °C. As shown in Figure S2a, the catalyst deactivates more rapidly during the first 12 h. At 400 °C, the NOX conversion rate decreases from 94.97% of the fresh sample to 63.06% of the T12 sample. Then the deactivation degree declines after 12 h. The deactivation degree in the first 12 h is higher than that of subsequent aging. The entire aging process could be divided into two parts: the fast deactivation stage and the slow deactivation stage, when samples are thermally aged at the same temperature. In addition, the active sites that work in high temperature (mainly around 400 °C) and low temperature (mainly around 250 °C) are different. As can be seen in Figure S2b, the relative deactivation degree of lower temperature is slightly higher than that of higher temperature. For example, the relative deactivation degree rises from 40.68% (T4) to 88.84% (T-24) at 250 °C, while it rises from 5% (T-4) to 60.54% (T-24) at 400 °C. Aged catalyst samples eventually maintain a NOX conversion rate of about 10% at lower temperatures. When the aging temperature reaches a certain value, the NOX conversion rate could be very sensitive to the increase of hydrothermal aging temperature. The temperature of 635 °C is only 10 °C higher than 625 °C, and 650 °C is only 15 °C higher than 635 °C. However, the highest conversion rate decreases from 68.25% of sample ht625-24 to 55.36% of sample ht635-24, and to 36.27% of sample ht650-24. In addition, the conversion rate of sample ht650-12 is very close to the conversion rate of sample ht635-24. The sample ht63524 has a 3% higher conversion rate at 450 °C. At other 5089
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2141), though the signal is below the detection limit in fresh samples. This phenomenon indicates that the WO3 crystallites disperse very well in the fresh sample and the dispersibility gets worse after 12 h’ deterioration. That could affect the dispersion of V2O5 and impact the number of active sites in return.33 The relationship between BET surface area and NOX conversion rates at 250 and 450 °C is displayed in Figure 3a. There is a significant positive correlation between the BET specific surface area and NOX conversion rate. In samples ht650-4, ht650-8, and ht650-24, the conversion rate decreases from 42.55 to 25.12% and finally to 9.03% at 250 °C, and the BET surface area also decreases from 43.52 to 19.85 m2/g and finally to 4.48 m2/g. The correlation coefficient of 450 °C is a little lower. When the NOX conversion rate is higher, the BET specific surface area is also higher. Therefore, BET surface area reduction is proportional to the decrease of the NOX conversion rate. The reduction of the BET surface area is the main reason for the decrease of the conversion rate during aging. The relationship between the BET surface area and average anatase particle dimension perpendicular to the (101) plane is displayed in Figure 3b. The decrease of the BET surface area is inversely proportional to the increase of particle sizes. Larger crystallite sizes mean worse sintering and agglomeration. Literature studies have pointed out that the reduction in the BET surface area is due to anatase sintering and phase transition.20,35 That is to say, sintering and aggregation of anatase aggravate the deactivation condition by reducing the BET surface area. Sintering has been reported to have negative impacts on the dispersion of vanadium species.35 Consequently, the effects of sintering and aggregation during hydrothermal aging are inevitable and should be highlighted. The proportion of rutile TiO2 is obtained based on formula 3. Rutile appeared in sample ht650-12 with a fraction of 3.52%, the number dashes to 29.23% in sample ht650-24. Remarkably, the content of rutile is 4.68% in sample ht635-24, much lower than that in sample ht650-24. In addition, hydrothermal deterioration of 48 h at 600 °C results in no trace of rutile. As a result, phase transition of TiO2 is greatly affected by hydrothermal temperature, especially when the temperature is relatively high. Meanwhile, the proportion of the rutile phase is enforced by aging time. However, the catalyst has lost most of its activity before rutile accumulates massively. After 16 h’ aging, the proportion of rutile increases greatly, and the catalyst deactivates slowly. It is unreasonable to attribute most of catalyst inability to anatase phase transition. In fact, V2O5 has strong interactions with TiO2, which is related to the active sites.40 Meanwhile, anatase TiO2 is more active than other supports.41 Thus, phase transition must have interrupted those V−O−Ti connections and caused some loss to the catalyst activity, but it is not as deadly as sintering. 2.3. Active-Site Variations. The mechanism of vanadiumbased catalysts has been frequently discussed in literatures.41−43 In situ FTIR was used to determine the changes in active sites after NH3 adsorption at 100 and 200 °C. Studies have shown that NH3 is adsorbed on Brønsted acid sites as ammonium ions42 and coordinated over Lewis acid sites.44 Typical results are shown in Figures 4, 5, S4 and S5. The bands at 1240 cm−1 are related to the symmetric bending vibrations of N−H bonds on Lewis acid sites,44 whose asymmetric modes appear at 1609 cm−1.44,45 Correspondingly, the stretching modes are found at 3178 and 3245 cm−1 for coordinated ammonia on Lewis acid sites and 3028 and 2808 cm−1 for the
temperatures, the differences between the two samples do not exceed 0.5%. This means that a rise of 15 °C could save about 12 h of aging when the temperature is high enough. This conclusion can be confirmed by sample ht600-48. Though the aging time is up to 48 h, the conversion rate is higher than the sample aged at 650 °C for 8 h. Particularly, the conversion rate of sample ht600-48 is 54.36% and sample ht650-8 is 45.66% at 300 °C. Therefore, the higher the aging temperature, the greater it impacts the NOX conversion rate. Many literatures have pointed out the competitive adsorption of H2O and NH3,32,34 and the beneficial effects of H2O on the selectivity of SCR reaction.39 However, few studies have shown how H2O affects hydrothermal aging. H2O has no significant effect on the conversion rate during hydrothermal aging, as shown in Figure 1b. After 4 h of aging, the conversion rates of hydrothermally aged sample ht650-4 and the thermally aged sample T-4 are very close. The biggest difference situates at 250 °C, and the conversion rate of sample T-4 is 4% higher. After 12 h of aging, the conversion rate of sample T-12 is about 8% higher than that of sample ht650-12 at 400 °C. When the aging time is as long as 24 h, the conversion rates are closer again, and the maximum difference does not exceed 3.5%. Therefore, it can be said that H2O has little effect on the conversion rate of NOX. 2.2. Sintering, Phase Transition, and BET Surface Area. Madia et al.9 showed the relationship between the BET specific surface area and the catalyst particle size. He stated that BET surface area was consistent with the calculated anatase particle values before pore clogging. Thus, the anatase transformation, particle size growth, and BET surface area of the catalyst were explored. As shown in Figure 2, the anatase TiO2 phase predominates in all the samples (2θ = 25.26, 37.779, 47.98, 53.88, 55.001,
Figure 2. XRD patterns of fresh and hydrothermal aging samples. The rhombus represents the anatase TiO2 phase, the heart shape represents the rutile TiO2 phase, and the black dot represents the WO3 phase.
62.66, 68.741, 70.22, and 75.98, JCPDF, PDF84-1285). No phase transition occurs at the beginning. With the prolongation of deactivation time and the increase of hydrothermal temperature, the presence of rutile can be clearly identified in sample ht650-24 (2θ = 27.419, 36.06, and 54.3, JCPDF, PDF86-0148). Evidences for WO3 crystallite are found in aged samples (2θ = 23.081, 23.561, and 24.06, JCPDF, PDF 715090
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Figure 3. (a) BET surface area vs NOX conversion rate at 250 and 450 °C for fresh and hydrothermally aged samples. (b) Average anatase particle dimension perpendicular to the (101) plane vs BET surface area for different aged samples.
Figure 4. In situ FTIR spectra of the fresh sample and hydrothermally aged samples ht650-4, ht650-8, ht650-12 at 100 °C. A lot of active sites were lost during hydrothermal aging. The intensity changes a lot. This graph is the superposition of the mentioned samples with different ordinates. Some active sites would be unclear if the same ordinate was used. It is the same with Figures 5, S4 and S5. The intensity differences can be seen in Figure S6.
Figure 5. In situ FTIR spectra of NH3 adsorption of thermally aged samples T-4, T-8 and T-12, and the fresh sample at 100 °C with different ordinates (the same with Figure 4). “L” refers to Lewis active sites and “B” refers to Brønsted active sites.
indicates that good dispersion of VxOy species on TiO2 supporter48 has been affected. Taking the results of Section 2.2 into account, impacts of sintering and BET surface area are key factors in the dispersion and distribution of active sites. On the other hand, V−OH active sites are very important in Topsøe’s catalytic cycle of SCR.42 The disappearance directly affects the catalyst NOX conversion ability. In hydrothermally aged samples, V−OH active sites do not disappear until the samples are aged for 24 h at 650 °C. Therefore, H2O can slow down this process in hydrothermal aging. In samples T-20 and T-24, new bands appear at 3745 cm−1 (3730 cm−1 in sample ht650-24 instead), indicating some free OH groups.42 These free OH groups can come from surface tungsten31 or titanium. They do not appear until the rutile phase emerges, indicating that restructuring must occur on the surface of the catalyst because of phase transformation. Sample fresh and sample T-24 show broad bands at around 3200 cm−1, while other thermally aged samples show bands at 3178 and 3245 cm−1, and hydrothermally aged samples show bands at 3170, 3200, and 3248 cm−1. There is no perfect explanation for this phenomenon. As previously mentioned,
ion form of ammonium adsorbed on Brønsted acid sites.45−47 All the samples show bands at 1425 and 1676 cm−1 (small shifts will be discussed later). They can be assigned to asymmetric and symmetric bending vibrations of NH4+ species on Brønsted acid sites, respectively.44,48 Some blue shifts occurred in aged samples. In Figures S4 and S5, the active sites at 1240 cm−1 shift to 1242 cm−1(T-8), 1245 cm−1 (T-12, T16), 1247 cm−1 (T-20), and 1260 cm−1 (T-24). It also happens in hydrothermal aging samples. The bands located at 1670 cm−1 also shift from 1670 cm−1 (fresh, T-4) to 1675 cm−1 (hydrothermal aging samples). Studies14 have shown that blue shift (to higher frequencies) would imply agglomeration of vanadium because of sintering. Thus, hydrothermal aging makes acid sites move to more stable location because of sintering and agglomeration. The negative spectra 3654 cm−1 (V−OH) are detected in sample fresh and T-4, however, they disappear in samples T16, T-20, and T-24, as shown in Figures S4 and S5. This 5091
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bands at 3245 cm−1 (3248 cm−1)31,45 are in the N−H stretching region and can be assigned to coordinated ammonia on Lewis acid sites. Bands at 3178 cm−1 (3170 cm−1)31,45 are NH4+ ions adsorbed on the Brønsted acid sites of the surface. One possible explanation is: at first, two peaks of Lewis acid sites and Brønsted acid sites interact and mix with each other and then broad bands appear at around 3200 cm−1,13 as detected in fresh samples. However, one kind of acid sites is more sensitive to temperature. Thus, after aging, the peaks of Lewis acid sites and Brønsted acid sites displayed separately. After prolonged deactivation, a great loss of both Lewis acid sites and Brønsted acid sites causes the two signals to mix with each other again, for example, the weak signal detected in sample T-24. The loss rate is different because of H2O existence in the hydrothermal aging process, so it is not the same as thermal aging samples. In situ FTIR detection is a semiquantitative experiment. Zhu et al.31 used the peak intensity to calculate the number of acid sites. On the basis of that, the peak intensities of Lewis acid sites (1240 cm−1) and Brønsted acid sites (1425 cm−1) after hydrothermal and thermal deactivation were calculated at 100 and 200 °C, as shown in Figures 6 and S6. For Lewis acid sites,
temperature. In contrast, Brønsted acid sites were dominant in the presence of H2O molecules and were highly affected by temperature. Lietti et al.49 proposed that Lewis-bonded NH3 species were thermally more stable than ammonium ions. This article proves their opinions. In summary, the Brønsted acid sites are more susceptible and more unstable during hydrothermal aging and thermal aging. The reduction in NOX conversion rate is strongly dependent on the stability of the acid sites. The presence of H2O makes Brønsted acid sites a little more stable. 2.4. Changes of NH3 Storage Capacity and Vanadium Ratio. Few studies have been reported using TPD instruments to study the hydrothermal deactivation of catalyst so far. Some yellow vanadium was found during hydrothermal and thermal aging experiments at the outlet of the quartz tube, the same as description in the literature.15 The volatilization of vanadium is too difficult to quantify.7 Nonetheless, it indicates that a certain degree of acid sites has been lost. The acid site amount and intensity of several aged samples were detected by NH3-TPD and displayed in Figure 7, 8, and S7. It cannot be ignored that
Figure 7. NH3-TPD spectra of fresh and hydrothermally aged samples. The peaks and NH3 storage capacity change a lot after aging. Figure 6. Percentage of active-site disappearance (%), percentage (%) = 100 × (peak intensity differences between 100 and 200 °C)/peak intensity at 100 °C, calculated according to Zhu et al.31 For all the aged samples, the reduction of Lewis active sites is in the left and Brønsted active sites in the right. Overall, Brønsted active sites are more sensitive to the characterization temperature.
the reduction in fresh samples is 10.12%, while the minimum decrease in aged samples is 20.65%. For Brønsted acid sites, decrease in fresh samples is 42.38%, while the minimum reduction for the aged samples is 68.73%. Therefore, the active sites in unaged samples are less sensitive to characterization temperature. Considering all the aged samples, the average reduction ratio of Lewis acid sites is less than the average reduction ratio of Brønsted acid in Figure 6. For samples with similar NOX conversion rates (ht650-12 and ht635-24), samples aged at higher temperature are more sensitive to the characterization temperature. Similarly, samples with higher conversion rates are less sensitive to the characterization temperature. Lin and Bai et al.32 argued that Lewis acid sites were dominant in the absence of H2O molecules, and they were also less sensitive to
Figure 8. Integral area of the NH3 mass signal vs abatement of NOX conversion rates of fresh and hydrothermally aged samples. The NOX conversion ability depends highly on NH3 storage capacity. 5092
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Table 1. Valence State of Vanadium and Atomic Percentage on the Catalyst Surface Determined from Deconvoluted XPS Spectra samples
fresh
ht650-4
ht650-8
ht650-12
ht650-24
T-4
V4+ (eV) V5+ (eV) V4+/(V4+ + V5+) (%) samples
515.61 516.92 51.59 T-8
516.24 517.05 53.91 T-12
515.86 516.98 57.39 T-24
516.15 517.14 55.07 ht600-48
515.65 516.71 63.07 ht625-24
515.58 516.58 52.97 ht635-24
V4+ (eV) V5+ (eV) V4+/(V4+ + V5+) (%)
515.9 517.05 47.53
515.79 517.2 52.13
515.59 517.16 47.36
516.12 517.18 54.36
515.87 517.21 66.37
515.64 516.77 75.82
correspond to V4+ and V5+ species, respectively.9,51−53 Notably, most of aged samples have larger binding energies (except for sample T-4 and sample ht635-24). This could be related to the morphological changes of vanadium during aging. Vanadium is not so well dispersed on the surface of TiO2 after deactivation. In addition, the values of V4+/(V4+ + V5+) increase in hydrothermally aged samples. Larger V4+/(V4+ + V5+) ratio would generate more free electrons. It also illustrated that V4+ species have more stable structures.54 For thermally aged samples, no significant laws of change in the ratio of V4+/(V4+ + V5+). This could result from the peaks of vanadium affected by peaks of O 1s satellites. Both Brønsted and Lewis active sites have V4+ and V5+, trends of V4+/(V4+ + V5+) are different for hydrothermally and thermally aged samples. It also verifies the different loss ratio of Brønsted acid and Lewis acid sites during deactivation.
the intensity of the NH3 signal decreases tremendously, falling from 6 × 10−10 a.u. in the fresh sample to about 3 × 10−10 a.u. in sample ht650-4. Limited by instruments, the consumption of NH3 is difficult to quantify precisely, but the reduction of the integral area of the NH3 mass signal in TPD detection has the same trend as the abatement of NOX conversion rates in Figures 8 and S7b, hinting a close tie between the number of active sites and the NOX conversion ability of the SCR catalyst. In short, one of the biggest reasons for catalyst deactivation is the loss of active sites. In Figure 8, the decrease of NH3 integral area and the decrease of NOX conversion rates are not 100% linearly related. The conversion rate of sample ht650-8 is lower than that of sample ht600-48, but the integral area of sample ht6508 is higher. Similarly, the conversion rates of sample ht650-12 and sample ht635-24 are similar, but sample ht650-12 has a larger integral area. In Figure S7b, there is no such difference when the aging temperature is the same. The number of active sites reduces significantly in the first 12 h. Thereafter, the reduction rate slows down, in accordance with the deactivation of catalyst NOX conversion ability. It shows better consistency at lower temperatures. In Figure S7a, the fresh sample exhibits a broad peak around 290 °C, indicating the presence of medium-to-strong acid sites.50 Later, the peak of sample T-4 is around 248 °C and the peak of sample T-8 is 228 °C. The peaks of sample T-12 and T-24 are approximately 212 °C. Thus, NH3 desorption peaks shift to lower temperature. The above results indicate that after aging, the strong adsorption sites of NH3 disappeared a lot, and the proportion of weak adsorption sites among the remaining active sites becomes larger. The active sites which adsorbed NH3 weakly, mainly affect the catalyst performance at lower temperature. Although ammonia desorption amount of the catalyst is positively correlated with the NOX conversion rates, samples are not completely identical at different aging temperatures. When the deactivation temperature is high enough, NH3 desorption amount is higher than that aged at lower temperature and longer aging time, though it may have a lower NOX conversion rate. Interestingly, peaks of sample ht600-48, ht625-24, and ht635-24 shift more to the left than the samples of the same NOX conversion rates. It reflects the fact that although high temperature plays a major role in the aging process, the aging time changes the proportion of active sites. Compared hydrothermally and thermally aged samples of the same deactivation time, peaks of the two are not so different. Thus, the influence of H2O on active sites during hydrothermal aging is not particularly large. XPS measurements were performed to investigate the surface binding energies and valence states of various aged species. The results are shown in Table 1. The binding energies of V 2p3/2 at 515.5−516.2 and 516.5−517.3 eV in the table
3. DISCUSSION As described in Section 2.1, the relative deactivation degree at lower temperatures is higher. As mentioned in Section 2.3, Brønsted acid sites are more sensitive to temperature and are greatly lost during thermal aging. The TPD results have proved the significant loss of strong adsorption sites. The large loss of related acid sites makes the conversion rate at lower temperatures (mainly around 250 °C) decrease more than the higher temperatures (mainly around 400 °C). Therefore, the Brønsted acid is a strong adsorption site for the adsorption of NH3 and contributes a lot to the NOX conversion ability at lower temperature. Topsøe 55 proposed the reaction mechanism of the vanadium-based SCR catalyst based on Brønsted acid sites, and Ramis56 elaborated the mechanism model based on Lewis acid sites. Combined with the results of hydrothermal and thermal aging in this research, both Brønsted acid sites and Lewis acid sites are involved in the reaction. A most likely mechanism is shown in Figure 9. Ammonia can be adsorbed on Brønsted acid and Lewis acid sites. The ammonia adsorbed on Brønsted acid sites reacts with NO with the help of VO and produces N2 and H2O, as well as V4+−OH and V5+−OH. Ammonia adsorbed on the Lewis acid site reacts with NO and generates V4+−OH, N2 and H2O. Finally, V4+−OH can be reduced to VO with the aid of O2. The working temperature range of these two acid sites is not exactly the same. Different active sites participate in SCR reactions with different activation energies, resulting in biased reactions. The main working temperature of each active site is also different. The loss of active sites in aging is the biggest reason for deactivation, but changes in the proportion of active sites also result in changes of reaction products and conversion rates. 5093
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Thermal deactivation causes sintering, which leads to the decrease of the BET surface area. This in turn causes the reduction of the NH3 adsorption amount and the changes of active sites. The different ratios and great loss of active sites after aging should be responsible for hydrothermal deactivation. In hydrothermal aging, high temperature is the key factor of catalyst deactivation. In the early stage of aging, the high temperature causes the catalyst carrier to sinter and the particles to grow. Sintering has negative impacts on the dispersion of vanadium species and reduction of the surface area, indirectly affecting the number and distribution of active sites. Another result of high temperature is anatase phase transition. Although this has been repeatedly mentioned in the previous studies, it turns out to be less important than sintering because it does not show up until the catalyst has lost most of its activity. However, it probably influences the surface reconstruction. Speaking of aging time, it can greatly accelerate the degree of deactivation. However, doubling the aging time at lower temperature does not lead to phase change, for example, sample ht600-48. Therefore, the effect of aging time is not as great as the temperature. Nonetheless, differences in aging time can change the proportion of the active sites. It speeds up the reduction of strong adsorption sites for NH3. Refers to the role of H2O, the conversion rates of samples hydrothermally and thermally deactivated under the same conditions do not have big differences. However, H2O inhibits the loss of Brønsted acid sites a little and accelerates sintering and loss of surface area a little.
Figure 9. Deactivation mechanism of NH3-SCR reaction on vanadium-based catalysts. Both Brønsted acid sites and Lewis acid sites are involved in the SCR reaction. Aging decreases the number of active sites and changes the ratio of different active sites, affecting the catalytic cycle.
On the basis of the above catalytic mechanism, the hydrothermal deactivation mechanism in this research can be summarized as follows: in the process of hydrothermal aging, sintering of the carrier leads to the reduction of the BET surface area. The BET surface area has a great relationship with NOX conversion rates than the NH3 adsorption capacity, comparing the results in Figures 3a and 8. Thus, decrease of active sites and changes in the ratio of active sites probably originates from BET surface area reduction. More exactly, the decrease in sintering and BET surface area results in the reduction of NH3 adsorption amount and loss of active sites. In the process of aging, different kinds of active sites have different antiaging abilities. Different ratios of remaining active sites cause the conversion rate of the catalyst to be different in the high temperature range and the low temperature range, which is proved by FTIR and XPS results. Comparing the thermal and hydrothermal aging samples deactivated at 650 °C of the same time, such as the NOx conversion rate in Figure 1b, the active site changes in Figures 4 and 5, it can be found that H2O only slightly accelerates the sintering and the BET surface area reduction and increases the stability of Brønsted acid sites a little. The value of V4+/(V4+ + V5+) rises in Table 1 also prove that. In short, high temperature is the main reason that affects the conversion rates. The impacts of H2O are not so great. In addition, although the catalyst remains highly active after 48 h hydrothermal aging at 600 °C, its peak in TPD experiments shift obviously and the value of V4+/(V4+ + V5+) differs from the fresh sample. Hence, the time of deactivation can change the ratio of active sites.
5. EXPERIMENTAL SECTION The catalyst provided by the China National Heavy Duty Truck Group Co. Ltd. is a commercial V2O5-WO3/TiO2 catalyst, which has been widely used in China. The catalyst consists of 2.45% V2O5, 9.6% WO3, and 86.6% TiO2. V2O5 is the active component. As the stabilizer, WO3 increases the number of NH3 adsorption sites57 and prevents anatase from transforming into rutile.20 TiO2 acts as the carrier. The catalyst was prepared by an incipient wetness impregnation method. At first, oxalic acid and NH4VO3 were dissolved in pure water and stirred for 30 min. This solution was then mixed with anatase TiO2 and WO3 powder and stirred for 1.5 h. After that, the mixture was dried and carefully ground. The final step was to calcine the catalyst at 550 °C for 3 h. The catalyst was pressed and sieved prior to the aging experiments. Hydrothermal and thermal deactivation experiments were performed on the flow reactor shown in Figure S1. The feed gas mixture contained 350 ppm NO, 350 ppm NH3, 10% O2, and the remaining N2. The space velocity was 40 000 h−1. Quartz sand was used to reduce the gas flow resistance and would be sieved out after the aging experiments. Each sample contained a mixture of 1.2 g of V2O5-WO3/TiO2 catalyst and 0.8 g of quartz sand. Samples were hydrothermally aged at 600 °C for 48 h, 625 °C for 24 h, 635 °C for 24 h, 650 °C for 4, 8, 12 h, and for 24 h and thermally aged at 650 °C every 4 h from 4 to 24 h. The unaged catalyst sample is labeled “Fresh”. Hydrothermally aged samples are labeled “htT-t”, where “T” represents the aging temperature and “t” refers to the aging time (hours). For example, sample ht600-48 represents samples aged at 600 °C for 48 h. Thermally aged samples at 650 °C for different hours are labelled “T-4”, “T-8”, “T-12”, “T-16”, “T-20”, and “T-24” respectively.
4. CONCLUSIONS In this paper, a commercial Vanadia-SCR catalyst was hydrothermally and thermally deactivated at 600,625, 635, and 650 °C for 4 h to as long as 48 h. Samples were characterized by performing XRD, BET, XPS, in situ FTIR, and NH3-TPD experiments. Main findings are as follows: Both Brønsted acid and Lewis acid sites are involved in the SCR reaction. The two acid sites have different main operation temperatures and antiaging abilities. Brønsted acid sites are more likely to disappear in hydrothermal and thermal aging. They contribute more at lower temperatures. Thus, the conversion rates of the lower temperature decrease more after aging. The more stable Lewis acid sites may work in the entire evaluation temperature range. It needs further study. 5094
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with He gas (50 mL/min) for 90 min, the reactor temperature was raised to 500 °C at a constant heating rate of 10 °C/min. Although H2O has been proved to convert NH3 species to NH4+ species,25,31 it is not allowed to add H2O into the experiments of FTIR and NH3-TPD for the device protection.
The SCR catalytic performance tests were carried out in the same flow reactor with a mixture of 80 mg catalyst samples and 160 mg quartz sand. The feed gas consisted of 450 ppm NO, 450 ppm NH3, 10% O2, 7.8% H2O, and the balance N2. The concentrations of NOX were recorded every 50 °C between 200 and 450 °C at atmospheric pressure. Data were continuously recorded for over 10 min in the steady-state condition and the average values were used to calculate the NOX conversion rate. The corresponding space velocity was 3 × 105 h−1. NOX conversion rates (%) were obtained using Formula 1, where C refers to the concentration. NOX conversion rate (%) =
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03572.
C NOin − C NOout − C NO2out
Introduction of the fixed-bed quartz reactor, graphs of thermally aged samples catalytic performance, XRD patterns, NH3-TPD spectra, and active-site variations of both thermal and hydrothermal aging samples (PDF)
C NOin (1)
Phase transition and sintering were characterized by XRD measurements on a PANalytical X’Pert3 diffractometer. Samples were scanned from 10° to 80° (2θ) in steps of 0.02° s−1 with the Cu Ka anode (λ = 0.15405 Å) at 40 kV and 40 mA. The crystallite dimension D was calculated using Scherrer equation
kλ D= β cos θ
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Corresponding Author
*E-mail:
[email protected]. ORCID
Xingwen Li: 0000-0003-2429-8617 Dongwei Yao: 0000-0001-7698-514X Lai Wei: 0000-0003-3035-2851 Biao Liu: 0000-0001-6254-6306
(2)
where k is a constant depending on the shape of the particle, λ is the wavelength of X-ray radiation, β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the position. The average dimension Danatase(101) refers to the anatase particles perpendicularly to the (1 0 1) plane. The proportion of rutile in aged samples was estimated by following equation XR = (1 + 0.794Ia /Ir)−1
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key R&D Plan of China [grant number 2017YFC0211103], the National Hightech R&D Program of China (863 Program) [grant number 2013AA065301] and the ZJU-UIUC Institute Research Program. The authors would like to thank the China National Heavy Duty Truck Group Co. Ltd for providing the commercial V2O5-WO3/TiO2 catalyst.
(3)
where Ia and Ir are the intensity of (101) and (110) plane reflections for anatase and rutile, respectively. In situ FTIR experiments were performed on a Nicolet 6700 FTIR spectrometer at 100 and 200 °C to detect the active-site changes. The skeletal spectra were recorded in the region of 800−4000 cm−1 at 4 cm−1 resolution. All samples were ground to 100 meshes and were pretreated in a stream of N2 at the temperature of 350 °C for 1 h. Then they were cooled to 100 and 200 °C and saturated with 1000 ppm NH3 for 1 h. Thereafter, the physical adsorption was removed using a 100 mL/min N2 flow before recording the FTIR spectra for 50 min. Time-resolved data were obtained according to the available literature.31 XPS is a surface-sensitive analytical technique providing compositional data and elemental oxidation states. XPS measurements were performed on VG ESCALAB MARK II, using Mg Ka (1253.6 eV) X-ray beam. The C 1s line at 284.6 eV from carbon was used as a reference. The binding energies were calculated with software XPS Peak 4.1. N2 adsorption isotherms for the BET surface area were measured at 77 K with a Micromeritics Trista II. Samples were pretreated for 12 h at 150 °C under vacuum. NH3-TPD experiments were conducted on a fixed-bed reaction system using a chemisorption instrument AutoChem II 2920. Desorption of NH3 was recorded by an online QIC20 mass spectrometer. Prior to the test, 46 mg samples were pretreated in a He stream (50 mL/min) at 450 °C for 45 min. The samples were then cooled to 60 °C and purged for 30 min in 10% NH3/He flow (50 mL/min). After sweeping samples
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REFERENCES
(1) Bosch, H. Formation and control of nitrogen oxides. Catal. Today 1988, 2, 369−379. (2) Lamas, M. I.; Rodríguez, C. G. Emissions from marine engines and NOx reduction methods. J. Mar. Res. 2012, 9, 77−82. (3) Kröcher, O.; Elsener, M. Chemical deactivation of V2O5/WO3− TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies. Appl. Catal., B 2008, 77, 215−227. (4) Chen, J. P.; Yang, R. T. Role of WO3 in mixed V2O5-WO3/TiO2 catalysts for selective catalytic reduction of nitric oxide with ammonia. Appl. Catal., A 1992, 80, 135−148. (5) Cho, C. P.; Pyo, Y. D.; Jang, J. Y.; Kim, G. C.; Shin, Y. J. NOx reduction and N2O emissions in a diesel engine exhaust using Fezeolite and vanadium based SCR catalysts. Appl. Therm. Eng. 2017, 110, 18−24. (6) Oliveri, G.; Ramis, G.; Busca, G.; Escribano, V. S. Thermal stability of Vanadia−Titania catalysts. J. Mater. Chem. 1993, 3, 1239− 1249. (7) Hu, S.; Herner, J. D.; Shafer, M.; Robertson, W.; Schauer, J. J.; Dwyer, H.; Collins, J.; Huai, T.; Ayala, A. Metals emitted from heavyduty diesel vehicles equipped with advanced PM and NOX emission controls. Atmos. Environ. 2009, 43, 2950−2959. (8) Odenbrand, C. U. I. Thermal stability of vanadia SCR catalysts for the use in diesel applications. Chem. Eng. Res. Des. 2008, 86, 663− 672. 5095
DOI: 10.1021/acsomega.8b03572 ACS Omega 2019, 4, 5088−5097
ACS Omega
Article
(28) Chen, L.; Li, J.; Ge, M. DRIFT study on cerium-tungsten/ titania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590−9596. (29) Gasior, M.; Haber, J.; Machej, T. Evolution of V2O5-TiO2 catalysts in the course of the catalytic reaction. Appl. Catal. 1987, 33, 1−14. (30) Zhu, M.; Lai, J.-K.; Tumuluri, U.; Ford, M. E.; Wu, Z.; Wachs, I. E. Reaction Pathways and Kinetics for SCR of Acidic NOx Emissions from Power Plants with NH3. ACS Catal. 2017, 7, 8358− 8361. (31) Zhu, M.; Lai, J.-K.; Tumuluri, U.; Wu, Z.; Wachs, I. E. Nature of Active Sites and Surface Intermediates during SCR of NO with NH3 by Supported V2O5-WO3/TiO2 Catalysts. J. Am. Chem. Soc. 2017, 139, 15624−15627. (32) Lin, C.-H.; Bai, H. Adsorption Behavior of Moisture over a Vanadia/Titania Catalyst: A Study for the Selective Catalytic Reduction Process. Ind. Eng. Chem. Res. 2004, 43, 5983−5988. (33) Wang, C.; Yang, S.; Chang, H.; Peng, Y.; Li, J. Dispersion of tungsten oxide on SCR performance of V2O5-WO3/TiO2: Acidity, surface species and catalytic activity. Chem. Eng. J. 2013, 225, 520− 527. (34) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. T. Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalystsA review. Catal. Today 2011, 175, 147−156. (35) Nova, I.; Lietti, L.; Beretta, A.; Forzatti, P. Study of the sintering of a deNOx commercial catalyst. Stud. Surf. Sci. Catal. 2001, 139, 149−156. (36) Casanova, M.; Schermanz, K.; Llorca, J.; Trovarelli, A. Improved high temperature stability of NH3-SCR catalysts based on rare earth vanadates supported on TiO2-WO3-SiO2. Catal. Today 2012, 184, 227−236. (37) Maunula, T.; Kinnunen, T.; Kanniainen, K.; Viitanen, A.; Savimaki, A. Thermally Durable Vanadium-SCR Catalysts for Diesel Applications; SAE Technical Paper, 2013; No. 2013-01-1063. (38) Chapman, D. M. Behavior of titania-supported vanadia and tungsta SCR catalysts at high temperatures in reactant streams: Tungsten and vanadium oxide and hydroxide vapor pressure reduction by surficial stabilization. Appl. Catal., A 2011, 392, 143− 150. (39) ToPsøE, N.-Y.; Slabiak, T.; Clausen, B. S.; Srnak, T. Z.; Dumesic, J. A. Influence of water on the reactivity of vanadia/titania for catalytic reduction of NOx. J. Catal. 1992, 134, 742−746. (40) Bulushev, D. A.; Kiwi-Minsker, L.; Zaikovskii, V. I.; Renken, A. Formation of Active Sites for Selective Toluene Oxidation during Catalyst Synthesis via Solid-State Reaction of V2O5 with TiO2. J. Catal. 2000, 193, 145−153. (41) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl. Catal., B 1998, 18, 1− 36. (42) Topsoe, N. Y.; Dumesic, J. A.; Topsoe, H. Vanadia-Titania Catalysts for Selective Catalytic Reduction of Nitric-Oxide by Ammonia. J. Catal. 1995, 151, 241−252. (43) Liu, Q.; Liu, Z.; Li, C. Adsorption and Activation of NH3 during Selective Catalytic Reduction of NO by NH3. Chin. J. Catal. 2006, 27, 636−646. (44) Ramis, G.; Yi, L.; Busca, G. Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study. Catal. Today 1996, 28, 373−380. (45) Lin, C.-H.; Bai, H. Surface acidity over vanadia/titania catalyst in the selective catalytic reduction for NO removal-in situ DRIFTS study. Appl. Catal., B 2003, 42, 279−287. (46) Sun, D. K.; Liu, Q. Y.; Liu, Z. Y.; Gui, G. Q.; Huang, Z. G. Adsorption and oxidation of NH3 over V2O5/AC surface. Appl. Catal., B 2009, 92 (3), 462−467. (47) Xi, Y.; Ottinger, N. A.; Liu, Z. G. New insights into sulfur poisoning on a vanadia SCR catalyst under simulated diesel engine operating conditions. Appl. Catal., B 2014, 160−161, 1−9.
(9) Madia, G.; Elsener, M.; Koebel, M.; Raimondi, F.; Wokaun, A. Thermal stability of vanadia-tungsta-titania catalysts in the SCR process. Appl. Catal., B 2002, 39, 181−190. (10) Asako, T.; Kai, R.; Toyoshima, T.; Vogt, C.; Hirose, S.; Nakao, S. Evaluation of Hydrothermally Aged Vanadia SCR on High-Porosity Substrate; SAE Technical Paper, 2016. (11) Marberger, A.; Elsener, M.; Ferri, D.; Kröcher, O. VOx Surface Coverage Optimization of V2O5/WO3-TiO2 SCR Catalysts by Variation of the V Loading and by Aging. Catalysts 2015, 5, 1704− 1720. (12) Girard, J. W.; Montreuil, C.; Kim, J.; Cavataio, G.; Lambert, C. Technical Advantages of Vanadium SCR Systems for Diesel NOx Control in Emerging Markets. SAE Int. J. Fuels Lubr. 2008, 1, 488− 494. (13) Nova, I.; dall’Acqua, L.; Lietti, L.; Giamello, E.; Forzatti, P. Study of thermal deactivation of a de-NO x commercial catalyst. Appl. Catal., B 2001, 35, 31−42. (14) Rasmussen, S. B.; Abrams, B. L. Fundamental chemistry of VSCR catalysts at elevated temperatures. Catal. Today 2017, 297, 60− 63. (15) Xi, Y.; Ottinger, N.; Liu, Z. G. Effect of Hydrothermal Aging on the Catalytic Performance and Morphology of a Vanadia SCR Catalyst; Sae Technical Paper, 2013; No. 2013-01-1079. (16) Maunula, T.; Kinnunen, T.; Kanniainen, K.; Viitanen, A.; Savimaki, A. Thermally Durable Vanadium-SCR Catalysts for Diesel Applications; SAE International Journal of Engines, 2013; No. 201301-1063. (17) Li, Z.; Li, J.; Liu, S.; Ren, X.; Ma, J.; Su, W.; Peng, Y. Ultra hydrothermal stability of CeO2 -WO3/TiO2 for NH3 -SCR of NO compared to traditional V2O5-WO3/TiO2 catalyst. Catal. Today 2015, 258, 11−16. (18) Beale, A. M.; Lezcano-Gonzalez, I.; Maunula, T.; Palgrave, R. G. Development and characterization of thermally stable supported V−W−TiO2 catalysts for mobile NH3−SCR applications. Catal., Struct. React. 2012, 1, 25−34. (19) Went, G. T.; Leu, L. J.; Rosin, R. R.; Bell, A. T. The effects of structure on the catalytic activity and selectivity of V2O5/TiO2 for the reduction of NO by NH3. J. Catal. 2010, 23, 492−505. (20) Choung, J. W.; Nam, I.-S.; Ham, S.-W. Effect of promoters including tungsten and barium on the thermal stability of V2O5/ sulfated TiO2 catalyst for NO reduction by NH3. Catal. Today 2006, 111, 242−247. (21) Reddy, B. M.; Ganesh, A.; Reddy, E. P. Study of Dispersion and Thermal Stability of V2O5/TiO2−SiO2 Catalysts by XPS and Other Techniques. J. Phys. Chem. B 1997, 101, 1769−1774. (22) Went, G. T.; Leu, L.-j.; Bell, A. T. Quantitative structural analysis of dispersed vanadia species in TiO2 (anatase)-supported V2O5. J. Catal. 1992, 134, 479−491. (23) Wachs, I. E.; Deo, G.; Weckhuysen, B. M.; Andreini, A.; Vuurman, M. A.; Boer, M. D.; Amiridis, M. D. Selective Catalytic Reduction of NO with NH3 over Supported Vanadia Catalysts. J. Catal. 1996, 161, 211−221. (24) Djerad, S.; Tifouti, L.; Crocoll, M.; Weisweiler, W. Effect of vanadia and tungsten loadings on the physical and chemical characteristics of V2O5-WO3/TiO2 catalysts. J. Mol. Catal. A: Chem. 2004, 208, 257−265. (25) Lai, J.-K.; Wachs, E. A Perspective on the Selective Catalytic Reduction (SCR) of NO with NH3 by Supported V2O5−WO3/TiO2 Catalysts. ACS Catal. 2018, 8, 6537−6551. (26) Nova, I.; dall’Acqua, L.; Lietti, L.; Giamello, E.; Forzatti, P. Study of thermal deactivation of a de-NOx commercial catalyst. Appl. Catal., B 2002, 35, 31−42. (27) Kompio, P. G. W. A.; Brückner, A.; Hipler, F.; Manoylova, O.; Auer, G.; Mestl, G.; Grünert, W. V2O5-WO3 /TiO2 catalysts under thermal stress: Responses of structure and catalytic behavior in the selective catalytic reduction of NO by NH3. Appl. Catal., B 2017, 217, 365−377. 5096
DOI: 10.1021/acsomega.8b03572 ACS Omega 2019, 4, 5088−5097
ACS Omega
Article
(48) Li, X.; Li, X.; Yang, R. T.; Mo, J.; Li, J.; Hao, J. The poisoning effects of calcium on V2O5-WO3/TiO2 catalyst for the SCR reaction: Comparison of different forms of calcium. Mol. Catal. 2017, 434, 16− 24. (49) Lietti, L.; Ramis, G.; Berti, F.; Toledo, G.; Robba, D.; Busca, G.; Forzatti, P. Chemical, structural and mechanistic aspects on NOx SCR over commercial and model oxide catalysts. Catal. Today 1998, 42, 101−116. (50) Zhang, Y.; Guo, W.; Wang, L.; Song, M.; Yang, L.; Shen, K.; Xu, H.; Zhou, C. Characterization and activity of V2O5-CeO2/TiO2ZrO2 catalysts for NH3-selective catalytic reduction of NOx. Chin. J. Catal. 2015, 36, 1701−1710. (51) Dong, G.-j.; Bai, Y.; Zhang, Y.-f.; Zhao, Y. Effect of the V4+(3+)/ V5+ratio on the denitration activity for V2O5−WO3/TiO2 catalysts. New J. Chem. 2015, 39, 3588−3596. (52) Biesinger, M. C.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887−898. (53) Guo, X.; Bartholomew, C.; Hecker, W.; Baxter, L. L. Effects of sulfate species on V2O5/TiO2 SCR catalysts in coal and biomass-fired systems. Appl. Catal., B 2009, 92, 30−40. (54) Li, W.-Z.; Gao, F.; Li, Y.; Walter, E. D.; Liu, J.; Peden, C. H. F.; Wang, Y. Nanocrystalline Anatase Titania-Supported Vanadia Catalysts: Facet-Dependent Structure of Vanadia. J. Phys. Chem. C 2015, 119, 15094−15102. (55) Topsoe, N. Y. Mechanism of the selective catalytic reduction of nitric oxide by ammonia elucidated by in situ on-line fourier transform infrared spectroscopy. Science 1994, 265, 1217−1219. (56) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction. Appl. Catal. 1990, 64, 259−278. (57) Amiridis, M. D.; Duevel, R. V.; Wachs, I. E. The effect of metal oxide additives on the activity of V2O5/TiO2 catalysts for the selective catalytic reduction of nitric oxide by ammonia. Appl. Catal., B 1999, 20, 111−122.
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