Journal of Nanomaterials
Volume 2015 (2015), Article ID 273968, 7 pages
http://dx.doi.org/10.1155/2015/273968

Research Article

Promotional Effect on Selective Catalytic Reduction of NOx with NH₃ over Overloaded W and Ce on V₂O₅/TiO₂ Catalysts

Seunghee Youn, Inhak Song, and Do Heui Kim

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 151-744, Republic of Korea

Received 30 January 2015; Revised 3 March 2015; Accepted 4 March 2015

Academic Editor: Zhi Chen

Copyright © 2015 Seunghee Youn et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

W and Ce are known to be a good promoters to improve selective catalytic reduction (SCR) activity for V₂O₅/TiO₂ catalysts. This work aimed at finding the optimum ratio and loading of promoters (W and Ce) on V₂O₅/TiO₂ catalyst in order to improve SCR reactivity in low temperature region and to minimize N₂O formation in high temperature region. In addition, we changed the order of impregnation between W and Ce precursors on V₂O₅/TiO₂ catalyst during the preparation and observed its effect on SCR activity and N₂ selectivity. We utilized various analytical techniques, such as N₂ adsorption-desorption, X-ray diffraction (XRD), and temperature-programmed reduction with hydrogen (H₂ TPR) to investigate the physicochemical properties of catalysts. It was found that W- and Ce-overloaded V₂O₅/TiO₂ catalyst such as W/Ce/V/TiO₂ (15 : 15 : 1 wt%) showed the most remarkable DeNOx properties over the wide temperature region. Additionally, this catalyst significantly suppressed N₂O formation during SCR reaction, especially in high temperature region (350–400°C). Based on the characterization results, it was found that such superior activity originated from the improved reducibility and morphology of W and Ce species on V₂O₅/TiO₂ catalyst when they are incorporated together at high loading.

1. Introduction

Selective catalytic reduction of NOx with NH₃ (NH₃-SCR) as reductant is well-known technology for removing NOx emissions from stationary source or diesel engine vehicles operated under highly oxidizing condition [1–5]. Generally, vanadia-titania-based NH₃-SCR catalysts have been used in stationary sources due to high resistance to SO₂, while Cu-based zeolite is being used for mobile source [6]. In spite of a high NOx conversion between 300 and 400°C, the side reactions such as the oxidation of NH₃ to N₂O or NO and toxicity of the evaporated vanadium at high temperature are the major problems of V₂O₅/TiO₂ catalysts to overcome [7, 8].

Meanwhile, International Maritime Organization (IMO) announced the NOx regulations (Tier III) on vessels, which will take effect from 2016 [9]. It makes catalyst community pay attention to the DeNOx technology for marine application. In particular, the SCR catalyst for marine application requires higher DeNOx activity in the low temperature range (200–250°C) due to the low temperature exhaust from marine engine. In addition to needs to lower the SCR operation temperature, a concern for the emissions of N₂O has been growing, since N₂O has about 300 times greater greenhouse gas effect than CO₂. During NH₃ SCR reaction, N₂O is known to be produced in the following reaction:

In commercial catalysts, the loading of vanadia is generally as low as 1–1.5 wt% whereas the loading of promoters is about 6–10 wt%, respectively [3]. W-V/TiO₂ or Ce-V/TiO₂ is well-known SCR catalytic system for commercial application. However, there is no previous study regarding the overloading effect (higher than 10 wt%) of promoters on V₂O₅/TiO₂ SCR catalysts to our knowledge. Therefore, the aim of this study is to find the optimum condition to prepare V₂O₅/TiO₂ SCR catalyst with high DeNOx activity in low temperature and minimized N₂O formation by loading the excessive amount of promoters (W and Ce) compared with vanadium loading. Additionally, we tried to change the order of impregnation between two promoters on V₂O₅/TiO₂ while focusing on the characteristics of catalysts. We also investigate whether the synergetic effect of promoters exists when they are loaded together on V₂O₅/TiO₂. To sum up, the effect of impregnation order and the ratio between promoters and vanadium in M/V₂O₅/TiO₂ catalyst on the NOx conversion and the N₂O selectivity during SCR reaction were addressed in this study.

2. Experimental

2.1. Catalysts Preparation

All catalysts were prepared by applying wet impregnation method of vanadium and promoter solution on anatase titania. Anatase TiO₂ powder (DT-51 Millennium Chemicals) was used as support. Ammonium metavanadate (99%, Sigma Aldrich) was dissolved in diluted oxalic acid solution (0.5 M) to produce the solution of vanadium precursor [10]. The samples with 1 wt%, 3 wt%, and 5 wt% V₂O₅ loading on TiO₂ were prepared. We used two promoters, W and Ce, to be impregnated on V₂O₅/TiO₂ catalyst. Ammonium tungstate (99.99%, Sigma Aldrich) and cerium(III) nitrate hexahydrate (99.99%, Sigma Aldrich) were used to produce precursor solution which was dissolved in diluted water. After drying at 105°C for 12 h, promoter precursor solution was added to V₂O₅/TiO₂ in a rotary evaporator again. After a series of impregnation processes, catalysts were dried and then calcined at 400°C for 4 h in air.

Sample was designated as the following notation M1/M2//VT ( :  : ). M1 and M2 showed the promoters and sequence of impregnation, where M2 is firstly impregnated on V₂O₅/TiO₂, followed by the impregnation of M1 on M2/V₂O₅/TiO₂. and meant the weight loading of each promoter and is the weight loading of vanadium on TiO₂. For example, W/Ce/VT (15 : 15 : 1) indicated V₂O₅/TiO₂ catalyst with 1 wt% of vanadium loading which are promoted with W and Ce promoters with 15 wt% each.

2.2. Characterization

The specific surface area of catalysts was measured by ASAP 2010 (Micromeritics) apparatus and calculated by using BET method. The samples about 0.1 g were degassed at 250°C for 12 h before measurement. After the pretreatment step, N₂ adsorption-desorption isotherms were obtained at −196°C. The X-ray diffraction (XRD) patterns were obtained by using a Rigaku mode 1 Smartlab with Cu Κα radiation ( nm). 40 kV and 50 mA of voltage and current were applied and the XRD patterns were collected in a 2-theta range from 20 to 80° at a speed of 10°/min [11].

H₂ TPR was used to measure the reducibility of metal oxides by detecting the hydrogen consumption at elevated temperature with a thermal conductivity detector (TCD) in a BEL-CAT-BASIC (BEL Japan Inc.). Prior to the analysis, 0.03 g samples were pretreated at 350°C for 1 h in a flow of 30 mL/min Ar. After cooling down to room temperature, samples were then exposed to 5% H₂/Ar until the temperature reached 800°C at the rate of 10°C/min.

2.3. Reaction Test

SCR activity was measured in a fixed-bed quartz tubular reactor. Catalysts were sieved to 300–500 μm in diameter then loaded in the reactor. The composition of inlet gas was 500 ppm NO, 500 ppm NH₃, and 2% O₂ and was balanced with N₂. Space velocity of inlet gas was maintained to be 40,000 h⁻¹. We raised reaction temperature from 150°C to 400°C in increments of 50°C with the rate of 10°C/min. NOx concentration of outlet gas was analyzed by using NOx chemiluminescence analyzer (Model 42i High level, Thermo Scientific). Also, Fourier transform infrared (FT-IR) spectroscopy was applied to observe the N₂O concentration in the gas. We used the average data of 16 scans at a resolution of 1.0 cm⁻¹. A Nicolet 6700 (Thermo Scientific), with 2 m gas analysis cell heated to 120°C to exclude the effect of H₂O, was used for gas phase analysis. N₂ selectivity referred to in this study was calculated by using the following equation [2]:

3. Result and Discussion

Two representative elements, W and Ce, were chosen as promoters to V₂O₅/TiO₂. The order of impregnation and loading was changed to find the optimum catalyst. All results of prepared catalysts were displayed in Table 1. Since high SCR activity at low temperature and low N₂O formation at high temperature are regarded as two main requirements for novel SCR catalysts, we chose NOx conversion at 200°C and N₂O concentration at 400°C to compare the samples we tested in this study. Firstly, Figure 1 showed NOx conversion and N₂O concentration of various samples containing 3 wt% V₂O₅ with different amounts of W and Ce. Each SCR reaction result was compared with standard SCR catalysts with same vanadium loading.

Table 1: conversion at 200°C and N₂O concentration at 400°C obtained on all the samples we tested in this study.

Sample
conversion (%) at 200°C
N₂O concentration (ppm) at 400°C

1 wt% V₂O₅/TiO₂
10
15.4

3 wt% V₂O₅/TiO₂
55
41.3

5 wt% V₂O₅/TiO₂
67
42.1

W/VT (9 : 3)(a)
67
36.2

Ce/VT (9 : 3)
63
10.8

W/Ce/VT (9 : 9 : 3)
58
14.4

Ce/W/VT (9 : 9 : 3)
51
13.2

W/VT (9 : 1)
34
6.9

Ce/VT (9 : 1)
41
5.6

W/Ce/VT (9 : 9 : 1)
36
5.0

Ce/W/VT (9 : 9 : 1)
33
5.2

W/VT (15 : 1)
28
6.7

Ce/VT (15 : 1)
44
3.7

W/Ce/VT (15 : 15 : 1)(b)
68
4.2

Ce/W/VT (15 : 15 : 1)(c)
55
3.7

(a), (b), and (c) were chosen as the promising catalysts.

(a)

(b)

Figure 1: (a) NOx conversion and (b) N₂O concentration of various samples containing 3 wt% vanadium and 9 wt% promoters.

As shown in Figure 1, the addition of 9 wt% promoters on 3 wt% V₂O₅/TiO₂ (W/VT (9 : 3) or Ce/VT (9 : 3)) demonstrated the slightly higher NOx conversion than 3 wt% V₂O₅/TiO₂ catalysts not only in the low temperature region (250°C) but also in the high temperature region (400°C). However, W/Ce/VT (9 : 9 : 3) or Ce/W/VT (9 : 9 : 3) catalyst, which were expected to show synergetic effect of two promoters, showed similar SCR activity in low temperature region below 250°C to standard sample, implying that no synergism between two promoters exists at this condition. It must be pointed out that all W- and Ce-containing catalysts displayed lower N₂O formation at 400°C than 3 wt% V₂O₅/TiO₂. 3 wt% V₂O₅/TiO₂ catalyst showed 81% N₂ selectivity whereas Ce-containing catalysts showed about 91%.

The ratio of promoter to vanadium was increased by lowering the vanadium loading to 1 wt% to see whether the promotional effect is enhanced. As shown in Figure 2, all the promoter-containing catalysts demonstrated considerably improved NOx conversion, especially in low temperature range, compared with 1 wt% V₂O₅/TiO₂, implying that the promotional effect is remarkably enhanced over the catalyst with the low vanadium loading (1 wt%). As observed in the Figure 1, however, the addition of two promoters, for example, W/Ce/VT (9 : 9 : 3), did not give rise to the enhanced NOx conversion over all temperature ranges.

(a)

(b)

Figure 2: (a) NOx conversion and (b) N₂O concentration of various samples containing 1 wt% vanadium and 9 wt% promoters.

Since we observed the promotional effect of adding promoters (9 wt% Ce or W) on SCR activity, we further increased the amount of promoters to 15 wt%. As shown in Figure 3, the increase in W loading from 9 wt% to 15 wt% resulted in the rather decrease in the activity at 200°C, while that of Ce loading showed almost the same activity. More drastic enhancement of activity in the low temperature region was observed for the samples such as W/Ce/VT (15 : 15 : 1) or Ce/W/VT (15 : 15 : 1). For example, W/Ce/VT (15 : 15 : 1) catalyst displayed 68% of  NOx conversion at 200°C which was the best activity among all catalysts. This value was as high as that of W/VT (9 : 3) catalyst; however, more important thing to note would be even lower N₂O production (30 ppm versus 5 ppm) at 400°C.

(a)

(b)

Figure 3: (a) NOx conversion and (b) N₂O concentration of various samples containing 1 wt% vanadium and 15 wt% promoters.

Noteworthy is that the synergism between Ce and W elements is observed only at higher ratio of promoters to vanadium. For the case of W/Ce/VT (9 : 9 : 1) catalyst, there is no promotional effect upon simultaneous loading of two promoters. On the other hand, we observed confident synergistic effect over W/Ce/VT (15 : 15 : 1) sample, especially the NOx conversion in the low temperature region, although there is not much difference in N₂O production. It must be pointed out that, for most cases, the change of the impregnation order of W and Ce did not reveal remarkable effect on NOx conversion and N₂O formation during SCR reaction. According to the references, WO₃/TiO₂ [12] and CeO₂/TiO₂ [13] catalysts showed low emission of N₂O at high temperature, although the reason of low N₂O formation was not fully identified. Generally, vanadium-containing catalysts usually emit more N₂O than vanadium-free catalysts while the former has higher deNOx activity than the latter. Therefore, the result of reduced N₂O formation on vanadium-containing catalysts the same level as nonvanadium metal oxide catalysts on TiO₂ was a remarkable accomplishment of our study.

Figure 4 displayed how overloaded W- and Ce-containing catalysts improve SCR activity compared with pure V₂O₅/TiO₂ catalysts. W/Ce/VT (15 : 15 : 1) and Ce/W/VT (15 : 15 : 1) showed slightly higher NOx conversion than 5 wt% V₂O₅/TiO₂ catalyst over all temperature ranges; more importantly, they produced about 88% less N₂O at 400°C. It is worth to mention that the higher activity and excellent N₂ selectivity can be obtained at low vanadium loading, which is regarded as the toxic element.

(a)

(b)

Figure 4: NOx conversion and N₂O concentration of promising catalysts compared with 1 wt%, 3 wt%, and 5 wt% V₂O₅/TiO₂.

We applied various analytic techniques to investigate which characteristics made the difference in SCR reaction among variety of samples with different amount of promoters. Firstly, BET surface area of catalysts was measured by using N₂ adsorption-desorption method. Even samples containing overloaded Ce and W, in other words, Ce/W/VT (15 : 15 : 1) and W/Ce/VT (15 : 15 : 1) catalysts, showed the BET surface area of 60 m²/g and 66 m²/g, respectively. Since pure TiO₂ has the surface area of 87 m²/g, it is confirmed that the overloading of W and Ce does not change the textural property of the catalyst too much, for example, by the blockage of pore.

As presented in Figure 5(a), XRD patterns were obtained to investigate the crystallinity of various catalysts containing 3 wt% of vanadium. Only anatase TiO₂ phase was observed in these catalysts with the peaks at 2 theta equal to 26°, 36°, 37°, 48°, 54°, 55°, 63°, 69°, 70°, and 75° [14, 15], except for the Ce/VT (9 : 3) sample in which very small peak at 28.5° arising from CeO₂ phase was brought out. This means that most of W or Ce promoters exist in the amorphous form or in a highly dispersed state on TiO₂ support.

(a)

(b)

(c)

Figure 5: XRD patterns of catalysts with a ratio of promoter : vanadium—(a) 9 : 3, (b) 9 : 1, and (c) 15 : 1 (•: CeO₂, ▴: WO₃).

When the vanadium loading decreased to 1 wt%, all catalysts also showed only TiO₂ main peaks, except for Ce/W/VT (9 : 9 : 1) catalyst displaying small CeO₂ peaks at 2 theta equal to 28°, 34°, 47°, and 56°, as demonstrated in Figure 5(b). However, the sample with the reversed impregnation order, in other words, W/Ce/VT (9 : 9 : 1) catalyst, does not show any peaks other than TiO₂, implying that the impregnation order affects the crystallinity of the sample. As presented in Figure 5(c), all samples displayed small peaks in XRD patterns arising from either CeO₂ or WO₃. Specifically, broad CeO₂ peaks were observed in all 15 wt% Ce-containing catalysts. The samples containing both 15 wt% W and 15 wt% Ce, in other words, Ce/W/VT (15 : 1) and W/Ce/VT (15 : 15 : 1) catalysts, had lower crystalline CeO₂ phase compared with Ce/VT (15 : 1), implying that W aided in dispersing CeO₂ in the catalyst. In addition, crystalline WO₃ phase [16] was observed in the XRD pattern of W/VT (15 : 1) catalyst, which might have a negative effect on the activity as shown in Figure 3. However, the WO₃ phase was not detected for the samples such as W/Ce/VT (15 : 15 : 1) and Ce/W/VT (15 : 15 : 1) catalysts, where both Ce and W promoters were incorporated. Summarizing XRD patterns and activity measurement, it can be clearly confirmed that the synergistic effect observed in the samples such as W/Ce/VT (15 : 15 : 1) and Ce/W/VT (15 : 15 : 1) catalysts originated from the formation of highly dispersed Ce or W elements in the catalysts.

H₂ TPR is a widely used technique to investigate the redox properties of the catalysts. The H₂ TPR profiles of the prepared catalysts are shown in Figure 6. For the case of W/VT (15 : 1) sample, two H₂ consumption peaks at 500°C and 750°C were shown, which were assigned to the reduction of vanadium oxide and tungsten oxide, respectively [17]. On the other hand, Ce/VT (15 : 1) sample had the peak at 475°C, arising from the reduction of Ce⁴⁺ to Ce³⁺ reaction. The reduction behavior of the elements has changed when two promoters were incorporated together (W/Ce/VT or Ce/W/VT). It was reported that the introduction of Ce not only improved the oxygen storage capacity but also enhanced the redox activity by interaction between W and Ce species. The improvement of redox activity is confirmed by comparing the peak area of each sample. The peak area increased in the following order: W/VT (15 : 1) < Ce/VT (15 : 1) < W/Ce/VT (15 : 15 : 1) < Ce/W/VT (15 : 15 : 1). It is noticeable that two-promoter-containing catalysts showed larger peak area than only one promoter impregnated on V₂O₅/TiO₂. Samples which have larger peak area had more reducible form of metal oxide on their surfaces assisting higher NOx reduction capability. Furthermore, the shift of the reduction peak of W/Ce/VT (15 : 15 : 1) and Ce/W/VT (15 : 15 : 1) compared with W/VT and Ce/VT indicated the close interaction between Ce and W species, resulting in the different Ce or W species on the surface. In addition, the fact that W/Ce/VT (15 : 15 : 1) showed higher NOx conversion than Ce/W/VT (15 : 15 : 1) in low temperature region can be explained by the difference in reduction behavior of catalyst. Therefore, the simultaneous introduction of W and Ce could create a new center for oxygen storage and release, which can act as an active site for SCR reaction.

Figure 6: H₂ TPR profile of various samples containing 1 wt% vanadium and promoters.

4. Conclusion

The promotional effect of loading W and Ce on V₂O₅/TiO₂ catalyst was investigated while changing different loading ratio between promoters and vanadium and the order of loading promoters in order to find the optimum catalyst for SCR reaction. Among the catalysts, W/Ce/VT (15 : 15 : 1) catalyst demonstrated the most desirable reaction behavior with high NOx conversion and less N₂O formation during SCR reaction. It was found that highly dispersed CeO₂ and WO₃ phases were present even at high loading of W and Ce. Combined XRD and H₂ TPR results confirmed that the simultaneous impregnation of W and Ce on V₂O₅/TiO₂ at high loading leads to the change in the reducibility and morphology of elements in catalysts that eventually facilitated SCR reaction in low temperature region and minimized the amount of N₂O formed in high temperature region.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This project is supported by the R&D Center for reduction of Non-CO₂ Greenhouse Gases (0458-20140019) funded by Korea Ministry of Environment (MOE) as Global Top Environment R&D Program.

References

L. Zhang, W. Zou, K. Ma et al., “Sulfated temperature effects on the catalytic activity of CeO₂ in NH₃-selective catalytic reduction conditions,” The Journal of Physical Chemistry C, vol. 119, no. 2, pp. 1155–1163, 2015.

D. W. Kwon, K. B. Nam, and S. C. Hong, “The role of ceria on the activity and SO₂ resistance of catalysts for the selective catalytic reduction of NOx by NH₃,” Applied Catalysis B: Environmental, vol. 166-167, pp. 37–44, 2015.

P. Forzatti, “Present status and perspectives in de-NOx SCR catalysis,” Applied Catalysis A: General, vol. 222, no. 1-2, pp. 221–236, 2001.

M.-J. Jeon, S. H. Park, J. M. Kim et al., “Removal of NOx at low temperature over mesoporous α-Mn₂O₃ catalyst,” Journal of Nanoscience and Nanotechnology, vol. 14, no. 3, pp. 2527–2531, 2014.

Q. Tian, H. Liu, W. Yao et al., “SO₂ poisoning behaviors of Ca-Mn/TiO₂ catalysts for selective catalytic reduction of NO with NH₃ at low temperature,” Journal of Nanomaterials, vol. 2014, Article ID 904649, 6 pages, 2014.

L. Ma, Y. Cheng, G. Cavataio, R. W. McCabe, L. Fu, and J. Li, “In situ DRIFTS and temperature-programmed technology study on NH₃-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts,” Applied Catalysis B: Environmental, vol. 156-157, pp. 428–437, 2014.

S. Roy, B. Viswanath, M. S. Hegde, and G. Madras, “Low-temperature selective catalytic reduction of NO with NH₃ over Ti0.9M0.1O2−δ (M = Cr, Mn, Fe, Co, Cu),” The Journal of Physical Chemistry C, vol. 112, no. 15, pp. 6002–6012, 2008.

X. Mou, B. Zhang, Y. Li et al., “Rod-shaped Fe₂O₃ as an efficient catalyst for the selective reduction of nitrogen oxide by ammonia,” Angewandte Chemie International Edition, vol. 51, no. 12, pp. 2989–2993, 2012.

M. Magnusson, E. Fridell, and H. H. Ingelsten, “The influence of sulfur dioxide and water on the performance of a marine SCR catalyst,” Applied Catalysis B: Environmental, vol. 111-112, pp. 20–26, 2012.

J. M. Cho, M. H. Sun, T. H. Kim, and S. J. Cho, “Formation of nanotubule, nanorod and polycrystalline nanoparticles TiO₂ by alkaline hydrothermal transformation of anatase TiO₂,” Journal of Nanoscience and Nanotechnology, vol. 10, no. 5, pp. 3336–3340, 2010.

X. Chen, C. Cen, Z. Tang et al., “The key role of pH value in the synthesis of titanate nanotubes-loaded manganese oxides as a superior catalyst for the selective catalytic reduction of NO with NH₃,” Journal of Nanomaterials, vol. 2013, Article ID 871528, 7 pages, 2013.

M. Kobayashi and K. Miyoshi, “WO₃-TiO₂ monolithic catalysts for high temperature SCR of NO by NH₃: influence of preparation method on structural and physico-chemical properties, activity and durability,” Applied Catalysis B: Environmental, vol. 72, no. 3-4, pp. 253–261, 2007.

W. Shan, F. Liu, H. He, X. Shi, and C. Zhang, “An environmentally-benign CeO₂-TiO₂ catalyst for the selective catalytic reduction of NOx with NH₃ in simulated diesel exhaust,” Catalysis Today, vol. 184, no. 1, pp. 160–165, 2012.

G.-J. Dong, Y.-F. Zhang, Y. Zhao, and Y. Bai, “Effect of the pH value of precursor solution on the catalytic performance of V₂O₅-WO₃/TiO₂ in the low temperature NH₃-SCR of NOx,” Journal of Fuel Chemistry and Technology, vol. 42, no. 12, pp. 1455–1463, 2014.

W. Yu, X. Wu, Z. Si, and D. Weng, “Influences of impregnation procedure on the SCR activity and alkali resistance of V₂O₅–WO₃/TiO₂ catalyst,” Applied Surface Science, vol. 283, pp. 209–214, 2013.

S. Zhang and Q. Zhong, “Surface characterization studies on the interaction of V₂O₅–WO₃/TiO₂ catalyst for low temperature SCR of NO with NH₃,” Journal of Solid State Chemistry, vol. 221, pp. 49–56, 2015.

Q. Wan, L. Duan, K. He, and J. Li, “Removal of gaseous elemental mercury over a CeO₂-WO₃/TiO₂ nanocomposite in simulated coal-fired flue gas,” Chemical Engineering Journal, vol. 170, no. 2-3, pp. 512–517, 2011.