Environmental Technology Selective catalytic reduction (SCR) of NO ...

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Selective catalytic reduction (SCR) of NO by urea loaded on activated carbon fibre (ACF) and CeO2/ACF at 30 °C: The SCR mechanism Zheng Zeng b

a b

, Pei Lu

& Xiaopeng Fan

a b

, Caiting Li

a b

, Guangming Zeng

a b

, Xiao Jiang

a b

, Yunbo Zhai

a

a b

a

College of Environmental Science and Engineering, Hunan University , Changsha , 410082 , China b

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education , Changsha , 410082 , China Accepted author version posted online: 29 Sep 2011.Published online: 04 Nov 2011.

To cite this article: Zheng Zeng , Pei Lu , Caiting Li , Guangming Zeng , Xiao Jiang , Yunbo Zhai & Xiaopeng Fan (2012) Selective catalytic reduction (SCR) of NO by urea loaded on activated carbon fibre (ACF) and CeO2/ACF at 30 °C: The SCR mechanism, Environmental Technology, 33:11, 1331-1337, DOI: 10.1080/09593330.2011.626799 To link to this article: http://dx.doi.org/10.1080/09593330.2011.626799

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Environmental Technology Vol. 33, No. 11, June 2012, 1331–1337

Selective catalytic reduction (SCR) of NO by urea loaded on activated carbon fibre (ACF) and CeO2 /ACF at 30 ◦ C: The SCR mechanism Zheng Zenga,b† , Pei Lua,b∗† , Caiting Lia,b† , Guangming Zenga,b , Xiao Jianga,b , Yunbo Zhaia,b and Xiaopeng Fana,b a College

of Environmental Science and Engineering, Hunan University, Changsha 410082, China; b Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, China

Downloaded by [Arizona State University], [Pei Lu] at 15:09 08 January 2014

(Received 17 July 2011; accepted 16 September 2011 ) Selective catalytic reduction (SCR) of NO by urea loaded on rayon-based activated carbon fibre (ACF) and CeO2 /ACF (CA) was studied at ambient temperature (30 ◦ C) to establish a basic scheme for its reduction. Nitric oxide was found to be reduced to N2 with urea deposited on the ACF and CA. When oxygen was present, the greater the amount of loaded urea (20–60%), the greater the NOx conversions, which were between 72.03% and 77.30%, whereas the NOx conversions were about 50% when oxygen was absent. Moreover, when the urea was loaded on CA, a catalyst containing 40% urea/ACF loaded with 10% CeO2 (UCA4) could yield a NOx conversion of about 80% for 24.5 h. Based on the experimental results, the catalytic mechanisms of SCR with and without oxygen are discussed. The enhancing effect of oxygen resulted from the oxidation of NO to NO2 , and urea was the main reducing agent in the SCR of loaded catalysts. ACF-C was the catalytic centre in the SCR of NO of ACF, while CeO2 of urea-loaded CA was the catalytic centre. Keywords: NO reduction; urea; CeO2 ; activated carbon fibre; catalytic mechanism

1. Introduction Nitrogen oxides (NO, NO2 and N2 O) remain a major source of air pollution, and contribute to photochemical smog, acid rain, ozone depletion and greenhouse effects[1]. In spite of large efforts to reduce the emission of NO [2,3], the NO concentration in urban areas is not reduced low enough to satisfy the environmental standards [4]. Especially, the NO pollution of areas near heavy traffic, highways and busy traffics cross-sections is claimed to cause several health problems to people living in urban streets [5,6]. However, economic and effective ways to reduce NO emissions have not been proposed and practically evaluated yet [7,8]. Selective catalytic reduction (SCR) has been successfully applied for several decades to reduce NOx emissions. In order to convert NO contained in flue gas into N2 , reducing agents must be employed, such as NH3 , CO, H2 and a variety of hydrocarbons [9–11]. Although a lot of reducing agents have been utilized in SCR of NO, ammonia is the most widely researched and used [12]. A number of SCR catalysts, such as noble metals, zeolites and transition metal (V, Mn, Fe, Co, Ni and Cu) oxides loaded on different carriers [13–19], have been researched in the past few decades. However, most of the catalysts can only yield a desirable NO conversion when the temperature is over 150 ◦ C, and the NH3 -SCR technological process needs a system to control the NH3 . Therefore, recent efforts have been ∗ Corresponding † Zheng

made to develop desirable catalysts for low temperature SCR of NO without NH3 [20]. Recently, Shirahama et al. [21] developed a pitch-based activated carbon fibre (ACR) with urea deposited on it, which showed very high NO2 removal activity even at room temperature. However, the catalysts they researched had very low SCR activity on NO. Therefore, the efficient SCR of NO at ambient temperature (about 30 ◦ C) is a subject that attracts the attention of many researchers. Furthermore, there are at least two key processes that need to be improved in the reduction of NO. One is the process of NO oxidation, and the other is the process of efficient deoxidization of NOx . It has been reported that NO2 can be purified by ACF; however, NO2 cannot be completely deoxidized to N2 on the surface of ACF [21]. Meanwhile, some researchers [22,23] have reported that ceria (CeO2 ) has a special electronic transition ability which is propitious to the oxidation of NO. Urea can be easily decomposed on many supports such as ACF and silica, and has become one of the most practical reducing agents [20,21] for NO reduction when the temperature and space are limited. The current authors have studied the versatility of ACF and ACF modified by HNO3 , La2 O3 and CeO2 in the SCR of NO with NH3 from 150–450 ◦ C [24,25], and a high activity of the catalysts was achieved at low temperature.

author. Email: [email protected] Zeng, Pei Lu and Caiting Li contributed equally to this work as co-first authors.

ISSN 0959-3330 print/ISSN 1479-487X online © 2012 Taylor & Francis http://dx.doi.org/10.1080/09593330.2011.626799 http://www.tandfonline.com

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P. Lu et al. Table 1. Code

Sample

SAa (m2 /g)

PVb (cm3 /g)

ADc (nm)

ACF UA10 UA20 UA30 UA40 UA50 UA60 CA10 UCA4 UCA5

ACF 10% urea/ACF 20% urea/ACF 30% urea/ACF 40% urea/ACF 50% urea/ACF 60% urea/ACF 10% CeO2 /ACF 40% urea-10% CeO2 /ACF 50% urea-10% CeO2 /ACF

1479.08 1467.57 1380.91 1353.98 1338.24 1094.68 1089.36 1435.66 1405.28 1312.94

0.7618 0.7583 0.7507 0.7290 0.7199 0.5916 0.5550 0.6789 0.6719 0.6416

2.06 2.07 2.17 2.15 2.15 2.16 2.04 1.89 1.91 1.95

a surface


Properties of prepared samples.

area; b pore volume; c average diameter.

The present paper describes the reduction of NO with urea deposited on ACF and CeO2 /ACF (CA) at ambient temperature to establish a novel process to decrease NO. 2. Experimental 2.1. ACF and urea/ACF preparation The rayon-based ACF, with a high surface area, was supplied by Nantong Carbon Fibre Co., China. The ACF was dried at 100 ◦ C for 2 h in a drying oven. The ACF was dipped into aqueous urea solution for 24 h. Then, the ACF was filtered from the urea solution. Thus, urea was impregnated on to the ACF. For example, 0.5 g of ACF was dipped into 50 mL of aqueous urea solution (0.125–2.0 mol/L), and, after the solution was stirred sufficiently, it was stored in the solution for 24 h at room temperature. The solution was separated as much as possible from the ACF by filtration without washing. The ACF was vacuum-dried at 50 ◦ C. In this way, various concentrations (10%, 20%, 30%, 40%, 50% and 60%, w/w) of urea were deposited on to ACF, and the catalysts prepared were referred to as UA10, UA20, UA30, UA40, UA50 and UA60, respectively. 2.2.

CeO2 /ACF and urea–CeO2 /ACF preparation

The catalyst of 10% CeO2 and ACF (CA) was prepared, as described in previous papers [24,25], by dipping the ACF into a certain concentration of Ce(NO3 )3 solution for 24 h. Afterwards, the samples were dried at 50 ◦ C and pyrolysed at 350 ◦ C for 3 h in inert conditions. Urea–CeO2 /ACF (UCA) was prepared by depositing urea on to the prepared CA. In this study, catalysts of UCA3 (30% urea), UCA4 (40% urea) and UCA5 (50% urea) were prepared. The surface area, pore volume and average diameter of the ACF and the series of catalysts were measured by the BET method, and are listed in Table 1. 2.3. Measurement of reactivity The reaction of NO in mixed gas (N2 , O2 and NO, respectively, controlled by a compressed gas cylinder) with

prepared catalysts was carried out in a fixed reactor as described in detail in previous papers [24,25]. The catalyst was packed into a stainless-steel tube of 20 mm diameter. The composition and flow rate of the mixed gas were adjusted with a mass flow meter. The concentrations of NO at the inlet and NOx (NO and NO2 , respectively) at the outlet of the reactor were measured continuously by a NOx analyser (Testo 350 Xl, Testo Co., Germany).

3. Results and discussion 3.1. Reactivity of 500 ppm NO with urea supported on rayon-based ACF Figure 1 shows the breakthrough profiles of 500 ppm NO in dry air through reaction with urea of variable mass supported on rayon ACF at 30 ◦ C. In these experiments, with the increase in urea, the NOx conversions of the six samples were increased and their breakthrough times also became longer. For UA10, its NOx conversion was about 71% after 9.1 h. After that the NO conversion sharply decreased owing to the consumption of urea and the saturation of NO3 adsorption [21], as shown in Figure 1a. Guo et al. [26] reported that NO could be oxidized to NO2 on ACF. Adapa et al. [27] also found that NO could be oxidized by ACF. Among the NO purification processes in this study, NO was oxidized to NO2 and NO3 through its disproportionation over ACF [28]. Furthermore, urea was suggested to reduce the oxidative products to N2 on ACF. Therefore, for the samples of UA, with the existence of O2 , ACF and urea, the mechanism of NO reduction was partly different from those reported. The greater the amount of loaded urea (20–60%), the greater the NOx conversions, which were between 72.03% and 77.30%, since more urea could play an important role in reducing NOx . Moreover, the breakthrough time of those samples became significantly longer than that of UA10, as shown in Figure 1b–f, since the consumption time of urea depended on its mass. However, compared with UA50, UA60 did not contribute to any further improvement in NOx conversion, as shown in Figure 1f, which resulted from the fact that the oxidation of NO into NO2 was essential in

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(b) 100

100

80

80 Removed NOx

60

NO x (%)

NO x (%)

Removed NOx

40

60

40

NOx at outlet 20

0

0 0

2

4 6 8 Time on stram (h)

(c)

10

0

12

2

4

6

(d)

8 10 12 14 Time on stram (h)

80

80

18

20

Removed NOx

Removed NOx NO x (%)

60 NO x (%)

16

100

100

40 NOx at outlet

20

60

40

NOx at outlet

20

0

0 0

4

8

(e)

12 16 Time on stram (h)

20

0

24

4

8

(f)


24

28

100

100

80

Removed NOx

80

Removed NOx

60

60 NO x (%)

NO x (%)


NOx at outlet

20

40

NOx at outlet

20

40

20

NOx at outlet

0

0 0

5

10


30

0

5

10

15

20

25

30

35

40

Time on stram (h)

Figure 1. Breakthrough profiles of NO over urea-impregnated ACF. Sample: 0.5 g; relative humidity = 0%; temperature = 30◦ C; NO: 500 ppm; O2 : 21%; N2 : balance; flow rate = 225 mL/min. (a) UA10, (b) UA20, (c) UA30, (d) UA40, (e) UA50, (f) UA60.

the reaction, which required more open surfaces of ACF for NO oxidation. Meanwhile, the amount of urea also affected the NO removal ability of urea/ACF. Obviously, urea itself also needed a bigger specific surface area to purify NO efficiently. However, the surface area of ACF was limited. Therefore, the sample prepared by depositing

50% urea on ACF was suitable for purifying NO at ambient temperature. Figure 2 shows the breakthrough profiles of 500 ppm NO with samples of 50% and 60% urea supported on ACF without oxygen at 30 ◦ C. In Figure 2a, with 50% urea deposited on ACF, NO removal was about 48% for 34.6 h. When 60%

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80

80

60

60

40 Removed NOx 20

NOx at outlet

40 Removed NOx 20

NOx at outlet

0

0


100

NOx (%)

NOx (%)

(a) 100

0

10

20

30

0

10

Time on stram (h)

20 30 Time on stram (h)

40

Figure 2. Breakthrough profiles of NO over urea-impregnated ACF without oxygen. Sample: 0.5 g; relative humidity = 0%; NO: 500 ppm; O2 : 0%; N2 ; balance; flow rate = 225 mL/min; temperature = 30◦ C. (a) UA50, (b) UA60. Table 2.

Calculation of mole ratio of supplied NO to samples.

Weight of sample (g) Urea rate on sample (%) CeO2 rate on sample (%) Inlet NOx concentration (ppm) Inlet O2 concentration (%) Amount of urea on sample (mol) Breakthrough time (BTT) (h) Total volume of supplied NOx until BTT (L) Total volume of supplied net NOx until BTT (L) Ratio of reacted NOx to supplied net NOx Volume of removed NOx (L) Amount of removed NOx (mol) Mole ratio of reacted NOx to urea over ACF

urea was deposited on ACF, it provided a steadier removal of NO at about 50% for about 37.5 h. Furthermore, 60% deposition of urea on ACF did not show any further improvement in the NO removal (Figure 2b), which was consistent with the results shown in Figure 1f. Moreover, with more urea, the breakthrough time of UA60 was longer than that of UA50, which also accorded with results shown in Figure 1. Table 2 summarizes the material balances of the reaction by urea deposited on the ACF, with and without O2 in the carrier gas, under dry conditions. The amount of urea remaining on the ACF after the reaction was negligible. About 1.65 mole of NO were reduced with one mole of urea when the O2 concentration was 21%, whereas the mole ratio of reacted NO to urea was 1.19 without oxygen. Though Shirahama et al. [21] reported that the influence of O2 was small on the reactivity of NO with urea on ACF, Guo et al. [26] reported that NO could be oxidized to NO2 , and the oxidation reaction was greatly influenced by O2 concentration when the O2 concentration was lower than 5%. According to the experimental results, the NOx conversion

UA50

UA50

UCA4

0.5 50 0 500 21 0.0042 30.4 410.4 0.2052 75.11 0.1541 0.0069 about 1.65

0.5 50 0 500 0 0.0042 34.6 467.1 0.2335 47.82 0.1117 0.0050 about 1.19

0.5 40 10 500 21 0.0033 24.5 330.8 0.1654 80.73 0.1335 0.0060 about 1.79

of UA50 of 21% O2 concentration was much higher than that of UA50 without O2 . Therefore, it could be deduced that the influence of O2 in the reaction was decided by the sample characteristics. In this study, the reactivity of NO with urea on the ACF was significantly affected by O2 concentration. 3.2.

Reactivity of 500 ppm NO with urea supported on CeO2 /ACF

Figure 3 illustrates the reactivity of NO with urea supported on CA. It can be easily seen that, with CeO2 , the NOx conversion of UCA can be significantly increased. Ceria has been studied extensively for its oxygen storage and redox properties. Above all, the most important property of ceria is as an oxygen reservoir [29–31], which stores and releases oxygen via the redox shift between Ce4+ and Ce3+ under oxidizing and reducing conditions, respectively. Ceria could enhance the oxidization of NO to NO2 [23]. Therefore, comparing Figure 3 with Figure 1c–e, the conversion from NO to NO2 might be propitious for the reduction of NO. The

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free energy (G) of formation (25 ◦ C, 100 kPa) were also calculated.

100

NOx conversion (%)

80

3.3.1. With oxygen 60

6NO + 2CO(NH2 )2 = 5N2 + 2CO2 + 4H2 O G1 = −1615.92 kJ/mol

40

2NO + O2 −→ 2NO2

(2)

G3 = −2995.524 kJ/mol

(3)

NO + NO2 + 2CO(NH2 )2 = 2N2 + CO2 + 2H2 O

0 0


G2 = −70.48 kJ/mol

6NO2 + 4CO(NH2 )2 = 7N2 + 4CO2 + 8H2 O

UCA3 UCA4 UCA5

20

(1)

10

20 Time (h)

30

Figure 3. Breakthrough profiles of NO over urea-impregnated CA. Sample: 0.5 g; relative humidity = 0%; temperature = 30◦ C; NO: 500 ppm; O2 : 21%, N2 : balance; flow rate = 225 mL/min.

G4 = −791.89 kJ/mol

(4)

4NO + 2CO(NH2 )2 + O2 = 4N2 + 2CO2 + 4H2 O G5 = −1798.28 kJ/mol

(5)

2NO2 + 2CO(NH2 )2 + O2 = 3N2 + 2CO2 + 4H2 O breakthrough times of UCA3 and UA30 were almost the same, the breakthrough times of UCA4 and UA40 were almost the same, and the breakthrough times of UCA5 and UA50 were almost the same. Furthermore, although the longevity of UCA3 was much shorter than the other two UCA catalysts, the NOx conversion by it was up to 85.5%, which was higher than those of UCA4 and UCA5. This resulted from the amount of urea loaded, since too much loaded urea resulted in not only a decrease in the surface area and pore volume of the ACF, but also a covering of CeO2 , the catalytic oxidization centre, which further influenced its electron transfer ability. Thus, in order to balance the inconsistency, the amount of urea should be optimized. The sample prepared by depositing 40% urea on CA was suitable for purifying NO at ambient temperature. Table 2 summarizes the material balances of the reaction of NO with urea on the CA in the carrier gas under the dry condition. The amount of urea remaining on the ACF was negligible after the reaction. For UCA4, the molecules of NO reduced with one molecule of urea was about 1.79.

3.3.

NO conversion mechanisms

Many researchers have studied and suggested the mechanisms of SCR of NO, including the catalytic mechanism and the thermal mechanism [27,32–34]. However, most of those mechanisms discussed were not at ambient temperature. Shirahama et al. [21] discussed the conversion mechanism of NO2 with urea at room temperature. However, the reactions between NO, O2 and urea were not further discussed in his research. Furthermore, the experimental conditions and samples were partly different from those of the present study. When it comes to the components of the reaction gases and samples, the following reactions might take place on the surface of the ACF and/or CeO2 during the NO purifying process, and their standard Gibbs

G6 = −1410.64 kJ/mol

(6)

2NO2 −→ NO3 ad + NO

G7 = —

(7)

2NO2 + O2 −→ 2NO3 ad

G8 = —

(8)

2NO3 ad + 2CO(NH2 )2 −→ 3N2 + 2CO2 + 4H2 O G9 = —

(9)

The enhancing effect of oxygen resulted from the oxidation of NO to NO2 . Furthermore, there were some differences between UA and UCA in the redox processes of NO, which were decided by the components of the samples. For UA, as other authors reported [26,27], NO could be oxidized to NO2 on the surface of the ACF, shown as reaction (2). Meanwhile, reactions (3)–(6) could also take place on the surface of the ACF. Moreover, the effect of urea directly reducing NO was small [21], so reaction (1) was not the main process. Evaluated by the Gibbs free energy of formations (25 ◦ C, 100 kPa), the main reaction on the surface of the ACF was reaction (5), not only because its G5 was −1798.28 kJ/mol but also reactions (3), (4) and (6) were limited by reaction (2) because G2 = −70.48 kJ/mol. Though Shirahama et al. [21] also suggested that reactions (7)–(9) could also take place, reactions (7)–(9) mainly took place when there were metal catalysts, as other researchers reported [13,16–19]. So it is suggested by the authors of the present study that those reactions could take place on the surface of the UCA. Therefore, the reactions between UA and NO with O2 were reactions (1)–(6), and they can be summarized as follows: 3NO + 7NO2 + 11CO(NH2 )2 + 3O2 = 21N2 + 11CO2 + 22H2 O G10 = −7515.40 kJ/mol

(10)

The mole ratio of NO to urea in reaction (10) was about 1.82, which was similar to the calculated ratios in Table 2.


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For the reaction between UCA and NO, because of the presence of CeO2 – the catalytic centre in the reaction, the rate of NO oxidization to NO2 was faster than that of UA. Reaction (2) can be enhanced by UCA because of the addition of CeO2 , which provided a good electron transfer orbit. Obviously, CeO2 could significantly enhance the effect of reaction (2), which made reactions (3), (4) and (6) take place completely. Thus, the NO oxidation activity over the catalysts played an important role in improving the catalytic activity since reaction (2) and (6) could simultaneously take place, significantly enhancing NOx conversion. Therefore, with the same amount of urea loaded, UCA could yield a higher NOx conversion than that of UA, and the reactions between UCA, NO and O2 were (1)–(9). Therefore, the reaction of NO and urea on the surfaces of CeO2 and ACF could be summarized as follows:

co-adsorb physically with O2 on the carbon surface. Therefore, the NOx conversion was much smaller than when O2 was present.

4. Conclusion The SCR activity of ACF could be greatly improved at ambient temperature (30 ◦ C) when oxygen was present, leading to the oxidation of NO to NO2 . Urea could be the best reducing agent for an efficient catalytic ability. When both NOx conversion and breakthrough time are taken into consideration, for UA, UA50 could yield about 75% for 30.4 h whereas, for UCA, UCA4 could yield about 80% for 24.5 h, since CeO2 could act as an electron transfer station. The catalytic activity and stability of UA50 and UCA4 at higher temperature should be investigated in future research.

11NO + 13NO2 + 15CO(NH2 )2 + 4O2 = 27N2 + 15CO2 + 30H2 OG11 = −9843.18 kJ/mol

(11)

Also, it could be calculated that the mole ratio of NO to urea in reaction (11) was about 1.73, which was also consistent with the calculated ratio in Table 2. Since reactions (7)–(9) were only part of the NO reduction, the way of NO reduction of UCA was almost the same as that of UA. Therefore, the mole ratio of reacted NO to urea, calculated in Table 2, was a little higher than that of reaction (11).

Acknowledgements This work was supported by the National Natural Science Foundation of China (50878080, 51108169, 50908080), the Key Scientific and Technological Special Project K0902006-31 of Changsha City in China, the National Undergraduate Innovating Experiment Project (521611185) and the Scientific and Technological Project of Hunan Province in China ([2008]GK3118). The authors would like to acknowledge the help of Dr Yide He.

References 3.3.2. Without oxygen 6NO + 2CO(NH2 )2 = 5N2 + 2CO2 + 4H2 O G1 = −1615.92 kJ/mol

(1) ACF

6NO + 4CO(NH2 )2 + 6R − O −→ 6R − +7N2 + 4CO2 + 8H2 O G12 = —

(12)

On one hand, in reaction (1), the mole ratio of reacted NO to urea is 3. On the other hand, reaction (12) takes place resulting from the oxygen-containing functional groups of ACF [28,35,36], and here the mole ratio of reacted NO to urea is 1.5. Guo et al. [26] reported a similar phenomenon. However, according to Table 2, the mole ratio calculated by the experiments was less than 3 and, in the case of UA50, was less than 1.5. Therefore, reactions (1) and (12) were not the major reactions. Actually, since a suitable surface area and a well-developed porosity are thought to be important for sorption, ACF played an important role in the removal of NOx . However, the effect of ACF directly adsorbing NO was small [21]. The presence of surface oxygen groups on the carbon may assist slightly the NO adsorption when oxygen gas is absent. However, when oxygen is present, both physical and chemical adsorption of NO were significantly enhanced [36], which led to the conclusion that NO may

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