Selective Catalytic Reduction of NO with NH3 over Ce–Mo–Ox

Catalyst

Xiaoliang Li • Yonghong Li

Received: 17 May 2013 / Accepted: 10 September 2013 / Published online: 12 October 2013

� Springer Science+Business Media New York 2013

Abstract A novel type of Ce–Mo–Ox catalyst prepared by

the facile coprecipitation method has been utilized for the

selective catalytic reduction of NO with NH3. The catalyst

showed excellent activity, prominent resistant to the space

velocity in a temperature range from 200 to 400 �C and

exhibited high SO2/H2O durability at 300 �C. XRD and

in situ DRIFTS proved that the strong interactions between

CeOx and MoOx in the catalyst could be the main reason for

the excellent NH3-SCR catalytic performance.

Keywords Cerium � Molybdenum � Selective

catalytic reduction � NH3

1 Introduction

Nitrogen oxides (NOx) emitted from stationary and mobile

sources could lead to acid rain, photochemical smog, ozone

depletion, greenhouse effects and endangering human

health [1]. Selective catalytic reduction of NOx with NH3

(NH3-SCR) has been proved to be the most effective

method for controlling NOx emissions. The most widely

used catalysts for this process are V2O5-WO3 (MoO3)/TiO2

[2–4]. However, there are some disadvantages related to

these catalysts, such as the toxicity of vanadium species to

environment, the relatively narrow operation temperature

window and the rapid decrease of SCR activity at high

temperature [5, 6]. Therefore, more and more researchers

are focusing on development of novel catalysts to sub-

stitute the vanadium based catalysts in recent years.

Cerium oxides are widely accepted as a promising

candidate due to their outstanding oxygen storage-release

capacity and excellent redox properties in NH3-SCR

reaction. Recently, many kinds of promising Ce-based

catalysts were reported, Ceria modified MnOx/TiO2 was

reported by Wu et al. [7] as a superior catalyst for NO

reduction with NH3 at low temperature, Ce/TiO2 were

prepared by He and coworkers [8] and Gao et al. [9] using

different methods as effective catalysts for the NH3-SCR

reaction, Ce–W mixed oxides were studied by Li and

coworkers [10] and Liu and coworkers [11] which exhib-

ited high NO conversion at a broad temperature range from

200 to 400 �C, Ce–P-O catalyst was also investigated by

Yang et al. [12] which showed excellent NH3-SCR activity,

K2O and SO2/H2O durability. In addition, MoO3 has been

used as a stabilizer and promoter to enhance the NOx

removal activity in the traditional V2O5-MoO3/TiO2 cata-

lyst for many years. Lietti et al. [13] found that the strength

of Lewis acid was enhanced by the addition of MoO3 to

V2O5/TiO2 catalyst and that the V2O5-MoO3/TiO2 catalyst

exhibited higher reactivity with respect to V2O5/TiO2 or

MoO3/TiO2. Up to now, fewer researches on the molyb-

dena-based catalysts for the selective catalytic reduction of

NOx were reported comparatively.

Based on the idea of the excellent oxidation reduction

activity of CeO2 and the super promoting effect of MoO3 in

the V2O5-MoO3/TiO2 catalyst. The Ce–Mo–Ox catalysts

were synthesised by the facile coprecipient method in this

work.

X. Li � Y. Li (&)

Key Lab for Green Chemical Technology of Ministry of

Education, School of Chemical Engineering and Technology,

Tianjin University, Weijin Road 92, Tianjin 300072, China

e-mail: yhli@tju.edu.cn

Y. Li

National Engineering Research Center for Distillation

Technology, Tianjin 300072, China

123

Catal Lett (2014) 144:165–171

DOI 10.1007/s10562-013-1103-6

2 Experimental

2.1 Catalyst Preparation

The Ce–Mo–Ox catalysts were synthesized by the copre-

cipitation method using (NH4)2Ce(NO3)6 and (NH4)6Mo7

O24�5H2O as precursors and 25 wt% NH3�H2O as precipi-

tator. The aqueous solutions of (NH4)6Mo7O24�5H2O with

equal weight H2C2O4�2H2O were added into deionized

water. After the (NH4)6Mo7O24 was dissolved completely,

the (NH4)2Ce(NO3)6 was added into the aqueous solution

with the required molar ratio (Ce/Mo = 4:1, 3:1, 2:1, 1:1,

1:2). Excessive 25 wt% NH3�H2O solution was dropped into

the mixed solution under vigorous agitation until pH = 10.

The precipitated solids were collected by filtration and

washed with deionized water, followed by drying at 110 �C

for 12 h and subsequently calcination at 500 �C for 5 h in the

static air atmosphere. Finally, the catalysts were crushed and

sieved to 40–60 meshes. The catalyst was marked as Cea-

MobOx, where the a/b represented the Ce/Mo molar ratio.

For parallel test purpose, the conventional V2O5-WO3/

TiO2 was also prepared. The V2O5-WO3/TiO2 catalyst with

2.5 wt% V2O5 and 10 wt% WO3 was prepared by the con-

ventional impregnation method using NH4VO3,

(NH4)10W12O41, H2C2O4�2H2O as precursors and anatase

TiO2 as support. After impregnation, the excess water was

removed at 90 �C, subsequently the sample was dried at

110 �C for 12 h and then calcined at 500 �C for 5 h in the

static air condition [11, 12].

2.2 Characterization of Catalysts

The BET (Brunauer–Emmett–Teller) surface areas of the

catalysts were obtained from N2 adsorption/desorption

analysis at 77 K using a Micromeritics Tristar-3000 sys-

tem. Prior to the N2 physisorption, catalysts were degassed

at 300 �C for 4 h. Surface areas were determined by the

BET equation in 0.05–0.35 partial pressure range.

The crystallinity of the catalysts was measured by means

of powder XRD patterns obtained on a Rigaku D/max 2500

X-ray diffractometer with Cu Ka radiation, scanning

between 10� and 85� at a step of 5o/min.

Visible Raman spectra were carried out at room tem-

perature on a Spex 1877 D triplemate spectrograph with

spectral resolution of 2 cm-1. A 532 nm DPSS diode pump

solid semiconductor laser was used as the excitation source

and the power output was about 40 mW.

H2-TPR (Hydrogen-temperature programmed reduction)

profiles were measured by using a Micromeritics Auto

Chem II 2920 instrument. In each experiment, a 50 mg

sample was loaded into the quartz reactor and pretreated in

Ar (50 mL/min) at 300 �C for 1 h. The sample was then

cooled to room temperature under flowing N2. The sample

was reduced starting at room temperature and increasing up

to 900 �C in a gas mixture of 10 % H2/Ar at 10 �C/min.

The consumption of H2 was monitored continuously by

using a thermal conductivity detector.

The in situ DRIFTS experiments were performed on a

FT-IR (Fourier transform infrared) spectrometer (Nicolet

Nexus 670) equipped with an in situ diffuse reflection

chamber and high sensitivity mercury–cadmium–telluride

(MCT) detector cooled by liquid nitrogen. An about 30 mg

sample was finely ground and placed in the in situ cham-

ber. The mass flow controllers and temperature controller

were used to simulate the reaction conditions. Prior to each

experiment, the catalyst was heated at 500 �C for 30 min in

a flow of 10 % O2/N2 and then cooled to 200 �C. The

background spectrum was recorded and subtracted from the

sample spectrum. All the spectra were collected with an

accumulating 100 scans at a resolution of 4 cm-1.

2.3 Activity Test

The SCR activity measurements were carried out in a fixed

bed quartz reactor (inner diameter 10 mm) under atmo-

spheric conditions. A K-type thermocouple (o.d.1 mm) was

directly immersed into the catalyst bed from the bottom of

the reactor and connected to a programmable temperature

controller to monitor the reaction temperature. The com-

position of the model flue gas was: 500 ppm NO, 500 ppm

NH3, 5 % O2, 5 % H2O (when used), 100 ppm SO2 (when

used), balance N2, and 300 mL/min total flow rate, yielding

a GHSV range from 25,000 to 100,000 h-1 based on dif-

ferent volume of the catalyst. The water vapor was intro-

duced by an injection pump(LSP01-1A, Longer Pump

Inc)and an evaporator. The concentrations of NO and NO2

in the inlet and outlet gas were measured by an online

chemiluminescent NO/NOx analyzer (Model KM9106,

Kane Inc). The data were collected after 1 h when the SCR

reaction had reached a steady state. NO conversion was

calculated by using: NO conversion = (1 - [NOx]out/

[NOx]in) 9 100%; with [NOx] = [NO] ? [NO2].

3 Results and Discussion

3.1 Activity Test

Figure 1a presented the NO conversion over CeaMobOx

catalyst for the selective catalytic reduction of NO by NH3

under different molar ratio of Ce to Mo. Obviously, CeOx

and MoOx exhibited poor NO conversion in the whole

temperature range separately. However, when the Ce/Mo

molar ratio being 1:2, the catalytic activity was greatly

improved and the reaction temperature range was broad-

ened from 250 to 400 �C with more than 65 % NO

166 X. Li and Y. Li

123

conversion was achieved. As the Ce/Mo molar ratio

increased to 1:1, the reaction activity was further improved

in the whole test temperature scope. When the Ce/Mo

molar ratio being 2:1, the best catalytic activity was

attained. However, further increasing the Ce/Mo molar

ratio decreased the NO conversion at both low and high

temperatures, which probably be associated with the

decrease of active sites on the surface of the catalyst [10].

In order to evaluate the SCR performance of Ce–Mo–Ox

catalyst, the Ce2MoOx catalyst was chosen to compare with

the prepared 2.5 wt%V2O5-10 wt%WO3/TiO2 catalyst. As

showed in Fig. 1b, the V2O5-WO3/TiO2 catalyst exhibited

good activity with nearly 100 % NO conversion from 300 to

400 �C. The Ce2MoOx catalyst presented much higher SCR

activity than V2O5-WO3/TiO2 catalyst especially below

300 �C. Although the high temperature SCR activity above

350 �C showed a little decrease, the Ce2MoOx catalyst

showed excellent catalytic activity in a wider operation

temperature window from 200 to 400 �C. Consequently, the

Ce–Mo–Ox catalyst is a promising candidate for the removal

of NO.

3.2 The Influence of GHSV on NH3-SCR Activity

The NH3-SCR catalyst usually undergoes different gas

hourly space velocity (GHSV) in the practical application,

especially for the treatment of NOx produced from mobile

sources. Thus the influence of GHSV on NO conversion

over the Ce2MoOx catalyst was investigated and the results

were exhibited in Fig. 2. Clearly, as the GHSV was

increased from 25,000 to 50,000 h-1, the catalytic activity

decreased significantly in the range of 100 to 225 �C, but

no distinct effect on the higher temperature activity. When

the GHSV was further increased to 100,000 h-1, the cat-

alytic activity decreased markedly in all temperature range

especially below 250 �C. However, more than 90 % NO

conversion was still attained during the temperature range

from 250 to 400 �C. These results indicated that the Ce–

Mo–Ox catalyst was effectively resistant against the high

space velocity.

3.3 The Influences of SO2 and H2O on NH3-SCR

Activity

Since SO2 gas and water vapor in the combustion exhaust

often lead to the deactivation of the SCR catalyst, the

influences of 100 ppm SO2 and/or 5 % H2O on the per-

formance of the Ce2MoOx catalyst were also studied at

300 �C. The effects of H2O and SO2 on the catalytic

behavior were illustrated in Fig. 3. Before adding SO2 and

H2O, the reaction had been stabilized for 3 h, after stop-

ping them the reaction lasted for 4 h. It could be seen that a

slight decline in NO conversion occurred after the 100 ppm

Fig. 1 a NH3-SCR activity over CeaMobOx catalysts with different

Ce/Mo molar ratios under GHSV of 50,000 h-1 b Comparison of NO

conversion over Ce2MoOx and 2.5 wt%V2O5-10 wt%WO3/TiO2

under GHSV of 50,000 h-1

Fig. 2 NH3-SCR activity over Ce2MoOx catalyst under different

GHSV

Selective Catalytic Reduction of NO with NH3 167

123

SO2 was added for 1–6 h, then the NO conversion became

stable. After removing SO2 the conversion was restored to

its original level. When 5 % water vapor was introduced

into the stream at 300 �C, the NO conversion was kept

above 98.5 % during the tested period and after stopping it,

the conversion was recovered to be 100 %. When the

100 ppm SO2 and 5 % H2O were injected into the feed

gases at the same time, the NO conversion decreased much

more severely compared with only 100 ppm SO2 or 5 %

H2O, which might be related to the deposited of ammo-

nium sulfate on the surface of the catalyst and blocked the

active sites [14]. However, the conversion was still main-

tained at a relatively high level with about 90 % NO

conversion attained during the measured period. The above

results suggested that the catalyst had excellent SO2/H2O

durability.

3.4 BET and XRD Results

The BET surface areas of 2.5 wt%V2O5–10 wt%WO3/

TiO2 catalyst, CeaMobOx catalyst, CeOx and MoOx were

shown in Table 1. The BET surface area of the V2O5-WO3/

TiO2 was much larger than that of Ce2MoOx, yet the SCR

performance of the Ce2MoOx was much higher than that of

V2O5-WO3/TiO2 in the temperature range from 100 to

300 �C. This result indicated that the BET surface area was

not the crucial factor to affect the SCR catalytic activity. At

the same time, it also could be seen that the values of the

CeaMobOx catalysts were between CeOx and MoOx, which

might be due to the formation of mixed oxide phase, yet the

NH3-SCR performances of the Ce–Mo–Ox catalysts were

much higher than that of the CeOx and MoOx, implying

that the Ce and Mo oxides species were not mixed simply.

Meanwhile, the XRD was carried out to investigate the Ce-

Mo-Ox catalysts and the patterns were displayed in Fig. 4.

As the Ce/Mo being 1:2, it was found that there existed

three types of crystal phases in the catalyst and some new

peaks appearing at 2h values of 25.27�, 28.54� and 47.63�which could be assigned to Ce2Mo3O13 (PDF31-0332).

When the Ce/Mo molar ratio was raised to 1:1, another

new Ce2(MoO4)3 (PDF30-0303) phase was detected. While

the Ce/Mo molar ratio greater than 1:1, only cerianite CeO2

phase was discovered. This suggested that molybdenum

oxide species were mainly in highly dispersed state. Fur-

thermore, the grain sizes of CeO2 on the Ce–Mo–Ox and

CeOx were calculated by the Scherrer equation and the

results were shown in Table 1. The minor CeO2 crystallite

size was acquired in the Ce–Mo–Ox catalyst, which

Fig. 3 NH3-SCR activity over

Ce2MoOxcatalyst in the

presence of H2O/SO2 at 300 �C

under GHSV of 50,000 h-1

Table 1 BET surface aera and CeO2 crystallite size of the catalysts

Sample BET surface

area (m2/g)

Crystallite size

of CeO2a (nm)

CeOx 71.75 11.86

Ce4MoOx 37.99 9.34

Ce3MoOx 30.13 8.99

Ce2MoOx 31.50 8.48

CeMoOx 18.89 10.46

CeMo2Ox 17.49 11.34

MoOx 1.30 –

V2O5-WO3/TiO2 65.23 –

a CeO2 crystallite size calculated by Scherrer equation from XRD

results

168 X. Li and Y. Li

123

testified that the existence of molybdenum oxide could

lower the crystallinity size of CeO2 thus enhance the dis-

persion of CeO2 on the catalyst surface. The above results

hinted that the synergistic effect exists between Ce and Mo

oxides in the Ce-Mo-Ox catalyst.

3.5 Raman Spectra Analysis

The Raman spectrum was also employed to analyze the

CeOx, MoOx and Ce–Mo–Ox catalysts, and the results were

showed in Fig. 5. The spectrum of MoOx showing eight

Raman bands peaked at 217, 245, 284, 337, 378, 665, 819

and 995 cm-1, are attributed to MoO3 [15, 16]. For the

CeOx, the band at 463 cm-1 is due to the Raman active F2g

mode of CeO2, the typical band of a fluorite structural

material [17]. No MoOx species were detected in the Ce-

Mo-Ox catalysts. In addition, the band intensity of CeO2 on

Ce-Mo-Ox was lower than that on pure CeOx, which

proved the particle size was smaller owing to the inhibition

of crystallization by Mo doping. These analytical results

were in good agreement with the XRD patterns. It has been

reported that the key active phase in CeO2/TiO2 and Ce–

W–Ox catalyst was the nanocrystalline CeO2 [8, 10]. In

combination with the XRD and Raman results analysis in

this study, the main active component on the surface of the

Ce–Mo–Ox catalyst was also the highly dispersed nano-

crystalline CeO2.

3.6 H2-TPR and In Situ DRIFTS Spectra Analysis

The H2-TPR was measured to investigate the redox activity

of CeOx and Ce2MoOx, the profiles were showed in Fig. 6.

For the CeOx, two reduction peaks were detected, the first

peak at about 510 �C corresponded to the reduction of

surface Ce4? to Ce3? and the peak at higher temperature

could be attributed to the reduction of bulk CeO2 [7, 18].

For the Ce2MoOx catalyst, the TPR profile showed one

peak at around 577 �C, the peak area of the Ce2MoOx

catalyst was about two times larger than that of the CeOx,

which indicated that the Ce2MoOx catalyst owned much

stronger oxidation reduction ability and further strength-

ened the SCR reaction performance.

The in situ DRIFTS was performed to study NH3

adsorption and NO?O2 adsorption on the CeOx, MoOx,

Ce2MoOx and V2O5-WO3/TiO2 catalysts at 200 �C

respectively. As the in situ DRIFTS exhibited in Fig. 7,

after the NOx adsorption, obviously almost no band was

found on the V2O5-WO3/TiOx catalyst. After the NH3

adsorption, some bands at 1,224, 1,420, 1,601, 3,164,

3,257, 3,356 and 3,637 cm-1 were detected, the band at

1,224/1,601 cm-1 could be assigned to coordinated NH3

on lewis acid sites [19], while the band at 1,420 cm-1 can

Fig. 4 XRD profiles of CeaMobOx serial catalysts

Fig. 5 Raman spectra of CeaMobOx serial catalysts (kex = 532 nm)

Fig. 6 H2-TPR profiles of CeOx and Ce2MoOx

Selective Catalytic Reduction of NO with NH3 169

123

be attributed to ionic NH4? species on Brønsted acid site

[20]. The bands at 3,100–3,400 cm-1 due to N–H

stretching vibration modes and one negative band around

3,637 cm-1 belonged to the surface O–H stretching were

also seen [21, 22]. However, the intensity of all the bands

was much weaker than that of Ce2MoOx, which might be

the main reason for the lower SCR activity. In the mean-

while, after the NH3 or NOx adsorption, almost no band

was detected on the MoOx. When the CeOx and Ce2MoOx

were exposed to NH3 separately, some weak bands were

detected on the surface of CeOx, however, the NH3

adsorption capability was enhanced on the Ce2MoOx sur-

face evidently, several bands at 1,182, 1,421, 1,543, 1,591,

1,668 cm-1 were found, the band at 1,182 cm-1 could be

attributed to coordinated NH3 on lewis acid site, the bands

at 1,668/1,421 cm-1 due to ionic NH4? on Brønsted acid

sites [20, 23]. In the N–H stretching region, two bands were

found at 3,261 and 3,380 cm-1 [21]. Another two negative

bands at 3,621, 3,663 cm-1 were also detected [24, 25],

which could be assigned to the hydroxyl consumption.

After introducing NO?O2 into the DRIFT cell, the inten-

sity of the bands on the Ce2MoOx catalyst showed no

remarkable decreased compared to the CeOx, some bands

appeared at 1,598, 1,572, 1,540, 1355, 1,237 and

1,210 cm-1, the bands at 1,598, 1,237 and 1,210 cm-1

were assigned to bridging nitrate species [26–28], the

bands at 1,572 and 1,540 cm-1 were attributed to bidentate

nitrate species and monodentate nitrate respectively [29,

30]. Based on the above analysis, the weaker adsorption

capacity of V2O5-WO3/TiO2 to the NOx and NH3 was the

main reason for the decrease of the SCR activity. At the

same time, there were some synergistic effect between

CeOx and MoOx. Besides the adsorbed NH3 and NOx

species could react with each other on the surface of the

Ce2MoOx catalyst, which is beneficial to enhance the

deNOx activity. The coexistence of CeOx and MoOx with

synergistic effect in the Ce–Mo–Ox catalyst were crucial

for achieving excellent NH3-SCR catalytic activity.

In conclusion, the novel Ce–Mo–Ox catalyst prepared by

coprecipitation method presented super NH3-SCR activity

in a relatively wide temperature range from 200 to 400 �C,

and strong resistance to the high space velocity as well as

anti-poisoning of SO2 and H2O. The excellent catalytic

performance was attributed to the synergistic effect

between Ce and Mo oxides species. Further studies for

understanding the reaction mechanism are in progress.

Acknowledgments We acknowledge gratefully the financial sup-

port of the Program of Universities’ Innovative Research Terms (No.

IRT0936).

References

1. Bosch H, Janssen F (1988) Catal Today 2:369

2. Busca G, Lietti L, Ramis G, Berti F (1998) Appl Catal B 18:1

3. Rriche MA, Hug P, Baiker A (2000) J Catal 192:400

4. Ramis G, Yi L, Busca G (1996) Catal Today 28:373

5. Liu F, Asakura K, He H, Liu Y, Shan W, Shi X, Zhang C (2011)

Catal Today 164:520

6. Orsenigo C, Lietti L, Tronconi E, Forzatti P, Bregani F (1998)

Ind Eng Chem Res 37:2350

7. Wu Z, Jin R, Liu Y, Wang H (2008) Catal Commun 9:2217

8. Xu W, Yu Y, Zhang C, He H (2008) Catal Commun 9:1453

9. Gao X, Jiang Y, Fu Y, Zhong Y, Luo Z, Cen K (2010) Catal

Commun 11:465

10. Shan W, Liu F, He H, Shi X, Zhang C (2011) Chem Commun

47:8046

11. Chen L, Li JH, Ablikim W, Wang J, Chang HZ, Ma L, Xu JY, Ge

MF, Arandiyan H (2011) Catal Lett 141:1859

12. Li F, Zhang Y, Xiao D, Wang D, Pan X, Yang X (2010)

ChemCatChem 2:1416

13. Lietti L, Nova I, Ramis G, Acqua L, Busca G, Giamello E,

Forzatti P, Bregani F (1999) J Catal 187:419

14. Xu W, He H, Yu Y (2008) J Phys Chem C 113:4426

Fig. 7 In situ DRIFTS of a NH3 adsorption b NO?O2 adsorption on

the CeOx, MoOx, Ce2MoOx and V2O5-WO3/TiO2 catalyst at 200 �C

170 X. Li and Y. Li

123

15. Srilakshmi C, Chew W, Ramesh K, Garland M (2009) Inorg

Chem 48:1967

16. Seguin L, Figlarz M, Cavagnat R, Lassegues JC (1995) Spetro-

chimica Acta Part A 51:1323

17. Reddy BM, Khan A, Yamada Y, Kobayashi T, Loridant S, Volta

J (2003) J Phys Chem B 107:5162

18. Chen L, Li JH, Ge MF (2009) J Phys Chem C 113:21177

19. Chen L, Li J, Ge M (2010) Environ Sci Technol 44:9590

20. Shan W, Liu F, He H, Shi X, Zhang C (2012) Appl Catal B

115–116:100

21. Busca G, Larrubia MA, Arrighi L, Ramis G (2005) Catal Today

107–108:139

22. Montanari T, Bevilacqua M, Resini C, Busca G, Pirone R, Ru-

oppolo G (2000) J Porous Mater 14:291

23. Smirniotis PG, Pena DA, Uphade BS (2001) Angew Chem Int Ed

40:2479

24. Zhang X, Shen Q, He C, Ma C, Cheng J, Li L, Hao Z (2012) ACS

Catal 2:512

25. Chen H, Wang X, Sachtler WMH (2000) Phys Chem Chem Phys

2:3083

26. Li L, Zhang F, Guan N, Richter M, Fricke R (2007) Catal

Commun 8:583

27. Su Y, Amiridis MD (2004) Catal Today 96:31

28. Marth MS, Wokaun A, Baiker A (1992) J Catal 138:306

29. Cordoba LF, Sachtler WMH, Correa CM (2005) Appl Catal B

56:115

30. Desikusumastuti A, Staudt T, Gronbeck H, Libuda J (2008) J

Catal 255:127

Selective Catalytic Reduction of NO with NH3 171

123