TiO2-Supported Binary Metal Oxide Catalysts for Low-temperature Selective Catalytic Reduction of NOx with NH3

CHEM. RES. CHINESE UNIVERSITIES

Available online at www.sciencedirect.com

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2008,24(5),615-619 Article 10 1005-9040(2008)-05-615-05

TiOrSupported Binary Metal Oxide Catalysts for Low-temperature Selective Catalytic Reduction of NO x with NH3 WU Bi-jun':", LIU Xiao-qin', XIAO Ping 2 and WANG Shu-gang! 1. Chemical Engineering Institute, Nanjing University ofTechnology, Nanjing 210009, P. R. China; 2. State Power Environmental Protection Research Institute ofChina Guodian Corporation, Nanjing 210031, P. R. China Abstract Binary metal oxide(MnO x-A/Ti0 2) catalysts were prepared by adding the second metal to manganese oxides supported on titanium dioxide(Ti02), where, A indicates Fe20J, WOJ, MoO J, and Cr20J' Their catalytic activity, N2 selectivity, and S02 poisonous tolerance were investigated. The catalytic performance at low temperatures decreased in the following order: Mn-WITi0 2>Mn-FeITi02>Mn-Cr/Ti02>Mn-Mo/Ti02, whereas the N 2 selectivity decreased in the order: Mn-Fe/Ti0 2> Mn-W/Ti0 2 > Mn-Mo/Ti0 2 > Mn-CrITi02· In the presence of 0.01% S02 and 6% H 20, the NOx conversions in the presence of Mn-W/Ti0 2, Mn-Fe/Ti0 2, or Mn-Mo/Ti0 2 maintain 98.5%, 95.8% and 94.2%, respectively, after 8 h at 120°C at GHSV 12600 h- 1• As effective promoters, WOJ and FezOJ can increase N 2 selectivity and the resistance to S02 of MnOjTi02 significantly. The Fourier transform infrared(FTIR) spectra of NH J over WOJ show the presence of Lewis acid sites. The results suggest that WOJ is the best promoter of Mn0xlTi0 2, and Mn-W/Ti0 2 is one of the most active catalysts for the low temperature selective catalytic reduction of NO with NH J. Keywords Selective catalytic reduction of NO with NH J; Low-temperature selective catalytic reduction; Binary metal oxide catalyst; FTIR; NHJ-TPD

1 Introduction Selective catalytic reduction(SCR) with NH 3 is an effective technology for abatement nitrogen oxides emission from stationary sources. The commercial catalysts for this process are V20s/Ti02 mixed with W0 3 or Mo0 3, which are active within a temperature window of 300-400 "C. It is necessary for electric power plants to locate the SCR reactor upstream of the electrostatic precipitator, i.e., between the economizer and air pre-heater for avoiding to reheat the flue gas as well as deposit ammonium sulfate salts on the catalysts[I,2]. The life of catalysts will be decreased because of the existance of enormous fly ash. Actually, in industry, the catalyst components are made up ceramic honeycomb or metal plate to reduce the pressure drop!". For the reasons mentioned above, scientists are strongly interested in developing high active catalysts at low temperatures for stationary source NO x SCR with NH 3. Among them, manganese oxides has receiced considerable attention. Pure MnOx , natural manganese ore, as well as MnOxlAh03 were found to exhibit certain activities between 150 to 250 °C[4-6J.

Several Ti0 2-supported transition metals such as V, Cr, Mn, Fe, Co, Ni, and Cu oxides were evaluated by Donovan[?l at 120 and 175°C, among which MnO xITi02 provides the best catalytic performance. As a prospective catalyst for low temperature selective reduction of NO x with NH 3, MnO xlTi0 2 has been further investigated. Qi and Yang[8 J found that Fe-MnlTi02 was high active in a range from 120 to 180°C, and the mixed oxides of Mnt), and Ce02 provided even higher NO conversion and N 2 selectivity at 120°C than Fe_MnlTi02[9,lOJ. Recently, we characterized and investigated the mechanism of catalytic reaction with self-prepared Fe-Mn/Ti02 by the aid of FTIR and in situ IR[lIl. After the experiments on catalytic performance over various loadings and calcination temperatures of MnO xITi0 2, we found the optimal preparation conditions for the single manganese oxides supported on TiOl 2l. In this article, four binary metal oxides (MnO x-A/Ti02) were prepared and evaluated, aiming at improving N 2 selectivity and S02 tolerance of MnO xlTi02 •

*Corresponding author. E-mail: wubijun@neprLcom Re.feived July 23, 2007; accepted December 21,2007. Copyright © 2008, Jilin University. Published by Elsevier Limited. All rights reserved.

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616

2 2.1

Experimental Preparation of Catalysts

The catalyst MnOx/Ti02 was impregnated by incipient wetness. A certain amount of manganese nitrate aqueous solution was mixed with Ti0 2(P25, Degussa) after its purification and activation. The mixture was added to ammonium carbonate slowly with stirring to form Mn(OH)x deposited Ti0 2. After filtering and washing, the obtained solid sample was first dried at 120°C and then calcinated at a required temperature for 5 h. Finally, the catalyst was crushed, sieved to collect the 20-40 mesh fraction, and pushed into pellets with a diameter of 0.15-0.2 em. Mn-Fe/Ti02 and Mn-Cr/Ti0 2 were prepared by co-precipitation. Iron nitrate or chromium nitrate were dissolved in water with manganese nitrate and then mixed with Ti0 2; then ammonium carbonate was slowly added to the mixture. The subsequent processes such as drying, calcinating, and tabletting were repeated as mentioned above. Mn-W/Ti0 2 and Mn-Mo/Ti02 samples were prepared by co-precipitation combined with impregnation by incipient wetness with Mn(OH)x deposited Ti0 2; then, the samples were added to ammonium tungstate or ammonium molybdate aqueous solution, and stirried at 80°C until the water was evaporated completely. Finally, the identical processes as mentioned above were repeated. 2.2

Catalytic Activity Experiment

The SCR activity measurement was carried out in a fixed-bed reactor loading 1 mL of catalyst(""='0.99 g). The reactor was heated by a furnace, which was connected to a temperature programmer with a thermocouple inserted in the catalyst bed: The reaction stream was diluted from pre-diluted gases 1% NOlHe and 1% NH3lHe(supplied by Nanjing Weichang Special Gas Co.) by He. The typical reaction conditions were 0.05% NO(about 5% N0 2 in it) , 0.05% NH 3, 4.8% 02, balanced with He, 6% water vapor evaporated from deionized water and 0.01% S02(when used), 215 mLlmin total flow rate, and gas hourly space velocity(GHSV)=12600 h- 1(ambient conditions). The NO and N0 2 concentrations of the inlet and outlet streams were monitored using a flue gas analyzel(Model KM9106, Kane, England). The outlet stream was absorbed with a phosphoric acid solution

Vol.24

to prevent from spoiling chemical sensors of the analyzer by NH 3 . The byproduct ofN20 was analyzed by a gas chromatograph(GC-9790) at 80°C with Porapak Q column and TCD. Since the reaction was carried out at a low temperature, it is necessary to balance the NO x concentrations of the inlet and outlet gases to avoid the interference of adsorption by catalysts. For each group measurement, at the very beginning of experiment, the catalysts were purged by He for 0.5 h on heating; then, NO gas entered the reactor until the NO x concentration deviations of the inlet and outlet were less than 3%. The data were collected after reaching the required temperature and the steady state for 90-120 min. 2.3

Characterization of Catalysts

The specific surface areas of the catalysts were measured by means of nitrogen adsorption at 77 K via the BET method using Belsorp II(BEL Co., Japan) after pretreatment at 473 K under vacuum. The pore volume and pore size of catalysts were calculated from the adsorption and desorption isotherms. The chemisorptions of catalysts were performed with FTIR(Thermo Nicolet, America). 0.1% NH3IHe was introduced and flowed at 30 mLlmin after the sample was pretreated with He at 25°C for 30 min. NH 3- TPD studies were carried out by Che BET 3000(Quantachrome, America). A sample of 50 mg was pretreated at 300°C for 1 h in a high pure He stream and lowered to 100°C; then, pure NH 3 was introduced at a flow rate of 30 mLlmin for 0.5 h. Physisorbed NH 3 was removed by He, then the signals were recorded by TCD while heating at 10°C Imin to 700°C and maintaining at that temperature for 1 h to ensure complete ammonia desorption.

3 Results and Discussion 3.1

Catalytic Activity and Selectivity of MnOxl

Ti0 2

Fig.1 shows the NO x conversion on different manganese loadings of MnO xlTi0 2 from 5% to 25% varying with reaction temperature. The catalytic activity increases with the amount of manganese. When the Mn loading reaches 15%, the NO x conversion varies from 93.0% to 98.3% in a range of 80 to 160°C, and the yield is nearly 100% over 25% MnO xlTi0 2 at 120°C. The N2 selectivities on three loadings of MnO xITi0 2(l5%, 20%, 25%) are all 100% below

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WU Bi-jun et al.

No.5

100 DC(Fig.2), then reduce sharply above 120 DC with the increase of Mn amount. At 180 DC, the N 2 selectivities over 15% and 25% MnO/Ti0 2 are as low as 96% and 93.3%, respectively.

: :

100 ~

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.9

e

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96 92

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Mass fraction of catalyst: ...... 5% ...... 10% -+- 15% ....,...20% -+- 25%

88 84 80

100

80

140 120 Temperature/X'

160

NOx conversions on different loadings of MnOJTiO z

Fig.I

NO 0.05%, NH 3 D.05%,O 2 4.8%, balance with He. 100

.-.~

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...... 25% ...... 20% -+- 15%

3.2

Promoting the Effect of Binary Metal Oxides

3.2.1 Promoting the Activity Fig.4 shows the performance of four binary metal oxides for low temperature SCR NO x with NH 3. The catalysts contain 15%MnOx-l0%A/Ti02, in which A represents Fe203, W0 3, Mo0 3, and Cr203. Among them, Mn-W/Ti02 is the most active with 94.5% and 99.6% NO x conversions at 80 and 160 DC. Second is Mn-Fe/Ti0 2; NO x conversions achieve 93.5% and 98.8%, respectively, at 80 and 160 DC, higher than the corresponding ones on the same loading of MnO xITi0 2• The other two catalysts also have certain activities for low temperature SCR. The NO x conversion increases obviously with temperature over Mn-Mo/Ti0 2 although it is less active at low temperatures. The catalytic activity for the four mixed oxides at low temperatures decreases in the following order: Mn-W/Ti0 2

120

100

160

140

100

NO 0.05%, NH 3 0.05%, 0 24.8%, balance with He.

When the gas stream has 0.01% S02 and 6% H 20, the NO x conversion drops quickly to 53% after 8 h at 120 DC, as seen in Fig.3, and small amounts of

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/II.

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200

Temperature/X'

NOx conversion on binary metal oxides

Fig.4

nary metal oxides is presented in Fig.5. Mn-Fe/Ti0 2, Mn-W/Ti0 2, and Mn-Mo/Ti0 2 have high selectivities in the range of temperature. Their N 2 selectivities only decrease slightly from 100% to 99.2%, 98.7%, and 98.0%, respectively, when the temperature rises from

4

5

6

7

8

100

NO x conversion on 25% MnOJTiO z in the presence of SOz and HzO

NO 0.05%, NH 3 0.06, 024.8%, SO, 0.01%, H20 6%, 120°C.

It can be seen that although increasing the Mn loading of MnO/Ti0 2 can raise NO x conversion at low temperatures, the N 2 selectivity decreases. Be-

-----

~~

sides, the tolerance of MnO/Ti0 2 to S02 needs to be improved.

95 90

.... Mn-WffiO, ...... Mn-MolfiO, ... Mn-CrlfiO,

85 80

-

.~---=====t::::::::=t=!

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Timelh

Fig.3

Mn-Cr/Ti02

3.2.2 Promoting on Selectivity The N 2 selectivity vs. temperature of the four bi-

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NO 0.05%, NH 3 0.050/0, 0 24.8%, balance with He.

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80 70

MnO xlTi0 2

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60

the

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.9

Nz selectivities on different loadings of MnOjTiO z

S02 can deactivate low-temperature range.

~

'"

180

TemperaturezC

Fig.2

Mn-Fe/Ti02

Mn-MolTi0 2.

~

80

>

L-

80

L.-

120

I,--_ _--...J'-------'

160

200

Temperature/X'

Fig.5

Nz selectivity on mixed metal oxides NO 0.05%, NH 3 0.050/0, 024.80/0, balance with He.

CHEM. RES. CHINESE UNIVERSITIES

618

100 to 200 "C. At 180 "C, the three samples abovementioned show 99.6%, 99.2%, and 98.5% product selectivity for N2, at least, 4.5% higher than that of 25%MnO x/Ti0 2. However, the N 2 selectivity of MnCr/Ti02 is 4.5% lesser than that of 25% MnO x/Ti02. In the range of reaction temperature, the N2selectivity of the four binary metal oxides decreases in the following order: Mn-Fe/Ti0 2 > Mn-W/Ti0 2 > Mn-Mo/ Ti0 2> Mn-Cr/Ti0 2. Improving S02 Tolerance When 0.01% 802and 6% H20 were added to gas stream, the performances of the four catalysts exhibited obvious differences at 120°C. As seen in Fig.6, the NO x conversion on Mn-Fe/Ti02 decreases slightly in 2 h from 97.5% to 95.8%, then tends to stabilize, raises slightly, and tends to stabilize from the beginning of 96.6% to 98.5% in 8 h on Mn-W/Ti0 2, and remains around 94.2% on Mn-Mo/Ti0 2. However, the performance of Mn-Cr/Ti02 is completely different from that of the three catalysts mentioned above; the NO x conversion drops rapidly from 69% to 48.2% in 8 h, while MnOx/Ti0 2 deactivates about 50% after 8 h. The resistances of the four binary metal oxides to 802 3.2.3

decrease in the following order: Mn-W/Ti02 >

Mn-Fe/Ti0 2 > Mn-Mo/Ti0 2 > Mn-Cr/Ti0 2 • Mn-W/ Ti02 has excellent tolerance, and the NO x conversion maintains 98.5% under the condition: NO 0.05%, NH3 0.06%, 8020.01%, H20 6%, 120 -c Until now, the highest resistance of catalyst to 802 has been reported in the literatures. Nearly all the activity of 2% MnlAh03 was lost when 0.005% 802 was added to a standard reaction mixture[13]. The results from Fe-Mn/Ti02 and MnOx-Ce02 binary oxides reported by Qi[7,9] show that the NO x conversion decreased quickly from nearly 100% to 85% and 95% , respectively, at the same concentration of 802.

; i i ;

100 90

1 ~

....... Mn-Fe/Tifj, ..... Mn-Wffi02 ....... Mn-Moffi02 ~ Mn-Crrri02

80 70

o z;

y~

--y~

60 50

o

2

468 Timelh

Fig.6 Promoting effects of HzO and SOz on NOx conversion NO 0.05%, NHl 0.06, O 2 4.80/0, S02 O.oI%, H20 6%, 120 DC.

Vol.24

3.3 Results of Characterization Surface Area, Pore Volume and Pore Size The BET surface areas, pore volumes, and pore sizes of four binary metal oxides are summarized in Table 1. Among them, Mn-Fe/Ti0 2 has the largest surface area of 65.38 m2/g, second is Mn-W/TiO z with 61.07 m2/g. Mn-W/Ti0 2 has the least pore volume and size, of 0.297 cm3/g and 19.48 nm, respectively. This will partly explain the high N z selectivity and stronger tolerance of Mn-W/TiO z to 802. The pore sizes of Mn-Mo/Tio- and Mn-Cr/Ti0 2 are considerably larger than those of others. Perhaps this is one of the reasons of their poor N z selectivity and activity. Generally, the surface area, pore volume, and pore size of MnFe/TiOz are the closest to that of 25%MnOjTiOz in the four binary metal oxides. 3.3.1

Table 1

Characterization of various catalysts BET surface areal (m2'g- I)

Pore volume/ (cm3.g- I )

Pore size/nm

Mn-Fe/Ti02

65.38

0.332

20.31

Mn-W/Ti0 2

61.07

0.297

19.48

Mn-Mo/Ti02

47.53

0.353

29.71

Mn-Cr1Ti02

56.89

0.341

29.98

25%MnOx/Ti 0 2

69.25

0.355

20.49

Sample

~TI~ oflV1l3 We found that most ofNH 3is mainly adsorbed in coordinated form(NH 3) on Lewis acid sites of Mn, and some of it is adsorbed on Brenst acid sites in the form of NH/ in the experiment of FTIR over Mnf), and FeZ03, while there is no obvious band over FezOlI]. However, the presence of a prominent band near 3450 cm~1 indicates that NH 3 is adsorbed in the form ofimine(-NH z) on W6+, in the recent investigation, as shown in Fig.7. Unlike Lietti's results of NH 3 FTIR over W0 3/TiO z, which show three bands from 3200 to 3400 em-I, the prominent bands also indicated that NH 3 is adsorbed in coordinated form on Lewis acid sites near 1200 and 1603 em-I. Besides, there also exist the bands ofNH/[14,15] at 1440 and 1660 em-I.

3.3.2

Based on the number and size of the bands observed over W0 3/TiO z, which exceeds that over V z05/TiOz, it is believed that Lewis and Brenst acidities of W0 3/TiOz are even stronger than those of V z05/TiOz. Fig.8 presents the spectra of NH 3 adsorption on MnO x. It shows a strong band owing to the coordinated form ofNH3 at 1605 cm- I [16] and four bands of imine from 3200 to 3600 cm'" near 3247, 3430, 3490 cm', and 3548 em-I as well as a weak band ofNH/

No.5

619

WU Bi-jun et al.

NHJHe He 1000

1200

1400

1600

1800

2000

2600 2800 3000 3200 3400 3600 3800 4000

p/cm- 1

Fig.7

FTffi spectra ofW03 exposed to NH3, balance with He

NHJHe

He

He llOO 1200 1300 1400 1500 1600 1700 1800

2800

3000

3200

3400

3600

3800

P/cm- l

Fig.S FTIR spectra of MnO x exposed to NH3 , balance with He

near 1640 em-I. From Figs.7 and 8, it can be concluded that metal manganese ion is the main catalyst while tungsten ion is the promoter in the system of Mn-W/Ti02 • Unlike iron in Mn-Fe/Ti02, the tungsten ion also provides part of Lewis acid centers, which are important active sites for low temperature SCR.

3.3.3 ]vllJ-1"JDl)

~

[2] Busca G., Lietti L., Ramis G., et al., Appl. Catal. B: Environmenta, 1998,18(112), 1

2006, 35(ll), 59, 64 [4] Kapteijn E, Singoredjo L., Andreini A., Appl. Catal. B: Environmental, 1994, 3(1), 173

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u, Panagiotis G. S., J.

Catal., 2004, 221(2),

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~

[l] Bosch H., Janssen E, Catal. Today, 1988, 2(4), 369

51( 2), 93

25

~c

References

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The chemisorption characteristics of NH 3 over the binary metal oxides were further probed by NH 3-TPD experiments. The temperature and number of desorption peaks for NH 3 over binary metal oxides rise obviously compared with those on MnO xITi02 ; as seen in Fig.9, the spectra of Mn-Fe/Ti0 2, Mn-W/Ti0 2, and Mn-Mo/Ti0 2 present three peaks, where, the temperature of the third one is higher than 500°C. The curve of Mn-Cr/Ti0 2 gives two peaks, but the temperature of the second rises above 600°C. It sug gests that the addition of second component such as

::i

Fe203, W0 3, Mo0 3 or Cr203 to MnO xlTi0 2 can enhance the strength of acidity; besides Fe203, W0 3, and Mo0 3 can increase the number of active sites.

Mn-Mo

15

Mn-W

10

Mn-Fe

5

51 [12] Wu B. J., Liu X. Q., Wang S. G., et al., J. Comb. Sci. and Tech., 2008, 14(3),221 [13] Kijlstra W. S., Biervliet M., Poels E. K., et al., Appl. Catal. B, 1998,

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o

131

100

200

300 400 500 Temperaturel'C

600

700

Fig.9 NH3- TPD spectra of mixed metal oxides supported on Ti02

[IS] Lietti L., Alemany J. L., Forzatti P., et al., Catal. Today., 1996,

29(1-4),143 [16] Lu Y. Q., Deng Z. H., Practical Infrared Spectrum Decipherment and Analysis, Electron Industry Press, Beijing, 1989, 176