Selective catalytic reduction of NO with ammonia on titania pillared clays

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.

2973

SELECTIVE C A T A L Y T I C R E D U C T I O N OF NO W I T H A M M O N I A ON TITANIA P I L L A R E D CLAYS Lee, D.-K. and Kim, S.-C. Department of Chemical Engineering/School of Environment Protection, Environment and Regional Development Institute, Environmental Biotechnology Research Center, Gyeongsang National University, Kajwa-dong 900, Jinju, Gyeongnam 660-70 l, Korea.

ABSTRACT TiO2-pillared clays were synthesized by pillaring TiO2 onto the pure bentonite. As the pillaring agent was used a solution of partially hydrolyzed Ti-polycation which had been prepared by adding TiCl4 into HC1 solution. Successful intercalation of TiO2 could be achieved, and the physical properties such as d00~ spacing, surface area and pore volume were influenced by the concentration of HC1, TIC14 and Ti/clay ratio. The d00~ spacing and surface area of the TiO2-PILCs increased up to 29.8A and 389m2/g, respectively. The pore volume also increased up to 0.25cm3/g which was significantly higher than 0.06cm3/g of the unpillared clay. The N2 adsorption isotherms showed the presence of both the micropores and mesopores in the pillared clays. TPD and FTIR analyses showed the wide presence of the Bronsted acid site which is essential to the SCR of NO with NH3. When Fe203 was incorporated into the TiOz-PILCs, the Bronsted acidity increased significantly, and even the complete conversion of NO could be achieved with the Fe203/TiO2-PILCs at the temperature window between 375-400 ~

INTRODUCTION While clays have two-dimensional layered structures, pillared clays have three-dimensional network structures like those of zeolites. The main goal of the pillaring process has been and continues to be that of producing new and inexpensive materials having properties complementary to those of zeolite (pore size and shape, acidity, redox properties, etc.) [1-6]. Pillared clays (PILCs) are nano-composite materials with open and rigid structures obtained by linking robust, three-dimensional species to a layered host. Why PILCs are of wide interest for catalytic applications is clear when thinking about possible controlling of physical and chemical properties. The removal of NO from stack gases is an important step towards controlling air pollution. With increasing strict NOx emission regulations some form of post-combustion NOx removal is necessary. The most popular commercial technique for removing NOx is so-called selective catalytic reduction (SCR) of NO by NH3. In the SCR process NO is reduced in the presence of NH3 to N2 and H20. The catalyst typically used in the SCR process for NO removal is V205 supported on TiO2. TiO2-PILCs could also be promising catalyst supports for SCR of NO because the presence of Bronsted acid sites for the adsorption of NH3 is important to SCR reaction and intercalating TiO2 between the SiO2 tetrahedral layer is a unique way of increasing acidity of TiO2 support [7]. The outstanding features that TiO2-PILCs have large pore sizes allowing further incorporation of active ingredients without hindering pore diffusion and have high thermal and hydrothermal stability among pillared clays make their catalytic applications more practical [8,9]. In this study TiOz-pillared clays were synthesized under different conditions and iron was doped onto the prepared TiO2-PILCs. The iron-doped TiO2-PILCs were used as catalysts for SCR of NO with NH3, and their catalytic performances were investigated. EXPERIMENTAL

Starting materials The starting clay for the preparation of TiOz-PILCs was a purified bentonite powder (DongYang Bentonite Co.). Only particles of clay with a size less than 2 were used in the pillaring process. The chemical analysis of the bentonite was SiO2 52.33%, A1203 16.79%, Fe203 3.84%, TiO2 0.11%, Na20 1.28%, MgO 1.94%, CaO 1.69%, K20 0.54% by weight. The CEC(cation exchange capacity) value of the bentonite

2974 was found to be 96meq per 100g of clay. Its specific surface area and pore volume as determined from nitrogen adsorpsion isotherm were 33mR/g and 0.06cma/g, respectively. The H2 adsorption of the unpillared clay was close to type I isotherm which is generally observed for the solids containing micropores.

Synthesis of the TiO2-PILCs As the pillaring agent for the synthesis of TiOz-PILCs was used a solution of partially hydrolyzed Ti-polycation which had been prepared by adding TiCl4 into HCl solution under constant stirring. The mixture was then diluted by slow addition of deionized water under stirring to reach final Ti concentrations ranging from 0.32 to 0.82M. HCl concentration corresponding to 0.11-0.6M were used in the preparation. The prepared solutions were aged at room temperature for 12h prior to their use. A bentonite suspension in deionized water was mixed with these pillaring solutions under rigorous stirring at room temperature for 12h to have Ti/clay ratio in the range 3.9-10mmol/g clay. The mixture was filtered and washed by centrifugation with deionized water until it was chloride-free. The suspension was then dried at 120 ~ for 24hr and the resulting sample was calcined at 300 ~ for 6h. Highly dispersed iron clusters on the TiO2-PILCs were prepared through carbonyl introduction method. Iron carbonyl(Fe(CO)5) dissolved in n-pentane was physically dispersed onto the TiOz-PILCs. The TiO2-PILCs was dehydrated in vacuo at 300~ The impregnation took place in an evacuated sealed cell over a period of 12h at -10~ The mixture was then warmed slowly to room temperature in vacuo over a period of 2h and was maintained under a dynamic vacuum (10 2 Torr) for another 24h. The prepared samples were finally calcined with flowing 1% O2/He at 300~ for 12hr. The Fe loading of the prepared samples was about 2wt%.

Characterization The X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuKct radiation. Surface areas and pore volumes were determined by using nitrogen as the sorbate at 77K in a static volumetric apparatus (Micromeritics ASAP 2010). Temperature-programmed desorption (TPD) of ammonia was carried out in a quartz tubular reactor. 0. lg samples were loaded in the reactor and were pretreated in flowing helium while heating at 10~ up to 500~ After being maintained for 30min at this temperature, the samples were cooled to 150~ and saturated for 30min in an ammonia stream (l,000ppm NH3 in Ar). The samples were then allowed to equilibrate in a helium flow at 150~ for l h. The TPD was performed by ramping the temperature at 10~ /min from 25~ to 500~ The exit stream from the reactor was analyzed with a mass spectrometer. Infrared spectra were recorded with a Digilab FTS-80 FTIR spectrometer. About 50mg of the sample was pressed into a self-supporting wafer and placed inside a infrared cell similar to the one designed by Hicks et al.[l 0]. The pretreatment and NH3 adsorption procedure were the same as TPD experiments. Ferromagnetic resonance (FMR) spectra of iron species in the iron-doped TiO2-PILCs were recorded at X-band frequencies on a Varian E-4 spectrometer. DPPH was used as a standard to determine g-values. Quartz sample tube was designed for in situ operation.

SCR reaction SCR reaction with ammonia was carried out in a quartz tubular reactor. The composition of reactants was 1,000ppm NO, 1,000ppm NH3, 2% 02, 8% H20 and balance Ar. NO concentration was continuously measured by a chemiluminescent NOx analyzer (Thermo Environmental Instrument, Model 17C). Phosphoric acid solution ammonia trap was installed before the sample inlet to avoid the oxidation of ammonia in the converter of the NOx analyzer. RESULTS AND DISCUSSION

Characterization of the TiO2-PILCs The abbreviated symbols of the prepared TiO2-pillared clays and their synthesis conditions are listed in Table 1, and Figure 1 shows the XRD patterns of the clay and the synthesized TiOa-pillared clays. The d0ol peak for the unpillared clay was at 20=7.8. Upon intercalation the do01 peak of the unpillared clay was almost disappeared, which indicates that the whole clay was successfully pillared. In addition the do01 peak shifted toward lower 20 value at around 4 ~ corresponding to the increase in d001 spacing up to 29.4A as shown in Table 2.

2975 Table 1. Synthesis conditions of TiO2-pillared clays. Symbol

HC1 cone. [M]

TIC14 cone. [M]

mmol Ti/g clay

TiO2-PILC-A TiO2-PILC-B TiO2-PILC-C TiOz-PILC-D TiO2-PILC-E

0. l 1 0.28 0.60 0.11 0.11

0.82 0.82 0.82 0.32 0.41

10.0 10.0 10.0 3.9 5.0

(A) m m

(B) r

(c)

g m

.r Ib,,B

r

(E)

w m

(D)

-,41,,,,l m

c

(F)

o

t'o

:30

2'0

4'0

50

20 Figure 1. XRD patterns of the clay and the prepared TiO2-PILCs ((A)TiO2-PILC-A, (B)TiO2-PILC-B, (C)TiOz-PILC-C, (D)TiO2-PILC-D, (E)TiOz-PILC-E, (F) clay).

Table 2. Summarized physical properties of the TiO2-PILCs. Symbol

d001 (A)

Surface area (m2/g)

Pore volume (cm3/g)

clay TiO2-PILC-A TiO2-PILC-B TiO2-PILC-C TiO2-PILC-D TiO2-PILC-E

11.3 29.4 29.2 28.8 24.8 25.6

33 389 321 298 169 184

0.06 0.25 0.21 0.21 0.13 0.16

Einaga[11] suggested the existence of titanium hydroxocomplex [(TiO)sOH12] 24+ during TIC14 hydrolysis and polymerization. Sharygin et al.[12] proposed that the titanium hydroxocomplex is the main intermediate to be transformed into hydrated titanium dioxide. The chemistry of titanium is very complex, and the

2976 pathways of the TIC14 hydrolysis and polymerization cannot be presently elucidated. The TiO2 pillaring process is, however, believed to proceed via the formation of titanium hydroxocomplex. As shown in Figure 1 and Table 2, there was a slight decrease in the d001 spacing with decreasing concentration of TIC14 and increasing HCI concentration. Nabivanets and Kudritskaya [13] showed that the polymerization increases with increasing concentration of titanium and chloride, and proposed that the polymeric species are probably linked by chloride. When the titanium concentration is low, the possibility of the formation of polymeric species becomes lower. The aforementioned decrease in the d001 spacing with decreasing TIC14 concentration seems to be due to the lower possibility of formation of the polymeric species. If the polymeric species are linked by chloride as suggested by Nabivanets and Kudritskaya [13], the d001 spacing will increase with increasing HC1 concentration. Contrary to the suggestion, the d001 spacing decreased slightly with increasing HC1 concentration. Accordingly the polymeric species are not believed to be linked by chloride, but HC! seems to play a role on inhibiting the hydrolysis process. The interlayer spacings of the TiO2-PILCs ranged from 24.8 to 29.4A. These values are much higher than those of other published pillared clays. This large pore size is desirable for the application of the TiO2-PILCs as catalyst supports. All the prepared TiO2-PILCs showed typical type II nitrogen adsorption isotherms, which indicated the presence of both micropores and mesopores in the pillared samples. In addition the hysterisis loop of the TiO2-PILCs exhibited H3 type, revealing the slit-shaped pore structure. The incorporation of the titanium between the silicate layers increased both the surface area and pore volume significantly. The prepared TiO2-PILCs had surface areas higher than 160m2/g (the surface area of the unpillared clay was 33m2/g). The pore volumes of the pillared samples were at around 0.2cm3/g which was remarkably higher than 0.06cm3/g of the unpillared clay. This intercalation effect on surface area and pore volume was especially outstanding when the pillaring process was performed under the lowest HCI concentration and the highest TiCI4 concentration (TiO2-PILC-A sample).

Acidity analysis of the TiO2-PILCs and iron-doped TiO2-PILCs The NH3 TPD spectra of the unpillared clay and the TiO2-PILCs are shown in Figure 2. While the desorbed amount of NH3 from the unpillared clay was almost negligible, two distinct desorption peaks at around 220~ and 330~ were obtained from the TiO2-PILCs which did clearly chemisorb substantial amount of the strongly bound NH3. Moreover the intensity of the higher temperature desorption peak was much weaker than that of the lower temperature peak.

= m

e~

=2 e~ 0 e~ l

o

1;o

2;0

3;0

4;0

Temperature(~ Figure 2. TPD spectra of NH3 on the TiO2-PILCs ((A)TiO2-PILC-A, (B)TiO2-PILC-B, (C)TiO2-PILC-C, (D)TiO2-PILC-D, (E)TiO2-PILC-E).

2977 FTIR spectra of the adsorbed NH3, as shown in Figure 3, could be used to investigate the nature of acid sites. The band at around 1450cm ~ came from the asymmetric bending vibration of NH4 + on Bronsted acid sites and the asymmetric bending mode of ammonia on Lewis acid sites appeared at approximately 1620cm -1. .........................

,,

:

:::

......

:::

.............

O

1750

!SfO 1550 1450 Wave nu mbed 9 )

1350

Figure 3. FTIR spectra of the adsorbed NH3 on the TiO2-PILCs((A) TiO2-PILC-A, (B)TiO2-PILC-B, (C)TiO2-PILC-C, (D)TiO2-PILC-D, (E)TiO2-PILC-E). TiO2 alone did not show strong acidity. The bulk mixed oxides, especially SiO2-TiO2 mixed oxide, developed a greater acidity than individual phases [14]. The acid sites in the TiO2-PILCs are believed to locate mainly at the interface between silicate layers and TiO2 pillars. Since the presence of acid sites for the adsorption of NH3 is important to SCR reaction with NH3, the TiO2-PILC samples were used as catalyst supports.

t~

O tO

(1

9

.....................

0

:! . . . . . . . . . . . . . . . . . . . . . . . . . . .

100

!

............................

! ...............................................

. ! ...............................................

NO 300 400 Temperat. re (* C)

500

Figure 4. TPD spectra of NH3 on the iron-doped TiO2PILCs((A) iron-doped TiO2-PILC-A, (B) iron-doped TiO2-PILC-B, (C) iron-doped TiO2-PILC-C, (D) iron-doped TiO2-PILC-D, (E)Fe203/TiO2-PILC-E).

II

t 750

I

t !650

..

I

t 550

:1. . . . . . . . . . . . . . . . .

1450

1350

Wavenumber(cm "t) Figure 5. FTIR spectra of the adsorbed NH3 on the iron-doped TiO2-PILCs ((A) iron-doped TiO2-PILC-A, (B) iron-doped TiO2-PILC-B, (C) iron-doped TiO2-PILC-C, (D) iron-doped TiO2-PILC-D, (E) iron-doped TiO2-PILC-E).

2978

100

..j ~ ~

80 A

c

._o

60

> =

o o 0

40

/'7

"=w

iron-doped TiO2-PILC-A

........9 ....... 9 iron-doped TiO2-PILC-B

Z

2O

---~---

iron-doped TiO2-PILC-C

---V ....

iron-doped TiO2-PILC-D

m

200

250

--1 - -

iron-doped TiO2-PILC-E I

,

300

350

!

400

Temperature(~ Figure 6. Temperature dependence of NO conversion on the iron-doped TiO2-PILCs (0.2g catalyst, 480cm3(STP)/min flow rate). Figure 4 and 5 exhibit the results of NH3 TPD and FTIR spectra of NH3 on the iron-doped TiO2-PILCs, respectively. By the presence of iron the intensity of the higher temperature peak in NH3 TPD as well as Bronsted acid peak in FTIR spectra increased significantly. By comparing the FTIR spectra the NH3 desorption peak at around 330~ seems to be due to the NH3 adsorbed on Bronsted acid sites.

SCR reaction on the iron-doped T i O 2 - PILCs The iron-doped TiO2-PILCs were employed as catalysts for the SCR of NO with ammonia. Figure 6 shows the plots of NO conversion versus reaction temperature. When iron was impregnated on the TiO2-PILC-A having the highest values of surface area and pore size, complete conversion of NO could be achieved in the temperature window of 375-400~ At this temperature window the TiO2-PILC-A alone showed just 25% NO conversion. The incorporation of iron must have enhanced the SCR activity remarkably. Since the adsorption of NH3 on acid sites is essential for the SCR of NO [15-20], the high SCR activity of the iron-doped TiO2-PILCs must be due to the presence of acid sites. The state of iron in the iron-doped TiO2-PILC-A was analyzed with FMR at different detection temperatures (Figure 7). At-l$0~ a strong paramagnetic Fe3+ signal was detected at g=4.3 together with the weaker signal at g=2.1. The signal at g=2.1 became stronger and its band width became narrower with increasing detection temperature, which is a typical characteristics of finely dispersed ferromagnetic species such as zero-valent iron particles and magnetite, Fe304 [21,22]. Although the exact state of iron could not be answered, both the Fe 3§ cation and finely dispersed iron particles is believed to be present in the TiO2-PILC-A. CONCLUSION TiO2-pillared clays were synthesized under different HCI and TIC14 concentrations. Upon intercalation the d00~ spacing increased up to 29.4A depending on the concentrations of HCi and TIC14. All the prepared TiO2-PILCs had both the micropores and mesopores. The surface area, pore volume and d001 spacing increased with increasing TiCI4 and decreasing HCI concentration. The prepared TiO2-PILCs showed the

2979 presence of both the Bronsted and Lewis acid sites which were believed to be formed mainly at the interface between silicate layers and TiO2 pillars. When iron was doped onto the TiO2-PILCs, the acidity, especially Bronsted acidity, increased significantly. This wide presence of Bronsted acid sites made the iron-doped TiO2-PILCs successful catalyst for the SCR of NO with ammonia.

g 4.3

2.32.0

! 9

! .

I ~

9 9

. .

. .

-150~

25~

150~

9

ldoo'

9

3000

o'oo

H(gauss) Figure 7. FMR spectra of the iron-doped TiO2-PILC-A detected at different temperatures.

ACKNOWLEDGEMENT This work was supported by a grant from the Korea Science and Engineering Foundation (KOSEF) to the Environmental B iotechnology Research Center (grant #" R15-2003-012-02002-0). REFERENCES 1. Canizares, P., Valverde, J.L., Sun Kou, M.R. and Molina, C.B., Microporous and Mesoporous Materials, 29 (1999) 267. 2. Yang, R.T. and Baksh, M.S.A., AIChE J., 37 (1991) 679. 3. Cheng, L.S. and Yang, R.T., Ind. Eng. Chem. Res., 34 (1995) 2021. 4. Vaccari, A., Catal. Today, 41 (1998) 53. 5. Barrault, J., Abdellaoui, M., Bouchoule, C., Majeste, A., Tatibouet, J.M., Louloud, A., Papayannakos, N. and Gangas, N.H., Appl. Catal. B, 27 (2000) L225. 6. H.L. Delcastillo, A. Gil and P. Grange, Catal. Lett., 36 (1996) 237. 7. Bernier, L.F. Admaiai and P. Grange, Appl. Catal., 77 (1991) 219. 8. Sterte, J., Catal. Today, 2 (1998) 219. 9. Del Castillo, H.L. and Grange, P., Appl. Catal. A, 103 (1993) 23. 10. Hicks, R.F., Kellner, C.S., Savatsky, B.J., Hecker, W.C. and Bell, A.T., J. Catal., 71 (1981) 216. 11. Einaga, H., J. Chem. Soc. Dalton Trans., (1974) 1917. 12. Sharygin, L.M., Vovk, S.M. and Gonchar, V.F., Russ. J. Inorg. Chem., 33 (1988) 970. 13. Nabinavets, B.I. and Kudritskaya, L.N., Russ. J. Inorg. Chem., 12 (1969) 611. 14. Shibata, K., Kiyoura, T., Kitagawa, J., Sumiyoshi, T. and Tamabe, K., Bull. Chem. Soc. Jpn., 46 (1973) 2985.

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