New CoO−SiO2-Sol Pillared Clays as Catalysts for NOx Conversion

Chem. Mater. 2002, 14, 3823-3828

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New CoO-SiO2-Sol Pillared Clays as Catalysts for NOx Conversion Jin-Ho Choy,*,† Hyun Jung,† Yang-Su Han,† Joo-Byoung Yoon,† Yong-Gun Shul,‡ and Hyun-Jong Kim‡ National Nanohybrid Materials Laboratory, School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-742, Korea, and Inorganic Material Laboratory, Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea Received February 11, 2002. Revised Manuscript Received June 3, 2002

New CoO-SiO2-sol pillared montmorillonite (Co-SiM) is synthesized by interlamellar hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the presence of Co2+ ion via an alkylammonium intercalated montmorillonite. The silicate layers of Na+-montmorillonite are expanded in advance with hexadecyltrimethylammonium cation up to ∼22 Å, followed by reaction with a mixed sol solution of TEOS and CoCl2‚6H2O in the presence of n-dodecylamine as a cotemplate. A microporous Co-SiM is obtained after removing the organic templates at 550 °C. The nitrogen adsorption-desorption isotherm for the pillared sample reveals that a large number of micropores are created between the silicate layers, giving rise to a high BET specific surface area (SBET ) ∼570 m2/g) and a pore volume (V ) ∼0.65 mL/g). According to the X-ray photoelectron and X-ray absorption spectroscopy, the incorporated cobalt species is found to be CoO, which is homogeneously dispersed on the surface of the SiO2 pillars. The Co-SiM exhibits an excellent NO conversion rate of ca. 50% at 200 °C, which continuously increases up to 80% at 500 °C.

1. Introduction Recently, the design and synthesis of inorganic porous materials with a controlled pore structure have been of great interest because of their potential applications as catalysts, supports, selective adsorbents, separating agents, and porous matrixes for the encapsulation of specific functional molecules.1-3 Among various porous compounds, pillar interlayered clays (PILCs) have been highly attractive due to their controllable pore dimensions and specific catalytic properties depending on the type of silicate layers and pillaring agents.4,5 To obtain tailored microporous PILCs, various metal oxide pillars, such as Al2O3,6 ZrO2,7 TiO2,8 SiO2,9 Fe2O3,10 SiO2TiO2,11 SiO2-Fe2O3,12 and SiO2-Cr2O3,13 have been successfully introduced between the silicate layers. * To whom all correspondence should be addressed. Tel: +82-8806658. Fax: +82-872-9864. E-mail: [email protected]. † Seoul National University. ‡ Yonsei University. (1) Corma, A. Chem. Rev. 1997, 97, 2373. (2) Vaughan, D. E. W. Catal. Today 1988, 2, 187. (3) Herman, S. M.; Ze´lia, P. L.; Rodrigo, L. O.; Wander, L. V.; Cintia, O.; Lucas, J. M. Biomacromolecules 2000, 1(4), 789. (4) Barton, T. J.; Bull, L. M.; Klemperer, W, G.; Loy, D. A.; McEnaney, B.; Misono, M.; Monson, P. A.; Pez, G.; Scherer, G. W.; Vartuli, J. C.; Yaghi, O. M. Chem. Mater. 1999, 11, 2633. (5) Ishizaki, K.; Komarneni, S.; Nanko, M. Porous Materials; Kluwer Academic Publishers: Norwell, MA, 1998; pp 181-199. (6) Shabtai, J.; Rosell, M.; Tokarz, N. Clays Clay Miner. 1984, 32, 99. (7) Bartley, G. J. J.; Burch, E. Appl. Catal. 1985, 19, 175. (8) Sterte, J. P. Clays Clay Miner. 1986, 34, 658. (9) Endo, T.; Motrand, M. M.; Pinnavaia, T. J. Clays Clay Miner. 1980, 28, 105. (10) Burch, R.; Warburton, C. I. Appl. Catal. 1987, 33, 395. (11) Choy, J. H.; Park, J. H.; Yoon, J. B. J. Phys. Chem. B 1998, 102, 5991. (12) Han, Y. S.; Choy, J. H. J. Mater. Chem. 1998, 8, 1459.

More recently, tremendous effort has been expended in the application of porous materials as a selective reduction catalyst of nitrogen oxide compounds, socalled NOx, which is a major atmospheric pollutant responsible for acid rain, photochemical smog, and eventually ozone depletion.14-16 Conventionally, the deNOx reaction has been achieved over various catalysts such as precious metals17 (Pt, Pt/Rh, Pd-Au, etc.), metal oxides18 (SrFeO3, CoAl2O4,YBa2Cu3O7, etc.), and metal ion-exchanged zeolites (Mn+-ZSM-5, Mn+ ) Cu2+, Co2+, etc.).14,19 Among them, Mn+-ZSM-5-type catalysts, especially Co-ZSM-5, have been found to possess a high activity and selectivity for NO decomposition in the presence of hydrocarbons under lean combustion conditions.20,21 However, Mn+-ZSM-5-type materials have suffered from a poisoning effect due to their cross channel-like pore structure and limited available pore size (∼5 Å).14 Thus, the catalytically active centers are often blocked by the deposition of residual carbon during hydrocarbon combustion of exhaust gas, which rapidly decreases the NOx conversion rate as the running cycle increases. From this viewpoint, PILCs can be regarded as promising inorganic porous supports, because they have a larger pore size than that of conventional (13) Han, Y. S.; Yamanaka, S.; Choy, J. H. Appl. Catal. A 1998, 174, 83. (14) Shelef, M. Chem. Rev. 1995, 95, 209. (15) Fritz, A.; Pitchon, V. Appl. Catal. B 1997, 13, 1. (16) Ciambelli, P.; Corbo, P.; Migliardini, F. Catal. Today 2000, 59, 279. (17) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (18) Meubus, P. J. Electrochem. Soc. 1977, 124, 49. (19) Iwamoto, I.; Maruyama, K.; Yamazoe, N. Seiyama, T. J. Chem. Soc., Chem. Commun. 1972, 615. (20) Sun, T.; Fokema, M. D.; Ting, J. Y. Catal. Today 1997, 33, 251. (21) Li, Y.; Armor, J. N. Appl. Catal. B 1995, 5, L257.

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zeolites, and moreover, pore dimension is easily tunable through the proper selection of synthetic parameters. In the present study, therefore, primary attention is paid to the preparation of a de-NOx catalyst by pillaring of mixed metal oxide sol particles between silicate layers of clay. For this purpose, codoped SiO2 sol PILC (CoSiPILC) is prepared via an organic template route and the interlayer pillar structure is systematically investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and nitrogen adsorption isotherm measurement, along with a preliminary test on the de-NOx catalytic activity. 2. Experimental Section Materials. Na+-montmorillonite (Kunipia F, Kunimine Corp.) was used as a starting material with the chemical formula of Na0.35K0.01Ca0.02(Si3.89Al0.11)(Al1.60Mg0.32Fe0.08)O10(OH)2‚nH2O and a cation exchange capacity (CEC) of 100 mequiv/100 g. Hexadecyltrimethyl alkylammonium bromide ((CH3)3(CH2)15N+Br- Merck, 99%: Q+Br-), and dodecylamine (CH3(CH2)11NH2, Aldrich, 95%) were used as organic template and cotemplate, respectively. CoCl2‚6H2O (Hayashi, 95%) and tetraethyl orthosilicate (Aldrich, 98%: TEOS) were used as inorganic pillar precursors. Sample Preparation. First, Na+-montmorillonite was preswelled with hexadecyltrimethylammonium cation (Q+) by conventional ion exchange reaction. The reaction was carried out with a 2-fold excess of Q+ ions to the CEC in an aqueous solution at 80 °C for 12 h. After repeating the reaction twice, excess surfactants were thoroughly removed by washing with distilled hot water and then dried under an ambient atmosphere. The inorganic pillaring solution was prepared separately by mixing TEOS, CoCl2‚6H2O, and dodecylamine in the molar ratio of 80:1:10, followed by hydrolyzing at 50 °C for 30 min until it became a blue, clear sol solution. Then, the prepared pillaring solution (12 mL) was mixed with 1 g of Q+clay and reacted for 48 h at room temperature under continuous stirring and N2-bubbling. The reaction product was then separated by centrifugation and washed thoroughly with a mixed solution of ethanol and water (1:1 in volume percent), to remove nonintercalated species on the clay surfaces, and then dried in air for 48 h. Finally, it was calcined at 550 °C for 4 h in an ambient atmosphere to remove the organic templates. Sample Characterization. Powder XRD patterns were measured by a Philips PW1830 diffractometer with Ni-filtered Cu KR radiation (λ ) 1.5418 Å). A simultaneous thermogravimetry-differential thermal analysis (TG-DTA) was performed on a Rigaku TAS-100 with a heating rate of 10 °C/min under an ambient atmosphere. FT-IR was recorded using a Bomem Michelson MB-102-C15 FT-IR spectrometer by the standard KBr disk method. Elemental analysis was carried out with an inductively coupled plasma (ICP) method (Shimazu ICPS-1000IV) for the samples fused with lithium metaborate at 900 °C and dissolved in 3% HNO3 solution. Adsorption-desorption isotherms were measured volumetrically at the liquid nitrogen temperature (77 K). The pillared samples were degassed at 300 °C for 2 h under vacuum prior to the sorption measurements. XPS was obtained at room temperature by the EscaLab 220-IXL system using Al KR radiation with a step size of 0.05 eV under base pressure ∼1 × 10-8 Torr. The peak position was calibrated against the C 1s peak located at 284.8 eV. The Co K-edge X-ray absorption spectroscopy was carried out with synchrotron radiation using the extended X-ray absorption fine structure (EXAFS) spectroscopy facilities installed at the beam line 10B of the Photon Factory at Tsukuba (KEK), operated at 2.5 GeV with ca. 350-400 mA of stored current. The data were collected in transmission mode at room temperature. Absorbance was measured with the ionization chamber filled with N2 (25%) + Ar (75%) and N2 (100%) for incident and transmitted beams, respectively. The

Choy et al. data analyses for experimental spectra were performed by the standard procedure as previous described.22-24 Photon energies of all X-ray absorption near edge structure (XANES) spectra were calibrated by the first absorption peak of the Co metal spectrum (E0 ) 7709 eV). The inherent background in the data was removed by fitting a polynomial to the preedge region and extrapolating it through the entire spectrum, from which it was subtracted. The absorbance µ(E) was normalized to an edge jump of unity for comparing the XANES features directly with one another. For EXAFS analysis we employed the UWXAFS 2.0 code.25 In the course of nonlinear least-squares curve fitting between the experimental EXAFS spectrum and theoretical one, calculated by ab initio FEFF6 code,26,27 the structural parameters such as coordination number (Ni), bond distance (Ri), Debye-Waller factor (σi2), and threshold energy difference (∆E0) were optimized as variables. The amplitude reduction factors of Co-O and Co-Co pairs were obtained from the reference compound; CoO and that of Co-Si pair was fixed to 0.9, because we could not obtain any cobalt silicate reference compound. Selective Catalytic Reduction of NO with CO. The steady-state selective catalytic reduction of NO with CO over CoSi-PILC catalyst was carried out in an isothermal fixedbed continuous flow quartz reactor (o.d. ) 1/4 in.) under atmospheric pressure. Catalyst (0.2 g) was packed in a quartz reactor and pretreated with helium gas at 300 °C for 1 h. Then the reaction gas mixture containing 4000 ppm NO, 4000 ppm CO, and 5% oxygen in helium was fed to the catalyst with a total flow rate of 170 mL/min, corresponding to gas hourly space velocity (GHSV) ) 15 000 h-1. The flow rate of the gas was regulated by the mass flow controller (GMC 1000) and the evolved gases were analyzed by a mass spectrometer (HALO 201 RC system).

3. Result and Discussion Chemical Analysis. The elemental composition of CoSi-PILC was analyzed to estimate the pillar content of SiO2 and CoO. Assuming that the Al content of the aluminosilicate layer remains unchanged during the pillaring reaction, the pillar composition of the CoSiPILC is found to be (CoO)0.1(SiO2)6.4 on the basis of the O10(OH)2 anionic basic unit of the clay, and the weight percent of Co is estimated to be 0.82 wt %. The chemical formula of the Co-ZSM-5 prepared by ion exchange reaction28 is also determined to be (CoO)1.4[(AlO2)(SiO2)131], corresponding to a Co weight percent of 0.83 wt %. Thermal Analysis. TG and DTA curves for the airdried CoSi-PILC are shown in Figure 1. Three-step weight losses are observed in the TG curve. The first step occurring below 100 °C with a weak endothermic peak in the DTA curve can be assigned to the dehydration of surface adsorbed water. Second, the large weight (22) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: Berlin, 1986; pp 114-157. (23) Konigsberger, D. C.; Prins, R. X-ray absorption. Principles, applications, techniques of EXAFS, SEXAFS and XANES; Wiley: New York, 1988; pp 444-457. (24) Winick, H.; Xian, D.; Ye, M.; Huang, T. Applications of synchrotron Radiation; Gordon and Breach: Langhorne, PA, 1989; pp 135-223 (25) Stern, E. A.; Newville, M.; Ravel, B.; Yacoby, Y.; Haskel, D. Physica B 1995, 208 & 209, 117. (26) Rehr, J. J. Jpn. J. Appl. Phys. 1993, 32, 8. (27) Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. Rev. Lett. 1992, 69, 3397. (28) For comparison of de-NOx catalytic activity with CoSi-PILC, Co-ZSM-5 was also prepared from synthetic NH4+-ZSM-5 (Si/Al ) 136.5) by conventional ion exchange method by adding 1 g of NH4ZSM-5 to 200 mL of a 0.05 M solution of cobalt acetate (Wako, 97%) at 80 °C for 4 h. After the ion-exchange, sample was thoroughly washed with distilled water and then dried overnight at 120 °C.

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Figure 1. TG-DTA curves of the CoSi-PILC sample dried at room temperature.

Figure 2. Powder X-ray diffraction patterns for (a) the (CH3)4(CH2)15N+-intercalated clay (Q+-clay) and (b) CoSi-PILC dried in air and (c) calcined at 550 °C for 4 h.

loss in the temperature range of 150-450 °C with an intense exothermic peak corresponds to the oxidative decomposition of organic templates between silicate layers. An additional weight loss at around 600 °C with a weak endothermic peak is due to the dehydroxylation of silicate layers. From this result, an appropriate calcination temperature can be suggested as 550 °C to prevent the collapse of silicate layers upon dehydroxylation and to remove the organic moieties completely burnt in the interlayer space, which is also confined by FT-IR measurements.29 Powder X-ray Diffraction Analysis. Figure 2 represents the XRD patterns for the samples obtained at various stages of pillaring reaction. Upon intercalation of Q+ ion into Na+-montmorillonite (a), the basal spacing of silicate layers expands to 21.2 Å, indicating that the interlayered Q+ molecules are in a pseudo-triple arrangement between silicate layers.30 The basal spac(29) The symmetric and asymmetric stretching vibrations peaks of methylene group and the deformation of the N-C peak of organic templates near 2850, 2960, and 1470 cm-1 completely disappear as a result of the calcination process. (30) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529.

Figure 3. (A) The nitrogen adsorption-desorption isotherms curve for the CoSi-PILC calcined at 550 °C for 4 h. (B) Pore size distribution curve for the CoSi-PILC.

ing further increases to 35.2 Å upon reacting with pillaring solution (b). In this stage, the dodecylamine molecules would be adsorbed into the interlayer space and solvate the interlayered Q+ molecules, giving rise to the rearrangement of Q+ ion from parallel to perpendicular orientation.30 At the same time, the precursory inorganic pillar species would also be incorporated into the interlayer spaces by hydrophobic interaction between the interlayer Q+ molecules and the ethyl groups in TEOS. Cobalt species ligated by amine ligands may also be intercalated between the silicate layer simultaneously. After being introduced, the inorganic species would then be hydrolyzed in the interlayer owing to the presence of base catalyst (dodecylamine) and some residual interlayer water [about 3.65% (w/w) from TG analysis of Q+-clay]. On heating the air-dried sample at 550 °C for 4 h (c), the basal spacing decreases slightly to 33.1 Å due to the contractions of the pillar species upon forming stronger chemical bonds in the pillar itself, or between aluminosilicate layer and pillar. In the calcined CoSi-PILC, no distinctive XRD peaks are observed, which is mainly due to the high dispersion state of cobalt species such as CoO or Co3O4. Nitrogen Adsorption-Desorption Isotherms. Figure 3A represents the typical nitrogen adsorptiondesorption isotherm of the CoSi-PILC calcined at 550

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Figure 4. X-ray photoelectron spectrum of the CoSi-PILC calcined at 550 °C for 4 h.

Choy et al.

Figure 5. Co K-edge XANES spectra for (a) Co3O4, (b) CoO, and (c) CoSi-PILC calcined at 550 °C for 4 h. The inset shows magnified preedge features.

°C. The isotherm can be characterized as a hybrid type between types I and IV in the BDDT (Brunauer, Deming, Deming, and Teller) classification,31,32 which is indicative of the coexistence of micro- and mesopores. The hysteresis loop resembles type B in de Boer’s classification,31 corresponding to the adsorbents with slit-shaped pores between parallel layers. Furthermore, the narrow hysteresis loop indicates that the pores formed in the pillared clays are quite open.30 Concerning the pore dimension, the nearly linear portion of the adsorption curve in the P/Po region from 0.02 to 0.25 reflects the existence of supermicropores (14-20 Å) or small mesopores (20-25 Å),31 which is quite consistent with the dimension expected from the expanded gallery height (∼23.5 Å) upon pillaring. In the P/Po region from 0.0 to 0.4, the adsorption isotherm gives a good fit with the BET equation as well as with the Langmuir equation, even though a better fit is obtained with the BET equation for a limited number of nitrogen adsorption layers. The estimated BET specific surface area (SBET) and total porosity (Vt) for the calcined sample are 570 m2/g and 0.65 mL/g, respectively. The pore size distribution curve calculated by the BJH (Barrett, Joyner, and Halenda) method31,32 using the absorption branch is plotted in Figure 3B. As shown in the figure, the average pore size is estimated to be ca. 22 Å, which is in good agreement with that expected from XRD. Judging from the width at half-maximum (∼10 Å) of the distribution curve, the pores are quite uniform in size. X-ray Photoelectron Spectroscopy. To probe the oxidation and coordination states of the cobalt species in the pillared sample, XPS measurement was performed, as presented in Figure 4. The two major peaks observed at 782 and 798 eV are assigned to Co 2p1 and 2p3 transitions, respectively.33 It is noteworthy here that characteristic satellite peaks appeared at ∼6 eV higher energy compared to the major ones, unambiguously substantiating the presence of Co2+ species, since Co3+

species do not give such strong shake-up peaks.34,35 Furthermore, the observed energy separation (∼6 eV) between the satellite and the main Co 2p3/2 peaks of Co2+ ion supports the fact that the Co2+ ions in pillars are stabilized in octahedral sites rather than in tetraheral ones.36 X-ray Absorption Spectroscopy. More detailed microscopic information on the cobalt species present in the PILC could be obtained by Co K-edge X-ray absorption spectroscopy. Figure 5 shows the Co K-edge XANES spectra for CoSi-PILC together with the reference compounds, including CoO and Co3O4. The intensity and the energy position of the preedge peak around 7709 eV in the XANES spectra involve information on the local geometry and valent state of cobalt species, respectively. As shown in the inset feature, for instance, the reference compound, Co3O4, where one-third of cobalt species are stabilized in tetrahedral sites, shows a strong preedge peak. On the other hand, the peak of CoO, where all cobalt species are located in octahedral sites, is very weak. The peak intensity of CoSi-PILC is very similar to that of CoO, indicating that the cobalt species are stabilized in octahedral sites, which is quite consistent with the above XPS observation. The oxidation state of the cobalt species in the PILC turns out to be divalent, since the preedge position is equal to that of CoO and no absorption peak at 7711 eV, corresponding to the trivalent cobalt, is observed, as shown in the XANES spectrum of Co3O4. However, it is worthwhile to note here that the preedge peak intensity of the present PILC is slightly larger than that of reference CoO, which may be due to some coordinatively unsaturated sites giving rise to a breakdown of regular octahedral symmetry. Figure 6 represents the k3-weighted EXAFS spectra and their Fourier transforms (FTs) for CoSi-PILC and the reference compounds, CoO and Co3O4. In the FTs of the reference compounds, the first peak appearing

(31) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic: London, 1983. (32) Allen, T. Particle Size Measurement, 4th ed.; Chapman and Hall: London, 1980. (33) Sun, T.; Trudeau, M. L.; Ying, J. Y. J. Phys. Chem. 1996, 100, 13662.

(34) Inaba, M.; Kintaichi, Y.; Haneda, M.; Hamada, H. Catal. Lett. 1996, 39, 269. (35) Chuang, T. J.; Brundle, C. R.; Rice, D. W. Surf. Sci. 1976, 59, 413. (36) Thomas, S. M.; Bertrand, J. A.; Occelli, M. L.; Stencel, J. M.; Gould, S. A. C. Chem. Mater. 1999, 11, 1153.

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Figure 7. Comparison between the experimental (solid line) and the theoretical (symbol) inverse Fourier transforms: (a) CoO and (b) CoSi-PILC calcined at 550 °C for 4 h. Table 1. EXAFS Fitting Results for CoO and CoSi-PILC Calcined at 550 °C for 4 h sample CoO

bond

Co-O Co-Co CoSi-PILC Co-O1 Co-O2 Co-Si1 Co-Co Co-Si2 a

Figure 6. (A) The k3-weighted EXAFS oscillation for (a) CoO and (b) CoSi-PILC calcined at 550 °C for 4 h. (B) The Fourier transforms (solid line) and fitting results (symbol) for (a) CoO and (b) CoSi-PILC calcined at 550 °C for 4 h.

at ∼1.8 Å (nonphase shift corrected) in the FT feature corresponds to the nearest oxygen neighbors, and the next peak at ∼3 Å (nonphase shift corrected) is mainly due to the (Co-Co) pairs. However, it should be noted here that the FT amplitude due to the second neighbors in CoSi-PILC becomes drastically weaker than that in CoO and Co3O4, reflecting the fact that the cobalt oxide cluster is very small and/or some light elements exist in the second coordination sphere. To obtain further structural information, nonlinear least squares curve fitting was carried out for CoSiPILC and the reference compound CoO (Figure 7). The fitted structural parameters (Table 1) for the reference CoO is quite consistent with those in the previous work37 within the error limits. For CoSi-PILC, our first attempt to fit the experimental curve in the FT range of 1.0-2.3 Å based on the single oxygen shell was not satisfactory, suggesting that the coordination environment of oxygens around the absorbing element is nonequivalent. Under the assumption that two different oxygen shells are in the first neighbor, we could obtain a good fitting result (Table 1); two oxygen atoms are (37) Huffman, G. P.; Shah, N.; Zhao, J.; Huggins, F. E.; Hoost, T. E.; Halvorsen, S.; Goodwin, Jr. J. G. J. Catal. 1995, 151, 17.

distance (Å)

Na

σ2 (10-3 Å-2)b

E0 shift (eV)

2.13(1) 3.00(0) 1.96(1) 2.12(1) 2.97(3) 3.43(8) 4.36(3)

6.0 12.0 1.9(2) 3.3(4) 2.3(6) 1.1(3) 9.6(2)

7.6(2) 8.1(1) 0.8(1) 1.7(2) 14.4(7) 12.4(6) 27.2(3)

-0.5(5) -0.9(3) -3.6(7) 0.01(12) -4.4(21) -7.5(17) 3.0(15)

Coordination numbers. b Debye-Waller factor.

coordinated around the Co atom with an average bond distance of 1.98 Å, while three or four oxygens are coordinated to the Co atom with a slightly larger bond distance (2.13 Å). Here the former oxygens would be supplied from the terminal hydroxyl group in the micropores, while the latter are from the pillars, which means that most of the cobalt species are distributed on the pillar surfaces rather than in the bulk. This argument is further supported by the curve-fitting result for the second next metallic neighbors (Table 1); a better fit could be obtained by introducing (Co-Si) shells rather than pure Si shells or Co ones. If the Co species are distributed homogeneously throughout the pillars, the second metallic shell has to be composed of Si, since the Si molar content in the pillar is over 60 times greater than the Co molar content. Thus, the high probability of finding Co in the second metallic shell indicates that the Co species is rather confined to the surface region of the pillars. On the basis of the above XANES and EXAFS analyses, it can be concluded that small CoO clusters are homogeneously distributed on the surface of the SiO2 pillar.38 It is, therefore, highly expected that such a Co-pillared compound may show excellent activity for the de-NOx reaction. Selective Catalytic Reduction of NO with CO. Figure 8 shows the NO conversion rate as a function of reaction temperature over CoSi-PILC at a GHSV of 15 000 h-1. The catalytic activity of a representative deNOx catalyst like Co-ZSM-5 (SBET ) 210 m2/g, Co ) 0.83 w/w %) is also plotted for comparison. It is (38) Choy, J. H.; Jung, H.; Yoon, J. B. J. Synchr. Rad. 2001, 8, 599.

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are needed, the homegeneous distribution of CoO clusters on the surfaces of open micropores in the pillared clays is primarily responsible for the excellent de-NOx catalytic activity even at such a low temperature. 4. Conclusions

Figure 8. Selective catalytic activity of CoSi-PILC for the de-NOx reaction.

noteworthy here that 50% NO conversion could be achieved even at about 200 °C in the case of CoSi-PILC. On the other hand, the Co-ZSM-5 exhibits virtual inactivity for the de-NOx reaction in this temperature region. Furthermore, the maximum NO conversion rate reaches at least up to ∼85% at around 500 °C. Even though further systematic studies on the de-NOx conversion reaction including the decomposition mechanism

A novel de-NOx catalyst based on the pillaring of mixed metal oxide particles between the silicate layers of clay is prepared by the organic templating method. The CoSi-PILC has an extremely large BET specific surface area (∼570 m2/g) and large micropores or small mesopores with a dimension of 15-25Å. According to the XPS and XAS analyses, it is found that the cobalt species are stabilized in the form of the CoO cluster and distributed quite homogeneously on the surfaces of SiO2 pillars. Remarkably, the porous catalyst shows a high catalytic activity for the de-NOx reaction; the NO conversion rate reaches 50% at a temperature as low as 200 °C and 80% at the optimum reaction temperature of 550 °C. Acknowledgment. This study was supported by the National Research Laboratory program (NRL ‘99). We thank Prof. M. Nomura for his help in synchrotron radiation experiments conducted at the Photon Factory. CM020201X