J. Phys. Chem. B 2001, 105, 9805-9811
9805
Pore-Size Control of Cobalt Dispersion and Reducibility in Mesoporous Silicas Andrei Y. Khodakov,*,† Anne Griboval-Constant,† Rafeh Bechara,† and Franc¸ oise Villain‡ Laboratoire de Catalyse de Lille, UniVersite´ des Sciences et Technologies de Lille, Baˆ t. C3, Cite´ Scientifique, 59655 VilleneuVe d’Ascq, France, and Laboratoire de Chimie Inorganique et Mate´ riaux Mole´ culaires, UniVersite´ P. et M. Curie, 4 Place Jussieu, 75252 Paris, France ReceiVed: May 24, 2001
The effect of porosity in a wide range of mesopore diameters (dp ) 20-330 Å) on the dispersion and reducibility of cobalt species in mesoporous silicas is examined using nitrogen adsorption, X-ray diffraction, thermogravimetric analysis, and in situ X-ray absorption. It is shown that modification of mesoporous silicas by cobalt via aqueous impregnation results in small Co3O4 crystallites located in the pores of silicas. The sizes of these crystallites increase with increasing mesopore diameters. In situ X-ray absorption and thermogravimetric analyses show that the reduction of Co3O4 crystallites in hydrogen leads to CoO and Co metal particles. The porous structure of the supports strongly affects the extent of cobalt reduction. It is found that smaller particles in the narrow pores (20-50 Å) are much more difficult to reduce to metal species than larger ones situated in the broad pores (>50 Å) of mesoporous silicas.
Introduction It is known that the porous structure of catalytic supports can strongly influence the catalytic behavior of many catalysts in a large number of reactions. The irregular spacing and the broad distribution of pore sizes in traditional mesoporous oxides usually used as catalyst supports make it difficult to study the impact of pore sizes on the structure and properties of supported metal phases. The recently discovered periodic mesoporous silicas MCM-411-4 and SBA-155-7 represent a new class of inorganic oxides. Their surface areas are approaching 1000 m2/g and the pore-size distributions in periodic mesoporous silicas are very narrow.8 The pore sizes from 20 to 300 Å can be adjusted at the stage of synthesis of these materials using different surfactants. Thus, periodic mesoporous silicas can be considered as model supports, which allows the effect of pore sizes on the properties of supported metal phases to be studied. Cobalt catalysts are known to be efficient in the FischerTropsch synthesis. Active metal catalysts are usually produced by the reduction of catalyst precursors containing oxidized cobalt species. It is generally assumed that the Fischer-Tropsch synthesis involves Co metal atoms. The number of metal sites available for the catalytic reaction depends on the Co content in the catalysts, the sizes of the Co particles, and their reducibility.9-12 The efficient control of cobalt dispersion and reducibility in silica-supported catalysts would lead to the design of new metal supported catalysts with improved catalytic properties, such as activities and selectivities. At present, mostly empirical methods with poor reproducibility are used to prepare cobalt catalysts with desired metal dispersions. The effect of catalyst modification with different promoters on cobalt reducibility and on electronic properties of supported cobalt particles has been the subject of a large number of publications. It has been shown, for example, that the introduction of a small amount of noble metals, such as Pt and Ru, * To whom correspondence should be addressed. E-mail: andrei.
[email protected]. Fax: +33 3 20 43 65 61. † Universite ´ des Sciences et Technologies de Lille. ‡ Universite ´ P. et M. Curie.
improves Co reducibility.13-17 Much less information is available about the effect of porosity on the structure of supported Co species and their reduction properties. Previously, Castner et al.18,19 suggested that support porosity could influence the reducibility of supported Co3O4 particles. The Co reducibility was correlated with the ease of water removal during the CoO f Co reduction step. Because only two amorphous silicas were used in that study,18,19 it was rather difficult to draw more general conclusions about the effects of the porosity. Earlier, we showed20 that, in silica-supported catalysts prepared by the sol-gel technique, the ease of Co reduction depends on the Co particle size, decreasing from larger to smaller particles. This work addresses the effect of pore sizes in the range from 20 to 330 Å on the dispersion and reducibility of cobalt species supported by mesoporous silicas. Both periodic mesoporous and commercial mesoporous silicas are used as catalytic supports. The pore structure of both the supports and the catalysts was characterized by nitrogen adsorption. Information about local and long-distance coordination of Co atoms in oxide precursors and in reduced catalysts and about their reducibility was obtained by X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Experimental Procedures 1. Catalyst Preparation. MCM-41 type silica (SI20; Table 1) was synthesized as described in refs 3 and 4. A total of 1 g of a 28% NH4OH solution was added to 21 g of a 25% solution of cetyltrimethylammonium chloride under stirring. This solution was combined with 5.3 g of tetramethylammonium hydroxide pentahydrate (97%), followed by the addition of 5.6 g of fumed silica (Cab-o-sil M-5, Cabot) and 11.4 mL of water under stirring. The reacting mixture was transferred into a hermetically closed polypropylene flask and heated in an oven at 333 K for 24 h. The resulting gel was washed with distilled water, then dried at room temperature for 24 h, and calcined at 773 K for 6 h. The rate of temperature ramping was 1 Κ/min. SBA-15 silicas (SI42 and SI91; Table 1) were obtained using as a template block copolymer (P123), poly(ethylene glycol)-
10.1021/jp011989u CCC: $20.00 © 2001 American Chemical Society Published on Web 09/18/2001
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TABLE 1: Synthesis Procedures and Adsorption Properties of Mesoporous Silicas silica SI20
source of SiO2
SI42
fumed silica, Cab-o-sil TEOS
SI91
TEOS
synthesis procedure surfactant cetyltrimethylammonium chloride poly(ethylene glycol)block-poly(propylene glycol)block-poly(ethylene glycol), av Mn ) 5800 poly(ethylene glycol)block-poly(propylene glycol)block-poly(ethylene glycol), av Mn ) 5800
ref
SBET (m2/g)
TPVa (cm3/g)
pore diameter in silicas (Å)
3, 4
742
0.59
≈20
7
679
0.78
42
6
887
1.91
91
213
0.84
330
Cab-o-sil M-5 a
TPV is the total pore volume.
block-poly(propylene glycol)-block-poly(ethylene glycol), with average Mn of ca. 5800 (Aldrich, 43,546-5). First, the P123 copolymer was dispersed in diluted HCl under stirring. In the synthesis of SI91, dimethylformamide was also added. After P123 was completely dissolved, tetraethyl orthosilicate (TEOS) was added to the solution. The mixture was kept at 313 K under stirring for 24 h. Then, it was transferred to a hermetically closed polypropylene flask and heated in an oven at 373 K for 48 h. The silicas were washed with distilled water, dried at room temperature, and then calcined in an oven at 773 K. The details of the synthesis procedures are presented in Table 1 and references therein. Prior to cobalt introduction, a commercial fumed silica (Cabo-sil M5, Cabot) was agglomerated by wetting and then dried in an oven at 393 K overnight. Cobalt was introduced to the silicas by aqueous incipient wetness impregnation using solutions of cobalt nitrate prepared to obtain 5 wt % Co content in the final catalysts: CoSI20, CoSI42, CoSI91, and CoCab (Table 3). The samples were dried in an oven at 373 K overnight and then calcined in the flow of dry air at 773 K for 5 h. Co contents in the samples were measured by atomic absorption at the Service Central d’Analyses du CNRS (Vernaison, France). 2. Catalyst Characterization. Surface Areas and Pore-Size Distributions. The BET surface areas and pore-size distributions in mesoporous silicas were measured using nitrogen absorption at 77 K. Prior to the experiments, the samples were outgassed at 473 K for 5 h. The isotherms were measured using a Micrometrics ASAP 2010 system. The total pore volumes (TPV) were calculated from the amount of vapor adsorbed at a relative pressure close to unity, assuming that the pores are filled with the condensate in the liquid state. The pore-size distribution curves were calculated from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) formula.21 XRD. XRD patterns were recorded at room temperature by a Siemens D5000 diffractometer using Cu KR radiation. The objective was to measure Co3O4 particle sizes by XRD line broadening. Average Co3O4 particle sizes were calculated by the Sherrer equation22 using a Co3O4 XRD peak at 2θ ) 65.3°. Instrument line broadening was corrected by calibration using a mechanical mixture of bulk crystalline Co3O4 and silica. TGA. The extent of Co reduction in hydrogen was measured using a Sartorius 4102 electronic ultramicrobalance equipped with a controlled-atmosphere cell. Co oxidized catalysts (2025 mg) were placed in a quartz crucible and were dehydrated in dry air at 773 K for 3 h. Then, the samples were cooled to room temperature. After that treatment, the catalysts were heated from room temperature to 773 K in a flow of hydrogen. The rate of temperature ramping was 1 K/min. The extent of
TABLE 2: Coordination Numbers and Interatomic Distances in Co3O4, CoO, and Co Metal Phases compound
R (Å)
N
compound
R (Å)
N
Co-Co Co-Co
2.85 3.35
4 8
Co-O
3.69
8
3.57
6
Co3O4 Co-O CoO
1.89 1.98
4 1.33
Co-O Co-Co
2.13 3.02
6 12
Co-Co
2.51
Co fcc or hcp 12 Co-Co
CoO
reduction was calculated from the weight loss in the atmosphere of hydrogen, assuming stoichiometric reduction of Co3O4 to metallic cobalt. The data were corrected by subtracting the weight losses of silica supports treated under the same conditions. XAS. X-ray absorption measurements were carried out at the 42 beamline in LURE (Orsay, France), using synchrotron radiation from the DCI storage ring running at 1.85 GeV with an average current of 250 mA. The XAS data were taken in the transmission mode through an Si(111) channel-cut monochromator using two ionization chambers for X-ray detection. The monochromator was calibrated by setting the first inflection point of the K-edge spectrum of Co foil at 7709 eV. The XAS unit, equipped with a heater, a water-cooling system, thermocouples, and a gas manifold, allows in situ treatment of samples at temperatures 298-773 K. The apparatus is identical to that designed by Lytle.23,24 The catalysts were reduced in hydrogen using a temperature program (298-773 K, 5 Κ/min). The length of time for measuring an X-ray absorption spectrum (76008400 eV) was about 25 min.Crystalline Co3O4, CoO, and Co foil were used as standard compounds for the analysis of X-ray absorption results. Their structural data25 are presented in Table 2. Co3O4 has a direct spinel-type structure, where Co2+ and Co3+ cations have tetrahedral and octahedral oxygen coordination, respectively. The coordination numbers for an average Co atom in Co3O4 (Table 2) were obtained by multiplying the coordination numbers of the Co2+ and Co3+ ions by their relative concentrations in the spinel structure. CoO crystallizes in a fcc NaCl-like structure. In that structure, Co2+ ions are situated in the center of the octahedrons where they are surrounded by six oxygen atoms. Two crystal structures (R and β) are known for metallic Co: hexagonal close-packed (hcp) and fcc. In both crystal phases, however, the interatomic distances and Co coordination numbers for the first and the second coordination spheres are the same (Table 2). The X-ray absorption near-edge structure (XANES) spectra, after background correction, were normalized by the edge height. The Co K edge EXAFS of reduced samples was analyzed using
Co Dispersion and Reducibility the standard data analysis procedure (SIMPLEX software package).26 The EXAFS signal was first transformed from k space to r space (k3, Hanning windows, 3.6, 3.9, 9.9, and 12 Å-1) to obtain the radial distribution function (RDF). The EXAFS signal, for one or several coordination shells, was isolated by inverse Fourier transform of the RDF over appropriate regions and fitted using the single scattering EXAFS equation. Cobalt coordination numbers (Ni), Debye-Waller factors (σi), interatomic distances (Ri) for each coordination shell, and a difference in the origin of photoelectron kinetic energy between a catalyst and the reference compounds were used as fitting parameters. Ab initio simulations for CoO and Co metal phases, assuming single and multiple scattering, were carried out using FEFF7 code27,28 to check the possible influence of multiple scattering on EXAFS. The simulations did not reveal any effect of multiple scattering on the RDFs of these compounds at interatomic distances smaller than 3.6 Å. It is generally assumed that the uncertainty of coordination numbers determined from EXAFS could be less than 20% when least-squares fitting involves only a single well-resolved coordination shell. This uncertainty can be higher for least-squares fittings which involve several coordination shells because of high correlation between coordination numbers and other parameters in data refinement. These were the reasons why, in the present paper, we use EXAFS to obtain only qualitative information about the structure of Co species. Note that no quantitative conclusions are made from EXAFS about the diameters of Co particles, the extent of Co reduction, or concentrations of different phases. Results 1. Porous Structure of Mesoporous Silicas. The isotherms of nitrogen adsorption and desorption on mesoporous silicas and the corresponding pore-size distribution curves calculated using the BJH method21 are shown in Figures 1 and 2. The BET surface areas, TPV, and average pore diameters are presented in Table 1. The BET surface areas in all periodic mesoporous silicas (SI20, SI42, and SI91) were higher than 650 m2/g, whereas the BET surface area of the Cab-o-sil silica was much lower (∼213 m2/g). The shape of the isotherms of the SI20 sample (Figure 1a, curve 1) is typical for MCM-41 silicas. It corresponds to the adsorption of N2 on the walls of narrow mesopores. The average pore diameter calculated using the BJH method was less than 25 Å. The surface area, pore volume, and average pore diameter of SI20 silica are in good agreement with those of Beck et al.2 and Chen et al.3 Upon nitrogen adsorption, SI42, SI91, and Cab-o-sil silicas produce irreversible type IV isotherms29 with a H1 hystheresis loop (Figure 1a,b) that is typical of mesoporous materials with 1D cylindrical channels. The isotherms of mesoporous silicas exhibit a sharp reflection in P/P0 range characteristic of capillary condensation within mesopores.6,7 The P/P0 position of the inflection points is related to a diameter in the mesopore range. The BJH pore-size distributions (Figure 2) were narrower for the SI42 and SI91 periodic mesoporous silicas than for commercial Cab-o-sil M5. The average pore diameters ranged from 42 Å in SI42 silica to 330 Å in Cab-o-sil M5. The porous structure of mesoporous silicas remains almost intact after modification with cobalt. Table 3 shows that cobalt introduction results only in a slight decrease in BET surface areas and in pore volumes relative to the initial silicas. The average pore diameters of silicas were also smaller after Co incorporation. 2. Cobalt Species in Oxidized Catalysts. The XANES spectra and moduli of Fourier transform of EXAFS of the
J. Phys. Chem. B, Vol. 105, No. 40, 2001 9807
Figure 1. Nitrogen adsorption-desorption isotherms: (1) SI20, (2) SI42, (3) SI91, and (4) Cab-o-sil.
Figure 2. Pore-size distribution curves calculated from nitrogen desorption isotherms: (1) SI20, (2) SI42, (3) SI91, and (4) Cab-o-sil.
oxidized catalysts and bulk Co3O4 are presented in Figure 3. The XANES spectra of oxidized Co supported silicas are practically identical; they resemble the spectrum of bulk Co3O4. Two peaks at 1.5 and 2.5 Å and a shoulder at 3.1 Å are observed in moduli of Fourier transform of EXAFS (Figure 3b). In accordance with previous reports,20,30 the first peak at 1.5 Å is attributed to CoO coordination shells for octahedral Co3+ and tetrahedral Co2+ sites (Table 2). These two CoO coordination shells are usually not resolved in the Fourier transform moduli.20,30 The peak and shoulder situated at 2.5 and 3.1 Å, respectively, were assigned to two CoCo coordination shells in Co3O4 (Table 2). Comparison of both XANES spectra and moduli of Fourier transform shows a similar local coordination of Co atoms in bulk Co3O4 and oxidized Co supported catalysts. The XRD patterns of Co supported catalysts are presented in Figure 4. Only the Co3O4 crystalline phase was detected. The XRD peaks were much broader for Co3O4 crystallites in narrowpore silicas (CoSI20 and COSI42). The Co3O4 crystallite sizes
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TABLE 3: Cobalt Catalysts Supported by Mesoporous Silicas
Co catalyst
Co content (wt %)
CoSI20 CoSI42 CoSI91 CoCab
5.67 6.95 5.39 4.75
adsorption properties of Co catalysts BET surface TPV pore diameter area (m2/g) (cm3/g) (Å) 462 586 674 206
0.46 0.62 1.11 0.76
≈20 43 75 230
Figure 3. XANES spectra (a) and moduli of Fourier transform of EXAFS (b) for oxidized Co catalysts: (1) CoSI20, (2) CoSI42, (3) CoSI91, (4) CoCab, and (5) bulk Co3O4.
Figure 4. XRD patterns of Co catalysts: (1) CoSI20, (2) CoSI42, (3) CoSI91, and (4) CoCab.
(Table 3) were calculated from the widths of XRD peaks using the Sherrer equation (2θ ) 65.30°).22 Table 3 shows that the sizes of Co3O4 crystallites depend on the average pore diameters in mesoporous silicas; larger Co3O4 crystallites are found in large-pore silicas. A relation between the sizes of supported Co3O4 crystallites and diameters of pores in mesoporous silicas is presented in Figure 9.
Co3O4 crystallite diameter from XRD (Å, 2θ ) 65.30°) 8 at R ) 2.50-2.55 Å; Table 4). Metal CoCo coordination numbers were found to be much lower in CoSI20 and CoSI42 samples than in bulk metal. Discussion XRD measurements showed that the sizes of the Co3O4 particles supported by mesoporous silicas depended on the diameters of the pores. The diameters of the Co3O4 particles ranged from less than 40-50 Å for CoSI20 catalysts to 200 Å in CoCab (Table 3). They are comparable with the pore sizes of the supports. It is known that limitations and approximations of the Scherrer equation could result in a slight overstating of the crystallite sizes calculated from XRD peak widths.33 It can be suggested, therefore, that the diameters of Co3O4 crystallites
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TABLE 4: Results of EXAFS Fitting for Co Catalysts Reduced at 773 K in Hydrogena Co-O coordination in CoO CoSI20 CoSI42 CoSI91 CoCab CoO Co foil a
CoCo coordination in Co metal
CoCo coordination in CoO
NCoO
RCoO (Å)
σ
NCoCo
RCoCo (Å)
σ
NCoCo
RCoCo (Å)
σ
∆2 × 103
4.6 1.6 0.6 0.8 6 0
2.08 2.05 2.09 2.09 2.13
0.07 0.04 0.04 0.04
3.1 5.3 8.0 8.2 0 12
2.55 2.52 2.50 2.51
0.05 0.04 0.05 0.05
4.2 4.4 2.8 3.1 12 0
3.04 3.01 2.95 2.95 3.02
0.03 0.06 0.08 0.09
4.5 9.2 2.1 0.7
2.51
N and R are a calculated coordination number and an interatomic distance, σ is a Debye-Waller factor, and ∆2 is a fit residual.
Figure 9. Dependences of Co3O4 particle sizes and extents of Co reduction at 673 K on the pore diameters of mesoporous silicas.
could be a little smaller than those shown in Table 3 and that Co3O4 crystallites would perfectly fit the pore sizes of mesoporous silicas. Because the diameters of Co3O4 nanoparticles are limited by the pore sizes, it seems reasonable to suggest that Co3O4 particles are mainly encapsulated into the support pores rather than located on the outer surface of the supports. This suggestion is in good agreement with the data of Iwamoto et al.,34 who found that MCM-41 materials could be used as cages and stabilizers for nanoparticles of iron oxide. They also found that the band gap of supported iron oxide, evaluated from the absorption edges of the UV-Vis spectra, was changed by the quantum size effect and controlled by the pore diameters of MCM-41 mesoporous silicas. The suggestion about the preferential localization of Co atoms inside pores of the mesoporous silicas is also consistent with the data of Jentys et al.,35,36 who studied the localization of Co metal particles introduced via impregnation in MCM-41 silicas with pore diameters between 29 and 36 Å. X-ray absorption and TGA data show that the reduction of Co3O4 to Co metal proceeds in two steps: Co3O4 f CoO f Co. The conversion of Co3O4 to CoO starts at 473-523 K, whereas the reduction of CoO to metal proceeds at temperatures higher than 573 K. The TGA data show that the Co3O4 f CoO transition is affected to a much lesser extent by the porous structure of supports and Co3O4 particle sizes than the second step of the reduction: CoO f Co. These observations are consistent with the previous works16,19,31 and our earlier data.20,32 Comparison of Co catalysts supported by large- and small-pore silicas shows that the CoO f Co reduction proceeds at much lower temperatures in large-pore silicas than in small-pore supports. The different reducibility is likely to be related to the different sizes of Co particles located in the pores of different diameters. It is known19,20,32 that smaller Co particles are much more difficult to reduce than larger ones. The most probable interpretation of this phenomenon is related to metal-support
interaction. In smaller particles, the interaction between metal and support is usually much stronger than that in larger ones.20 The suggestion about different reducibilities of Co3O4 particles located in the mesopores is confirmed by X-ray absorption measurements. XANES spectra, presented in Figure 7, show that Co K absorption near-edge spectra of the catalysts treated in hydrogen at 773 K shifts from the CoO-type shape in the CoSI20 sample to the shape more typical for bulk metallic cobalt in CoCab. This shift indicates an increase in the extent of Co reduction with an increase in the diameters of silica pores and cobalt oxide particle sizes. This suggestion is also consistent with the TGA data about the extent of cobalt reduction in mesoporous silicas (Table 3). Much lower coordination numbers for the CoCo nearestneighbor metal coordination shell at R ) 2.50-2.55 Å, characteristic for metallic cobalt, were obtained from EXAFS analysis for narrow-pore CoSI20 and CoSI42 catalysts. In largepore samples, nearest-metal-metal-neighbor coordination numbers (NCoCo >8) were more similar to those in bulk metal (Table 4). Lower nearest-neighbor metal coordination numbers measured from EXAFS are usually interpreted in terms of smaller sizes of metal clusters. Assuming cuboctahedron symmetry of the supposed Co metal clusters and total reduction of the Co species, the nearest-neighbor metal coordination number of 5 would correspond to the Co metal clusters of a 7 Å diameter.37 Note that for the metal particles larger than 15-20 Å the nearestneighbor metal coordination numbers are not sensitive to the sizes of metal clusters.37 Previous reports17,19 show that reduction of Co3O4 crystallites in hydrogen results in the diameters of reduced metal particles being slightly smaller than the sizes of the initial Co3O4 crystallites. Using XPS and TEM with a controlled-atmosphere cell attachment, Castner et al.19 showed, for example, that upon reduction of the Co3O4 supported particles to metallic Co the particle sizes decrease by 30-50%. Schanke et al.17 converted the sizes of supported Co3O4 particles to the corresponding cobalt metal particles according to the relative molar volumes of metallic cobalt and Co3O4. The resulting conversion factor for the diameter d of a given Co3O4 particle being reduced to metallic cobalt is d(Co0) ) 0.75d(Co3O4).17 Therefore, the sizes of reduced Co metal particles supported by mesoporous silicas are probably about 30% smaller than those of the initial Co3O4. Taking into consideration the diameters of the Co3O4 crystallites and the pore diameters in mesoporous silicas, it seems reasonable to suggest that the diameters of supported Co metal particles could be significantly larger than 10 Å. The most probable interpretation of lower nearest-neighbor metal coordination numbers (NCoCo) in small-pore catalysts seems to be related not to the smaller sizes of the reduced Co metal clusters but to the lower extent of reduction of CoO to metallic Co for smaller particles in narrow-pore silicas. Note that the coordination numbers obtained from EXAFS represent the coordination of an average Co atom in a given catalyst. Because unreduced
Co Dispersion and Reducibility catalysts contain significant concentrations of the CoO phase, the nearest CoCo metal coordination numbers calculated from EXAFS, even for the metal clusters of the same sizes, would be much lower in unreduced catalysts than in completely reduced ones. Because EXAFS coming from a CoO coordination shell is usually much less intense than that coming from a CoCo coordination shell, the presence of considerable concentrations of unreduced oxide results in much less intense features in the RDF than the same amounts of metal phases. Figure 9 shows the dependences of the sizes of Co3O4 supported particles and the extent of Co reduction on the diameters of pores in mesoporous silicas. The dependences of these two parameters have very much in common. An increase in the pore diameters results both in an increase in the sizes of the cobalt particles and in the reducibility of the Co species. An increase in the diameters of the cobalt particles with pore diameters seems to be an argument in favor of the preferential localization of most of the cobalt oxide particles in silica pores. In narrow pores, the interaction between small particles of cobalt oxide and silica would be much stronger than that in largepore supports. This interaction results in lower reducibility of Co particles in small-pore silicas. Conclusions The use of periodic mesoporous silicas as catalytic supports allowed the effect of pore structure on the dispersion and reducibility of supported Co particles to be studied. It was shown that impregnation of mesoporous silicas with cobalt nitrate, followed by calcination at high temperatures, resulted in Co3O4 particles, mainly encapsulated in the silica pores. Analysis of X-ray absorption and TGA data suggests that the reduction of Co3O4 crystallites to metallic Co particles proceeds in two steps with CoO as an intermediate. An increase in the pore sizes of the supports leads to both an increase in Co3O4 particle sizes and the reducibility of intermediary CoO particles. The extent of Co reduction rises sharply as the pore sizes in mesoporous silicas become larger than 50 Å. On the basis of these data, it is evident that the porosity of catalytic supports can control both the sizes of the metal and oxide particles and their hydrogen reduction properties. Acknowledgment. The authors thank C. Guelton for TGA measurements. The authors acknowledge the Laboratoire pour l’Utilisation du Rayonnement Electromagne´tique (LURE), Orsay, France, for the use of the beamline for X-ray absorption measurements. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.;
J. Phys. Chem. B, Vol. 105, No. 40, 2001 9811 McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (4) Holmes, S.; Zholobenko, V.; Thursfield, A.; Plaisted, R. J.; Candy, S. C.; Dwyer, J. J. Chem. Soc., Faraday Trans. 1998, 94, 2025. (5) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (6) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (7) Luan, Z.; Hartmann, M.; Zhao, D.; Zhou, W.; Kevan, L. Chem. Mater. 1999, 11, 1621. (8) Biz, S.; Occelli, M. Catal. ReV.sSci. Eng. 1998, 40 (3), 329. (9) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. AdV. Catal. 1993, 39, 221. (10) Iglesia, E.; Soled, S. L.; Fiato, R. A. J. Catal. 1992, 137, 212. (11) Li, F.; Bartholomew, C. H. J. Catal. 1985, 92, 376. (12) Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984, 85, 78. (13) Batley, G. E.; Ekstrom, A.; Johnson, D. A. J. Catal. 1974, 34, 368. (14) Guczi, L.; Hoffer, T.; Zsoldos, Z.; Zyade, S.; Maire, G.; Garin, F. J. Phys. Chem. 1991, 85, 802. (15) van’t Blik, H. F. J.; Koningsberger, D. C.; Prins, R. J. J. Catal. 1986, 97, 210. (16) van’t Blik, H. F. J.; Prins, R. J. Catal. 1986, 97, 210. (17) Schanke, D.; Vada, S.; Blekkan, E. A.; Hilmen, A. M.; Hoff, A.; Holmen, A. J. Catal. 1995, 156, 85. (18) Castner, D. G.; Watson, P. R.; Chan, I. Y. J. Phys. Chem. 1989, 93, 3188. (19) Castner, D. G.; Watson, P. R.; Chan, I. Y. J. Phys. Chem. 1990, 94, 819. (20) Khodakov, A.Y.; Lynch, J.; Bazin, D.; Rebours, B.; Zanier, N.; Moisson, B.; Chaumette, P. J. Catal. 1997, 168, 16. (21) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (22) Cullity, B. D. Elements of X-ray Diffraction; Addision-Wesley Publishing Company: London, 1978. (23) Sinfelt, J. H.; Via, G. H.; Lytle, F. M. Catal. ReV.sSci. Eng. 1984, 26, 81. (24) Short, D. R.; Mansour, A. N.; Cook, J. M.; Sayers, D. E.; Katzer, J. R. J. Catal. 1983, 82, 299. (25) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1960. (26) Michalowicz, A. Me´thodes et Programmes d’Analyse des Spectres d’Absorption des Rayons X. Ph.D. Thesis, Universite´ Paris, Val de Marne, France, 1990. (27) Ankoudinov, A. L. Relativistic Spin-dependent X-ray Absorption Theory. Ph.D. Thesis, University of Washington, Seattle, WA, 1996. (28) . Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B: Condens. Matter 1995, 52, 2995. (29) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (30) Huffmann, G. P.; Shah, N.; Zhao, J.; Huggins, F. E.; Hoost, T. E.; Halvorsen, S.; Goodwin, J. J. Catal. 1995, 151, 17. (31) Ernst, B.; Bensaddik, A.; Hilaire, L.; Chaumette, P.; Kiennemann, A. Catal. Today 1998, 39, 329. (32) Bechara, R.; Balloy, D.; Dauphin, J.-Y.; Grimblot, J. Chem. Mater. 1999, 11, 1703. (33) Ganesan, P.; Kuo, H. K.; Saaverda, A.; DeAngelis, R. J. J. Catal. 1978, 52, 319. (34) Iwamoto, M.; Abe, T.; Tachibana, Y. J. Mol. Catal. A: Chem. 2000, 155, 143. (35) Jentys, A.; Pham, N. H.; Vinek, H.; Englisch, M.; Lercher, J. A. Catal. Today 1998, 39, 311. (36) Jentys, A.; Pham, N. H.; Vinek, H.; Englisch, M.; Lercher, J. A. Microporous Mater. 1996, 6, 13. (37) Benfield, R. J. Chem. Soc., Faraday Trans. 1992, 88 (8), 1107.
