High Cobalt Content Mesoporous Silicas - ACS Publications

Chem. Mater. 2004, 16, 2805-2813

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High Cobalt Content Mesoporous Silicas Jamal El Haskouri,† Sau´l Cabrera,‡ Carlos J. Go´mez-Garcıá,§ Carmen Guillem,† Julio Latorre,† Aurelio Beltrań,† Daniel Beltrań,† M. Dolores Marcos,| and Pedro Amoro´s*,† Institut de Cie` ncia dels Materials de la Universitat de Valencia (ICMUV), P.O. Box 2085, 46071-Valencia, Spain, Laboratorio de So´ lidos y Quı´mica Teo´ rica, Instituto de Investigaciones Quı´micas UMSA, Cota-Cota, Calle nο. 27, La Paz, Bolivia, Instituto de Ciencia Molecular de la Universitat de Valencia (ICMOL), Doctor Moliner 50, 46100-Burjassot, Valencia, Spain, Departamento de Quı´mica, Universidad Polite´ cnica de Valencia, Camino de Vera s/n, 46071-Valencia, Spain Received February 13, 2004. Revised Manuscript Received May 13, 2004

Silica-based MCM-41-like mesoporous materials with high cobalt content (∞ g Si/Co g 23) have been synthesized through a one-pot surfactant-assisted procedure from aqueous solution using a cationic surfactant (CTMABr ) cetyltrimethylammonium bromide) as structural directing agent, and starting from molecular atrane complexes of Co and Si as inorganic hydrolytic precursors. This preparative technique allows optimizing the dispersion of the Co guest species in the silica walls. The mesoporous nature of the final materials is confirmed by XRD, TEM, and N2 adsorption-desorption isotherms. They display unimodal and relatively narrow pore size distributions, whereas their pore array evolves from ordered hexagonal (H0) to wormhole-like (W) as the Co content increases. A careful spectroscopic (UV-visible and NMR) and magnetic study of these materials shows that, regardless of the Si/Co ratio, Co atoms are organized in well-dispersed, uniform CoO nanodomains (ca. 3 nm) partially embedded within the silica walls. These materials, which show superparamagnetic behavior, can be referred to as mesoporous CoO-MCM-41 nanocomposites.

Introduction Theoretical and applied interest in transition-metalsubstituted microporous materials is mainly due to their real and potential applications in catalysis, which is related to the presence in the framework of redox-active sites.1 Indeed, dealing with cobalt derivatives, it was found that Co-containing zeolites and ALPOs (such as Co-ZSM-5, Co-beta, CoAPO-11, CoAPO-5, etc.) exhibit high activity and selectivity in a variety of redox reactions, such as, for example, ammoxidation of ethane to acetonitrile, selective catalytic reduction (SCR) of NO, or oxidation of saturated hydrocarbons.2-6 The activity of these catalysts is interpreted in terms of the cobalt ability to exchange between the Co(II) and Co(III) oxidation states. Cobalt redox chemistry is a particularly well-explored item. In a logical way, materials of this kind have been the object of a variety of spectroscopic * To whom correspondence should be addressed. Phone: +34-963543617. Fax: +34-96-3543633. E-mail: [email protected]. † Institut de Cie ` ncia dels Materials de la Universitat de Valencia. ‡ Instituto de Investigaciones Quı´micas UMSA. § Instituto de Ciencia Molecular de la Universitat de Valencia (ICMOL). | Universidad Polite ´ cnica de Valencia. (1) Hartmann, M.; Kevan, L. Chem. Rev. 1999, 99, 635. (2) Campa, H. C.; De Rossi, S.; Ferraris, G.; Indovina, V. Appl. Catal. 1996, 8, 315. (3) Li, Y.; Slager, T. L.; Amor, J. N. J. Catal. 1994, 150, 388. (4) Li, Y.; Amor, J. N. J. Catal. 1994, 150, 376. (5) Chisuka, H.; Tabata, T.; Okada, O.; Sabatino, L. M. F.; Bellusi, G. Catal. Lett. 1997, 44, 265. (6) Li, Y.; Amor, J. N. J. Chem. Soc. Chem. Commun. 1997, 20, 2013.

studies addressed to elucidate the geometry of the active sites.7-10 On the other hand, it is well-known that the discovery of the M41S family of mesoporous silicas, besides its inherent impact, resulted in an opportunity for expanding the available pore sizes typical of zeolites to the mesopore range.11 Owing to their large pore sizes, MCM41-like mesoporous materials might be efficient in catalytic reactions involving bulky organic substrates.12-14 However, the neutral character of the Si framework proper of pure mesoporous silicas implies limitations on their applicability in catalysis (or in areas such as molecular sieving and adsorption). Hence, the incorporation of different elements to the silica materials promptly was explored to modulate their catalytic properties. Thus, a very large variety of mesoporous materials has been prepared using a diversity of procedures conceived to incorporate catalytically active species inside or on the silica walls. In short, these (7) Verberckmoes, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. Microporous Mesoporous Mater. 1998, 22, 165, and references therein. (8) Deˇdecˇek, J.; Wichterlova´ J. Phys. Chem. B 1999, 103, 1462. (9) Sˇ poner, J.; C ˇ ejka, J.; Deˇdecˇek, J.; Withterlova´, B. Microporous and Mesoporous Materials 2000, 27, 117. (10) Drozdova´, L.; Prins, R.; Deˇdecˇek, J.; Sobalı´k, Z.; Wichterlova´ J. Phys. Chem. B 2002, 106, 2240. (11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (12) Corma, A. Chem. Rev. 1997, 97, 2373. (13) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem. Int. Ed. 1999, 38, 56. (14) On, D. T.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal. A: Gen. 2001, 222, 299.

10.1021/cm049772a CCC: $27.50 © 2004 American Chemical Society Published on Web 06/19/2004

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Table 1. Selected Synthetic Parameters and Physical Properties of Co-MCM-41 Mesoporous Materials

sample

Si/Co (precursor)

Si/Coa (solid)

1 2 3 4 5

∞ 50 40 30 20

∞ 155(2) 98(2) 49(2) 23(3)

Si/Cob

a0 (Å)c (as-synthesized)

a0 (Å)c (calcined)

SBET (m2g-1)

pore diameter (Å)d

wall thickness (Å)

∞ 153.0 99,2 48.2 22.5

43.94 43.55 43.93 43.95 47.18

39.20 38.03 38.01 40.96 41.37

1161 909 662 526 479

26.6 25.0 24.8 24.7 25.0

12.7 13.0 13.2 16.3 16.4

a Values averaged from EPMA of ca. 50 different particles (final porous materials). Spot area of ca. 1 µm. Statistical ESDs in parentheses. Values calculated from XRF. c Cell parameters calculated assuming a MCM-41-like hexagonal cell (a0) 2d100/31/2). d BJH pore diameters calculated from the adsorption branch of the isotherms.

b

procedures can be referred to as one-pot methods (implying co-condensation of the active moieties together with silicon precursors during the synthesis of the parent mesostructured material) or postsynthesis treatments (by using methods such as ion-exchange, impregnation and deposition, or grafting techniques).14 When compared to mesoporous silicas including transition metals such as Ti or V, attention paid to Cocontaining related materials has been relatively limited. Nevertheless, Co atoms have been incorporated (through co-condensation or impregnation procedures) in mesoporous silicas such as MCM-41,15-22 HMS,23,24 and SBA1.25 It is remarkable that, for a given set of precursors, cobalt loads achieved by impregnation are usually higher than those resulting from co-condensation. Moreover, in this last case, the effect (relatively low amount of Co atoms entering the silica walls) is more pronounced when working at acid pH values, which has been tentatively attributed to the higher solubility of the Co species.25 In most cases, and particularly when Co atoms are introduced through impregnation techniques, the metallic centers occupy extraframework positions,19,20,22,24 this favoring the formation of cobalt oxide nanocrystals. This fact has been exploited to prepare Fischer-Tropsch catalysts with high surface area and mesoporous texture.24 In practice, these catalysts, in which Co is supported on mesoporous silica, are more efficient for CO conversion than Co supported on silica gel. In any case, it must not be forgotten that both the amount of accessible cobalt atoms (forming small aggregates) and a good dispersion of the active sites are usually required to achieve good catalytic performances. We report here on a one-pot reproducible surfactantassisted procedure that has allowed us to synthesize high-cobalt-content mesoporous silicas displaying high (15) Jentys, A.; Pham, N. H.; Vinek, H.; Englisch, M.; Lercher, J. A. Microporous Mater. 1996, 6, 13. (16) Carvalho, W. A.; Varaldo, P. B.; Wallau, M.; Schuchardt, U. Zeolites 1997, 18, 408. (17) Be´land, F.; Echchahed, B.; Badiei, A. R.; Bonneviot, L. Stud. Surf. Sci. Catal. 1998, 117, 567. (18) Echchahed, B.; Badiei, A. R.; Be´land, F.; Bonneviot, L. Stud. Surf. Sci. Catal 1998, 117, 559. (19) Deˇdecˇek, J.; Zˇ ilkova´, N.; C ˇ ejka, J. Stud. Surf. Sci. Catal. 2000, 129, 235. (20) Suvanto, S.; Hukkama¨ki, J.; Pakkanen, T. T.; Pakkanen, T. A. Langmuir 2000, 16, 4109. (21) Parvulescu, V.; Su, B.-L. Catal. Today 2001, 69, 315. (22) Wei, M.; Okabe, K.; Arakawa, H.; Teraoka, Y. New J. Chem. 2002, 26, 20. (23) Fu, Z.; Yin, D.; Chen, Y.; Yin, D.; Guo, J.; Xiong, C.; Zhang, L. Stud. Surf. Sci. Catal. 2001, 135, 312. (24) Yin, D.; Li, W.; Yang, W.; Xiang, H.; Sun, Y.; Zhing, B.; Peng, S. Microporous Mesoporous Mater 2001, 47, 15. (25) Vinu, A.; Deˇdecˇek, J.; Murugesan, V.; Hartmann, M. Chem. Mater. 2002, 14, 2433.

chemical homogeneity (good dispersion of active sites) and a considerable amount of accessible Co centers. Our synthesis method takes advantage of using a triethanolamine-water reaction medium, which allows harmonizing the hydrolytic reactivity of the inorganic precursors. Experimental Section Synthesis. The method is based on the use of a cationic surfactant (CTMABr ) cetyltrimethylammonimum bromide) as structural directing agent or supramolecular template (and, consequently, as a porogen after template removal), and a hydro alcoholic reaction medium (water:2, 2′,2′′-nitrilotriethanol or triethanolamine, N(CH2-CH2-OH)3, hereinafter TEAH3), in which the presence of the polyalcohol is key to balance the hydrolysis and condensation reaction rates of the Co and Si sources, which finally results in a high dispersion of the Co atoms throughout the final material. Chemicals. All reagents [CTMABr, tetraethyl orthosilicate (TEOS), CoCl2, TEAH3, and NaOH] were used as received from Aldrich. Preparative Procedure. In a typical synthesis leading to the Si/Co ) 49 mesoporous material (Sample 4 in Table 1), 0.49 g (12.3 mmol) of NaOH was dissolved at 60 °C in 23 mL (172 mmol) of TEAH3. After a few minutes, 10.5 mL (45.3 mmol) of TEOS and 0.20 g (1.5 mmol) of CoCl2 were added while stirring, and the mixture was heated at 150 °C for 5 min. The resulting solution was cooled to 100 °C, and 4.68 g (12.8 mmol) of CTMABr was added while stirring. Then, 80 mL (4.44 mol) of water was added with vigorous stirring at a mixing temperature of 60 °C. After a few minutes, a pale-blue suspension resulted. This mixture was aged at room temperature for 24 h. The resulting mesostructured powder was filtered off, washed with water and ethanol, and air-dried. Finally, to open the mesopore system, both the surfactant and TEAH3 were removed from the as-synthesized solid by calcination at 550 °C during 5 h under static air atmosphere. Table 1 summarizes the main synthesis variables and physical data. In all cases, the molar ratio of the reagents in the mother-liquor was adjusted to ca. 2:x:7:0.50:0.50:180 Si/ Co/TEAH3/NaOH/CTMABr/H2O, with variable amounts (x) of the Co precursor species. Physical Measurements. All solids were analyzed for Co and Si by electron probe microanalysis (EPMA) using a Philips SEM-515 instrument. Si/Co molar ratio values averaged from EPMA data corresponding to ca. 50 different particles of each sample are summarized in Table 1. All bulk samples were also analyzed for Co and Si by X-ray fluorescence (XRF) using a Pico TAX TXRF spectrometer (see Table 1). X-ray powder diffraction (XRD) data were recorded on a Seifert 3000TT θ-θ diffractometer using Cu KR radiation. Patterns were collected in steps of 0.02° (2θ) over the angular range 1-10° (2θ) for 25 s per step. Electron microscopy study (TEM) was carried out with a Philips CM-10 electron microscope operating at 100 kV. Samples were gently ground in dodecane, and microparticles were deposited on a holey carbon film supported on a Cu grid. Surface area and pore size values were calculated from nitrogen adsorption-desorption isotherms (-196 °C) recorded

High Cobalt Content Mesoporous Silicas on a Micromeritics ASAP-2010 automated instrument. Calcined samples were degassed for 15 h at 130 °C and 10-6 Torr prior to analysis. Surface areas were estimated according to the BET model, and pore size dimensions were calculated by using the BJH method. 29Si MAS NMR spectra were recorded on a Varian Unity 300 spectrometer operating at 79.5 MHz, and using a magic angle spinning speed of at least 4.0 kHz. Room-temperature diffuse reflectance spectra were registered using a Shimazdu UV-Vis 2501PC instrument equipped with an integrated sphere coated with BaSO4, which also is used as standard. To gain insight on the site geometry and accessibility, as well as on the reactivity of the Co species, we have recorded different series of UV-Vis spectra corresponding to dehydrated and rehydrated samples. Samples were dehydrated during 4 h at 300 °C. The rehydration process was monitored vs time. Thus, the dehydrated samples were placed in a cell of controlled humidity and the spectra were recorded at different times on the same sample. All the magnetic measurements were performed on powder samples (sealed in plastic bags) with a SQUID susceptometer (Quantum Design MPMS-XL-5), except the magnetization measurements at 20 K, which were measured on magnetic fields of up to 9 T in a Quantum Design PPMS-9 equipment. DC susceptibility measurements were done in the temperature range of 2-300 K with an applied magnetic field of 1000 G (0.1 T) for all the samples except the one with less Co content, where a magnetic field of 1 T was used. AC susceptibility measurements were performed in the temperature range 2-25 K with an alternating field of 0.395 mT oscillating at frequencies of 1, 10, 110, 332, and 997 Hz. Hysteresis cycles of all the samples were measured after zero field and field (2 T) cooling at temperatures of 2, 3, 4, and 5 K with applied magnetic fields in the -5 to 5 T range. Zero field cooled (ZFC), field cooled (FC), and remnant magnetization (RM) measurements where performed as usual: the sample is cooled in zero field down to 2 K and then a field of 50 G is applied and the susceptibility is measured from 2 to 25 K (ZFC) and from 25 to 2 K (FC). Finally, the magnetic field is switched off and the susceptibility is measured again from 2 to 25 K (RM).

Results and Disscussion Synthesis Strategy. Our synthesis strategy is based on the so-called “atrane route”, a simple preparative technique whose main points have been previously described in detail.26-28 In fact, such a method has allowed us to successfully prepare a diversity of mesoporous single oxides (Si MCM-41,27 MCM-48,27 SBA-8,29 UVM-7;30 Al ICMUV-1;31 Ti ICMUV-432) and mixed oxides (Si-Ti, Si-V, Si-Al, Si-Zr).26-28,30,33-36 The “atrane route” is based on the use of complexes which (26) Cabrera, S.; El Haskouri, J.; Guillem, C.; Latorre, J.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Solid State Sci. 2000, 2, 405. (27) Amoro´s, P.; Beltrań, A.; Beltrań, D.; Cabrera, S.; El Haskouri, J.; Marcos, M. D. Patent WO 01-72635. (28) El Haskouri, J.; Cabrera, S.; Calde´s, M.; Alamo, J.; Beltrań, A.; Marcos, M. D.; Amoro´s, P.; Beltrań, D. Int. J. Inorg. Mater. 2001, 3, 1157. (29) El Haskouri, J.; Cabrera, S.; Calde´s, M.; Guillem, C.; Latorre, J.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Chem. Mater. 2002, 14, 2637. (30) El Haskouri, J.; Ortiz de Za´rate, D.; Guillem, C.; Latorre, J.; Calde´s, M.; Beltrań, A.; Beltrań, D.; Descalzo, A. B.; Rodrı´guez, G.; Martıńez, R.; Marcos, M. D.; Amoro´s, P. Chem. Commun. 2002, 330. (31) Cabrera, S.; El Haskouri, J.; Alamo, J.; Beltrań, A.; Beltrań, D.; Mendioroz, S.; Marcos, M. D.; Amoro´s, P. Adv. Mater. 1999, 11, 379. (32) Cabrera, S.; El Haskouri, J.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Solid State Sci. 2000, 2, 513. (33) Cabrera, S.; El Haskouri, J.; Mendioroz, S.; Guillem, C.; Latorre, J.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Chem. Commun. 1999, 1679. (34) El Haskouri, J.; Cabrera, S.; Gutierrez, M.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Chem. Commun. 2001, 309.

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include triethanolamine-like species as ligands. The use of TEAH3 keeps the oxide forming elements (Si and Co in the present case) in homogeneous solution. These mixtures can be used to prepare mesostructured materials by reaction with the surfactant aggregates. In short, this preparative strategy is designed to optimize the dispersion of Co guest species in the mesoporous host matrix. Characterization. We have used EPMA to check the chemical homogeneity of the resulting solids. Summarized in Table 1 are the data of the bulk chemical composition of the samples (XRF) together with the corresponding EPMA Si/Co molar ratio values, which have been averaged in each case from data of ca. 50 different particles. As can be noted, there is a very good correspondence between XRF and EPMA results. Besides this, the low values of the calculated standard deviations allows us to reasonably state that all samples are chemically homogeneous at micrometric level with a regular dispersion of cobalt species (and silicon) throughout the inorganic walls at the spot size (ca. 1 µm). Also, the absence of XRD peaks (characteristics of cobalt oxides) at high 2θ values confirms that no extra wall phase segregation occurs. Indeed, the preparative procedure has allowed us to achieve a good level of heteroelement dispersion even in the case of the solids relatively rich in cobalt. The Co content in the final solids is significantly lower than that present in the starting solutions (a synthesis feature that must be outlined in contrast to the results obtained when using the same procedure for incorporating other heteroelementssAl, Ti, or Zrsto the silica matrix).33-36 This fact could be tentatively attributed to the well-known affinity of Co(III) for nitrogen donors (or, in other words, to the effect of the presence of moderate to strong field ligands in the stabilization of the Co(III) oxidation state in solution).37 It must be pointed out that, in the absence of water, TEAH3 is capable of forming stable amine-trialkoxo complexes by acting as a tetradentate tripod ligand, whereas in aqueous solution TEAH3 essentially behaves as a tertiary amine.38 Thus, regardless of the ultimate nature of the initial Co(II) containing species, taking into account both the low stability in aqueous solution of the Co(II)-TEAH3 complexes (log K ) 1.73 for the 1:1 complex)39 and the fact that they can be easily oxidized by air to stable Co(III)-TEAH3 complexes,40 it is reasonable to assume that, after water addition, a certain Co amount is stabilized in solution in the form of Co(III)-TEAH3 complexes. In fact, after filtration of the mesostructured solids, the resulting solutions have an intense red-wine color typical of moderate- to strong(35) El Haskouri, J.; Ortiz de Za´rate, D.; Pe´rez-Pla, F.; Cervilla, A.; Guillem, C.; Latorre, J.; Marcos, M. D.; Beltrań, A.; Beltrań, D.; Amoro´s, P. New J. Chem. 2002, 26, 1093. (36) El Haskouri, J.; Cabrera, S.; Guillem, C.; Latorre, J.; Beltrań, A.; Beltrań, D.; Marcos, M. D.; Amoro´s, P. Chem. Mater. 2002, 14, 5015. (37) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, Principles of Structure and Reactivity, 4th ed.; Harper & Row: New York, 1993. (38) Naiini, A.; Young, V.; Verkade, J. Polyedron 1995, 14, 393. (39) Smith, R. M.; Martell, A. E. Critical Stability Constants. Volume 6: Second Supplement, Plenum Press: New York, 1989, p 205. (40) Lin, J. M.; Shan, X.; Hanaoka, S.; Yamada, M. Anal. Chem. 2001, 73, 5043.

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Figure 2. Selected TEM micrographs: (a) sample 2; (b) sample 4; and (c) sample 5. Figure 1. Low-angle XRD patterns of (a) mesostructured and (b) mesoporous materials.

field Co(III) complexes.41 In short, the stabilization of Co atoms in the form of Co(III)-TEAH3 complexes results in a significant decrease of the Co(II) species in solution capable of entering into the silica walls. Shown in Figure 1 are the low-angle XRD patterns corresponding to both the as-synthesized mesostructured solids and the mesoporous materials obtained after template removal (calcination). With the exception of the mesoporous material with the highest cobalt content (sample 5, Si/Co ) 23), all solids display XRD patterns with at least one strong ((100) reflection) diffraction peak (and some other features of lower intensity), which is characteristic of mesostructured/ mesoporous materials. In the case of the mesostructured solids (Figure 1a), apart from the (100) reflection, we can observe three other resolved small signals ((110), (200), and (210)) in the patterns of the materials in which Co content (molar ratio) is in the ∞ g Si/Co g 49 range. Their observation is clearly indicative of highly ordered hexagonal (Ho) pore systems. However, a clear loss of order occurs in the case of the solid with the highest Co content (Si/Co ) 23). In this case, the XRD pattern displays only one broad signal (what is characteristic of a disordered hexagonal (Hd) array). Although the same general features are also observed in the XRD patterns of the calcined (mesoporous) samples (Figure 1b), it can be concluded that surfactant removal implies a relative loss of order in the pore system. In this context, the loss of X-ray intensity in the pattern corresponding to the solid with the highest Co content must be attributed to a significant loss of order (more than to phase cancellation phenomena associated with (41) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: New York, 1984; p 1303.

the introduction of scattering material into the pores or a partial collapse of the mesostructure). In short, in the case of the mesoporous solids, the order of the pore system evolves with the Co content (to finally become a highly disordered wormhole-like (W) array) according to the following sequence: Ho (∞ g Si/Co g 98) f Hd (Si/Co ) 49) f W (Si/Co ) 23). On the other hand, we can see (Table 1) that the cell parameter values (a0) also show an increasing tendency with the Co content, with this occurring for both the mesostructured and mesoporous solids. A priori, such an evolution suggests that Co atoms should be effectively incorporated into the inorganic walls, according to the literature.42 As indicated above, this behavior might be understood as resulting from the interaction of the heteroelement in the growing of the silica network in one-pot (cohydrolysis) synthesis methods. TEM images (Figure 2) fully correlate with XRD observations. Thus, only one type of particle morphology (with an ordered or disordered hexagonal pore system) is observed for each sample and the formation of massive crystalline silicon or cobalt collapsed bulk oxides can be discarded. Mesoporosity of the Co-MCM-41 materials is further illustrated by the N2 adsorption-desorption isotherms (Figure 3). In all cases, the curves show one well-defined step at intermediate partial pressures (0.2 < P/P0 < 0.5) characteristic of Type IV isotherms, which should be due to the capillary condensation of N2 inside the mesopores. On the other hand, although the progressive incorporation of Co atoms practically does not affect the pore size (estimated by using the adsorption branch of the isotherms and applying the BJH model) of the final materials, it has a significant effect on the BET surface area, pore volume, and pore size distribution (see Table 1). Thus, as the Co content increases, both the BET area (42) Sayari, A. Chem. Mater. 1996, 8, 1840.

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Figure 4. Diffuse reflectance UV-visible spectra of anhydrous mesoporous samples: (a) sample 2, (b) sample 3, (c) sample 4, and (d) sample 5. Table 2. Selected UV-Visible Spectroscopic Data 4A (F) 2

sample

E1 (cm-1)a

E2 (cm-1)a

E3 (cm-1)a

nonaccessible/ accessible ratiob

2 3 4 5

14925 15228 14877 15169

16585 16567 16449 16448

18890 18889 18830 18776

0.31 0.29 0.30 0.29

a 4A

Figure 3. N2 adsorption-desorption isotherms for: (a) sample 2, (b) sample 3, (c) sample 4, and (d) sample 5. The insets show the BJH pore size distributions from the adsorption branch of the isotherms.

and pore volume decrease in a gradual way, whereas relatively wider pore size distributions are observed (ranging from 22 to 30 Å for sample 2 to 18-39 Å for sample 5). In good agreement with the aforementioned XRD and TEM results, such an evolution clearly indicates a relative loss of order and regularity in the pore system. Finally, the thickness of the pore walls (Wt ) a0 - φBJH) also increases with the Co content, which should be in agreement with the incorporation of Co atoms in the mesostructure and might explain the decrease of the SBET. Shown in Figure 4 are the UV-Vis spectra of some selected Co-MCM-41 samples dehydrated at 300 °C under air atmosphere. In all cases, the spectra display an intense and complex absorption band in the VIS region (15 000-20 000 cm-1). This band clearly includes three absorption peaks, and can be deconvoluted in three components (Table 2). This triplet can be unambiguously assigned to the 4A2(F) f 4T1(P) transition of

f 4T1(P) triplet

Calculated values from spectral deconvolution. f 4T1(P) anhydrous /hydrated samples.

b

Total area

2(F)

Co(II) ions in tetrahedral environments. In any case, the blue color clearly observed in all the dehydrated samples may be considered as a fingerprint of tetrahedral Co(II)O4 sites. On the other hand, it must be said that UV-Vis spectra registered in previous studies on related materials (where cobalt atoms were included in the silica pores by using exchange and/or grafting techniques) include absorption bands which have been associated with the presence of octahedral Co(II) sites.7 The (pale pink) octahedral Co(II) complexes have a multiple absorption band in the visible near 20 000 cm-1 (whose main component is usually associated with the 4T (P) f 4T transition). Even considering the fact that 1g 1g this band is weak (around 2 orders of magnitude less intense that those associated with tetrahedral environments), we do not observe any absorption band close to this energy value in the spectra of the Co-MCM-41 materials. Finally, it must be noted that all the spectra in Figure 4 show a wide and low intensity absorption band at ca. 27 000 cm-1. The presence of such a band also has been observed in the spectra of other Cocontaining molecular sieves such as CoAPOs and CoSBA-1.25 As possible origin of this band, it has been suggested either the presence of Co(III) sites in the framework or distortion-induced charge transfer effects (without change in the cobalt oxidation state). Oxidation to Co(III) of a certain amount of Co atoms has been proposed in cases in which an intensity growth of the band is observed after calcination of the molecular sieve. Moreover, this effect occurs associated with a visible evolution of the color of the material from blue to green. In our case, the intensity remains practically unchanged along the dehydration-rehydration cycles, which allows us to discard a perceptible presence of Co(III) atoms in the solids (that maintain their characteristic blue color). In addition, the absence of significant changes in the

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Figure 5. Diffuse reflectance UV-visible spectra of the samples 5 (left) and 3 (right): (a) dehydrated samples, (b) hydrated samples.

Figure 6. 29Si MAS NMR spectra (recorded under ambient conditions) of selected mesoporous materials: (a) sample 1; (b) sample 5.

normalized (to the Co content) spectra is consistent with the presence of a single Co(II) environment. So far, our results and subsequent conclusions should be similar to those in previous reports on related materials.16,25 This notwithstanding, the study of the rehydration process (UV-Vis spectroscopy) together with the analysis of the 29Si NMR spectra of the solids raised some interesting questions concerning the nature of the cobalt centers (isolated or clustered). After rehydration, the UV-Vis spectra of the CoMCM-41 materials (Figure 5) maintain the general features that characterize those of the anhydrous samples (one complex absorption band with three maxima in the 15 000-20 000 cm-1 range). However, when compared with the spectra of the corresponding anhydrous samples, we can observe that the intensity of the main band at 15 000-20 000 cm-1 significantly decreases. In contrast, the relative intensity of the component at lower energy (ca. 19 000 cm-1) increases, which only can be interpreted in terms of a new low intensity absorption contribution associated with new chromophores. In any case, it must be stressed that these dehydrationrehydration processes are reversible (i.e., the water molecules implied in such a reaction show a certain lability), a fact that has been previously observed in CoAPO materials.1,7 A reasonable explanation for this behavior is to consider that there is a certain proportion of (tetrahedral) Co atoms at the wall surface which are able to expand their coordination sphere by accepting two water molecules. In this way, they should become octahedral centers, while keeping their original position. Such a process would allow direct relation of the significant reduction of the intensity of the 4A2(F) f 4T (P) band to the decrease in the concentration of 1 Co(II)O4 sites. Taking into account the low absorption molar coefficients of transitions associated to octahedral chromophores (with regard to tetrahedral ones), the only predictable effect associated with the increase of octahedral sites is a slight increase of the absorption intensity around 20 000 cm-1. However, the numerical deconvolution of the spectra did not allow us to identify any additional Gaussian contribution. In any case, it is possible to estimate the proportion of accessible Co sites by simply assuming that the intensity of the bands associated with octahedral Co chromophores is practically negligible when compared to those due to tetra-

hedral ones. If so, the area under the Vis curve should be proportional to the number of non-hydrated tetrahedral Co centers (Table 2). It is worth emphasizeing that this simple analysis significantly results in a nonaccessible (tetrahedral)/accessible (octahedral) Co atoms ratio practically constant and close to 2:7, regardless of the Co content. The fact that such a ratio could be constant along the entire (and large) studied compositional range is difficult to understand on the basis of a structural model involving isolated Co(II)O4 entities statistically distributed in the pore walls. In fact, taking into account the relative sizes of the Co(II)O4 chromophores and the walls, it could be expected that such a statistical distribution would lead to an increasing number of accessible Co atoms as the cobalt content increases. On the other hand, this experiment also supports the idea that Co atoms are located and organized in a similar way for all the explored compositions. Shown in Figure 6 are the 29Si MAS NMR spectra of some selected Co-MCM-41 samples. They clearly reveal that there are not significant differences in the Si environments related to the Co content. As the Co content increases, the only appraisable spectral variation is a slight broadening of the signals, which is accompanied by a noticeable intensity decrease. Such an effect can be fundamentally related to dipolar interactions of Si atoms with the nearby paramagnetic Co atoms.43 The broadening of the signals beyond the detection limit in itself is indicative of the formation of Si-O-Co links between the silica matrix and the Co centers,43 without segregation at micrometric scale, according to TEM and XRD data. In the case of the sample with the highest cobalt content (Si/Co ) 23), the fact that we still obtain an intense and well-resolved spectrum cannot be explained assuming a model of cobalt total dispersion, in which case a significantly higher signal decrease should be expected. The observed behavior requires an alternative model. The most acceptable explanation is based on the formation of small nanoclusters of cobalt oxide partially embedded in the silica walls. The growth of cobalt oxide nanodomains would limit the formation of Si-O-Co bonds, which (43) Csu, M.; Marincola, F. C.; Lai, A.; Musinu, A.; Piccaluga, G. J. Non-Cryst. Solids 1998, 232-234, 329.


Chem. Mater., Vol. 16, No. 14, 2004 2811

Table 3. Magnetic Parameters of the Prepared Samples sample

Si/Co (solid)

TB (K)a

Tm′′ (K)b

T m′ (K)c

TRM (K)d

Hc (mT)

Ea (K)

2 4 5

155 49 23

14-16 15-17 17-19

3.9-5.1 3.5-4.6 3.8-5.1

5.4-6.8 4.8-6.3 5.2-6.6

13 13 13

75 80 85

123 107 106

a Temperature range from the χ′′ signal in the 1-997 Hz frequency range. b Temperature range of the maxima in χ′′. c Temperature range of the maxima in χ′. d Temperature at which RM vanishes.

Figure 8. Thermal variation of the in-phase (filled symbols, left scale) and the out-of-phase signals (empty symbols, right scale) of the AC susceptibility of sample 5 at 1 Hz (b), 10 Hz (9), 110 Hz ([), 332 Hz (2), and 997 Hz (1).

Figure 7. Thermal variation of the in-phase (filled symbols) and the out-of-phase signals (empty symbols) of the AC susceptibility of samples 2 (b), 4 ([), and 5 (2) at 997 Hz.

would allow us to understand the relatively low variability of the 29Si NMR spectrum with the Co content. In any case, there were the magnetic measurements that finally allowed us to confirm that Co atoms arrange in the form of oxide nanoparticles. Magnetic Study. To clarify the nature of the Co species in the solids, we have performed a complete magnetic study for all the materials. The DC magnetic susceptibility shows, in all cases, a Curie-Weiss behavior at high temperatures (> 30 K) with a small negative Weiss constant, which is a consequence of the reduction of the magnetic moment due to the spin-orbit coupling expected for Co(II) ions. More important is the fact that the magnetic susceptibility always shows a sharp increase below ca. 15 K. Although this behavior is typical of long range ordered materials, other possibilities, such as superparamagnetism, cannot be excluded. To determine which of these possibilities is the correct one, we carried out AC measurements on all the samples at different frequencies and low temperatures. In all cases, these measurements (see Table 3) show a frequency-dependent maximum in both the in-phase (χ′) and the out-of-phase (χ′′) signals at low temperatures (4.8-6.8 K and 3.5-5.1 K, respectively), with a small shoulder (also frequency-dependent) at higher temperatures (Figure 7). Increasing the frequency of the oscillating magnetic field results in displacements of the positions of the χ′ and χ′′ maxima (Tm′ and Tm′′, respectively) and that of the shoulder toward higher temperatures (Figure 8). The presence of an out-ofphase signal besides the frequency dependence of the χ′ and χ′′ maxima is typical of superparamagnetic systems (and also spin glass systems). In a superparamagnet the high and anisotropic spins show slow relaxation dynamics and at a low enough temperature (defined as the blocking temperature, TB) they cannot follow the oscillating magnetic field, giving rise to an energy absorption that is reflected in the appearance of an out-of-phase signal. This effect appears at higher temperatures when the frequency of the oscillating field

Figure 9. Hysteresis cycles at 2 K for samples 2 (9), 4 (O), and 5 (b).

increases, given that the thermal energy needed to follow the oscillating field becomes higher at high frequencies. Considering the blocking temperature as that where χ′′ becomes nonzero, we can observe that TB is very similar for all the samples and varies from 16 to 19 K at 997 Hz (Figure 7, Table 3). This suggests that the particles have similar sizes in all the samples. Furthermore, in all cases, the low Tm value suggests that the size of the particles must be very small (see below).44 The AC measurements exclude also the hypothesis of homogeneous distribution of the simple Co(II) ions in the Si framework (since this figure would produce a typical paramagnetic behavior without any out-of-phase signal nor a maximum in the in-phase one). A confirmation of the superparamagnetic behavior comes from the analysis of the dependence of the temperature of the maximum in χ′′ (Tm′′) with the frequency of the oscillating field. Thus, all the samples follow an Arrhenius law, ν ) ν0‚exp(-Ea/kBTm′′), with ν0 values of the order of 1012 Hz and activation energies around 100 K (Table 3). These values are typical of superparamagnetic systems and exclude a possible spin glass behavior.45 Besides the sharp increase in the DC susceptibility and the frequency dependent χ′ and χ′′ signal, a third characteristic feature of a superparamagnetic behavior is the presence of hysteresis cycles in the isothermal magnetizations below the blocking temperature, as it is observed in all the samples at low temperatures (Figure 9). The coercive fields observed at 2 K are of the same order in all the samples (Table 3), which also supports the idea that the sizes of the superparamagnetic particles are similar in all the measured samples. The small differences observed can be due to many factors besides particle sizes, such as (44) Ramos, J.; Millań, A.; Palacio, F. Polymer 2000, 41, 8461. (45) Mydosh, J. A. Spin Glasses: An Experimental Introduction; Taylor and Francis: London, 1993.

2812


El Haskouri et al. Table 4. Particle Size as Determined from Magnetic Measurements

b

Figure 10. ZFC, FC, and RM for sample 5.

anisotropy, environments and degree of hydration of the particles, and even the interparticle distance. All these factors may affect the relaxation rate of the spins and, therefore, the coercive field of the samples. It is worth mentioning here that the hysteresis cycles are almost identical for all the samples when they are cooled in the presence of a magnetic field of 2 T or when they are cooled in zero fields. Furthermore, the field-cooled samples do not exhibit any exchange field (shift of the hysteresis cycle),excluding the presence of CoO particles with a Co core.46 A fourth feature characterizing a superparamagnetic material is the splitting of the zero field (ZFC) and field cooled (FC) susceptibilities, as well as the presence of a remnant magnetization (RM) below the blocking temperature (Figure 10). The temperatures of the splitting of the ZFC and FC susceptibilities, and those where the RM cancel (TRM), are also very similar for all the samples (Table 3) and confirm the presence of superparamagnetic behaviors in all the samples. Note that these temperatures are slightly lower than the TB values obtained from the AC measurements (since they are obtained with a DC field). In conclusion, the magnetic measurements (DC, AC, hysteresis cycles, and ZFC, FC, and RM measurements) strongly support a superparamagnetic behavior in all the samples and exclude the idea that the simple Co(II) ions are homogeneously distributed in the Si framework. Besides supporting the presence of small superparamagnetic particles, the magnetic measurements may give very useful information about the size of these particles. There are at least two possible ways to determine the particle size using the magnetic data. Indeed, the magnetization measurements above the blocking temperature can be used to determine the number of uncompensated spins in the particle (n) by fitting the magnetization data to the Langevin function: L(x) ) coth (x) - 1/x, where x ) (MH)/(kBT) and M ) nµBgS.47 The fitting to the Langevin function allows, therefore, the determination of n, which depends on the volume of the particle since the number of uncompensated spins of a particle with N spins can be 3 approximated as xN.48 From the ionic radii of the Co2+ and O2- ions (79 and 126 pm, respectively) we can estimate that each spin occupies an approximate volume of 0.01 nm3. Thus, it is possible to estimate the particle (46) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogue´s, J. Nature 2003, 423, 850. (47) MacHenry, M. E.; Majetich, S. A.; Artman, J. O.; DeGraef, M.; Staley, S. W. Phys. Rev. B 1994, 49, 11358. (48) Sato, M.; Takada, S.; Kohiki, S.; Babasaki, T.; Deguchi, H.; Oku, M.; Mitome, M. Appl. Phys. Lett. 2000, 77, 1194.

sample

Si/Co

Ea (K)

d (nm)a

n

d (nm)b

2 4 5

155 49 23

123 107 106

3.2 3.0 3.0

11.3 11.0 11.2

3.0 2.9 3.0

a Determined from the E data (E ) KV) with K ) 106 erg‚cm-3. a a Determined from the magnetization data at 20 K (above TB).

volume from the number of uncompensated spins in the superparamagnetic CoO particles. This estimation leads to very similar particles sizes in all the samples ranging from 2.9 to 3.0 nm (Table 4). A second way to estimate the size of the superparamagnetic particles uses the activation energies obtained from the Arrhenius fit of the frequency-dependent maxima in χ′′ (see above), as this activation energy is equal to the product of the particle volume times the energy density of the magnetic anisotropy (K): Ea ) K V.49 Although there are discrepancies on the exact value of K for CoO, an order of magnitude of 106 erg‚cm-3 is commonly accepted.49-51 With this value for K, the particle volumes and sizes (3.0-3.2 nm) so-obtained are very similar for the three considered materials. In addition, they are in very good agreement with those determined from the isothermal magnetization data above the blocking temperature (Table 4). In any case, the important result of these two analyses is the order of magnitude of the particle size (around 3 nm), rather than the exact value. Note also that the particle size obtained in our samples is similar to those obtained by other authors (with CoO particles of about 3 nm that display very similar magnetic behaviors and blocking temperatures).48 Superparamagnetism is a relevant feature of these materials. In practice, interest in uniform nanometric particles is related to the novel properties that arise in systems intermediate in size between isolated atoms or molecules and bulk materials.52 Notwithstanding, special properties of nanoparticulate materials strongly depend on morphological aspects. Thus, control of the size and shape of the particles and the attainment of good dispersity levels are essential requirements.53 In this context, the use of MCM-41-like materials to stabilize highly dispersed metal or metal oxide particles overcomes the tendency of nanoparticles to form large aggregates. Looking for magnetic materials, a diversity of guest species (semiconductors, metals, metal oxides, clusters, organic molecules, and even single-molecule magnets) have been hosted in M41’s matrixes.54-66 (49) Takada, S.; Fujii, M.; Kohiki, S.; Babasaki, T.; Deguchi, H.; Mitome, M.; Oku, M. Nano Lett. 2001, 1, 379. (50) Hughes, A. E. Phys. Rev. 1971, 3, 877. (51) Gruyters, M. J. Magn. Magn. Mater. 2002, 248, 248. (52) Alivisatos, A. P. Science 1996, 271, 933. (53) Hyeon, T. Chem. Commun. 2003, 927, and references therein. (54) Yang, C.; Sheu H.-S.; Chao, K.-J. Adv. Funct. Mater. 2002, 12, 143. (55) Coleman, N. R. B.; Morris, M. A.; Spakling T. P.; Holmes, J. D. J. Am. Chem. Soc. 2001, 123, 187. (56) Gao, F.; Lu, Q.; Llu, X.; Yan Y.; Zhao, D. Nano Lett. 2001, 1, 743. (57) Coradin, T.; Larionova, J.; Smith, A. A.; Rogez, G.; Cle´rac, R.; Gue´rin, C.; Blondin, G.; Winpenny, R. E. P.; Sanchez, C.; Mallah, T. Adv. Mater. 2002, 14, 896. (58) Clemente-Leońa, M.; Coronado, E.; Forment-Aliagaa, A.; Amoro´s, P.; Ramı´rez-Castellanos, J.; Gonza´lez-Calbet, J. M. J. Mater. Chem. 2003, 13, 3089. (59) Clemente-Leoń, M.; Coronado, E.; Forment-Aliaga, A.; Martıńez-Agudo, J. M.; Amoro´s, P. Polyhedron 2003, 22, 2395.


However, as far as we know, there were only two previous publications dealing with the magnetic properties of nanometric cobalt oxides hosted in MCM-41 solids, and the nanocomposites were prepared in both cases by postsynthesis impregnation treatments.48,49 Although soaking really is a common procedure to functionalize mesoporous silicas, this approach (in contrast to the “atrane route” exploited here) is multistep and, more important, frequently leads to necking or even blocking of the pore system. Concluding Remarks The relevant conclusion from the magnetic and spectroscopic studies is that, regardless of the Si/Co ratio, the Co atoms are organized in similar and homodisperse CoO particles with sizes close to 3 nm. Thus, CoOMCM-41 seems to be a more correct formulation than Co-MCM-41 in order to emphasize the small phase segregation at nanometric scale in the final materials described here. Assuming a sphalerite structure (with ionic radii of 0.079 and 0.126 nm for Co2+ and O2-, respectively), these nanoclusters should contain ca. 200 CoO units. Taking into account the ratio between the “non-accessible” and “accessible” Co atoms of 2/7 that we have estimated by UV-Vis spectroscopy, we can postulate a model for these CoO-MCM-41 nanocomposites in which the CoO nanodomains are partially embedded (ca. the 25%) in the silica matrix. The “accessible” Co atoms should be located at the nonembedded part of the particles and forming the pore surface. The remaining Co atoms would correspond to the “non-accessible” centers located in the inner part of the embedded CoO nanoparticles (see Figure 11). In short, we present here a simple, reproducible, onepo, surfactant-assisted approach for obtaining super(60) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950, and references therein. (61) Murray, S.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2000, 12, 1339. (62) Ko¨hn, R.; Paneva, D.; Dimitrov, M.; Tsoncheva, T.; Mitov, I.; Minchev C.; Fro¨ba, M. Microporous. Mesoporous Mater. 2003, 63, 125. (63) Gross, A. F.; Diehl, M. R.; Beverly, K. C.; Richman, E. K.; Tolbert, S. H. J. Phys. Chem. B 2003, 107, 5475. (64) Ko¨hn, R.; Fro¨ba, M. Z. Anorg. Allg. Chem. 2003, 629, 1673. (65) Zhang, L.; Papaefthymiou, G. C.; Ying, J. Y. J. Phys. Chem. B 2001, 105, 7414. (66) Garcıá, C.; Zhang, Y.; DiSalvo, F.; Wiesner, U. Angew. Chem. Int. Ed. 2003, 42, 1526.

Chem. Mater., Vol. 16, No. 14, 2004 2813

Figure 11. Proposed schematic model for the CoO-MCM-41 superparamagnetic nanocomposites.

paramagnetic mesoporous materials with high cobalt content and well-dispersed uniform CoO nanoparticles within the inorganic walls without blocking of the pore system. Assuming the difficulty with which Si atoms could be replaced by divalent Co species in the mesoporous walls, the nucleation and growth of the CoO nanoparticles and the silica condensation must be considered as independent and noncompetitive chemical processes. Thus, the homogeneity in size, nature, and dispersion of the CoO nanoparticles might be understood by assuming that the growing process of the nucleated CoO species generated by hydrolysis of the Co(II)-TEAH3 precursor complexes is blocked by the relatively slower condensation of the mesostructured silica network. In other words, (under the experimental Si/Co concentrations) the relative time availability for the nucleation and growing processes of the CoO particles is constant and determined by the silica polymerization process. The CoO-MCM-41 materials may be described as nanocomposites of CoO clusters on mesoporous silicas. This nanometric organization of highly dispersed and accessible CoO nanodomains could be of catalytic interest in Fischer-Trosch and ODH processes. There are other potential applications in areas such as microelectronics, but also applications as original as the separation of magnetically labeled biomolecules could be envisaged. Acknowledgment. This research was supported by the Ministerio de Ciencia y Tecnologıá (under grants MAT2002-04329-C03-01 and MAT2003-08568-C03-01), the Generalitat Valenciana (GRUPOS03/099), and the Agencia Espanõla de Cooperacioń Internacional. J.E.H. thanks the Ministerio de Educacioń, Cultura y Deporte for a postdoctoral grant. CM049772A

High Cobalt Content Mesoporous Silicas - ACS Publications

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