J Nanopart Res (2011) 13:3489–3501 DOI 10.1007/s11051-011-0270-x
RESEARCH PAPER
Iron–cobalt nanocrystalline alloy supported on a cubic mesostructured silica matrix: FeCo/SBA-16 porous nanocomposites Daniela Carta • Maria F. Casula • Salvatore Bullita • Andrea Falqui • Anna Corrias
Received: 9 September 2010 / Accepted: 28 January 2011 / Published online: 12 February 2011 Ó Springer Science+Business Media B.V. 2011
Abstract A series of novel nanocomposites constituted of FeCo nanoparticles dispersed in an ordered cubic Im3m mesoporous silica matrix (SBA-16) have been successfully synthesized using the wet impregnation method. SBA-16, prepared using the non-ionic Pluronic 127 triblock copolymer as a structuredirecting agent, is an excellent support for catalytic nanoparticles because of its peculiar three-dimensional cage-like structure, high surface area, thick walls, and high thermal stability. Low-angle X-ray diffraction, N2 physisorption, and transmission electron microscopy analyses show that after metal loading, calcination at 500 °C, and reduction in H2 flux at 800 °C, the nanocomposites retain the wellordered structure of the matrix with cubic symmetry of pores. FeCo alloy nanoparticles with spherical shape and narrow size distribution (4–8 nm) are homogeneoulsy distributed throughout the matrix and they seem in a large extent to be allocated inside the pores.
D. Carta (&) M. F. Casula S. Bullita A. Corrias Dipartimento di Scienze Chimiche and INSTM, Universita` di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato, Cagliari, Italy e-mail:
[email protected] A. Falqui Istituto Italiano di Tecnologia I.I.T., Via Morego 30, 16163 Genoa, Italy
Keywords SBA-16 mesoporous silica Sol–gel Nanosized FeCo alloy Self-assembly Triblock copolymers Metallic alloys
Introduction Nanoparticles of transition metals and their bimetallic alloys (Fe, Co, Mn, Ni, Mo) have attracted much attention in recent years for their high potential in catalysis. Nanoparticles have different properties compared to the bulk, strongly related to their small particle size, high surface-to-volume ratio, and morphology. Therefore, it is important to avoid agglomeration and coalescence of nanoparticles. To do so, nanoparticles can be coated (Turgut et al. 1998), prepared using colloidal methods (Choi et al. 2003), or synthesised by co-decomposition of appropriated organometallic precursors (Desvaux et al. 2005; Li et al. 2001). An alternative approach is to disperse the nanoparticles in a high surface area porous matrix, where they should be ideally isolated and uniformily distributed. The dispersion of transition metals nanoparticles and their bimetallic alloys in a porous matrix is of particular interest in catalysis: the matrix not only acts as a support material but also plays an active role in the catalytic process; in fact, the porosity of the matrix influences the accessibility of the active catalytic particles, affecting selectivity and
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activity. In particular, ordered mesoporous silicas ˚ have with uniform pore sizes between 20 and 500 A attracted much interest as a support for catalytic nanoparticles because of their regularly arranged pore structure and high specific surface area. Ordered mesoporous silica are commonly prepared through sol–gel using a surfactant-templated synthetic procedure. Amphiphilic block copolymers are widely used as structure-directing agents. In particular, nonionic triblock polymers such as poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide), Pluronic, are particularly suitable because of their low cost, availability, and biodegradability (Zhao et al. 1998a). Block-copolymers form self-assembled micellar aggregates in solution. Monomeric and oligomeric species that arise from hydrolysis of a silica precursor interact with the surface of the micelles by Van der Waals or electrostatic forces and then condense on the surface of the micelles leading to the final silica. After removal of the organic template, the silica maintains the ‘‘negative’’ structure of the organic template, with an ordered pore structure generated by the calcinations of the micelles (El Haskouri et al. 2008). Until now, the research on ordered mesoporous catalyst supports has been mainly focussed on the two-dimension (2D) mesoporous silica, possessing an hexagonal array of ordered mesopores. However, the corresponding cubic mesoporous silica, having a cubic array of mesopores developing in three-dimension (3D), has the potential to be more efficient in catalysis and absorption/separation processes. This is because the hexagonal pore arrangement is formed by parallel channels that can be accessible only in one direction (Salis et al. 2005). On the other hand, the cubic pore arrangement is formed by a cage-like system of pores that are all interconnected providing an easy accessibility for the diffusion of reactants in all directions. Moreover, cubic mesoporous silica has a better thermal stability (Grudzien et al. 2007), an advantage for hightemperature applications, thicker walls, and enhanced resistance to local pore blockage in comparison to channel like-silica (Sierra et al. 2008). Among the cubic mesoporous silicas, the so-called SBA-16 is considered the most interesting. SBA-16 is formed by an arrangement of spherical empty cages having a body-centered cubic symmetry—bcc (Im3m). Each cage is connected to eight neighbouring cages by narrow openings forming a 3D system of mesopores network (Li et al. 2009). Thanks to the 3D
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cage-like cubic arrangement of mesopores, the high surface area, and the thermal stability, SBA-16 appears to be the ideal material for catalytic supports. In particular, the peculiar structure of SBA-16 makes it an ideal support for metal/metal oxide nanoparticles that can be accommodated in the spherical nanocages of the 3D structure; nanoparticle growth is limited by the restricted space of the pores, and aggregation and agglomeration are limited by the small pore entrances that limit the motion of nanoparticles (Yang et al. 2009). Unlike the hexagonal form, cubic mesoporous silica has not been studied extensively, probably because it can only be produced in a narrow range of conditions (Zhao et al. 1998a). In particular, very little work has been done on the synthesis of metal nanoparticles dispersed in a SBA-16 matrix. Only few works dealing with nanoparticles of Ni–Cu alloy (Li et al. 2009), Co– Mo–W (Huirache-Acun˜a et al. 2009), Pd (Yang et al. 2010), Ni (Park et al. 2004), CuO (Dong et al. 2008), Fe (Jermy et al. 2009), TiO2 (Ma et al. 2010), iron oxide (Tsoncheva et al. 2006; Yiu et al. 2010), Au (Lee et al. 2009; Sun et al. 2009), Au–Pd alloy (Chen et al. 2010), and Ti (Shah et al. 2009; Shen et al. 2007) supported on SBA-16 were recently presented. Each of these materials can be used for a different catalytic application. Among all the possible catalytic applications of cubic mesoporous silica-based nanocomposites, the production of carbon nanotubes (CNTs) is quite promising. Films of nanocomposites of Fe nanoparticles dispersed in SBA-16 have been prepared and used as catalyst for the growth of vertically aligned CNTs (Zheng et al. 2001; Huang et al. 2003) and Fe-filled CNTs (Shi et al. 2005). However, it has been shown that isolated Fe nanoparticles are less effective for the growth of CNTs than FeCo alloy nanoparticles (Zhu et al. 2003). In this study, we have synthesised bulk nanocomposites containing FeCo alloy nanoparticles dispersed in a SBA-16 support by wet impregnation of a previously prepared SBA-16 matrix with a solution of Fe and Co nitrates.
Experimental Synthesis The synthesis of SBA-16 was performed according to Zhao et al. (1998a). Pluronic F127 (P127, Aldrich)
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block copolymer, formed by a sequence of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO106-PO70-EO106) units with high molecular weight (Mav = 12,600) and high EO/PO ratio, was used as a structure-directing agent. About 4.0 g of P127 was added to a mixture of water (30 g) and HCl 2 M (120 g) under stirring at room temperature. About 8.5 g of TEOS (Aldrich 98%) was then added to the solution that was left under stirring at room temperature for 20 h. The solution was aged for 2 days at 80 °C. The precipitate of SBA-16 was separated from the mother solution by centrifugation, washed with distilled water to remove H? and Clions, and dried at room temperature. To remove the organic template (P127), the pure SBA-16 was then calcined in air at 500 °C; calcination was performed by heating in air up to 500 °C with an heating step of 1 °C/min and by keeping the final temperature of 500 °C for 6 h. Nanocomposites were prepared by wet impregnation of SBA-16 (0.3 g) with an aqueous solution of Fe(NO3)39H2O (Aldrich, 98%) and Co(NO3)26H2O (Aldrich, 98%) (10 mL), vigorously stirred for 24 h. Different nanocomposites were prepared varying the total metal molar concentrations of Fe and Co ions (0.4 and 0.8 M) and the Fe:Co ratio (1:1 and 2:1). The impregnated SBA-16 was separated from the solution by centrifugation. Afterwards, the impregnated SBA-16 was calcined in air at 500 °C to transform the metal nitrates in metal oxides. The formation of FeCo alloy nanoparticles was obtained by reduction of the metal oxides dispersed in the matrix by heating the nanocomposites in H2 flux with an heating step of 10 °C/min up to 800 °C and held at 800 °C for 2 h. The products calcined at 500 °C and reduced at 800 °C will be hereafter indicated as FexCoy_Z_500 and FexCoy_Z_r800, respectively, where x:y is the Fe:Co ratio and Z indicates the overall metal molar concentration. The sample indicated as Fe2Co1_0.4_D has been prepared using an overall metal molar concentration of 0.4 M but using a 20 mL of ions solution, a double volume with respect to the Fe2Co1_0.4 sample. Characterisation The mesostructure of the silica matrix and of the nanocomposties was studied by low-angle X-ray diffraction (XRD) recorded on a small-angle X-ray
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scattering (SAXS) apparatus S3-MICRO SWAXS camera system (HECUS X-ray Systems, Graz, Aus˚ ) was provided by tria). Cu Ka radiation (1.542 A a GeniX X-ray generator, operating at 30 kV and 0.4 mA. A 1D-PSD-50 M system (HECUS X-ray Systems, Graz, Austria) containing 1,024 channels of width 54.0 lm was used for detection. Scattering patterns were acquired for 3,600 s. Wide-angle XRD patterns were recorded on a X3000 Seifert diffractometer equipped with a graphite monochromator on the diffracted beam using Cu-Ka radiation within the range 15–80° (2h). Surface areas, pore sizes, and pore volumes were obtained from N2 adsorption–desorption measurements at 77 K recorded on a Sorptomatic 1990 System (Fisons Instrument). Surface area was estimated using the Brunauer-Emmett-Teller (BET) model (Brunauer et al. 1938; Barret et al. 1951; Rouquerol et al. 1999) and pore size and pore volumes were estimated using the Barret-JoynerHalenda (BJH) method (Lecloux and Pirard 1979). Transmission electron microscopy (TEM) micrographs were recorded either on a JEOL 200CX microscope operating at 200 kV or on a Jeol 1011 microscope operating at 100 kV and equipped with a W thermionic electron source.
Results Low-angle XRD The low-angle XRD pattern of SBA-16 treated at 500 °C is reported in Fig. 1A. It shows a strong ˚ and reflection corresponding to a d spacing of 95.0 A two weak reflections corresponding to d spacing of ˚ and 54.8 A ˚ . The reflections can be indexed as 67.1 A (110), (200), and (211) corresponding to a bcc Im3m structure. The unit cell parameter was calculated to ˚. be a = 134 A The low-angle XRD patterns of the nanocomposites calcined at 500 °C are shown in Fig. 1B and those of the samples reduced at 800 °C in Fig. 1C. An intense peak, indicating the presence of an ordered mesostructure, is present in all nanocomposites. Nanocomposites calcined at 500 °C show a strong peak, ascribed to the (110) reflection, corre˚ . The unit cell sponding to a d spacing of 93.6 A ˚ , very parameter was calculated to be a = 133 A
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Fig. 2 Wide-angle XRD of nanocomposites calcined at 500 °C: (a) Fe2Co1_0.4_500, (b) Fe2Co1_0.4_D_500, (c) Fe2Co1_ 0.8_500, (d) Fe1Co1_0.4_500, and (e) Fe1Co1_0.8_500
In addition to the intense (110) reflection, Fe2 Co1_0.4_500, Fe1Co1_0.4_500, and Fe1Co1_0.4_ r800 show an additional weak reflection in a position consistent with a cubic Im3m symmetry. Wide-angle XRD
Fig. 1 Low-angle X-ray diffraction patterns of A SBA-16 calcined at 500 °C; B (a) Fe2Co1_0.4_500, (b) Fe2Co1_ 0.8_500, (c) Fe1Co1_0.4_500, and (d) Fe1Co1_0.8_500; C (a) Fe2Co1_0.4_r800, (b) Fe2Co1_0.4_D_r800, (c) Fe2Co1_ 0.8_r800, (d) Fe1Co1_0.4_r800, and (e) Fe1Co1_0.8_r800
˚ observed for pure SBAsimilar to the value of 134 A 16. The nanocomposites reduced at 800 °C also show a strong peak ascribed to the (110) reflection ˚ , with a corresponding to a d spacing of 89.9 A contraction of the unit cell parameter down to ˚. a = 127 A
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The wide-angle XRD patterns of the nanocomposites calcined at 500 °C and reduced at 800 °C are shown in Figs. 2 and 3, respectively. The halo around 2h * 20–30° observed in all nanocomposites arises from the amorphous silica matrix. The XRD patterns of nanocomposites after calcination at 500 °C do not show any intense Bragg peak. Some weak and broad features can be observed in Fe2Co1_0.8_500 and Fe1Co1_0.8_500, the samples obtained using the most concentrated solution, but they are difficult to be assigned to specific phases. After reduction at 800 °C, a peak at 2h * 45°, ascribed to the formation of the FeCo alloy phase (PDF-2 card 48-1816), is observed in all nanocomposites. In the samples obtained using the most concentrated solution, Fe2Co1_0.8_r800 and Fe1Co1_0.8_r800, additional reflections because of the FeCo alloy phase at 2h * 65° and 2h * 82° can also be observed. N2 physisorption analysis The physisorption isotherm of pure SBA-16 is shown in Fig. 4A. It can be classified as type IV with an H2
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Fig. 3 Wide-angle XRD of nanocomposites reduced at 800 °C: (a) Fe2Co1_0.4_r800, (b) Fe2Co1_0.4_D_r800, (c) Fe2Co1_ 0.8_r800, (d) Fe1Co1_0.4_r800, and (e) Fe1Co1_0.8_r800
type hysteresis loop, characteristic of ink-bottle mesopores, indicative of a cage-like cubic porous structure. The surface area, as assessed by the BET method, is 952 m2 g-1, and the total pore volume VP is 0.60 cm3 g-1. The shape of the isotherm reveals the presence of an important fraction of micropores, the amount of N2 adsorbed at very low relative pressures (P/P° \ 0.1) being more than 60% of the overall N2 uptake. The pore size distribution, as obtained by the Barret-Joyner-Halenda (BJH) analysis of the adsorption and of the desorption branches, indicates a narrow distribution of pore and neck size, respectively. As a consequence of the shape of the hysteresis loop, the pore size distribution ˚ , whereas the average pore neck is centered at 46 A ˚ size is 35 A. In Fig. 4B, the N2 adsorption–desorption isotherms of Fe2Co1_0.8_500 (a) and Fe2Co1_0.8_r800 (b) are shown. Similarly to the pure SBA-16 matrix, Fe2Co1_0.8_500 also shows a type IV isotherm with an H2 hysteresis loop, which closes just above P/P0 relative pressure of about 0.45, indicating that the nanocomposites still maintain the cubic cage-type porous structure of the matrix. Likewise pure silica, Fe2Co1_0.8_500 exhibits a contribution from microporosity, although to a minor extent compared to SBA-16. In the nanocomposite, the amount of N2 adsorbed at relative pressures \0.1 accounts for 50% of the overall nitrogen uptake. The surface area as
Fig. 4 N2 absorption–desorption isotherms for A SBA-16 and B Fe2Co1_0.8_500 (a) and Fe2Co1_0.8_r800 (b)
assessed by the BET method is 470 m2 g-1, and the total pore volume VP is 0.60 cm3 g-1. Fe2Co1_0.8_ r800 also shows a type IV isotherm with an H2 hysteresis loop, even if the loop is less defined. Therefore, the cubic mesostructure of the matrix is preserved even after reduction at 800 °C. However, the surface area and pore volume show a significant decrease, being 98 m2 g-1 and 0.10 cm3 g-1, respectively. The main textural parameters for SBA-16 and for a selection of nanocomposites as obtained by physisorption measurements are summarized in Table 1. TEM TEM micrographs of SBA-16 are shown in Fig. 5. A highly ordered arrangement of mesopores that is consistent with a cubic mesostructure can be observed. ˚ and Pore diameter was estimated to be around 50 A ˚ wall thickness around 44 A. TEM micrographs of nanocomposites having a Fe:Co molar ratio of 2:1 and 1:1, after calcination at 500 °C, are shown in Figs. 6 and 7, respectively. Views taken along different directions are shown. Images show a very similar morphology to SBA-16 with a regular arrays of mesopores having size
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Table 1 Physical properties of the pure SBA-16 and selected nanocomposites from physisorption data: in the pores, having an ink-bottle shape, the size of the pore was calculated from the Surface area (m2 g-1)
adsorption branch, whereas the size of the neck was calculated from the desorption branch
Pore volume (cm3 g-1)
˚) Pore size (A
˚) Pore neck Size (A
SBA-16
952
0.60
46
35
Fe2Co1_0.4_r800
157
0.30
45
34
Fe2Co1_0.8_500
470
0.60
46
35
Fe2Co1_0.8_r800
98
0.10
45
34
Fig. 5 TEM images of SBA-16
˚ and wall thickness 45 A ˚ . In particular, in the 40–45 A c and d images of Fig. 7, the cubic porous structure viewed along the [100] direction is evident. It has to be noted that in the Fe1Co1_0.8_500 sample, some spherical nanoparticles having dimensions in the range 4–8 nm can be observed embedded in the matrix. In particular, they are revealed as bright spots in the dark field images f and h of Fig. 7. TEM micrographs of nanocomposites having a Fe:Co molar ratio of 2:1 and 1:1 after reduction in H2 at 800 °C are shown in Figs. 8 and 9, respectively. Regular arrays of pores can still be observed confirming that the ordered structure is retained even after the formation of the FeCo alloy. Only the Fe2Co1_0.4_D_r800 sample (Fig. 8c, d) shows a more disordered worm-like mesoporosity. FeCo alloy nanoparticles in the range 4–8 nm homogeneously dispersed on the SBA-16 support are clearly visible in
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the bright field images of all nanocomposites as dark spots. The presence of nanoparticles is even more evident in the dark field images where they appear as bright spots (Figs. 8l, and 9d, f, and h).
Discussion The synthesis of the ordered cubic mesoporous silica used in this work as a support for the FeCo alloy nanoparticles is not an easy task. The success of the synthesis depends on several key parameters such as pH and surfactant concentration. If basic or neutral conditions are used, either amorphous silica or silica gel, both with a disordered array of pores, is obtained. If relatively mild acidic conditions are used (pH 2–6), no silica precipitation is observed. SBA-16 can be prepared under relatively high acid concentration;
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Fig. 6 TEM images of Fe2Co1_0.4_500 (a, b), Fe2Co1_0.4_D_500 (c, d), and Fe2Co1_0.8_500 (e, f)
however, in these conditions, a rapid mesophase assembly occurs and the control of the textural and structural parameters is more difficult (Kleitz et al. 2006). Therefore, SBA-16 can be prepared only in a narrow range of compositions and using dilute surfactant concentration. SAXS, TEM, and physisorption analysis show that in our conditions (pH \1 and P127 concentration around 3 wt%), SBA-16 cubic mesoporous silica was successfully obtained. The SBA-16 prepared in the current work shows some differences from the SBA-16 reported in the original paper by Zhao et al. (1998a). Pore size and pore volume for the SBA-16 prepared in this work ˚ and 0.60 cm3 g-1, respecwere found to be 46 A ˚ tively, compared to the corresponding values of 54 A and 0.45 cm3 g-1 reported in the original paper. Moreover, the unit cell value of SBA-16 calculated ˚ ) was found to be smaller using SAXS data (134 A ˚ ). than the value reported in the original paper (166 A However, Van Der Voort et al. (2002) have already shown that SBA-16 characteristics are very sensitive
to synthesis parameters such as stirring temperature and time, aging temperature and time, and TEOS/ surfactant ratio. In particular, they have reported that pore size, pore volume, and unit cell vary with stirring and aging time, finding pore size values in ˚ , pore volume values in the range the range 47–56 A 0.37–0.61 cm3 g-1, and unit cell values in the range ˚. 121–134 A Synthesis was performed at room temperature, in order to obtain thick walls; in fact, higher temperature synthesis increases the hydrophobicity of EO units, decreasing the thickness of silica walls (Zhao et al. 1998a). The thickness of the silica walls, calculated ˚. from TEM images, was found to be around 44 A This value is significantly larger than the values ˚ ), commonly found in MCM-41 materials (10-15 A made using conventional cationic surfactants, leading to a better hydrothermal stability (Zhao et al. 1998b) without the need of post-synthesis stabilization treatments of the inorganic walls (Beck et al. 1992). The pore size of the silica support, which depends on
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Fig. 7 TEM images of Fe1Co1_0.4_500 (a, b) and Fe1Co1_0.8_500 (c–h)
the chain length of the hydrophilic part of the P127 surfactant (EO units) forming the core of the micelles burnt after calcination, was determined by the analysis of the TEM images and N2 adsorption– desorption results. According to TEM analysis, pore ˚ in good agreement with size was found to be 50 A N2 adsorption–desorption analysis which indicates highly ordered cubic mesopores with a narrow size distribution around the average pore diameter of 46 ˚ , as calculated from the adsorption branch of SBAA 16. The EO units of the P127 surfactant are also responsible for the high percentage of micropores present in the SBA-16 which is ascribed to the
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penetration of the EO chains into the silica walls, generating intrawall porosity (Van Der Voort et al. 2002). The important contribution from micropores in the SBA-16 also affects the high surface area value (952 m2 g-1). The SBA-16 was found to be a very good support for catalysts composed of FeCo alloy nanoparticles distributed throughout the mesoporous matrix. N2 physisorption analysis shows that the structure of the matrix does not change significantly after metal loading followed by calcination at 500 °C; the hysteresis loop of Fe2Co1_0.8_500, closing at relative pressures of about 0.45, is very similar to the one
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Fig. 8 TEM images of Fe2Co1_0.4_r800 (a, b), Fe2Co1_0.4_D_r800 (c, d), and Fe2Co1_0.8_r800 (e–l)
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Fig. 9 TEM images of Fe1Co1_0.4_r800 (a– d) and Fe1Co1_0.8_r800 (e–h)
observed for the SBA-16 silica matrix, typical of inkbottle shape mesopores related to a cubic cage-like structure. As expected, in the nanocomposite sample, a decrease in the surface area was observed, which is also associated to a decrease in the relative contribution of microporosity. The mesostructure is still retained after the reduction treatment at 800 °C under H2 flow, although in this case a significant surface volume and pore volume reduction is observed. The decrease in the surface area and pore volumes can be ascribed both to the effect of thermal treatment and to the presence of the nanoparticles within the silica
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porous structure. In particular, the decrease in surface area and pore volume is higher in Fe2Co1_0.8_r800 than in Fe2Co1_0.4_r800, which is consistent with the former sample having a higher nanoparticle loading than the latter. The N2 physisorption and TEM results are also in agreement with low-angle XRD. A strong diffraction peak at low angle is observed for all nanocomposites after calcination at 500 °C and after reduction at 800 °C, confirming that the mesoporous ordered structure is preserved quite well after calcination and after the formation of the FeCo alloy. However, it
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has to be noted that intensity ratio between the first peak and the other reflections in the nanocomposites is lower than that of pure SBA-16. Therefore, the long-range order is less evident in the nanocomposites than in the pure matrix, especially after reduction at 800 °C. This could be an indication of a loss of mesoscopic order in the nanocomposites; however, this would be in disagreement with N2 physisorption and TEM analysis that clearly give evidence of the retaining of the ordered mesoporous structure. It has been reported that the loss of peak intensity could be because of the decreasing contrast between walls and pores of the matrix caused by the filling of the pores (Tsoncheva et al. 2006; Schu¨th et al. 2001; Zhu et al. 2010). This is a further indication that part of the nanoparticles is allocated inside the mesopores of the support. Assuming that the Im3m bcc structure is retained in all the nanocomposites, as suggested by TEM images, the strongest reflection is due to the (110) set of planes observed for SBA-16. For the nanocomposites calcined at 500 °C, the position of this reflection is very similar to the pure SBA-16, with a very similar unit cell parameter. For the nanocomposites reduced at 800 °C, a slight contraction of the cubic lattice is observed, the position of the strongest (110) reflection corresponding to a unit ˚. cell parameter of 127 A There is not clear evidence of the intermediate Fe and Co phases present after calcination at 500 °C that form the FeCo alloy on reduction at 800 °C. Wideangle XRD patterns do not give any significant information because of the absence of strong Bragg peaks; only the XRD patterns of Fe2Co1_0.8_500 and Fe1Co1_0.8_500 (for which nanoparticles can be seen at TEM) show some weak peaks but no unambiguous assignment can be given. However, the absence of strong Bragg peaks indicates that the intermediate phases containing Fe and Co formed at 500 °C by (hydrolysis and) decomposition of nitrate precursors are highly disordered and/or dispersed. Wide-angle XRD patterns of samples reduced at 800 °C show that the final FeCo-SBA-16 nanocomposites are obtained only on reduction. The main peak observed in all nanocomposites at 2h * 45° and the weaker peaks observed at 2h * 65° and 2h * 82° in Fe2Co1_0.8_r800 and Fe1Co1_0.8_ r800 are typical of bcc a-Fe and bcc FeCo-alloy. It is not easy to distinguish between a-Fe and bcc FeCoalloy on the basis of the XRD patterns, the lattice
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parameters being very similar. However, EXAFS and XANES data (not reported here) confirmed the formation of bcc FeCo alloy. As expected, the intensity of the FeCo alloy peak increases with the increase of the molar concentration of the solution used to impregnate the SBA-16 matrix. It has to be noted that the intensity of the FeCo alloy peak in Fe2Co1_0.4_r800 and Fe2Co1_0.4_D_r800 is very similar. Therefore, the amount of FeCo adsorbed during impregnation is not influenced by the volume of the solution used to impregnate the SBA-16 matrix. TEM analysis shows that FeCo alloy nanoparticles have a spherical shape and a narrow size distribution (4–8 nm), quite similar for all nanocomposites. Nanoparticles seem to be homogeneously dispersed in the matrix and no agglomeration or coalescence is observed. Therefore, the size of nanoparticles does not vary with increasing the FeCo loading of the catalyst. This result confirms the stabilisation of nanoparticles because of the cage-like porous structure of the matrix. Furthermore, it is a very promising result as it suggests that the FeCo loading in SBA-16-based nanocomposites can be increased without coalescence of nanoparticles.
Conclusions Nanocomposites formed by FeCo nanoparticles dispersed in a cubic mesoporous silica matrix SBA-16 were prepared using the wet impregnation technique. SBA-16 shows to be a very good support for the preparation of FeCo-based catalysts because of its thermal stability, high surface area, thick walls, and 3D-cage-like structure. Comparison of TEM, SAXS, and N2 absorption–desorption of the pure SBA-16 matrix with the nanocomposites confirmed that the ordered cubic mesoporous structure is preserved after metal loading and post-synthesis reactive thermal treatments to promote the alloy formation. The amount of alloy deposited on SBA-16 increases with the concentration of the solution used for the impregnation. TEM investigation points out that the bcc FeCo alloy nanoparticles formed on reduction are homogeneously distributed throughout the matrix and they seem to be allocated in a large extent inside the pores. In agreement, spherical FeCo nanocrystals with similar size and relatively narrow size distribution (4–8 nm) are observed for all the samples.
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3500 Acknowledgments This work has been carried out within the framework of the project ‘‘NANOCAT’’ funded through the SEED call of the Italian Institute of Technology (IIT). We would like to thank Sardegna Ricerche for the free access to the SAXS apparatus and Prof. M. Monduzzi and Dr. S. Lampis for their help in SAXS measurements.
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