Iron oxide nanoparticles within the pore system of mesoporous SBA-15 in different acidity: the synthesis and characterization

DOI 10.1007/s10971-007-1534-x

Iron oxide nanoparticles within the pore system of mesoporous

SBA-15 in different acidity: the synthesis and characterization

Yi-Chun Lin

Received: 9 July 2006 / Accepted: 2 January 2007 / Published online: 6 April 2007 C

Springer Science+ Business Media, LLC 2007

Abstract Direct hydrothermal method is employed for in-corporating iron into the pore structure of SBA-15. The re-sultant materials were analyzed by X-ray diffraction (XRD) patterns, N2sorption isotherm and X-ray photoelectron spec-troscopy (XPS). The characterizations of XRD patterns and XPS revealed that iron nanoparticles were present as highly dispersed nanoclusters in the well-ordered mesoporous chan-nels of SBA-15. The characterizations of t-plot reveal only microporous channels of SBA-15 are confirmed to be filled with iron nanoparticles, leaving the mesopores unaffected. The supported material still maintained its ordered meso-porous structure similar to SBA-15 and possessed high sur-face area, large pore volume and uniform pore size. Keywords SBA-15 . Catalyst support . Fe2O3 nanoparticles . Mesoporous silica

1 Introduction

In the last decades, the synthesis and characterization of nano-sized materials have been of great interest because of

C.-Y. Liu (
)· J.-P. Leu

Department of Materials and Engineering, National Chiao-Tung University,

1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC e-mail: [email protected]

C.-F. Chen

Institute of Material and System Engineering, MingDao University,

ChangHua 523, Taiwan, ROC Y.-C. Lin

Department of Chemistry, National Tsing Hua University, 101, Sec 2, Kuang Fu Rd., Hsinchu 300, Taiwan, ROC

their specific electronic, magnetic, optical, biological and mechanical properties compared to the bulk materials [1–4]. Template-assisted synthesis has a number of interesting and useful features for the preparation of nanostructures, since the size and shape of the desired materials can be easily di-rected using a well-defined template matrix [5,6]. Among the porous materials, the mesoporous silica materials included uniform pore size distribution (2–30 nm) and high surface areas have been widely applied as host for loading catalyst [7], polymer [8], metals [9] and semiconductors [10]. Vari-ous nanoparticles have been incorporated into the frames of SBA-15 such as Al [11], Ti [12] and Zr [13–15]. In particular, their large pore size makes the nanoparticles-incorporated SBA-15 attractive as catalysts for reactions. In compari-son to M41s, the SBA-15 materials which exhibit the high pore size and thick pore wall synthesized under the higher acidic conditions. Unfortunately, the synthesis conditions of SBA-15 were usually acidic, therefore, it was difficult to embed the nanoparticles. The amount of nanoparticles in-corporated into the frames of SBA-15 is very low. More-over, Newalkar and co-workers indicated the ordering of mesostructure still decreased markedly as the Zr/Si atomic ratio in the synthesis gel was greater than 5% and the mesostructure was nearly disorganized when the ratio ap-proached 10% [15]. Li and co-workers reveal that mesostruc-ture is still maintained when iron oxide nanoparticles are supported into SBA-15 at mild acidic conditions [16]. Fr¨oda and co-workers reported Fe-supported SBA-15 with differ-ent precursors in dry ethanol via post-synthesis procedure and also checked the direct hydrothermal methods [17]. They found that different strategies of synthesis lead to sig-nificant changes in the bonding and environment of iron species within the silica materials. Nevertheless, some re-porters showed nanoparticles supported on the SBA-15 pre-pared by impregnation or grafting methods are not suitable

to prepare catalysts with highly isolated catalytic active sites. The studies did not present the direct hydrothermal synthe-sis of the Fe/SBA-15 materials. The main purpose of this work is to investigate the characterization and synthesis of Fe-nanoparticle-incorporated SBA-15 materials. The appli-cations of multiple characterization technologies, i.e. pow-der X-ray diffraction (XRD), N2 absorption spectroscopy and X-ray photoelectron spectroscopy (XPS), proved that nanostructured Fe has been formed within the pore system of SBA-15.

2 Experimental methods 2.1 Sample synthesis

The SBA-15 materials were synthesized by the traditional hydrothermal method using the triblock copolymer P123 (EO20-PO70-EO20, Aldrich) as the surfactant. In a typi-cal synthesis, 4 g P123 was dissolved in 30 g of deion-ized water at 30–40◦C and stirred until transparent solution was obtained. Then, 90 g of different concentration of hy-drochloric acid was added and stirred for 2 h. After 2 h, the tetraethyl orthosilicate (TEOS) and the amount of approxi-mate Fe(NO3)3· 6H2O was added into the slurry. Then, the resultant solution was stirred for 20 h. The resultant solu-tions were quickly poured into a large polypropylene bottle. The mixture thus obtained was heated for the formation of the surfactant–silica mesophases under static conditions at 373 K. The solid products were filtered, washed with deion-ized water and dried at oven overnight. In order to remove the organic surfactant, the samples were calcined at 560◦C for 6 h at a heating rate of 1 K/min.

2.2 Materials characterization

XRD patterns were obtained by a Bede/D1 diffractometer using the Cu Kα radiation (1.543 Å) with a voltage of 40 kV and a current of 40 mA. The nitrogen adsorption–desorption isotherms were measured at 77 K using NOVA 1000e system in static measurement mode. The samples were degassed at 250 ◦C for 5 h under vacuum before measurement. The specific surface areas were calculated by Brunauer–Emmett– Teller (BET) method based on the adsorption branches. The pore diameter and pore size distribution were measured from desorption braches by the Barrett–Joyner–Halenda (BJH) method. The nanoparticles embedded analysis by XPS, using Al Kα X-ray source.

3 Results and discussion

Figure 1 show that the XRD patterns of Fe incorporated SBA-15 with Fe/Si ratios between 0.01 and 0.1. It can be

Fig. 1 Powder XRD patterns of Fe-loaded SBA-15 with various Fe/Si

ratios

seen that the XRD patterns reveal three well-ordered peaks at 0.9◦, 1.6◦and 1.8◦, indicating well-resolved (1 0 0), (1 1 0) and (2 0 0) peaks, as shown in Fig.1. The intensities of XRD patterns would also decrease and d spacing also shifts to small angle with the increase in iron loading. It seems that the absence of the prominent peaks revealed the mesostruc-ture would collapse with high Fe loading. These effects have already been discussed in previous studies dealing with metal oxide phase within mesoporous silica [17,18]. XRD patterns show that iron oxide nanoparticles embedded the SBA-15 templates under various acidity conditions. Well-ordered structures were observed at strongly acidic condi-tions. It can be seen in Fig. 2 that intensities decreased slightly with the decreasing acidic concentrations. The strong reduction of the intensities of the XRD peaks is due to

Fig. 2 Powder XRD patterns of Fe-loaded SBA-15 under various acidity conditions

Fig. 3 High-angle XRD patterns of Fe-loaded SBA-15 with various Fe/Si ratios

Fig. 4 XPS of Fe/SBA-15

increasing destructive interferences by filling the pores with rising amount of the iron oxide nanoparticles. In general, the introduction of scattering material into the pores lead to an increased phase cancellation between scattering from the wall and the pore regions and therefore to reduced scattering intensities for the Bragg reflections [19]. Figure3indicated the diffraction pattern of different iron loaded. The weak and broad reflection at 2 ∼ 23◦corresponds to almost amor-phous SBA-15. The absence of these prominent reflections in case of the nanostructured Fe2O3nanoparticles indicated that no crystalline bulk materials have been formed outside the pore system. The results also showed that Fe2O3 nanopar-ticles were dispersed uniformly in the frame of SBA-15 [20]. It reveals that protons and Fe ions are probably in a compet-itive position in self-assembly of the organic template and inorganic precursors. The XPS result of Fe nanoparticle is shown in Fig.4. In the case of Fe/SBA-15, the binding ener-gies are located at 710.0, 711.6, 714.9, 724.3 and 728.1 eV. The binding energies shifted slightly, which implies that the Fe phase in SBA-15 might be different from iron species.

As a whole, the peak position is consistent with other re-searches [20, 21]. Wandelt et al. announced that the XPS characteristic of Fe 2p2/3 spectra is located at 707± 0.2 eV, 710 ± 0.2 eV and 711 ± 0.2 eV, respectively [22]. One peak located at 710.0 eV was assigned to Fe2+_{. The peak at} 711.3 eV was assigned to Fe3+_{. These peaks are consistent} with other researches [23]. Because the binding energy of Fe metal appears generally around 706.4 eV and those of the oxides are between 710 and 723 eV [24]. Accordingly, the binding energies of 714.9 eV in Fig.4indicate that surface energy can be assigned to Fe phase of Fe-incorporated SBA-15. Several reporters showed the peak located at 715 eV iden-tified the surface peak [25]. Droubay and Chambers claimed that the surface peak is mostly caused by the Fe3+_{cations at} the surface ofα-Fe2O3, which affects the binding energy of numerous samples [26]. In the bulk of iron oxides, it could be eliminated as the cause of the high binding energy surface peaks [27]. The Fe components anchored inside SBA-15 by the reaction with silanol group were oxidized through the calcinations step. However, it is believed that the iron ox-ide which is formed by Fe-impregnated SBA-15 might be at a different oxidation state than activated iron. Figure5(a) shows the N2adsorption–desorption isotherm and pore size distribution curves determined by BET. The method gives information on the specific surface area and the pore di-ameter. These materials show type IV isotherm typical for the mesoporous materials [28]. A well-defined step occurs at about p/p0= 0.4; this is associated with the filling of the mesopores due to capillary condensations. All materials ex-hibit a sharp step at p/p0= 0.6–0.8, which demonstrates a uniform mesophase distribution. It shows that the addition of crystalline entities to mesostructured materials does not interfere with the formation of the mesoporous silica, but it disturbs the hexagonal ordering to some extent. In or-der to unor-derstand the textural properties of the mesoporous composites, the nitrogen isothermal sorption technology was employed to investigate the iron species effect on the pore structure as well as to determine the location of Fe in SBA-15. Figure5(a) shows the adsorption isotherm of SBA-15 and Fe/SBA-15, with the total surface areas found to be 1091 and 912 m2g−1, respectively. In Fig.5(a), the surface area was found to decrease somewhat from 1091 to 912 for Fe/SBA-15. Even though the SiO2–Fe composition was ob-served to decrease in surface areas, this was still relatively high. Pore size distribution determined by BJH equations is shown in Fig. 5(b). It indicates that the mesoporous sizes are changing slightly from 7.00 to 6.68 nm in diameter for both SBA-15 and Fe/SBA-15. These results also show that the Fe particles do not occupy the mosoporous. From the adsorption–desorption curves, t-plots are estimated depend-ing on the known adsorption behavior of the materials and that of non-porous standard silica (in Fig.6) [29]. Distinct change for SBA-15 is observed, illustrating the existence of

Fig. 5 Nitrogen sorption isotherms (a) and pore size distribution (b) of SBA-15 and Fe/SBA-15

Fig. 6 t-Plot analysis of SBA-15, SBA-15-ultra and Fe/SBA-15 from nitrogen adsorption isotherms

Table 1 Surface areas and pore size distribution of SBA-15 and Fe/SBA-15 under various acidity conditions

Total surface Mesoporous Microporous Samples areas (m2_{/g) surface area (m}2_{/g) surface area (m}2_/g)

SBA-15 1091 980 111

Fe/SBA-15 611 611 0

Fig. 7 Typical TEM images of the nanoparticles embedded onto the ordered mesoporous silica

microporous. On the contrary, for Fe/SBA-15, no distinct change is detected. This verifies that the micropores have disappeared after treatment of iron nitride. The microporous channels of SBA-15 are filled with Fe. Table 1 shows a de-tailed surface and pore size between SBA-15 and Fe/SBA-15. As shown in Table1, there is a slight difference in total surface area between SBA-15 and Fe/SBA-15. The microp-orous area can be observed whereas the mesopmicrop-orous surface area is almost unchanged. It is reasonable to suggest that only the microporous are filled with Fe for Fe/SBA-15 with the surface area of the mesoporous being almost the same as the original host SBA-15. On the other hand, the decrease in surface areas is likely to be principally due to plugging sup-port pores by Fe loaded [30]. In conclusion, the t-plot reveals that Fe nanoparticles are incorporated into the microporous of SBA-15. Figure7shows the nanoparticles embedded onto the pore of SBA-15. It should be noted that the small amounts of iron compounds were observed in the pore of SBA-15. The diameter of these particles was equal to that of pore of SBA-15. Taking into account the XRD results, the incorporated iron species may have poor crystallinity.

4 Conclusion

In summary, highly ordered mesoporous Fe incorporat-ing SBA-15 with different Fe contents were successfully

synthesized. Direct hydrothermal method is employed on the inner surface of self-ordered mesoporous SBA-15. The Fe loading did not affect the surface areas and pore size dis-tribution but affected the mesostructure. Mesoporous silica SBA-15, which has high surface area and highly ordered structure, has attracted much attention as a metal nanoparti-cle support. Through the analysis of the resultant materials, we can find that only the micropores of SBA-15 are filled with iron metal. Therefore, a high surface area is retained after the incorporation of iron into the channels of SBA-15. The preparation method appears to be viable for template synthesis of various metal-modified mesoporous silicas with high surface areas.

Acknowledgment The authors would like to acknowledge the

finan-cial support of the National Science Council of Taiwan for this work. Acknowledgements are also extended to Center for Nano Science and Technology, NCTU for XRD experiment.

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