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Journal of Materials Science

, Volume 43, Issue 1, pp 244–253 | Cite as

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

  • Zhida Huang
  • Wolfgang Bensch
  • Wilfried Sigle
  • Peter A. van Aken
  • Lorenz Kienle
  • Tonya Vitoya
  • Hartwig Modrow
  • Thorsten Ressler
Open Access
Article

Abstract

MoO3 was dispersed onto mesoporous SBA-15 by using ammonium heptamolybdate as MoO3 source. The formation of MoO3 was carried out by heating the loaded material to 500 °C for 3 h in air. Below 13 wt% Mo loading, no reflections of MoO3 occur in the X-ray powder patterns and even for high MoO3 contents, the intensities of the reflections are much lower than expected for fully crystalline material. A detailed XAFS analysis reveals that at low Mo contents, the metastable hexagonal modification of MoO3 is formed despite the high calcination temperature of 500 °C. It is highly likely that the nanosize of the particles and the interaction between MoO3 and SBA-15 stabilize the metastable modification of the material. Nitrogen physisorption experiments show the typical type-IV isotherms indicating that the mesoporosity of the materials is preserved despite the large amount of MoO3. Transmission electron micrographs demonstrate the presence of MoO3 inside the SBA-15 support. The Raman spectra display a remarkable size-dependent intensity loss and several features give evidences for a bond formation between nano-sized MoO3 particles and the silica support. Moreover, the spectroscopic details suggest the formation of (MoO3)n oligomers.

Graphical Abstract

MoO3 nanoparticles are successfully introduced into the pores of mesoporous SBA-15. Up to about 13 wt% Mo the material is amorphous and even for higher loadings a large amount of MoO3 is still not crystalline. Nitrogen physisorption and transmission electron microscopy evidences that the mesoporosity of the material is retained. At low Mo loading, the metastable hexagonal modification of MoO3 appears to be stabilized by the interaction with the SBA-15 support material (XAFS).

Keywords

MoO3 XANES Spectrum MoO3 Nanoparticles MoO3 Particle MoO3 Species 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Introduction

In the past few years, periodic silica-based mesoporous materials such as MCM-41 and SBA-15 were used as templates for the syntheses of shape-selective new materials. The mesoporous materials are synthesized via the polymerization of silica around periodic cationic amphiphiles (MCM-41) and neutral block copolymer (SBA-15). SBA-15 possesses well-ordered hexagonal arrays of mesopores with pore diameters between 3 and 30 nm [1, 2] and SBA-15 possesses thicker walls (31–64 Å) compared to MCM-41. Therefore, SBA-15 exhibits a higher thermal stability, being claimed to be stable for at least 48 h in boiling water [1]. Using SBA-15 with the narrow pore diameter distribution as a template for the synthesis of nanoparticles, products are obtained reflecting the diameters of the pores and maximal particle lengths in accordance with the channel lengths of the template material. Such incorporated nanoparticles may have interesting optical, magnetic, and catalytic properties. In addition to the studies of the formation of metal nanoparticles and nanowires, such as Pt [3, 4, 5], Pd [6, 7, 8, 9], Au [3, 4, 5, 10, 11], Ag [3], and Ti [12], the formation of binary nanomaterials such as PbS [13], CdS [14], TiN [15], and GaN [16] within the nanoscale channels of these mesoporous materials have been explored. More recently, highly ordered quantum wires of Zn1−x Mn x S [17] and Cd1−x Mn x S [18] have been successfully synthesized within the mesopores of SBA-15.

Molybdenum-based catalysts have been investigated intensively [19, 20, 21, 22, 23, 24] because of their importance in the technical processes, such as propene metathesis [25, 26], propene oxidation [27, 28], methanol oxidation [29], ethanol oxidative photochemical reaction, and most importantly in hydrodesulfurization (HDS) processes, which represent one of the largest groups of catalytic steps in petroleum and petrochemical reactions. Recently, SBA-15-supported MoO3 materials were used as an intermediate for the preparation of MoS2 catalysts. Different synthesis strategies were applied like thermal treatment of SBA-15 impregnated with ammonium heptamolybdate (AHM) [30], thermal spreading of MoO3 [31], or sonochemical decomposition of Mo(CO)6 in organic solvents [32]. The main aim of these investigations was the preparation of active HDS catalysts. The formation of MoO3 and especially the structures of the supported oxide were not investigated. The MoO3 species are formed within a confined space and the highly interesting question arises whether a metastable modification of the oxide is formed depending on particle sizes as observed for other oxides like TiO2. The thermal decomposition of bulk AHM in different gas atmospheres was recently investigated by in-situ X-ray absorption spectroscopy (XAFS) and in-situ X-ray diffraction [33]. Under static condition AHM is decomposed first to an amorphous phase followed by the formation of ammonium tetramolybdate (ATM) at about 230 °C. With increasing temperature, first a mixture of α- and hexagonal MoO3 is formed and finally pure orthorhombic α-MoO3 is observed at about 360 °C. In the present study, AHM was decomposed to prepare MoO3 species within the pores of SBA-15. Several samples containing different amounts of MoO3 are analyzed and characterized applying various analytical methods including Raman spectroscopy, N2 physisorption, transmission electron microscopy (TEM), high-angle annular darkfield (HAADF) technique combined with energy-dispersive X-ray analysis (EDXS), X-ray scattering experiments, and XAFS. From the results of these studies, a complete picture of the structures of MoO3/SBA-15 composite materials is obtained.

Experimental section

Sample preparation

Synthesis of pristine mesoporous SBA-15

About 8 g of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer (Aldrich, pluronic, P-123) was dissolved in 240 g of water and 28.6 g of concentrated HCl at 30 °C on a water bath. After dropwise addition of 16 g of tetraethyl orthosilicate (TEOS), the reaction mixture was stirred for 24 h at 30 °C. The resulting gel was transferred into a Teflon bottle and heated to 80 °C for 24 h to obtain SBA-15 with 6 nm pores. The resulting white powder was filtered and washed with deionized water, and the surfactant was removed by Soxleth extraction at 78 °C with a mixture of 970 mL of ethanol and 30 mL of concentrated hydrochloric acid. After washing with ethanol, the white powder was dried at RT for about 1 week.

Synthesis of MoO3/SBA-15

Typically, 1 g of SBA-15 was stirred in aqueous solutions of ammonium heptamolybdate (NH4)6Mo7O24 4H2O (AHM) with different concentrations at pH 7–8. After stirring at room temperature for 18 h, the products were filtered without washing. The impregnated SBA-15 was calcined at 500 °C for 3 h in air to obtain MoO3. This calcination temperature is much higher than the usual thermal decomposition temperature of AHM (ca. 350 °C) [34], at which MoO3 is formed in air. At lower temperatures, AHM is found not completely to be decomposed to MoO3, which indicates that AHM is located in the pores and not on the outer surface of the material. The silica walls and small pore diameters can impede the removal of the decomposition products of AHM, such as NH3 and H2O, and consequently a higher formation temperature is required. Materials containing different Mo concentrations were prepared using appropriate concentrations of AHM, i.e., 0.03, 0.06, and 0.09 M solutions. The synthesized products are abbreviated as Mo/SBA-15(A), Mo/SBA-15(B), and Mo/SBA-15(C) with Mo loadings of 8, 13, and 17 wt%, respectively, and were stored in dried air.

Sample characterization

Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer at room temperature using Cu Kα radiation generated at 40 kV and 30 mA. Fourier transform (FT) Raman spectra were recorded with a Bruker FT-106 Raman module, equipped with a Ge detector cooled by liquid nitrogen, and connected to a Bruker FT-IR 66 interferometer and a continuous wave diode-pumped Nd:YAG Laser with a radiation wavelength of 1,064 nm (9398.5 cm−1). The laser power was set to 300 mW and the spectral resolution was 2 cm−1. Nitrogen adsorption measurements were carried out at 77 K on a Sorptomatic 1990 adsorption analyzer. Before the measurements, the samples were outgassed for at least 24 h at 120 °C. The BET surface areas were calculated from p/p 0 = 0.03–0.3 in the adsorption branch; the BJH pore size distributions were calculated from the desorption branch.

For TEM, dried SBA-15 and MoO3/SBA-15 powders were sonicated in dichloromethane for several seconds, dropped on the TEM grids, and dried on carbon-film-coated copper grids (Ted Pella) in air. Transmission electron microscopy microphotographs were obtained on a JEOL JEM-2000 FX electron microscope operating at 200 kV. HAADF images were obtained in a VG HB501UX dedicated scanning transmission electron microscope. In this imaging mode, the brightness is proportional to Z n , where Z denotes the atomic number and n ≈ 1.7. This allows distinguishing Mo from the SBA-15 support. The Mo-content was determined by EDXS with a spatial resolution of about 1 nm.

The XAS experiment was performed at the INE beamline at the ANKA 2.5 GeV synchrotron radiation facility. For details about the instrumentation at this beamline see Ref. [35]. For energy monochromatization, a Lemonnier-type [36] double-crystal monochromator equipped with Ge(422) crystals was employed, leading to a minimum energy step width of 0.2 eV. For calibration, photon energy of 20 keV was assigned to the first inflection point of the K-edge spectrum of a molybdenum foil. The absorption spectra of the powder samples: MoO3/SBA-15 with 8 wt% molybdenum loading and MoO3 reference were recorded at the Mo–K edge (20 keV) in transmission mode between 19.799 keV and 20.853 keV with energy steps of 0.5 eV and 2 s integration time per step.

X-ray absorption fine structure (XAFS) analysis was performed using the software WinXAS v3.1 [37]. Background subtraction and normalization were carried out by fitting linear polynomials to the pre-edge and the post-edge region of the absorption spectra, respectively. The extended X-ray absorption fine structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic background, m 0(k), where k denotes a reciprocal space vector. The pseudo-radial distribution function FT(χ(k)*k 3) was calculated by Fourier transforming the k 3-weighted experimental χ(k) function, multiplied by a Bessel window, into reciprocal space. EXAFS data analysis was performed using theoretical backscattering phases and amplitudes calculated with the ab-initio multiple-scattering code FEFF7 [38]. Single scattering and multiple scattering paths in the α-MoO3 model structure were calculated up to 5.0 Å with a lower limit of 2.0% in amplitude with respect to the strongest backscattering path. EXAFS refinements were performed in reciprocal space simultaneously to magnitude and imaginary part of a Fourier-transformed k 3-weighted and k 1-weighted experimental χ(k) using the standard EXAFS formula [39]. Structural parameters that are determined by a least-squares EXAFS refinement of a MoO3 model structure to the experimental spectra are (i) an overall shift of E 0, (ii) Debye-Waller factors for single-scattering paths, and (iii) distances of single-scattering paths. Coordination numbers (CN) and S 0 2 were kept invariant in the refinement.

Results and discussion

Powder X-ray diffraction

Figure 1 shows the X-ray powder diffraction patterns of MoO3/SBA-15 displaying the three characteristic reflections (100), (110), and (200) of hexagonal mesoporous SBA-15 with space group P6mm. The powder diffraction patterns indicate that the primary structure of SBA-15 consists of well-ordered channels, which are maintained after the formation of MoO3 nanoparticles inside the channels. In addition, the phase cancellation between the pore walls and the guest species MoO3 leads to a decrease of the intensity of Bragg reflections, which is more pronounced for higher MoO3 contents. This phenomenon is well known [17, 40] and was also observed in [30].
Fig. 1

XRD patterns of (a) SBA-15, (b) Mo/SBA-15(A), and (c) Mo/SBA-15(C) with Mo loadings of 8 wt% and 17 wt% respectively

In Fig. 2, wide-angle XRD patterns of the MoO3/SBA-15 samples along with a diffraction pattern of a physical mixture of MoO3 and SBA-15 are displayed. Below 13 wt% Mo, no reflections of crystalline MoO3 occur in the patterns and only a broad feature caused by SBA-15 is observed, indicating that the crystallite size of the MoO3 particles is below the coherence length of X-ray scattering of roughly 5 nm (see Fig. 2, trace (a) with 8 wt% Mo as an example). Some weak reflections of α-MoO3 are visible at a loading of 13 wt% Mo, and the reflection positions agree well with those of bulk MoO3. For higher loading than 13 wt% Mo, the reflections of α-MoO3 become more intense. But even for 17 wt% Mo in the SBA-15 pores, the reflection intensity is much lower compared to the data of the mechanical mixture which has only 7 wt% Mo. The average particle size of MoO3 can be estimated from the peak width using the Scherrer formula. The average crystallite size estimated from the reflection at 2θ = 23.4° is about 30 nm for high Mo loading of 17 wt%. Comparing the size of MoO3 with the pore diameters of the SBA-15 material suggests that the MoO3 crystallites are located partly outside the channels of SBA-15. The amount of crystalline dispersed MoO3 can be estimated from the intensity of the XRD reflection located at 2θ = 23.4°. Since the intensity of Mo/SBA-15(C) with 17 wt% Mo is nearly identical with that of a mechanical mixture of 2.5 wt% Mo (not shown), one may estimate that the amount of nanocrystalline MoO3 for this sample is close to 14.5 wt% Mo. Indeed, the maximal loading achieved can be evaluated to 1.9 μmol m−2 (surface area of SBA-15 is about 850 mg−1) for a fully-dispersed MoO3, which is in good agreement with the value determined by Blanchard et al. (2 μmol m−2) for SBA-15 support [31]. The results of the XRD investigations clearly indicate that the MoO3 species are finely dispersed and a similar observation was made by Dhar et al. [30].
Fig. 2

XRD patterns of (a) Mo/SBA-15(A) (8 wt% Mo), (b) Mo/SBA-15(B) (13 wt% Mo), (c) Mo/SBA-15(C) (17 wt% Mo), and (d) of a physical mixture (7 wt% Mo)

Nitrogen physisorption

Figure 3 displays the N2 adsorption–desorption isotherms (at 77 K), the pore-size distribution of SBA-15 and the MoO3-impregnated SBA-15 materials with different Mo concentrations. The numerical data are listed in Table 1. For all materials, a type-IV isotherm is observed, which is the typical hysteresis loop of mesoporous materials [41], indicating that the incorporation of Mo species does not destroy the structure of mesoporous SBA-15. The pore sizes were calculated from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model, which is an appropriate method for determining changes in the pore-size distribution [17, 18, 42] and the Barrett-Emmett-Teller (BET) equation was applied for calculation of specific surface areas. The isotherms have the following characteristic features: (1) the capillary condensation step, which is caused by the decrease of the pore sizes, typically at the relative pressure p/p 0 = 0.6–0.7 for pristine SBA-15, shifts to lower relative pressure p/p 0 = 0.4–0.6 after introduction of MoO3 particles into the pores (Fig. 3a); (2) the amount of adsorbed nitrogen decreased significantly, which is associated with the smaller specific surface area; (3) the pore volumes decrease with increasing amount of impregnated Mo in SBA-15 (Table 1) and the mean pore diameter (Fig. 3b) decreases as evidenced by the slight shift of the main peak in pore-size distribution curves to smaller values; (4) adsorption of intra-wall micropores at low relative pressures decreases with the increasing loading of MoO3; (5) a small sub-step at the relative pressure at about 0.45 can be observed.
Fig. 3

Nitrogen absorption/desorption isotherms of Mo/SBA-15(A) (8 wt% Mo), Mo/SBA-15(C) (17 wt% Mo) (a), and BJH pore-size distribution (b)

Table 1

BET surface area, mean pore diameter D, and wall thickness t (calculated from t = aD [18], a = cell parameter), for the pristine SBA-15 silica and the MoO3/SBA-15 host–guest compounds

Sample

Mo loading wt%

Mesopore/micropore volume (cm3 g−1)

BET surface area (m2 g−1)

Mean pore diameter D (nm)

Wall thickness t (nm)

 

Normalized

 

Normalized

SBA-15

0

1.08/0.17

1

920

1

5.96

4.31

Mo/SBA-15(A)

8

0.65/0.13

0.68/0.86

597

0.74

5.34

4.93

Mo/SBA-15(B)

13

0.55/0.11

0.63/0.80

505

0.68

5.11

5.16

Mo/SBA-15(C)

17

0.45/0.08

0.56/0.61

397

0.58

5.02

5.25

\( \hbox{Normalized}(X)=X_{\rm MoO_{3}}/((1-y) X_{\rm{SBA}-15}),\) where X is specific surface area or pore volume and y is weight fraction of MoO3

From the isotherms, it can be concluded that the mesoporosity of the support is still preserved after the incorporation of MoO3 particles into the SBA-15 materials. The important decrease in pore volume (mesopores and micropores) and the reduced mean pore sizes (see Table 1) indicates the filling of both the mesopores and the micropores by MoO3 particles. The two-step desorption branch [43] and the decrease in pore volume might be attributed to the presence of a pore-blocking effect by MoO3, because the formation of MoO3 particles inside the pores of SBA-15 would not significantly affect the pore volume due to the high density of MoO3 (4,700 kg cm−3). This blocking effect becomes more pronounced with increasing Mo loadings.

Transmission electron microscopy

Transmission electron microscopy was performed to analyze the microstructure of the samples. For all samples with different Mo loadings, the hexagonally ordered pore structure of the SBA-15 host was maintained after inclusion of MoO3 and calcination. This is exemplified by the bright-field images of Fig. 4 (loading: 17 wt% Mo, two distinct magnifications), which were recorded perpendicular to the channel axes. The presence of MoO3 inside the pores can be demonstrated by a combined approach of HAADF and EDXS. The HAADF images of Fig. 5 (17 wt% Mo) were recorded in two distinct orientations of the channels. For the image recorded perpendicular to the channel axes (Fig. 5a), randomly distributed bright spots are preferably aligned along the channel axes. In case of images along the channels axis (i.e., [001]) (Fig. 5b), bright spots fill the space of the open SBA-15 pores. Figure 6 shows EDXS spectra acquired from the bright spots (a) and from darker areas (b). It is obvious that the intensity of the Mo-L peak is significantly higher at the position of the bright spots. Occasionally, the locations of the bright spots do not fit with the periodicity of the host. The most probable interpretation here is to assume the presence of MoO3-nanoparticles on the surface of the host. But it appears that most MoO3-nanoparticles are located inside the SBA-15 pores.
Fig. 4

TEM images with different magnifications recorded for the sample with 17 wt% Mo

Fig. 5

HAADF images of Mo/SBA-15 (17 wt% Mo) in an orientation (a) perpendicular to and (b) along the channel axes. MoO3-nanoparticles show bright contrast as indicated by the arrows in (b)

Fig. 6

Energy-dispersive X-ray spectra of (a) MoO3 particle and (b) SBA-15

Raman spectroscopy

Raman spectroscopy is one of the effective methods to probe crystalline and amorphous nanoscale oxides [23, 24]. Raman spectra of the impregnated samples before calcination are displayed in Fig. 7, and the characteristic bands of the [Mo7O24]6− anion are clearly visible for the sample with 8 wt% Mo. The strong band at 943 cm−1 is due to the symmetric stretching of the short Mo–O terminal bonds of [Mo7O24]−6 along with a weak and medium band at 898 cm−1 [22]. The bending vibration at 362 cm−1 and deformation band at 320 cm−1 of the terminal Mo=O groups are also observed. The intensity of these bands increases with the amount of incorporated AHM.
Fig. 7

Raman spectra of (a) SBA-15, (b) Mo/SBA-15(A) (8 wt% Mo), (c) Mo/SBA-15(B) (13 wt% Mo), and (d) Mo/SBA-15(C) (17 wt% Mo) before calcination at 500 °C

In the Raman spectra of calcined Mo/SBA-15 along with a spectrum of the mechanical mixture (Fig. 8), the characteristic features of bulk MoO3 are seen. Considering only Mo–O bonds up to 2 Å, MoO3 chains are directed along the crystallographic c axis by corner sharing of MoO4 tetrahedra [44]. The Raman spectrum of MoO3 between 82 and 995 cm−1 was investigated by Py and Maschke [45], who carried out a study on single crystals and performed valence force-field calculations. The band at 994 cm−1 was assigned to the symmetric stretching mode of the terminal Mo=O groups and the signals at 819 and 665 cm−1 to the vibrations of the Mo–O–Mo bridge. The doublet at 290–280 cm−1 corresponds to the two wagging modes of the terminal Mo=O groups.
Fig. 8

Raman spectra of (a) Mo/SBA-15(A) (8 wt% Mo), (b) Mo/SBA-15(B) (13 wt% Mo), (c) Mo/SBA-15(C) (17 wt% Mo) after calcination, and (d) of a mechanical mixture (10 wt% Mo)

In the spectrum of Mo/SBA-15(A) with 8 wt% Mo, only two bands are visible at 819 and 994 cm−1 as weak signals. For low loadings, the predominant silica-supported molybdenum species were observed to be isolated molybdate tetrahedra [21]. However, no bands at 988 and 620 cm−1 characteristic of MoO 4 2− were detected in our sample, indicating that MoO3 should be formed as the majority species under our experimental conditions. At high loadings, the observed wave numbers for MoO3/SBA-15 materials are similar to that found for the physical mixture. However, some features in the Raman spectra should be highlighted. The Raman spectra of MoO3/SBA-15 materials exhibit a significant intensity loss compared to the physical mixture with the same Mo content. The intensity reduction may be explained by a reduced effective scattering volume of dispersed MoO3 particles. Such a size-dependent intensity change was also reported by Mestl et al. [20]. Furthermore, the spectra show a change of the relative intensity of the MoO3 characteristic modes at 819 and 994 cm−1, which may be associated with the weak interaction between the highly distorted small MoO3 particles and the SBA-15 support. The typical intensity ratio I 994/I 819 for Mo/SBA-15(B) and Mo/SBA-15(C) is 0.45, which is smaller than that of the mechanical mixture of 0.64. An identical feature was observed by Braun et al. [19] and Jeziorowski et al. [46], who prepared the silica supported MoO3 by thermal spreading (I 994/I 819 = 0.49) and the impregnation method (I 994/I 819 = 0.46), respectively. In addition, an interesting phenomenon in the Raman spectra is the change in relative intensity of the two wagging modes at 290 (B2g) and 280 cm−1 (B3g). In the single crystal and the physical mixture spectra, the B3g wagging mode has only 2/3 of the intensity of the B2g mode. For all the MoO3/SBA-15 samples, the B3g wagging mode is as intense as the B2g signal and the two modes are hardly resolved. This observation is consistent with a phenomenon, which has been discussed by Braun et al. [19] in the spectra of Mo/Al2O3 and Mo/SiO2 sample. The two wagging modes of the terminal Mo=O groups are parallelly polarized to the direction of the chain of the tetrahedral MoO4 units, i.e., directed along the c-axis. Therefore, the change in the intensity ratio for this doublet may be attributed to the distortions along the MoO4 chains. Finally, a weak shoulder appears at about 953 cm−1 corresponding to the Mo=O vibration of a polymeric molybdate species interacting with the support [24].

XAFS

The XRD patterns confirm the formation of α-MoO3 at high Mo-loadings, and Raman data indicate the interaction of size-reduced MoO3 with the silica support. However, the modification of MoO3/SBA-15 at low concentration of Mo stayed unknown and was furthermore examined by XAFS. The Mo–K edge XANES spectra of MoO3/-SBA-15(A) (8 wt% Mo), α-MoO3, and hex-MoO3 are depicted in Fig. 9. The similarity of the local structure around the Mo atoms in α-MoO3 and hex-MoO3 (Table 2) is readily visible in a very similar near-edge structure. However, in particular the feature at ∼20.03 keV (arrow in Fig. 9) appears to be more reduced in the spectrum of hex-MoO3 than in that of α-MoO3. Compared to the references, the XANES spectrum of MoO3/SBA-15 exhibits a similar overall shape. While the number of peaks in the XANES spectrum of MoO3/SBA-15 is the same as in those of α-MoO3 and hex-MoO3, the amplitude of the peaks is significantly reduced. This holds particularly for the peak at ∼20.03 keV. In total, the Mo–K edge spectra of MoO3/SBA-15 seem to correspond to a local structure similar to that of α-MoO3 and/or hex-MoO3. The reduced amplitude of XANES can be attributed to a considerably reduced ordering of small particles.
Fig. 9

Mo–K edge XANES spectra of MoO3-SBA-15 (8 wt% Mo), α-MoO3, and hex-MoO3

Table 2

Type and number (N) of atoms at distance R from the Mo atoms in α-MoO3 and hex-MoO3 model systems compared to experimental distances and EXAFS Debye-Waller factors

Type

N

Model α-MoO3

Model hex-MoO3

hex-MoO3

MoO3-SBA-15

R (Å)

R (Å)

R (Å)

σ 22)

R (Å)

σ 22)

Mo–O

1

1.67

1.68

1.68

0.0010

1.68

0.0010

Mo–O

1

1.73

1.73

1.75

0.0035

1.75

0.0030

Mo–O

2

1.95

1.97

1.97

0.0035

1.99

0.0065

Mo–O

1

2.25

2.20

2.21

0.0012

2.23

0.0030

Mo–O

1

2.33

2.39

2.38

0.0026

2.39

0.0021

Mo–Mo

2

3.44

3.31

3.33

0.0050

3.30

0.0080

Mo–Mo

2

3.70

3.73

3.70

0.0026

3.79

0.0083

Mo–Mo

2

3.96

4.03

3.95

0.0031

4.00

0.0101

Experimental parameters were obtained from a refinement of a α-MoO3 model structure to the experimental Mo–K edge XAFS χ(k) of hex-MoO3 and MoO3/SBA-15 (8 wt% Mo) (k-range from 3.1 Å−1 to 14.3 Å−1, R range from 0.9 Å to 4.0 Å, E 0 (MoO3/SBA-15) = −1.5 eV, E 0 (hex-MoO3) = −1.8 eV, residual = 6.9, N ind = 24, N free = 15). The uncertainty in the distances amounts to about 0.03 Å

The Fourier-transformed XAFS χ(k)*k 3 of MoO3/SBA-15(A) (8 wt% Mo), α-MoO3, and hex-MoO3 are shown in Fig. 10. Compared to the XANES spectrum, more distinct differences can be seen in the FT(χ(k)*k 3) of α-MoO3 and hex-MoO3. Particularly the shape of the peaks between 1.0 Å and 2.0 Å (not phase-shift corrected) that correspond to Mo–O distances and the position of the peak at ∼3 Å that corresponds to Mo–Mo distances differ significantly. Compared to the references, the overall shape of the FT(χ(k)*k 3) of MoO3/SBA-15 appears to resemble more closely to that of hex-MoO3 than to that of α-MoO3.
Fig. 10

Fourier transformed Mo-K edge XAFS χ(k)*k 3 (not corrected for phase-shifts) of MoO3-SBA-15 (8 wt% Mo), α-MoO3, and hex-MoO3

For a more detailed analysis of the XAFS data, the theoretical FT(χ(k)*k 3) calculated for an α-MoO3 model structure was refined using the experimental spectra of MoO3-SBA-15 and hex-MoO3 (Fig. 11). Therefore, the most prominent Mo–O and Mo–Mo single scattering paths were included. The refinement of the model structure to the spectrum of MoO3/SBA-15 results in a very good agreement between the experimental and theoretical spectrum (Fig. 11). The structural parameters obtained are given in Table 2. Besides small differences in the local structure around the Mo atoms in α-MoO3 and hex-MoO3, a significantly shortened Mo–Mo distance of 3.3 Å exists in hex-MoO3 compared to 3.4 Å in α-MoO3. This short Mo–Mo distances account for the shift in peaks at higher distances as visible in the FT(χ(k)*k 3) of the two references. Similar to hex-MoO3, MoO3/SBA-15 exhibits a short Mo–Mo distance at ∼3.3 Å and a slightly prolonged Mo–O distance at ∼2.4 Å. The good agreement between the structural parameters of MoO3/SBA-15 and hex-MoO3 indeed indicates the presence of hex-MoO3 inside the pore structure of SBA-15. The strongly reduced amplitude in the FT(χ(k)*k 3) at higher distances may be caused by very small particles or a predominantly two-dimensional structure of the hexagonal MoO3 modification in SBA-15. Treating AHM in air at 500 °C results in the formation of α-MoO3, as previously reported [33], and hex-MoO3 is formed as an intermediate phase at ∼350 °C. However, hex-MoO3 transforms upon further heating into the more stable α-MoO3 modification. For the materials with low Mo-loading studied here, the metastable hexagonal modification of MoO3 appears to be stabilized by the interaction with the SBA-15 support material.
Fig. 11

Experimental and theoretical Fourier transformed Mo–K edge XAFS χ(k)*k 3 (not corrected for phase-shifts) of MoO3-SBA-15 (8 wt% Mo)

Conclusions

AHM dissolved in water is a useful Mo precursor, which can be readily introduced into the channels of SBA-15 without destroying the structure of mesoporous SBA-15 materials (XRD, TEM, and N2 physisorption). The formation of the [Mo7O24]6− anion is observed in Raman spectra prior to calcination. After conversion of AHM to MoO3 by a heat treatment, the Raman spectra show strong effects of the reduced dimensions of MoO3 nanosized particles onto the vibrational properties. The appearance of a weak shoulder at about 953 cm−1 indicates that MoO3 particles on the pore surface of SBA-15 form (MoO3)n oligomers. The intensity of the reflections in the XRD patterns with increasing Mo content, the reduction of the pore volumes and of the surface areas obtained from the N2 physisorption experiments reveal that MoO3 particles are introduced into the channels of SBA-15. This is also confirmed by the application of HAADF technique combined with EDXS. In the wide-angle XRD patterns, reflections of α-MoO3 appear only at high loadings of more than 13 wt% Mo, i.e., for lower MoO3 contents the particle size is far below the coherence length of X-rays. This is further evidenced by comparing the powder pattern of a physical MoO3/SBA-15 mixture (7 wt% Mo) with that of SBA-15 materials with MoO3 in the pores. The reflections of the mixture are much more intense than that of the material containing 17 wt% Mo. For higher Mo loading than 13 wt%, the reflections of α-MoO3 become more intense with increasing MoO3 content. However, at low Mo loading, the metastable hexagonal modification of MoO3 appears to be stabilized by the interaction with the SBA-15 support material, which is clearly confirmed by the results of the EXAFS data analysis. Further experiments are under way to test the new materials in different catalytic reactions and as sensor materials.

Notes

Acknowledgements

The financial support of the Deutsche Forschungsgemeinschaft (DFG), the Fonds der chemischen Industrie (FCI), and the State Schleswig-Holstein is gratefully acknowledged.

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Copyright information

© Springer Science+Business Media, LLC 2007

Authors and Affiliations

  • Zhida Huang
    • 1
  • Wolfgang Bensch
    • 1
  • Wilfried Sigle
    • 2
  • Peter A. van Aken
    • 2
  • Lorenz Kienle
    • 3
  • Tonya Vitoya
    • 4
  • Hartwig Modrow
    • 4
  • Thorsten Ressler
    • 5
  1. 1.Institut für Anorganische ChemieUniversity of KielKielGermany
  2. 2.Max-Planck-Institut für MetallforschungStuttgartGermany
  3. 3.Max-Planck-Institut für FestkörperforschungStuttgartGermany
  4. 4.Physikalisches Institut der Universität BonnBonnGermany
  5. 5.Institut für Chemie, Fachgruppe Anorganische und Analytische ChemieTU BerlinBerlinGermany

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