Spectroscopic study of molybdenum oxides supported on silica

MATERIALS CHEb,M&WiSND ELSEVIER

Materials Chemistry and Physics 43 (1996) 140-144

Spectroscopic study of molybdenum

oxides supported on silica

J. Mhdez-Viva Depaitamento

& Quimica,

Universidad

Autdnoma

Metropolitana-Iztapalapa,

AP 55-534,

0.9340 Mesico

DF,

Mesico

Received23 December 1994;accepted9 June 1995

Abstract Molybdenum-doped silica (b:l-1.0 kit:%) was prepared via the sol-gel process, using HC! as catalyst. The solids were studied by Fourier-transform infrared spectroscopy (FT-IR) and diffuse reflectance. The degree of hydroxylation of the Si02 surface was different in the non-catalyzed solids, compared to the HC! catalyzed, after calcining at 600 “C, by FT-IR. UV bands were only found by the diffuse reflectance study for all the solids. Location of the charge transference bands observed in the spectra suggests that the structure of acid-catalyzed samples is different from non-catalyzed ones. An X-ray diffraction study showed only amorphous SiO, in all the solids, including the calcined at 900 “C. It was not possible to determine the MO structures by this technique because of the low MO content. Keywords:

Infrared spectroscopy; Molybdenum oxides; Silica

1. Introduction The study of molybdenum oxides supported in silica is interesting due to theiT application in several fields: natural gas processing and chemical and pollution control industries [l-3]. The traditional approach on the preparation of the solids consists in the deposition of the catalytically active molybdenum oxide on the surface of an oxide support by an iinpregnation method [4]. The spectroscopic study of the surface structure of the obtained catalysts is important because the nature and reactivity of the support largely determines the catalytic properties of the solid [5,6]. The goal of the present study was to prepare and characterize Mo/SiO, powders obtained by an alternative method, the sol-gel process [7]. Characterization was done by Fouriertransform infrared (FT-IR) and diffuse reflectance (UV-Vis) spectroscopies.

2. Experimental Si(OC,H,), (O.S33 mol) (Aldrich, 98%) was dissolved in 0.256 mol EtOH(Baker) in a flask at room temperature. The mixture was stirred and heated to reflux. A MoOCl, (Pfaltz and Bauer) in EtOH solution (Mo/EtOH molar ratio, 1.664) was added by dropping. 0254-0584/96/$15.000 1996 Elsevier ScienceS.A. All rights reserved ssD10254-0584(95)01609-X

The’ final MO content was 0.1 wt.%. The resulting solution was refluxed 24 h. To hydrolyze the sol, H20 in EtOH was added. The H,O/Mo molar ratio was 300 and the resulting pH was 4.0. The solution was refluxed 24 h more. The excess of solvent was slowly evaporated. The resulting powder was dried in an oven at 100 “C for 24 h. This was for the MO-0.1 sample. Following the above mentioned procedure additional samples were prepared using 3.2 x lo-* mol HCl (Baker, 37 wt.%) as catalyst during the hydrolysis step, according to the sol-gel process [7]. The final MO content of these samples was 0.1 wt.% (labelled MO-0.1 HCl), 0.5 wt.% (MO-0.5 HCl) and 1.0 wt.% (MO-1.0 HCl) and the final pH was 2.0 in all cases. The samples were thermally treated in air atmosphere at 300, 600 and 900 “C for 24 h. FT-IR spectra were recorded in a Perkin Elmer Model 1600 spectrophotometer as KBr pellets in the 200-4000 cm-l region. UV-Vis results were obtained in a Varian Cary 1 spectrophotometer in the range 12 500-50 000 cm-‘, using a diffuse reflectance attachment and MgO as reference. 3. Results and discussion The initially transparent Si sol turned into a pale yellow color when the MoOCI, solution was added. The

J. MPndez-

Vivar

/Materials

Chemistry

Cd)

(c)

lb)

(a)

L 4

I

I

3

WAVE NhvlBER

/

I

I

0.4

,,3 x IV

(cm-‘)

Fig. 1. FT-IR spectra of MO-0.1 sample: (a) xerogel, and calcined at (b) 300 “C, (c) 600 “C and (d) 900 “C.

color intensified during reflux. These changes in color indicated the reduction of Mo(VI) from MoOCI, to MO(V) species [8]. The acid-catalyzed yellow sols changed into pale green immediately after the addition of HCl. Color intensification was observed with the increasing of the MO content. In this case mixed-valence molybdenum compounds were obtained [9- 111. The powders dried at 100 “C, named xerogels, were white (MO-0.1) and beige (all the acid-catalyzed samples) in color. Thermal treatment led to additional changes in color. Non-catalyzed (MO-0.1) samples after calcination at 900 “C appeared white, whereas the corresponding acid-catalyzed samples (MO-O. 1 HCl) were black in color. 3.1. Infrared spectra Fig. 1 presents the FT-IR spectra of MO-0.1 solid samples after thermal treatment. In Fig. l(a) the spectrum of the xerogel (solid treated at 100 “C) appears. The wide band centered on 3500 cm-’ corresponds to Si-OH bonds [12]. A shoulder appears at 3200 cm-‘; it corresponds to O-H bonds [12]. The band at 1685 cm-’ is attributed to O-H bending [ 13-151. A sharp weak band is observed at 1400 cm-’ that corresponds to MO-O bonds [16]. The intense signal at 1090 cm-’ is due to Si-0 stretching bonds [ 171. The band at 940

and Physics

43 (1996)

140- 144

141

cm-’ is assigned to Si-OH of silanol groups weakly bonded to the silica surface [17]. The sharp band at 800 cm-’ and the weak band at 670 cm-’ both correspond to MO-O-MO bands [18]. The band at 560 cm-’ has been assigned to MO-O vibrations [19]. The 0-MO-Cl stretching band was found at 450 cm-’ as an intense signal [ 191. FT-IR spectra of the MO-0.1 sample calcined at 300, 600 and 900 “C appear in Fig. l(b), (c) and (d), respectively. The major differences of these spectra compared to the xerogel are as follows. (i) The bands at 3500, 3200, 1685 and 940 cm-’ gradually disappear with the thermal treatment. This change is due to the dehydroxylation of the surface of the powder. The effect is dramatic for the sample treated at 600 “C (Fig. l(c)), where these bands completely disappear. (ii) The band at 1090 cm-’ slightly shifts to 1100 cm-’ for the sample calcined at 900 “C (Fig. l(d)). This is the result of the stabilization of the SiO, network. (iii) The band at 560 cm-’ gradually disappears, turning into a shoulder at 600 “C and it was not detected at 900 “C. This is attributed to partial separation and crystallization of MO from SiOZ, and it will be discussed later. The band initially found at 450 cm-‘, unfolds at 600 “C into three bands at 460, 445 and 404 cm-r (Fig, l(c)). These peaks are now assigned to vibration of rings formed by four, five and six siloxane groups [20]. For MO-0.1 calcined at 900 “C, the band at 468 cm-’ corresponds mainly to MO-O in Mo*O,, formed after MO separates from Si02 and crystallizes as MOO,, which melts at 795 “C [ 18,211. Contributions from siloxane bands (previously at 460 cm-‘, 445 cm-’ and 404 cm-‘) also may exist. The crystalline phase change from Moo3 to Mo,O,, has been demonstrated in Ref. [21], where the same MO precursor, MoOCl,, was used. Fig. 2 shows the FT-IR of MO-1.0 HCl powders treated at 100 “C (Fig. 2(a)), 300 “C (Fig. 2(b)), 600 “C (Fig. 2(c)) and 900 “C (Fig. 2(d)). Comparing Fig. 2 to Fig. 1 the effect of the catalyst HCl can be understood. In Fig. 2(a), a band appears at 1642 cm-’ corresponding to O-H bending [ 13- 151. The band is also present in Fig. 2(b) and (c), In all cases the band is stronger compared to the corresponding one in Fig. 1. This can be interpreted as the increasing degree of hydroxylation of the sample. This effect can be attributed to the acid catalyst. Thermal treatment of the MO-1.0 HCl sample led to a reduction of the bands as can be seen in Fig. 2(b) and (c). An important change is observed in the band initially found at 794 cm-’ in Fig. 2(a). This in Fig. 2(c) turns into two sharp and intense bands at 812 and 800 cm-‘. These bands correspond to MO-O-MO bonds in crystalline MOO, [21]. For this sample, phase separation is macroscopically observed. MOO, (black color) separates from SiOZ (white color). Fig. 2(d) corresponds to the FT-IR of MO-1.0 HCl calcined at 900 “C. The spectrum is typical of SiOz, showing bands at 1100

142

J. M&dez-Vivnr

/Materials

Chemistry

rind Physics

43 (1996)

140-144

(d)

(b)

ca

(a)

(b)

-I

(a)

4

3

2

I

o’., x IO3

WAVE NUMBER (cm-‘)

5.0

.5x10’

WAVE NUMBER (cm-‘)

Fig. 2. FT-IR spectra of MO-1.0 HCl: (a) xerogel, and calcined at (b) 300 "C, (c) 600 “C and (d) 900 “C.

Fig. 3. Diffuse reflectance spectra of solids calcined at 300 ‘C: (a) MO-0.1 and (b) MO-1.0 HCI.

cm-’ and 470 cm-‘. The additional strong band at 794 cm-’ corresponds to MO-O-MO bending in the new MO crystalline phase, Mo,O,, [ 18,211. According to the FT-IR results the effect of increasing the MO concentration predominates over the effect of the catalyst; i.e. differences between the FT-IR spectra of MO-0.1 and MO-0.1 HCl were not clear. The FT-IR results show that phase separation of MO from SiOZ occurs after calcination at 600 “C. An additional phase transformation for MO occurs at 795 “C [21], so after calcination at 900 “C, Mo80,, is obtained. SiOZ do not exhibit crystallization, and it is amorphous in all cases, including the solids calcined at 900 “C. Metal-oxide support interaction is weaker when silica is the support, compared to other supports such as alumina, titania and magnesia [22]. This weak interaction gives as a result in this work phase separation between MO and Si. Previous results from Leyrer et al. [23] show that calcination of physical mixtures of MOO, and Si02 did not produce homogeneous spreading of crystalline MOO, on SiOZ.

not show significant differences when non-catalyzed samples were compared to the HCl-catalyzed ones. In Fig. 3 the diffuse reflectance spectra of MO-0.1 (Fig. 3(a)) and MO-1.0 HCI (Fig. 3(b)), both calcined at 300 “C, appear. For both samples the bands appear in the UV region. For MO-0.1 a shoulder is observed at 3.7 x IO4 cm-‘. This is a charge transference band 0 +Mo, corresponding to ‘B, -+“B,(II) [24]. In the MO-1.0 HCI (Fig. 3(b)) spectra two slight signals appear. The first is a shoulder at 3.3 x IO4 cm-‘, assigned to ‘B, +2B2(II), also. The second band is at 4.5 x lo4 cm-’ and corresponds to 2B, + ‘E(IV) [24]. The spectra evolve as can be seen in Fig. 4, corresponding to the same samples calcined at 600 “C. The modification occurs only for MO-O. 1 (Fig. 4(a)), where the band shifts to 3.6 x IO4 cm-*. In Fig. 5 the spectra of solids calcined at 900 “C appear. Fig. 5(a) shows the MO-0.1 spectra. A single band appears, and it is better defined compared to the corresponding samples at lower calcination temperatures. It is at 4.5 x lo4 cm-’ and it is assigned to 2B2+2E(IV). In Fig. 5(b) the spectrum of MO-1.0 HCl exhibits two bands, both at higher energy regions compared to the corresponding spectra obtained at 600 “C. Bands appear now at 3.5 x lo4 cm-’ and 4.65 x IO4 cm-‘. These results suggest that the MO-1.0 HCl solid crystallizes faster than MO-O. 1. This can be attributed to the catalyst and to a higher MO concentration on the

3.2. DljGse rejlectnnce

All the results discussed in this section correspond to the 25 000-50 000 cm-’ region. This is because all the bands appeared in this zone. The spectra of xerogels did

J. Mhdez-

Viaar /Materials

Chemistry

and Physics

43 (1996)

140- 144

(b)

5.0

WAVE NUMBER

215

143

(b)

I

x10’

5 1

(cm-‘)

5 XIO’

WAVE NUMBER (cm”)

Fig. 4. Diffuse reflectance spectra of solids calcined at 600 “C: (a) MO-0.1 and (b) MO-1.0 HCI.

Fig. 5. Diffuse reflectance spectra of solids calcined at 900 “C: (a) MO-0.1 and (b) MO-1.0 HCI.

former sample. According to Kim et al. [25], the MOO,/ SiO, catalyst primarily contains crystalline MOO, because of the lower density and reactivity of the silica surface O-H groups. It is important to point out also that the nature and surface distribution of the molybdenum-containing species depend on the preparation method, molybdenum loading, calcination temperature, etc., as they control the physicochemical and catalytic properties of these solids. In order to compare the structure of both catalyzed and non-catalyzed solids, an X-ray diffraction (XRD) study was done for samples calcined at 300 and 900 “C. Unfortunately the MO content (O.l- 1.0 wt.%) was below the limit to be detected by this technique, using a Siemens D500 diffractometer. In all cases only amorphous SiO, was found. According to Rajagopal et al. [26], crystalline MOO, on SiOZ has been found for an MO loading of 2.4 wt.%, from XRD. This is a very high loading compared to the maximum employed here (1.0 Wt.%). From the previous discussion, FT-IR and UV-Vis spectroscopies show that the most appropriate samples to be tested in the future as catalysts are those calcined at 300 “C, where MO is still bonded to the Si network. Dehydroxylation of the SiOZ surface plays an important role in the stability of SiO,-supported MO, because after calcination at 600 “C both phase separation and silica

dehydroxylation occur. The existence of hydrogen bridging (MO . . OH-Si) on hydroxylated samples explains the stability of the solids calcined at 300 “C.

4. Conclusions HCl used as catalyst in sol-gel SiO,-supported MO significantly modified the spectroscopic (FT-IR and diffuse reflectance) properties of Si02. MO crystallization in MOO, and Mo80,, was detected by FT-IR. Based on the shifting of the charge transference bands of calcined samples, it can be proposed that the structure of the catalyzed solids is different compared to the non-catalyzed. The MO content was too low to be detected by XRD.

References [l] M.A. Bafiares and J.L.G. Fierro, Catal. Lett., J 7 (1993) 205. [2] Q. Sun, J.I. Di Cosimo, R.G. Herman, K. Klier and M.M. Bashin, Catal. Lett., 15 (1992) 371. [3] CL. Thomas, in Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970. [4] D.S. Kim, I.E. Wachs and K. Segawa, J. Catal., 146(1994) 268. [5] K.Y.S. Ng and E. Gulari, J. Caral., 9.2 (1985) 340. [6] L. Wang and W.K. Hall, J. Catal., 77 (1982) 232.

144

J. Mendes-Vivar

[7] C.J. Brinker and G.W. Scherer, in Sol-Gel istry

and Physics

of Sol-Gel

Processing,

/Materials Science:

Chemistry

the Chem-

Academic Press, San

Diego, 1990. [8] M.L. Larson and F.W. Moore, Inorg. Chem., 5 (1966) 801. [9] K.H. Tytko and U.1Trobish, in Katscher and F. Schroder (eds.), Gmelin Handbook of Inorganic Chemistry, Vol. B3a, Springer, Berlin, 1987, p. 1. [lo] C. Sanchez, J. Livage, J.P. Launay, M. Fournier and Y. Jeannin, J. Am. Chem. SOL, 104 (1982) 3194. [ll] K.H. Tytko and 0. Glemser, Adv. Inorg. Chem. Radiochem., 19 (1976) 239. [12] B.A. Morrow, IA. Cody and L.S.M. Lee, J. Phys. Chem., 80 (1976) 2761. [13] B. Bridge and M.D. Pate], J. Non-Cryst. Solids, 91 (1987) 27. [14] K. Nakamoto, in Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 3rd edn., 1978. [ 151 M.L. Larson and F.W. Moore, Inorg. Chem., 5 (1966) 5.

and Physics

43 (1996)

[16] J. Mendez-Vivar,

140-144

A. Campero, J. Livage and C. Sanchez, J. 121 (1990) 26. [ 171 T. Lopez, L. Herrera, J. MCndez-Vivar, P. Bosch, R. G6mcz and R.D. Gonzalez, J. Non-Cryst. Solids, 147 and 148 (1992) 773. [IS] J. MCndez-Vivar, Inorg. Chim. Acta, 179 (1991) 77. [19] J. MCndez-Vivar and T. Lopez, Mater. Clrox. Plrys., 34 (1993) 101. [20] F.L. Galeener, J. Non-Cryst. Solids, 49 (1982) 53. [21] J. Mendez-Vivar, T. Lopez, A. Campcro and C. Sanchez, Larlgmuir, 7 (1991) 704. 1221 S.R. Stampfl, Y. Chen, J.A. Dumesic, N. Chunming and C.G. Hil, J. Catal., 105 (1987) 445. [23] J. Leyrer, D. Mey and H. KnGzinger, J. Catal., 124 (1990) 349. [24] H.B. Gray and CR. Hare, Zno/*g. C/x,q 1 (1962) 363. [25] D.S. Kim, I.E. Wachs and K. Segawa, J. Catal., 146 (1993) 268. [26] S. Rajagopal, H.J. Marini, J.A. Marzani and R. Miranda, J. Catal., 147 (1994) 417. Non-Cryst.

Solids,