V2O5-SiO2 Catalyst Prepared by the sol–gel Process in the Oxidative Dehydrogenation of n-butane

1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmonet al., editors,

669

V205-SIO2 Catalyst P r e p a r e d by the sol-gel P r o c e s s in the O x i d a t i v e D e h y d r o g e n a t i o n o f n-butane E.L. S h a m ab, V. Murgia ab, J.C. Gottifredi ab and E.M. Farffin-Torres a a INIQUI-CONICET, Consejo de Investigaci6n

b Facultad de Ingenieria Universidad Nacional de Saita, Buenos Aires 177, 4400 - Saita, Argentina ABSTRACT Vanadium silicate gels (V205-8iO2) were prepared by the sol-gel method by hydrolysis of vanadium acetilacetonate and silicon alkoxide. The changes in the vanadium species and its interaction with SiO2 network upon heat treatment at various temperatures was studied by means of XRD, XPS and IR. The catalytic behavior for the oxidative dehydrogenation of nbutane was studied. It was found that hydration degree of gel derived catalyst strongly influenced its catalytic activity. 1. INTRODUCTION Sol-gel methods have been used by different authors working on catalysts preparation (1). The advantages of the applications of these methods to the design of catalytic materials have been already described in the literature (1,2). Catalysts based on supported vanadium oxide are frequently used in selective oxidation reactions due to their considerable activity for the oxidation of aromatics, alkanes, and alcohols (3-5). Differences in their catalytic behaviour are generally explaines on the basis of the nature and distribution of vanadium species, which are influenced by the vanadium loading, the preparation procedure and the acid-base character of the support (6-9). The interaction of vanadia with silica and the structure of the VOx units on the support have been studied by a number of investigators (5,7,9-17). It has been observed that in general highly dispersed monomeric surface vanadia species were found on silica-supported catalyst with very low loadings of vanadia (7,12-13). Crystalline V205 has been detected on catalysts well bellow the so-called "monolayer" coverage as the vanadium loading was increased. The formation of crystalline V205 in the V205/SIO2 system reflects the weaker interaction of vanadium oxide with the SiO2 supports relative to others like A1203 and TiO2 (5,14,18).

670 To obtain a catalyst with V-SiO2 interactions stronger than those developed in impregnations solids we have studied the preparation of these materials by means of the solgel process. Neuman et al. have previously prepared amorphous metallosilicalite xerogels by the sol-gel method using metal alkoxides precursors (4). This work have shown that vanadium silicate xerogel are active catalysts for the activation of aqueous hydrogen peroxide for a variety of reactions including epoxidation of alquenes, oxidation of secondary alcohols to ketones and the hydroxylation of phenol. In the present work we report the obtention of VOx-SiO2 catalyst from vanadium acetyl acetonate and tetraethoxysilane. These catalysts were characterized and evaluated in the oxidative dehydrogenation of n-butane.

2. E X P E ~ M E N T A L

2.1. Catalyst Preparation Tetraethoxysilane (TEOS) and vanadium acetyl acetonate (VAA) were used as precursor to prepare gels containing 1.4 weight % of vanadium. The catalysts preparation is based on a two-step procedure. In the first stage VAA was dissolved in methanol, given rise to a clear green solution, and mixed under stirring with TEOS. In the second stage this mixture was refluxed at least two hours. Then hydrolysis and condensation were completed to form the gel by the addition of one water equivalent and ammonium hydroxide (25%). The gel is then washed with water and dried at 60 ~ to obtain a fine yellow green powder. The dried gel was calcined in air increasing the temperature with a speed of 10~ min -i and then kept at a fmal temperature of 500, 600 or 700~ during 16 hours. Samples prepared in this way were named SiVA-x ~ were x ~ is the calcination temperature or SiVA-TS. The gel was also activated by a sudden heating at 700 ~ during a few minutes and then calcined at 500 ~ and named SiVA-TS (TS = thermic shock). A reference catalyst with the same vanadium content was also prepared by impregnating SiO2 (Aldrich 300 m 2 g-l) with ammonium methavanadate solutions. The solid named 1.4 V/SiO2 was dried at 60 ~ and calcined for 16 hours at 500~

2.2. Catalyst Characterisation X-ray diffi'action (XRD) patterns were collected in a Rigaku diffractometer DMax-IIC using Cu kc~ radiation (3~ = 0.1549 nm). Diffuse reflectance spectra (DR) in the UV-Visible region were obtained with a GBCUV/Vis 918 equipped with an integer sphere and using BaSO4 as blank reference. Infrared spectra were recorded between 4000 and 400 cm-1 with a Bruker IFS 88 on samples dispersed in KBr and pressed into thin wafers. The XPS spectra were taken in a Physical Electronics 5700 ESCA spectrometer. The exciting radiation was Mg Kc~ (1253.6 eV). The binding energies (BE) were calculated with respect to C 1s peak set at 284.5 eV.

671 2.3. Catalytic Test N-butane oxidehydrogenation (ODH) studies were performed in an isothermal stainless steel microreactor (1I) 0.9 cm) fed with a mixture 15/20/65 of oxygen, n-butane and nitrogen respectively at a constant rate of 153 ml min l. In all cases a constant amount of catalyst (0.4 g ) diluted in 0.8 g of quartz was charged in the reactor. Particle sizes were 500-800 g m . Two layers (0.5 cm height) of quartz particles one above and one below the bed completed the reactor. Temperature was recorded from a moving centreline thermocouple (type K). The resulting catalytic volume and W/F were 1.2 cm 3 and 6.26 g cat h/tool (n-butane) respectively. The reaction temperature range was varied (400-600 ~ to achieve different conversion levels. Blank tests on the same reactor showed no activity in all the range of temperatures studied. Reactants and reaction products were analysed by on-line chromatography using a Varian 3700 and a Shimadzu GC-3BT chromatographs for heavy and light products respectively. A 25% dimethyl sulfolane in Chomosorb P 60/80 (15m) and 32% silicone oil DC 200/50 in Chomosorb W 60/80 (5m) nylon column was used in the first case and a molecular sieve 5A 60/80 stainless-steel column (2 m) in the second case.

3. RESULTS

3.1. Catalyst Characterisation The 1.4 V/SiO2 catalyst and SiVA calcined materials show XRD patterns of amorphous solids with a very broad diffraction line centre near 4.06 A characteristic of amorphous silica. In the case of the gel dried at 60 ~ (SiVA-60~ four low intensity diffraction lines at 5.876 A; 4.131 A; 3.761A. and 3.160 A were also observed. These lines corresponds to a vanadium hydrated gel developed on the surface of SlOE (19-20). The DR spectra in the UV-Vis region of SiVA solids are shown in Fig.1. SiVA-60~ is a green yellow powder, (Fig. 1.A) and shows an broad absorption band at 229.3 nm related to SiO2 gels, and two shoulders centred at 288.5 and 347.8 nm corresponding to V 5+ in tetrahedral environment (3,4). After calcination at 500 ~ a broad absorption band with two maxima at 265.0 and 389.3 nm are observed corresponding to V 5+ ions with tetrahedral and octahedral co-ordination respectively. When the calcination temperature is increased at 600~ the presence of only an absorption band at 280.0 nm shows that the V 5+ ions in tetrahedral environment are the principal species present in this solids.( Fig. 1C and 1D). The DR spectrum of SiVA-TS sample (Fig. 1E) is very similar to that of SiVA-500~ showing that the thermal treatment employed in this case does not produce a noticeable change in the developed vanadium species. UV-Vis DR spectra of SiO2 and 1.4V/SIO2 catalyst before and after calcination together with SIVA-500~ are presented in Fig.2. 1.4V/SiO2 before calcination (Fig. 2B) present an absorption band typical of SiO2 (213 nm) and the presence of both tetrahedral (268 nm) and octahedral (broad band in the 350-500 nm region) V s+ species. After calcination (Fig. 2C) SiO2 band disappear and tetrahedral and octahedral V 5+ species are only observed. This spectrum is very close of SiVA-500~

672

till (J

z

-D

El

D~ 0

9C -B

w

~D

2OO

4OO

WAVELENGTH

600

{n m )

A

BOO

Fig.1 DR UV Vis spectra of (A) SIVA60~ (B) SiVA-500~ (C) SiVA-600~ (D) SiVA-700~ and (E) SiVA-TS

----- A

260

I

4o0

....

I

I

'

WAVELENGTH {nrn)

obo

Fig.2 DR UV Vis spectra of (A) SiO2; (B) 1.4V/SiO2 60~ (C) 1.4V/SiO2 500~ and (D) SiVA-500~

Infrared spectra of SiVA materials in the 2000 to 400 cm -1 region are depicted in Fig.3. On the SiVA-60~ spectrum (Fig.3A) we can see the following absorption bands: 1636 cm l belong to physisorbed water; 1400 cm -1 and 669 cm 1 belong to residual ammonia, 1080 cm l and the shoulder at 1217 cm-1 assigned to v -Si-O-(Si) vibration; 958 cm l assigned to v-SiO-(H) and v-Si-O-V, 800 cm -1 belong to ring structures and valence vibration of silicon; and 519, 509 and 459 cm-1 assigned to O-Si-O bending deformation (22,23). The IR spectra of calcinated SiVA show that the intensity of the band at 1639 cm ~ decrease for sample heated at 500~ (Fig.3B) and disappear for higher calcination temperatures. The bands corresponding to v-Si-O-(H) vibrations becomes narrower and shift to 1082, 1103, and 1107 cm -1 for SiVA-500~ SiVA-600~ and SiVA-700~ respectively. The band corresponding to v-Si-O-(H) is very sensible to the heat treatment and shift to 970 cm l for SiVA-500~ and to 933 cm -] for samples calcined at higher temperatures. The intensity of bands belonging to residual ammonia (1400 and 669 cm -1) decrease with the calcination temperature being completely absent for SiVA-700~ (Fig.3D). In any case the Si-O-Si bending deformation bands shift from the corresponding values observed for SIVA60~ The IR spectrum of SiVA-TS sample (Fig.3E) is very similar to that of SiVA-500~ but bands belonging to residual ammonia are completely absent in this case. Infrared spectra of SiO2 and calcined V1.4/SiO2 are presented in Fig.4. Commercial SiO2 support present the same bands observed for SiVA-600~ (Fig.4A). When vanadium is

673 introduce on the support the v-Si-O-(Si) and v-Si-O-(H) band shift slightly to lower wavenumbers due to the weak interaction of vanadia with the silica support.

E la,l m

m

2006 ..... ~6bo ' ~2~ ' 8 o o WAVENUMBER (r

'

Fig. 3. IR spectra of (A) SiVA-60~ (B) SiVA-500~ (C) SiVA-600~ (D) SIVA700~ and (E) SiVA-TS

4oo 2oo0

~

~2oo

WAVENUMBER

800

(era - I )

400

Fig. 4. IR spectra of (A) SiO2; (B) 1.4V/SiO2 500~ and (C) SiVA-500~

Bands related to vanadia species like g o 4 or V=O, could not be distinguish in the IR spectra due to the overlapping of this bands with the corresponding to the SiO2 support. The binding energies determined from XPS spectra together with the V/Si atomic ratios are presented in Table 1. Table 1. Binding energies and V/Si atomic ratios determines for SiVA and 1.4V/SIO2 catalysts BE (eV) Catalyst

V2p3/2

Si2p

SiVA-60~ SiVA-500~ SiVA-600~ SiVA-700~ SiVA/TS 1.4V/SiO2/60~ 1.4V/SiO2/500~

515.50 516.30 516.20 516.70 516.40 516.40 516.20

102.60 103.12 103.10 103.00 103.25 102.50 102.80

Surface Atomic Ratio V/Si 0.38 0.048 0.054 0.066 0.050 0.057 0.050

674 The most relevant features of the BE values of the V2p3/2 levels for the SiVA catalysts are as follows: (i) For SiVA-60~ the low BE values observed indicate that VO + species can be present in dried gels before calcination (24). ii) When SiVA gel is calcined BE corresponding to VO4 species were observed for all the calcination temperatures, iii) For impregnating catalyst BE of VO4 units were also determined. Regarding to BE of the Si 2p levels it can be observed that in the case of SiVA-60~ gel silicon may be present forming ring or chain silicates (24). When SiVA gel is calcined BE typical of SiO2 gel were detected. The atomics ratios V/Si were determined from the integral of the signal corresponding to V2p3n and Si 2p levels. These atomic ratios can be taken as a measure of the relative dispersion of vanadium in this support (25). From values reported in Table 1 we can observed that SiVA-60~ gel present a high vanadium surface concentration showing that vanadium is essentially distributed on the surface of SiO2 particles or the silicates formed upon hydrolysis. After calcination at 500~ the V/Si ratio strongly decrease indicating that a new solid organisation is achieved and V is now also dispersed in the pores of a S i Q network. When calcination temperature is increase to 600 and 700 ~ an increase of surface vanadium dipersion is produced as consequence of the loss of specific surface area due to dehydration of SiO2 network and also to vanadiumsegregation from the micropores to the surface of the support 1.4V/SiQ catalyst calcined at 500 ~ and SiVA-500~ present near the same vanadium dispersion, so we can expect to observe similar catalytic behaviours for these two materials. 3.2. Oxidative Dehydrogenation of n- butane The predominant products in the oxidation of n-butane were dehydrogenation products (1butene, cis- and trans-2-butene and 1,3-butadiene) and combustion products (carbon oxides). Small amounts of lower hydrocarbons were also detected. Figure 5 shows the dependence of conversion on reaction temperature. Over the range of studied temperatures the conversion was found to be higher on the SiVA-500~ and SiVA-TS catalyst than the SiVA-600~ SiVA-700~ and 1.4V/SIO2 samples.

30 25. 20o

"~ 15 L_

> C 10 o

35O

i

~

- -

zOO

i

|

5O 500 55O Temperature (*C)

600

Fig.5. Variation of n-butane conversion % with the temperature of" (11) SiVA-500~ SiVA-600~ (A) SiVA-700~ (v) SiVA-TS and (O) 1.4V/SIO2

(O)

675 Table 2 shows the DHG selectivity values measured at the reaction conditions investigated for these catalysts. As indicated in this table, the 1.4V/SiO2 sample was the more selective catalyst, followed by SiVA-500~ solid. However, the conversion levels were lower for the impregnated catalyst than those observed for SiVA-500~ sample. SiVA-TS catalyst developed near the same activity of SiVA-500~ solid but its DHG selectivity is lower. SiVA-600~ and SiVA-700~ were the less selective catalyst of the studied series. Table 2. Reaction Data for SiVA and 1.4V/SiO2 Catalyst Catalyst

Temp (~

Conversion %

Selectivity % Cracking 1.14 4.72 4.71

DHG 44.16 54.54 53.74

SiVA-500~

494 567 575

17.28 26.29 26.01

Combustion 54.7 40.74 41.55

SiVA-600~

492 566 574

5.39 16.43 17.93

92.74 83.55 77.7

1.77 1.,89

7.26 14.68 20.41

SiVA-700~

494 567 575

2.77 11.63 12.71

87.75 82.09 73.15

2.59 2.49

12.25 15.53 24.37

SiVA-TS

494 567 574

15.49 27.07 28.11

66.06 47.63 46.05

1.29 3.7 3.83

32.65 48.67 50.12

1.4 V-SiO2

490 567 574

2.87 15.53 17.16

23.34 32.67 32.84

2.83 3.29

76.66 64.5 63.87

4. D I S C U S S I O N

From DR, FTIR and XPS results we can conclude that the vanadium acetyl acetonate is completely hydrolysed under the reaction condition employed in the synthesis of vanadium silicate gels. A vanadium oxihydroxide layer interacting with a silicate oligomer derived from TEOS hydrolysis is obtained after drying the gels at 60 ~ After heat treatment at 500~ a rearrangement of gel network takes place and a dispersion of VO4 unit in a SiO2 gel network is obtained. From IR results it can be observed that the absorption band belong v-Si-O-(H) which normal absorption takes place around 980 cm -1 (for highly ordered dehydrated silica) is strongly shift to 958 cm ~ for SiVA-60~ This shifting can be related to the substitution of hydrogen for vanadium in the silanol groups, which can produce a change in the position of

676 the band more pronounced than in the case of deuteration of silanol groups (26). However the observed absorbance position shift could also be due to the formation of metastable intermediate products of polycondensation of silica gel, for example trimmer or tetramer (23). This is the most probable situation because when vanadium silicate gel is calcined at 500~ (SiVA-500~ a shift to 970 cm -1 was observed for v-Si-O-(H) band related with a polycondensation of silica gel network and the formation of VO4 units on the surface of support (23). While, if the absorption band at 958 cm -1 should be related with Si-O-V groups this mild heat treatment must not produce any change on its position and intensity. Furthermore XPS and DRX results agree with these conclusions, because for SiVA-60~ material the BE energies of V2p3/2 level correspond to the presence of VO + species (probably a vanadium oxihydroxide from XRD), and the BE for Si2p belong to tetramers of SiO2. The SiVA-60~ powder is then a silicate highly hydrated cover by a layer of a vanadium oxihydroxide gel, as denoted by the position of the broad absorption band at 1080 and 1216 cm -1 belong to v-Si-O-Si in tetrahedron silica well organised network and the high V/Si surface atomic ratio determined from XPS studies. The calcination of SiVA materials at 500 ~ produce only a slight shift of the v-Si-O-Si band to 1082 cm -1 without changes in the band intensity. However, the v-Si-O-H band shifts to 970 c m 1 showing that at this temperature a solid state reaction between the vanadium oxihydroxide layer and the silica oligomers takes place. In consequence the v-Si-O-H band loss intensity and shifts to wavenumbers values typical of VO4 bridged by water molecules on silica (3). This is also confirm by the BE of V2p3/2 level and the disappearance of the vanadium oxihydroxide diffraction lines in the XRD patterns. This model is consistent with the conclusions of Yoshida et al. (15) deduced from XANES/EXAFS spectroscopy and also with that of Narayama et al. (27) by spin-echo modulation. However, octahedral species are also present, because a broad band in the region of 400-550 nm was observed in the UV-Vis spectrum of SiVA-500~ (Fig.lB), which is assigned to a change transfer band in VO6 clusters (28). From XPS, DR and IR studies it can be seen that the catalyst obtained by calcination of SiVA gel at 500~ presents the same surface vanadium dispersion and is structural similar to the reference catalyst 1.4V/SIO2/500~ The principal differences between both catalyst are the structure arrangement and hydration degree between the SiO2 network obtained by the solgel synthesis and the commercial silica. When the calcination temperature increase the v-Si-O-Si band becomes more intense and narrow and shift to higher wavenumbers values (1102-1103 cm-1). The observed increase in the definition and intensity of this band is produce when water is release from SiO2 network producing that S i-O-H groups change to S i-O-Si groups. These results show that the silica network developed by sol-gel process is really well organised, because only in this case this behaviour is observed. For randomly crosslinked SiO2 gels negative shift of the band and a strong intensity decrease must be observed. Another fact that confirm the former observation is that the 800 cm l band related with ring silicate structures is unaltered upon heat treatment. For SiVA-600~ and SiVA-700~ catalyst it is also observed that the band at 970 cm -l belong to v-Si-O-V absorption shift to 933 cm -1. As in these solids the dehydration degree is strong we can assign this band to V=O species adsorbed on basic sites. This band is more

677 intense and narrower than that observed for SiVA-500~ because there no more or very few contribution of silanols groups to the absorption as was confirmed from v-Si-O-Si band shape. SIVA-TS catalyst is very similar in structure and vanadium dispersion to SiVA-500~ oxide. The only difference between these materials is that the hydration degree seems to be lower in the SiVA-TS solid as shown in IR and DR results. In DR spectra, it can be seen that a more important contribution of tetrahedral vanadium species seems to be present. This probably can arise because the short heat treatment at 700~ produce an important dehydration allowing that a more important fraction of octahedral species transforms to tetrahedral co-ordination. Reaction Studies

As was shown from characterisation results SiVA-500~ SiVA-TS and 1.4V/SiO2/500~ present almost the same surface vanadium species and dispersion. It can therefore be expect to observe the same catalytic behaviour for this three solids. As predicted selectivity towards dehydrogenation products is very close for the three catalysts. However, from Fig.5 it can be seen that 1.4V/SiO2/500~ is markedly less active than both SiVA catalyst. This difference in catalytic activity can be related with the hydration degree of SiO2 supports. Results reported by Le Bars et al. (29) show that acidic hydroxyl groups present on V/SiO2 catalyst could be related with hydrated forms. These authors also report that an increase in the acidity of silica-supported vanadium oxides enhance the rates of oxidative dehydrogenation of ethane but does not affect the selectivity to ethene. Then, as in SiVA-500~ and SiVA-TS catalyst the hydration degree is higher than that observed for 1.4V/SiO2 oxide, the SiVA oxides must be more acidic and consequently more active as was determined. For SiVA oxides calcined at 600 and 700~ the hydration degree is lowered and SIVA700~ oxide behave as impregnated catalyst, confirming that activity is controlled by the acidic hydroxyl groups content. In the case of SiVA-600~ and SiVA-700~ catalyst changes in selectivity were also observed. However, the vanadium dispersion is higher for these solids. Then, changes in selectivity must be correlated with the increase in VO4 groups after dehydration. Then it becomes clearly that both octahedral and tetrahedral species must be present on vanadium-silica catalyst to achieved good selectivities. CONCLUSIONS Our research results show that vanadium-silica catalyst prepared by sol-gel method are active catalyst for the oxidative dehydrogenation of n-butane. This catalyst was more active that an impregnated catalyst with the same vanadium loading. Differences in activity were attributed to the development of a hydrated vanadiasilica catalyst when sol-gel process is used. Derived sol-gel catalyst with good activity and selectivity are obtained when the vanadiumsilica gels were calcined at 500~ Selectivity toward dehydrogenation products was proven to depend on the ratio of tetrahedral to octahedral vanadium species. Both type of species must be present to obtain good selectivities to olefin production.

678 REFERENCES 1. M.A. Cauqui and J.M. Rodriguez-Izquierdo, J.Non-Cryst. Solids, 1475.148(1992)724. 2. G.M. Pajonk, Appl.Catal.,72 (1991) 217. 3. P. Concepcidn, J.M. L6pez Nieto, J. P6rez Pariente, J. Molec. Catal. A: Chem., 99 (1995) 173. 4. R. Neuman and M. Levin-Elad, Appl. Catal. A: Gen., 122 (1995) 85. 5. S.T. Oyama, K.B. Lewis, A.M. Carr and G.A. Somorjai, Proced. 9th Intern. Congr. Catal., 3 (1988) 1473. 6. T. Blasco, J.M. L6pez Nieto, A. D6joz and M.I. Vfizquez, J. Catal., 157 (1995) 271. 7. S. Narayam and B. Prabhu Prasad, J. Molec. Catal. A: Chem., 96 (1995) 57. 8. E.A. Mamedov and V. Cort6s Corberfin, Appl. Catal. A: Gen., 127 (1995) 1. 9. T. Kataoka and J.A.Dumesic, J. Catal., 122 (1988) 66. 10.L. Owens and H.H. Kung, J. Catal., 144 (1993) 202. 11.J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 102 (1986) 52. 12.G. Deo and I.E. Wachs, J. Phys. Chem., 95 (1991) 5889. 13.S.T. Oyama, G.T. Went, K.B. Lewis, A.T. Bell and G. Somorjai, J. Phys. Chem., 93 (1989) 6786. 14.M. Akimoto, M. Usami and E. Echigoya, Bull. Chem. Soc. Japan, 51 (1978) 2195. 15.S. Yoshida, T. Tamaka, Y. Nishimura, H. Mizutani and T. Funabiki, Proceed. 9th Intern. Congr. Catal., 3 (1988) 1473. 16.D. Miceli, F. Arena, A. Parmaliana, M.S. Sawrell and V. Sokolovskii, Catal. Lett., 18 (1993) 283. 17.M.M. Koranne, J. G. Goodwin Jr. and G. Marcelin, J. Catal., 148 (1996) 378. 18.J.E. Wachs and F.D. Hardcastle, Peoced. 9th Intern. Cong. Catal., 3 (1988) 1449. 19.F. Babonneau, P. Barboux, F.A. Josien and J. Livage, J. Chem. Phys., 82 (1985) 48. 20.M. Khairy, D. Tinct and H. Van Damme, J. Chem. Soc. Chem. Commun., (1990) 856. 21.J.G. Eon, R. Olier and J.C. Volta, J. Catal., 145 (1994) 318. 22.J.J. Fripiat, A. Leonard et N. Barak6, Bull. Soc. Chim. France, (1963) 122. 23.D. Niznansky and J.L. Rehspringer, J. Non- Cryst. Solids, 180 (1995) 191. 24.J. Chastain (Eds.) , Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corporation, 1992. 25.J.M. L6pez Nieto, G. Kremenic and J.L.G Fierro, Appl. Catal., 61 (1990) 235. 26.Ji.G. Fierro (Eds), Spectroscopic Characterisation of Heterogeneus Catalyst, Catalysis and Surface Science Series, Vol.57, Elsevier, Amsterdam, 1990. 27.M. Narayoma, C.S. Narasimham and L. Kevan, J. Catal., 79 (1983) 237. 28.H. Praliaud and M.V. Mathieu, J. Chim Phys., 73 (1976) 689. 29.J. Le Bars, J.C. V~drine, A.Auroux, S. Trautmann and M. Baems, Appl. Catal. A, 88 (1992) 179.