4780
J . Phys. Chem. 1986, 90, 4780-4784
significantly by the water desorbed from the oxides in the bed which could only be removed by slow diffusion and convection. In contrast, in a dry O2stream monolayer dispersion did not occur. Thus, not only is water vapor necessary for the dispersion to occur but also the volatilization of MOO, and its gas-phase transport can be rejected as possible mechanisms. The surface free energy change-assuming a mechanism of wetting of one solid by another-would certainly also depend on the surface hydration. However, the most plausible explanation of the effect of water vapor on the dispersion process can be based on the MOO, chemistry described by Glemser and co-worker~.~%~' These authors have shown that solid Moo3 reacts with water vapor according to the following equilibrium: MOO&) + H,O(g)
MoO,(OH),(g)
F=
(1)
The oxy hydroxide has an appreciable vapor pressure, much higher than that of MOO, itself, in the temperature regime applied. The gas-phase transport in the two-bed experiment can thus easily be rationalized, and gas-phase transport may indeed play a significant role during monolayer dispersion. However, surface diffusion with a concentration gradient providing the driving force for the surface transport of the M O O ~ ( O Hspecies )~ on the hydrated y-A1203 cannot be excluded. This process may in fact be the dominating transport mechanism in the physical mixtures, although the relative contributions of gas-phase and surface diffusion cannot be evaluated from the present data. Additional experimental work is required and presently carried out. Based on the solid/gas equilibrium (l), the surface chemistry which leads to the formation of surface molybdates can easily be rationalized. It is reasonable to assume that the M o 0 2 ( 0 H ) , species react with surface hydroxyl groups: Mo02(OH),(g)
+ 20H-(surface)
-
M~O,~-(surface)+ 2H20(g) (2)
The formation of monomeric tetrahedral Moo4,- in the initial stages of the dispersion as described above (see Figure 6) is thus easily explained. At higher surface densities of these monomeric (29) Glemser, 0.;Wendlandt, H. G. Angew. Chem. 1963, 75, 949. (30) Glemser, 0.;Wendlandt. H. G. Ado. Inorg. Chem. Radiochem. 1963, 5, 215. (31) Glemser, 0.;von Haesseler, R. Z . Anorg. Allg. Chem. 1962, 316, 168.
species, a two-dimensional condensation with creation of Mo-0Mo linkages and eventually the formation surface polyanions is a plausible process which has also been shown to occur in catalysts impregnated conventionally from aqueous solution.'*20It should also be mentioned that Sonnemans and Mars32and Fransen et al.33 have already applied the volatilization of MOO, by water vapor to deposit molybdenum species on the surface of various oxide supports. These authors have followed the movement of the concentration profile of the oxy hydroxide through a column containing the support oxide. These experiments, however, do not seem to have found much attention, and they have certainly not been considered in connection with monolayer dispersion or spreading until now. In conclusion, a necessary step for monolayer dispersion to occur in physical mixtures of crystalline MOO, and y-Al,O, seems to be the formation of MoO,(OH)~in the presence of water vapor. The transport mechanism is probably due to both surface diffusion in a concentration gradient and gas-phase diffusion. It is to be expected that the same mechanism is operative in mixtures of M o o 3 with other support oxides. These results shed some doubt on the interpretation of the dispersion phenomenon as wetting of one solid by a second solid under the action of forces of surface tension, at least for the presently studied system (Mo03/y-A1203)under the experimental conditions applied. It may be interesting to note that V,O5 also forms volatile oxy hydroxides under very similar reaction cond i t i o n ~ .One ~ ~ may ~ ~ ~therefore speculate that water vapor might also play an important role in the V2O5/TiO2system studied by Haber and c o - w ~ r k e r s and ' ~ ~ that ~ an analogous mechanism might be operative in this system.
Acknowledgment. This work was supported by grants from the Deutsche Forschungsgemeinschaft and from the Fonds der Chemischen Industrie. M.I.Z. is indebted to the Alexander von Humboldt Foundation for a research grant. Registry No. Moo3, 1313-27-5; CO, 630-08-0; H,O, 7732-18-5. (32) Sonnemans, J.; Mars, P. J . Curd. 1973, 31, 209. (33) Fransen, T.; van Berge, P. C.; Mars, P. Presented at the 1st International Symposium on Scientific Basis for the Preparation of Heterogeneous Catalysts, Brussels, 1979; paper D5. (34) Glemser, 0.; Muller, A. 2. Anorg. Allg. Chem. 1963, 325, 220. (35) Yannopoulos, L. N. J . Phys. Chem. 1968, 72, 3293.
Catalytic Activity and Structure of MOO, Highly Dispersed on SiOl Takehiko Ono, Masakazu Anpo, and Yutaka Kubokawa* Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: January 30, 1986)
The structure of Mo-Si oxides has been investigated by XRD, FT-IR, laser Raman, and photoluminescence techniques. At low Mo content, an X-ray amorphous phase is formed, which is characterized by surface molybdates dispersed on SiOz. The surface concentration of amorphous M o o 3 as well as the rate of oxidative dehydrogenation of CzH50Hshows a maximum at about 1 atom % Mo. The change in the rate runs parallel with the change in the concentration of amorphous MOO,, i.e. surface polymolybdate. The concentration of tetrahedrally coordinated Mo ions in Mo-Si oxides determined by the photoluminescence technique shows a maximum again at about 1 atom % Mo content, becoming zero in the range above 3 atom % Mo content. A good parallelism between the concentration of tetrahedrally coordinated Mo ions and the rate of the metathesis reaction of propene is observed.
Introduction The carrier effect is one of the most important problem in heterogeneous catalysis. When a metal oxide is on a carrier at small concentrations, its character is seriously modified, resulting in a change in its catalytic activity and selectivity. Although such a phenomenon has been observed by a number of workers, there remain various unresolved problems. Along this 0022-3654/86/2090-4780$01.50/0
line, we have investigated the structure of V-Ti' and Mo-Ti2 oxides by XRD and IR techniques and shown that at low V or Mo content an amorphous phase is formed, which is characterized (1) Nakagawa, Y.; Ono, T.; Miyata, H.; Kubokawa, Y. J. Chem. SOC., Trans, I 1983, 79, 2929, (2) Ono, T.; Nakagawa, Y.; Miyata, H.; Kubokawa, Y. Bull. Chem. SOC. Jpn. 1984, 57, 1205.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4781
MOO, Dispersed on Si02 TABLE I: Composition of Mo-Si Oxide Catalysts
surface catalyst area, m2/g Mo(0.5)-Si 41 40 Mo(1)-Si Mo(3)-Si 38 Mo(5)-Si 36 Mo(O.012)400 Si(grafted)" Mo(O.11)400 Si(grafted)"
Mo, M a 3 , Mo, Mo, atom % wt % rnmol/g pmol/m2 0.08 2 0.5 1.2 0.16 4 2.4 1 12 6.8 0.47 3 0.89 25 5.8 13 0.005 0.012 0.029 0.002 0.11
0.26
0.018
0.045
"See text. Prepared by the CVD method. by vanadate or molybdate dispersed on T i 0 2 in a monolayer. The resulting distorted structure brings about the weakening of V=O or M-0 bond, Le., enhanced activity for oxidative dehydrogenation of C2H50H. Furthermore, it has been shown in our laboratory that the photocatalytic reaction over MOO, supported on porous Vycor glass is closely associated with its photoluminescence attributed to the charge-transfer process (Mo6+=02-)
& (Mo5+-0-)* hv'
where tetrahedrally coordinated Mo ions play a significant role., In addition, it has been reported that tetrahedral Mo ions are important in the oxidation of alkenes over supported MOO, catalyst4 In the present work, in order to obtain a better understanding of the supported MOO, catalyst, we have undertaken an investigation of the structure of Mo-Si oxides by combining IR, laser Raman and XRD techniques with photoluminescence techniques, and we have attempted to clarify its correlation with the activity for oxidative dehydrogenation of C2H50Has well as the metathesis reaction of propene.
Diffraction angle 28Idegree Figure 1. X-ray diffraction intensities of MOO,in Mc-Si oxide catalysts (A) and in mechanical mixtures (B). The indices (hkl) denote the crystal planes of Moo3. Each diffraction line was integrated at 0.05' intervals of the diffraction angle for 20 s.
A
.-"-
$1 / I:50
Experimental Section Materials. The catalyst M o 0 3 / S i 0 2 (Merck, Fractosil 500, 41 m2/g) was prepared by the ordinary impregnation method using at pH 6-7 and an aqueous solution of (NH4)6M07024-4H20 subsequent drying followed by heating in air at 723 K for 22 h. The molybdenum oxide grafted onto S O 2was prepared by the reaction of gaseous MoC15 with dehydrated SiOz (Spherosil, XOA400,400 m2/g) at 773 K. The amount of Mo fixed on S i 0 2 was determined from the change in the concentration of MoC15 before and after the fixation by colorimetry. Details are found in the l i t e r a t ~ r e . ~Catalyst compositions used are shown in Table . I. C3H6and H2were of extrapure grade from the Takachiho Shoji Co. Commercial tank oxygen was purified by low-temperature distillation. C 2 H 5 0 Hof 99.5% purity was used without further purification. Apparatus and Procedure. X-ray diffraction patterns of the catalysts were obtained on a Rigaku Denki RAD-rA diffractometer using Cu K a radiation. A stepscanning method was applied for the quantitative analysis of the X-ray diffraction patterns. IR, FT-IR, and Raman spectra were recorded with a Hitachi G2 IR spectrometer, a Nicolet 7199 FT-IR spectrometer, and a JASCO N R - 1000 laser Raman spectrometer, respectively. Photoluminescence and its excitation spectra were measured with a Shimadzu RF-501 spectrofluorophotometer with color filters to eliminate scattered light of 77 K. Details of the experiment have been described e l s e ~ h e r e . ~ . ~ (3) Anpo, M.; Tanahashi, I.; Kubokawa, Y . J . Chem. Soc., Faraday Tram. 1 1982, 78, 2121. Anpo, M.; Tanahashi, I.; Kubokawa, Y. J . Phys. Chem. 1982, 86, 1. Anpo, M.; Kondo, M.; Kubokawa, Y.; Louis, C.; Che, M., to be submitted for publication. (4) Anpo, M.; Kubokawa, Y . J. CaraL 1982, 75, 204. (5) Louis, C.; Che, M.; Bozon-Verduraz, F. J. Chim. Phys. 1982, 79, 803. Che, M.; Dyrek, K.; Louis, C. J . Phys. Chem. 1985, 89, 4531.
>5
c
Mo Iatom 'lo Figure 2. Crystallinity of MOO, (A) and concentration of amorphous Mo oxide on SiOl (B) as a function of Mo content.
The C z H 5 0 Hoxidative dehydrogenation was carried out in a closed circulation system (dead volume of ca. 290 cm,). The C3H6 metathesis reaction was carried out in the same system at 473 K using 1-2 m2 catalysts, which had been reduced by H, at 753-823 K. The reaction products were quantitatively analyzed by gas chromatography. Results and Discussion Structure of Mo-Si Oxides. IR and Raman Spectra and XRD Studies. X-ray diffraction patterns of Mo-Si oxides showed only lines due to MOO,. The S i 0 2 used was found to be amorphous. N o lines due to a new compound between MOO, and Si02were observed. The intensities of the diffraction lines due to MOO, were compared with those of mechanical mixtures of MOO, and SO2. For Mo(0.5)-Si and Mo(1)-Si (the value in parentheses is the atom 5% Mo present) with very small intensities, the comparisons were carried out by a step scanning method using a RAD-rA diffractometer. The results are shown in Figure 1. For Mo(O.5)-Si the MOO, is mainly present as a X-ray amorphous phase, while its fraction is reduced to 80% with Mo(1)-Si. The
4782 The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
Ono et al. eV I
4.96
,
3.54
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5
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I
1100 loo0 900 800 Wavenumberl c d l
700
,\,
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250
350
450
5 50
Wavelength / nm Figure 5. Photoluminescence spectrum of Mo(1)-Si oxide catalyst at 77 K. Excitation spectrum was monitored at the 500-nm emission band. Photoluminescence spectrum was measured by 280-nm excitation.
1000
900
Wavenumber/&' Figure 3. IR and FT-IR spectra of Mo-Si oxide catalysts. (A) IR spectra: (a) Si02, (b) Mo(O.S)-Si, (c) Mo(1)-Si, (d) Mo(3)-Si, (e) Mo(S)-Si, and (f) Mo(20)-Si. (B) FT-IR spectra: (a) Mo(O.S)-Si, (b) Mo( 1)-Si. The absorption due to S i 0 2 was subtracted from those of Mo(O.S)-Si and Mo( 1)-Si catalysts. Values in parentheses denote atom % Mo.
1000 900 800 700 Wavenumber/crn-l Figure 4. Raman spectra of Mo-Si oxide catalysts: (a) Mo(1)-Si, (b) Mo(0.8)-Si, and (c) Mo(O.S)-Si. 1100
fraction of crystalline MOO, in Mo-Si oxides is shown in Figure 2 as a function of Mo content. The fraction increases with increasing Mo content, approaching about 90-95% at 3 and 5 atom % Mo. Since no line broadening was observed with Mo( 1)-Si to Mo(5)-Si oxides, their crystal sizes will be above 200-500 nm. IR spectra of Mo(0.5)- and Mo(1)-Si show bands at 800 and 1140 cm-' due to SiO, alone (Figure 3). In addition to those bands, the bands at 995 and 880 cm-' are observed with Mo-Si oxides containing more than 3 atom % Mo. FT-IR spectra have been studied with Mo(0.5)- and Mo( 1)-Si. Figure 3 shows the spectra obtained after subtraction of the absorption due to Si02. With Mo(O.5)-Si the bands appear at 995,960-40,925, and 905
cm-l. The band at 995 cm-' increases in intensity with Mo(1)-Si. Figure 4 shows the Raman spectra of the Mo-Si oxides. The bands due to MOO, appear at 997 and 820 cm-I. With Mo(OS)-Si the broad band at 980-950 cm-' appears in addition to the bands due to MOO,. The appearance of the bands similar to those in the FT-IR and Raman spectra of Mo(O.5)-Si (Figures 3 and 4) has been reported by a number of workers as follows: Raman spectra of Mo-Si oxides have been investigated by Cheng6 and Jeziorowskii: who observed bands at 981, 951, and 883 cm-I and at 970, 956, and 884 cm-l, respectively. These bands have been attributed to the surface polymolybdate species. A similar conclusion has been drawn by Seyedmonirs who observed bands at 970 and 925 cm-' in the FT-IR spectra of Mo-Si oxides. Recently we have shown that with V-Ti' and Mo-Ti2 oxides containing a low V and Mo content V205 or MOO, is highly dispersed on supports and present as an amorphous phase which is characterized by surface vanadate or molybdate. In addition, it has been r e p ~ r t e dthat ~ . ~polymolybdate ions such as Mo,O,,~and Mos0264-in solution exhibit Raman peaks at 960-950 cm-'. Thus, it is concluded that the amorphous phase observed by XRD studies in the present work is characterized by surface polymolybdate species dispersed on S O 2 . From the data shown in Figure 1, the amount of X-ray amorphous Moo3 per unit area has been calculated for each Mo-Si oxide (Figure 2B). Thomas et al.1° have also reported the presence of noncrystalline molybdate on SiOz using XRD and XPS techiques. In this work, the amount of X-ray amorphous MOO, increases with increasing Mo content, passes through a maximum, and then decreases. Under the assumption the amorphous MOO, is dispersed in a monolayer, its amount of per unit area can be regarded as the number of Mo ions present on the surface of SO,. If the (100) and (001) planes of MOO, are assumed, the number of Mo ions on the surface of MOO, crystals is calculated to be ca. 13 gmol/m2. Accordingly, with Mo(1)-Si which contains the maximum amount of amorphous MOO, (3.2 gmol/m2 of S O 2 ) and with Mo(O.5)-Si (2 wmol/m2 of SiOJ, the coverage of Mo ions present on the surface of S i 0 2 is estimated as about 25% and 15%, respectively. A further increase in the Mo content would result in remarkable crystallization to MOO,, although the X-ray amorphous phase is still present at 3 or 5 atom % Mo. As far as the results of XRD, IR, and Raman data are concerned, with Mo(0.5)- and Mo( 1)-Si catalysts, the X-ray amorphous phase seems to be mainly polymolybdate. Above 3 ( 6 ) Cheng, C. P.; Schrader, G. L. J . Coral. 1979, 60, 276. ( 7 ) Jeziorowskii, H.; Knozinger, H.; Grange, P.; Gajardo, P. J . Phys. Chem. 1980, 84, 1825. ( 8 ) Seyedmonir, R. S.; Abdo, S.; Howe, R. F. J . Phys. Chem. 1982, 86, 1233. (9) Griffith, W. P.; Lesniak, P. J. B. J . Chem. SOC.A 1969, 1066. (10) Thomas, R.; van Oers, E. M.; de Beer, V. H. J.; Moulijn, J. A. J . Catal. 1983, 84, 275.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4783
MOO, Dispersed on SiOz
ET' t 0.4 0.4 E
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Figure 6. Relative intensity of the photoluminescence spectra of Mo-Si oxide catalysts. (A) (a) Mo(1)-Si, (b) Mo(OS)-Si, (c) Mo(3)-Si, and (d) Mo(5)-Si. (B) Relative photoluminescence intensities as a function of Mo content. Experimental conditions were same to those in Figure 5.
atom % Mo, it may contain fine particles of MOO, (25 nm) in addition to polymolybdate. Raman spectra of Mo( l)-Si, however, show very high intensities of crystalline MOO, in spite of the presence of surface polymolybdate. This might be attributed to the fact that the crystalline M o o 3 shows much higher Raman intensities than surface polymolybdate. Chan et al." have reported that the intensity of the Raman bands of the crystalline WO, is much higher than that of the surface tungsten oxide species by two orders of magnitude. A similar situation seems to be likely in the Mo-Si oxide catalysts. Photoluminescence Studies. As shown in Figure 5 , Mo(1)-Si oxides show the photoluminescence spectrum having an emission maximum at 480 nm together with an excitation maximum at 290 nm. This is similar to the photoluminescence of Mo oxide supported on porous Vycor glass,j which has been attributed to the charge-transfer process (Mo6+=02-)
& (M$+-O-) * hd
It has been also reported that tetrahedrally coordinated Mo species on supports exhibit a maximum in the excitation at 260-280 nm.12'33 Accordingly, it is concluded that the photoluminescence observed with Mo( 1)-Si oxides is attributable to tetrahedrally coordinated Mo species, i.e., the intensity of the photoluminescence of Mo-Si oxides is proportional to the concentration of tetrahedral Mo ions. Figure 6A shows the photoluminescence spectra obtained with Mo-Si oxides of various compositions. Figure 6B shows the (11) Chan, S . S.; Wachs, I. E.; Murrell, L. L. J . C a r d . 1984, 90, 150. (12) I.wasawa, Y.; Ogasawara, S. Bull. Chem. SOC.Jpn. 1980, 53, 3709. Kazanskii, V. B. Kinet. Coral. 1983, 24, 1338. (13) Anpo, M.; Suzuki, T.; Tanaka, F. Yamashita, S.; Kubokawa, Y. J . Phys. Chem. 1984, 88, 5778. Kubokawa, Y.; Anpo, M. Adsorption and Catalysis on Oxide Surfaces, Che, M.,Bond, G. C., Ed.; Elsevier: Amsterdam, 1985; p 127.
In Mo(0.5)- and Mo( 1)-Si, isolated tetrahedral Mo ions should be present separately in a manner similar to that with Mo oxide grafted on S O 2 . Comparison of Figures 6B and 2 shows that the concentration both of the tetrahedral Mo ions and of the X-ray amorphous M o o 3 pass through maxima at similar Mo content. It seems likely that tetrahedrally coordinated Mo species are formed on a special part of the amorphous Mooj. As shown in Figure 6, the photoluminescence intensity remarkably decreases in the region above 3 atom % Mo, suggesting that a change in coordination of Mo ions from tetrahedral to octahedral takes place, in a manner similar to that observed with Mo-oxides supported on porous Vycor glass.I3 Such a change in coordination would result from a decrease in the coordination unsaturation and/or decreased covalency of the Mo-oxygen bond with increasing Mo (14) Iwasawa, Y.; Nakano, Y.; Ogasawara, S. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 2968. Iwasawa, Y.; Nakamura, T.; Takamatsu, K.; Ogasawara, S. J . Chem. SOC.,Faraday Trans. 1 1980, 76, 939.
4784
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
and cis- and trans-2-butene, the ratio of three butenes being nearly in thermodynamic equilibrium, i.e., 13:32:55 for 1:cis:trans at the reaction temperature (473 K).15 With increasing Mo content, the rate increases, passes through a maximum, and then decreases in a manner similar to that for oxidative dehydrogenation. However, above 3 atom % Mo content the features of both reaction are different; the rate becomes zero for the metathesis reaction. Such a feature resembles closely the change in concentration of tetrahedrally coordinated Mo species rather than that of amorphous MOO,. Accordingly, it is concluded that tetrahedral Mo ions constitute the active species for the metathesis reaction. The appearance of activity after the reduction of the catalyst suggests that the active species is
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content, as described previ0us1y.l~ Futher work is necessary to settle the problem. Catalytic Activity for Oxidative Dehydrogenation of C2H50H and Metathesis Reaction of C3H6. Figure 7 shows the rate of oxidative dehydrogenation of C 2 H 5 0 Hand selectivity toward C H 3 C H 0 formation as a function of Mo content. The major product is C H 3 C H 0 , the remainder being CH3COOC2H5and (C2H5)2O. The selectivity toward C H 3 C H 0 formation is more than 90%. The rate passes through a maximum around 1 atom % Mo and then decreases, but it still remains considerable in the range 3-5 atom % Mo. As described above, the concentration of the X-ray amorphous phase shows a maximum at similar Mo content, suggesting that amorphous MOO,, Le., polymolybdate, plays a significant role in oxidative dehydrogenation. The considerable activity observed in the range above 5 atom % Mo suggests that crystalline MOO, also participates in the reaction, though its activity is less than amorphous MOO,. As described above, with Mo(O.5)-Si where 100% MOO, is present as an amorphous phase, the number of Mo ions on the surface of SiOl is about 15% of the number of Mo ions on the surface of crystalline Moo3. Since the rate of oxidative dehydrogenation is 0.26 pmol m-2 min-' for Mo(O.S)-Si and 0.77 @mol m-2 min-' for crystalline MOO,, the efficiency of Mo ions for oxidative dehydrogenation with amorphous MOO,, i.e. polymolybdate, is about twice the corresponding efficiency with the crystalline Moo3. The results obtained with the metathesis reaction are shown in Figure 8. The products were composed of ethene, I-butene,
O=Mo
/O-
'0-
The metathesis reaction was examined with Mo(O.1 1)-Si(grafted) oxide comprised of tetrahedral Mo ions alone. As would be expected, a remarkably high activity for butenes formation (1.3 lmol m-* h-I) was observed, being about 30 times higher than that with impregnation catalysts. One of the reasons Mo-Si(grafted) is active for the metathesis reaction might be the isolated character of its tetrahedral Mo ions, which facilitates formation of the carbene intermediate and/or suppression of the decay of the carbene intermediates. The results described above suggests that a similar isolated situation would be realized for tetrahedral Mo ions present on amorphous MOO, in the Mo-Si oxides prepared by the impregnation method. The results clearly indicate that tetrahedrally coordinated Mo species are not as active for oxidative dehydrogenation of alcohol. Such features would be expected from the recent work of Iwasawa et alei6who reported that the isolated tetrahedral Mo ions is less active for oxidative dehydrogenation as compared to species where two adjacent Mo ions are present. Finally, it should be stressed that the present work confirms the important role of amorphous MOO, in oxidative dehydrogenation.
Acknowfedgment. The authors thank Mr. Hiroyuki Kamisuki and Miss Tomoko Hamada for carrying out part of the experiments. The authors thank Dr. Koji Ohta (Government Industrial Research Institute, Osaka) for FT-IR measurements and helpful discussion. Registry No. Moo3, 1313-27-5; C 2 H 5 0 H , 64-17-5; C3H6, 115-07-1. (15) Meyer, E. F.; Stroz, D.G.J . Am. Chem. SOC.1972, 94, 6344. (16) Iwasawa, Y.;Tanaka, H.Proc. Inf. Congr. Cufal.,8fh 1984, 4, 381.
