J . Phys. Chem. 1986, 90, 4775-4780 13CH2=CHCD0 and CD2=CH13CH0. Each of the two deuterium signals in CD2=CH13CH0 will be split into a doublet due to the vicinal coupling between D and 13C. The coupling constants ( J C D ) can be calculated from the results obtained for 3JCHby Vogeli et al.*' from the following equation: 3JCD,calcd
=
3JCH
YH/YD
(3)
where y H and y D are the gyromagnetic ratios for a proton and a deuteron, respectively. The experimental results presented in Table VI are in good agreement with those calculated from eq 3. The coupling constant for the trans deuterium is 2.4 H z (2.5 from 3JCH) and 1.6 Hz (1.6 from 3JcH)for the deuterium cis to the carbonyl carbon. Using these coupling constants, it is possible to assign the chemical shift at 6.42 ppm to the deuterium trans to the carbonyl carbon (D,) and the chemical shift at 6.24 ppm to the cis deuterium (D2). The small signal at 6 6.38, Figure 8c, can be assigned to acrolein with a deuterium a t C-2 by comparing the results obtained for the oxidation of CH2=CDCH3 (Figure 11). Only one signal, corresponding to CH2==CDCH0, is detected (6 6.38). The DEFT I3C spectrum (Figure 12) also indicates that a deuterium atom is located a t C-2. This very minor product is produced by a mechanism not yet explained. The ratio of the various forms of deuterated acrolein, CD2= CHCHO and CH2=CHCD0, were calculated by comparing the amount of D2 (preferred due to better resolution) or D, with that of -CDO. The data summarized in Table VI1 indicate that a 50:50 ratio was obtained for the oxidation of CD2=CHCH3, CH2=CHCD3, and I3CH2=CHCD3 over iron-antimony catalysts. This suggests that a common intermediate is formed from CD2=CHCH3 and CH2=CHCD3. As suggested earlier, this intermediate is a symmetrical ?r-allylic intermediate.
4775
This 5 0 5 0 ratio also agrees with the results reported by Burrington et al." They observed very nearly equal amounts of CD2=CHCH0 and CH2=CHCD0 produced during the oxidation of CH3CH=CD2 over antimony-based catalysts. They suggest that this ratio is a result of an irreversible addition of oxygen to the r-allyl intermediate to form the o-allyl intermediate. This sequence provides a reasonable explanation for our suggestion that with antimony-based catalysts oxygen is added before the second hydrogen is abstracted. Another possible explanation of the 50:50 ratio of the deuterated acrolein would be rapid double bond isomerization of propylene before the *-allylic intermediate is formed. This possibility can be ruled out, however, by examining the position of D (or I3C) in the recovered unreacted propylene. The data in Table VI1 indicates that less than 8% of deuterium scrambling takes place before reaction. The I3C data indicates that less than 2% double bond isomerization takes place. Thus, the evidence is quite conclusive that the 50:50 ratio cannot be explained by a rapid isomerization of propylene.
Conclusions The selective oxidation of propylene over iron-antimony catalysts is kinetically and mechanistically very similar to the selective oxidation of propylene reported for USb3010.13The reaction proceeds via a redox mechanism and the selective lattice oxygen is confined to a thin surface layer, only several monolayers thick. The abstraction of an allylic hydrogen from propylene to form a r-allyl intermediate is the rate-limiting step and oxygen addition to form a cr-allyl intermediate occurs before the abstraction of a second hydrogen. Carbon dioxide is formed almost exclusively by the further oxidation of acrolein. Registry No. Sb204,1332-81-6;FeSb04, 15600-71-2;D,,7782-39-0; propylene, 1 15-07-1.
Solid/Solid Interactions. Monolayer Formation in Mo03/AI,03 Physical Mixtures J. Leyrer, M. I. Zaki,+ and H. Knozinger* Institut fur Physikalische Chemie, Uniuersitat Miinchen, 8000 Miinchen 2, West Germany (Received: January 30, 1986)
The so-called monolayer dispersion in physical mixtures of Moo3 and y-AI2O3has been studied in the temperature range 723-823 K by laser Raman spectroscopy and low-temperature infrared spectroscopy of adsorbed carbon monoxide. It is shown that monolayer dispersion or spreading only occurs in the presence of water vapor, which volatilizes Moo3with formation of Mo02(0H),. The dispersion mechanism in this system is considered as surface diffusion of M O O ~ ( O Hin) ~a concentration gradient with additional contributions from gas-phase transport. An explanation of the dispersion phenomenon as wetting of one solid by another solid under the action of the surface tension is considered less likely.
Introduction Molybdate catalysts supported on alumina represent one of the technologically most important class of solid catalysts. They are usually prepared by impregnation of the alumina support from aqueous solution of ammonium heptamolybdate. It is well-known today1v2that the molybdenum species in these catalysts in their oxide precursor state can be described as surface analogues of molybdenum polyanions. Xie and co-workers first showed3that heating a physical mixture of crystalline M o o 3 and y-AI2O3at temperatures near 670 K for typically 24 h led to the disappearance of the X-ray diffraction pattern of Moo3. This observation was interpreted as monolayer dispersion of MOO, on the surface of y-A1203,and a monolayer 'Permanent address: Chemistry Department, Faculty of Science, Minia University, El-Minia, Egypt.
0022-3654/86/2090-4775$01.50/0
capacity of 0.12 g of Mo03/100 m2 was estimated. In a series of subsequent papers, the same group reported supportive data for the same oxide combination4-* and for crystalline M o o 3 (1) Jeziorowski, H.; Knozinger, H. J . Phys. Chem. 1979, 83, 1166. (2) Leyrer, J.; Vielhaber, B.; Zaki, M. I.; Zhuang, Shuxian; Weitkamp, J.; Knozinger, H. Mater. Chem. Phys. 1985, 13, 301. (3) Liu, Yingjun; Xie, Youchang; Ming, Jing; Liu, Jun; Tang, Youqi Cuihua Xuebao 1982, 3, 262. (4) Liu, Ying-Jun; Xie, Youchang; Li, Ce; Zou, Zhi-yang; Tang, Youqi Cuihua Xuebao 1984, 5, 234. ( 5 ) Xie, Youchang; Gui, Linlin; Liu, Yingjun; Zhao, Biying; Yang, Naifang; Zhang, Yufen; Guo, Quinlin; Duan, Lianyun; Huang, Huizhong; Cai, Xiaohai; Tang, Youchi Proc. Int. Congr. Catal., 8th. 1984 1984, 5, 147. (6) Xie, Youchang; Gui, Linlin; Liu, Yingjun; Zhang, Yufen; Zha, Biying; Yang, Naifang; Guo, Quinlin; Duan, Lianyun; Huang, Huizhong; Cai, Xiaohai; Tang, Youchi In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 139.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
physically mixed with other oxide supports such as Si02,5,6*839 Ti02,53839and Mg0.9 Monolayer dispersion was also found for a few other oxide combinations and for some salts mixed with y-A1203.536 Haber and c o - ~ o r k e r s I @studied ~~ the same phenomenon for the V205/Ti02system. They were able to show that V 2 0 5 was dispersed on the surface of anatase but not on that of rutile. Xie et aL6 interpreted the monolayer dispersion of one oxide on the surface of a second (support) oxide by the reasonable assumption that the total free energy will decrease due to the formation of surface bonds between the ions of the dispersed oxide and the support surface in combination with the entropy increase which is induced by changing an ordered three-dimensional crystalline phase into a less-ordered two-dimensional monolayer. The transport mechanism was assumed to be surface diffusion in a concentration gradient. This type of dispersion mechanism was rejected by Haber et al.'2913on the basis of their experimental data for the V 2 0 5 / T i 0 2system. They argued that surface diffusion would be extremely slow in the temperature regimes of interest due to the usually high values of the lattice energy of oxides. In contrast, the change in surface free energy is considered to be the driving force for the rapid migration of one oxide on the surface of another this process being the manifestation of wetting of one solid oxide by another solid oxide under the action of forces of surface tension. Both research groups which have hitherto reported on the phenomenon of monolayer dispersion or wetting were unable to analyze the nature and structure of the dispersed oxide due to the experimental techniques applied (X-ray diffraction, XPS). It is therefore the goal of this work to find complementary information on the structure of the dispersed species. This study concentrates on the Mo03/yA1203system, and in contrast to previous reports, the experiments are carried out under controlled conditions, either in dry flowing 0, or in a flow of moist 0, saturated with water vapor at room temperature. The surface structure was analyzed by means of laser Raman spectroscopy and low-temperature IR spectroscopy of adsorbed carbon monoxide.
Experimental Section Materials. y-A120, was prepared by calcination in air of the hydroxide (Condea) at 1048 K for 24 h. The N, BET surface area of the resulting oxide was 145 m2 g-l. MOO, was a Merck product (AR grade). 02,H2, and C O were from Linde and had a nominal purity of 99.998%. 0, and Ar were further dried by passing them through a cold trap filled with molecular sieve 4A (Merck); H2 and C O were further purified by oxisorb. Preparation Methods. Physical mixtures of MOO,and y-Al2O3 were prepared by first tumbling and then grinding the mixture in an agate mortar. A quantity of 9 wt % Moo3 was used, which is close to but certainly below the monolayer capacity of the -y-A1203used. These mixtures were treated in different ways: (a) The powder was heated in air in a porcelain crucible. This procedure was used in a few experiments so as to reproduce the conditions applied by other research groups. (b) The powder was pressed into the stainless steel sample holder of the Raman cell, and heat treatments were carried out in situ in controlled atmospheres. The Raman spectra were recorded in dry argon. (c) Larger quantities of the powder were placed into one of the beds of a two-bed quartz reactor shown in Figure 1, and heat (7) Xie, Youchang; Yang, Naifang; Liu, Yingjun; Tang, Youqi Sci. sin., Ser. E (Engl. Transl.) 1983, 25, 337. (8) Gui, Linlin; Liu, Yingjun; Guo, Quinlin; Huang, Huizhong; Tang, Youqi China Sei., E 1985, 6, 509. (9) Liu, Ying-jun; Xie, You-chang; Xie, Gang; Tang, You-qi Cuihua Xuebao 1985, 6, 101. ( I O ) Haber, J. In Surface Properties and Catalysis by Non-Metals; Bonnelle, J. P., Delmon, B., Derouane, E., Eds.; Reidel: Dordrecht, Boston, MA, 1983; p 1. (1 1) Haber, J. Proc. I n f . Congr. Catal. 8th, 1984 1984, 1 , 85. (12) Haber, J. Pure Appl. Chem. 1984, 56, 1663. (13) Haber, J.; Machej. T.; Czeppe, T. Surf. Sci. 1985, 151, 301
Leyrer et al.
A
0,
0
E -4-
*
----It-------
I$
VI
VI
cn Figure 1. Scheme of two-bed reactor. TABLE I: Strongest Raman Bands of Moo3, Moot-, and Mo7OU6 compd
wavenumber/cm-'
Moods) MoO?-(aq)" Mo70*46-(aq)b
117, 129, 158, 284, 667, 820, 996 3 18, 846, 896 218, 359, 895, 938
Reference 23.
Reference 24
treatments were again carried out in controlled atmospheres. The resulting materials were used for the preparation of self-supporting wafers for IR spectroscopy. In a separate experiment, pure Moo3was placed in the lower bed (up stream) of the reactor shown in Figure 1 while y-A120, was contained in the upper bed. This experiment was designed to test whether gas-phase transport could play a role. The 0, flow (20 cm3 mi&) could be saturated with water vapor at 295 K (20 Torr) by using a temperature-controlled saturator which was described previo~sly.'~ Raman Spectroscopy. Laser Raman spectra (LRS) were recorded in the wavenumber range 100-1200 cm-' by a computer-controlled Cary 82 spectrometer equipped with a triple monochromator. The in situ Raman kell and procedures were described in detail earlier.I5 The 514.5-nm line of a Spectra Physics Model 165 Ar+ ion laser was used for excitation. A laser power of typically 30-35 mW at the sample was applied. The spectral slit width was 6 cm-', and the wavenumber accuracy was f 2 cm-l. Infrared Spectroscopy. Infrared transmission spectra were recorded in the wavenumber range 2300-1900 cm-' by using a Perkin-Elmer Model 580 B spectrophotometer. The spectral resolution was 5.3 cm-' in this wavenumber range, and the wavenumber accuracy was =t1 cm-'. Self-supporting wafers (20 =t 2 mg ern-,) were mounted in an in situ cell, the principle of which was reported earlier.16 The low-temperature version of this cell will be described in a forthcoming paper." The wafers could be treated in situ in flowing O2 (50 cm3 m i d , 673 K, 1 h) to remove contaminations or in flowing H2 (50 cm3 mi&, 773 K, 2 h) for reduction. Details of the low-temperature adsorption of C O have been described earlier.1s*19 (14) Knozinger, H.; Ress, E. Z . Phys. Chem. (Frankfurt) 1967, 54, 136. (15) Vielhaber, B. Ph.D. Thesis, University of Miinchen, 1985. (16) Knozinger, H. Acta Cienf. Venez.,Suppl. 2 1973, 24, 76. Knozinger, H.; Stolz, H.; Buhl, H.; Clement, G.; Meye, W. Chem.-Ing.-Tech.1970, 42, 548. (17) Kunzmann, G.; Ertl, G.; Knozinger, H., to be submitted for publi-
cation
Monolayer Formation in Mo03/A1203
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4777
z0-l
I
1200
d
A
800 ---ASR
a
coo
I
0
I cm -'
Figure 3. Raman spectra of physical mixture 9 wt % Mo03/y-Al,03 (a) and after thermal treatment at 723 K for 30 h in air in an open crucible (b).
W a v e n u m b e r /cm-l
Figure 2. Infrared carbonyl spectra at 77 K of CO adsorbed on y-A1203 and on a 10.5 wt 76 Mo03/y-Al,03 catalyst: (a) y-AI2O3,02-treated at 723 K, 40 Torr of C O (b) same after evacuation at 77 K (c) y-Al,O,, H,-treated at 723 K, 40 Torr of CO; (d) same after evacuation at 77 K (e) catalyst, 0,-treated at 723 K, 40 Torr of CO; (f) same after evacuation at 77 K; ( 9 ) catalyst, after H2 reduction at 773 K, 40 Torr of CO; (h) same after evacuation at 77 K.
Results and Discussion Reference Data. Table I summarizes the most prominent and Raman bands of MOO, and of the molybdate anions MOO-: M07024b for comparison purposes. The characteristic wavenumber regions of molybdate species are 200-250, 300-370, 700-850, and 900-1000 cm-',which are to be assigned respectively deformation, terminal Mo=O bending, antito Mo-0-Mo symmetric Mo-0-Mo stretching, and symmetric and antisymmetric terminal stretching The Raman spectra of conventionally prepared molybdate catalysts are very similar to those of polymolybdate anions and have been interpreted in terms of surface analogues of highly condensed polyanions.',2s20 This interpretation was based on the appearance of the 220-cm-I band, which suggests the presence of Mo-0-Mo linkages, and on the high wavenumber of the Mo=O terminal stretching mode in the 940-960-cm-' range.'S2 Jannibello et aL2' have argued against this interpretation by mentioning that both monomeric and polymeric species should be present on the alumina surface after calcination. These authors emphasized the difficulty of distinguishing between Mo-O-Mo and Mo-0-A1 linkages, the more so as spectra of well-characterized molecular compounds containing similar M e A I bonds are not available. Jannibello et aL2' concluded that the high wavenumber values in the Mo=O stretching region were not sufficient to justify the polymerized surface species of octahedrally coordinated molybdenum-oxygen species, since a similar wavenumber shift was observed and attributed to an increase of the polarization of terminal Mo=O bonds of monomeric tetrahedral structures as a result of pro(18) Delgado, E.; Fuentes, G. A.; Hermann, C.; Kunzmann, G.; Knozinger, H. Bull. SOC.Chim. Belg. 1984, 83, 135. (19) Zaki, M. I.; Vielhaber, B.; Knozinger, H. J . Phys. Chem. 1986, 90,
3116. (20) Knozinger, H.; Jeziorowski, H. J . Phys. Chem. 1978, 82, 2002. (21) Jannibello, A.; Marengo, S.; Tittarelli, P.; Morelli, G.; Zecchina, A. J . Chem. Soc., Faraday Trans. 1 1984, 80, 2209.
gressive surface dehydration. Analogous conclusions were drawn by Stencel et a1.22 However, recent angular correlation s t ~ d i e s , ~ , ~ ~ have provided additional unequivocal evidence for the fact that condensed polymeric surface species with molybdenum in octahedral coordination are the majority species (typically 85-90%) in Mo/y-AI2O3catalysts after calcination at 770 K. Hence, the Raman spectra of the samples studied here will be interpreted based on the original arguments. It has been shown recentlyIgthat the dispersion of the molybdate species can be tested by low-temperature IR spectroscopy of adsorbed CO. An example is given in Figure 2 for the CO adsorption at 77 K on unmodified y-A1203and on a 10.5 wt 7% Mo0,/y-A120, catalyst prepared conventionally by impregnation from aqueous solution. The bands observed in spectrum a for unmodified 7-AI2O3in the presence of gas-phase CO are to be assigned to physically adsorbed CO (2143 cm-I) and to H-bonded CO (2152 cm-1)19925 and CO bonded to coordinatively unsaturated AI3+ sites (2188 cm-1).19,26 The high relative intensity of the 2152-cm-' band as compared to the band at 2143 cm-' is to be emphasized. Evacuation at 77 K removes these two bands, and the only species retained is the A13+coordinated CO (band at 2196 cm-I in spectrum b). The spectra (c and d) obtained after H, treatment of the unmodified y-AI2O3are identical. In contrast, the MoO3/y-AI2O3catalyst in the oxidized state does not contain the A13+-C0complex, and the H-bonded species is significantly reduced in abundance relative to the physically adsorbed CO (spectra e and f i n Figure 2). This result clearly demonstrates that the molybdate species are well-dispersed on he 7-A120, support surface and that they have substituted the alumina hydroxyl groups to a large extent. H2 reduction (spectra g and h in Figure 2) clearly leads to additional bands which are assignedlg dicarbonyl species (2145 and 2133 to weakly bonded Mo~+(CO)~ cm-I) and to CO coordinated terminally to coordinatively unsaturated Mo4+ sites (2185-2187 cm-I). This latter species is stable toward evacuation at 77 K. The reducibility of Mo6+ to only Mo4+ as indicated by the CO adsorption spectra is also considered as evidence for the highly dispersed character of the molybdate layer.'9s27 (22) Stencel, J. M.; Makovski, L. E.; Sarkus, T. A.; De Vries, J.; Thomas, R.; Moulijn, J. A. J . Catal. 1984, 90, 314. (23) Maller, A.; Weinstock, N.; Mohan, W.; Schlapfer, C. W.; Nakamoto, K. Appl. Spectrosc. 1913, 27, 251. (24) Mattes, R.; Bierbiisse, H.; Fuchs, J. Z . Anorg. Allg. Chem. 1971,385, 230. (25) Beebe, T. B.; Gelin, P.; Yates, J. T. Surf. Sci. 1984, 148, 526. (26) Della Gatta, G.; Fubini, D.; Ghiotti, G.; Morterra, C. J. Cafal.1976, 43, 90.
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Leyrer et al.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
I 1'
jjl I
1200
-
800
e x2
1
0
LOO
b
AvR cm-'
Figure 4. Raman spectrum of physical mixture after thermal treatment in dry flowing oxygen at 723 K for 30 h.
a
1200
-
800
LOO AV,/Cm-'
I
0
Figure 6. Raman spectra of the physical mixture (9 wt % Mo03/yA1203) after thermal treatment at 723 K in flowing moist oxygen for increasing time periods: (a) untreated sample; (b)-(e) treated for 2, 10, 24, and 30 h, respectively.
v
rd _ -__.--
I
rn
m
Wave n um b e r / cm-1 Figure 5. Infrared carbonyl spectra at 77 K of CO adsorbed on physical ' M O O ~ / Y - A ~ ~(a)-(d) O ~ ) . after treatment at 723 K for mixture (9 wt % 36 h in a flow of dry oxygen: (a) 40 Torr of CO; (b) evacuated; (c) after H2reduction at 773 K, 40 Torr of CO; (d) same after evacuation. (e)-(h) after treatment at 723 K for 36 h in a flow of moist oxygen: (e) 40 Torr of CO: (f) after evacuation: (8) after H2 reduction at 773 K, 40 Torr of CO; (h) same after evacuation.
Monolayer Dispersion of Physical Mixtures. A physical mixture containing 9 wt % MOO, gives a Raman spectrum at room temperature (Figure 3, spectrum a) which is identical with that of crystalline Moo3. When this mixture is thermally treated at 748 K for 24 h in a static air atmosphere in an open crucible, spectrum b is observed. This spectrum is clearly assigned to a surface plymolybdate based on the arguments given above. The (27) Abart, J.; Delgado, E.; Ertl., G.; Jeziorowski, H.; Knozinger, H.; Thiele, N.; Wang, X. Zh.; Taglauer, E. Appl. Card. 1982, 2, 155.
most reasonable interpretation of this observation is in fact monolayer dispersion in agreement with the results reported by Xie and co-workers3-* for similar experimental conditions. The present result, however, is complementary to the previous ones in that it permits a structural description of the dispersed species. When the same physical mixture (9 wt % Mo0,/A1203) was calcined in situ in the Raman cell in an atmosphere of dryflowing O2 at 723 K for 30 h, the Raman spectrum in Figure 4 was obtained. This spectrum is identical with that of pure crystalline MOO, (see Figure 3, spectrum a), and hence, this experiment suggested that dispersion or spreading of the MOO, on the y-A1203 surface did not occur. This implication is supported by the results of CO adsorption at 77 K, which are shown in Figure 5 , spectra a-d. The low-temperature infrared spectra of this physical mixture after thermal treatment in dry flowing 0, at 723 K for 36 h are identical with those observed for unmodified y-A1203 (Figure 2, spectra a-d) in both the 0,- and the H,-treated states. Therefore, after treatment in dry O2neither the alumina hydroxyl groups are consumed nor are the coordinatively unsaturated AI3+ sites blocked for CO adsorption. Also, the reduction of MOO, is not detected. Both these observations support the idea that the physical mixture remains unchanged under these conditions, in particular, the dispersion of the MOO, component remains unchanged; spreading does not occur in contrast to the thermal treatment in an open crucible in a static air atmosphere. Figure 6 shows a series of Raman spectra which were obtained for the 9 wt % Mo03/AI,03 physical mixture, when it was thermally treated in situ in the Raman cell at 723 K in aflow of moist O2 (water vapor pressure, 20 Torr) for increasing time periods (spectra b-e). After 2 h two bands at 915 and 950 cm-' of comparable intensity are detected besides the characteristic features of crystalline MOO,. After 10 h, the 950-cm-' band becomes the dominating band, and it is the only strong and somewhat asymmetric band after 24 h besides relatively weak
Monolayer Formation in Mo03/A1203
MOO, features. These latter bands have completely disappeared after a prolonged heat treatment for 30 h, and the spectrum then only contains bands at 218, 358, and 950 cm-', which very much resemble the characteristic normal modes of a surface polymolybdate species (see Table I). The weaker band, which is observed a t 915 cm-' on samples having experienced a shorter thermal treatment, is slightly higher in wavenumber than the M e 0 terminal stretching mode of the monomeric tetrahedral MOO^^- anion (898 cm-l). However, as suggested earlier,21,28the interaction with the support surface may sufficiently distort the tetrahedral symmetry of the anion so as to lead to rehybridization and enhanced double-bond character in the Mo=O bonds which are not directly engaged in the bonding to the surface. Hence, the 915-cm-' band can best be explained as being due to the intermediate formation of monomeric molybdate species a t shorter thermal treatment periods. Assuming spreading in the presence of water vapor in the reaction atmosphere, this means that the molybdenum species initially are deposited on the y-AlZO3 surface as monomeric MOO^^- anions which then-at increasing thermal treatment periods and, hence, increasing surface density-polymerized to form the surface polyanions. The phenomenon of spreading under these experimental conditions, namely moist atmosphere, is clearly supported by the low-temperature carbonyl spectra of adsorbed C O (spectra e-h, Figure 5). Spectrum e for C O adsorbed a t 77 K on the sample in its oxidized state does not show any clear evidence for the A13+-C0 complex with a band at around 2188-2196 cm-l (compare spectrum a in Figure 2), and the abundance of the H-bonded species (2152 cm-I) is strongly reduced relative to the physically adsorbed CO. On evacuation, only a very weak band.at 2196 cm-I pertains, indicating only a very low concentration of A13+-C0 complexes. Hence, these data clearly suggest the-at least partial-coverage of the yAl2O3 surface by molybdate species, this supporting the conclusion drawn from the Raman spectra. Spectra g and h in Figure 5 also show that Mo6+can be reduced to Mo4+ as evidenced by the appearance of the band at 2185 cm-I which is suggestive of a Mo4+-C0 surface complex. Again, this reducibility relates to the formation of the polyanionic surface structures and supports the monolayer dispersion. Clearly, these experiments demonstrate the importance of water vapor for the dispersion of molybdate species on the surface of y-Al2O3 to occur. It is, however, not clear whether the migration of the relevant species occurs via surface diffusion or through the gas phase. The experiment in the twebed reactor shown in Figure 1 was designed with the goal to test the possibility of transport contributions through the gas phase. The MOO, was placed in the bed upstream while the 7-A1203support was located in the bed downstream. It was to be expected that deposition of molybdenum species on y-Al2O3 should only be possible if volatile molybdenum compounds were formed and transported through the gas phase by the O2 stream. The Raman spectrum did not show any characteristic Mo=O vibrational bands on y-A1203 when the experiment was carried out at 823 K for 96 h in a flow of dry 02.The absence of any detectable coverage of the y-Al2O3 surface was also demonstrated by the fact that the low-temperature carbonyl infrared spkctra after this treatment (spectra a 4 in Figure 7) were identical with those observed in Figure 2 (spectra a-d) for pure, unmodified y-A1203. However, when an O2flow saturated with water vapor was applied at the same temperature for the same time period, the Raman spectrum shown in Figure 8 was obtained. Although the signal/noise ratio of this spectrum is poorer than those of the spectra shown in Figure 6, a band at 952 cm-I is undoubtedly recognized, this band being characteristic of the terminal Mo=O stretching mode of polyanionic surface molybdates; the spectrum therefore is indicative of some molybdate deposition on the y-A1203 by gas-phase transport. The low-temperature carbonyl infrared spectra e-h in Figure 7 are also supportive of this phenomenon. Although less pronounced than in the previous experiments in the (28) Wang, L.; Hall, W. K. J . Card. 1980, 66,251.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4779
Wavenumber /cm-l
Figure 7. Infrared carbonyl spectra at 77 K of CO adsorbed on Mo/yA1203prepared by gas-phase deposition in the two-bed reactor after treatment in dry [(a)-(d)] and moist [(e)-(h)] flowing oxygen: (a) 40 Torr of CO; (b) same after evacuation; (c) H2reduction at 773 K, 40 Torr of C O (d) same after evacuation; (e) 40 Torr of CO; (fj same after evacuation; ( 9 ) Hzreduction at 773 K, 40 Torr of CO; (h) same after
evacuation. U
m
0, I
1200
-
800 LOO A3,/cm-'
I
0
Figure 8. Raman spectrum of sample prepared by gas-phase deposition in a flow of moist O2 at 823 K (96 h).
presence of water vapor, the typical effects indicating the partial coverage of the y-A1203 surface as described above (reduced abundance of H-bonded species, reduction of Mo6+ to Mo4+) are detected. Again the importance of water vapor is demonstrated by this experiment, and possible contributions of gas-phase transport to the dispersion mechanism are documented.
Conclusions The present results are in agreement with those reported by Xie and his c o - w o k e r ~ , ~indicating -~ the possibility of monolayer dispersion of molybdenum oxygen species on the surface of yA1203 in a physical mixture with crystalline Moo3. They are complementary to Xie's results insofar as the structure of the dispersed species could be identified as polymolybdate anions. However, it was also demonstrated that the dispersion only occurs in the presence of water vapor, while it could not be observed in a dry atmosphere in the temperature regime applied here, Le., T I820 K. A certain though uncontrolled vapor pressure of water is undoubtedly present when the thermal treatment is carried out in a static atmosphere in an open crucible. In this case the water vapor is provided by the humidity of air and probably even more
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
