6638
J. Phys. Chem. 1987, 91,6638-6642
5 ) . On the catalyst V 2 0 s on Ti02, small amounts of N 2 and N 2 0 were formed (Table 11); this results from the reoxidation of the reduced catalyst surface with NO according to reactions 5 and 6 of Figue 6. Going from V2Os (SBET = 10.5 m2/g) to V205 on TiOz (SBET = 45 m2/g) the selectivities of N 2 I 8 0toward N 2 0 and l5NNI8Otoward I5NNO decrease 50%. This implies that in case of a lower surface area, lattice oxygen becomes more and more involved in the production of N 2 0 . The density of vacancies is thus enhanced. Conclusions A more detailed mechanism based on the experimental results with nitrogen and oxygen tracers has been proposed for the reaction of N O with NH, in the presence of oxygen. Almost every phenomenon observed could be explained by the proposed reaction mechanism. (i) During the reduction reaction most of the N 2 and of the N 2 0 are formed from the nitrogen atoms of nitric oxide and ammonia for reaction both in the presence of and in the absence of oxygen. The N 2 0 molecule formed consists primarily of the atoms from one molecule of N O attached to a nitrogen atom from NH3.
(ii) It is suggested that two molecules of N O adsorb on a doublet oxygen vacancy with simultaneous release of N 2 and N 2 0 into the gas phase. When molecular oxygen is present, the formation of N, and N 2 0 from N O is suppressed. Under reducing conditions, some N 2 is formed from N O and under oxidizing conditions, N 2 is formed from ammonia. (iii) During the reduction reaction, N O shows scrambling with the surface of the catalyst and ammonia is oxidized to molecular nitrogen. This phenomenon is currently being studied further by using labeled NO. (iv) N 2 0 is produced from the reaction of one molecule N O with the nitrogen atom from ammonia on two different sites of the catalysts by a reaction sequence involving gaseous nitric oxide and chemisorbed ammonia and by denitration of the surface. The same holds for the formation of water, which is produced by dehydration of the catalyst surface, as a result of the reaction of N O and NH3. (v) The oxygen vacancies are reoxidized more rapidly by O2 than by NO; this explains the enhancing effect of oxygen on the rate of the overall reaction of NO, NH3, and 02. Registry No. NO, 10102-43-9;NH,, 7664-41-7;02,7782-44-7;V20,, 1314-62-1;Ti02, 13463-67-7.
Physical Adsorption on Patchwlse Heterogeneous Surfaces. 5. Phase Transitions in Krypton Films on Graphitized Carbon Black in the Temperature Range 104-129 K Toan P. Vot and Tomlinson Fort, Jr,*$ Department of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, Pennsylvania 1521 3 (Received: March 17, 1987; In Final Form: June 23, 1987)
Adsorption isotherms of krypton on graphitized carbon black were measured with a high-precision volumetric adsorption apparatus in the temperature range of 104.5-129.0 K. Three of the isotherms were measured up to more than five statistical adsorbed layers. Isosteric heats and equilibrium spreading pressures were calculated from the adsorption isotherms and aided interpretation of the data. The existence of both a fluid to in-registry solid and an in-registry solid to out-of-registry solid phase transition in the adsorbed krypton films is clear below 122 K. At temperatures of 122 K and above the fluid krypton changes continuously to the out-of-registry solid. Thus, a phase diagram enclosing the in-registry solid adsorbed film was defined, The dispersion force contribution to the surface free energy of graphitized carbon black was calculated to be 154.7 mN/m.
I. Introduction Experimental studies of gas adsorption have revealed a variety of phase transitions in the adsorbed films’-“ if the substrate is nearly “homogene~us”.~One such solid substrate is graphite. Graphitized carbon black (gcb) and some of its variations expose almost exclusively the (0001) lattice plane and consequently have nearly homogeneous surfaces. If the dimension of the adsorbed gas molecule is compatible with that of the graphite lattice, there may be more than one two-dimensional solid phase. It has been observed that, near the completion of a monolayer, adsorbed krypton may form a twodimensional solid in-registry with the graphite l a t t i ~ eas ~ .well ~ as a closepacked film. The phase transition from two-dimensional (2-D) fluid to 2-D in-registry solid, and the rearrangement of the 2-D inregistry solid to form a close-packed film, appear as two steps on the vapor pressure isotherms near the coverage corresponding to monolayer completion. These transitions were first observed by Thomy and Duval,’ then by Larher,2 and, in this laboratory, by Putnam and Fort3 over a temperature range from 77.3 to 108.7 K. ‘Present address: Calgon Carbon Corporation, Pittsburgh, PA 15230. ‘Present address: California Polytechnic State University, San Luis Obispo, CA 93407.
The present work was intended primarily to extend our study of krypton adsorption on graphitized carbon black to higher temperatures in an attempt to find the temperature above which we postulated6 that the adsorbed 2-D fluid crystallizes directly to the adsorbed 2-D close-packed solid. It was also our goal to define more carefully the part of the 2-D phase diagram which enclosed the registered solid phase.6 Finally, from determination of spreading pressures for adsorbed krypton films up to pressures near saturation, we wanted to confirm our previously reported3 estimate for the dispersion force contribution to the surface energy of graphitized carbon black. To these ends, seven new adsorption isotherms were obtained in the temperature range of 104-129 K. Three of the isotherms were measured up to more than five statistical adsorbed layers. Special attention was paid to the phase transition portion of the isotherms, namely, the range of coverage from 100 to about 135 (1) Thomy, A.; Duval, X.J. Chim. Phys. 1970, 67, 1101. (2) Larher, Y. J. Chem. SOC.,Faraday Trans. 1 1974, 70, 320. (3) Putnam, F. A.; Fort, Jr., T. J . Phys. Chem. 1975, 79, 459. (4) Butler, D. M.; Huff, G. B.; Toth, R. W.; Stewart, G. A. Phys. Reu. Lett. 1975, 35, 1718. ( 5 ) Thomy, A,; Regnier, J; Duval, X . Colloq. Int. C.N.R.S. 1972, No. 201. (6) Putnam, F. A,; Fort, Jr., T.; Griffiths, R. B. J. Phys. Chem. 1977, 81, 2171.
0022-3654/87/2091-6638$01 .50/0 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6639
Krypton Adsorption on Graphitized Carbon Black
t
P
I
1 I
I D
I
I O
I
I
I
I
1
:
l
I
116 I
0
I
200
It
1
1
400 PRESSURE
1
,
1
600 TORR
I
I
800
Figure 1. Krypton-graphitized carbon black adsorption isotherms at 110.14,114.14,and 122.02 K. The solid circle is a desorption check at the end of the experiment.
pmol/g of adsorbent. Calculation of isosteric heats of adsorption aided interpretation of the experimental data. 11. Experimental Section
The adsorbent used in this study was the same as that used by Putnam and Fort,3 namely, Sterling FT graphitized carbon black FT-D5 (Cabot Corp., Boston, MA). Research grade krypton of 99.99% purity was obtained from Air Products and Chemicals Co., Emmaus, PA. The volumetric adsorption apparatus used in this work was, with a few modifications, the same as that used by Putnam and Fort3 and is described e l ~ e w h e r e . ~An additional capacitance manometer (MKS Instruments, Burlington, MA) that measures pressures up to 10 Torr was added to the system to attain higher resolution in the pressure measurement in the range of 1-10 Torr. The mercury diffusion pump was found to work most efficiently if its cooling jacket was maintained at lower than tap water temperatures. So, the 70:30 alcohol-water solution fed to the jacket was precooled to 10 'C by passage through a refrigerated chiller (Polyscience Corp., Niles, IL). Adsorption measurements were made at 104.49 K (to check the technique) and at seven new temperatures: 110.14, 114.14, 117.14, 120.18, 122.02, 125.04, and 129.00 K. The relative uncertainty in pressure measurement was estimated to be about 0.0006. The adsorption data may be found in the M.S. and Ph.D. Theses of Toan P. Vo, at Carnegie-Mellon University. The calculation scheme has been described7 and will not be repeated here. The virial equation of state for krypton truncated after the second term* was used in the adsorption calculations. 111. Results and Discussion A . Adsorption Isotherms. 1. Multilayer Region. Figure 1 shows three complete adsorption isotherms at 110.14, 114.14, and 122.02 K. The isotherms show the stepwise character expected for adsorption on homogeneous surfaces. The definition of the steps decreases as temperature increases. The saturation pressures at these temperatures were calculated from the equations given by Freeman and H a l ~ e y . They ~ are 303.6,464.2, and 901.1 Torr, respectively, and are shown as dashed vertical lines. In obtaining the saturation pressure of liquid krypton at 122.02 K, it was assumed that the equation given by Freeman and Halsey for the temperature range of 115.6-121.0 K is also applicable at 122.02 K without introducing large error. One desorption check at the end of a run was done for the 114.14 K isotherm. No hysteresis was found. In view of this result, and the results of previous experiments by Putnam and Fort,3 it was judged unnecessary to check for hysteresis in the other two isotherms. Further assurance (7) Drzal,L.T.; Putnam, F. A,; Fort, Jr., T.Rev. Sci. Instrum. 1974, 45, 1331. (8) The second virial coefficient of krypton gas was obtained from J. A. Barker, IBM Corp., San Jose, CA. (9) Freeman, M. P.; Halsey, Jr., G. D. J . Phys. Chem. 1956, 60, 11 19.
I12
ul l t // 110.14
'O6.8
108
,
,
K I.o
1.2
1.4
P/ PT
Figure 2. Krypton-graphitizedcarbon black adsorption isotherms showing the 2-D fluid to in-registry solid phase transition. PTis the pressure at the phase transition.
was found in the fact that, in many experiments which are not reported here, frequent desorption checks were made to check for possible nonequilibrium adsorption and no hysteresis was ever observed. 2. Submonolayer Region-Phase Transitions. The isotherms up to one monolayer exhibit the usual S shape which is characteristic of adsorption on homogeneous surfaces. Each of the isotherms measured at temperatures less than about 122 K shows a clear, abrupt step in the range of coverge above 105 pmol/g. This step was observed earlier for krypton, xenon, nitrogen, and methane on various kinds of graphitesI4 and has been attributed to a phase transition from 2-D fluid to 2-D solid in-registry with the graphite lattice. For krypton adsorbed on graphite, this 2-D in-registry solid corresponds to the krypton atom localized on top of one out of every three hexagonal graphite lattices. A schematic is shown in ref 3. The coverages at which this phase transition occursat 110.14, 114.14, 117.14,and 120.18 K a r e 109.0, 110.8, 112.2, and 114.0 pmol/g, respectively. The corresponding pressures at the phase transition are 4.08, 9.96, 18.88, and 36.25 Torr, respectively. The transition becomes less pronounced as temperature increases, as is most clearly shown in Figure 2 where amount adsorbed is plotted as a function of pressure/transition pressure. It appears to be of the classical second-order type although at 120.18 K the slope of the isotherm at the transition is not definitely discontinuous. For each isotherm measured at temperatures less than about 122 K, another phase transition occurs at coverages higher than the fluid to in-registry solid phase transition. This second transition appears as a small step near monolayer completion at 110.14 K and moves toward lower coverage as temperature increases. The coverages at which this second phase transition occur are 130.4, 127.4, 121.4, and 117.0pmol/g for isotherms at 110.14, 114.14, 117.14, and 120.18 K, respectively. The corresponding pressures a t the phase transition are 51.75, 64.4, 39.78, and 39.908 Torr, respectively. The magnitude of this phase transition is much less than that of the fluid to in-registry solid phase transition. It also becomes less pronounced and spreads out over a larger pressure range as temperature increases. Figure 3 shows the amount adsorbed in the region of this phase transition as a function of pressure/transition pressure. This second phase transition was first observed by Thomy et aL5 in krypton adsorption vapor pressure isotherms on exfoliated graphite in the temperature range of 77.3-96.3 K and was explained as the rearrangement of the in-registry solid phase to an
6640 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Vo and Fort
c? 122
- 121
n
%Lz 133
0
cn n U
c
132
2 3
0
5 131 130
129
17 0.9
1.0 P/ PT
1.1
1.2
Figure 3. Krypton-graphitized carbon black adsorption isotherms showing the in-registry solid to out-of-registry solid phase transition. PT is the pressure at the phase transition.
c?
2 I30 2
I
520 -
n
# 110 Lz 0 g 100 U I-
.
90 -
p
z
2 80 -
0
c
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90
100 TEMPERATURE
110 ,K
ISOTHERM KINKS, THIS WORK
:~~{~~SSIi~L~JRTPEAKS, ISOTHERM KINKS THOYV AND D U V i L
120
130
Figure 4. Phase diagram of krypton adsorbed on graphite near monolayer completion.
out-of-registry close-packed structure. Later, Putnam, Fort and Griffiths? by carefully obtaining the compressibility of adsorbed krypton films, also found clear evidence for this transition at temperatures of 94.72-104.49 K. The results of Thomy et al. and of Putnam, Fort, and Griffiths were combined in a phase diagram published by the latter authors. For temperatures of about 122 K and higher, only one phase transition was found in the present work. The coverages at which this phase transition occurs increase rapidly with increasing temperature. It appears at 115.4 Kmol/g at 122.02 K and 118.2 Kmol/g at 125.05 K. At 129.00 K there is still evidence of this phase transition at coverage of about 123.5 pmol/g although it appears only as a small rise in the isotherm over a wide pressure range. This single phase transition at 122.02 K or higher is interpreted as a direct transition from 2-D fluid to 2-D closepacked krypton. Thus, at temperatures higher than 122.02 K the in-registry solid probably exists only momentarily and locally as the adsorbed krypton passes from the 2-D fluid to the 2-D close-packed film. All the phase transitions defined in this study are combined with the results of Putnam, Fort, and Griffiths6 and of Thomy et al. and are shown in Figure 4. It can be said that, with this work, the portion of the phase diagram for adsorbed krypton on
I
I
I
1
I
0.3 0.4 0.5 PRESSURE , TORR Figure 5. Very low coverage region of krypton-graphitized carbon black adsorption isotherms at (1) 110.14, (2) 114.14, (3) 117.14, (4) 120.18, (5) 122.02, ( 6 ) 125.04, and (7) 129.00 K. 0
0.I
0.2
TABLE I: Temperature Dependence of Intercepts of Isotherms in the Very Low Coverage Reeion intercept, std dev, woVg w”/g temp, K 110.14 114.14 117.14 120.18 122.02 125.05 129.00
0.295 0.285 0.274 0.254 0.257 0.250 0.210
0.002 0.004 0.002 0.019 0.010 0.004 0.004
graphite near the monolayer completion has been completely defined. 3. Very Low Coverage Region. Figure 5 shows the very low coverage region of the isotherms of this study. In this region, all isotherms first exhibit a concavity toward the pressure axis and then become nearly linear over a small coverage range. The concavity of the isotherms near the origin has been interpreted as caused by filling of the strong sites on the graphite surface. Graham’O estimated the extent of strong sites of his graphitized carbon P33 and Graphon surfaces by extrapolating the linear portion of his nitrogen adsorption isotherms to zero pressure. He estimated that the contribution of the strong sites to the P33 and Graphon surfaces amounted to 0.1% and 1.25%, respectively. As pointed out by Putnam and Fort,3 Graham’s method is not an appropriate way to estimate the extent of strong sites. From the results of a virial coefficient analysis, they estimated the extent of strong sites of this graphitized carbon black surface to be 0.5% of the total monolayer capacity. This work confirms their point of view. If the present isotherms are extrapolated to the zero pressure axis by using the simple linear least-squares method applied to the nearly linear portion of the isotherms, the intercept is found generally to decrease with increasing temperature. Table I shows the variation of the intercepts with temperature and their estimated standard deviations. The fact that the intercepts decrease with increasing temperature shows that there is a distribution of the energies of the strong sites, a point that was dismissed by Graham. B. Adsorption Thermodynamics. 1 . Isosteric Heat. If the gas phase above the surface is ideal, the isosteric heat, qst,is defined as
a In P
= -R[
m],.
where R is the gas constant, P is the pressure, T i s the absolute (10) Graham, D. J . Phys. Chem. 1957, 61, 1310.
The Journal of Physical Chemistry, Vol. 91, NO. 27, 1987 6641
Krypton Adsorption on Graphitized Carbon Black I
1
1
1
r
I
120 70
Y i: 100
60
I
ri
-
80
8
60
n W m
50
40
v)
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E 70
I- 40
3
a
z
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20
50
IO
z 9 40 w
0
U
a
20
a 0 -4
30
-2
-3
-I In P
0
I
2
3
Figure 6. The 110.14 K isotherm. The solid line is drawn through the data points, and crosses are the interpolated points. 2o I
I
I
A
22
IO
0
tc
0
6 20 ?
i IO
20
30
40
50
60
Figure 8. Spreading pressure of krypton adsorbed film at 110.14, 114.14, and 117.14 K.
3
I8 IQ
2
16
0 U
E
14
U J
0
52 12 IO I
0
1
80 AMOUNT ADSORBED,
40
I
I20
pMOL/G
Figure 7. Comparison of isosteric heats calculated for different temperature ranges: (-) calculated from isotherm isotherms at 110.14, 114.14, and 117.14 K; (---) calculated from isotherms at 122.02, 125.05. and 129.00 K.
temperature, and nu is the amount adsorbed. In order to calculate the isosteric heat, the isotherms must be interpolated at constant amount adsorbed. The interpolation was performed by using the method of Akima.1',12 Figure 6 shows the result of Akima's method of interpolation applied to the 110.14 K isotherm. The solid line was drawn through the data points which are not shown, and the crosses are the interpolated points. After pressures at fixed amount adsorbed were obtained, the best straight line was drawn in the (In P)-(l/nplane by using the method of least squares. The slope and its standard deviation were determined for the calculation of the isosteric heat. The standard errors in the heats rarely exceed 150 J/mol, a significantly larger figure than that of Putnam and Fort3 because the adsorption measurements were not carried out at equally spaced reciprocals of temperature in the present work. Figure 7 compares isosteric heat curves obtained from data taken at temperatures above and below the limit of stability of solid in-registry adsorbed krypton on graphite. For both situations, as coverage increases, isosteric heat first falls due to saturation of strong sites and then rises because of increased opportunities (11) Akima, H. Commun. ACM 1974, 17, 18. (12) Putnam, F. A.; Fort, Jr., T. J . Phys. Chem. 1982, 86, 4974.
for lateral interactions between adsorbed molecules of krypton. In the region of the phase transitions the curves are no longer similar because the transitions are reflected in the heats. Thus, the strong peak A in curve 1 is characteristic of the transition from 2-D fluid to 2-D in-registry solid. Peaks B and C in curve 1 are manifestations of the 2-D in-registry solid to 2-D out-of-registry solid phase transition in the isotherms at 110.14 and 114.14 K, respectively. (The similar phase transition at 117.14 K is not seen because it is too weak.) Peak D in curve 1 occurs at the end of the portion of the isotherms with minimum slope. Peak a in curve 2 represents the 2-D fluid to 2-D out-of-registry solid phase transition at temperatures of 122.02 K and higher. The reduced, in comparison to peak A of curve 1, magnitude of this peak shows that this is a relatively weak phase transition as is shown in the adsorption isotherms. The broad shoulder b may be due to some small amounts of adsorption in the second layer. The isosteric heats in the multilayer region are not shown but are similar to, though less well-defined than, those shown in ref 3 (Figure 5 ) . 2. Spreading Pressure. The spreading pressure of krypton on graphitized carbon black was calculated from the Gibbs adsorption equation x
=
e
r P n u d In P
2% J O
where x is the spreading pressure and A is the surface area per gram of adsorbent. In this study 34 was taken to be 11.4 m2/g as estimated by Putnam and Fort3 for this graphitized carbon black. Again, the method of curve fitting of Akima was used so as to allow the integration to be performed analytically. Typical spreading pressure curves are shown in Figure 8. The two-dimensional fluid to in-registry solid phase transition can be seen, and its magnitude clearly decreases with increasing temperature. It has been found that the Frenkel-Halsey-Hill adsorption isotherm equation describes the steps in the adsorption isotherm of krypton on graphitized carbon black weIl.l3 Therefore, this equation can be used in conjunction with the Gibbs adsorption equation to obtain an expression for the spreading p r e s s ~ r e . ' ~ (13) Singleton, J. H.; Halsey, Jr., G. D.Can. J . Chem. 1955, 33, 184. (14) Putnam, F. A. MS Thesis, Case Western Reserve University, 1975.
J . Phys. Chem. 1987, 91, 6642-6648
6642
Here, rSat is the spreading pressure of the thick adsorbed film at saturation, Ac is the net interaction energy in the first layer, nu, is the monolayer capacity, NA is Avogadro’s number, and a is the area per molecule. This is the equation of a parabola in the A-a plane with its vertex at ( a = 0, T = rut).This equation, therefore, can be used to extrapolate the spreading pressure curves in the multilayer region at 110.14, 114.14, and 122.02 K to the saturation vapor pressure of krypton to obtain rsat.This equation also suggests that if A is plotted against a2,a straight line should result. A good straight line is obtained only over a limited region near was found to the top of the isotherms. An average value of rSat be 75.4 mN/m. C. Surface Energy. The dispersion contribution to the surface free energy of graphitized carbon black was calculated from the equation given by Fowkes.lS Fowkes showed that for a thick film of an adsorbate which wets an adsorbent (4) Here, 72 and 7 : are the dispersion force contributions to the surface free energy of the adsorbent and the adsorbate, respecis the surface free energy of the adsorbate. tively, and for krypton with a value It can be assumed that ya = estimated to be 52 mN/m.I6 ysd, then, is found to be 154.7 mN/m. Putnam and Fort3 found a value of 151 mN/m for this quantity in the temperature range of 94.72-104.49 K. The
agreement between the two values is good. They may be compared with the value of 123 mN/m for gcb at 77 K cited by Fowkes’’ based on analysis of nitrogen adsorption data of Me1r0se.I~ Melrose had to use data from two different studies in his calculations, which in fact may have caused an uncertainty in his value of rat.Although was found to be slightly higher in the present work than obtained by Putnam and Fort3 for the same graphitized carbon black at lower temperatures, it is strongly believed that, like the surface energy, 7; should decrease with increasing temperature. The decrease may be very small in the temperature range investigated, and some experimental errors may have masked the effect. D. Surface Area. The adsorption isotherms at 104.51, 110.14, and 114.14 K show clear portions having minimum slopes at coverages of 123.2-125.2 Fmol/g. If the ends of these portions are interpreted as corresponding to complete, no-vacancy monolayers of in-registry adsorbed krypton, then from the lattice constant of the graphite surface the surface area of this graphitized carbon black is found to be 11.9 m2/g, which is in excellent agreement with Putnam and Fort’s value of 11.4 i 0.4 m2/g.3 If the end of the in-registry solid to out-of-registry solid-phase transition is taken to be a complete close-packed monolayer with no second layer adsorption, then the monolayer capacity is found to be 131.2 i 1.2 pmol/g from these isotherms. Taking the lattice parameter of the fcc krypton crystal at 110.14 K to be 5.81 A,** the area per molecule is 14.62 A2 and the surface area of this graphitized carbon black is 11.5 m2/g. This value is in excellent agreement with those reported above. Registry No. Kr, 7439-90-9.
~~
(15) Fowkes, F. M. In Chemistry and Physics of Interfaces; American Chemical Society: Washington, DC, 1965. (16) Benson, G. D.; Claxton, T. A. J. Phys. Chem. Solids 1964, 25, 367.
(17) Melrose, J. C. Adv. Chem. Ser. 1964, No. 43, 172. (18) Pollack, G. L. Reu. Mod. Phys. 1964, 36, 748.
Study of Oxidic and Reduced Alumlna-Supported Molybdate and Heptamolybdate Species by in Situ Laser Raman Spectroscopy E. Payen,+ J. Grimblot, and S. Kasztelan* Laboratoire de catalyse hdtZrog2ne et homog2ne (U.A. CNRS 4021 and Laboratoire de spectrochimie Infrarouge et Raman (L.P. CNRS 2641), Universitd des Sciences et Techniques de Lille Flandres-Artois, F-59655 Villeneuve d’Ascq Cedex, France (Received: April 27, 1987)
Alumina-supported molybdate and heptamolybdate species have been prepared by monitoring the molybdenum loading and characterized during their preparation, reduction, and reoxidation by in situ laser Raman spectroscopy. Reduction and reoxidation of the Ni- or Co-promoted heptamolybdate have also been investigated. It is shown that upon these treatments the supported species remain stable and have a similar behavior. The changes in the spectra observed can therefore be attributed to chemical effects rather than to structural modifications. A global interpretation of the Raman bands of supported molybdenum oxide and reduced species is discussed. In particular, the terminal Mo-0 band wavenumber shifts have been discussed in terms of three different effects, namely, ligand heterogeneity, coordinative heterogeneity, and oxidation number of Mo ions.
Introduction
The understanding of the structure of alumina-supported oxomolybdenum catalysts in their oxide state owes much to results obtained by using laser Raman spectroscopy (LRS).I4 In particular, LRS demonstrated that as the molybdenum loading increases, monomeric molybdate species and then a polymolybdate phase are deposited on alumina whereas crystallites of M o o 3 appear after saturation of the so-called monolayer coverage of the alumina s ~ r f a c e . ’ ~ ~ ~ ~ - ~ From the LRS studies of the genesis of these catalysts the nature of the polymolybdate phase has been proposed to be hepta*To whom correspondence should be addressed. t Laboratoire de spectrochimie Infrarouge et Raman.
0022-3654/87/2091-6642$01.50/0
molybdate aggregate^.'.^^'.'^ This has been confirmed by recent studies by time differential perturbated angular correlation of these (1) Jeziorowski, H.; Knozinger, H. J . Phys. Chem. 1979, 83, 1166 and references therein for earlier literature. ( 2 ) Payen, E.; Barbillat, J.; Grimblot, J.; Bonnelle, J. P. Spectrosc. Lett. 1978, 11, 997. ( 3 ) Sombret, B.; Dhamelincourt, P.; Wallart, F.; Muller, A. C.; Bouquet, M.; Gromangin,J. J . Raman Spectrosc. 1980, 9, 291. (4) Zing, D. S.; Makowsky, L. E.; Tisher, R. E.; Bown, F. R.; Hercules, D. M. J . Phys. Chem. 1980,84, 2898. (5) Wang, L.; Hall, W. K. J. Catal. 1980,60, 251; J . Catal. 1982, 77, 232. (6) Cheng, C. P.; Schrader, G . L. J . Catal. 1979, 60, 276. (7). Kasztelan, S.;Grimblot, J.; Bonnelle, J. P.; Payen, E.; Toulhoat, H.; Jacquin, Y . Appl. Catal. 1983, 7 , 91. (8) Giordano, N.; Bart, J. C. J.; Vaghi, A,; Castellan, A,; Martinotti, G. J . Catal. 1975, 36, 81.
0 1987 American Chemical Society
