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
Alumina-Supported Molybdate and Heptamolybdate Species catalysts." However, such a conclusion depends in particular on the calcination temperature conditions. At high temperature such species can be destroyed and generate other phases such as A12(M004)3.1'6312 Recently, it has been shown by several groups that some bands in the Raman spectra of Mo-, W-, Re-, and V-supported oxometalate catalysts are sensitive to the presence or absence of water.I2-l8 Thiseffect has been attributed either to the interaction of adsorbed water with the surface oxometalate specie^'^,'^-'^ or to a hydration-dehydration process of these species.10s18Interestingly, Raman band positions have also been found to be sensitive to I8O exchange,I4to NH3 and D 2 0adsorption, and to the presence of promoter ions (nickel or cobalt).16,18 These results illustrated nicely the potential of LRS to investigate the reactivity of these systems in the oxide state. Comparatively, no extensive LRS study of the reduction of these catalysts has appeared although Wojciechowski et al.I9 and Payen et aLzo have reported preliminary accounts of such studies on alumina-supported Mo and W catalysts. In the present work we prepared model molybdate- and heptamolybdate-supported catalyst by monitoring the molybdenum loading of the r-Al,O, support. Then LRS has been used to study the surface chemistry of these species during the preparation steps, Le., from impregnation to calcination, reduction, and reoxidation. In addition, the effect of the addition of nickel or cobalt promoter ions on the reduction and reoxidation of the supported heptamolybdate catalysts has also been investigated.
Experimental Methods Catalysts. The supported Mo catalysts were prepared by a pore-filling impregnation of r-Al,O, extrudates (238 m2 g-I) with an ammonium heptamolybdate solution. After drying at 383 K overnight, the samples were calcined for 2 h in air at 773 K. The promoted catalysts were prepared with the Mo catalysts calcined at 623 K for 2 h. Then the promoter salt solutions (cobalt or nickel nitrates) were added by a pore-filling impregnation followed by drying at 383 K overnight and calcination at 773 K for 2 h. The catalysts will be designated by their loading in wt % of oxide (Le., MOO,, COO,and NiO) followed by the symbol of the element. The samples 4Mo, 14M0, 3Co14M0, and 3Nil4Mo have been prepared. In addition to these laboratory-made catalysts, two industrial catalysts HR306 (3Co14Mo) and HR346 (3.6Ni14Mo) from Procatalyse have also been studied. A M o o 2 sample from "ICN Products" has been used as a reference sample after pretreatment under N z at 873 K. Catalyst Pretreatments. The reduction step was carried out in situ by using the samples as received or after calcination under dry oxygen at 720 K for 2 h. After cooling, the reduction was performed by raising the temperature (heating rate 5 K min-') to the desired final temperature under the flow of the reducing gas mixture. Similarly, reduction by deuterium has also been performed on the sample pretreated under N2/D20overnight at (9) Dufresne, P.; Payen, E.; Grimblot, J.; Bonnelle, J. P. J . Phys. Chem. 1981, 85, 2344. (10) Payen, E.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. Polyhedron 1986, 5, 157. (1 1) Vogdt, C.; Butz, T.; Lerf, F.; Knozinger, H. Proc. Int. Congr. Catal., Berlin, 8rh 1984, 111-1 17. (12) Chan, S. S.;Wachs, I. E.; Murrell, L. L.; Hall, W. K. J . Phys. Chem. 1984, 88, 5831. (13) Stencel, J. M.; Makowski, L. E.; Sarkus, T. A,; de Vries, J.; Thomas, R.; Moulijn, J. A. J . Catal. 1984, 20, 304. (14) Marcinkowska, K.; Ordrigo, L.; Kaliagine, S.; Roberge, P. C. J . Card. 1986, 97, 75. (1 5) Chan, S. S.;Wachs, I. E.; Murrell, L. L.; Dispenziere, N. C.; J. Catal. 1985, 92, 1.
(16) Stencel, J. M.; Makowski, L. E.; Diehl, J. R.; Sarkus, T. J . Catal. 1984, 95, 414. (17) Wang, L.; Hall, W. K. J . Catal. 1983, 82, 177. (18) Payen, E.; Kasztelan, S.;Grimblot, J.; Bonnelle, J. P. J . Raman Spectrosc. 1986, 17, 233. (19) Wojciechowski, W.; Pawlowska, M. Chem. Stosow. 1980, 24, 181. (20) Payen, E.; Kasztelan, S.;Grimblot, J.; Bonnelle, J. P. J . Mol. Spectrosc. 1986, 143, 259.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6643 920
h
f
e
d Air-773 K 898 C
b a
-II
1 1000
'
800
Solution pH=ll
cm''
Figure 1. Raman spectra of the monomeric molybdate species in solution and on the alumina support surface during the preparation of the 4 wt % MoO3/y-Al20, catalyst. TABLE I: Some Assignments (in an-')of Raman Bands of Iso- and Heteropolyanions in Aqueous Solution vibrational modes
(Mo-0,) (MG-OJ
usym "asym
(MG-0-Mo) uIym (Mo-0,) bend (MG-0-Mo) def
Mo70246- MOO?931
898
900 860
840
360 220
320
PMo120403- SiMo,20404995 980 890 25 1
980 963 883 250
room temperature. Reoxidation of the reduced samples has been performed in situ with dry oxygen or wet air at different temperatures. Laser Raman Measurements. The Raman spectra have been recorded on the Raman microprobe Mole (Jobin-Yvon) equipped with a measurement cell described elsewhere.21 The 488-nm line of an Ar+ laser was used with a power at the sample of about 1 mW. Photon-counting detection and data processing were employed to improve the sensitivity and signal-to-noise ratio. The spectra have been recorded systematically with the sample at room temperature. This procedure allows to avoid temperature modification of the Raman spectra and to freeze the sample in an intermediary state. Results The Supported Molybdate Species. The supported monomeric molybdate species can be obtained for amounts of Mo lower than 5.5 wt % MOO, or ca. 1 atom of Mo nm-2 on a 250 m2 g-' alumina support.',8 The evolution of the Raman spectra of a catalyst loaded with 4 wt % MOO, at different steps of preparation is reported in Figure 1. This preparation starts with a solution a t pH 5.5 containing the heptamolybdate anion. For the purpose of comparison, spectrum l a of the molybdate species in basic solution (pH 11) has also been reported. The Raman spectra of that solution correspond to the unperturbated molybdate species showing a major band at 900 cm-', a small broad peak at 840 cm-I, and, not reported in Figure 1, the characteristic 320-cm-I band. The assignment of these bands taken from the literature has been reported in Table I. The wet impregnated support gives the Raman spectrum 1b, which is similar to the molybdate solution spectrum la. This result (21) Payen, E.; Dhamelincourt, M. C.; Dhamelincourt, P.; Grimblot, J.; Bonnelle, J. P. Appl. Spectrosc. 1982, 36, 30.
6644
Payen et al.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 940
ll
a)+Air-298K
940
C
1005
b-
a)+02-673
K e)R e h yd r a t ed
4
1000
600
200
cm-1
Figure 2. Raman spectra of the dehydrated (02,720 K) monomeric molybdate supported catalyst after reduction and reoxidation.
shows that a transformation of the heptamolybdate into the molybdate anion has occurred during the wetting of the support. In spectrum l b the 900-cm-' band has a shoulder at higher wavenumber which may correspond to a partly bound species. This point is confirmed by the Raman spectrum I C of the dried catalyst. The main band is now shifted to 920 cm-', a band also observed by Jeziorowski et al.' on a dry 3 wt % Mo03/y-A1203 catalyst. Upon calcination under air, the main band is shifted to 945 cm-] and a broad shoulder at 850 an-'appears, giving the classical spectrum Id. Such an air calcination has recently been found by several groups to be not sufficient to provide a fully dehydrated calcined sample. This latter state is obtained by in situ calcination and storage under dry oxygen. Upon such a treatment, the Raman spectrum l e is obtained showing the main band shifted to 1005 cm-l whereas only a very broad band centered around 900 cm-l can still be distinguished. At this point one may wonder whether the original monomer species is still present intact on the support surface. A way to check this point is to hydrate the sample calcined after dehydration. Upon exposure to air, a Raman spectrum similar to spectrum Id is obtained. Further exposure to water vapor for a few days gives the Raman spectrum If showing clearly the band at 920 cm-' present on the Raman spectrum IC of the dried sample. Therefore, such a reversibility can be considered as a demonstration of the presence of the same species whose local environment has been modified all along these treatments. Then reduction by H2 at various temperatures has been undertaken on both the hydrated and the dehydrated monomeric molybdate-supported samples. The former gives no exploitable spectra due to a strong fluorescence background. This phenomenon can be due to the presence of either impurities or fluorescent hydroxyl groups according to Jeziorowski et a1.22 It can also be recalled here that the low Mo loading of this sample makes it difficult to record a decent spectrum under the negative influence of such a perturbation. The Raman spectrum 2a of the reduced dehydrated sample has been reported in Figure 2. On that spectrum, bands can be detected at 1060, 800, and 300 cm-'. It has been checked that neither the cell nor the alumina support, treated under the same conditions, gives these Raman bands. To our knowledge, these bands have been mentioned only by Wojciechowski et al.I9 To check that we are still dealing with the molybdate species in a reduced state, reoxidation of the sample has been performed under both dry oxygen and wet air. After reoxidation by dry oxygen the Raman spectrum 2b, very similar to spectrum l e of the calcined dehydrated sample, is obtained. Moreover, after reoxidation by wet air spectrum 2c of the partially hydrated sample is recovered. Notably, the detection of the 320-cm-I band in that spectrum confirms the presence of the monomeric species. The Supported Heptamolybdate Species. The Raman spectra obtained during the preparation of the 14 wt 7% Mo03/AI,03 (22) Jeziorowski, H.; Knozinger, H. Chem. Phys. Lett. 1977, 51, 519.
a
J 1000
Solution pH15
'
8bO
'0-'
Figure 3. Raman spectra of the heptamolybdatespecies in solution and on the alumina support surface during the preparation of the 14 wt % MOO3/y-A1@3 catalyst.
sample are reported in Figure 3. The Raman spectrum 3a of the solution containing the heptamolybdate anion shows a net difference similar to spectrum l a of the monomeric molybdate species. The main band is now at 940 cm-' with a broad shoulder at 900 cm-'. Interestingly, bands at 360 and 220 cm-l, not reported here, can also be used to distinguish between the monomeric molybdate and the heptamolybdate species.' The assignment of the bands of both species can be compared in Table I. After impregnation of the alumina with a solution containing mainly the heptamolybdate species, a mixture of molybdate (900 cm-l) and heptamolybdate (940 cm-') species can be distinguished in the Raman spectrum 3b of the wet catalyst. However, after drying the bands of the molybdate species have disappeared and only the bands characteristic of the heptamolybdate remain (spectrum 3c). Upon calcination under wet air the Raman spectrum 3d is obtained. A net change from the heptamolybdate Raman spectrum 3c of the dry sample occurs with two sharp bands, characteristic of MOO,,appearing at 820 and 995 cm-]. Besides these bands, a band is observed at 960-970 cm-l and a broad shoulder at 850 cm-l. Such a spectrum is usually considered to be characteristic of the so-called polymolybdate p h a ~ e . ' $ ~After * ~ >cal~ cination under dry oxygen only trace amounts of Moo3 are formed, and as for the sample containing monomeric molybdate species, a net shift from 965 to 1000 cm-I of the main band occurs due to the dehydration of the sample. It can also be checked that this calcination step is reversible as after extensive rehydration the spectrum of the heptamolybdate anion is restored (spectrum 3f). Once again the recovery upon hydration of the spectra of the original species supports the idea that the spectral transformations observed are the results of modifications of the local environment of the Mo ion in the same species rather than changes of the nature of the species. The spectra obtained after reduction of the hydrated and dehydrated samples have been reported in Figure 4. First, the effect of the reduction temperature has been investigated on the hydrated sample (spectra 4a-4e). After reduction at mild temperature (413 K, spectrum 4a) no major differences with the calcined sample can be found. Then at higher temperatures and similarly to the monomeric molybdate-supported sample, spectra 4b-4d obtained show the following transformation: disappearance of the 970-cm-' band; appearance of peaks in the 850-750-cm-' range and of a small 1000-cm-l band whereas the 360-cm-' band shifts to 300
Alumina-Supported Molybdate and Heptamolybdate Species
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6645
950
CoMo 1 01 0 6 0O
h
620
h
6
0
-
H p 593 K
e)+ 0 2 - 7 7 3 K
D2- 593K
e)+ Air-298 K
9 'Oo0
l
760
o
w
3
;
o
H2-593K
f :
O
H2-773 K
m
Hp- 773 K
~
0
6
0 NiMo ~
0
H2-673K H2-773 K
C
770
1000
b
840
H2-593 K io00 -
1000
600
200
cm-1
Figure 4. Raman spectra of the hydrated and dehydrated (02,720 K)
heptamolybdate supported catalyst after reduction and reoxidation. 198
*
,
1000 600 Figure 5. Raman spectra of Moo2.
'
2;)o
cm-'
cm-I. A rather drastic reduction (773 K, spectrum 4e) does not lead to further evolution. After reduction the dehydrated sample gives the Raman spectrum 4f showing different features as compared to the hydrated sample (spectra 4d and 4e). In particular, bands at 1060 and 760 cm-' appear quite similar to the dehydrated 4 wt % M a 3 sample. All these spectra of reduced samples remain very different from the Moo2 spectrum reported in Figure 5 and are therefore characteristic of a particular reduced species. For a reduction at 593 K, the substitution of H2 by D2 does not lead to drastic changes of the spectra (compare spectra 4b and 4c). However, the 850-75O-cm-' broad band is now centered a t 770 cm-I instead of 840 cm-I. Rather than involving the presence of M d H group sensitive to the isotopic shift, this effect is likely due to a slightly more reduced state which does not depend on the use of H2or D2but on the time of reduction and the initial state of the sample as will be seen later. Reoxidation of the reduced sample by wet air at room temperature gives spectrum 4g containing bands of reduced (300 and 850 cm-I) and oxidized (970 cm-') species. Further reoxidation at 773 K (spectrum 4h) restores the spectrum of the heptamolybdate species as observed on the dry catalyst. These observations stress the stability of the supported heptamolybdate species upon reduction. The Supported Promoted Heptamolybdate Species. The effect of the addition of Ni or Co cation on the reduction and reoxidation of the 14 wt % MOO, catalyst, i.e., the supported heptamolybdate sample, has been investigated. After reduction, the promoted hydrated catalyst gives the spectra 6 a - 6 ~and 6 6 g with the four bands already noticed for the reduced heptamolybdate species.
A
300
D 2 - 593 K
H2-593K
,
1000 600 ' 200 cm-1 Figure 6. Raman spectra of hydrated and dehydrated (02, 720 K) Co-
or Ni-promoted heptamolybdate supported catalysts after reduction. HR346 and HR306 industrial catalysts have been used for experiments d, f, and g, respectively. However, the 1000-cm-I band remains small and the 760- and 300-cm-' bands are now broad. Furthermore, a reduction at 593 K leads to the almost complete disappearance of the lOOO-cm-' band for both the CoMo and NiMo catalysts (spectra 6g and 6c, respectively). These features likely characterize a more reduced state. For both the NiMo and CoMo catalysts the exchange of H2 by D2does not lead to changes of the spectra (compare spectra 6a, 6b, 6e, 6f, and 6g). Clearly, both promoters have similar effects and do not induce major modifications relative to the Mo sample. The dehydrated samples give the Raman spectra 6d and 6h for the CoMo or NiMo sample, respectively, which are different from the hydrated sample because of the presence of the 1060-cm-' band already detected for the dehydrated molybdate- and heptamolybdate-supported species. Again it appears that the dehydration of the sample has an effect on the Raman spectra obtained after reduction whereas the presence of the promoter does not lead to noticeable transformation. The reoxidation of these reduced CoMo and NiMo samples has been investigated, and the Raman spectra have been reported in Figure 7. In both cases the original spectra of the oxide state, either hydrated or dehydrated, are recovered and no other phase can be distinguished. Noticeably, no differences between Co- and Ni-promoted catalysts have been found.
Discussion Preparation of the Model Catalysts. It is well-known that in the pH range of the molybdenum solution used for the support impregnation (Le., 2 < pH < 11) two major anions exist: the molybdate M o o t - and the heptamolybdate In aqueous solution these species are in equilibrium according to the following reaction M070246-
+ 4H20
7M0042- + 8H'
with log K = 57.7 in 3 M NaC104 a t 298 K.23 In the solution wetting the support during the impregnation step the modification of the monomer/heptamer (M/H) ratio will (23) Tsigdinos, G. A.; Chen, H. Y.; Streusand, B. J. Ind. Eng. Chem. Prod. Res. Deu. 1981, 66, 251.
-
6646 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Payen et al. SCHEME I
CoMo
360
i h
gL J.
Air-773K
Air-573K
0 2 -423 K
SCHEME I1
Air-423K
f 360
Ni Mo Air-298 K Air-773 K
0 2 -573 K 0 2 -373 K
a
J 1000
600
'
w 200
Air-298 K
cm-'
Figure 7. Raman spectra of reduced NiMo and CoMo catalysts of Figure 6 after reoxidation.
depend on both the pH and the molybdenum concentration. The alumina support is well-known to have a strong basic effect that changes the M / H ratio of the solution which contains initially the heptamolybdate species (pH 5.5). This has been observed in this work as in the case of the low Mo concentrated sample, all the heptamer has been transformed into the monomer species during the wetting step (Figure 1). In the case of the 14 wt % MOO, sample this effect still occurs but seems attenuated. However, this is not the only effect that determines the nature of the fixed species. The other one is the decrease of the ratio M / H during the drying step. This evolution occurs because of the acidification of the solution due to its higher concentration of either ammonium ions or Mo ions. In addition, as drying occurs, the Mo concentration increases, favoring also the presence of the heptamolybdate as the main species. Thus, the preparation of alumina-supported molybdate and heptamolybdate species can be realized by choosing the appropriate Mo loading. Stability of the Supported Species. The reported results strongly suggest that, in the range of calcination and reduction temperatures used (up to 773 K), both molybdate and heptamolybdate species are stable. After calcination this is illustrated by the reversibility upon rehydration of the spectra of the oxide. After reduction MOO, is not detected and is likely not formed within the experimental conditions used in this work. In addition, reoxidation always leads to the original spectra of the fixed species. This stability enhancement may seem surprising as it is known that bulk molybdate or heptamolybdate salts transform into molybdenum trioxide or MOO, a t rather low temperature of calcination or reduction (573-673 K). Such a stability enhancement likely results from the support interaction. Indeed, recent results on silica-supported heteropolyanions have shown a 250 K increase in the decomposition temperature of the heteropolyanionic structure.z4 This effect is evidently of interest as it allows to attribute the Raman spectral modifications detected in this work to chemical effects rather than to structural modifications. (24) Moffat, J. B.; Kasztelan, S.J . Catal., in press.
6C 5c 4C 100Oc m-1 10 0 0 - 1020cm-1 1040 -1 0 60 cm- '
Raman Bands of the Oxide State. In the supported molybdate and heptamolybdate oxide catalysts the main effect of the various treatment of the preparation is the shift of the stretching vibration of the M d 2 bond between 900 and 1010 cm-I. In addition, both the monomeric and heptameric species give similar spectra after calcination. Thus, a unified interpretation of those Raman band shifts should therefore be found. In the following the spectral modifications detected will be discussed by considering two effects that should influence band wavenumber in the oxide state, Le., ligand and coordinative h e t e r ~ g e n e i t y . ~ ~ It is known that molybdenum in its +6 oxidation state is a versatile ion which can adopt various configurations between tetrahedral and octahedral environments. In the latter case molybdenum always adopts a distorted configuration with rather short and long Mo-0 bonds. According to Goodenough,26 the different main environments of the molybdenum ion in an oxide state are given in Scheme I, where the subscripts t and b denote terminal and bridge, respectively. In that scheme, four entities are distinguished on the basis of the number of terminal Mo-0, bonds. Another way to consider these entities is in terms of ligand heterogeneity as proposed by Zecchina et al.25as each octahedral Mo ion has a different oxygen "ligand" such as terminal and bridged oxygen. In such entities, as the number of Mo-0, bond decreases, a shortening of the remaining ones can be expected, leading to an increase of the wavenumber of the corresponding band as demonstrated by Cotton and Wing.27 This effect is observed in Table I when comparing the Mo-0, bond symmetric stretching mode of the iso- and heteropolyanions in aqueous solution. The free molybdate anion has four Mo-0, bonds and gives a band at 898 cm-' (type I), the heptamolybdate with two Mo-0, bonds give a band at 937 cm-' (type 111), and the 12-phosphomolybdic or 12-silicomolybdic acids with one Mo-O, bond give a band at 995 and 980 cm-l, respectively (type IV). In the solid state Moo3 also has a band at 995 cm-' corresponding to a type IV group. This effect can also be found when comparing model compounds possessing different numbers of Mo-O, bonds such as Mo03F3' (915 cm-I), Mo02F," (951 cm-I), and MoOF4 (1048 cm-1).28 By analogy, the intermediate type I1 group (three Mo-O, bonds) can be proposed to lead to a band at ca. 920 cm-]. Furthermore, there is a second effect influencing the Raman band position of a terminal bond that needs to be considered, Le., the coordinative heterogeneityz5or existence of coordinatively unsaturated Mo6+ions such as those given in Scheme 11, with X = Mo, AI, Co, Ni and 0 = vacancy. A decrease of the coordination number should lead to an increase of the wavenumber of the Mo-0, band. Such an effect can be observed in model com(25) Zecchina, A.; Coluccia, S.; Cerruti, L.; Borello, E. J . Phys. Chem. 1971, 75, 2783. (26) Goodenough, J. B. In Proceedings of the 4th International Conference on fhe Chemistry of Molybdenum; Barry, H . F., Mitchell, P.C. H., Eds.; Climax Molybdenum: Ann Arbor, MI, 1982; p 1. (27) Cotton, F. A.; Wing, R. M. Inorg. Chem. 1964, 4, 867. (28) Schmidt, K. H.; Muller, A. Coord Chem. Reo. 1974, 14, 115.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6647
O,?/OH yo6+
?X
-
?/OH
q?
H9,o
,M","fM","-,Mc('
0 I Mo"
0I 0I 0I 0I 00 00 0000 OH I I I I I I I I I
xxxxxxxxxxxxx
go 9099 /I\ \
k0
6r
xxxx
6C o r a
+
OH
x
/I\\
-????
xxxx
-??099
+
?
//I\\
xxxxx
+
H20
x
5c
pounds with Mo5+ possessing different coordinations such as MoOCI~~(950 cm-'), MoOCl, (ca. 1000 cm-I), and MoOCl, (1013 cm-1).28 An attribution of a 1000-1020-cm-' band to Mo6+5, can therefore be proposed by analogy with model compounds MoOC14(1027 cm-I) and MoOF, (1048 cm-1).28 A higher wavenumber (ca. 1040-1060 cm-I) is likely for Mo6+&although no reference compounds have been found in the literature. The shifts toward higher wavenumber of the main band of the molybdate or heptamolybdate anions in interaction with the alumina surface during the preparation can therefore be tentatively assigned to a decrease of the number of M A , bonds according to Scheme 111, with X = Mo, Al, Co, and Ni. In that scheme the dehydration-hydration process generating species IV looks like a bridge or bond creation-rupture process as proposed earlier.18 However, this process results more likely from the creation of vacancy according to Scheme IV, with X = Mo, Al, Co, Ni and = vacancy. In that case both intra- or intermolecular dehydration can be considered whether or not X belongs to the same isopolyanion or eventually to the support. Both the ligand and coordinative heterogeneity effects lead to an increase of the Mo-O, wavenumber as observed for both the monomer and the heptamer species. Evidently, some differences in the band position between the supported molybdate and heptamolybdate species are to be expected because of the presence of the neighbor Mo ion in the heptamolybdate relative to the isolated molybdate. This is the case in the calcined hydrated (940 and 950 cm-I) and dehydrated (1005 and 1000 cm-I) molybdate and heptamolybdate samples, respectively. However, this difference of Mo-0, band position is quite small, making the distinction between both supported species difficult. In addition to the bands previously discussed, bands centered a t 860 cm-I and in the range 300-360 cm-I can be seen in the various spectra of the calcined catalyst reported in Figures 1 and 3. It is likely that several modes contribute to the 860-cm-I broad band, in particular Mo-0-Mo ones for the polymeric species. However, that band being present in the supported molybdate species (spectra Id and le), it can be proposed that the M&AI group also gives a band in that region in accordance with the suggestions of several a u t h o r ~ . l ? ~Further -~ support to this attribution can be found in recent literature when comparing the Raman spectra in the region 750-900 cm-I of Mo species on several supports. Clearly, a band sensitive to the nature of the support can be found at 850 cm-I for Mo/Zr02, 800 cm-' for Mo/TiO,, 790 cm-l for Mo/CeO,, and 850 cm-I for Mo/Si0,29 and be attributed to the Mo-0-X group. The 320- and 360-cm-' bands of the hydrated catalysts are usually assigned to the bending mode of the Mo-0, bond in the molybdate and heptamolybdate species, respectively (Table I). These bands allow an easy distinction between both species as pointed out by Knozinger et al.Iqz9 In the heteropolyanions this (29) Leyrer, J.; Vielhaber, B.; Zaki, M. I.; Shuxian, Z . ; Weitkamp, J.; Knozinger, H. Mater. Chem. Phys. 1985, 13, 301.
band is found at 250 cm-' (Table I), showing a net effect on the frequency resulting from the isolation of the terminal Mc-0, bond. In model compounds such as MoOF4 the corresponding band can be found a t 294 cm-'.28 For the supported catalysts in the dehydrated state the Mc-0, bending mode tends to shift to 300 cm-', which confirms the appearance of a single Mo-0, bond. Finally, the 220-cm-' band assigned to the deformation mode of the Mo-0-Mo bridge (Table I) is characteristic of the heptamolybdate and can be used to identify that species.'-zg In Schemes 1-111 the existence of O H groups has been considered. This is not surprising as the ammonium ion after decomposition should be replaced by a proton. The presence of these protons is furthermore supported by other results such as pyridine chemi~orption.~ However, this means that the acid form of both species and in particular the heptamolybdic acid exist on the support surface. While molybdic acid is a known compound, it appears that heptamolybdic acid, unknown in solution and in the solid state, would be stabilized by the support. The existence of the M d H group is of importance when the problem of the interaction between the oxomolybdenum species and the promoter ions is considered. Indeed, Stencel et a1.I6 have recently evidenced an interaction between Mo and Ni species in a LRS study of the effect of Ni cation addition on dehydrated calcined NiMo catalysts. Interestingly, they observed a decrease of the ratio of the intensities of the 1000-cm-I over 860-920-cm-' bands which may result from either the disappearance of the terminal Mc-0 bond (1000 cm-') or the creation of the Mc-O-Co or Mo-0-Ni bond (expected at ca. 860-920 cm-I). In addition, Kasztelan et al. have evidenced a direct interaction between Mo and Ni or Co ions by low-energy ion-scattering s p e c t r o s ~ o p y . ~ ~ These authors have suggested that the interaction resulted from a cation exchange giving supported Ni or Co salts of oxomolybdate species. In Schemes 1-111 this interaction has been implicitly considered by taking X as Co, Ni, or other cations. Raman Bands of the Reduced State. To interpret the spectral modifications observed upon reduction, the variation of the oxidation number of the Mo ions has to be taken into account in addition to the previously defined ligand and coordinative heterogeneity effects. These effects result not only from the reduction process of the samples but also from the dehydration reaction occurring simultaneously in particular for the hydrated sample. The molybdate, heptamolybdate, and promoted heptamolybdate species all give similar spectra after reduction. Therefore, a common interpretation has to be proposed as for the oxide state. In the following a tentative assignment is suggested. The main features of the Raman spectra of the reduced samples are the disappearance of the intense bands in the 900-1000-~m-~ range and generation of very characteristic bands at 1000 cm-I, in the 750-850-cm-' range, and at 300 cm-'. In addition, on the reduced dehydrated samples a band at 1060 cm-I remains. It is well-admitted that the wavenumber of the stretching mode of the M d , bond of reduced Mo ions decreases relative to Mo6+ ions to give bands in the 1000-900-~m-~ range.28J' Therefore, the presence of the 1000-cm-' broad band in Raman spectra of reduced hydrated samples suggests that the terminal Mo-0, bond still remains in the reduced state although it tends to disappear upon drastic reduction. This band can, however, also be attributed to a Mo6+-0, bond as seen previously, but the existence of this oxidation number appears unlikely after such reduction pretreatment as shown by XPS by Zingg et aL4 The broad intense band at 300 cm-' supports the conclusion that M A , groups still exist. Although this band can result from the creation of isolated Mo6+-0,, it is likely that the broadening corresponds to the presence of Mo5+-0, and Mo4+-0, bonds. The bands in the 750-860-cm-I range can be assigned to either bridged Mo-0-Mo or Mo-0-A1 bonds with reduced Mo ions. As both the molybdate and heptamolybdate give the same spectra, it can be suggested that the major contribution to these bands (30) Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. J. Phys. Chem. 1987, 91, 1503. (31) Mitchell, P. C. H. Q.Reu. 1966, 20, 103.
J. Phys. Chem. 1987, 91. 6648-6658
6648
is the Mo-O-A1 groups with the band at 850 cm-' corresponding to MoS+-O-Al and the band at 760 cm-' corresponding to MO~+-O-A~. The presence of the 1060-cm-' band on reduced dehydrated samples is intriguing. This is a very high wavenumber for a M d , bond which can only be found in M o O F , . ~ * ~As ~ ] it is unlikely that such a band corresponds to a reduced Mo ion, it can be suggested here to assign this band to a very distorted environment of residual Mo6+ ions leading to a short Mo-0, bond. This environment can result from a decrease of the coordination number giving Mo6+&. This species appears only after reduction of dehydrated samples which seems therefore more difficult to reduce. This suggests that the presence of water or O H groups favors the reduction. Such an observation has already been reported for the s u l f ~ r i z a t i o n . ~ ~ As this intermediate species would be very sensitive to hydration, it is possible that it cannot be easily seen on the hydrated sample. From these observations it can be proposed that as a first step reduction would proceed through Mo6+5cor Mo6+&ions possessing vacancies allowing hydrogen chemisorption and lead to various coordinatively unsaturated Mo5+and Mo4+and eventually to Mo ions in low oxidation states. No net perturbation of the observations made on the Mosupported sample has been detected as a result of the promoter addition. However, the presence of the promoter ion seems likely to increase the reduction rate because hydrogen dissociation is easier on these ions. Conclusion
Within the experimental conditions used in this work it is shown by in situ LRS that alumina-supported molybdate and heptamolybdate species remain stable during preparation, reduction, (32) Amoldy, P.; Van Den Heijkant, J. A. M.; De Bok, G. D.; Moulijn, J. A. J . Catal. 1985, 92, 35.
and reoxidation. This demonstrates a marked increase of stability relative to the bulk compounds induced by the support. Therefore, the Raman spectral modifications detected can be assigned to chemical effects such as ligand heterogeneity, coordinative heterogeneity, and variation of the oxidation number. In the oxidic state the terminal M d bond stretching frequency increases from the wet to the calcined dehydrated catalyst. This frequency is therefore strongly dependent on the environment of the Mo-0, bond. The increase of the frequency results from stronger interactions with the support surface leading to isolation of one terminal Mo-0 bond and/or vacancy generation in particular by dehydration. The other bands have been assigned according to literature proposals. In particular, the band at 860 cm-' has been assigned to the Mo-0-A1 bond. After reduction no M o o 2 is detected, and molybdate, heptamolybdate, and promoted heptamolybdate species all give similar spectra with strong modification relative to the oxide. Bands detected at 1000 cm-', at 300 cm-I, and in the 850-750-m-' range have been assigned to M O ~ + ~ ~ +and - O , M O ~ + ~ ~ + - Ogroups, -A~ respectively. On reduced dehydrated samples a band at 1060 cm-' has been detected and attributed to a very distorted Mo6+-0, bond stretching mode resulting from residual Mo6+with 4-coordination. The reduction has been found enhanced by both the presence of water and promoter ions. The band assignment of reduced species proposed in this work remains in part speculative due to the lack of Raman data in the literature, and further work is necessary in order to reach a more complete interpretation. However, these results illustrate the potentiality of in situ LRS to study the reactivity of supported species in particular upon reduction.
Acknowledgment. We are indebted to the Institut Francais du Petrole for financial support. Registry No. Moo:-, 14259-85-9; M o , O ~ ~ ~12274-10-1; -, Mo, 7439-98-7;Ni, 7440-02-0;Co, 7440-48-4.
A Quantum Chemical Study of ZnO, Cu/ZnO, Cu20, and CuO Clusters and CO Chemisorption on Zn0(0001), CuZn0(0001), and Cu/Zn0(0007) Surfaces Jose A. Rodriguez and Charles T. Campbell*+ Chemistry Department, Indiana University, Bloomington, Indiana 47405 (Received: April 27, 1987, In Final Form: July 2, 1987)
Copper/zinc oxide mixtures show strong synergistic effects when used together as catalysts, particularly in methanol synthesis and water-gas-shift reactions. We have employed semiempirical quantum-mechanical calculations (INDO) to study the electronic properties of ZnO, CuO, and Cu20clusters (126 atoms), of Cu adsorbed on or substituted in these ZnO clusters, and of CO chemisorbed on ZnO(0001) and Cu/ZnO clusters. The results are discussed in light of models previously proposed to explain the unique properties of Cu/ZnO catalysts, which often involve TU+" impurities in (on) the ZnO lattice. We use mainly the calculated charge on the Cu atom and its interaction with CO to address the electronicproperties of Cu substituted in and adsorbed on ZnO clusters. Our results for neutral clusters indicate that, with respect to atomic charge, this Cu is quite similar in nature to the Cu atoms of bulk CuO or the Zn atoms of ZnO, where the metal has a formal oxidation state of +2. The Cu site in these ZnO clusters shows unique electron affinity properties, accepting a major fraction of the added electron density for anionic clusters. The mechanisms of CO chemisorptive bond formation on Cu( loo), Zn0(0001), and Cu-doped ZnO are compared and contrasted based on the present results and those in the literature.
I. Introduction
Catalysts based upon Cu/ZnO mixtures are very useful in a number of reactions of current or potential industrial importance: hydrogenation of ethylene (C2H4 H2 C2H6),' oxidation of C O (2CO O2 2C02),2methanol synthesis (CO 3H2 CH30H),3,4water-gas shift (CO H 2 0 C 0 2 + H2),5and methanol steam reforming ( C H 3 0 H H 2 0 C 0 2 3H20).6
+
-
+ +
-
+
--
+ +
-
+Alfred P. Sloan Research Fellow
0022-3654/87/2091-6648$01.50/0
As such, these catalysts have been the subject of intense fundamental and practical research, especially with respect to methanol synthesis. 334,7-19
(1) Kokes, R. J. Acc. Chem. Res. 1973, 6, 226. (2) Dwyer, F.G. Catal. Rev. 1972, 6, 261. (3) Kung, H. H. Catal. Rev.-Sci. Eng. 1980, 22, 235. (4) Klier, K. Adu. Catal. 1982, 32, 243. (5) Newsome, D. S. Cafal.Rev.-Sci. Eng. 1980, 21, 275. (6) Kobayashi, H.; Takezawa, N.; Minochi, C. J . Cafal. 1981, 69, 487.
0 1987 American Chemical Society
