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alloy crystallites over the entire composition range of a binary system, despite the fact that the phase diagram exhibits a wide miscibility gap. However, some theories predict surface enrichment by segregation of Cu to sites with low coordination number such as ledges and kinks, even if in the bulk the metals are completely mi~cib1e.l~ Obviously reliable experimental data are needed to discriminate between the different models. With respect to the catalytic properties the results are qualitatively similar in both systems. The catalytic activity decreases with increase of the Cu content, but in a different manner for the methanization and Fischer-Tropsch reactions. The Fischer-Tropsch synthesis of higher hydrocarbons is apparently more sensitive to the introduction of Cu than the methanization reaction. It is noteworthy that the dependency of the turnover number on the temperature is neither a function of the Cu/Ni or Cu/Ru ratio, nor of the H2/C0 ratio, but solely of the properties of the active metal. Since the actual rates depend markedly on the metal ratios this means that there is no “ligand effect” but only a “geometric effect”, Le., the number of active metal atoms in the average ensemble at the surface determines whether the reaction can or cannot
H. Knozinger and H. Jeziorowski
occur. The minimal number of active metal atoms in the “Fischer-Tropsch ensemble” is apparently larger than in the “methanization ensemble”. Acknowledgment. The authors thank Professor Dr. G. C. A. Schuit for helpful suggestions, stimulating discussions, and critically reading the manuscript and W. V. Herpen for technical assistance.
References and Notes V. Ponec, Cafal. Rev., 11, 41 (1975). M. A. Vannice, Catal. Rev., 14, 153 (1976). J. H. Slnfelt, Acc. Chem. Res., 10, 15 (1977). G.C. A. Schuit and N. H. de Boer, Red. Trav. Chlm., Pays-Bas, 70, 73 (1951). G. C. A. Schuit and N. H. de Boer, Recl. Trav. Chlm., Pays-Bas, 72, 103 (1953). P. W. Selwood, J . Catal., 42, 148 (1976). G. C. Bond and B. D. Turnharn, J . Cafal., 45, 128 (1976). J. C. M. Harberts, A. F. Bourgonje, J. J. Stephan, and V. Ponec, J . Cafal., 47, 92 (1977). S. Engels, P. Hartmann, R. Malsch, and M. Wilde, Z. Chem., 16, 330 (1976). S. D. Robertson, S. C. Kloet, and W. M. H. Sachtler, J . Catal., 39, 234 (1975). J. A. Dalmon, G. A. Martin, and B. Imelik, Surface Sci., 41, 587 (1974). D. F. Ollis, J. Catal., 23, 131 (1971). J. J. Burton, E. Hyman, and D. G. Fedak, J. Catal., 37, 106 (1975).
Raman Spectra of Molybdenum Oxide Supported on the Surface of Aluminas H. Knozinger” and H. Jeziorowskl Institut fur Physikalische Chemle, Universitat Munchen, 8000 Munchen 2, West Germany (Received April 4, 1978)
Raman spectra of molybdenum oxide, supported at different concentrations on the surfaces of 7- and 7-A1203, have been recorded. The dominant species formed, presumably in a “monolayer”, is a polymeric aggregate of molybdenum-oxygen octahedra. The two supports influence the structure of the supported molybdenum-oxygen phase in distinct manners. Structure-determiningfactors are probably the surface areas and pore diameters and also the nature and structure of exposed crystallographic planes of the spinel lattice.
Molybdenum oxide supported on yA1203is an interesting catalytic system1p2and, promoted by Co2+and/or Ni2+,in the oxidized form it is the precursor for most important hydrodesulfurization catalyst^.^+^ Despite the wide-spread use and technical importance of this catalytic system, the detailed structure of the molybdenum oxide supported on the alumina surface is still not well understood. Massoth5has recently described a model for the Mo03/A1203 system which suggests the occurrence of one-dimensional chains of Mooz species, the third 0 associated with Mo being placed in vacancies of the alumina support. For the sulfided catalyst forms obtained from oxidized Co0-Mo03/A1203 catalyst precursors, different models have been put forward in the past, namely, the monolayer model? the intercalation model? and the model of synergy by c ~ n t a c t .Furthermore, ~ controversal opinions regarding the coordination of molybdenum in the oxidized catalysts appear in the literature.“ll Very recently, Brown and c o - ~ o r k e r s have ~ ~ J ~shown that Raman spectroscopy can be applied to obtain vibrational spectra of the supported molybdenum oxide phase. These authors used commercial catalysts, the preparation conditions of which were unknown. Thus, a correlation between the observed spectra and preparation conditions was not possible and, moreover, some bands could not be assigned unequivocally. We have therefore prepared a series of molybdenum oxide catalysts supported on both 7-and 7-A1203,the Raman 0022-3654/78/2082-2002$0 1.OO/O
spectra of which were recorded. The r-Al2O3 support (Catapal) was from GirdlerSudchemie AG. The 7-A1203was prepared from aluminum isoproxide. Both materials were calcined in air at 1073 and 1023 K, respectively, before use. Their respective BET surface areas were 97 f 10 and 150 f 10 m2/g after this treatment. The supported molybdenum oxide catalysts were prepared by wet impregnation at pH 6 of the alumina carriers. These were soaked in a quantity of distilled water, which was sufficient to just fill the total pore volume and which contained the appropriate amounts of ammonium paramolybdate (NH4)6M07024 to produce catalysts containing 4, 8, and 12 wt % of Moo3. The impregnated material was left a t room temperature for 15 h, dried a t 383 K for 15 h, and then calcined in air a t 923 K for 2 h. These samples were pressed into a ring of a rotating sample holder. The Raman spectra were recorded on a Cary 82 spectrometer equipped with a triple monochromator. The 514.5-nm line from a Spectra Physics Model 165 Ar+ laser was used for excitation. The spectral slit width was typically 4 cm-l and a laser power of approximately 60 mW was used. The samples were rotated at a frequency of approximately 60 Hz. The wavenumbers obtained from the spectra are accurate to within f 2 cm-l. Some typical spectra are shown in Figure 1, and the positions of Raman bands observed for the different samples are summarized in Table I together with the 0 1978 American Chemical Society
Raman Spectra of MOO, on AI,O, Surfaces
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978 2003
TABLE I: Raman Bands (cm-l) of Molybdenum Oxide Supported on q - and r-Al,O, and of Reference Compoundsa wt % MOO, on y-Al,O, 4% 117 W,SP 1 2 3 W,SP 1 3 0 VW,SP 134 W,SP
8% 117 w,sp 123 w,sp 130 vw,sp 134 w,sp
223 vw
-376 vw,br
-371 vw,br
wt % MOO, on s-Al,O, 12%
4%
8%
12%
MOO,
116 w,sp
117 m,sp 123 w,sp 1 3 1 vw,sp 140 w,sp 154 vw,sp
117 m,sp 123 w,sp 1 3 1 vw,sp 140 w,sp 157 vw,sp
116 m,sp
117 s
115 w
129 w,sp
129 s
134 w
158 s 199 w 218 w
197 vw,sh
- 223 m,br 252 w
158 m,sp 198 vw 217 vw 220 w,br 245 vw 284 m,sp 291 w,sp
-377 w,br
339 w,sp 367 vw 370 w,br 380w,sp
138 vw,sp
247 W . 284 s 292 m 337 m 367 w
Mo,0,,6- solid
220 m,br 245 w,sh 308 vw 337 vw 363 w,br
380w 474 vw 549 vw 572 vw,sh 633 vw
-870 w,sh
820 vw,sh 845 w 858 w 873 w
665w,sp 821 s,sp 865 m 882 m
866 w,br
963 s,br
969 s,br
667m 820 vs 838 vw,sh 859 w 879 m,sp 888 m,sp 901 w,sh 906 w,sp 913 vw,sh 931 vs
898 w,sp 957 m,br
965 m,br
970 s,br
950 s,br 970 sh
998 s,sp
996 vs
a Al,(MoO,), (most prominent bands): 381 s,sp; 1004 s,sp; 1026 m,sp. vw = very weak; w = weak; m = medium strong; s = strong; vs = very strong; br = broad; sp = sharp; sh = shoulder,
Raman bands of several reference compounds. The most important observations can be summarized as follows: (1)Hydroxylated alumina supports usually show a very strong fluorescence background which has been shown to be due to electronic excitations of surface hydroxide ions.14 This fluorescence is strongly decreased or even eliminated after deposition of the molybdenum oxide phase. As shown by IR spectroscopy,15 surface hydroxyl groups of the support are largely eliminated during the preparation of the supported catalysts by reaction with ammonium paramolybdate. The fluorescence background is thus reduced. (2) At Raman shifts between 50 and 150 cm-l a series of sharp lines can be observed in all samples which must be assigned as lattice modes of the alumina support. A clear distinction between q- and y-A1203cannot be made although the relative intensities of the lattice modes differ slightly for the two modifications. (3) The most prominent features in the spectra of all supported catalysts are broad bands in the range between 950 and 970 cm-l which are accompanied by broad bands or shoulders at 860-870 cm-l, and close to 370 and 220 cm-l. The latter two bands are weak and relatively illdefined at the lower concentrations. On the yA1203 support these four bands are the only detectable bands. The most intense band at the highest Raman shifts, which appears at 970 cm-l on the catalyst cdntaining 12-wt % Moo3, seems to shift very slightly to lower Raman shifts at decreasing Moo3 content, On the 7-A1203the spectra show more complicated variations as the Moo3 content is reduced, namely the broad bands at 970 and 870 cm-l are split at the lower MOO, contents (see Figure 1 and Table I). An assignment of these spectral features will be given below.
Figure 1. Raman spectra of molybdenum oxide supported on q-A120& (A) 12 wt % Moo3; (B) 4 wt % MOO,; (C)8 wt % Moo3.
(4)On the catalyst containing 12 wt % MOO, on q-A1203 bulk Moo3 is clearly formed as evidenced by the occurrence of a series of sharp lines (Figure 1)which exactly correspond to the Raman spectrum of MOO, (see Table I). ( 5 ) On the catalyst containing only 4 wt % MOO, on q-A1203a relatively weak sharp band is observed at 898 cm-l together with a shoulder near 820 cm-l. These bands
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The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
have to be assigned as 5l and ZZ modes, respectively, of MOO^^-, although the 53 and 54 modes could not be detected. The band at 898 cm-l belongs to a species Al, while the ij2 mode belongs to species E and the Z3 and 54 modes to species F2 in the free Mood2-ion.16 On the surface, the M o o t - tetrahedra are distorted which leads to a splitting of all bands belonging to degenerate species in the free ion. These bands could therefore escape detection at the low levels of concentration of MOO^^- present in the sample. (6) One might expect the formation of bulk A12(M004)3. If this compound is formed at all, then its concentration must be extremely low since the characteristic Raman lines, namely, those slightly above 1000 cm-l (see Table I), could not be detected in any of the catalysts. This observation is in agreement with the conclusions of Pott and Stork,17who suggested that N2(M004)3may be formed on dry impregnation but not on wet impregnation of the alumina support. The typical broad and asymmetric band near 970 cm-l has been assigned by Brown and co-workers13as a molybdenum-oxygen stretching fundamental associated with octahedrally coordinated molybdenum. The exact nature of the band was, however, not unequivocally ascertained. As shown above, the band at 950-970 cm-l appears jointly with bands at 860-870 cm-l and at 370 and 220 cm-l (see Table I). Very similar features, although, not unexpectedly, not at exactly the same wavenumbers, are observed for the M070246- ion in ("4)6M07024. The bands are relatively sharp in the solid, but appear broadened in aqueous solution, from which the alumina has been impregnated. The most prominent bands of the Mo70246-ion are found a t 930-940 cm-l (see Table I), Le., a t lower wavenumbers as compared to the characteristic band of the supported catalysts. It has been shown, however, that an intense broad band occurs a t 963 cm-l for solid ("4)4M080z64H20 together with bands at 916, 360, and 200 cm-l.18 Thus, the spectra of the supported catalysts very closely resemble those of polymeric molybdenumoxygen units, although the degree of aggregation is not known. Such species are apparently formed exclusively on the three r-Al,03 supported samples and they are preferentially formed on the two 7-A120, supported samples containing 4 and 8 wt % Moo3. According to Lindqvist the M0@246- ion is built up by seven edgesharing Moo6 0 ~ t a h e d r a . lAssuming ~ a similar structure for the supported material, a preferential octahedral coordination of molybdenum is suggested for the catalysts studied. For the Moo3 concentrations applied, this conclusion is in accord with the results of Giordano et al.ll On the basis of a recent surface model of alurninas2Oand of IR spectroscopic data of supported molybdenum oxide catalysts, it has been concluded15 that the molybdenum-oxygen polyhedra may indeed be aligned in parallel rows on the support surface. The 0-0 distances within the octahedra of the M070246 ion and distances between oxygen atoms of octahedra which share corners match rather well the distances between neighboring oxygen atoms within a row and between oxygen atoms of every second of these parallel rows in (111)faces of the spinel lattice of alumina. I t seems therefore possible that structures similar to those of the ion are being formed on alumina surfaces in such a way that several Moo6 octahedra are anchored via oxygen bridges onto the support surface, the oxygen atoms of these bridges being aligned in parallel rows. At present, it cannot conclusively be decided whether all octahedra of the polymeric surface species are directly bound to the surface or whether some three-dimensional character as in the original polyanion
H. Knozinger and H. Jezlorowski
is maintained. For reasons of stability one would probably expect most of the polyhedra to be bound to the alumina surface. The differences between catalysts supported on 7- and y A l z 0 3can be explained as follows: the 7-A1203support has a larger surface than the 7-A1203. Consequently, 7-A1203must have narrower pores. Under the impregnation conditions at pH 6, Mo70246-ions exist exclusively in aqueous solution.18 These large ions can block the narrow pores of 7-Alz03, so that molybdenum oxygen species cannot be deposited within the pore volume. At higher loadings, not all the molybdenum oxide can be spread out over the accessible surface in a "monolayer", so that bulk MooB is formed upon calcination of the catalyst containing 12 wt % Moo3 supported on 7-Alz03. On 7-A1203,on the contrary, despite of the smaller total surface area the same amount of Moo3 can be spread out over a larger accessible surface area. Moreover, if the model described above is correct, not all the low index crystallographic planes do equally well match the dimensions of the polymeric aggregate. The distribution of exposed crystallographic planes differs on the surfaces of the two alumina modifications.20 This fact may explain the differences in the spectra at low Moo3 concentrations, namely, the splitting of the bands near 960 and 870 cm-l on the 7-A1203supported samples, which my be assigned as being due to the formation of polymeric molybdenum-oxygen polyhedra at varying degrees of aggregation. At low loadings a very small portion of the molybdenum-oxyen species is being held as distorted MOO:tetrahedra. A quantitative estimate cannot be made of the fraction of molybdenum in tetrahedral coordination. It is, however, quite possible that this state does not entirely disappear a t higher loadings but that it is being overwhelmed by the spectra of the more abundant polymeric species. It should be mentioned that the present conclusions deviate from those drawn by Ashley and Mitchell8 from their UV diffuse reflectance spectra. These authors suggested that M o o t - tetrahedra were being formed on the alumina support. The results of Giordano et al.ll are closer to the present data. These differences cannot be explained easily. They may be due in part to slightly differing preparation conditions and the fraction of tetrahedral species as obtained from UV spectra may be overestimated due to higher extinction coefficients of tetrahedral than of octahedral species. It has been shown that Raman spectroscopy can provide very important information on the structure of supported oxide phases. Additional work is in progress in this laboratory and further results including studies on promoted molybdenum oxide on alumina will be published elsewhere in the near future.
Acknowledgment. Financial support of this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. References and Notes (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10)
W. K. Hall and F. E. Massoth, J . Catal., 34, 41 (1974). M. Lo Jacono and W. K. Hall, J. ColloklInterface Sci., 58, 76 (1977). G. C. A. Schuit and B. C. Gates, AlChE J . , 19, 417 (1973). V. H. J. De Beer and G. C. A. Schuit, "Preparation of Catalysts", B. Delmon, P. A. Jacobs, and G. Poncelet, Elsevier, Amsterdam, 1976, p 343. F. E. Massoth, J . Catal., 36, 164 (1975). R. J. H. Voorhoeve, J . Catal., 23, 236 (1971). 0. Hagenbach, Ph. Courty, and B. Delmon, J. Catal., 31,264 (1973). J. H. Ashley and P. C. H. Mitchell, J . Chem. SOC. A , 2730 (1969). G. N. Asmolov and 0. V. Krylov, Kinet. Katal., 11, 847 (1970). J. M. J. G. Lipsch and G. C. A. Schuit, J . Catal., 15, 174 (1969).
Conformations of EDA Complexes of TCNE (1 1) N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan, and G. Martinotti, J. Catal., 36, 81 (1975). (12) F. R. Brown and L. E. Makovsky, Appl. Spectrosc., 31, 44 (1977). (13) F. R. Brown, L. E. Makovsky, and K. H. Rhee, J . Catal., 50, 162, 385 (1977). (14) H. Jeziorowski and H. Knozinger, Chem. Phys. Lett., 51, 519 (1977). (15) P. Ratnasamy and H. Knozinger, J. Catai., in press.
The Journal of Physical Chemktry, Vol. 82, No. 18, 1978 2005
(16) A. Muller, N. Weinstock, N. Mohan, C. W. Schiipfer, and K. Nakamoto, Appl. Spectrosc., 27, 257 (1973). (17) G. T. Pott and W. H. J. Stork in ref 4, p 537. (18) J. Aveston, E. W. Anacker, and J. S. Johnson, Inorg. Chem., 3 , 735 (1964). (19) I. Llndqvlst, Ark. Kern., 2, 325 (1951). (20) H. Knozinger and P. Ratnasamy, Cafal. Rev. Sci. Eng., 17, 31 (1978).
Spectroscopic Studies on the Conformations of Electron Donor-Acceptor Complexes of Tetracyanoethylene Michael J. Mobley,+ Klaus E. Rleckhoff, and Eva-Maria Voigt” Depaflments of Chemistry and Physics, Simon Fraser Universlty, Burnaby, British Columbia V5A 1S6,Canada (Received September 19, 1977; Revised Manuscript Received June 12, 1978) Publication costs assisted by the National Research Council of Canada
The temperature dependence of the relative intensities of the double charge-transfer (CT) absorption bands of electron donor-acceptor complexes of tetracyanoethylene (TCNE) with substituted benzenes, naphthalene, and pyrene in dichloromethane solution,in inert polymer films of polyethylene and poly(methy1methacrylate), and as polymer-TCNE films of polystyrene, poly(4-methylstyrene),and poly(4-methoxystyrene) is reported. Films were studied in the range from 1.5 to 330 K. The results obtained show that two stable isomeric structures exist for each complex with an asymmetrically substituted benzene, naphthalene, or pyrene as donor, corresponding to maximum overlap for the highest and second highest donor orbital, respectively, with the same lowest empty acceptor orbital of TCNE. Association constants and energy separations for the two complex conformations are determined. The relative stabilities of the separate isomers in the complexes studied are found to be dependent upon the electron donating strength of substituents on the benzene ring, as well as on the solvating environment. Low temperature film studies indicate a temperature dependence for the oscillator strengths associated with CT transitions,
I. Introduction The question of the molecular structure of the ground electronic state of the generally weak r / r electron donor/acceptor (EDA) complexes in solution has been a matter of considerable experimental and theoretical interest over the past 3 decades. Theory, of course, has addressed itself mainly to the already complex situation of “isolated” 1:l EDA interactions (Le., gas phase encounters at low pressures). Using Mulliken’s well-known charge transfer theory of EDA complexes, maximum overlap between r-electron donor and acceptor molecular orbitals will lead to preferred sandwich geometries of the complexes when charge transfer is a strongly contributing interaction. However, in weak EDA complexes the contribution from actual charge transfer in the ground state is generally small and various other interactions must be taken into account whose relative contributions are difficult to assess.l This difficulty has been acerbated due to the lack of primary experimental information. In a recent publication we presented the first direct experimental evidence that r/r EDA sandwich structures exist in solution by studying the temperature dependence of the relative absorption intensities of EDA complexes which have more than one charge transfer band, specifically complexes of tetracyanoethylene (TCNE) as acceptor and suitably substituted benzenes as donors.2 In the present study we have extended our previous investigation and more importantly, we have obtained experimental evidence of significant contributions to the ground state EDA complex structures by intra- and intermolecular Department of Chemistry, Arizona
State University, Tempe, Ariz.
85281. 0022-365417612082-2005$0 1.OO/O
interactions other than charge transfer. We determined in various environments the relative stability of the two most stable isomeric conformations which we have shown to exist for each appropriate TCNE/donor complex. The two stable isomeric structures correspond to geometric configurations maximizing the probability for each of the two CT transitions of a given complex as discussed by Voigt3 and Zweig4 and as shown in Figure 1. The figure depicts the general case of any mono- or para-disubstituted benzene with electron-donating substituents: the Y configuration giving rise to the low energy band and the X configuration to the high energy band. Theoretical calculations by Lippert, Hanna, and Trotter5 based upon the formalism of MurrelF have previously predicted two stable rotational isomers corresponding to the X and Y configurations of Figure 1for the p-xylene-TCNE complex. The calculations predict that the exchange repulsion interaction, which contributes a 4-kcal/mol barrier between conformations, is of primary importance in determining the complex geometry. In contrast, perturbation calculations by Herndon and Feuer’ using CND0/2 determined molecular orbitals have predicted only a single preferred conformation for the p-xylene-TCNE complex with the X and Y configurations corresponding to energy maxima. Holder and Thompson8 have interpreted the relative CT band intensities of increasingly sterically hindered alkylbenzenes with TCNE in terms of these multiple configurations. They concluded that para substitution by bulkier groups favored the X configuration of the complex as increased steric hindrance would apparently result from the Y configuration. Sakurai and Kirag found that the relative intensity of the double CT bands had approximately linear dependence on the Q+ 0 1978 American Chemical Society
