Langmuir 1994,10, 3094-3104
3094
Spectroscopic Studies (UV-vis and FTIR) of CO and Ethene Molecular Complexes and of Ethene Oligomerization on a-Cr203 Surfaces Domenica Scarano, Giuseppe Spoto, Silvia Bordiga, Luca Carnelli, Gabriele Ricchiardi, and Adriano Zecchina* Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Univemita di Torino, Via Pietro Giuria 7,I-10125 Torino, Italy Received March 4,1994. I n Final Form: May 30, 1994@ The W-vis and FTIR spectra of CO, adsorbed at the surface of a-CrzOs microcrystals, were preliminarily studied to gain information on the nature of the actiye sites (coordination number and local structure) present on the most abundant (0112), (21161,and (1120) faces. On these faces two main families of Cr3+ (5-fold and, to a lesser extent, 4-fold coordinated) are present, which form, with carbon monoxide, a-n 1:l adducts, with predomigant u character. The molecular adsorption of ethene involves the 5-fold C$+ sites ofthe predominant(0112)faces,giving a CZ"coordinationcomplex, where ethene interacts with the electronwithdrawing Cr3+ centers, through a o-bond, perpendicular to the plane of the olefinic system. On high index faces,the ethene molecule oligomerizes,the polymerization activity of reduced samples being definitely larger than that of stoichiometric samples. The presence of a small fraction of reduced centers on higher index faces and on edges is responsible for the increased polymerization rate and for the formation of longer polymeric chains. The problem of the interaction of the hydrocarbon chains, formed at the catalytic centers, with the Cr3+sites of the predominant (but not catalytically relevant) faces is also considered, by studying the interaction of a model hydrocarbon (n-heptane)with the clean surface. The effect of CO redosing on the a-CrzO3 surfaces, precovered by hydrocarbon chains, is studied and the remarkable difference in the frequency and intensity of the CO peaks is discussed. oxygen bridges, are the key structures conferring on the Philips catalyst its well-known high activity. Chromium-based systems like Cr/SiOz and Cr/Al203are Modeling the behavior of these catalysts is complicated active catalysts in ethene p~lymerization.l-~ In particular by the amorphous (Si021 or defective nature (y-AlzO3)of Cr/SiOz prepared by impregnation of Si02 with chromic the support where the Cr ions are grafted or embedded acid (followed by reduction in CO, Hz, or CzH4 itself) is the and by the small number of sites really active in the base ofthe Phillips c a t a l y ~ t . ~Cr-based ~~-~~ active ~ centers polymerization reaction. Despite these difficulties, the are also involved in the Union Carbide catalyst prepared basic chemistry of anchored Cr sites toward simple ligand through interaction of CrCpz with surface ~ i l a n o l s . ~ ~ ~ molecules has been elucidated in recent times, principally Similarly mild activity is found in other Cr-based systems by means of FTIR spectroscopy.6 The sensitivity of the like Cr/SiOz prepared via different p r e c ~ r s o r s ~ or- Cr/ ~~~ modern FTIR spectrometers is such that even the chemA l ~ o In ~ .all ~ these systems the valence, coordination istry.of active ions representing only a very small fraction states, and the number of Cr sites (nuclearity) of the active of the total (and presumably more directly involved in the centers are debated. In fact, depending upon the prepapolymerization) is becominggradually more clear. In this ration method, the precursor, and the support (SiOz, respect the use of fast scanning techniques has allowed y-AlzO,), the surface can contain mainly Cr2+or C$+ or also the investigation of some ofthe aspects ofthe initiation both1-3,7,9 under isolated o r clustered forms. For many of mechani~m.~.~ these systems the first step of the ethene activation is All the systems discussed so far only deal with isolated considered to be the formation of CzH4-Cr2+ (Cr3+) or clustered Cr'+ ions anchored with the surface of oxidic molecular complexes. Indeed, some of the vibrational supports. characteristics of such species have been discussed in Little, on the contrary, is known about the interaction detaiL4 However, the transformation mechanism of the of CzH4 with a-CrzO3, where the,Cr3+ions form ordered weak molecular complexesinto strongly adsorbed species, two-dimensional arrays of potentially active centers, playing a vital role in the initiation steps of the polymhaving well-defined structures (because the system is erization, is still debated. Some authors also hypothesize crystalline). In view of the previous considerations, the that, in some cases, pairs of Cr2+sites, linked through knowledge of the interaction of CzH4 with this system could help to elucidate the possible role of clusters of Cr3+ * Author to whom correspondence should be addressed. ions as potential catalytic centers. Moreover, as it is, in Abstract published in Advance A C S Abstracts, July 15, 1994. principle, conceivable to reduce the surface under mild (1) McDaniel, M.P. ddu. Catal. 1986,33,47. (2)Ghiotti, G.; Garrone, E.; Zecchina, A. J . Mol. Catal. 1988,46,61. conditions (with subsequent formation of Crzi- centers (3)Karol, F.J.;Brown, G. L.; Davison, J. M. J . Polym. Sci. 1973,11, embedded in a surface layer of CS+centers),the a-Crz03413. CzH4 system appears as a versatile training school for (4) Ghiotti, G.; Garrone, E.; Zecchina, A. J . Mol. Catal. 1991,65,73. (5) Spoto, G.; Bordiga, S.; Garrone, E.; Ghiotti, G.; Zecchina, A. J . further investigations on the chemistry of Cr-based Mol. Catal. 1992,74,175. systems. This approach is realistic because the surface (6) Zecchina, A.; Spoto, G.; Ghiotti, G.; Garrone, E. J . Mol. Catal. properties of a-Crz03 (in terms of type of exposed faces 1994,86,423. and spectroscopy of adsorbed CO) are well-known in our (7)Zecchina, A.;Spoto, G.; Bordiga, S. Faraday Discuss. Chem. SOC. 1989,87,1. laboratory.1°-12
Introduction
@
(8) Hogan, J. P. J . Polym. Sci., Polym. Chem. E d . 1970,8,2637. (9) Otero Arean, C.; Escalona Platero, E.; Spoto, G.; Zecchina, A. J .
Mol. Catal. 1989,56,211.
(10) Scarano, D.; Zecchina, A.; Reller,
0743-7463/94/2410-3094$04.50/0
0 1994 American Chemical Society
A.Surf. Sci. 1988,198,11.
Complexes and Oligomerization on a-CrzOs Surfaces
Langmuir, Vol. 10, No. 9, 1994 3095
In this paper the following topics are discussed: (a) structure of the C2H4-Cr3+ molecular adducts formed on the various exposed faces; (b) the polymerization activity of stoichiometric samples; (c) the effect of mild reduction in H2 on the polymerization activity; (d) the structure of the growing (living) polymeric chains (as studied by means of fast scanning techniques) on the working catalyst. In relation to the points b-d, the spectra of CO on a-Cr203 before and after ethene adsorption are also discussed.
Experimental Section High surface area a-CrzO3 specimens (BET specific surface area 40-70 m2 g-l) have been prepared by exothermic decomposition of (NH&Cr207, as previously described.1° For UVvis-near IR reflectance measurements, the sample, in the form of powder, was inserted in an all silica cell, where both thermal treatments at high temperature and reflectance spectra at room temperature can be carried out under controlled atmosphere. For IFttransmission measurements the polycrystalline samples were compressed in the form of pellets and inserted in a cell where both the high temperature treatments, needed to clean the surface, and the in situ IR spectra at 77 K can be carried out. Two different activation procedures have been adopted: following the first procedure the samples were outgassed for 3 h at 993 K under high vacuum (1.33 x Wa) to eliminate the adsorbed impurities; followingthe second one, the samples were first reduced for a few minutes in Hz at 973 K and then activated at 993 Kunder vacuum (1.33 x P a ) . After these treaments two different samples have been obtained: the first one, with green color, was in fully stoichiometric form; the second one, with green-blackcolor,was in slightly reduced form. The activity of the surface Cr3+ ions was first investigated by in situ measurement of spectra of adsorbed CO. The UV-vis-near IR spectra have been obtained with a Cary 5 spectrometer equipped with a reflectance attachment, while the IR spectra were obtained with a Bruker IFS 48 F'l'IR spectrometer, equipped with a cryogenic MCT detector (2 cm-1 resolution). The time resolved spectra of ethene polymerization were obtained immediately after gas dosage (and for a total contact time not exceeding 30 and 6 min for the stoichiometric and reduced samples, respectively). M e r CzH4 gas removal, both pellets were then contacted again with CO at 298 K, to probe the surface situation after the ethene polymerization. The morphologicaland microstructural characterizations were carried out by means of high-resolution transmission electron microscopy (HRTEM) with a Jeol JEM 2000 EX microscope equipped with a top entry stage. The samples for this analysis were prepared by suspending the powder in isopropyl alcohol and by successivelydispersing them on a coppergrid coatedwith a holey carbon film. Modeling of the shape of the polyhedra, reproducing the microcrystallineparticles, ofthe extensionofthe Werent crystal terminations, and of the surface local structure was carried out by means of the software program Insight 11, distributed by BiosymTechnologies,Inc.,running on a SGI 4D/35 workstation.l3 Results and Discussion (1) Morphology of the Microcrystals. A detailed investigation of the shape of the microcrystals and of their evolution upon the thermal treatments has been carefully reported elsewhere12and the reader is referred to it for more details. In this investigation we shall report only the major conclusions. The dimensions of the microcrystals are in the 20-40 nm interval, corresponding to (11) Scarano, D.; Spoto, G.;Bordiga, S.;Ricchiardi, G.;Zecchina, A. J.Electron Spectrosc. Relat. Phenom. 1993,64/65, 301. (12) Scarano, D.; Zecchina, A.; Bordiga, S.; Ricchiardi, G.;Spoto, G . Chem. Phys. 1993,177, 547. (13)Insight11 User Guide, version 2.2.0;Biosym Technologies: San Diego, CA, 1993. Catalysis User Guide, version 2.0;Biosym Technologies: San Diego, CA, 1993.
/
5-fOld
1
S-fold
\
5-fold 4-fold
ogure 1. Protome polyhedronwith theindexedfaces: (Oli2),
(21161, and (1120) (the predominant (0112) faces are more extended);the local structure of C?+ ions on the three dominant faces is also evidenced.
a surface area of ~40-70m2 g-'." Although these particles are irregularly shaped, with only a few straight edge traces, they are single crystals, as shown by the remarkably constant presence of interference planes fringes. According to our recent analysis,12 based on the measure of the crossing planes fringes, observed on welloriented crystals, and on the computer modeling of the shape of the microcrystals, we have concluded that the particles-expose preval-ently (0112) faces and, to a lesser extent, (2116) and (1120) faces. The ideal shape of the most representative microcrystals, as deduced from high-resolution electron microscopy, is reported in Figure 1. The shape can be defived-from a co-mmon precursor polyhedron, where the (01121,(21161, (1120) facelets are equally represented, by means of a computer graphics procedure already described.12 The (0112) and (1120) plcnes expose 5-fold coordinated Cr3+ ions, while on the (2116) both 5-fold and 4-fold coordinated sites are present; 4-fold coordinated sites are also expected a t the edges. The fraction of these edge sites does not exceed a few percent of the total. A detailed representation of the local structures of the adsorbing centers on these dominant faces is given-in Figure 1. We can notice that although C 9 + ions on (0112) and (1120)faces are both five-coordinated, the second ones are expected to be less reactive, owing to their location in a n inward shifted position. Indeed it has been demonstrated12that, unlike the other Cr3+centers, they do not coordinate CO at room temperature. (2) Optical Properties of the Ci+Surface Ions and Their Modification upon CO Adsorption. Following the considerations outlined in [ 1_1,121, microcrystalline a-CrZO3 prevalently exposes (01121, (21161, _and (1120) facclets, where the cs+ions are in &fold [(0112),(2116)) (1120)] and 4-fold [(2116)]coordination. In the first case the local symmetry is C4" (Cr& while in the second one it is likely D 4 h ((21-4~). As the coordination states and local symmetries of the surface ions are different from those of the bulk (where C9+ are in 6-fold coordination and the approximate local symmetry is Oh), the associated d-d transition^'^ are expected to have sensibly different frequencies and intensities.
Scarano et al.
3096 Langmuir, Vol. 10, No. 9, 1994 Chart 1
3 oh
c4v
4T1p -_--_- __ _ _ - -
4P
4P
- - - - - _ _ _- -_- - _ _
2
4E
1
C 3
t I 0
1 n
3
Y
1
,k
,
0
20000
10000
6000
wavenumbers (cm- 1)
Figure2. Diffuse reflectance spectra of stoichiometrica-CrzOa before (a) and after (b) CO adsorption (P= 5.32 kPa). Inset expanded view of low frequency region. In order to check this point and to gain further information on the surface structures, we have performed diffuse reflectance spectra of a-CrzO3before and after CO adsorption (where the coordination number is supposed to increase by 1, by insertion of a predominantly a-ligand: vide infra). The resulting spectra are shown in Figure 2. The bands a t ~ 2 000 3 and 18 000 cm-' (unaffected by the adsorption of surface ligands) are the well-known
44g - 4T,,(F)
44g4T,,(F) M
23 000 cm-'
M
18 000 cm-'
transitions of the bulk Cr3+ions in O h symmetry. These bands are distinctly composite, because of trigonal field splitting effects.15 In Figure 2 we can also observe that, besides the d-d transitions of the bulk Cr3+ions in Oh symmetry, a broad maximum at 10 700 cm-l (with shoulders a t 14 000, 12 000, and 8000 cm-'; see also inset in the figure) is clearly visible, which cannot be attributed to octahedral ions. The existence of these shoulders can be more clearly demonstrated by studying the first derivative. This absorption is partially eroded when CO is dosed on the surface and fully restored upon outgassing at room temperature (inset in Figure 2). This behavior is clearly associated with the CO ligands insertion a t the surface coordination vacancies, to farm Cr3+-C0 adducts with prevailing a-character (vide inpa). Therefore we conclude that the broad absorption in the 14000-8000 cm-l range is associated with the d-d transitions of the surface Cr3+ ions. (14)Figgis, B.N. Introduction to ligand fields; Interscience Publishers, J. Wiley & Sons Inc.: New York, 1966; p 222. (15)Hush, N. S.; Hobbs, R. J. M. Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Interscience Publishers, J. Wiley & Sons: New York, 1968; Vol. 10, p 259.
Although the C 9 + ions located on the surface are only a very minor fraction of the total number explored by the incident radiation (for particles with the dimensions indicated before, the fraction of ions located a t the surface being only a few percent of the total), nevertheless the associated d-d bands in the above mentioned interval can be observed because the local symmetry of the vast majority of the surface ions is C4"(Le. lower than O h ) and hence the associated d-d transitions are Laporte allowed and consequently are 1 to 2 orders of magnitude more intense.16 The adsorption of CO, being accompaniedby the increase of (i) the coordination number and of (ii) the symmetry (square pyramidal pseudo octahedral) causes a shift of the d-d transitions toward the values more typical of the octahedral coordination, with simultaneous abrupt decrement of the intensity (the combination of the effects i and ii is expected to induce a decrement of the specific intensity of a t least 1 order of magnitude, so causing the total disappearance of the surface modes). This explains why the erosion of the surface bands is not accompanied by any apparent increase of absorptions a t higher frequency. Having established that the 14000-8000 cm-l absorption is associated with surface ions, we can try now a more detailed assignment. For sake of simplicity we shall assume that the sites responsible for the absorption are essentially the 5-fold coordinated Cr3+5cin local C4" symmetry, because they are present on all faces. Consequently we shall concentrate only on the optical properties of such ions. In order to perform this in a semiquantitative way, we recall that on passing from octahedral to square pyramidal symmetry the scheme of levels changes as shown in Chart 1,i.e. the 4TzEand 4T1, levels split into doublets (E Bz; E Az) with disappearance of the g symbols (due to the absence of the inversion center). The energy levels diagram for the square pyramidal CrI1IIv(d3)chromophore can be desumed from that (better known) of Co1I&(d7)17(Chart 2). If Dq = 1700 cm-l (i.e. the same value observed for a-CrzOB) and a splitting effect not exceeding 3000 cm-I are assumed,l5J7JBthen the first transitions
-
+
+
- 4E
4B1 ~~
a n d 4B,
- 4B,
~~
(16)Zecchina,A,;Spoto, G.;Coluccia, S.; Guglielminotti, E. J.Chem. Phys. lSS4,88, 2575. (17) Ciampolini, M. In Structure and Bonding; Hemmerich, P., et al., Eds.;Springer-Verlag: Berlin, 1969; Vol. 6, p 52. (18)Douglas, B. E.; Hollingsworth, C. A. In Symmetry in bonding and spectra. A n introduction; United Kingdom Edition, published by Academic Press Inc.: San Diego, CA, 1985; p 274.
Complexes and Oligomerization on a-CrzO3 Surfaces Chart 2
0
0.4
0.8
1.2
1.6
Dq, kK
are expected in the 11000-14000 cm-l interval, while the remaining ones are certainly a t -i, 2 20 000 cm-l (hence not visible, because they are overshadowed by the bands of the Cr3+of the bulk). This result is in good agreement with the experimental data. Having explained the band at 10 700 cm-' in terms of d-d transition of the Cr3+5c ions, a t this point, we have to explain why the adsorption of CO does not cause its full destruction. First of all we have already report_ed1° that a t room temperature the coverage of CO on (0112) faces is only 0.7 and that the full coverage is reached only a t 77 K. -Second, the Crbeions emerging on the less abundant (1120) faces do not coordinate CO at all at room temperature. Because of these facts, the incomplete erosion of the band centered a t 10 700 cm-' can be readily accounted for. The analysis made so far was based on the simplified hypothesis that the spectra are dominated by the C r 3 t species. However, we know that C 1 3 t chromophores are also present in smaller percentage. Their presence can explain the shoulder a t ~ 8 0 0 cm-': 0 in fact if a reduction of the crystal field similar to that usually found on passing from 6-fold to 4-fold coordinated ions is hypothesized, a transition a t a frequency as low a s 8000 cm-' finds full explanation.
(3)IR Spectra of CO Adsorbed on Clean Surfaces. A preliminary IR investigation of the surface properties of the two differently prepared a-Crz03 specimens was carried out by using CO as probe, at 298 K, as reported in Figure 3. The IR spectra of CO on stoichiometric (Figure 3a) and reduced samples (Figure 3b) show a main peak a t 21812183 cm-I (0 x 0)and 2165-2167 cm-' (Om, z 0.7) and two satellites a t 2170-2173 cm-' and 2175-2177 cm-l. On the basis of the detailed discussion, already made for sintered samplesl2 (where the same band, but with narrower profile, is present), we attribute the main peak a t ~2165-2167 cm-' to the interaction oCC0 with C 9 + ions on the most abundant and stable (0112) faces. The frequency of the main peak changes gradually with the coverage: this effect is associated with the gradual modifications of the static and dynamic interactions occurring among CO species adsorbed on the (0112) flat
Langmuir, Vol. 10,No. 9,1994 3097 facelets, as already thoroughly discussed,l2 for sintered (and hence more ordered) specimens. At low coverages the CO molecule, adsorbed in end-on form on 5-fold coordinated C 9 + ions, is isolated (singleton) and its stretching frequency is entirely determined by the nature of the CO-surface bond. The high value of the associated CO stretching frequency, with respect to the CO gas (Aij = +38-40 cm-9, indicates that the interaction is primarily electrostaticin nature, associated with the positive electric field localized at the C13+sites. Similarly to what observed on sintered specimens,lzwhen the adsorption experiment is performed at 77 K(data not reported for sake ofbrevity), the main band becomes more intense, as the CO coverage on the (0112)faces is now complete. The main field-related effect on the CO stretch is that the field holds the adsorbate near the surface, which represents a uwall'' for the vibrating molecules, due to the Pauli r e p u l ~ i o n . ' ~As reported by some authorsz0for CO on 5-fold coordinated sites, the direct field effect alone could not be sufficient to j u s t i g the observed upward shifi and the stability of the complex a t room temperature. Consequently a u charge transfer to the substrate may contribute to the adsorption-induced blue shift of the CO frequency. Moreover, as already discussed in detail, for sintered (model) samples,12the possible presence of weak overlap interactions of the d-n type cannot be excluded. Indeed an indirect indication of the additional presence of small, but not negligible, d-n overlap interactions was inferred from the evaluation of a, (dynamic polarizability) of CO in the adsorbed state (method of the 1zCO-13C0diluted mixtures). In fact, as a, ( ~ . [ S p / d Q ldepends ~) very much upon the charge oscillation during the CO stretching motion, which is in turn favored by the presence of d-x overlap forces,z1a high value of this parameter (as found in the present case, for sintered samples9 is indicative of the presence of d-n exchange forces in the Cr3+-C0 complex bond. Moreover, the main peak shows a remarkably higher 0 at halfwidth (fwhm M 15 cm-l a t 8 z Om, and ~ 2 cm-l 8 % 0)compared to 1.5 and 6 cm-l on sintered materials. This can be readily explained in terms of inhomogeneous broadening effects, associated with the disorder and with the smaller extension of the facelets. In fact, being the frequency of the CO oscillators, adsorbed at the border of the facelets or near a step or defect, slightly different from that of CO adsorbed at the center of the faces, a severe broadening effect of the whole absorption band is well justified. The twoextra features a t 2170-2173 cm-' and ~ 2 1 7 5 2177 cm-' (Figure 31, which find correspondence in the two narrow, well-defined peaks observed at the same frequencies on the sintered samples,l2 are analogously assigned to 5-fold and 4-fold coordinsted ions exposed in parallel rows 0.365 nm apart on the (2116)facelets, which are present in smaller proportion (a contribution from 4-fold coordinates sites on edges is, of course, not excluded because of the smaller dimensions of the microcrystals and of their more irregular shape). On reduced samples the contribution of these extra peaks is slightly depressed: this likely means that the surface reduction takes place preferentially on higher index faces and/or a t the edge positions, because the Cr3+ ions, being in lower coordination, are more reactive. To complete the picture of the CO/a-CrzO3 system, we remindlz that the Cr3+ions on (1130) faces are not able to adsorb CO a t all a t room temperature. (19) Pacchioni, G.; Cogliandro, G.; Bagus, P. S. Surf. Sci. 1991,255, 344.
(20)Neyman, K.M.;Rosch, N. Chem. Phys. 1992,168, 267. (21)Seanor, D.A.;Amberg, C. H. J. Chem. Phys. 1965, 42, 2967.
3098 Langmuir, Vol. 10, No. 9, 1994
Scarano et al.
/ b
ia
I
I
Wavenumber (cm-1)
2x0
2200
2180 2160 Wavenumber icm-i 1
2140
2120
Figure 3. (a)FTIR spectra of W O adsorbed at 298 K on stoichiometric a-CrzOs. (b) FTIR spectra of l2C0 adsorbed at 298 K on reduced a-CrzOs. The different curves correspond t o increasing integrated intensities, I. Each spectrum is labeled by Ill0 ratio, where IO is the maximum intensity measured in our experiments and corresponding to PCO 5.32 Wa and 0 = 0.7. Weak absorptions in the 2000-1600 cm-' range (not illustrated for sake of brevity) are also observed upon CO contact. As these bands (always weak or very weak) are much stronger (in relative terms) on reduced samples, they are attributed to Cro(CO),zerovalent groups formed by interaction of CO with rare Croatoms or (CrO),clusters, generated during the reduction procedure, as already discussed.22The detailed attribution of these weak bands to Cro(CO),species was made possible by the study of the IR spectra obtained during the direct Cro(CO)6dosage on stoichiometric a - C r ~ O 3 To . ~ ~evaluate the concentration of these species, it is useful to recall that the extinction coefficient of the carbonylic bands in zerovalent carbonyls is a t least 1order of magnitude larger than that of CO on Cr3+ions; so a very low intensity of these bands (as in our case) is indicative that the concentration of these species is always very small even on reduced samples, where they can be more easily detected. As these species do not play any detectable role in the CzH4 polymerization, as demonstrated by the observation that a-CrzOs deliberately doped with CrO from Cr0(CO)6does not show an increased polymerization activity (results not shown for sake of brevity), they will not be discussed further. (4) C2H4 Adsorption and Polymerization TimeDependent IR Spectra. High-surface area a-Crz03 is quite effective in ethene polymerization a t room temperature, as shown by the sequence of spectra reported in Figures 4 and 5 , where the activities ofthe stoichiometric and reduced samples are compared. (22) Zecchina, A,; Spoto, G.; Scarano, D. J . Electron Spectrosc. Rel. Phenom. 1987,45,269. (23)Escalona Platero, E.; Otero ArBan, C.; Scarano, D.; Spoto, G.; Zecchina, A.Mater. Chem. Phys. 1991,29,347.
Figure 4 (a, b, and c) shows the time-dependent IR spectra obtained on stoichiometric samples, in the 31502650,1650-1100, and 1050-800 cm-l intervals, respectively, while Figure 5 (a, b, and c) shows the IR spectra obtained on the reduced samples, in the same frequency intervals (both obtained in presence of 5.32 W a of ethene). The two spectral sequences were recorded a t 10- and 7-s intervals and for total contact times of 30 and 8 min, respectively. For sake of clarity we shall discuss the initial spectra first (because they are mainly associated with molecularly adsorbed ethene); then the time-evolving spectra associated with the formation of the living oligomers will follow.
(a) The IR Spectra of Molecularly Adsorbed Ethene. As the 5-fold Cfl+ions are the most abundant, we shall assume, for simplicity, that the initial spectra are essentially dominated by the bands of the ethene molecule adsorbed on a single type of site, i.e. the CrbC ions ,c=c,
In favor ofthis hypothesis, we remind that on both a-Crz03 samples, stoichiometric and reduced, all modes of molecularly adsorbed ethene essentially constitute one single component (Figures4 and 5 ) ,so indicating that they mostly belong to ethene adsorbed on a single predominant site. From the inspection of the initial spectra, we also notice the following:
Complexes and Oligomerization on a-Cr203 Surfaces
Langmuir, Vol. 10, No. 9, 1994 3099 C
I
i
'
,
I
Figure 4. FTIR time resolved spectra of ethene polymerization reaction on stoichiometric a-Cr203: (a)C& and CH2 stretching modes region; (b! (C=C) stretching and d(CH2)modes region; (c) ya, (CH2)modes region. Continuous m e s (-) IR spectra taken at 10-sintervals m presence ofP = 5.32kPa of ethene; dashed m e (- - -), after a total contact time of 30 min and ethene removal by outgassing at room temperature; curve on the bottom, ethene gas.
b
1
T
Figure 6. FTIR time-resolved spectra of ethene polymerization reaction on reduced a-Cr203: (a)CH3 and CH2 stretching modes region; (b) (C=C) stretching and d(CH2) modes region; (c) ya, (CH2) modes region. Continuous curves (-) IR spectra taken at 7-s intervals in presence of P.= 5.32 kPa of ethene; dashed curve (- - -1 after a total contact time of 8 min and ethene removal by outgassing at room temperature; curve on the bottom, ethene gas.
(i)The vasp(CH2) (Bb)and vsym(CH2) (B1,) modes (3105 and 2989 cm-l in the gas phase") are shifted to lower frequency after interaction with C P k ions, giving rise to new bands at 3066 and 2955 cm-l (Figures 4a and 5a); a small downward shift from 1444 cm-l (gas) to 1443 cm-l (adsorbed state) is also found for the 6 (CH2) (B1,) mode, which also gains intensity (Figures 4b and 5b). (ii)The v(C=C) (AI,) and d(CH2)(Alg)modes, which are IR inactive in the free molecule and are responsible for the two Raman lines at 1623 and 1342 ~ m - l , 2become ~ IR active in the adsorbed due to the reduced symmetry, (24)Weidlein, J.; Miiller, V.; Dehnicke, K In Schwingungspektroskopie. Eine Einfiihrung; Georg Thieme Verlag: Stuttgart, New York, 1982; p 98.
giving rise to two weak bands at 1598 and 1336 cm-l (Figures 4b and 5b); if we consider that, according to some a u t h 0 r s , 2 ~ the - ~ v(C=C)and d(CH)vibrationsare strongly coupled, even in ethene itself, it seems probable that the proportion of C=C stretch and CH deformation, in each of these bands, depends on coordination. So, as the (25) Powell, D. B.; Scott, J. G. V.; Sheppard, N. Spectrochim. Acta 1972,28~,327. (26) Nakamoto,K In I n b r e d and Raman Spectra of Inorganic and COWdinatbn COmpOUndS; Inh?I'SCienCe Publication, J. wiley & sons, Inc.: New York, 1986; p 386. (27)Busca, G.; Ramis, G.; Lorenzelli, V.; Janin, A.; Lavalley, J. C. Spectrochim. Acta 1987,43A,489. (28) Busca, G.; Lorenzelli,V.; Ramie, G.; Saussey, J.; Lavalley,J. C. J. Mol. S t r u t . 1992,267, 315.
3100 Langmuir, Vol. 10, No. 9,1994
Scarano et a1. Table 1
ethene gas"
ZnOb a-CrzOs
Va,ymm(CH)
Vsy"(CH)
B2u
B1u
2989 IR 2960 IR -2955 IR
3105 IR 3057 IR -3066 IR
Fe20sb 3080 IR [(C2H4)Pt1'C13]- C 3079 IR, R [(CZH~)A~I(BF~)E [(C2Ha)Cu1Clld 3076 R a
2975 IR 2920 IR, R 2924 R
v(C=C)
+
6(CH)
+
ydCH)
&CH) Ai, 1623 R 1599 R -1598 IR
Y(C=C) Ai, 1342 R 1340 R 1336 IR
&CHI B1, 1444 R 1440 IR 1443 IR
1618 R 1515 IR, R 1579 R 1538 IR
1340 R 1243 IR, R 1320 R 1275 IR
1439 IR 1426 IR, R 1411 IR
B3u
949 IR 1002 IR 992 IR 990 IR 975 IR, R 971 R 940 IR 948 R
B2g
943 R 964 IR
1010 IR, R 990 R 960 IR
J. Weidlein e t al.24 G. Busca et a1.28 D. B. Powell e t al.25 M. B i g ~ r g n e . ~ ~
neous complexes, the downward shifts of the v,,,(CHd strength of the coordination increases, the proportion in the low frequency band of C=C stretch increases. It is (Bz,), v,,(CHz) (Bd, and v(C=C) (AI,) and the upward useful a t this point to compare the IR spectra of ethene shiR of the ylU(CH2)(B3J modes can be used to gain on a-Cr203 with those of the homogeneous complexes of information on the nature of the bond between the surface ethene with unsaturated metal centers (Table 1). In and the ethene. In Table 1the frequencies of some typical particular in the silver-ethene homogeneous complex, molecular homogeneous and heterogeneous complexes of [(C2H4)Ag1(BF4)1, where the ethene is only weakly perethene are reported for the sake of c o m p a r i s ~ n . ~ ~ , ~ ~ , ~ ~ turbed, the proportion of v(C=C) and d(CH) in the 1579We can see that the spectrum of ethene adsorbed on and 1320-cm-l bands is not very different from the a-CrzO3 strongly resembles that of homogeneous comproportion in uncoordinated ethene; on the other hand in plexes, where ethene is coordinated to positively charged the platinum-ethene complex, [(C~H4)Pt"C131-,the 1243transition-metal centers through a-n interaction and cm-' band has a considerably higher proportion of C=C where the u contribution is predominant. stret~hing.~~ As a final consideration, let us remark that the bands (iii) The y,(CH2), (B3J and (Bag),modes of ethene (949 due to coordinated ethene are reversible upon room and 943 cm-'1 are shifted upward to 992 and 964 cm-l, temperature outgassing and that their intensity decreases respectively, in the adsorbed phase (Figures 4c and 5c). with time during the polymerization process. This effect The band a t 992 cm-' is very strong; hence it can be is particularly evident for the strong 992-964-cm-' bands considered as the most characteristic feature of the (Figures 4c and 5c) and will be further discussed in the Cr3+5c-CzH4surface complex. The upward shift of these following. modes may be justified by the interaction of the ethene (b)The Living Polymers and the Oligomerization with electron-withdrawing centers Cr3+,along a direction Process. The evolution of the spectra occurring during perpendicular to the plane of the olefinic system. This the polymerization process will be discussed by analyzing causes the formation of a complex with Czv~ y m m e t r y , ~ ~ ,the ~ ~ spectral regions: 3150-2650 cm-l (CH3 and CH2 where the C-H bonds bend out of the ethene plane, far stretching modes Figures 4a and 5a); 1650-1100 cm-I from the surface. Due to such deformation, a strong (C=C stretching and d(CH2) modes, Figures 4b and 5b); variation of the dipole moment, perpendicularly to the 1050-800 cm-l (yw(CH2)modes Figures 4c and 5c). plane of the surface ethene complex, occurs. A similar (bl) Stretching Modes Region (3150-2650 cm-'1. behavior was found in matrix isolated alkene-hydrogen The formation of the living polymer, already in the first halide complexes, where a hydrogen bond with the stages of the reaction, on both surfaces (stoichiometric n-electrons along the direction normal to the C=C plane and reduced) is evidenced by the appearance of a pair of is f o ~ - m e d . ~ *In- ~this ~ case it is reported that, due to bands a t 2924-2918 and 2851 cm-I (Figures 4a and 5a), weakening of the bond (as the HX ligand withdraws a growing with time in a parallel way a t nearly constant small amount of x electron density), the C=C stretching rates. These bands are assigned to in-phase and out-ofmode is shifted to lower frequency, while the out of plane phase stretching vibrations of CH2 groups, belonging to bending motions of the hydrogens are shifted to higher the growing polymeric chains frequency and become more intense, due to steric hindrance X
I
It is noteworthy that the high-frequency peak is characterized by two components, a t %2924and 2918 cm-l, which > C L C < evolve with the reaction time in a different way: the former one is predominant at high degree of polymerization, while X = halide the latter one is more evident for the shortest contact time. We can hypothesize that the band at 2918 cm-' The intensity increase is so remarkable that the observabelongs to modes of CH2 groups in the immediate vicinity tion of a strong C-H out of plane wagging mode for the ofthe Cr center, while the band a t 2924 cm-l is associated C2H4-m complexes (973cm-l for HF, 957 cm-l for HC1),32 with the more distant CH2 groups. The rate of CH2 group particularly sensitive to the effect of the HX ligand, formation is 1 order of magnitude higher on reduced provides evidence for a n out of plane n - ~ o m p l e x . ~ In~ , ~ ~ samples; this demonstrates that (i)reduced sites are more conclusion, like for d-n and hydrogen bonded, homogeactive and (ii) a fraction of the polymeric chains is consequentlymuch longer, so making the modes associated (29) Herberhold, M. Metal n-complexes; Elsevier: Amsterdam, 1974; with -(CH2),- groups predominant. Vol. 2, part 2. The spectral sequence, shown in Figure 5a, is remark(30) Barnes, A. J.; Davies, B. J.; Hallam, H. E.; Howells, J. D. R. J. Chem. SOC.,Faraday Trans. 2 1973, 69,246. ably similar to those found for ethene on chromia-silica (31) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J.Am. Chem. SOC. and C r - ~ i l i c a l i t e , ~ where . ~ very strong bands, associated 1982, 104, 6180.
Y
(32) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J.Am. Chem. SOC. 1982, 76, 5767.
(33) Bigorgne, M. J. Organomet. Chem. 1978, 160, 345.
Complexes and Oligomerizationon a-CrzOs Surfaces with -CH2- groups in the long (living)polymeric chains, were the only important observable feature in the IR spectra, even in the very early stages of the ethene oligomerization process. The absence of any spectroscopic evidence of chain-terminating groups, like -CH3 and -CH=CHZ in living polymers on Cr-Si02 and Crsilicalite, has suggested a polymerization mechanism, proceeding through cyclic-likeintermediates.6 Following this hypothesis the polymeric species have both ends anchored to the same Cr2+site and hence no terminating groups are observed. Because of this similarity, we can hypothesize that like on Cr/SiOz, the most active centers present on reduced samples are coordinatively unsaturated Cr2+ions and that the long chains are generated on them. In the present case, however, we can notice that immediately after ethene dosage, a band a t 2883-2877 cm-l (with probable counterparts a t 1370-1350 cm-l) is also clearly observable (especially on activated samples). As these bands can be readily ascribed to -CH3 groups in terminal positions, this suggests that on stoichiometric a-Cr203 other initiation mechanisms, involving the formation of shorter *-(CHZ),-R (R = CH3) chains on less active Cr3+sites, are operating. The terminal groups are more easily observed on these samples, because, due to the reduced turnover number, the chains generated on Cr3+sites are shorter and the contribution of long chains originated a t Cr2+sites is practically absent. It is evident that the main difference between coordinatively unsaturated Cr2+and Cr3+ions is mainly the propagation rate (Le. the ability to insert new molecules in the growing chains). Ofcourse in both cases the initiation mechanism remains substantially unknown and deserves further investigations. Concerning the question of the location of these centers, not very much can be said a t this stage of the discussion. However, the problem will be considered in the following on the basis of further experimental data. Besides the previously discussed 2924-2918- and 2851cm-l bands, a broad absorption (also growing with time) in the 2800-2750-~m-~range can be observed on both samples. As this anomalous absorption falls a t frequencies lower than those of normal CH2 (CH3) groqps, we hypothesize that they belong to perturbed CH2 (CH3) groups interacting with the cationic Cr3+centers. In fact, the lowered frequency and the wide halfwidth of these absorptions can be readily explained in terms of a n agostic type i n t e r a ~ t i o n where , ~ ~ , ~the ~ hydrogen atom of the C-H group is polarized or a-bonded by empty orbitals of the positive cation R3 Rt R1-CH---Cr3*
This type of interaction is similar to that occurringbetween the hydrogen atom of dihydrogen or the C-H groups of methane and ethane and positively charged centers (M+), which leads to the formation of the adducts H-H-M+ and H&-H-M+ and H&-H2C--H-M+, as discussed by Kazansky et al.,36-38and causes a net decrease of H-H and C-H stretching frequency. The similarity of the chemical behavior of H-H and >C-H has been already stressed.39 (34) Crabtree, R. H.; Hamilton, D. G.Adu. Organomet. Chem. 1988, 28, 299. (35) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983,250, 395. (36) Kustov, L. M.; Kazansky, V. B. J . Chem. SOC.,Faraday Trans. 1991, 87 (161,2675. (37) Khodakov, A. Y.; Kustov, L. M.; Kazansky, V. B.; Williams, C. J . Chem. SOC.,Faraday Trans. 1992, 88 (21), 3251. (38) Khodakov, A. Y.; Kustov, L. M.; Kazansky, V. B.; Williams, C. J . Chem. SOC.,Faraday Trans. 1993,89 (91, 1393.
Langmuir, Vol. 10, No. 9, 1994 3101
3
0
3000
2800
2600
2400
2200
2 00
Wavenumbrr (cm-1)
Figure 6. FTIR spectrum of n-heptane on stoichiometric a-CrzOs before (continuous curve) and after W O adsorption (dashed curve). Inset: expanded view of the 6(CHz) modes region.
As a further observation, we underline that, in rel/ative terms, these absorptions appear more intense “I stoichiometric surfaces (Figure 4a) than on the reduced ones (Figure 5a). As on the stoichiometric surfaces the chains are undoubtly shorter, we infer that the most effective hydrogen bonds involve preferentially the C-H groups closer to the active centers. These considerations can be fully supported by considering the spectra of adsorbed n-heptane (Figure 6) and the effect of CO dosage on a-CrzO3 surfaces, partially covered by living polymers (Figures 8 and 9) (vide infra). In fact, as far as the n-heptane adsorption on stoichiometric a-CrzO3 is concerned, we notice that, besides the normal CH2 and CH3 stretching modes, a broad band a t 5 < 2800 cm-l is clearly present (Figure 6, continuous curve). The interpretation is straightforward: the > CH groups (of -CH3 andor -CH2 groups) of n-heptane interact with surface Cr3+centers causing a lowering of the stretching frequencies.
1
When the same experiment is performed with the reduced (39) Saillard, J. Y.; Hoffman, R. J. Am. Chem. SOC.1984,106,2006.
3102 Langmuir, Vol. 10, No. 9, 1994
a
Scarano et al.
I,
I
0
3000
2800
Wavenumber (cm-1)
b
Wavenumber ( c m - l )
Figure 7. (a, top) FTIR spectra of l2C0 adsorbed on clean (dashed curve) and on polymer covered surface (continuous curve) of stoichiometric a-CrzO3. (b, bottom) FTIR spectra of l2C0adsorbed on clean (dashed curve)and on polymer covered surface (continuous curve) of reduced a-CrzO3. samples, the results are nearly identical; this confirms that the observed interaction is involving mainly the Cr3+ ions (because they are the most abundant). This does not mean that the reduced centers are inactive; simply we
2400
2600
Wavenumber
2200
2 00
IC~I-' 1
Figure8. Modificationsof the FTIR spectrum of livingpolymer chains on stoichiometric a-CrzOs induced by CO: continuous curve, before W O adsorption; dashed curve, after l2CO adsorption ( P= 2.66 Wa). Inset: expanded view of the d(CH2) modes region. have not enough instrumental sensitivity to observe the interaction on these rare centers. (b2)In-Plane Bending Modes Region (1650-1100 cm-'). Besides the weakbands a t ~ 1 6 0 0 , 1 4 4 0and , 1340 cm-l, associated to v(C=C) (AIg),6(CHz)( B d , and 6(CHz) (AIg)of the molecularly adsorbed ethene, new bands start gradually to develop a t 1470-1455 cm-l (Figures 4b and 5b). By carrying on with the reaction, the intensity ofthe %1600-,1440-, and 1340-cm-l components decreases, as expected, because they belong to weakly coordinated ethene, while the two bands a t 1470 and 1455 cm-l (6(CHd of saturated chains) grow up. These two components, which behave similarly to the pair a t 2924 and 2918 cm-', grow with time a t different rates, and for the highest contact times the component located a t 1470 cm-l becomes predominant. The former band is associated with long polyethylene chains, while the latter one is particularly evident a t low degree of polymerization, when the polymer is shorter. It is most noticeable that, like the (CH2) stretching bands, also the 6(CHz)modes show a tail at lower frequency associated with the > CHz-Cr3+ interactions. A similar effect is also observed in Figure 6 for the d(CH3) modes of n-heptane adsorbed on stoichiometric samples (band a t 1352-1354 cm-l). Weak absorptions a t 1370-1350 and 1295 cm-l are also observed, which could be due to the yw and yt modes, respectively, of the CH2 groups in the polymer. It must be recalled that the assignment of the wagging modes a t 1370-1350 cm-I was rather troublesome and uncertain even for the pure p ~ l y m e r . *This ~ $ ~band ~ could be better assigned to bending modes of terminal CH3 groups: this hypothesis finds support in its having larger intensity on the simply activated samples, where the chains are shorter
Complexes and Oligomerization on a-CrzO3 Surfaces
1450
I
Langmuir, Vol. 10, No. 9, 1994 3103
1
Wavenumber (cm-1)
, 3000
2U'OO
2d00 2i00 Wavenumber (em-)1
2200
-4.
2
I
Figure9. Modificationsof the FTIR spectrum oflivingpolymer chains on reduced a-CrzOa induced by CO: continuous curve, before W O adsorption; dashed curve, after l2COadsorption ( P = 2.66 kPa). Inset: expandedviewofthe d(CHz) modes region. and where the -CH3 stretching modes were also found to be more intense.
(b3)Out-of-PlaneBending (orDeformation)Modes Region (1050-800 cm-I). By comparing the evolution with time of the two spectral sequences, we notice that the bands a t 992 and 964 cm-l, associated with ethene molecularly adsorbed on (0112)faces, gradually decrease and that this decrement is clearly associated, although only indirectly, with the formation of the polymer (Figures 4c and 5c). This phenomenon is more evident on the reduced samples (Figure 5c), because the longer polymeric chains (although originated _Onhigh index faces o r edges: vide infra) spread on the (0112)faces in an extensive way (which become gradually covered by hydrocarbon chains) and displace the weak ethene ligand) (Figure loa). ( 5 ) The IR Spectra of CO Adsorbed on a-Cr203 Precovered by HydrocarbonChains. In Figure 7 the spectra of CO adsorbed on both samples (stoichiometric and reduced) before (dashed curve) and after (continuous curve)the polymerization process are compared. The most evident feature of samples containing living polymers is the absence of the CO modes a t 2170-2180 cm-l, associated with the highest index faces and with the most defective (or low coordinated) sites. This means that the Cr ions located on these faces (and on edges) are directly involved in the initiation of the oligomerization process. In fact, due to the formation of living hydrocarbon chains bonded to them, these ions are consequently no longer available for the coordination of CO molecules, which are so adsorbed only a t the_most abundant, but inactive Cr3+ sites emerging on (0112) faces. (40) Krimm, S. Fortschr. HochpoZym.-Forsch. 1960, 2, 51. (41)Gussoni, M.; Castiglioni, C.; Zerbi, G. In SpectroscopyofAduanced Materials; Clark, R. J. H., Hester, R. E., Eds.; J. Wiley & Sons, Ltd.: 1991, cap. 5 , p 251.
This experiment clearly allows us to answer the question concerning the location of the active Cr3+centers. Moreover as this effect is common to both stoichiometric and reduced samples, this definitively confirms that Cr3+ions of high index faces and/or edges are able to initiate the oligomerization reaction and that the real difference between Cr2+and Cr3+is only in the turnover number of the CzH4 insertion (chains initiated a t the Cr2+sites grow faster and so become longer). The long polymeric chains originated on the Cr2+ ions a t the borders of the (0112) faces (where they i-ntersect high index faces) can easily spread over the (0112)faces, where they interact with the C$+ centers (vide infra), (Figure loa) and displace weakly bonded ethene. In the previous paragraphs we have explained the anomalous tail on the low frequency side of the C-H stretching and bending bands of anchored oligomers and adsorbed n-heptane, in terms of >CH-M+ (M+ = Cr3+ ) interactions. In this paragraph the correctness of this hypothesis is confirmed again on the basis of the results of the interaction of CO with samples precovered by hydrocarbon chains (Figures 8 and 9) and n-heptane (Figure 6). In fact, i t can be observed that the CO adsorption causes a remarkable erosion of the broad 2800-2750-cm-' bands and a simultaneous intensity increase of the asymmetric and symmetric modes of CH2 groups: From this experiment it can be inferred that the CO ligand insertion into the coordination sphere of Cr3+ ions destroys the > C-H-Cr3+ interaction and simultaneously increases the contribution of the unbonded >CH2 modes (indeed the two CH2 modes become more intense). This definitively shows that the broad absorptions ranging in the 28002750-cm-I interval are due to the interaction of >C-H groups of the livingpolymers with empty Cr3+surface sites. The same behavior is found in the bending modes region. In fact, upon CO adsorption the 1470-cm-l component increases its intensity, while that a t 1455 cm-l is partially eroded (inset in Figures 8 and 9). All these considerations allow us to conclude that the effect of CO adsorption can be described as shown in Figure lob. These conclusions are also fully supported by the results of the interaction of CO with a-CrzO3 surfaces precovered by n-heptane (Figure 6). In fact, also in this case, when a more strongly adsorbed ligand (CO) is dosed on the preadsorbed n-heptane, the bands a t 2800-2750 cm-l, associated with the -CH3 and/or -CH2 groups interacting with the Cr3+sites, are remarkably eroded. At the same time, in the bending region, the band a t 1352-1354 cm-l is destroyed by CO interaction, as expected (Figure 6). As this broad band undoubtedly belongs to a perturbed -CH3, this, inter alia, means that n-heptane interacts preferentially, with the Cr3+centers through the terminal -CH3 groups (see 1). The idea that CO removes the interaction of the -CH2- groups of the chain with the surface is also confirmed by the remarkable difference between the frequency of the CO peak before and after the polymerization on reduced samples (Figure 91, compared to stoichiometric samples (Figure 8). In fact, whije on reduced samples the peak frequency of CO on (0112) faces is affected by the presence of hydrocarbon chains (AV x -2.6 cm-l), the same does not happen on stoichiometric sample. This difference can be readily explained with the presence, on the reduced samples, of longerpolymeric chains, which, although not originated on (0112) faces, spread over the whole surface and hence perturb the CO frequency, simulating solvent-like effects, as illustrated in Figure lob. A merit of this model is that it explains not only the frequency shift but also why the integrated CO intensity
3104 Langmuir, Vol. 10,No. 9,1994
Scarano et al.
I
I
I
3
I
Figure 10. Model of a linear polymeric chain anchored on a Cr ion at the border of the (01x2)face and interacting with the Cr3+ centers of the sameiace before (a) and after l2C0adsorption (b). (a) The polymeric chain, having fixed origin and about 20% of hydrogens 1.9-2.2 A apart from the surface chromium ions, was energy minimized, following a molecular mechanics approach, by using a consistence valence force field (CVFF).13After the minimization procedure the modified distances between the surface ions and the hydrogens were still in the same interval. (b) The polymeric chain was energy minimized by fixing its origin and by considering the presence of a bidimensional distribution of fixed CO molecules. From our results, the repulsion between the CO molecules and the polymeric chain is well evidenced. The presence of CO causes the removal of the polymer from the surface, with subsequent destruction of the > CH-C$+ interactions.
is rsmaining substantially unaltered (the Cr3+ sites of (0112)faces are not directly involved in the oligomerization reaction).
Conclusions On the basis of a n earlier detailed investigation, based on both electron microscopy analysis and compukr graphic modeling, the predominant presence of (0112),(2116),and (1120)faces on a-Cr203 microcrystals, prepared by combustion of (NH&Cr207, was established.12 On these surfaces, two main families of sites (&fold Cr3+and, to a lesser extent, 4-fold C F ) are present, which form with carbon monoxide a-n 1:l adducts, with predominant a character. Upon CO adsorption, the coordination number and the local symmetry of the Cr3+ surface ions change; consequently the associated optical d-d transitions (in the 14000-8000-cm-1 range) shift toward higher frequencies with parallel dramatic decrease of intensity. The ethene-surface complexesinvolve the predominant 5-fold coordinated Cr3+ions and have spectra similar to those of homogeneous complexes. These complexes have approximate symmetry, because the ethene interacts with the electron-withdrawing centers Cr3+, through a a-type bond, perpendicular to the plane of the olefinic system. This structure is similar to that of matrix isolated alkene-hydrogen halide (XH) complexes (where a hydrogen bonding interaction between the n-electrons of the C=C group and the XH is present) and to that of homogeneous complexes (where ethene is coordinated to
positively charged transition metal centers, through a-n interaction, with predominant a contribution). On reduced samples, a very small fraction of reduced centers, presumably located on high index faces and edges, shows high activity in ethene polymerization. This activity is demonstrated by the fast development of the IR bands characteristic of long -(CHZ),- polymeric chains, which spread on the whole surface. Like on Cr/SiO2 systems, it is hypothesized that the polymerization centers are mainly Cr2+in low coordination state. The ethene polymerization proceeds, with lower rate, also on Cr3+ ions of high index faces and/or edges, originating shorter polymeric chains. This justifies the modest polymerization activity characteristic of stoichiometric samples. The >CH groups of the living polymers interact with the empty Cr3+ surface sites of low index faces; this perturbation is revealed by the presence of anomalously low frequency bands in both the stretching and bending region of the -(CH2)- groups. Subsequent CO adsorption destroys these interactions and the IR spectra of the living polymers become more similar to those of the normal polyethylene chains. The ability of the Cr3+ surface ions to perturb the spectra of the adsorbed hydrocarbons through agostic-type interaction is also verified for n-heptane.
Acknowledgment. This research was supported by the Consiglio Nazionale delle Ricerche, Progetto Chimica Fine, and by MURST.
