2028
J . Phys. Chem. 1989, 93, 2028-2034
Structural Rearrangements in Chemisorbed Hydrocarbon Layers: 1,3-Butadiene on Rh/Al,O, P. Basu and J. T. Yates, Jr.* Department of Chemistry, Surface Science Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: June 1, 1988)
Transmission infrared spectroscopy has been used to study the chemisorption of 1,3-butadiene(C4H6)on Rh/A1203. Specific interaction between the hydrocarbon and metal surface is first observed at -200 K as a variety of carbonaceous species with varying H content are produced in the adsorbed layer. The resulting structures are rich in CH, and CH3 groups. The spectra between 225 and 300 K are characteristic predominantly of a di-cr (MCHzMCHCH=CH2) structure that coexists with a C4 metallocycle species where the C4chain is symmetrically coordinated to a metal site. The ratio of unsaturated to saturated species is seen to decrease between 225 and 300 K. Both double bonds in the parent diene are entirely consumed by 300 K. The di-a species is thermally unstable at T 2 300 K. At higher temperatures (300-350 K), C-C bond scission in C4species leads to the formation of adsorbed ethylidyne, M3CCH3,which is thermally stable up to 450 K. Subsequent C-H bond scission, at high temperatures, in the ethylidyne structure as well as in C,H,(a) species yields a hydrogen-deficient carbonaceous residue, most likely a polymer.
I. Introduction Determination of the molecular structure and orientation of chemisorbed hydrocarbon species on metal surfaces is of considerable significance to technologies such as heterogeneous catalysis, lubrication, adhesion, and corrosion. The effective application surface vibrational spectroscopies such as transmission infrared (TIR) and electron energy loss (EEL) spectroscopies has provided detailed information about hydrocarbon surface species. A large body of literature now exists encompassing EEL vibrational data for a number of hydrocarbon species on the more ideal and simpler single-crystal metal surfaces, and there exist a number of organometallic complexes with analogous ligands.'-I6 As a result, the vibrational assignment of spectra from the more complex dispersed metal systems has become a less formidable task. Although a definitive spectral interpretation and the assignment of vibrational modes to specific adsorbate structures is dependent on the hydrocarbon/metal system, general schemes of deciphering the adlayer structure are now beginning to emerge and a reactivity pattern particularly for olefin adsorption is being recognized.lv2 The general finding is that at T C 250 K the C=C group can undergo severe distortion as the C atoms rehybridize from sp2 sp3 to form an adsorbed configuration labeled "di-a"
-
( I ) Bandy, B. J.; Chesters, M. A.; James, D. I.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Philos. Trans. R. SOC.London, A 1986, 318, 141. (2) Koestner, R. J.; VanHove, M. A,; Somorjai, G. A. J . Phys. Chem. 1983, 87, 203. (3) (a) Beebe, T. P.; Albert, M. R.; Yates, J. T., Jr. J. Catal. 1985, 96, 1. (b) Beebe, T.P.; Yates, J. T., Jr. J. Am. Chem. SOC.1986, 108, 663. (c) Beebe, T. P.; Yates, J. T., Jr. J . Phys. Chem. 1987, 91, 254. (4) Bent, B. E.; Mate, C. M.f Crowell, J. E.; Koel, B. E.; Somorjai, G. A. J . Phys. Chem. 1987, 91, 1492. (5) Avery, N. R. Surf.Sci. 1984, 146, 363. (6) Avery, N. R.; Sheppard, N. Proc. R. SOC.London, A 1986, 405, 1. (7) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J. Chem. Phys. 1980, 72, 5234. (8) Kesmodel, L. L. J . Chem. Phys. 1983, 79, 4646. (9) Avery, N. R.; Sheppard, N. Surf.Sci. 1986, 169, L367. (10) Avery, N. R.; Sheppard, N. Proc. R. SOC.London, A 1986,405.27. (11) Steininger, H.; Ibach, H. Lehwald, S . Surf. Sci. 1982, 117, 685. (12) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf.Sci. 1984, 146, 21 1. (13) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, 111, L747. (14) Skinner, P.; Howard, M. W.; Oxton, I. A.; Kettle, S. F. A.; Powell, D. B.; Sheppard, N. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1203. (15) Anson, C. E. Keiller, B. T.; Oxton, I. A,; Powell, D. B.; Sheppard, N. J. Chem. SOC.,Chem. Commun. 1983, 470. (16) Albert, M.; Yates, J. T., Jr. The Surface Scientist's Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987.
0022-3654/89/2093-2028$01.50/0
(MCH2MCHzfor CzH4).1,6,9,10 On certain metal surfaces this distortion is lacking and the molecule retains its structural integrity bonding to the metal via interaction with its 7r-b0nd.',~.'~-'~A combination "di-a/d' structure has recently been postulated for 2-butene on Pt( 111) where the two C atoms in the double bond are each bonded to a metal atom by a a-bond and to a third metal atom via a 7r-bond.gJ0 These intermediate structures all dissociate at T 300 K to form an alkylidyne structure [M3C(CH2),CH3] on the close-packed surfaces of most group VI11 metal^.^-'^^^'*-^^^^^ Further dehydrogenation of this species at high temperatures yields a variety of C,Hy species that lead eventually to a graphitic overlayer a t 600-700 K. In this study we have examined the temperature-dependent behavior of 1,3-butadiene chemisorbed on Rh/A1203to determine how two double bonds in a simple diene compare in behavior to the surface chemistry of monoolefins such as CzH4. The scope of the work was limited to examining the general trends in diene reactive chemistry on the Rh/A1203surface while diene behavior was also compared and contrasted with that of simple olefin adsorption on metal surfaces.
-
11. Experimental Section
Infrared spectra were measured with a purged Perkin-Elmer Model PE-783 infrared spectrophotometer coupled with a 3600 data acquisition system for data storage and manipulation. The stainless steel UHV-IR cell used for the spectroscopic measurements has been described p r e v i o ~ s l y . Basically, ~ ~ ~ ~ ~ it consists of a main cell body containing a CaFz disk that serves as a transparent holder for the powdered catalyst. The CaF, plate is held in place by a Cu support ring that also houses the sample cooling coils. The cell body is mounted between two CaF, optical windows sealed in standard stainless steel flanges, permitting IR measurements in the 4000-1000-cm-' spectral range. The IR cell is attached to an all-metal gas handling system and is maintained typically at a base pressure, P Q 1 X IO-* Torr, by a 20 L/s ion pump. Preliminary evacuation is achieved with a liquid-nitrogen-cooled sorption pump. The sample preparation procedure essentially consists of slurrying known amounts of Rh"'Cl,(aq) [Alfa Chemicals (37.5% (17) Soma, Y. J. Catal. 1979, 59, 239. (18) Prentice, J. D.; Lesiunas, A.; Sheppard, N. J. Chem. Soc., Chem. Commun.1916, 76. (19) Soma, Y. J. Chem. Soc., Chem. Commun.1976, 1004. (20) Koestner, R. J.; Frost, J. C.; Stair, P. C.; VanHove, M. A,; Somorjai, G. A. Surf.Sci. 1982, 116, 85. (21) Salmeron, M.; Somorjai, G. A. J. Phys. Chem. 1982. 86, 341. (22) Beebe, T. P.; Gelin, P.; Yates, J. T., Jr. Surf Sci. 1984, 148, 526. (23) Wang, H. P.; Yates, J. T., Jr. J . Catal. 1984, 89, 79.
0 1989 American Chemical Society
Structural Rearrangements in Hydrocarbon Layers
1
3080
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2029
3042
1
4
29167
Resolution 5 . 4 cm-'
PStPNs I x
1840
PSCAN I 1 x 10-6Torr
1590
l i
Torr
dk Resolution
1
5 4 cm-'
-(d)
170K xIO
(e) ZOOK
( d ) 170K ( e ) 200K J
3200
I
I
I
I
2800
3000
Wavenumber ( c m - l ) Figure 1. Infrared spectra for C4H6on 10% Rh/A1203in the v(CH) region recorded as a function of temperature between 90 and 200 K. Data acquisition time was 4 s/cm-l. Initially 1.3 X 10" C4H6 molecules were condensed/adsorbed on the Rh/AI2O3sample at 90 K.
Rh)] and A1203in a mixture of H 2 0and acetone. The resultant slurry is sprayed onto half a side of the hot (50 "C) CaF2 disk (sample holder) such that the solvents are flash evaporated, leaving a uniform deposit. The other half of the CaF2 plate is sprayed with metal-free A1203support material in an identical manner. The CaF, plate containing the deposited powders is then mounted inside the IR cell. When the cell is translated in the beam path, this "half-plate" method allows us to compensate for any chemistry occurring on the support itself that has been treated under conditions identical with those on the catalyst side. The following pretreatment steps are employed in preparing the sprayed catalyst deposit: (a) outgassing at 300 K; (b) RhCl, decomposition at 473 K for 3 1 2 h; and (c) reduction at 473 K by several successive exposures of -400 Torr of H2for 15 min-2 h. After each H2 exposure for a specific time period, the cell is evacuated for -30 min at 473 K. Following the final reduction with H2, the cell is outgassed overnight at 473 K. The cell temperature was controlled between 90 and 300 K (to *2 K) by adjusting the flow of the liquid N2 coolant into the sample support ring assembly. Above 300 K an external heating arrangement was used to heat the entire cell. Temperature and cell pressure during the spectrum scan for each experiment are given in the respective figures. Typically the spectra shown for temperatures above 300 K were obtained by heating the cell to the given temperature and cooling to 300 K to scan the appropriate spectral regions. The mass of Rh employed in these experiments ranges from 2.27 to 2.50 mg in separate sample preparations, yielding an average of 10 wt % loading. A total sample weight of 22-27 mg was sprayed uniformly over a half-plate (CaF2 disk) area of 2.53 cm2, yielding a final density (Rh A1203) of (8.7-10.7) X lo-, g / m 2 for various samples. IR evidence from CO adsorption expenments on identical 10 wt % Rh/A1203 preparations indicates that metallic Rh sites capable of terminal-CO and bridged-CO adsorption exist together for these catalyst surfaces. The A1203used was Degussa aluminum oxide C of surface area 99.6 m2/g as determined by N2 adsorption (BET). The H2used for catalyst reduction was obtained from Matheson in a highpressure cylinder at a purity of 99.995%. The C4H6,also obtained from Matheson in a high-pressure cylinder (purity 99.8%), was transferred to a glass/metal storage bulb under standard vacuum techniques. The deuteriated isotopes 1,l ,4,4-C4H2D4and C4D6 were obtained from MSD Isotopes at a purity of 98.6% and 98.9%, respectively. The isotopic gases were obtained in break-seal glass flasks and used without further purification. The spectra presented were obtained with a slit program, yielding a maximum resolution of 5.4 cm-'. The spectra were
+
1900
1700
1500 I300 Wavenumber ( c m - ' )
Figure 2. Infrared spectra for C4H6 on 10% Rh/A1203in the v(C=C) and 6(CH) regions recorded as a function of temperature between 90 and 200 K. Data acquisition time was 8 s/cm-'.
acquired at 1 point/cm-' with typical data acquisition times as reported in the respective figures. The data were smoothed as required with a 19-point smoothing routine. 111. Results and Spectral Interpretation A . C4H6Adsorption on Rh/AI2O3between 90 and 200 K . IR spectra for C4H6adsorbed on 10% Rh/A1203 a t 90 K and following sequential heating to 200 K are shown in Figure 1 for the 3200-2800-cm-' v(CH) spectral region. Each spectrum displayed is determined by subtracting the background spectrum (taken in a preliminary experiment prior to adsorption) from the spectrum obtained following adsorption. The spectra consist of several intense features and are quite complex as would be expected for a large molecule such as C4H6. Comparison of the spectra on the Rh/A1203 side with spectra on the A1203side indicate that the changes in the spectra of Figure 1 due to heating are caused by C4H6-derivedprocesses that occur on the high area A1203 support in the temperature range 90-200 K. With an increase in temperature the following spectral changes are observed: (a) The intensities of all vibrational features decrease over the entire temperature range primarily due to thermal desorption of molecular C4H6. This is substantiated by the pressure rise in the continuously pumped cell and by mass spectral analysis of the desorbing gas that showed the presence of only 1,3-butadiene. (b) No significant shifts in the frequencies of the vibrational features are observed. (c) All features become somewhat broader upon heating. The selective absorbance enhancement of the 2997-cm-' band relative to others at 200 K is due to the onset of chemical changes in the C4H6adlayer on the Rh surface that proceed at higher temperatures. Above 200 K the C atoms in the adsorbed diene rehybridize, resulting in the production of CH2- and CH3-rich structures that contribute to the intensity of the 2997-cm-' feature; this is discussed in detail in subsequent sections. The corresponding v(-) and 6(C--H) spectral regions are shown in Figure 2. Several intense and a few weaker features are observed in this region. The most dramatic change that occurs upon heating is the disappearance of the feature at 1840 cm-' between 120 and 150 K. This cannot be attributed to a simple desorption effect since the feature at 1590 cm-' only decreases by -40% in intensity and other features remain visible also. The other spectral change observed is the broadening of the features at 1288 and 1304 cm-'. The doublet observed at 1372 and 1388 cm-' at 90-120 K transforms to a sharp single feature centered at 1380 cm-' at T > 120 K. This feature and the feature at 1840 cm-' are associated with vibrations of the =CH2 group and the spectral changes observed suggest a change in the local geometry of the molecule at the surface. In general, the spectra are quite complicated due to the presence of a number of overtone and combination bands and the vibrational assignment for the adsorbed layer is to be considered as ap-
2030
Basu and Yates
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
TABLE I: Assignment of the Vibrationnl Modes for C a , Adsorbed on Rh/AI,O~ at 90 K by ComDarison with Those Observed for Other Systems
mode
C4H6 + Rh/Al,O,"
vibrational frequency, cm-' matrix solidzS
3080 3042 3025
3079 3041 3021
2991
2995
2967 2912
2968 2910
1840
1840
1634 (weak) 1590 1463 1372, 1380 1288,1304
1635 (weak) 1587 1377, 1369 1285
3069-3075 304C-3056
I
PSCAN 5 1 x
gas**24
-itResolution
3102 3056
5 . 4 cm" T
) I Abs
225K
250K
1612-1618 1471-1474 1372-1375 1216-1232
1643 1599 1442 1385 1296
300 K
b- ( d ) 3 5 0 K
'This work. assigned features listed. 'C4H6 condensed with metal atoms including Ni, Pd, and Fe.
proximate with appreciable mixing of modes occurring. A partial assignment of the IR features observed in Figures 1 and 2 for C4H6 on Rh/A1203 is given in Table I and is made on the basis of a comparison ith IR spectra for gas phasez4and solid C4H6" and for C4H6isolated in low-temperature matrices containing metal atoms.26 Contributions due to vibrations of -CH and =CH2 groups were confirmed in this work by adsorption of 1,1,4,4C4HzD4and C4D6. Identical spectral changes in the u(CH), 6(CH), and u(C=C) regions are also observed for the metal-free A1203support. The changes observed here are therefore related to spectroscopic changes in C4H6as one proceeds from multilayer condensed C4H6 to monolayer quantities of C4H6on Alz03. E . Adsorption of c4H6 on RhIAlZO3above 200 K . At temperatures above 200 K, spectral changes on the Rh/AlZO3sample begin to differ from those observed on the A1203sample due to the onset of chemical changes associated with the Rh surface. C4H6adsorption on Al2O3is negligible in the 200-450 K range. IR spectra obtained as a result of heating the low-temperature C4H6layer to various temperatures are shown in Figure 3 for the u(CH) spectral region. Each spectrum displayed was obtained following a background subtraction, as previously explained. A general trend is easily seen in Figure 3. In the range 225-300 K, C-H stretching modes indicative of sp2-hybridized C atoms are lost and C-H stretching modes indicative of sp3-hybridized C atoms are produced. The spectra shown in Figure 3a-q obtained between 225 and 300 K, consist of several weak features superimposed on a broad and intense background that is indicative of many overlapping v(CH) vibrational features making a definitive assignment almost impossible. As the temperature is increased to 300 K the following occur: (a) Several sharp features between 3000 and 2800 cm-' become apparent. (b) The broad background systematically shifts down in frequency as the contribution to the intensity at frequencies above 3000 cm-' diminishes considerably (shaded region in Figure 3). An appropriate assignment for several of the modes that have developed at 300 K (Figure 3c) is as follows: u,(=CHz), 3070 cm-I; v ( X H ) , 3010 cm-'; u,,(CH3), 2975 cm-I; v,(>CH2), 2938 cm-I; u,(CH,), 2902 cm-'; and uS(>CHz), 2882 cm-'. The assignment is made based upon vibrational data for various hydrocarbons adsorbed on supported metals and metal single crystals,4~6,10,1 7.27-32 (24) (25) 645. (26) (27)
Torr
I
Panchenko, Y . N. Spectrochim. Acta, Part A 1975,31A, 1201. Bondybey, V. E.; Nibler, J. W. Spectrochim. Acta, Part A 1973, 29A, Hisatsune, I. C. Specfrochim. Acta, Part A 1984, 40A, 391. Busca, G.J. Mol. Sfruct. 1984, 117, 103.
( f ) 450K J
.oo
I
I
1
3000
Wavenumber (cm-'
I
2000 )
Figure 3. Infrared spectra in the u(CH) region showing rehybridization of carbon atoms in the C4H6-derivedadlayer on 10% Rh/AI2O3. Data acquisition time was 30 s/cm-l.
Spectral changes that occur in the v(CH) region between 350 and 450 K are shown in Figure 3d-f. The most significant result is the change of the v,(CH3) feature that is shifted slightly from 2902 to 2893 cm-'. It is believed that the intense asymmetric band containing components at 2893 and 2882 cm-' that exist at 350 K is mainly indicative of the development of a CH, functionality of nominal frequency, vs(CH3) = 2880 cm-'. The selective attenuation of one vibrational feature compared to others is indicative of the presence of different adsorbate structures; the new species developed consist predominantly of CH3groups exhibiting us(CH3) E 2880 cm-l. At 350 K, any absorbance in the region above 3000 cm-I is essentially lacking. Upon further heating to 450 K, a decrease in intensity for all the features in the 3000-2800-cm-I region is observed, while a broad and weak IR band develops between 3000 and 3100 cm-'. The latter is indicative of unsaturated CH, spec!es remaining on the surface after depletion of CH2- and CH,-rich structures present at 250-350 K. Corresponding IR spectra for the u(C=C) region are shown in Figure 4. The 200 K spectrum is shown for comparison. The somewhat unique contour and shift in frequency are caused by the spectral subtraction technique; a vibrational mode of A1203 is present in this region (1590-1570 cm-') and shifts slightly upon adsorption. Thus, spectral contour changes between 1570 and 1500 cm-I are to be disregarded. Only the positive absorbance (shaded area in Figure 4) corresponds to the C=C stretching vibration. A dramatic decrease in intensity of the characteristic v(C=C), 1590-cm-' feature is observed while heating above 200 K, indicating a depletion of the C=C functional group. The final spectral region presented is that due to 6(CH) modes of the chemisorbed species (Figure 5). For comparison, the (28) (29) (30) (31) (32) (33)
Soma, Y . Bull. Chem. SOC.Jpn. 1977, 50, 2119. Avery, N. R. J. Catal. 1970, 19, 15. Morrow, B. A.; Shepard, N. Proc. R. Soc. London A 1969,311,391. Morrow, B. A,; Sheppard, N. Proc. R. Soc. London, A 1969,311,415. Sheppard, N.; Ward, J. W. J. Catal. 1969, 15. 50. Davidson, G. Inorg. Chim. Acfa 1969, 3(4), 596.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2031
Structural Rearrangements in Hydrocarbon Layers .I
I
I
15gO d
-6
P ,,,
5 1 x 10
Torr
Q,
u c 0
e 0
In
n
a
I
'
\
/-d
\/
1700
1600 1500 Wavenumber (cm-' ) Figure 4. Infrared spectra in the v(C=C) region showing the behavior of double bonds as a function of temperature in the C4H6-derivedadlayer on 10% Rh/AI2O3. Data acquisition time was 24 s/cm-'.
I
lO-'Torr
PsCAN5 I
ResoI~t#on
al
c U 0
225KIal
v) 0
250Kibl
n L n
a i fI 400K
300Klcl
I
I
1450
1
1450 Wovenumber ( c m - ' )
1350
I
1350
Figure 5. Infrared spectra in the 6(CH) region showing a temperaturedependent structural rearrangement in the C&-derived adlayer on 10% Rh/AI2O3. Data acquisition time was 24 s/cm-'.
corresponding spectra for C4D6adsorbed at 225 K are also shown (Figure 5, spectrum d). The oscillations due to 6(CH) can be grouped into two sets: (1) those that concomitantly increase in intensity (1410, 1340 cm-') as a function of temperature and are attributed to one type of species and (2) features that concomitantly decrease in intensity with temperature (1436, 1320 cm-l) due to a different adsorbed structure. Also, two other associated modes at 1380 and 1355 cm-I are observed. Comparison of the C4H6 and C4D6 layer in Figure 5 confirms that the features observed between 1450 and 1300 cm-' all result from C-H and not C-C vibrations. C=C vibrations in ?r-bonded complexes are also typically observed in this spectral region. Thus, a T bonded structure such as H
H
can be ruled out although analogous structures for simple olefins have often been p o ~ t u l a t e d . ~ - ~ * ~ ~ The task of assigning the features observed to specific surface structures is done by drawing analogies from the vast body of EEL data for hydrocarbons adsorbed on metal single crystals and from vibrational assignments of related organometallic complexes. The basis for assignments of vibrational modes appropriate to this work
are summarized in Table 11. The modes at 1436 and 1320 cm-l are due to one type of adsorbate structure and are assigned as follows: 1436 cm-', 6(CHz),,; 1320 cm-', 6(=CHz). These assignments are in agreement with those by Soma for di-a C4H6 on Ni/A120328and by Sheppard for the same structure on Pt(1 1l).IO On the basis of the above assignments and the general agreement of our IR features in the v(CH) and u(C=C) spectral regions with those observed by Soma28and Sheppard,'O we conclude that the characteristic vibrations observed between 225 and 300 K are those of di-a C4H6(MCH2MCHCH=CHz). The only point remaining to be addressed is the 1408-cm-' feature that is assigned to a 6(=CH) mode by Somaz8 for the di-a structure. In our spectra (Figure 5a-c), this feature does not appear to decrease concomittantly with other di-a associated features. This is because the formation of another adsorbate structure is initiated that contributes to an increasing fractional intensity between 1408 and 1410 cm-' (see Figure 5c-f). The features at 1355 and 1380 cm-l observed in Figure 5a-c are due to yet another adsorbate structure that coexists with the di-a species. The absorbance occurs in the region where modes due to deformations of C H 2 groups are Furthermore, comparison of the observed typically 0bserved.6*~~-~~ frequencies with those for cyclic ethers (such as C4H80)suggests that these could be the u(CH2) modes of methylene groups in a ring structure.34 These features persist in the I R spectrum even at higher temperatures (Figure 5d,e) with a gradual decrease in intensity, An extremely interesting spectral change that occurs when the adlayer is annealed is the gradual appearance of a relatively intense feature at 1340 cm-' that becomes the predominant band at 350 K (Figure 5d) accompanied by a relatively weaker feature at 1410 cm-'. These features can easily be assigned as 6,(CH3) (1340 cm-') and 6,,(CH3) (1410 cm-l) consistent with the assignments by others (see Table 11). It is interesting to note the striking similarity of the C4H6-derived layer at 350 K with that for ethylidyne (CCH3) formation from CzH4 on Rh/AlZO3,recently reported by Beebe and Yates3, (see also Table 11). On the basis of this excellent agreement, in both the v(CH) and 6(CH) regions, we attribute the spectrum at 350 K predominantly to the ethylidyne structure. Further heating of the adlayer above 350 K results in a continuous decrease in intensity of all features in the 6(CH) region as observed in Figure 5e-g. IV. Discussion A . Structural Nature of the Low-Temperature C4H6 Adlayer on Rh/Alz03. On the basis of the similarity between the spectra for C4H6on Rh/Al2O3 and those of solid C4H6and matrix-isolated C4H6(see Table I), it can be concluded that in the temperature range 90-200 K the molecule retains its structural integrity as C4H6 multilayers condense on the surface. Furthermore, no metal-specific chemistry is observed spectroscopically in this temperature range since the spectra acquired following adsorption of C4H6 on Rh/A1203 are identical with those for Alz03; the spectra are dominated by the species adsorbed on the high-area A1203support. Upon heating, the weakly adsorbed C4H6molecules desorb, leaving behind a chemisorbed layer on the metal at T > 200 K. At T 150 K a structural change occurs in the C4H6multilayer, substantiated by the disappearance of the 1840-cm-' feature due to oscillations of the (=CH2) group. This is purely a physical phenomenon not related to any specific chemical interaction with the Rh metal or A1203support since it was also found to occur in the bulk phase on an unreactive CaF2 substrate (that serves as the sample holder). The effect is observed at a temperature close to where solid C4H6melts (164 K). In low-temperature matrices of C4H6condensed with various metal atoms, Hisatsune26 observed a similar effect upon heating the matrix to 160 K and attributed it to the fomration of an "amorphous solid". Consistent with this, the C4H6 multilayer (34) Palm, A.; Bissell, E. R. Spectrochim. Acta 1960, 16, 459.
2032
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Basu and Yates
TABLE II: Spectral Assignments for a Variety of Adsorbed Molecular Structures following Reaction of Different Olefins with Metal Surfaces and for Organometallic Complexes with Analogous Ligands proposed molecular structure freq, cm-' approx assignt of vibr mode adsorption syst ref T,CqHn
di-o, C4H6
di-o, C4Hs
di-a, C4Hs
ethylidyne, +C-CH3
pyropylidyne, CH3CH2C
