Langmuir 1988,4, 999-1006
999
Raman Spectroscopy of Unsaturated Hydrocarbons on Supported Rhodium and Palladium W. L. Parker,*>!A. R. Siedle,t and R. M. Hextert Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105, and 3M Corporate Research Laboratories, S t . Paul, Minnesota 55144 Received December 29, 1986. I n Final Form: March 8, 1988 Raman spectra of unsaturated hydrocarbons adsorbed on functional hydrogenation catalysts are found to be considerably different than vibrational spectra obtained from unsaturated hydrocarbons adsorbed on single crystal surfaces under ultra-high-vacuum conditions. The vibrational spectra are assigned to group frequencies, which are then used to rationalize the probable surface species. These surface species agree with those that have been proposed as intermediates in the catalytic reduction of hydrocarbons.
Introduction Heterogeneous catalysis is an important means of effecting chemical transformations, yet many heterogeneously catalyzed reactions are not understood at a molecular level. It has long been believed that formation of surface complexes is critical to the ensuing chemistry. The nature of these surface complexes and the information retained in them dictate the chemistry and possible reaction paths of the adsorbate. A variety of molecular spectroscopies have been developed specifically to determine the nature of these surface complexes. Each technique has its limitations, and many are difficult to apply when studying high surface area metal catalysts. As an example, electrical charging effects make electron energy loss spectroscopy difficult on nonconducting surfaces. In contrast, Raman spectroscopy is a potentially powerful probe of adsorbed materials applicable to the analysis of catalysts operating a t high (>1Torr) pressures because neutral photons rather than charged particles are scattered. The surface chemistry of unsaturated hydrocarbons adsorbed on atomically clean single metal crystals has been studied with many of the techniques of modern surface science. The relation between the results of these studies and the actual bulk chemistry is sometimes puzzling. Of particular interest has been the study of simple Cz hy; drocarbons on the group VI11 transition metals. LEED, ARUPS,2 and HREELS3 spectroscopies have all revealed that small alkenes and alkynes both undergo a remarkable structural rearrangement to form surface alkylidyne (M=C-R) fragments in which a RC group is bonded to a triangular M3 array. The role of hydrogen in the rearrangement has yet to be completely elucidated. The group VI11 transition metals are frequently used as catalysts for the hydrogenation and condensation reactions of unsaturated molecules. It is well-known that metalcatalyzed hydrogenation of alkenes and alkynes results in cis 1,2 addition of hydrogen to carbon-carbon multiple bonds.4 The hydrogen migration required for the formation of the surface alkylidyne species destroys the original stereochemical information contained within the adsorbate prior to its reaction. Once the stereochemical information is obliterated by creation of an alkylidyne surface intermediate, it cannot be recovered in subsequent reactions. This leads to a conundrum: hydrogenation of a surface alkylidyne cannot result in a product having the University of Minnesota. 3M Corporate Research Laboratories. *North Dakota State University.
*
0743-7463/88/2404-0999$01.50/0
regiospecificity and stereospecificity known to occur in reactions catalyzed by supported metal catalysts. One proposal which surmounts this difficulty supposes that on platinum ethylidyne (=CCH3) is hydrogenated to form ethylidene (=CHCHJ. This, in turn, transfers its a-hydrogen to a molecule of ethylene that is weakly adsorbed in second layera5 However, Beebe and Yates have recently demonstrated that, on 10% Pd/A1203, rates of ethylidyne formation (and hydrogenation) are 2-3 orders of magnitude slower than that of ethylene hydrogenation and, further, that the rate of ethylene hydrogenation does not depend on precoverage of the surface with ethylidyne.6 Thus, surface alkylidyne groups, although observable in both ultra-high-vacuum and high-pressure (0.350 Torr) regimes, may not be relevant to catalytic reactions of alkenes and alkynes. The structures of adsorbate-derived surface organometallic complexes, particularly those consistent with established kinetic and stereochemical results, are the topic of this paper. The probable surface geometries of ethylene, acetylene, and related hydrocarbons on supported rhodium and palladium are inferred from the analysis of their Raman spectra.
Experimental Section The catalysts used were rhodium and palladium supported on a-alumina. The metal loadings were 10% by weight, and the samples were prepared according to literature method^.^ Reduction and dosing were performed in a stainless steel reactor and spectroscopic cell constructed from a UHV six-way cross. The adsorbate was introduced into the reactor from a stainless steel gas manifold, and the gas pressure was measured by using a capacitance manometer. Both the manifold and the reaction chamber were evacuated by roughing with liquid N2 cooled sorption pumps; high vacuum was achieved in the reaction cell by an 8 L/s ion pump (base pressure 5 X Torr). The Raman cell had no provision for temperature control, so all spectra were obtained at room temperature and under a static gas pressure. Raman spectra were obtained by using a previously described spectrometer8and were measured relative to either the 488.0- or (1) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180.
(2) Tysoe, W. T.;Nyberg, G. L.; Lambert, R. M. J. Phys. Chem. 1984, 88, 1960. (3) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (4) Morrison, R. T.;Boyd, R. N. Organic Chemistry, 3rd ed.; Allyn and
Bacon: Boston, 1973. (5) Godbey, D.; Zaera, F.; Yates, R.; Somorjai, G. A. Surf. Sci. 1986, 167, 150. (6) Beebe, T . P.; Yates, J. T., Jr. J . Am. Chem. SOC.1986, 108, 663. (7) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504.
0 1988 American Chemical Society
1000 Langmuir, Vol. 4, No. 4 , 1988
Parker et al.
31 50.00 21 50.00 150,""'"'""'1""""'"""""~
1 150.00
1004
1
150.00
100
70 n
,
1
1
60 v
50
z
z
n -n
40
30
3.2
2.8
2.4
2
1.6
1.2
0.8
0.4
0
(Thousands) DELTA WAVENUMBER
Figure 2. Raman spectrum of CzDzadsorbed onto 10% Pd/aA1203: pressure approximately 50 Torr, laser line 488.0 nm, laser power 200 mW.
O
L I , 31 5d.00
I
I
I
J
I
I
,
I
I
I
21 50.00
I
I
,
I
I
I
,
,
,
1 156.00
I
,
,
,
,
,
,
150100
Delta Wavenum ber Figure 1. Raman spectra of acetylenes adsorbed on supported rhodium: pressure 47 Torr, laser line 488.0 nm, laser power 200 mW. The peaks marked with asterisks correspond to combinations of the intense peaks. 514.5-nm line of an Ar+ ion laser. In all cases the laser was defocused; laser powers of 200 mW or less impinged on the interference filter. At higher fluence noticeable charring of the sample occurred. Acetylene was purchased from Matheson Gas Products (purified grade, 99.6% pure) and was purified by bubbling through a concentrated aqueous sodium nitrite solution (to remove acetone) and then through dry ice cooled 5A molecular sieves. It was stored at less than 1atm of pressure in a glass bulb. Benzene was ACS reagent grade and was distilled from CaH, on a vacuum manifold immediately before use. Hydrogen (Matheson research grade), deuterium (Matheson chemical grade), and ethylene (Union Carbide analyzed grade 99.97% C2H4)were used without further purification. Isotopically labeled hydrocarbons were obtained in glass break seal flasks from Merck, Sharpe and Dohme and were used without further purification. The a-alumina was Linde high purity abrasive ( r < 1wm). It was boiled in 30% H,Op for 2 h and then oven dried before impregnation with RhC13or PdCl, solutions. The BET surface area of the support was 76 m2/g.
Results and Discussion Vibrations in chemisorbed adsorbates are traditionally assigned by analogy to structurally characterized model compounds. In this paper, measured Raman bands are assigned to molecular motions, and these assignments are then used to determine the molecular geometry. This method of argument necessitates frequent reference to results, so we present the parts of this Results and Discussion section as unified. The results and discussion of each adsorbate are then treated separately, in the order (8) Parker, W. L.; Hexter, R. M.; Siedle, A. R. Chem. Phys. Lett. 1984, 107,96.
(1) acetylene, (2) benzene, and (3) ethylene. Acetylene. Raman spectra of CzHz,CzD2,and 13C2H2 adsorbed on alumina-supported rhodium are presented in Figure 1. For C,Hz, the spectrum consists of two strong bands at 1476 and 1096 cm-' and four bands of lower intensity at 3000,2588, 2208, and 978 cm-'. The band at 1476 cm-' shifts to 1408 cm-l in the spectrum of C2Dzand to 1451 cm-' in 13CzHz. The band a t 1096 cm-' shifts to 849 cm-l on deuteriation and to 1081 cm-' with 13C2H2. These shifts indicate that the 1096-cm-' band is most likely due to a C-H bending motion, and the band at 1476 cm-' is a CC stretchg (cf. 1974 cm-' in free C2H2). Of these, the correct assignment of the vcc vibration is of paramount importance, because it provides information about the bond order and orientation of the CC unit.l0 It is tempting to assign the weak band at 3000 cm-l to a CH stretch and to rationalize the "high energy" Raman bands at 2588 and 2208 cm-' as frustrated torsional and rocking modes. The bands exhibited by the labeled compounds make these assignments untenable. The 3000-cm-' band shifts to 2951 cm-l on deuterium substitution and to 2910 cm-' in 13C2H2.This isotopic shift is incompatible with a CH stretch assignment. Careful examination of these shifts in each of the acetylene spectra reveals that these bands result from combinations and overtones of the two strongest bands in each spectrum. The remaining weak band at 978 cm-l shifts to 729 cm-l on deuterium labeling but remains essentially unshifted on 13C labeling (981 cm-I). This type of spectral response to isotopic substitution is characteristic of a C-H motion.ll This band is therefore tentatively assigned to the antisymmetric bending mode, 6cH(as). In the spectrum of C2Dzadsorbed on either Pd/Alz03 (Figure 2) or Rh/A120, (Figure 1B) an additional sharp band occurs at 1181cm-l. This extra band is found only for adsorbed deuterioacetylene. It is independent of whether the metal particles were produced by hydrogen or deuterium reduction, and it does not correspond to a combination or difference of the two strong bands. We believe that this extra peak is due to a CzHD impurity. Because the deuterium isotopic purity of the CzD2is 9970, there is a finite amount of both CzHD and CzHzpresent in the cell, with C2HD being more abundant. The position of the band (1181 cm-l) indicates that it is not a contri(9) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf.Sci. 1977, 1, 1. (10) Evans,J.; McNulty, G. S. J. Chem. SOC. Dalton Trans. 1984,79. (11) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982.
Langmuir, Vol. 4 , No. 4 , 1988 1001
Raman Spectroscopy of Unsaturated Hydrocarbons
Table I. Vibrational Data (cm-') for Adsorbed Acetylenes and Representative Model Compounds'
%+ 3374 1974 3289 612
CzHzb
l.4" 1380 3020 1049
CzHz on Pt(III)C EELS 3000 1424 760
730 2701 1762 2439 537
1310 2215 844
505
2224 1248 568
Os3(CO)&CHJd
Coz(CO)~CzHze(CH3C)Co3(CO)$ CzHz on 10% Rh
2986 1331 3047 1470
3116 1402 3086 894
1051 963 811 2180 1282 2140 760
768 2359 1346 2297 751
742
602
2888 1161 2930 1420 1356 1004
assignment
1476 (1451)
vCH(S) "CC
vca(as) 1098 (1081) 981 (982)
M S )
6cH(as)
1161 2192 828 1031 1002
" A dash indicates no observation. Data in Darentheses are for I3C. blAuis cis bent, ref 45.
u2, r, ref
3. dVinylidene, ref 20.
eaz,r, ref
19. fEthylidyne, ref 34.
bution from C2H2,so it is likely due to C2HD. This assignment has some interesting aspects. Independent labeling with deuterium and 13Cproduces spectral shifts in both the CC stretch and the symmetric CH bend relative to protioacetylene, indicating that there is considerable mixing of the CC stretch and the CH bend normal modes, a phenomenon well documented for some molecular compounds.12 In free C2HD,the bending modes resolve into those due to almost pure CH or CD motions.13 These normal modes are very much unlike the symmetric and antisymmetric CH bends of C2H2. If the adsorbed molecule retains this separation there will be no normal mode of the adsorbate, which approximates the symmetry species of the symmetric CH bend, and therefore we should 0.6 4 --"7--"--I-----4 0 1 8 3 4 not expect the bending motions to mix well with the CC Bond Ordex stretch. This lack of mixing accounts for the lack of a red Figure 3. Carbon-carbon bond stretching frequency vs bond shift compared to normal acetylene. There is also a reorder. The line is a best fit of data from gaseous acetylene, versal in the relative intensity of the bands assigned to the ethylene, and ethane plotted against their conventional bond CC stretch and the CH bend in going from deuterio- to orders. protioacetylenes. This could be explained by the overlap of the CC stretches of the two species. Table I summarizes the Raman results for isotopically labeled acetylenes adsorbed on 10% Rh/A1203and offers vibrational data characteristic of model organometallic compounds for comparison. Isotope-labeling experiments and group frequency arguments lead to a plausible molecular accounting of the observed Raman bands. The correspondence between the vibrational spectra of adsorbed acetylene, the low-temperature phase of acetylene on Pt(lll),CO2(C0)&2H,) and cis-polyacetylene strongly indicates that they share common structural features. The data for C2H2(ads)are thus consistent with orientation of Figure 4. Artist's rendition of a,,a-bound acetylene. the C-C bond parallel to the metal surface and with the CC bond oriented perpendicular to the projection of the metals, the nature of the hydrocarbon unit is the same: the MM bond, i.e., a twofold site, for which the cluster anaC-C bond is essentially parallel to the metal surface, the logue is U ~ , ~ - ( C ~ H ~ ) C O or ~ ( interacting C O ) ~ , ~ ~with a angle is decreased from the 180° in free acetthree-fold Rh site as exemplified by O S ~ ( C O ) ~ ~ ( P ~ C ~ PCCH ~ ) . 'bond ~ ylene, and the CC bond order is approximately 1.9. FolA better agreement of the Raman data with the group lowing the theory of Weinberg,'6 the low C-C stretching frequencies in (C2H2)C02(C0)6 provides a basis for a slight frequency, smaller than that of ethylene, is taken as evipreference for the location of C2H2adsorbed on a twofold dence that the actual bond order is less than 2 (cf. Figure surface site. 3). This argues for a tilted arrangement to maximize ?r In each of the model compounds, and by analogy for back-bonding to a third surface atom, as shown in Figure acetylene adsorbed onto alumina-supported group VI11 4. There are no bands in the Raman spectra that are attributable to an alkylidyne species. The surface hy(12) Nakamoto, K. Infrared and Raman Spectra of Inorganic Comdrocarbon species observed by Raman spectroscopy is pounds, 3rd ed.; Wiley: New York, 1978. tightly bound since no significant changes in signal in(13) Herzberg, G. Infrared and Raman Spectra of Polyatomic Moletensity or peak position occur on evacuating the cell to 5 cules; Van Nostrand Reinhold New York, 1945.
W
(14) Iwashita, Y.; Tamura, F.; Nakamura, A. Znorg. Chem. 1969, 8, 1179. (15) Pierpont, C. G. Inorg. Chem. 1977, 16, 636.
(16) Felter, T. E.; Weinberg, W. H. Surf.Sci. 1981, 103, 265.
Parker et al.
1002 Langmuir, Vol. 4, No. 4, 1988 X lo-' Torr. It is also chemically active, as the intensity of the spectrum is diminished upon addition of 100 Torr of hydrogen a t 150 "C, consistent with reduction of C2H2(ads)to weakly (if at all) bound C&6. Resonance Raman spectroscopy has been used to detect polyacetylene on zeolites." The polymerization of acetylene on aluminosilicates suggests that kinetically stable cis-(CH), might be a source of intense Raman signals on alumina-supported metal catalysts. Two separate lines of evidence suggest that this is not the case in our system. First, although for supported Pd there is a dependence of Raman intensity on support, this intensity difference vanishes with excess hydrogen.I8 This effect is correlated with a change in the optical properties of P d when hydrogen is dissolved in the bulk rnetal.ls The low pressure of C2H2and the presence of H2 should favor the hydrogenation reaction and disfavor polymerization. Secondly, GC-MS analysis of the gas-phase products formed when C2D2/Pd/A1203is treated with deuterium shows that the major products are ethane (60%), n-butane (23%), cyclohexane (6%), n-hexane (5%), CBD1, (4701,and C, ( n = 3 and 5) hydrocarbons being formed with less than 0.3% abundance. We surmise that C&2 and C&,, arise from the hydrogenation and hydrogenolysis, respectively, of C6D6formed by the cyclotrimerization of acetylene, because the hydrogenolysis of polyacetylene should show no preference for formation of even- versus odd-numbered hydrocarbons, and one would also expect a predominance of higher molecular weight hydrocarbons. Indeed, catalytic hydrogenation of authentic (CH), has been shown to produce high molecular weight p01yethylene.l~ Raman spectroscopy has also been used to characterize CF3C2H,CF3C2CF3,and CH3C2CH3absorbed on Rh/A1203 and CH3C2CH3/Pd/A1203. Spectra obtained with this collection of alkyl- and perfluoroalkyl-substituted acetylenes permit assignment of the CC stretching frequencies to bands at 1572, 1540, and 1576 cm-', respectively. The sensitivity of the CC stretching frequency to the mode of ligationlo makes this assignment most useful in the determination of the likely geometry of the adsorbate. Hexafluoro-2-butyne is known to exhibit several different bonding modes in its organometallic complexes. In structurally characterized20 Ni(C0)2(CF3C2CF3),Ni3(C0)3(C8H8)(CF3C2CF3), and Ni,(CO),(CF3C2CF3)3, the 2butyne unit coordinates in a cr2, H, and uZ,xfashion.20 The CC stretching frequencies for these organometallic complexes are respectively 1905 cm-' (u2-C4F6),1852 and 1834 cm-' (r-C,F6), and 1564 cm-' (u2,r-C4F6).Comparison of these CC frequencies with that observed for CF3C2CF3 adsorbed on Rh/A1203indicates that the likely adsorption geometry of the 2-butyne unit is also u2,rTT. Because alkylidyne formation is possible only for terminal alkynes, it is significant that propyne and 2-butyne both yield similar and self-consistent Raman spectra and, therefore, presumably bond in similar ways to the rhodium surface. Also, because the C-F bond energy is so much greater than that between carbon and hydrogen, it is improbable that the perfluoroalkylacetylenes, which bond in a u2,r fashion, like their hydrogen-substituted analogues, undergo surface
(17) Heaviside, J.; Hendra, P. J.; Tsai, P.; Cooney, R. P. J.Chern. Sac., Faraday Trans. I 1978, 74, 2542. (18) (a) Parker, W. L.; Siedle, A. R.; Hexter, R. M. J . Catal. 1986,99, 482. (b) Fleischmann, M.; Graves, M. R.; Hill, I. R.; Robinson, J. R. Chern. Phys. Lett. 1983, 95, 322. (19) (a) Shirakawa, H.; Sato, M.; Hamano, A.; Kawakami, S.; Soga, K.; Ikeda, S. Macromolecules 1980,13,457. (b) Chacko, B.; Chien, J. C. W.; Karasz. F. E.: Heeger, A,: MacDiarmid, A. Am. Phys. SOC.Bull. 1979,24, 480. (20) Davidson, J.; Green, M.; Stone, F. G. A,; Welch, A. J . Am. Chem. SOC.1975, 97, 7490.
31 50.00 21 50.00 1 150.00 30~"'""'"~'"'"""""""'
150.00
+-
E
3 0
15
10
1
31 50.00
1
1
1
1
1
1
~
21 50.00
I
I
I
I
I
I
1 150.00
I
I
~
J
I
1
~
150.00
Delta Wavenum ber Figure 5. Raman spectra of (A) benzene a t its vapor pressure adsorbed on 10% Rh/a-A1203, (B) surface benzene only, and (C) surface benzene plus coadsorbed CO. T h e exciting laser was operated at 488.0 nm a t an input power of 100 mW.
rearrangement involving multiple C-F shifts to form terminal alkynes. Benzene. Acetylene adsorbed onto Pd(ll1) is known to form benzene, and GC-MS analysis, vide supra, of the gas-phase products formed when C2D2/Pd/A1203is treated with deuterium shows that the major products include those logically derived from benzene, e.g., C6DI4and C6D12. C2D6 is the expected product of the catalyzed addition of D2 to C2D2. C4 hydrocarbons are known byproducts formed during the cyclotrimerization reaction of CzH2on single-crystal Pd surfaces,21,22and so the presence of deuteriated C4hydrocarbons is to be expected. Alternately, C4 products could result from catalyzed CC bond cleavage in the hexanes. The Raman spectrum of benzene adsorbed onto 10% Rh/N2O3changes in the presence of coadsorbates. Spectra which illustrate this are shown in Figure 5. The spectrum of benzene on Rh/A1203,Figure 5A, agrees with the models for the adsorbed hydrocarbons as deduced from other studies (vide supra). This spectrum consists of four broad bands centered at 2907, 2729, 909, and 800 cm-l due to chemisorbed benzene and sharp peaks at 3063 and 992 cm-' which correspond to the most intense bands in the spectrum of liquid benzene. The latter are assigned to a physisorbed multilayer phase as inferred from earlier infrared data;23they are not apparent when the laser beam is focused on the sample holder instead of the catalyst (21) Haberland, H. Surf. Sci. 1983, 130, 245. (22) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. Surf. Sci. 1983,135, 128. (23) Little, L. H. Infrared Spectra of Adsorbed Species; Academic: New York, 1966.
1
~
~
~
~
Raman Spectroscopy of Unsaturated Hydrocarbons sample. The lines due to physisorbed benzene disappear upon evacuation of the cell (Figure 5B). The broad bands in the Raman spectrum from the chemisorbed benzene probably represent the same types of atomic motions, CH stretches (2907 and 2729 cm-l), and CC stretch (909 cm-') as the sharp bands. This assignment is consistent with a 76 bonding geometry found on single-crystalmetal surfaces by other spectroscopic techniques.% Subsequent addition of carbon monoxide to the system leads to attenuation of the C&&(adS) bands and the appearance of new peaks in the spectrum, due to C2H2(ads)(Figure 5C). This connection between surface benzene and surface acetylene is important in light of the proposal that the relevant surface species is polyacetylene, or perhaps some oligomer of acetylene. This polymerization is well-known on some Pt group metal surfaces,22v25-nespecially P d ( l l l ) , which catalyze, the cyclotrimerization of acetylene. In terms of the C-H ratio benzene and acetylene are indistinguishable, but chemically the difference is dramatic. In benzene the CC bond is nearly sacrosanct, the chemistry being dominated by bond-making and -breaking reactions of the CH bonds, whereas in acetylene the CC bond frequently plays a determining role in the chemistry of the molecule. Widespread interest in the surface chemistry of benzene is evidenced by the quantity of published work on its surface structure. Surface vibrational information for benzene adsorbed on metals and metal particles has been obtained by using transmission infrared spectroscopy,2e EELS,29neutron inelastic scattering,m and Raman spect r o ~ c o p y .A ~ ~recurring conclusion of this work is that the usual structure of benzene on a surface is one in which the molecular plane lies parallel to the idealized metal surface. Analysis of the low-energy electron diffraction data corroborates this conclusion and further indicates that the hydrogens are slightly tilted away from the surface out of the c6 plane.32 I t has been established that on P d ( l l 1 ) u2,~-C2H2 is a precursor of C6H6in the surface trimerization process.17 In contrast, the formation of benzene from ethylidyne must require multiple rearrangements. In the absence of incisive theoretical work, we consider that the carbon monoxide induced decyclotrimerization of benzene has its genesis in electronic factors. Indeed, recent LEED data for C,H,/CO/Rh(lll) and C6H6/ 2CO/Rh(lll) reveal that (1)CO rests on a threefold hollow site and (2) surface CO exerts an orientational effect on the benzene such that the adsorbed C6H6reorders with the c 6 ring moving from a bridge to a hollow site.32 In addition to this orientational effect, we assume that CO (a strong K acceptor) decreases net electron density on Rh, which is better compensated by three C2H2(ads)than by one C6&(ads). Alternatively put, the same features that cause benzene to be such a stable molecule also ensure that it is unable to act as an effective donor to an electron-deficient metal. In support of this, we note that decyclization is also facilitated by NO, a strong K acceptor, but not NH3, (24)Koel, B.E.;Crowell, J. E.; Mate, C. M.; Somorjai, G. A.J.Phys. Chem. 1984,88, 1988. (25)Reppe, W.;Schweckendiecke, W. J. Liebigs Ann. Chem. 1948, 560, 104. (26) Maittis, P. M. Acc. Chem. Res. 1976, 9,93. (27)Yureva, L. P.Russ. Chem. Rev. 1974, 43, 48. (28)Haaland, D.M. Surf. Sci. 1981, I l l , 555. (29)Lehwald, S.;Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. Tomkinson, J.; Candy, J. P.; Fouilloux, P.; Renouprez, (30)Jobic, H.; A. J. Surf. Sci. 1980, 95,496. (31)Parker, W.L.; Hexter, R. M.; Siedle, A. R. J. Am. Chem. SOC. 1985, 107,4584.
(32)VanHove, M. A.;Lin, R. F.; Somorjai, G. A. J. Am. Chem. SOC. 1986, 108, 2532.
Langmuir, Vol. 4, No. 4, 1988 1003 1650.00
1 150.00
650.00
1 0 0 ~ " " ' " " " ' " " " " " " " " " ~
150.00
f
0:
I
I
1656.00
I
I
I
I
O
I
I
,
1 156.00
,
I
I
I
I
I
t
,,,,,,,,,:
6501.00
15O:OO
Delta Wavenum ber Figure 6. Raman spectra of ethylenes adsorbed on 10% Rh/ a-A1203. Part A is adsorbed '3C2H4,part B is adsorbed CzD4,and part C is adsorbed C2H4.Spectra were measured relative to the 514.5-nm line of an Ar+ laser with incident power between 100 and 200 mW.
which is expected to be a u donor. Thus, substrate-metal binding energies may provide a thermodynamic driving force for otherwise endothermic and improbable reactions. Our GC-MS data, vide supra, indicate that catalytic conversion of C2H2to C6H6occurs on Pd/A1203 in analogy to results obtained with single-crystal Pd. Significantly, the Raman spectra of CzDzon Pd/A1203 (Figure 2) and Rh/A1203 (Figure 1B) are identical, providing further evidence that the same u2,7r adsorbate structure is generated at room temperature on the surfaces of both supported metals. While no subsequent chemistry occurs on either single-crystal or supported rhodium, that on Pd is more elaborate in that CC bond formation occurs. The fact that synthesis of C6H6from C2Hzis catalytic implies that the two hydrocarbons are in equilibrium, and in principle one could, under favorable conditions, generate C2H2from CBHe Several investigators have noted that surface benzene may be substantially distorted32and may under some circumstances actually "decyclize" to form surface a~etylene.~'Our Raman spectra for benzene coadsorbed with CO on 10% Rh/A1203, Figure 5, indicate that just such a transformation is occurring. Because C2H2cannot be readily desorbed from Rh surfaces33the reaction in this case is stoichiometric and not catalytic. Ethylene. Figure 3 shows Raman spectra of C2H4, C2D4, and 13C2H4adsorbed on Rh/A1203. Comparison of Figures 1 and 6 reveals that the Raman spectra of adsorbed (33)Acetylene is not detected among the gaseous products of this reaction, and we have independently observed the tenacity of the acetylene metal bonding.
1004 Langmuir, Vol. 4, No. 4 , 1988
Parker et al.
Table 11. Vibrational Data (crn-l) for Adsorbed Ethylenes and Representative Model Ligands C2H4S gauche l,2-C2H,Br2 Zeiss' salt C2H, on Rh(ll1) C2H4 - . 1074 549.9 351
CzD4 . . 1091 532.4 307.1
13C2H4 .~ 1080 545 304.8
ref 33 2990 1445 1120 1040 625 3080 660 2990 1390 949 711 3080 823 686
ethylene and acetylene are significantly different. For ethylene adsorbed on 10% Rh/A1203,major Raman active bands occur a t 1074,550, and 351 cm-'. These bands do not correspond to the EELS results,34vide supra, or to the strong Raman bands that are observed for C2H2(ads)(cf. Table I). The early surface spectroscopy of olefins adsorbed on (111)faces of Pt group metals36 was believed to support the hypothesis that the relevant surface species was a simple T complex. As the amount of molecular level information concerning these surface species grew, it became apparent that the assumption of a simple T complex was inadequate. Among the first indications of this inadequacy was the near coincidence of the LEED data for ethylene and acetylene separately adsorbed on Rh(ll1) a t room t e m p e r a t ~ r eas , ~well ~ as a similar correspondence in the EELS spectra of these two adsorbates measured under the same condition^.^^ The conclusion is that under these conditions ethylene and acetylene exist as a common surface species. Current evidence seems to favor a surface ethylidyne as the common structure.37 If the surface chemistry of hydrocarbons adsorbed onto a supported metal differs from that on a single crystal surface, a comparison of the surface vibrational information obtained from adsorbed ethylene and acetylene may reflect this difference. Comparison of the Raman spectra of adsorbed ethylene and acetylene (Figures 1and 6) establishes the difference in surface species, even if more detailed analysis is necessary to determine the surface species. Raman bands of normal ethylene appearing at 351 and 550 cm-' shift to lower frequency upon labeling with either deuterium or 13C and are a t least plausibly assigned as metal adsorbate vibration^.^ The remaining normal ethylene Raman band at 1070 cm-' is in the spectral region where CC stretching vibrations between sp2 carbons are expected, but this band does not shift to lower frequency on isotopic substitution of either carbon or hydrogen. Anomalous isotope effeds are frequently observed in bands which result from vibrational modes involved in a strong (34) (a) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J. Chem. Phys. 1980, 72,5234. (b) Koestner, R. J.; VanHove, M. A.; Somorjai, G. A. J. Phys. Chem. 1983,87, 203. (35) Demuth, J. E.; Eastman, D. E. Phys. Reu. Lett. 1974, 32, 1123. (36) Kesmodel, L. L.; DuBois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70,2180. (37) (a) Stuve, E. M.; Madix, R. J. J.Phys. Chem. 1985,89,3183. (b) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985,89, 105.
ref 34 3005 2953 1420 1278 1104 1019 898 550 231 91 3005 1420 1245 1104 836 589 355
ref 17 3019 1623 1342 3108 1236 3106 810 2990 1444 1007 943 949 331 407 310 183 339 210 161 121 92
ref 29 3000 2900 shoulder 1350 1130 880 450
Fermi re~onance.'~Stuve and M a d i ~ have ~ ~previously concluded that such a resonance exists between CH2 bending modes and CC stretching modes for ethylene adsorbed onto Pd(100) a t 80 K. A similar isotope effect occurs in the vibrational spectra of cis-1,2-C2H4Br2( u H / u D = 0.98 for C2H4 on Rh/A1203; u H / u D = 0.93 for C2H4Br2). On this basis the 1070-cm-' band is assigned as primarily a CC stretch. Our assignment would then be consistent with a surface species much like a saturated hydrocarbon. Unfortunately, a saturated CC unit does not uniquely determine the adsorption geometry. Table I1 lists vibrational data for representative model compounds that exhibit different types of saturated hydrocarbon units. All of the model compounds closely match the CC stretching frequency, but none of the candidate molecules has a CH-type vibration at 550 cm-'. The closest matches are the C-Br stretch in gauche 1,2-C2H4Br2and the ring stretch and deformation in C2H4S(625 cm-l). This deformation mode appears to be sensitive to the mass of the heteroatom as the band shifts from 862 cm-' in C2H4038 to 625 cm-' in C2H4S. If we compare this mode in C2H4Sand C2H40,we see that the frequency varies as (Mo/Ms)1/2.This would imply that the analogous ring deformation vibration in C2H4Rhwould occur a t 340 cm-', in reasonably good agreement with the observed band a t 351 cm-'. The C3 cyclic structure apparently does not exhibit a vibration with a frequency near 550 cm-l. This vibration best corresponds to a free C-M stretch, as found in (CH3),Sn12 (usnC = 553 ~ m - and 9 ~ ~ would be plausible in a four-membered cyclic structure. An ethylidyne surface structure may not be readily dismissed. The model complex, CH3CCo3(CO)g, has a CC vibrational mode in this region (1163 cm-'), and this mode also has an 'anomalous" response to deuterium labeling. The ethylidyne species has a M-C mode at 555 cm-', which shifts on deuteriation to 536 cm-' in reasonable agreement with adsorbed CzH4,though the CoCC bending vibration (analogous to a deformation mode in a cyclic molecule) is at 220 cm-'. Either an ethylidynic or a gauche substituted ethane (bimetallocyclobutane) species could account for the observed vibrational spectra, but the data are inconsistent (38) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; NBS National Standard Reference Series. (39) Maslowsky, E., Jr. Vibrational Spectra of Organometallic Compounds; Wiley-Interscience: New York, 1977.
Raman Spectroscopy of Unsaturated Hydrocarbons Table 111. Symmetry Analysis of Candidate Modes of Ligation vibration
symmetry
Langmuir, Vol. 4, No. 4 , 1988 1005 assign the surface structure of adsorbed ethylene as cyclic tetraatomic.
geometry
Conclusions CH2-CHz In each of the instances above, Raman spectra of hyvcc = 1271 cm-’ 81 ring deformations /Rh/ drocarbons adsorbed onto a supported metal under static gas pressure reveal hydrocarbon vibrations that are a2 CZupoint symmetry v5 = 877 cm-’ markedly different from those observed €or the same hyu6 = 890 cm-’ bl drocarbons adsorbed onto atomically clean metal single crystals under UHV conditions. The Raman data for Ethylidynic ethylene, acetylene, and their congeners adsorbed on CHI vCc = 1163 cm-’ a1 supported rhodium or palladium are a t least consistent uMC (symmetric) = 401 cm-l I al vMC (antisymmetric) = 555 e C with surface species in which the central CC bonds remain intact and are oriented parallel to the metal surface. This cm-’ R l l‘Rh conclusion has not been shared by all workers studying Rh chemisorbed hydrocarbons on supported metals. Slichter vMcc(deformation) = 220 e C3” point symmetry cm-’ et a1.43144have used NMR spectroscopy to study the adsorption of C2Hzand C2H4 on Pt crystallites supported on Cyclic Tetraatomicb y-al~mina.A ~ t~ 15 ? ~%~ hydrocarbon coverage, their exvcc = 1019 cm-’ a CHz - C H z periments indicate that room-temperature adsorption of U C B ~= 550 cm-I a I 1 acetylene produces two surface species, a u2,r form (23%) R h * * * . Rh and a vinylidene (77%). We view this result as corrobovCCBr (deformation) = 231 a C2 symmetry ration of the stability of the u 2 , r species on supported cm-‘ metals. We are unable to explain the difference between Model gauche 1,2-C2H4Br2 a Ethylene oxide. the NMR evidence and that derived from our Raman study, namely, the presence of only one surface species, with a r-bonded unit, as in Zeiss’ salt, K[PtC13(C2H4)].H20, but we may speculate that either coverage or differing or a vinylidene unit, another geometry that has been metals may play a role. This speculation is in accordance suggested for ethylene on Rh(ll1) surfacesa3 Again the with recent results of Albert and Yates, who found that 1,2-dimetalloethane appears to slightly better account for for ethylene adsorbed on supported palladium high the observed vibrations and is more like the surface inethylene pressures ( P > 7 Torr) inhibited ethylidyne fortermediate proposed on the basis of homogeneous orm a t i ~ n . The ~ ~ experiments reported in this paper used ganometallic chemical reactions. The lowered bond order significantly higher adsorbate gas pressures (P = 100 Torr). of either geometry would reduce the polarizability of the Analysis of the Raman spectra of normal and isotopically CC bond and would account for the reduced intensity of labeled acetylene permits the assignment of the critically the Raman spectra of the adsorbed ethylene relative to important CC stretching and CH bending vibrations. acetylene. Analysis of the observed group frequencies leads us to In the spectroscopy of free molecules, selection rules may conclude that the structure of the adsorbed species at room frequently be used to determine which of several possible temperature maintains a CC bond, that each carbon molecular geometries is the more likely. Several attempts participates in bonding to metal atoms, and that this bond have been made to analyze the spectroscopic selection rules is parallel to the surface defined by the metal atoms. In applicable for a molecule vibrating on a conducting surface. each instance this room-temperature surface geometry on Electrodynamics mandates that we consider not merely the supported metal a t saturation coverage corresponds the dipole moment of the adsorbed molecule but also its to the low-temperature surface structure observed at lower image in the conducting surface. Hallmark and Campionrn coverage of the adsorbate on a single crystal surface. This have recently used group theory in a qualitative analysis stability of the u2,r-C2H2on supported Rh at 300 K is in of the “molecule-image”system on a general metal surface, striking contrast to the facile rearrangement from u2,raccounting for the imperfect reflectivity of the metal, C2H2 to a surface alkylidyne a t 270 K on bulk, single without introducing the need for a new surface selection crystal Rh(ll1). The persistence of this surface species rule.41 This method produces the same “propensity rules” a t higher temperatures provides a straightforward and as the exact electrodynamic treatment of M o s k o v i t ~ . ~ ~ logical explanation for the observed cis-l,2 addition of The salient feature of the electrodynamic theories of hydrogen or deuterium to the C2 unit of unsaturated hysurface Raman spectroscopy may be stated as follows: drocarbons on supported metal catalysts, for the cyclothose modes which transform as the square of the surface trimerization of C2H2to yield C6H6,and for kinetic data normal are most likely to be enhanced at the surface; that for ethylene hydrogenation on supported palladium.6 is, the intensity should follow the rule 22 > X Z , Y Z >> The Raman spectrum of benzene adsorbed onto aluX2, yz. mina-supported rhodium crystallites indicates that the Table I11 summarizes the transformation properties of preferred surface produces an 7 6 bound hydrocarbon as vibrational modes corresponding to the observed vibrations frequently encountered in organometallic chemistry. The in the likely adsorbate geometries. In each of these point distortion of the adsorbed benzene on coadsorption of the groups the totally symmetric irreducible representation strong r acceptor ligand, such as CO, leads to the decytransforms as 22. Only one of the candidate geometries, clotrimerization and eventual surface acetylene formation. namely Cz, possesses modes that well match the observed spectroscopy and transform properly. On this account we Cyclic Triatomic”
(40) HallmmrV. M.; Campion, A. J . Chem. Phys. 1986, 84,2942. (41) Hexter, R. M.; Albrecht, M. G. Spectrochim. Acta, Part A 1979, 35A, 233. (42) Moskovits, M. J. Chem. Phys. 1982, 77,4408.
(43) Wang,P. K. Slichter, C. P.; Sinfelt, J. H. Phys. Reu. Lett. 1984, 53, 82. (44) Wang, P. K.; Slichter, C. P.; Sinfelt, J. H. J. Phys. Chem. 1985, 89, 3606. (45) Albert, M. R.; Yates, J. T., Jr., unpublished results. (46) Ingold, C. K.; King, G. W. J. Chem. SOC.1953, 2702.
Langmuir 1988,4, 1006-1020
1006
We believe that this is an illustration of a general principle that strong metal-adsorbate binding may provide a thermodynamic driving force for otherwise unanticipated reactions. In the absence of a Raman excitation profile we cannot reliably assert that the Raman spectra presented in this paper are the product of any surface enhancement. The signal levels are quite low, and at least the electrodynamic theories of the enhanced Raman effect predict small enhancements of the Raman scattering cross section for molecules adsorbed on d band metals such as rhodium and palladium. The sole indication of a "surface enhancement" is the sensitivity of the Raman spectrum of acetylene adsorbed onto supported palladium to free hydrogen,18which is known to modify the electrodynamic properties of that metal. Raman spectroscopy is a technique of growing relevance to the surface science community. Few techniques are applicable to such a wide variety of surface morphologies and adsorbate pressures. Even on a metal particle which
is believed to provide only a small enhancement of the surface Raman scattering, we have been able to measure Raman spectra. Significantly, we have seen that the surface species differs from that deduced from vibrational spectroscopy under UHV conditions. As surface science works to ascertain the relative contributions of different thermodynamic parameters in heterogeneous catalytic systems, we anticipate that surface Raman spectroscopy will grow in importance.
Acknowledgment. This work was supported in part by grants from the Science Research Laboratory of the 3M Central Research Laboratories, the Office of Naval Research, and the US. Army Research Office. We wish also to thank Professor J. T. Yates, Jr., for permission to refer to as yet unpublished work. Registry No. CzDz,1070-74-2;"C2Hz, 38950-61-7;CF&H, 661-54-1;CFBCZCF,, 692-50-2; CH&&H3,503-17-3;CzD4,683-73-8; I3CzH4, 6145-18-2;Rh, 7440-16-6; Pd, 7440-05-3;C2H2, 74-86-2; C&, 71-43-2;CzH4, 74-85-1.
A Quantum Chemical Study of the Adsorption of C 0 2 and OH on Cu and ZnO Surfaces and OH on Pt Surfaces Jose A. Rodriguez Chemistry Department, Indiana University, Bloomington, Indiana 47405 Received February 15, 1988. I n Final Form: April 1, 1988 The adsorptions of OH and COz on Cu and ZnO surfaces and OH on Pt surfaces are examined by employing semiempirical quantum chemical calculations (INDO/S and extended Huckel) and clusters of limited size (Cu,, Pt,, n = 14, 16,18; ZnmOm,m = 13). The present results indicate that the OH molecule is a net acceptor of electrons (u-donor, s-acceptor) when adsorbed on Cu, Pt, and ZnO surfaces. The mechanism of bonding of OH to the surfaces involves the 3u and 1s orbitals of OH, with negligible participation of the empty OH 4u orbital. Peak assignments for the ultraviolet photoemission spectra of OH on Pt(ll1) and OH on Cu(ll0) are discussed on the basis of these results. An increase in the work function of Cu, Pt, and ZnO surfaces upon OH adsorption is predicted. Five different forms of COz coordination on a-top sites of Cu(lOO), Cu(llO), Cu(lll), and ZnO(OOO1) are investigated. The results show that C02 is a net donor of electrons (u- and s-donor) when adsorbed in linear forms of coordination and a net acceptor (u-acceptor and n-donor) upon adsorption in bent coordinations. The influence of chemisorption effects upon the C-0 bond orders of CO, is examined. On the basis of these INDO/S results, the possible UPS spectra and induced changes in work function for COPadsorbed on Cu and ZnO surfaces are discussed and compared with experimental results for the C02/Ni(l10)and COZ/Fe(lll)systems. The bond order indices indicate that the 4s and 4p orbitals of the metal (Cu or Zn) are primarily responsible for the adsorption bond of OH and C02 on Cu and ZnO surfaces.
I. Introduction The objective of the present work is to study, from a quantum chemical point of view, the adsorptions of CO, and OH on copper and zinc oxide surfaces and of OH on platinum surfaces. A better understanding of these adsorption reactions is desirable due to the fact that they are present in important catalytic processes. Adsorbed OH is an important intermediate in the hydrogen-oxygen reaction (2H2 + O2 2Hz0) on Pt catalysts1vZand in the C 0 2 + H,) on water-gas shift reaction (CO + HzO -+
-
(1)Norton, P. R. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984;Vol. 4,Chapter 2. (2)(a) Mitchell, G. A,; White, J. M. Chem. Phys. Lett. 1987,135,84. (b) Hellsing, B.; Kasemo, B.; Ljungstrom, S.; Rosen, A.; Wahnstrom, T. Surf. Sci. 1987,189/190,851.
0743-7463/88/2404-1006$01.50/0
Cu/ZnO catalyst^.^ Carbon dioxide appears as a reactant or a product in a series of reactions catalyzed by mixtures of Cu/ZnO: oxidation of CO (2CO + O2 2C02),4 methanol synthesis (COz + 3H2 CH,OH + H20),5
- -
water-gas shift? and methanol steam reforming (CH,OH + HzO C02 + 3H2).6 In addition, it has been known for some time that a small amount of COz acts as a promoter in the production of methanol from synthesis gas (CO/H,) on Cu/ZnO cataly~ts.~ Recently, some authors' -+
(3)Newsome, D. S. Catal. Reu.-Sci. Eng. 1980,21,275. (4)Dwyer, F.G. Catal. Reu. 1972,6, 261. (5)Klier, K. Adu. Catal. 1982,31,243. Takezawa, N.; Minochi, C. J. Catal. 1981,69,487. (6)Kobayashi, H.; (7)(a) Chichen, G. C.; Denny, P. J.; Parker, D. G.; Spencer, M. S.; Whan, D. A. Appl. Catal. 1987,30,333.(b) Chichen, G.C.; Spencer, M. S.; Waugh, K. C.; Whan, D. A. J. Chem. SOC.,Faraday Trans. I 1987,83, 2193.
0 1988 American Chemical Society
