J . Phys. Chem. 1985,89, 105-112 Fluorine substitution in the methylene position strengthens the adjacent C-C bonds and weakens the opposite C=C bond. Fluorine atoms in the vinyl position also strengthen the adjacent C - C and C - C bonds while weakening the opposite C-C bonds. This pattern of the effect of fluorine substitution in cyclopropenes is the same as that found in cyclopropanes and cyclopropenyl cations.20 The qualitative pattern of changes in the C=C stretching frequencies discussed above is partly a force constant (electronic) effect exerted through the F matrix and partly a mass effect exerted through the G matrix. Since the force constants in perfluorocyclopropene have been restored almost to their cyclopropene values, the high frequency of 1940 cm-' in perfluorocyclopropene is almost all a G matrix effect. The naturally higher frequency of CF stretching in comparison to CCl or even C C stretching, as in a methyl-substituted cyclopropene, enhances the coupling of C=C and CF stretching frequencies. For chlorine substitution on cyclopropenes only general observations are possible at the present time. Since the C - C and C-C stretching force constants in perchlorocyclopropene are rather close to those in cyclopropene itself, two interpretations are possible. One is that chlorine substitution has little effect on the bond strengths in the cyclopropene ring. This outcome is what one might expect from considerations of electronic interactions. On the other hand, near cancellation of effects may be occurring as is seen in going from cyclopropene to perfluorocyclopropene. Force constants for some partly substituted chlorocyclopropenes (20) Craig, N. C.; Lai, R. K.-Y.; Penfield, K. W.; Levin, I. W. J . Phys. Chem. 1980,84, 899-906.
105
would be needed to distinguish between these two interpretations. The evidence of the strong influence of fluorine subsitution on the force constants of cyclopropenes adds to that found in similar studies for cyclopropanes and cyclopropenyl cations.20 This influence has also been observed in bond length studies in fluorocyclopropanes:' fluorocyclopropenes,lZ and more recently in fluoroethylene oxides?2 It has been explored in ab initio electronic energy c a l ~ u l a t i o n s . ~However, ~ further studies of the electronic energies in these systems are needed. Acknowledgment. This research was supported by grants from the Petroleum Research Fund, admistered by the American Chemical Society, the Research Corporation, and the National Science Foundation College Research Instrumentation Program (PRM-7911202). We thank Kenneth Felz and Glenn B. Yeffeth for some of the preliminary work with pentafluorocyclopropane and perfluorocyclopropene. Registry No. C3F4, 19721-29-0; C3HF3, 35305-22-7; C3H2F2, 56830-75-2. (21) Perretta, A. T.; Laurie, V. W. J . Chem. Phys. 1975, 62, 2469-73. (22) LaBreque, G.; Gillies, C. W.; Raw, T. T.; Agopovich, J. W. J . Am. Chem. Soc. 1984,106,6171-5. (23) Deakyne, C. A.; Allen, L. C.; Craig, N. C. J . Am. Chem. Soc. 1977, 99, 3895-902. (24) Note Added in Proof: Wiberg et al. (Wiberg, K. B.; Dempsey, R. C.; Wendoloski, J. J., preprint) have used ab initio calculations to guide the fitting of force constants to cyclopropene. Their force constants are 10.29 mdyn A-' for C=C stretching and 3.97 mdyn A-' for C-C stretching, the latter after transformation to internal coordinate space.
Bonding and Dehydrogenation of Ethylene on Palladium Metal. Vibrational Spectra and Temperature-Programmed Reaction Studles on Pd( 100) E. M. Stuvet and R. J. Madix* Department of Chemical Engineering, Stanford University, Stanford, California 94305 (Received: June 1 1 , 1984)
The adsorption and reaction of C2H4on clean Pd( 100) was studied with temperature-programmed reaction spectroscopy (TPRS) and high-resolution electron energy loss spectroscopy (EELS). Both di-a- and *-bonded forms of C2H4are stable on this surface at 80 K. The r-bonded form desorbs in a broad peak from 100 to 300 K, but the di-u-bonded species dehydrogenates into coadsorbed atomic hydrogen and a stable intermediate, inferred to be a vinyl species (CHCH2). Upon heating to between 275 and 300 K, CH groups were produced. The ethylidyne intermediate, previously reported on Pd( 11 l), Pt( loo), Ru( 11l), and Pd(ll1) surfaces, does not form on this surface. Atomic hydrogen recombined and desorbed at its normal temperature of 360 K, while condensation and dehydrogenation of CH groups occurred from 350 to 500 K. This condensation reaction produced a carbon overlayer with unsaturated carbon-carbon bonds that may be a precursor to graphitic carbon. CZH4 dehydrogenation on Pd( 100) is compared with dehydrogenation on other single-crystal metals. The factors that govern the ethylene dehydrogenation pathways on different surfaces are also discussed.
Introduction There has been a tremendous interest in the interactions of unsaturated hydrocarbons on well-defined group 8 metal surfaces in recent years. Progress in understanding these interactions has been made possible by high-resolution electron energy loss spectroscopy (EELS), which provides vibrational spectra for adsorbed intermediates that form during thermal processing of the hydrocarbon-metal systems. The amount of work performed in this area is extensive, and attention shall be focused on ethylene interactions here. Ethylene has been studied with EELS on Pt(111),'-5 Pt(100),6 Pd(111),7-8 Ni(lll),g-ll Ni(100),12 Ni[5( l l l ) X ( l 1 0 ) ] , 9 Rh(111),l3 R U ( O O ~ ) , ' ~Fe(110),16 .'~ W(100),17 Current address: Department of Chemical Engineering, University of Washington, Seattle, WA 98195.
0022-3654/85/2089-0105$01.50/0
Cu( 100),18and Ag(1 10).19 Gentle and MuettertiesB have recently investigated the reactions of alkynes, alkenes, and arenes on the ( 1 ) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (2) Steininger, H.; Ibach, H.; Lehwald, S.; Surf. Sci. 1982, 117, 685. (3) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977, I, 1 . (4) Baro, A. M.; Ibach, H. J . Chem. Phys. 1981, 74, 4194. ( 5 ) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180. (6) Ibach, H. 'Proceedings of the International Conference on Vibrations in Adsorbed Layers, Julich, 1978; p 64. (7) Gates, J. A.; Kesmcdel, L. L. Surf. Sci. 1983, 124, 68. (8) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1982, 120, L461. (9) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (10) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 83, 531. (11) Demuth, J. E.; Ibach, H. Surf. Sci. 1978, 78, L238. (12) Lehwald, S.; Ibach, H.; Steininger, H. Surf. Sci. 1982, 117, 342.
0 1985 American Chemical Society
-
106 The Journal of Physical Chemistry, Vol. 89, No. 1, 1985
(loo), (1 1l), and (1 10) faces of Pd with temperatureprogrammed reaction spectroscopy (TPRS). Ethylene is molecularly adsorbed on all of these metals at low temperature (80-150 K) except for Ru(OOl), on which CH2 may be formed at 80 K,14 and W(lOO), on which dehydrogenation occurs at 135 K. As a rule, ethylene is approximately sp3 rehybridized upon low-temperature adsorption. The exceptions to this are for Pd( 11 l ) , Cu( loo), and Ag( 110) on which a relatively small change from sp2 occurs. Apart from Ag( 110) and Cu( loo), ethylene dehydrogenation occurs at higher temperatures to form ethylidyne (CCH3) on Pt( loo), the (1 11) faces of Pt, Pd, and Rh, and possibly also on Ru(001),15and acetylene on the other surfaces. Both intermediates undergo further dehydrogenation and decomposition as the temperature is raised, usually forming CH, species and eventually producing a carbon adlayer. This work deals with the adsorption and thermal evolution of ethylene on clean Pd( 100) and is the first in a series of three papers about ethylene adsorption. Dehydrogenation pathways for C2H4 on well-defined metal surfaces under ultrahigh vacuum (UHV) are also considered in this paper. The second paper2] is concerned with how preadsorbed oxygen modifies the reactivity of C2H4 on Pd(100). Both of these papers use a new parameter, called the *a parameter, to characterize the adsorption of C2H4. The third paperZ2is a short note that specifically discusses the T U parameter as a useful guide for understanding ethylene adsorption.
295-
Stuve and Madix I
I
I
I
Products /C2H4 ( I O 0 K )
;
Pd (100)
C2H4 ( m l c :27)
XI0
Experimental Section
The experiments were performed in a LEED-Auger-EELS UHV chamber (base pressure = 5 X lo-" torr) that has recently been described.23 Matheson C P Grade C2H4was purified with a liquid-nitrogen trap to remove traces of acetylene before being admitted to the vacuum chamber through a glass capillary array molecular beam doser. Matheson Research Grade C2D4 (minimum 99% D) was used without further purification. The effective pressure in the ethylene beam was at least 500 times that of the torr during the dose, background, which was held to 3 X and was estimated as follows. The Pd(100) surface was found to saturate with about 0.4 monolayer (ML) of C2H4 after an exposure of 2 X low9torr s as determined from the background pressure during the dose. If the sticking coefficient were unity, torr s, or then the actual exposure must have been 1.4 X slightly more than 500 times the background exposure. Note that a smaller sticking coefficient would give an even higher enhancement factor. The beam dosing procedure minimized contamination of tha sample from ethylene cracking fragments emitted by the ion pump and ionization gauge. The mass spectrometer signal for H2 desorption was calibrated against the coverage of atomic hydrogen on the surface. This was done by measuring the H2temperature-programmed desorption (TPD) as a function of exposure. The recombination and desorption of hydrogen occurred in a single TPD peak at approximately 390 K that shifted to 360 K with increasing coverage, in good agreement with the work of Behm et al.,24who performed a more complete study of H2on Pd(100). The TPD curves were integrated and scaled according to two points: (1) a ~ ( 2 x 2 LEED ) pattern is obtained for an atomic hydrogen coverage of 0.5 ML,
100
300
500
700
Temperature ( K ) Figure 1. Temperature-programmed desorption spectra showing the desorption yield (Y)for ethylene and hydrogen following a 3-langmuir ethylene exposure to Pd(100) at 100 K. The heating rate was 15 K s-].
and (2) the saturation coverage of atomic hydrogen is 1.35 ML.24 The two calibration points were found to be self-consistent, and the hydrogen coverage vs. exposure was in good agreement with the earlier The sample was cleaned by one of two methods. The first employed repeated doses of oxygen at 300 K followed by heating to 1100 K to remove carbon.23 These oxygen adsorption/desorption cycles were repeated until a reproducible oxygen desorption curve was obtained. The oxygen cleaning procedure had to be repeated after every ethylene dose, since carbon was deposited on the surface as a result of dehydrogenation. It was discovered that oxygen cleaning caused the elastic peak in the EELS spectrum to broaden about 25 cm-'; to avoid this degradation of the vibrational spectra, the surface was cleaned by Ar ion sputtering for several hours at 800 eV and 300 K and annealed to 1200 K for 60 s before taking EELS data. Both cleaning procedures produced surfaces free of carbon according to the CO desorption test described by Behm et al.,25and experiments showed that ethylene desorption and dehydrogenation did not depend on whether the surface was cleaned with oxygen treatments or Ar ion sputtering.
(13) Dubois, L. H.; Castner, D. G.; Somorjai, G.A. J . Chem. Phys. 1980, 72, 5234. (14) George, P. M.; Avery, N. R.; Weinberg, W. H.; Tebbe, F. N. J . Am. Chem. Soc. 1982, 105, 1393. (15) Barteau, M. A.; Broughton, J. Q.; Menzel, D., to be submitted for publication. (16) Erley, W.; Baro, A. M.; Ibach, H. Surf.Sci. 1982, 120, 273. (17) Hamilton, J. C.; Swanson, N.; Waclawski, B. J.; Celotta, R. J. J. Chem. Phys. 1981, 74, 4156. (18) Nyberg, C.; Tengstal, C. G.; Andersson, S.; Holmes, M. W. Chem. Phys. Lett. 1982, 87, 87. (19) Back, C . ;de Groot, C. P. M.; Biloen, P. App. Sur$ Sci. 1980,6,256. (20) Gentle, T. M.; Muetterties, E. L. J . Phys. Chem. 1983, 87, 2469. (21) Stuve, E. M.; Madix, R. J., to be submitted for publication. (22) Stuve, E. M.; Madix, R. J., to be submitted for publication. (23) Stuve, E. M.; Madix, R. J.; Brundle, C. R., Surf. Sci., in press. (24) Behm. R. J.; Christmann, K.; Ertl, G. Surf.Sci. 1980, 99, 320.
The energy loss experiments were performed only in the specular direction with a beam energy of approximately 1 eV. The resolution varied between 70 and 110 cm-' (9-12 meV). All spectra were recorded with the sample at 80 K following an anneal to the temperatures indicated in the figures. Results
Ethylene Adsorption and Dehydrogenation. Thermal Desorption. Ethylene and hydrogen thermal desorption spectra following an ethylene exposure of 3 langmuirs (1 langmuir = 3.8 X I O l 4 molecules of C2H4cm-*) with the Pd(100) surface at 100 (25) Behm, R. J.; Christmann, K.; Ertl, G. J . Chem. Phys. 1980, 73,2984.
Bonding and Dehydrogenation of Ethylene on Pd
The Journal of Physical Chemistry, Vol. 89, No. I , 1985 107
Eo = I eV
13000r0 Eo = I eV
I;__
500 K
1667
1
13333
\
16000
r430
)-
450
K
300
K
275
K
- 00
K
2935 7 ~ 3 0 9 0
450K
r505
1667
11000
0 N 0
350 H
I
i
,
0
1
1
000
1
1
1
1
1
1
1
1
0
1600 2400 3200 400(
000
1600 2400 3200 4000
E n e r g y Loss (cm-')
Energy Loss (cm-ll
Figure 3. Electron energy loss spectra as a function of annealing temperature for the Pd(100) surface exposed to 3 langmuirs of C2D4at 80 K. Spectra were recorded at 80 K. TABLE I: Comparison of Vibrational Frequencies for C2H4, C,H,Br,. and Zeise's Salt uH/vD
adsorbed
1.32
2980
6(CH2)d Y(CC)~ CH2 def
1.58 0.93 1.39
1455 1135 920
v(PdC)
1.01
390
mode u(CH2)
frequency, cm-I C2H4Br20 Zeise's saltb 3005 2953 1420 1019 1278 1104 898
3079 3013 1515 1243 975
K are shown in Figure 1. H2 and C2H4 were the only desorbing species; in particular, no benzene, ethane, acetylene, or methane ( m / e 78, 30,25, and 16, respectively) was detected. These results are in good agreement with those of Gentle and Muetterties20 for this same system with the exceptions noted below. Ethylene desorption occurred in two peaks at 235 and 295 K. The desorption yield (Y) of ethylene was determined to be about 0.25 ML. For this estimate, it was assumed that the mass spectrometer sensitivity to the C2H4 cracking fragments of 24-29 amu was constant and equal to that of O2 for which a calibration was known.23 The assignment of the 235 K peak to a multilayer state by Gentle and Muetterties is not consistent with the fact that it attained a saturation coverage. The 235 K peak therefore rep-
1342 1623 1023
841
OGauche form, A modes only.26 bK[(C2H4)PtC13].27 only. dStrongly coupled modes; see text.
Figure 2. Electron energy loss spectra as a function of annealing temperature for the Pd(100) surface exposed to 3 langmuirs of C2H4 at 80 K. Spectra were recorded at 80 K. Panel a: 80-300 K. Panel b: 350-500 K.
gasc 3026
modes
resents a state of C2H4 that is more weakly adsorbed than the major portion of C2H4, which desorbed at 295 K. The small oscillations in CzH4intensity above 350 K were due to desorption of C2H4 from the liquid-nitrogen cooling lines. Hydrogen TPD peaks were clearly observed at 360 and 480 K. The total amount of hydrogen that desorbed was equivalent to an atomic hydrogen coverage (0,) of 0.47 M L according to the H2 calibration. Therefore, the coverage of ethylene that underwent complete dehydrogenation (0,,,,) was 0.12 ML, and the total coverage of ethylene at 100 K was approximately 0.37 ML. Hydrogen desorbs from the otherwise clean Pd( 100) surface at 360 K, so the low-temperature H2peak in Figure 1 is desorption limited. The reaction-limited contribution (labeled y in Figure 1) to H2 evolution following C2H4 adsorption was found by subtracting out the desorption-limited portion. This was done by constructing a desorption curve (labeled 0) for H2that was equal in amplitude at 3 6 0 K to the H2/C2H4desorption curve in Figure 1; the width of the curve was scaled according to the amplitude. From this deconvolution, a third H2/C2H4peak at 400 K was evident. The hydrogen yields in the /3 and y curves were found to be equal with this deconvolution procedure. Electron Energy Loss. The electron energy loss spectra recorded as a function of annealing temperature, following saturation coverage of ethylene at 80 K, are shown in Figure 2 (a and b) for C2H4 and Figure 3 for C2D4. The energy loss assignments for ethylene adsorption at 80 K are listed in Table I for C2H4 and
Stuve and Madix
108 The Journal of Physical Chemistry, Vol. 89, No. 1, 1985 TABLE II: Comparison of Vibrational Freuqencies for CZD4, C2D4BrBand Zeise's Salt
freauencv. cm-' mode
adsorbed
4CH2)
2215
2174
6(CDJd w(CC)d CD2 def
920 1220 660
1141 947 1013 79 1 71 1
v(PdC)
384
C2D4Br2n Zeise's saltb 2349 2224 962 1353 757
gasC 2251 98 1 1515 728
597
"Gauche form, A modes only.28 bK[(C2D,)PtC13].27 'A, modes only. dStrongly coupled modes; see text. Table I1 for C2D4. Because of the large degree of vibrational coupling between the C-C stretching (u(CC)) and CHI scissoring (6(CH2)) motions, the assignment of C2H4 vibrations must be made with care. The 1135- and 1455-cm-I peaks represent coupled u(CC)and 6(CH2) modes; a more complete discussion will be given below. The corresponding peaks for C2D4 were found at 920 and 1220 cm-I. The remaining assignments for C2H4 (C2D4) on Pd( 100) at 80 K are relatively straightforward: u(CH2) = 2980 (2215) cm-l; C H 2 deformations = 920 (660) cm-'; and u(PdC) = 390 (385) cm-I. There was little change in the vibrational spectrum upon heating to 200 K, but annealing to 250 K resulted in some dehydrogenation of the adsorbed C2H4, as evidenced by the appearance of a peak at 530 cm-I due to the u(PdH) stretching motion of adsorbed atomic hydrogen.29 The remainder of the 250 K spectrum is similar to the lower temperature spectra; the relative intensity of the 1135-cm-' peak is reduced with respect to the 1455-cm-l peak. Annealing to 275 K resulted in increased intensity of the 530-cm-I peak and a substantial decrease in the intensity of the 1135-cm-' peak. This strongly reduced intensity is evidence that a new species is formed on the surface at 275 K; the identity of this species will be discussed below. Further heating to 300 K resulted in the disappearance of both the 1135- and 1445-cm-I peaks and the development of a narrower peak at 935 cm-I. Hydrogen desorbed from the surface as it was annealed to 350 K (Figures 1 and 2b), and the 525-cm-l peak due to u(PdH) was absent in the 350 K spectrum. At this point it is worthwhile to discuss the phase formed by heating to 300-350 K. Figure 1 shows that, of the hydrogen atoms originally present in the C2H4 that dehydrogenated, half desorbed in the /3 H2/C2H4TPRS state and would therefore desorb upon heating to 350 K. The C / H ratio on the surface at this temperature must therefore be 1:l. Possible intermediates with this stoichiometry are CH, C2H2,and CCH2. Dehydrogenation to acetylene (C2H2)can be ruled out as Kesmode130 has shown that C2H2adsorbed on Pd( 100) at 300 K produces a strong peak at 1210 cm-I in the energy loss spectrum. Vinylidene (CCH2)can also be ruled out since such an intermediate should produce a peak near 1400 cm-' due to the C H 2 scissor mode as can, in fact, CHCH2 or species containing methyl groups. Therefore, even without using the C / H stoichiometry inferred from the TPRS results, CCH3(ethylidyne) and CH2 can be ruled out on the basis of the EELS result^.'.^*^* Furthermore, CHI a u l d not be the sole intermediate as hydrogen transfer to the surface has occurred. In addition, u(CH) is too low for the sp-hybridized metal alkyne, -CCH, and a bridging species such as
/" p"="/"
Pd
or
/"
c=c .I \
Pd
would be expected to show a C-C stretch of at least moderate intensity3 similar to that observed for adsorbed acetylene.30 The 350 K spectrum must, therefore, represent a methylidyne (CH) species: u(CH) = 2920 cm-', G(PdCH) = 925 cm-', and v(PdC) = 470 cm-'. The corresponding peaks for CD groups were found at 2250 and 7 15 cm-' and a shoulder was found near 420 cm-I. Identification of the C H species is further supported by comparison of the vibrational spectra on Pd(100) with C H groups on other single crystals and two organometallic complexes (Table 111). The spectrum of C H on Pd( 100) is in good agreement with all of the other compounds listed. The u(PdC) frequency of 470 cm-' is considerably higher than observed on Pd( 111) and Fe( 110) and indicates that methylidyne on Pd(100). Of the may not be adsorbed in the fourfold other possible locations for CH, the C-H stretching frequency of 2920 cm-I indicates that the C H groups are most probably adsorbed at twofold bridging sites. C H adsorption directly on top of a Pd atom would imply that the carbon was sp hybridized for which a C H stretching frequency near 3200 cm-I is expected. A CH group bound to two Fe atoms in the organometallic complex ($-C5H5)2(CO)2Fe2(w-CH)(p-CO) has been recently reported,34 but its vibrational spectrum was not characterized. With the identification of C H groups, the overall reactions that produce H2 desorption are as shown in Scheme I Thus, the y
Scheme I
y states
C2H4 -+ 2H
+ 2CH
(1)
2CH
+ Hqg)
(3)
-+
2C
H2/C2H4TPRS states are assigned to the dehydrogenation of C H groups, but this is not a simple reaction as can be seen by the two peaks in the y H2 desorption curve and the energy loss spectra. The spectra taken for annealing temperatures above 350 K indicate further reaction of the C H groups. A new peak appeared at 3060 cm-' at 400 K, and the 925-cm-' peak due to the C H bend was absent. This annealing temperature was well into the y H2 desorption peak, so that some of the CH groups had dehydrogenated. The 925-cm-I peak was not seen at 400 K most probably because of the low intensity of this mode and the reduced coverage of C H species. The shift of u(CH) to a higher frequency and the loss of the peak at 920 cm-' indicate that some unsaturated carbonqrbon bonds were formed by the condensation of the C H groups, which may be a precurser to the formation of graphitic carbon at higher temperatures. Annealing to 450 K resulted in two broad bands at low and high frequency that exhibited maxima at 430, 520, and 800 cm-', and 2935 and 3090 cm-', respectively. Most of the dehydrogenation had occurred below this temperature as evidenced by the low intensity of CH stretching modes. The low-frequency peaks may be due to skeletal modes in the carbon overlayer such as ring deformations if the compound is cyclic, or C-C bending modes if it is linear. (This carbon overlayer will hereafter be referred to as the condensed carbon overlayer.) Finally, only one loss at 410 cm-', assigned to the carbon overlayer, was observed upon heating to 500 K. Reaction of Ethylene and Methylidyne with Hydrogen. No reactions were observed between coadsorbed ethylene and hydrogen on Pd( 100) by either EELS or TPRS, in contrast to the study by Gentle and Muetterties,20who reported the hydrogenation of C2H4to C2H6on this surface. For these experiments ethylene coverages were varied from approximately 0.05 ML to saturation (0.4 ML at 100 K); hydrogen coverages were varied from 0.2 ML
Pd
(26) Shimanouchi, T. "Tables of Molecular Vibrational Frequencies, Consolidated"; Natl. Stand. Ref Data Ser. ( U S . ffatl. Bur. Stand.) 1950, Vol. 1, NSRDS-NBS 39. (27) Hiraishi, J. Spectrochim. Acta, Part A 1969, 25A, 149. (28) Neu, J. T.; Gwinn, W. D. J. Chem. Phys. 1950, 18, 1642. (29) Nyberg, C.; Tengstal, C. G. Surf Sci. 1983, 126, 163. (30) Kesmodel, L. L., submitted for publication in J. Chem. Phys.
(31) Howard, M. W.; Kettle, S. F. A,; Oxton, I. A.; Powell, D. B.; Sheppard, N.; Skinner, P. J. Chem. SOC.,Faraday Trans. 2, 1981, 77, 397. (32) Oxton, I. A. Rev. Znorg. Chem. 1982, 4 , 1. (33) Ibach, H.; Mills, D. L. "Electron Energy Loss Spectroscopy and Surface Vibrations"; Academic Press: New York, 1982. (34) Casey, C. P.; Fagan, P. J.; Miles, W. H. J. Am. Chem. SOC.1982, 104, 1134.
The Journal of Physical Chemistry, Vol. 89, No. 1 , 1985 109
Bonding and Dehydrogenation of Ethylene on Pd
TABLE 111: Comparison of the Vibrational Frequencies of CH Groups frequency, cm-l H
H
I
assignment
Pd(100)
Pd(l1 l)a
P t ( l 1l ) b
2940
3002
3100
925 470
762 307
850
u(CH)~ 6 (CH)g
4MC)
Reference 7. Bend.
Reference 4.
Reference 16.
3600L
U
a
L
-
J
1
0
,
I
I
000 Energy
I
I
I
I
J
I
I
1600 2400 3200 4000 LOSS
Fe(1 2960 3050 850 -350
(p,-CH)[Co(CO),],. Reference 31.
0 > e
(cm-')
Figure 4. Electron energy loss spectra depicting H/D exchange in methylidyne groups on Pd(100). Lower curve: saturation exposure of C2D4 at 80 K followed by annealing to 350 K to form CD groups. Upper curve: spectrum recorded after subsequent exposure to 2 X lo4 torr of H2for 30 min at 300 K. The peaks at 2235 and 2940 cm-I indicate the simultaneous presence of CD and CH groups. Spectra recorded at 80 K.
to saturation (1.35 ML]. Both orders of adsorption of ethylene and hydrogen were examined. Exchange of hydrogen between C D groups and H was found to occur slowly. Deuterated methylidyne groups were formed by adsorption of approximately 0.05 ML of C2D4at 80 K followed by annealing to 350 K, the EELS spectrum is shown in the lower curve of Figure 4. The 6(CD) bending mode was not detected because of the low methylidyne coverage, and the small peak near 500 cm-I is due to coadsorbed H. This sample was then exposed to 2 X lo4 torr of H2for 30 min at 300 K. After the hydrogen exposure, the sample was allowed to cool to 80 K before recording the energy loss spectrum (upper curve). The presence of both C D and CH stretching frequencies (2235 and 2940 cm-I, respectively) shows that hydrogen exchanges with deuterium within the methylidyne groups. Other reaction conditions were tried that did not result in isotope exchange. These were (1) C D formation as above followed by H2 exposure a t 200 K, (2) formation of a condensed carbon overlayer (by annealing the C2D4 exposed surface to 500 K) followed by H2 exposure a t 200 K, and (3) as in experiment 2, except for H2 exposure a t 300 K. Isotope exchange at 300 K (Figure 4) and not at 200 K (experiment 1) indicates that the C H bond in methylidyne is not labile at the lower temperature. Experiments 2 and 3 were performed to determine whether atomic carbon or methylene (CH,) was the transient intermediate in the exchange reaction. Experiment 3 rules out isotope exchange with the condensed carbon overlayer, but the reactivity of condensed
I
e
3041
2994
85 0 417
895 426
(p3-CH)[Ru(CO),],. Reference 32.
Stretch.
carbon may differ from that of atomic carbon. (Recall that there is a difference between adsorbed atomic carbon and the condensed carbon overlayer formed by annealing to 500 K.) It was not possible to isolate atomic carbon for reaction with H2 in these experiments since the EELS results showed that a condensed carbon overlayer was formed after CH dehydrogenation. Thus, the reactivity of atomic carbon with hydrogen is not known. The stability of C H in the temperature range 300-350 K strongly suggests, however, that the exchange occurs via the transient formation of CH,,,, and not by dehydrogenation of CH,,, to C,,, followed by rehydroganation.
Discussion Ethylene Adsorption at 80 K . The assignment of the u(CC) and 6(CH2) modes for adsorbed ethylene must be made with care. This point is discussed at length in the third paper of this series.22 The assignments of the energy losses at 80 K are given in Table I for C2H4 and Table I1 for CzD4. Table I shows that the 1135-cm-I peak is assigned to u(CC) and the 1455-cm-l peak is assigned to 6(CH2). In point of fact, these are strongly coupled modes and therefore the 1135-cm-' peak also represents the 6(CH,) mode, while the 1455-cm-" peak has some contribution from the u(CC) mode. Since these modes are strongly coupled, it is not possible to infer from the v(CC) frequency of 1135 cm-I that C2H4 adsorbed on Pd(100) is di-a-bonded. The vibrational spectrum of adsorbed ethylene is compared with the spectra of CZH4Br2(a model for di-a-bonded C2H4), Zeise's salt (a model for *-bonded C2H4), and gaseous C2H4 in Tables I and 11. The vibrational spectrum of adsorbed C2H4 is in slightly better agreement with C2H4Br2than with Zeise's salt as seen from the u(CH2) and 6(CH2) modes (Table I). Thus, it would appear that to the first approximation C2H4 is indeed strongly rehybridized on Pd( 100). A better measure of the hybridized state of adsorbed C2H4 is the T U parameter22defined by ~ a ( C 2 H 4 )=
(
16231;i;d
I + 1342 1342 - band I1 )/0.366 (4)
1515 -band I T U ( C ~ D=~ ) 1515
(
+
981 -band I1 )/0.366 98 1
(5)
where band I refers to the higher frequency and band I1 to the lower frequency of the u(CC)-6(CH2) pair. The ~a parameter is zero for gaseous C2H4, 0.38 for Zeise's salt, and unity for C2H4Br2.For C2H4 (C2D4) on Pd(100), then, the TU parameter is 0.78 (0.70). The extent of rehybridization from sp2 to sp3 lies roughly midway between Zeise's salt and C2H4BrZ. Ethylene desorption and dehydrogenation were observed at temperatures greater than 80 K following adsorption at 80 K. It is of interest, then, to understand why some of the ethylene reacts, while the rest desorbs. This question can be answered by examination of the EELS and TPRS measurements for C2H4on oxygen-covered Pd(100) (see Figure 5). (A more complete discussion of the interactions between 0 and C2H4 is given in the second paper of this series.21) Figure 5a shows the energy loss spectrum for C2H4 with 0.18 ML of atomic oxygen. This spectrum is typical of *-bonded C2H4, a result that can also be determined
Stuve and Madix
110 The Journal of Physical Chemistry, Vol. 89, No. 1 , 1985
0
+ C2H4 I Pd (‘IO01
b
m/e = 28
I
I
,
I
I
3 00
100
1
I
7 00
500 Temperature (Kl
a
occurred at this temperature. This rules out CH2 and CHCH, as the sole intermediates, since no dehydrogenation would have occurred in their formation. The energy loss fingerprint of CCH, is composed of two peaks of equal intensity at 1130 and 1350 cm-1,2 so CCH, can be ruled out on the basis of the 275 K spectrum alone. Kesmode130has studied acetylene (C2H2)on Pd(100) at 300 K with EELS and found two peaks, again approximately equal in intensity, at 935 and 1210 cm-’ to be characteristic of molecularly adsorbed C2H2. There was no loss near 1200 cm-’ at 275 K, so acetylene can also be ruled out as a dehydrogenation intermediate. Thus, only two possibilities remain: vinyl (CHCH2) and vinylidene (C=CH2). It is not possible to distinguish between these two species on the basis of the EELS data alone. If the deconvolution of the hydrogen desorption spectrum into the /3 and y states is accurate (based on H2 desorption from clean Pd(100)), the initial dehydrogenation step creates an intermediate with a C / H ratio of 1 with a loss of half the hydrogen initially present. Several schemes are possible which lead to the CH intermediate at 300 K involving CHCH2 or CCH2. Scheme I1
HCCH2 CH, Scheme
--
+ CH2 CH + H
(7)
+H
(8)
CH
(6)
I11 HCCHZ
+
HCCH
-
HCCH 2CH
(9)
Scheme IV 0
800
1600
Energy Loss
2LOO
3200
CCH2
LOO0
CH2
(ern-')
Figure 5. Electron energy loss (a) and temperature-programmed reaction (b) spectra for coadsorbed C2H4and atomic oxygen. Oxygen was adsorbed at 300 K to 0.18 ML in part a and to 0.25 ML in part b. Following oxygen adsorption, the sample was cooled to 80 K and exposed to 3 langmuirs of CzH4. The heating rate in b was 15 K s-l.
+
+
+ CH2 H + CH C
H+C-CH Scheme V
CCH2
-
CCH
CCH - C by the A U parameter of 0.30 for this system.35 Figure 5b shows the temperature-programmed reaction spectrum for C2H4 with 0.25 ML of atomic oxygen. Note that the overall shape of the C2H4 desorption peak in Figure 5b is similar to that of C2H4 on clean Pd(100) (Figure l ) , indicating similar binding energies of C2H4 on both the clean and oxygen-covered surfaces. The amount of C2H4 desorbing from the surface following saturation exposure was nearly the same both with and without coadsorbed oxygen. Despite these similarities in C2H4 desorption, almost no dehydrogenation occurred on the oxygen-covered surface; the yield of H2 was 0.01 M L from C2H4 in the presence of 0.25 ML of atomic oxygen vs. 0.47 ML H2 from C2H4 on the clean surface. These observations indicate that there are two forms of adsorbed C2H4 on clean Pd(100): x-bonded and di-a-bonded C2H4. The Abonded form desorbs between 100 and 300 K, whereas the di-abonded form undergoes dehydrogenation. Apparently the x bonded C2H4 was not resolved in the 80 K EELS spectrum for C2H4 on clean Pd(100) (Figure 2a) because the spectral features of ?r-bonded C2H4 overlap with those of di-a-bonded C2H4 and are less intense. First Stable Intermediate in C2H4 Dehydrogenation. In this section, we address the initial dehydrogenation step. The sharp drop in intensity of the 1135-cm-I peak at 275 K in Figure 2a indicates that a new species was formed upon annealing to this temperature. Among the possible intermediates (see Scheme 11) are methylene (CH,), vinyl (CHCH2), ethylidene (CHCH,), vinylidene (C=CH2), acetylene (C2H2),and ethylidyne (CCH,). The presence of atomic hydrogen at 275 K, and even at 250 K, indicates that appreciable dehydrogenation of the parent C2H4 (35) This value was determined for C2D4for which both band I and band I1 were observed; see ref 21.
(11) (12)
+H
+ CH
H+C-CH
(10)
(13) (14) (15)
Of these, only Scheme I1 involves neither acetylene nor atomic carbon as an intermediate. The EELS data rule out acetylene, and experiments discussed above strongly mitigate against the facile reaction of surface hydrogen and carbon. Thus, the vinyl intermediate is preferred. The EELS spectrum at 275 K in Figure 2a is assigned as follows: 2940 cm-I, v(CH2) and v(CH); 1445 cm-l, 6(CH2);915 cm-I, CH2 wag and G(CCH); and 530 cm-’, v(PdH). The v(CH) frequency of 2940 cm-I is consistent with a strongly interacting vinyl species (approximate sp3 hybridization); a hybridization state closer to sp2 would be expected to show a CH stretching frequency closer to 3000 cm-’. The vinyl species can be envisioned as bridging two metal atoms with each carbon bound to the surface. An experimental measurement of the C-C bond length and orientation would test this suggestion. The small feature near 1135 cm-I may be a remnant of the di-a-bonded C2H4,or it could be due to the CHCH2 intermediate. If the latter case were true, then this could be assigned to the u(CC) mode. It is interesting to note that a qualitatively similar spectrum was obtained for coadsorbed C2H4 and 0 on Pt( 111) after annealing to 325 K.2 Losses were found at 3080, 2980, 1420, 940, 480, and 300 cm-I, but the spectrum was not assigned. The 3080- and 2980-cm-’ peaks indicate that the C-H stretches involved a carbon atom of approximately sp2 hybridization and would be consistent either with a vinyl species without di-a-bonding to the surface or with vinylidene itself. Ethylene Dehydrogenation on Pd(100). The results show that dehydrogenation is the only reaction observed for C2H4on Pd(100). Ethylene and hydrogen were the only desorbing species subsequent to ethylene adsorption, and no evidence for hydro-
Bonding and Dehydrogenation of Ethylene on Pd
Scheme VI
-
80 K:
C2H4(g)
250-300 K:
di-a-C2H4
250-300 K:
HCCH2
250-300 K:
CH2-
-
TABLE I V Adsorption Energy of H2 (EadH2) and the First Stable C2H4 Reaction Intermediate on Single Crystals
di-u-C2H4
HCCH2
+H
+ CH2 CH + H
+
The Journal of Physical Chemistry, Vol. 89, No. I, 1985 111
CH
(16)
(19) (20) (21)
Ed"2, surface
kcal/mol
ref
W(100) Fe(1 l d , Pd(100) Ni(100) Ni(ll1) Pd( 111) Pt(ll1) Rh(ll1) Pt(100) Ru(O0 1)
32.5 26 24.5 23 23 20.8 19.5 18.6 15 14.5
39 40 24 41 41 42 43 44 45 46
intermediate
CHCH, C2H2 CCH3 CCH3 CCHj CCH3 CH2, CCH3"
"Two different reaction products have been p r o p o ~ e d . ' ~ * ' ~
360-525 K: complex reaction to yield a surface residue and Hz(g)
TABLE V TU Parameter and CH Stretching Frequencies for C2H4 at 80 K and the First Stable C,HI Dehvdroeenation Intermediate ~
surface
ref
v(CH), cm-'
Pt(ll1) Ru(001)
2 15
Ni(100)
12
Ni(ll1)
9
Pd( 100) Fe(ll0) Pd(ll1)
16 8
3000 3110 2910 2980 2780d 2950 2740d 2980 2960 2996 2780d
TU
parameter"
intermediate
0.92 0.85b
CCH3 CCH3, CHIc
0.83
C2H4
0.80
C2H2
0.78 0.55 0.43b
HCCH2 C2H2 CCH3
'See ref 22 for calculation of TU. bEnergy losses reassigned.22 c T w different ~ reaction products have been proposed.14,'5 dSoftened C H modes.
\
l H
/ C
X4-44Pl1111l.P111001 Pdlllll.Rhllll1 IRul00111
genated intermediates was obtained. Hydrogenation of ethylene to ethane was not found in this study, in contrast to the results reported by Gentle and Muetterties.*O Benzene formation from ethylene was also not detected, in agreement with Gentle and Muetterties, although acetylene trimerization to benzene has been found on both Pd( 100) and Pd( 111).20v36,37Scheme VI, then, details the reaction steps observed in this work for C2H4 on Pd(100); all species are adsorbed unless otherwise indicated. Ethylene adsorption into the di-a- and 7-bonded states occurs in steps 16 and 17, and ethylene desorption in step 18. The vinyl species are formed in step 19 and decompose to methylidyne in steps 20 and 21; the hydrogen formed from steps 19 and 21 desorbs in the ,f3 state in step 22. Methylidyne condensation/dehydrogenation occurs in step 23 with the hydrogen desorbing in the y state. A more precise definition of the C H condensation/dehydrogenation mechanism cannot be made. In a recent study of acetylene adsorption on Pd(100), Kesmode13' proposed that CCH species were formed by heating acetylene, adsorbed at 300 K,to 450 K. The EELS spectrum thus obtained showed peaks at 750 (strong), 1340 (weak), and 3000 (medium) cm-'. An acetylide (CCH) species was also found on Ag( 110) for which energy losses at 690 and 3250 cm-' were found.38 Though the EELS in Figure 2b shows (36) Sesselmann, W.; Woratschek, B.; Ertl, G.; Kuppers, J.; Haberland,
similar losses at 450 K, we believe that a more complex reaction to form condensed rings occurs. Dehydrogenation Pathwaysfor C2H4; General Considerations. The investigation of possible dehydrogenation pathways for C2H4 requires the consideration of three effects: the strength of the atomic hydrogen-to-metal (H-M) interaction, the strength of the carbon-to-metal (C-M) interaction (both in the adsorbed ethylene and the stable dehydrogenation intermediate), and the geometry of the surface. The H-M interaction is important at the first stage of reaction and, in the case of ethylidyne formation, also the last. On two hypothetical surfaces with the same C-M interaction and geometry, the surface with the stronger H-M interaction will promote reaction to I11 (Scheme VII), while I1 will be favored for a weaker H-M interaction, assuming a C-M interaction sufficient for C-C bond breakage. The bond energy of adsorbed atomic hydrogen EH-M, and consequently the heat of adsorption for hydrogen EadH2,is given by EH-M
= Y2(EadHz + 104 kcal/mol)
If the heat of adsorption for hydrogen is too weak, then reaction to form I1 should be favored, if reaction occurs at all. The heats of adsorption for the surfaces on which ethylene dehydrogenation has been studied are listed in Table IV. It can be seen that on ethylene reaction t o Ru(001), the surface with the lowest EadH2, CH2 is plausible; those surfaces with larger heats of adsorption maintain the C-C bond. Another measure of the H-M interaction for the reactant ethylene can be obtained from the v(CH) frequencies for adsorbed ethylene (Table V). Softening of the v(CH) frequencies below 2800 cm-' indicates a direct interaction between the metal and a hydrogen atom of adsorbed ethylene. Ni( loo), Ni(l1 l), and Pd( 111) all show C H softening and all three of these
H.Surf.Sci. 1983, 130, 245.
(37) Kesmodel, L. L., submitted for publication in J. Vac. Sci. Technol. (38) Stuve, E. M.;Madix, R. J.; Sexton, B. A. Surf.Sci. 1982, 123,491. (39) Schmidt, L. D. Catal. Reu.-Sci. Eng. 1974, 9, 115. (40) Bozso, R.; Ertl, G.; Grunze, J.; Weis, M. Appf. Surf.Sci. 1977, 1 ,
103
(41) Christmann, K.; Schober, 0.; Ertl, G.; Neumann, M. J. Chem. P h p .
-.--.
1974.60.4528. -. ---
(42) Conrad, H.; Ertl, G.; Latta, E. E. Surf.Sci. 1974, 41, 435.
(43) Gdowski, G. E.; Fair, J. A.; Madix, R. J. Surf.Sci. 1983, 127, 541. (44) Yates, Jr., J. T.; Thiel, P. A.; Weinberg, W. H. Surf.Sci. 1979, 84, 427. (45) Netzer, F. P.; Kneringer, G. Surf.Sci. 1974, 41, 435. (46) Peebles, D. E.; Schriefels, J. A.; White, J. M. Surf.Sci. 1982, 116, 117. (47) Lloyd, D. R.; Netzer, F. P. Surf.Sci. 1983, 129, L249
J. Phys. Chem. 1985, 89, 112-118
112
surfaces initially promote dehydrogenation via C H bond cleavage. Once intermediate I11 is obtained, the ease with which hydrogen can be transferred from the surface to the adsorbate (for example, in going from I11 to IX) will increase with a decrease in EedH2 and therefore CCH3should be favored on those surfaces with lower heats of adsorption. Inspection of Table IV shows that this trend indeed occurs. Summary
Both di-a- and ?r-bonded C2H4 are formed on the clean Pd(100) surface at 80 K. The r-bonded state (saturation coverage -0.25) desorbes between 100 and 300 K, but the di-a configuration (saturation coverage = 0.12) reacts to form a dehydrogenated intermediate suggested to be CHCH2 and atomic hydrogen be-
tween 250 and 275 K. Decomposition of CHCH2 occurred between 275 and 300 K, leaving CH groups and atomic hydrogen on the surface. Between 350 and 500 K condensation and dehydrogenation were observed among the CH groups that formed a carbon overlayer with unsaturated carbon-carbon bonds. Ethylene adsorbed at 80 K did not react with coadsorbed hydrogen, but hydrogen exchange in CD groups was observed after prolonged exposure to H2 at 300 K. Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (NSF-CPE 80 23815) and the Wheeler Foundation through the Institute for Energy Studies at Stanford. Registry No. C2H4,74-85-1; Pd, 7440-05-3; CHCH2, 2669-89-8.
A Determination of the Photochemical Quantum Yields from Two Excited Triplet States of Biacetyl Using Holography R. K. Grygier, P.-A. Brugger,' and D. M. Burland* IBM Research Laboratory, San Jose, California 951 93 (Received: June 15, 1984)
A holographic technique has been used to measure the quantum yield for photochemical product formation from two excited triplet states of biacetyl in a polymer host. The technique involves a simple measurement of the time taken for the hologram to reach its maximum diffraction efficiency. This measured time can be directly related to a photochemical quantum yield. The diffraction efficiency from the lowest triplet state has a yield of 0.045 0.010 measured in this way as compared to a value of 0.03 0.005 determined by a direct absorption method. The triplet state 13 000 cm-' above the lowest triplet state has a quantum yield of 0.7 0.02 determined by the holographic technique.
*
*
*
I. Introduction A holographic technique has been developed recently for the investigation of photochemical reactions in the solid state.2 The technique is particularly useful for investigating photochemical reactions that occur in two steps and require two photons, Le., photochemical production of a metastable state such as the lowest triplet followed by subsequent excitation to a more highly excited state from which photochemistry occurs.3 The technique is a zero-background technique and by comparison to direct absorption spectroscopy is much more sensitive to small photochemical change~.~ In this paper the use of the holographic technique for direct determination of the quantum yield of a photochemical reaction will be described. Examples of its use for both one- and two-step processes will be given. In treating the two-step process, it will be shown that the holographic technique can be used to obtain photochemical quantum yields for reactions that occur as a consequence of triplet-triplet absorption. The experimental reaction used to illustrate both one- and two-step processes involves the excitation and reaction of biacetyl in a polymer host matrix. Biacetyl undergoes a hydrogen abstraction upon one-step excitation of its lowest triplet state.5 The exact nature of its reaction from the higher triplet state excited in the two-step process is not known in detail although it has been shown to differ from the lowest triplet
reaction and to be consistent with an a-cleavage mechanism.6 Recently, Deeg et ale7have developed a holographic technique, complementary to the one described here, that permits one to obtain photochemical quantum yields when the index of refraction difference between reactants and products is known. By comparison the technique described in this paper requires a knowledge of the extinction coefficient at the hologram recording wavelength and, for a two-step process, the lifetime of the metastable state. In the next section the relationship between the kinetic rate equations for one- and two-step processes and hologram growth is described. Simple expressions for the relationship between hologram growth parameters and photochemical quantum yields are presented. In section I11 the details of the hologram experiment are described as is the preparation of biacetyl/polymer samples. In section IV the use of the holographic technique to obtain quantum yields for both one- and two-step processes is demonstrated from experimental measurements on biacetyl. Knowing the quantum yields from section IV, one can check the validity of the theoretical framework by calculating other features of hologram growth and comparing these calculated quantities with experiment. This is done in section V. As a result of this analysis, it is shown that one can obtain the index of refraction change between reactants and photochemical products at the hologram recording wavelength. 11. Relationship between Holography and Photochemistry
( 1 ) IBM Postdoctoral Fellow. Current Address: Institut de Chimie Physique, Ecole Polytechnique Federale, CH- 1015 Lausanne, Switzerland.
(2) Burland, D. M.; Bjorklund, G. C.; Alvarez, D. C. J. Am. Chem. Soc. 1980, 102,7117. Bjorklund, G. C.; Burland, D. M.; Alvarez, D. C. J. Chem. Phys. 1980, 73, 4321. (3) Brluchle, Chr.; Burland, D. M.; Bjorklund, G. C. J. Am. Chem. SOC. 1981, 103,2515. Schmitt, U.; Burland, D. M. J. Phys. Chem. 1983, 87, 720. (4) Burland, D. M. Acc. Chem. Res. 1983, 16, 218. (5) Monroe, B. M. Adu. Photochem. 1971, 8, 11.
0022-3654/85/2089-0112$01.50/0
A . General Features of Holography. A generalized diagram of the production of a hologram is shown in Figure 1. Two coherent beams are superimposed on the holographic sample. For (6) Schmitt, U.; Brugger, P.-A., unpublished results. (7) Deeg, F. W.; Pinsl, J.; Brluchle, Chr.; Voitlander, J. J . Chem. Phys.
1983, 79, 1229.
0 1985 American Chemical Society
