Mechanisms for ethylene hydrogenation and hydrogen-deuterium

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J . Phys. Chem. 1990, 94, 5090-5095

Mechanisms for Ethylene Hydrogenation and H-D Exchange over Pt( 111) Francisco Zaera Department of Chemistry, University of California, Riverside, Riverside, California 92521 (Received: December 13, 1989)

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TPD data for C2D4 and C2D,H chemisorbed over Pt( 11 I ) single-crystal surfaces have provided further insight into the mechanisms of ethylene hydrogenation and H-D exchange under ultrahigh vacuum. Desorption of CzD6from adsorbed C2D4clearly proves that ethylene undergoes self-hydrogenation by starting with a C-D bond breaking step which provides deuterium atoms to other ethylene molecules. This is the rate-limiting step for both hydrogenation and ethylidyne formation. Coadsorbed hydrogen (or deuterium) increases the ethane yield and lowers the apparent activation energy for that reaction. When H2 and C2D4 are coadsorbed, hydrogenation and H-D exchange occur simultaneously through the formation of a common intermediate which we believe is an ethyl group. We were able to measure all the relevant kinetic and thermodynamic parameters for these reactions and to determine that, even though adsorbed ethylene is less stable than ethyl moieties, chemisorbed ethyl groups prefer to go through a P-H elimination step to form ethylene because of the concurrent formation of adsorbed hydrogen.

Introduction Even though ethylene adsorption over metal surfaces has been studied extensively by many research the mechanistic details of its reactivity are still controversial. Molecular adsorption under vacuum is followed by a combination of parallel reactions around room temperature which include molecular desorption, self-hydrogenation, H-D exchange, and decomposition to form ethylidyne. The formation of ethylidyne has received particular attention, but no consensus exists still on the pathway by which this reaction takes p I a ~ e . l " - ~Additional ~ complications arise when we try to extrapolate the chemistry occurring under vacuum to catalytic p r o c e s s e ~ . ~ ~ * ~ ~ I n the present paper we address the problem of ethylene reactivity on Pt( 1 1 1) by using thermal programmed desorption (TPD). We report experiments done with either C2D4or C2D,H adsorbed on Pt( 1 1 I ) , by themselves or in the presence of coadsorbed hydrogen (or deuterium). W e have focused our attention on the mechanisms for ethylene hydrogenation and H-D exchange and have been able to extract the necessary kinetic and thermodynamic information to get an overall picture of ethylene reactivity on Pt( 1 1 1 ) . Experimental Section These experiments were carried out in a system described in detail e1~ewhere.l~It consists of an ultrahigh vacuum (UHV) stainless steel bell jar pumped with a turbomolecular pump to a Torr. This chamber is equipped base pressure of about 1 X with a quadrupole mass spectrometer capable of detecting masses in the 1-800 amu range. The ionizer of the quadrupole is located ( 1 ) Zaera, F; Somorjai, G.A. I n Hydrogen in Catalysis: Theoretical and Practical Aspects: Paal, Z., Menon, P. G.. Eds.; Marcel Dekker: New York, 1987. (2) Demuth, J . E. IBM J . Res. Deu. 1978, 22, 265. ( 3 ) Kesmcdel, L. L.: Dubois. L. H.; Somorjai. G . A. J. Chem. Phys. 1979. 70. 2 130. (4) Baro, A. M.; Ibach, H. J . Chem. Phys. 1981, 74, 4194. ( 5 ) Salmeron, M.; Somorjai, G.A. J . Phys. Chem. 1982, 86, 341. (6) Creighton, J. R.; White, J. M. Surf. Sei. 1983, 129, 327. (7) Felter, 7. E.; Weinberg, W. H. Surf. Sei. 1981, 103, 265. (8) Freyer, N.; Pirug, G.; bonzel, H. P. Surf Sei. 1983, 16, 487. (9) Skinner, P.; Howard, M. W.; Oxten, I . A,; Kettle, S. F. A,; Powerrl, D. B.; Sheppard, N. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1203. (10) Staininger. M.; Ibach, H.; Lehwald, S. Surf. Sei. 1982, 117, 685. ( 1 I ) Kang, D. B.; Anderson, A. B. Surf. Sci. 1985, 155, 639. (12) Stuve, E. M.; Madix. R. J. J . Phys. Chem. 1985, 89, 105. ( 1 3 ) Lloyd, K. G.; Campion, A.; White, J. M. Catal. Left. 1989, 2, 105. (14) Bent, B. E. Ph.D. Thesis, University of California, Berkeley, 1986. (15) Zaera, F.; Somorjai, G.A. J . Am. Chem. SOC.1984, 106, 2288. ( 1 6 i Beebe. T. P.. Jr.; Albert, M . R.; Yates, J. T., Jr. J . Caral. 1985, 96,

inside an enclosed compartment with a couple of apertures about 7 mm in diameter in the front and back for gas sampling and exit to the quadrupole rods, respectively. The sample is positioned within 1 mm of the front aperture, a setup that results in an enhanced sensitivity of about a factor of 5 for desorption from the front face of the crystal and additional discrimination against desorption from the edges and back of the crystal and from the supporting wires. The mass spectrometer is interfaced to a computer, allowing the detection of signals from up to 10 separate atomic mass units ( a m ) in a single thermal desorption experiment. The system is also equipped with a concentric hemisphere electron energy analyzer for Auger (AES) and X-ray photoelectron (XPS) spectroscopies and with an ion sputtering gun for sample cleaning. The sample manipulator allowed for resistively heating the crystal up to 1300 K and cooling to 90 K within 5 min. The thermal desorption experiments reported here were done using a heating rate of about 10 K/s. The platinum (1 1 1) single crystal was cut and polished by using standard procedures. It was cleaned under vacuum by a combination of oxygen treatments and sputtering-annealing cycles until no impurities were detected by either AES or XPS. Hydrogen was obtained from Matheson (ultrahigh purity, 99.999%) and used without further treatment. Deuterated ethylene was acquired from Matheson as well (99 atom % ' D) and also used as received. Trideuterioethylene was prepared by a gas-phase exchange reaction between CD2=CDCl (CIL, 98 atom % D) and tributyltin hydride. Over 70% conversion was achieved after the mixture was left in a sealed I-L bulb for a couple of months at room temperature. The resulting mixture was vacuum distilled at 178 K, and the purity of the final product was estimated to be higher than 95% by using gas chromatography-mass spectrometry. Also, the isotopic purity was calculated to be over 90% by using a test described in a previous report.I8 The thermal desorption experiments were performed after sample cleaning and cooling to below 100 K. To minimize background adsorption, the crystal was quickly flashed to 600 K and cooled to below 200 K prior to gas dosing. The time between flashing and dosing was of the order of 100 s. Since the backTorr and the residual gas ground pressure was about 1 X was mostly hydrogen, this corresponds to a H, exposure of about 0.01-0.02 langmuirs ( 1 langmuir = Torrss). Blank TDS experiments determined that preadsorbed hydrogen amounted to about 0.02-0.03 monolayers of atomic hydrogen. The TPD spectra in Figures 3,4, 5, 7, and 8 have been corrected for contribution due to the cracking of other compounds in the mass spectrometer ionizer. The ethane product distribution was deconvoluted from the data by using a recurrent method starting

1.

( I 7 ) Zaera. F. J. Vac. Sci. Technol. 1989. A7, 640

0022-3654/90/2094-S090$02.50/0

( 1 8 ) Zaera F , J 4m Chem Sot 1989. 111 4240

b 1990 American Chemical Society

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5091

H-D Exchange over Pt( 11 1)

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Figure

by assigning the signals at 36 and 35 amu to C2D6 and C2D5H, respectively, and subtracting their contribution to the other traces. The cracking patterns for all possible isotopically substituted ethanes were obtained from the l i t e r a t ~ r e , and ' ~ mass spectra for pure C2D4 and C2D3Hwere taken with our mass quadrupole. This method was successfully tested by making sure that no signal was obtained for light ethanes in the TPD results from a D2 + C2D4 coadsorption experiment.

Results Figure 1 shows the thermal programmed desorption spectra obtained after a 5-langmuir exposure of C2D4on Pt( 11 1). The trace for 36 amu, which corresponds to C2D6product of ethane self-deuteration, peaks at 3 11 K. The value for the activation energy of that desorption is about 18 kcal/mol, and the TPD area represents about 0.7%of the total ethylene desorption. The trace for 34 amu originates mainly from C2D6 cracking in the mass spectrometer, but a very small fraction corresponds to C2D4H2, the product of the hydrogenation of deuterated ethylene with hydrogen adsorbed from the background. The signal for 35 amu is also quite small and corresponds to C2D5H formation. The combined yield for C2D5H and C2D4H2is less than 20% of that of C2D6,proving that ethylene self-hydrogenation occurs mainly by addition of deuterium atoms coming from ethylene decomposition. We will discuss this point in more detail later. Figure 1 also shows TPD results obtained for 26, 3 I , and 33 amu. All these peaks correspond to C2D4 desorption, although the trace for 31 amu may contain a small contribution from C2D,H product of H-D exchange on C2D4. Ethylene desorption peaks at 258 and at 305 K, below either ethane or hydrogen desorption. We have shown in the past that ethylene self-hydrogenation is limited by ethylene decomposition (which is the source of hydrogen atoms), and that, therefore, hydrogenation can be enhanced by hydrogen coadsorption.20 This is illustrated in Figure 2 for C2D4. If deuterium is preadsorbed on Pt( 1 1 l ) , the ethane desorption peak shifts to lower temperatures, from 31 l to 302 K and then to 297 K . A low-temperature shoulder (265 K ) also develops in the spectra for high deuterium coverages. The total yield increased by a factor of 5 even though the ethylene dose was smaller in the coadsorption case. Factors of up to 50 in yield Y.; Pottie, R. F. Can. J . Chem. 1968, 46, 1741. (20) Godbey, D.: Zaera, F.;Yates, R.; Somorjai, G . A. Surf. Sci. 1986, 167, 150. (19) Amenomiya,

200

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Temperature / K

Temperature I K

C2D6TPD spectra from coadsorption experiments on Pt( 11 I ) . Deuterium was dosed prior to deuterated ethylene in all cases. Figure 2.

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Temperature / K Figure 3. Hydrogenation and H-D exchange desorption spectra from C2D4coadsorbed with either H2or D, on Pt(ll1). The traces for C2D3H and for C2D4H2have been corrected for contributions from other molecules due to their cracking upon ionization.

increase and activation energies of about 6 kcal/mol have been reported previously.20 If H2 is coadsorbed with C2D4 (instead of D2), H-D exchange reactions compete with hydrogenation. This is illustrated in Figure 3. The bottom trace shows again the desorption of C2D6 from an experiment in which a 5-langmuir D2 dose is followed by 0.5 langmuir of C2D4. The middle spectrum corresponds to the desorption of C2D4H2from an equivalent experiment with predosed hydrogen (after correcting for contributions from C2D5H and C2D6). The two spectra look similar, although the low-temperature shoulder is a little bit smaller for the case of coadsorbed hdyrogen. There is also an isotope effect in the rates of ethane formation, which is seen by a shift in the TPD maxima from 297 K for C2D6 to about 292 K for C2D4H2. The third spectrum in Figure 3 IS the result of C2D3H desorption from the same hydrogen coadsorption experiment as the middle trace (again, corrected for

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The Journal of Physical Chemistry, Vol. 94, No. 12, 1990

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Figure 5. C2D4 molecular desorption and H-D exchange T P D after coadsorption of H, and C2D, on Pt( 11 1). Doses are given for each trace.

(after data deconvolution)

contributions from other species). C2D3H,the product of H-D exchange on C2D4, displays a TPD similar to that for C2D4H2, indicating that exchange and hydrogenation proceed through the slow formation of a common intermediate. The maxima for C2D3H is about 5 K higher than for C2D4H2,and the yields for both species are comparable. We suggest that the common intermediate mentioned above is an ethyl moiety. Further evidence of this is given in Figure 4, which shows the desorption of C2D6,C2D5H,and C2D4H2from an experiment in which a small amount of hydrogen was preadsorbed on the surface. While C2D4H2is a direct product of C2D4 hydrogenation with coadsorbed H atoms, C2D5Hand C2D6formation requires the decomposition of some deuterated ethylene molecules in order to provide the needed deuterium. C2D,H2 desorption occurs in a broad temperature range centered around 298 K, while C2D5H desorption peaks at 304 K and C2Ds desorption peaks around 309 K. These results indicate not only that self-hydrogenation is limited by the reaction that provides hydrogen or deuterium atoms (ethylene decomposition), but also that hydrogenation occurs stepwise. This is the reason why C2D5H is produced, and why its desorption occurs at intermediate temperatures between C2D4H2and C2D6desorption. H-D exchange is also limited by the formation of the ethyl intermediate, as illustrated in Figure 5. If 0.5 langmuir of H2 is dosed prior to ethylene exposure, H-D exchange occurs only above about 270 K (maximum at 301 K), with a yield of about 7% of the total ethylene. At higher hydrogen coverages the C2D3H desorption maxima shifts to 298 K and has a shoulder around 250 K (top spectra); the yield amounts to about 15% of the total desorbing ethylene. The high-temperature position of the exchange peaks suggests that ethyl formation occurs around 290-300 K and that its rate depends strongly on the hydrogen (or deuterium) surface coverage. Notice also how the high-temperature side of the C2D4trace has been depleted in each case accordingly. Corroborating experiments were done by using C2D3H. Figure 6 shows TPD spectra from 5 langmuirs of C2D3Hon Pt( 11 1). In this case self-hydrogenation is manifested by the desorption of C2D~H (35 amu), C,D,H, (34 amu), and C2D,H3 (33 amu). The relative yields for all three products are determined by the ratio of H to D atoms produced in the ethylene decomposition. While we have previously shown that the H:D yield ratio from ethylidyne formation is about 1:2,18 here we obtained ratios of about 5(C2D3H3):5(C2D4H2):4(C2D5H). This discrepancy can be explained by noticing that what matters for self-hydrogenation is

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the initial yield of H or D from decomposition (in the low-temperature side of the H2, HD, and D, TPD spectra), and hydrogen production is favored there because of the isotope effect in ethylidyne formation.2'.22 Our results suggest that the yields for H and D are about equal in that temperature regime. Figure 6 also displays traces for 27 and 30 amu. The trace for 27 amu corresponds almost exclusively to molecular desorption, but the signal for 30 amu has a sizable contribution from exchanged ethylene, which accounts for the higher yield at high temperatures in that peak and for the shift of its maximum from 294 to 298 K. Figure 7 show the results from coadsorption experiments similar to those in Figure 4. In this case, if hydrogen is dosed prior to (21) Zaera, F.; Fischer, D. A.; Carr, R. G.;Kollin, E. B.; Gland, J . L. In Molecular Phenomena at Electrode Surfaces; ACS Symposium Series No. 378; Soriaga, M. P., Ed; American Chemical Society: Washington, DC, 1988. (22) Gland, J. L.; Zaera, F.; Fischer, D. A,: Carr, R.G.; Kollin, E. B. Chem. Phys. Lett. 1988. 151, 227.

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5093

H-D Exchange over Pt( 1 1 1)

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400

600

Temperature / K Figure 8. TPD from hydrogen-ethylene coadsorption systems on Pt(ll1) which highlight the products of successive H-D exchange followed by hydrogenation reactions. See text for details.

cessing. C2D3H, hydrogenation (to form C2D3H3)peaks at 290 K, but C2D4H2formation (which requires one D atom from ethylene decomposition) has its maximum at 301 K. The experiments with coadsorbed deuterium yield similar results: straight deuteration occurs in a broad temperature range around 290 K, while C2D4H2 (one hydrogen from decomposition) peaks at 293 K. These results not only corroborate that hydrogenation takes place stepwise, but also show the isotope effect for ethylene decomposition, since C2D4H2formation peaks at 293 K with coadsorbed deuterium ( H from decomposition) but at 301 K with coadsorbed hydrogen (D from decomposition). Finally, Figure 8 shows some results which illustrate that a small fraction of exchanged ethylene remains on the surface before desorbing. In all three cases displayed the isotopic composition of the desorbing ethane shown can only be the product of an exchange reaction followed by hydrogenation (or deuteration). For instance, the middle trace shows the desorption of C2D3H3 which results from a H-D exchange step on C2D4 (to produce C2D3H) followed by hydrogenation. Similar arguments apply to the other two cases. Even though the spectra are quite noisy (they have been corrected for all possible contributions from other compounds), it is clear that desorption occurs in two peaks around 250 and 300 K. The relative yields are a reflection of isotope effects in reaction rates: the upper trace involves an exchange where C2D3H forms C2D4Has intermediate, dehydrogenates to C2D4,and then deuterates to form C2D6. The C-H bond breaking in the exchange competes favorably with other reactions, and therefore the yield for the exchange and subsequent deuteration is high in the low-temperature peak. The opposite is true for the case illustrated at the bottom: the exchange involves a C-D bond breaking, which is slower, so almost no C2D2H4is seen in the low-temperature peak but a reasonable yield is seen at higher temperatures. The middle case has a larger low-temperature peak because in this case the C-D bond breaking does not have to compete with other C-H bond breaking reactions. The fact that exchanged ethylene has a reasonably long residence time before desorbing has also been evidenced by the detection of multiple exchange in D2 C2H4experiments.20

+

Discussion We have presented some new data to help clarify the mechanism of ethylene conversion reactions on Pt( 1 11). It is well-known that ethylene chemisorption at liquid nitrogen temperatures is molecular, with the C-C bond axis parallel to the surface.2 Then,

at temperatures of about 300 K, up to four parallel reactions take place on the surface, namely, molecular desorption, H-D exchange, hydrogenation to ethane, and decomposition to ethylidyne.' In the present report we will focus on the hydrogenation and exchange processes. Ethylene self-hydrogenation under ultrahigh vacuum has been reported p r e v i o ~ s l y . ~However, -~~ exposure of the surface to small doses of background H2 is unavoidable in any vacuum experiment, and it is known that coadsorbed hydrogen enhances ethane formation. In order to determine the source of the hydrogen atoms in ethylene hydrogenation, we have done TPD experiments using C2D4 instead of CzH4. Figure 1 shows that the main hydrogenation product is C2D6and not C2D4H2,clearly demonstrating that indeed some ethylene decomposition is required as the source of hydrogen (or deuterium) for ethane formation. It is also clear that such C-H (C-D) bond breaking reaction is the rate-limiting step, since coadsorbed hydrogen (or deuterium) lowers the temperature at which this reaction takes place (Figure 2 ) . We have previously reported an activation energy of about 6 kcal/mol for ethane formation when hydrogen is present on the surface, compared to about 16-18 kcal/mol for both ethylene self-hydrogenation and ethylidyne formation.20 A more interesting result is shown in Figure 3, which displays TPD traces for both hydrogenation and H-D exchange products from hydrogen coadsorbed with C2D4. The desorption spectra for C2D3H and C2D4H2are quite similar, implying that both reactions take place through a common intermediate. We propose that this intermediate forms by ethylene partial hydrogenation in a rate-limiting step, which then reacts further by following two competitive reactions to yield either ethane (hydrogenation) or ethylene (exchange). The higher temperatures required for the desorption of C2D3His a reflection of an isotope effect, since H-D exchange in this case requires a C-D bond breaking. Notice that the C2D3H TPD closely resembles the trace for C2Ds from CID,+D,. Further evidence that ethylene hydrogenation occurs in at least two consecutive steps is given in Figure 4. When small amounts of hydrogen are coadsorbed with C2D4, ethylene hydrogenation can occur either by direct H incorporation from the surface or by D addition after some ethylene decomposition. Direct hydrogenation, which produces C2D4H2,occurs at lower temperatures (23) Berlowtiz, P.; Megiris, C.; Butt, J. 8.; Kung 1 , 206.

H.H.Langmuir 1985,

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The Journal of Physical Chemistry, Vol. 94. No. 12. 1990

Zaera

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Hydrogenation ( t H )


Figure 9. Reaction scheme for C2 hydrocarbon fragments chemisorbed on Pt( 1 1 I ) below room temperature. The numbers correspond to activation energies for the elementary steps, in kcal/mol.

than CzDs production. However, if the hydrogen coverage in the coadsorption experiments is low enough, some ethylene may partly hydrogenate by incorporation of one surface hydrogen atom followed by subsequent addition of a deuterium atom which originates from C2D4decomposition. The product of this sequence, C2DSH,desorbs at an intermediate temperature, between C2D,H2 and C2D6production. These results argue not only for the stepwise addition of hydrogens into ethylene, but also for the formation of ethyl groups as intermediates. Ethyl groups can then either incorporate another hydrogen atom to form ethane or lose a hydrogen atom to go back to ethylene. Figure 7 provides further confirmation of this mechanism from experiments with C2D,H. We have proven that H-D exchange is also limited by the kinetics of ethyl formation. This is illustrated in Figure 5 , which shows that, even though ethylene desorbs molecularly in two stages (peaking at 258 and 305 K), most of the exchanged ethylene desorbs in the high-temperature range, around 300 K. Only at very high hydrogen coverages does some exchange start occurring at 250 K. In order to estimate the rate of ethyl formation, we can assume pseudo-first-order kinetics for the case where excess hydrogen is present on the surface. A detailed analysis of the TPD using both the peak maxima and their widths results in values of 15 f 1 kcal/mol and 11.5 f 0.5 for the activation energy ( E , ) and log A , r e ~ p e c t i v e l y . ~ ~ We have recently reported some IR and thermal desorption spectra from ethyl iodide which complements the data given here.25 Ethyl moieties can be formed on Pt( 1 1 1 ) surfaces starting from ethyl i ~ d i d e . ~These ~ ~ ' ethyl groups can be hydrogenated to form ethane, which desorbs at 255 K.2sJ* Again, when excess hydrogen is present on the surface, the reaction is pseudo-first-order in ethyl coverage, and the TPD data yields a value for the activation energy of ethane formation of about 12 kcal/mol. Additionally, some ethyl moieties undergo @-hydrideelimination to form ethylene,29 which desorbs in broad peaks centered at temperatures as low as 170 K. This would correspond to an activation energy of about 7 f 2 kcal/mol and to a low preexponential factor, log A = 8. The picture that develops from all these pieces of information is summarized in Figure 9. After initial adsorption, ethylene can desorb molecularly, decompose to form ethylidyne, or hydrogenate to form ethyl groups. The kinetic parameters for molecular desorption obtained from Figure 1 and 6 are E, = 11 f 1 kcal/mol and log A = 7.2 f 0.6.24They compare well with reported values in the literature,s even though our lower heating rates allowed us to detect two clear desorption stages. Ethylidyne formation occurs through the formation of a vinyl intermediate,'* with E , = 15.0 kcal/mol and A = 3.6 X 1Olo s-' for normal ethylene, and (24) Chan, C.-M.; Aris, R.; Weinberg, W. H. Appl. Surf: Sri. 1978, I . 360. (25) Zaera, F. Surf. Sci. 1989, 219, 453. (26) Zaera, F.; Hoffmann, H.; Griffiths, P. R. Vacuum, in press. (27) Lloyd, K. G.; Roop, B.: Campion, A.; White, J. M . Sur/. Sri. 1989 214, 227. (28) Zaera, F. J . Phys. Chem., submitted for publication (29) Zaera, F. J . Am. Chem. Soc. 1989, 1 1 1 , 4240

TABLE I: Bond Strengths (DA-,J DA+, kcal/mol bond (A-B) ___. H-H 104.2

H-CH=CH, H-CH?CH, H ,C-C H 3 H,C=CH, H2C-CH2 Pt--H Pt-C H ZC H, Pt-CHzCHZPt

106.1 100.7

88.8 172 86 61.5 32 49 (each di-u)

reference I _

33

calc from 34 calc from 35 calc from 3 3 36

approx calc from 31 this work this work

E , = 16.7 kcal/mol and A = 3.5 X 10" s-' for C2D4.2'322The third channel for ethylene reactivity, ethyl formation. may require an activation energy as high as 15 kcal/mol, although we believe that this is an upper limit and that the high-temperature peaks in the TPD are due to low surface coverages of hydrogen. In fact, both ethane formation and H-D exchange are observed at low temperatures, around 250 K, after high Hz doses (Figure 3). Perhaps a more realistic number for the activation of ethyl formation would be about 13 kcal/mol. To complete the diagram, we have established that ethyl moieties then either hydrogenate further to form ethane (E, = 12 kcal/mol) or dehydrogenate back to ethylene. This last step has an activation energy of about 7 kcal/mol, but an overall rate only about 2 times faster than the hydrogenation. The kinetic information given above also allows us to obtain thermodynamic values for some of the surface species involved in ethylene reactivity. Heat of reactions can be estimated as the difference in activation energies between the forward and the backward steps. Few additional numbers were incorporated in Figure 9 for that purpose. First, ethylene adsorption is considered to be ~nactivated.~ The activation energy for ethane dissociative adsorption has been estimated from catalytic H-D exchange experiment^^^ and it ranges between 12 and 26 kcal/mol (19 kcal/mol on Pt( 1 1 1)). Hydrogen adsorption is also considered to be unactivated, and its desorption has an activation energy of 19 k ~ a l / m o l . ~ lUsing all these values we were able to construct the Born-Haber cycle shown in Figure IO. We can see that ethyl moieties are more stable than adsorbed ethylene by about 3.5 kcal/mol. However, since an extra hydrogen atom is produced and adsorbed on the surface in the conversion of ethyl to ethylene, this reaction is in fact favored by about 6 kcal/mol. The increase in entropy is probably also an important factor driving this reaction. The numbers given in Figure 10 are reliable within few kilocalories, and a self-consistency check can be done by comparing the reported heat of reaction from ethane to ethylene (32.7 kcal/m01)~~ with the number obtained by adding the values in ~~~

~~~~

(30) Zaera, F.; Somorjai, G . A. J . Phys. Chem. 1985, 89, 3211. (3 1 ) Poelsema. B.; Mechtersheimerm, G.;Comsa, G . Surf: Sci. 1981, I 1 I , 519.

(32) Stull, D. R.; Westrum, E. F. Jr.; Sinke, G. C. The Chemical Thermodynamics and Organic Compounds; Wiley: New York, 1969. (33) Benson, S.W. Theoretical Kinetics; Wiley: New York, 1976. (34) Parmar, S.S.; Benson, S.W. J . Phys. Chem. 1988, 92, 2652. (35) Brouard, M.; Lightfoot, P. D.; Pilling. M. J . J . Phys. Chem. 1986, 90. 445. (36) CRC Handbook of Chemistry and Physics, 55th ed.; CRC Press: Cleveland, OH, 1974.

J . Phys. Chem. 1990, 94, 5095-5102 our cycle (31 kcal/mol). The agreement is fairly good. From this cycle we can also calculate bond strengths (Table I). Finally, our results also provide some useful information into the mechanism of ethylidyne formation. We have proposed that vinyl moieties are the unstable intermediates in the conversion of ethylene into ethylidyne,'* but several other mechanisms have been suggested in the l i t e r a t ~ r e . ' ~In' ~particular, Benti4 has suggested the hydrogenation of ethylene to ethyl as the initial step. In view of our results this scheme seems highly unlikely, at least on Pt( 1 1 l), because ethyl formation can take place at temperatures well below those needed for ethylidyne formation.

confirmed previous reports which show that the limiting step for such reaction is ethylene decomposition (the source for hydrogen atoms) and that hydrogenation can occur a t lower temperatures and with higher yields if hydrogen or deuterium is preadsorbed on the surface. H-D exchange experiments proved that hydrogenation takes place stepwise, with the first step being ethyl formation. Ethyl moieties can then either hydrogenate further to form ethane or dehydrogenate back to ethylene. We combined the results presented here with additional data from experiments with ethyl iodide in order to calculate activation energies and heats of reactions for the relevant steps in C2 hydrocarbon chemistry on Pt( 11 1). We concluded that ethyl groups are more stable than chemisorbed ethylene, but the conversion of ethyl into ethylene is driven by the concomitant formation of atomic hydrogen, which also adsorbs on the surface.

Conclusions

W e have shown that chemisorbed ethylene can undergo selfhydrogenation on Pt( 1 1 1 ) surfaces under ultrahigh vacuum. We


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I n Situ Infrared Spectroscopy of CO Adsorbed at Ordered Pt( 100)-Aqueous Interfaces: Double-Layer Effects upon the Adsorbate Binding Geometry Si-Chung Chang and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: December 14, 1989)

Surface infrared spectra for carbon monoxide adsorbed on ordered Pt(100) in 0.1 M HCIO, are reported as a function of CO coverage, 0.1 5 0 5 0.85, and electrode potential, -0.25 V 5 E 5 0.25 V vs SCE. Vibrational C-0 stretching bands, vco, corresponding to both terminal and 2-fold bridging geometries are observed, with intensities that depend markedly upon E and 0. For coverages formed by dosing with dilute (ca. 2 X M) CO solutions for varying times ( S I 5 min), alterations in 0 at positive potentials ( E k 0.1 V) yield dramatic changes in the site occupancy in that bridging and terminal features are dominant for 0 S 0.3 and 0 k 0.7, respectively. Qualitatively similar, yet less marked, 0-induced alterations in the CO binding geometry are observed at negative potentials ( E < 0 V vs SCE), where hydrogen rather than water constitutes the major coadsorbate. The potential-induced changes in site occupancy at near-saturation CO coverages (0 = 0.8-0.85), increasingly favoring terminal versus bridging CO at more positive potentials, are ascribed to the diminishing influence of dz-277; back-bonding under these conditions. The markedly different potential-dependent site occupancies observed at lower coverages are attributed to the influence of coadsorbed hydrogen and water. As for the corresponding Pt( 11 1) and Pt( 1 IO) electrochemical surfaces, coverage-dependent spectra for CO layers formed by electrooxidative stripping exhibit much smaller decreases in band frequency than for coverages attained directly by solution CO dosing. Combined with coverage-dependent spectral measurements of dipoledipole coupling by utilizing i2CO/i3C0mixtures, these results indicate that large CO islands are formed during adsorbate electrooxidation. Dissipation of these domains into adsorbate structures similar to those obtained by solution CO dosing occurs typically within ca. 15 min at -0.25 V. Significant differences are also observed in the voltammetric electrooxidation of subsaturated CO layers formed under "stripping" and "dosing" conditions, the latter but not the former exhibiting increasingly facile kinetics for lower initial coverages.

purpose is carbon monoxide, partly in view of the detailed body of vibrational and other structural information now available for CO at monocrystalline metal-UHV interfaces. A specific focus of our attention so far is on the detailed examination of infrared spectra in the C-0 stretching (vco) region as a function of CO coverage, 0, and electrode potential for platinum and rhodium crystals in acidic aqueous solutioni-2at high CO coverages. Under some conditions, the forms of the vco spectra do not differ greatly between the electrochemical and UHV environments.ii2 This similarity extends to the frequencies of the terminal and bridge-bound CO bands, vko and vko, in some corresponding electrochemical and UHV systems once allowance is made of the differing electrostatic potentials typifying the two surface environments.ib,d,2a Nevertheless, several features of the electrochemical infrared spectra serve to highlight the influences of the double-layer environment on the CO adsorbate structure. Marked differences in the &dependent uc0 frequencies and band shapes are obtained on Pt( 11 1 ) and Pt( 110) depending on whether the CO coverages are formed by dosing with dilute (ca. M) CO solutions for various times, or by electrooxidative stripping from saturated CO

A topic of recent and ongoing interest in our laboratory concerns the examination of adsorbates on ordered low-index faces of platinum and rhodium in aqueous electrochemical environments by means of infrared reflection-absorption spectroscopy (IRRAS).i-3 One objective of these studies is to explore the manner and extent to which the double-layer environment can influence surface bonding for simple adsorbates a t such stereochemically well-defined surfaces. A powerful strategy is to compare coverage-dependent electrochemical infrared spectra with corresponding results for the same adsorbate-surface combination in ultrahigh vacuum (UHV). The prototype adsorbate for this ( I ) (a) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92,6985. (b) Chang, S.-C.: Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1989, 93,5341. (c) Chang, S.-C.; Weaver, M. J. J , Chem. Phys. 1990, 92, 4582. (d) Chang, S.-C.: Weaver, M. J. Surf. Sci., in press. (2) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J . Chem. Phys. 1989, 90, 7426. (b) Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem., in

press. (3) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J . Electroanal. Chem. 1989,266, 317. (b) Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1989, 93, 7218. (c) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem., in press.

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0 1990 American Chemical Societv -

Mechanisms for ethylene hydrogenation and hydrogen-deuterium

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