J. Phys. Chem. 1992, 96,8529-8538 (46) W w a , Y.; Grgtzel, M. J. Chem. Soc., Faraday Trans. I 1988,84, 197. (47) Kobayakawa, K.; Nakazawa, Y.; Ikcda, M.; Sato, Y.; Fujishima, A. Ber. Bunsenges. Phys. Chem. 1990, 94, 1439. (48) Peterson, M.W.; Turner, J. A.; Nozik, A. J. J . Phys. Chem. 1991, 95, 221* (49) Henglein, A. Top. Curr. Chem. 1988,143,113; Chem. Rev. 1989,89, 1861. (50) Grltzel, M.,Ed.; Energy Resource through Photochemistry and Catalysis; Academic Press: New York, London. 1983. (51) Fox, M.A.; Chanon, M. Photoinduced Electron Transfec Elsevier: Amsterdam, 1988. (52) Russell, G. A. J. Am. Chem. Soc. 1957, 79. 3871. (53) Sonntag,C. v. The Chemical Basis of Radiation Biology; Taylor & Francis: London, 1987. (54) Urry, W. H.; Eiszner, R.J . Am. Chem. Soc. 1951, 73, 2977; 1952, 74, 5822. (55) Nesmeyanov, A. N.; Freidlina, R. Kh.; Fintov, V. I. Zzu. A h d . Nauk. SSR, Org. Khim. Nauk. 1951, 505. (56) Nesmeyanov, A. N.; Freidlina. R. Kh.; Zakharkin, L. I. Dokl. Akad. Nuuk S S R 1951,81, 199. (57) Wilt, J. W. In Free Rudicals; Kochi, J . K., Ed.; Wiley: New York, 1973; p 333. (58) Skell, P. S.;Shea,K. J. In Free Radicals; Kochi, J . K., Ed.; Wiley: New York, 1973; p 809. (59) Freidlina, R. Kh. Adu. Free Radical Chem. 1965, I , 21 1. (60) Beckwith, A. L. J.; Ingold, K. U. In Rearrangement in Ground and Excited States; de Mayo, P., Ed.; Academic Press: New York, 1980; p 161. (61) Hopkinson, A. C.; Lien, M.H.; Csizmadia, I. G. Chem. Phys. Lett. 1980, 71, 557. (62) Hoz, T.; Sprecher. M.;Basch, H. J . Phys. Chem. 1985, 89, 1664. (63) Edge, D. J.; Kochi, J. K. J . Am. Chem. Soc. 1972,91,6485. (64) Chen, K. S.; Elson, I. H.; Kochi, J. K. J . Am. Chem. Soc. 1973, 95, 5341.
8529
(65) Chen. K. S.; Tang, D. Y. H.; Montgomery, L. K.; Kcchi, J. K. J. Am. Chem. Soc. 1974, 96,2201. (66) Skell, P. S.; Traynbam, J. 0. Acc. Chem. Res. 1984, 17, 160. (67) Skell, P. S.; Pavlis, R. R.;Levis, D. C.; Shea, K. J. J. Am. Chem.Soc. 1973, 95, 6735. (68) Gasanov, R. G.; Ivanova, L. V.; Freidlina, R. Kh. Bull. Acad. USSR (Engl. Transl.) 1979, 28, 2618. (69) Goldin& B. T.; Radom, L. J. Chem.S a . , Chem.Commun. 1973,939. (70) Clark, T.;Symons, M. C. R. J. Chem. Soc., Chem. Commun. 1986, 96. (71) Onciul, A. v.; Clark, T. J. Chem. Soc, Chem. Commun. 1989, 1082. (72) Heller, H. G.; Langan, G. R. EPA News Lett. 1981, 71; J. Chem. Soc., Pcrkin Trans. 2 1981, 341. (73) Fricke, H.; Hart, E. J. In Radiation Chemiktv; Attix, F. H., Roach, W. C., Eds.; Academic Press: New York, London, 1966; Vol. 11, p 167. (74) Asmus, K.-D. Methods in Enzymology; Packer, L., Ed.; Academic Press: New York, 1984; Vol. 105, p 167. (75) Mao, Y. Ph.D. Thesis; Technical University Berlin, D 83, 1989. (76) Asmus, K.-D.; Lal,M.;MBnig, J.; Schiheich, C. In Oxygen Radicals in Biologv and Medicine; Simic, M. G., Taylor, K. A,, Ward, J. F., Sonntag, C. v., Eds.; Plenum: New York, London, 1989; p 67. (77) Farhartaziz; Ross, A. B., Eds. Selected Specific Rate Constants of Transients from Water in Aqueous Solution; Natl. Stand. Ref. Data Ser. NSRDSNBS 59; US. Department of Commerce: Washington, DC, 1977. (78) Lal, hi.;Schheich, C.; Mbnig, J.; Asmus, K.-D. Int. J. Radial. Biol. 1988,54,773. (79) Gerischer, H.; Heller, A. J . Phys. Chem. 1991. 95, 5261. (80) Moo, Y.;Schijneich, Ch.; Asmus, K.-D. Unpublished results. (81) Peyton, G. R.; Huang, F. Y.; Glaze, W. H. Emiron. Sci. Technol. 1991. 16. 448. (82) khuchmann, M.N.; Schuchmann, H.-P.; v. Sonntag, C. J . Phys. Chem. 1989, 93, 5320. (83) Gilbert, B. C.; Holmes, R. G. G.; Norman, R.0.C. J . Chem. Res. (M) 1977, 101.
Carbon-Halogen Bond Dissociation on Copper Surfaces: Effect of Alkyl Chain Length Jong-Liang Lin and Brian E. Bent* Department of Chemistry, Columbia University, New York, New York 10027 (Received: April 24, 1992; In Final Form: July 10, 1992)
Carbon-halogen bond dissociation in a series of straight-chain alkyl halides adsorbed on single crystal copper surfaces has been studied by high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and work-function measurements. For two or more carbons in the alkyl chain, the rate of carbon-halogen (C-X) bond diaoociaton is independent of chain length, despite the fact that the heat of molecular adsorption (assuming a first-order preexponential factor of lOI3 s-’ for desorption) increases at the rate of 1.3 f 0.2 (kcal/mol)/CH, group. The combination of these two effects produce a sharp transition in the branching between desorption and decomposition as a function of alkyl chain length. For alkyl chlorides the surface reaction path switches from desorption to decomposition between C6and C,;for the alkyl bromides the transition is between Cz and C3. All of the alkyl iodides decompose. The C-X bond dissociation rates decrease in the order C-I > C-Br > C-Cl, and the activation energies for C-X bond scission are approximately 15% of the gas-phase bond energies in each case. The chain length dependence for the heat of adsorption suggests that the molecularly adsorbed alkyl halides bind with their carbon chains approximately parallel to the surface plane, while surface vibrational spectra indicate that the alkyl groups formed by carbon-halogen bond scission reorient to stand upright on the surface. The independence of the C-X bond dimciation rate on alkyl chaii length suggests that reorientation of the carbon chain occurs after the transition state for C-X bond scission. The results are rationalized by using a modified h a r d - J o n e s picture of dissociative adsorption.
1. htroduction Carbon-halogen bond dissociation on metal surfaces is a key step in synthetic pracesses such as the Grignard reaction1and the Rochow process? yet surprisingly little is known about the details of this elementary surface reaction. In general, the reactivity of alkyl halides with metals de”in the order iodides > bromides > chlorides, as might be expected from the gas-phase carbonhalogen bond energies: C-I 55 kcal/mol; C-Br 70 kcal/mol; C-CI 85 kcal/mol. On the other hand, this qualitative reactivity trend reflects a competition between the rate of carbon-halogen bond dissociation and the rate of alkyl halide desorption from the metal surface. Bromomethane may be less reactive than iodomethane because it has a faster rate of desorption (lower heat of
=
=
Author
to
adsorption) as opposed to a slower rate of carbon-halogen bond dissociation. Based on the results to date, this is an open issue. For example, bmme and chloromethane desorb molecularly intact from Pt( 111) below 200 K, the temperature where the carboniodine bond in iodomethane d ~ o c i a t e s . ~Perhaps -~ the rates of C-Br and C-Cl dissociation are close to that for C-I dissociation and yet dissociation may not be observed because desorption predominates. Alkyl chain length also has an effect on carbon-halogen bond dissociation on metal surfaces. For two or more carbons, lengthening the alkyl chain does not change the gas-phase carbon-halogen (C-X) bond energy: but it could affect the rate constant for dissociating the adsorbed C-X bond (for example, by constraining the C-X bond to a particular orientation with res@ to the metal surface), and it certainly decreases the rate
=
whom correspondence should be addressed.
0022-3654/92/2096-8529S03.00/0
(B
1992 American Chemical Society
8530 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
of alkyl halide desorption. A dramatic example of these combined effects on the alkyl halide dissociation/desorption yield has recently been demonstrated for alkyl iodide adsorption on aluminum surfaces.' In that case, increasing the alkyl chain length from two to three carbons increased the reactive sticking probability by orders of magnitude, an effect attributed to an increase in the heat of molecular adsorption relative to the activation energy for carbon-iodine bond dissociation. The effect of alkyl chain length on the rates of dissociation and molecular desorption has yet to be determined quantitatively, however. In this paper, we report studies of alkyl halide desorption from and dissociation on single crystal copper surfaces as a function of alkyl chain length. We find that the rates for carbon-halogen bond dissociation decrease in the order C-I > C-Br > C-Cl and that the activation energies for dissociation are approximately 15% of the gas-phase bond energies in each case. Our results also provide the following picture of the dissociation event for submonolayer coverages of the alkyl halides: (1) the molecularly adsorbed alkyl halide lies approximately flat on the surface with a trans conformation about the CI-C2 bond, (2) this flat-lying adsorption geometry is maintained in the transition state for carbon-halogen bond dissociation, and (3), subsequent to the transition state, the alkyl chain reorients to form the coppercarbon bond and stand upright on the surface.
2. Experimental Section The experiments were performed in an ultrahigh-vacuum (UHV) apparatus pumped by turbomolecular (Balzers, 330 L/s), ion (200 L/s), and titanium sublimation pumps. The chamber is equipped with a single-pass cylindrical mirror analyzer (Perkin-Elmer) for Auger electron spectroscopy, a high-resolution electron energy loss spectrometer (McAllister Technical) for surface vibrational spectroscopy, an ion gun for sample cleaning, and a quadrupole mass spectrometer (Vacuum Generators SXP300, 0-300 amu) for temperature-programmed desorption/reaction studies. The mass spectrometer is located behind a differentially pumped shield containing a 2-mm diameter aperture for sampling molecules evolving from the center of the single crystal surface. The (1 11)-oriented Cu single crystal (0.6-cm2surface area, 2 mm thick, Come11 Materials Research Laboratory) was mounted on a molybdenum resistive heating element by using two small tantalum tabs spot-welded to the heater and bent over the edge of the crystal. The (100)-oriented Cu crystal (1 .0-cm2 surface area, 2 mm thick, Monocrystals Ltd.) was fastened to a similar heating element by cutting grooves in the edge of the crystal and using 0.25-mm chrome1 wire to tie the sample to the heater. The crystal/heater units were inserted into a copper sample holder connected via a copper braid to a liquid nitrogen reservoir for temperature control from 120 to 1100 K. Surface temperatures were measured by using a chromelalumel thermocouple (0.25mm wires) with the thermocouple junction wedged into a 0.5-mm diameter hole drilled 3 mm deep into the side of the crystal. The crystal was cleaned by ion bombardment and annealing in vacuum as previously described.8 The halogens that were deposited on the Cu(111) surface during the experiments reported herein could be removed by briefly annealing the surface at 980 K. All of the alkyl halides with the exception of bromomethane (99.576, Matheson) were obtained from Aldrich, and they were generally used as received. If the liquids were yellow, they were filtered over an alumina column (basic pH) to remove iodine and HI. The compounds were then stored in glass vials and shielded from light. Prior to introduction into the vacuum system, they were further purified and degassed by several freeze-pump-thaw cycles with liquid nitrogen. The purity of each compound was verified in situ by mass spectrometry. Gas dosing was achieved by backfilling the chamber. All exposures are given in langmuirs (1 langmuir = 1 X IO" Tows) and are uncorrected for differing ion-gauge sensitivites. In temperature-programmeddesorption/reaction (TPD/TPR) experiments, the adsorbate-coveredsurface was positioned lineof-slight to the mass spectrometer, about 1 mm from the sampling
Lin and Bent aperture described above. The surface was heated at 2.5 K/s, and single or multiple (up to three) ions were detected as a function of surface temperature. The high-resolution electron energy loss (HREEL)spectrometer, which consists of 127' cylindrical, single-pass electron monochromator, and analyzer sectors, was operated at a beam energy of 3-5 eV and a resolution (full width at half-maximum) of 70-90 cm-I. All spectra were taken in the specular direction (0, = Oout = 60' from the surface normal) either at room temperature or at 120 K after the sample was briefly annealed to the desired temperature. Changes in the surface work function were monitored by measuring the cutoff in the current to ground of a low-energy (5-10 eV) electron beam impinging at normal incidence on the Cu( 111) surface. The experimental setup is similar to that in ref 9 except that, because the experimental configuration prohibits positioning the crystal perpendicular to the electron beam from the HREELS monochromator, we used the beam from the Auger electron spectrometer as our electron source. The current to ground was approximately lo-* A. Based on the energy spread in the incident electron beam and our experimental reproducibility, we estimate that these work-functionchange measurements are precise to within 50 meV. Comparison of our measured workfunction change for CO adsorbed on Cu( 111) with the results reported in the literature for similar experiments with a Kelvin probelo suggests the experimental accuracy is also within these limits. 3. Results and interpretation Our results for the straight-chain alkyl halides on copper surfaces are presented below in the order of bromides, iodides, and chlorides. We chose this order of presentation because the bromides best illustrate all the points we make. The surface vibrational spectra and temperatureprogrammed reaction spectra of bromopropane establish that carbon-halogen bond dissociation is an elementary event; i.e., no C-H bond scission occurs concurrently. The C-Br dissociation temperatures were determined by a combination of vibrational spectra, work-function change measurements, and elastic electron scattering as a function of surface temperature. Selected results for the iodides and chlorides are compared with those of the bromides to establish the similarities and differences in the C-X dissociation pathways and kinetics. It should be emphasized that the studies here focus on submonolayer coverages of the alkyl halides. Evidence for coupling and disproportionation reactions of alkyl radicals p r o d u d during carbon-iodine bond scission at near saturation of the monolayer will be presented elsewhere. 3.1. Alkyl Bromides. Bromometheae. Bromomethane desorbs molecularly intact from Cu( 11 l), as evidenced by the temperature-programmed desorption (TPD) results in panel A of Figure 1. We attribute the peak at 135 K to desorption of bromomethane from the (1 11) terraces and the higher temperature shoulder extending up to 215 K to desorption from surface imperfections. The assignment of the higher temperature peak to surface defects is based on previous studies in which we found that an analogous high-temperature peak for desorption of alkenes from copper surfaces can be enhanced by sputtering the surface to increase the number of surface defects." The higher temperature for bromomethane desorption from these surface defects implies a larger heat of adsorption. The observation that this "defect peak" grows fmt as a function of exposure indicates that bromomethane can diffuse to these sites prior to desorption. At the adsorption temperature of 120 K,multilayers of bromomethane were not condensed on the surface, and there is even a measurable desorption rate for the monolayer, as evidenced by the nonzero intercept for the 7.5-langmuir TPD curve in Figure 1A. This conclusion is consistent with studies on Pt( 11l), in which desorption of bromomethane multilayers occurred with a TPD peak temperature of 126 K.3 The monolayer desorption peak temperatures of 165 K on Pt( 111) and 135 K on Cu(ll1) correspond (assuming first-order preexponential factors of 1013s-I) to heats of adsorption of 9.7 and 8.1 kcal/mol, respectively. The
Carbon-Halogen Bond Dissociation
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8531 Cu(ll1) I C2H5Br 160 K
1
120 K
> c
8L
+
135K I
235
(A) CH,Br Adsorption CH3Br Desorptlon
I
K
mle=27
I
I
I
I
I
150
200
250
300
TEMPERATURE (K)
TEMPERATURE (K)
Figure 1. (A) Temperature-programmedmolecular desorption spectra for CH3Br from Cu(ll1). The CH,Br desorbing at 135 K comes from the (1 11) terraces; the higher temperature shoulder extending to 225 K reflects desorption from surface defects. No multilayen condense at the adsorption temperature (120 K). (B) Specular high-resolution electron energy loss (HREEL) spectrum of CH,Br after a 5-langmuir exposure on Cu(111) at 120 K to form a monolayer. The observation of both the symmetric and antisymmetric methyl deformation modes (1270 and 1415 cm-I, respectively) in the specular HREEL spectrum indicates that the adsorption symmetry for CH,Br is less than CjO.
CH3Br HREELS spectra on Pt( 1 1 1) and Cu( 1 1 1) are also similar. In both cases, there is evidence for an interaction between the C-Br bond and the surface based on a red shift in the C-Br stretching frequency from 61 1 cm-I in the gas phase to 565-570 cm-I on the surface. The Cu( 1 1 1) HREELS spectrum is shown in panel B of Figure 1. Except for the C-Br stretching mode at 570 cm-', the observed vibrational frequencies are essentially the same as thase for gas-phase CH3Br,I3which indicate that CH3Brinteracts with the surface primarily through the halogen end of the molecule. The fact that both the symmetric and antisymmetric methyl bending modes at 1270 and 14 1 5 are ohserved in the specular HREELS spectrum indicates that the adsorption symmetry is less than C3, and therefore that the C-Br bond is inclined from the surface normal. This interpretation is consistent with those made for CH3Br/Pt( 1 1 1)3 and CH3C1/Cu(1 lO)I4 based on HREELS studies. Reflection absorption infrared studies of CH3Br/Pt( 11 1) indicate that,for CoVefaBesabove half a monolayer, the molecules stand upright on the surface with their C-Br bond along the surface n0rma1.I~ &o"e. The temperatureprogrammed desorption results for m / e = 110 in Figure 2 show that the majority of bromoethane desorbs from Cu( 11 1) molecularly intact. We attribute the 120 K peak for the 8-langmuir exposure to desorption from the multilayer, the 160 K peak to desorption from the monolayer, and the shoulder at higher temperature to desorption from surface defects. Some decomposition at low coverage is evident from the comparison at the bottom of Figure 2 of the m / e = 27 and 110 TPR/TPD spectra for exposures 0.5 and 0.4 langmuir, respectively. If only molecular desorption occurs, we expect these two ions to have the same peak profiles because m / e = 27 is a cracking fragment of the bromoethane molecular ion ( m / e = 110). The increased relative intensity of m / e = 27 at 235 K for low exposures of bromoethane is evidence for dissociation, presumably at defect sites. The decomposition product that gives rise to m / e = 27 has been identified from the cracking pattern as ethylene. The fraction of bromoethane decomposing to ethylene was quantified by plotting the area of the molecular desorption peak as a function of exposure; the extrapolated peak area goes to zero at an exposure of 0.2L, 5% of the exposure required to achieve a monolayer.16 Assuming coverage-independent kinetic parameters and a preexponential
Figure 2. Temperatureprogrammeddesorption and reaction spectra for C2H5Bradsorbed on Cu( 111). The peaks at 120 and 160 K for m/e = 110 reflect molecular desorption from the multilayer and monolayer, respectively. The trace for m / e = 27 indicates that a small amount of C2H5Brdecomposes at surface defects to form ethylene. Cu(l11)
/CZHSB~
755 I
fl
tu0
I
I\ 0
(A) 15 L, 115
U 1000
i
(6) 4 L, 120 K 1-
2000
-
K
WI 3000
ENERGY LOSS (cm-1) Figure 3. Specular HREEL spectra of C2H,Br on Cu( 111) after (A) a 15-langmuirexposure to form multilayers and (B) a 44angmuir to form a monolayer. The intense peaks for the CH2rocking mode at 755 cm-I
and for the out-of-plane CH,rocking mode at 995 cm-'indicate that the Br-CC plane is oriented approximately parallel to the surface.
factor of lOI3 s-l, the 25 K higher peak temperature for bromoethane desorption relative to that for bromomethane (160vs 135 K) corresponds to an increase of 1.25 kcal/mol in the bamer for desorption. This result suggests that the alkyl chain lies parallel to the surface plane, an inference supported by the HREELS spectra presented below. HREELS spectra of multilayer and submonolayer C2H5Brare shown in parts A and B of Figure 3 respectively. Overall, these spectra are quite similar, and they are assigned in Table I by comparison with those for C2H5Br(g). The most significant differences between the multilayer and monolayer spectra are the shift of the C-Br stretching frequency from 600 cm-' in the multilayer to 540 c m - I in the monolayer, the increase in the relative intensity of the 755-cm-] peak, and the peak at 1015 cm-I in the multilayer as opposed to 995 cm-I in the monolayer. These spectral differences can be interpreted as follows. The 45-cm-' red-shift in the C-Br stretching frequency in the monolayer is analogous to that observed for bromomethane and indicates an interaction between the C-Br bond and the surface. The increased intensity at 755 cm-'(a CH2rocking mode), the increased intensity at 995 cm-l (an out-of-plane CH3 rocking mode), and the de-
Lin and Bent
8532 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 TABLE I: Comparison of the Vibrational Frequencies in the Fingerprint Region for Bromoethane Adsorbed on Cu(ll1) witb Tbose for Bromoetbaw in the Cas Phase vibrational freuuencv.“ cm-’ symmetry multilayer/ monolayer/ valence coordinate type in C, Cu( 11 1)b Cu(11 1)C gas” mode description“ symmetry a’ CH2 scissor 1435 s 1440 s 1451 m CH, d-deform a’ CH, d-deform a” 1386 m a’ CH, s-deform 1245 m 1245 m 1252 vs a’ CH2 wag 1248 calc CH2 twist a’’ 1060 vw 1015 m a’ CH3 rock 964 s a’ C C stretch 995 m CH, rock a’’ 755 m 775 s 770 m CH2 rock a’’ 540 m 583 vs 600 m C-Br stretch a’ ~~
“Reference 13. 15 langmuirs of C2H5Brat 115 K; this work. c 4 langmuirs of C2H5Brat 120 K; this work. d~ = weak; m = medium; s = strong; v = very; calc = calculated.
creased relative intensity at 1015 cm-’ (an in-plane CH3 rocking mode) all suggest that the Br-C-C plane lies approximately parallel to the surface. This adsorption geometry is shown adjacent to the spectrum in Figure 3A. 1 - B m m p ” While the majority of adsorbed bromcethane desorbs molecularly intact from Cu(l1 l), 1-bromopropanepredominantly dissociates. This fact is readily evidenced by the lack of molecular desorption at low exposures.I6 It is also evident in the HREELS spectra in Figure 4. The spectra in parts A and B are for a multilayer and submonolayer of 1-bromopropaneat 120 K, while the spectrum in part C is attributable to propyl groups formed by C-Br dissociation. The spectrum in part D for a submonolayer of propylene on Cu( 11 1) at 120 K is shown for comparison because propylene is the predominant decomposition product that evolves from the surface (see below). We discuss first the submonolayer and multilayer spectra of 1-bromopropanein Figure 4A,B. The observed vibrational frequencies are compared in Table I1 with those measured by infrared spectroscopy for 1-bromopropane in the gas phase. The approximate valence coordinate descriptions given for the normal modes are based on normal coordinate calculation^.^^ Note that two molecular conformations with distinct vibrational frequencies are possible. These conformations differ by rotation about the CI-C2 bond; i.e., the methyl group is either next to (gauche) or away from (trans) the bromine atom. The most significant difference between these conformers in the infrared is the C-Br stretching frequency, which occurs at 562 cm-’ for the gauche and 644 cm-’ for the trans conformer. Based on the 575-cm-l peak for 1-bromopropane in Figure 4A, we can conclude that gauche conformers are present in the multilayer. This observation suggests an amorphous layer since both trans and gauche con-
(C) n-CaH7Br 4 L, 195 K
757 p65
1
770 (A)
i
n-C3H7Br 15 L, 120 K
multilayerr
m
0
1000
2000
3000
ENERGY LOSS (cm-1)
Figure 4. Specular HREEL spectra of Cu( 11 1) after exposures of (A) 15 langmuirs of l-C3H7Brat 120 K to form a multilayer, (B) 4.5 langmuirs at 120 K to form a monolayer, (C) 4 langmuirs at 195 K to form C,H,(,, and Br(,), and (D) 4 langmuirs of C3Hs (propylene) at 120 K. The relative intensities of the vibrational modes for the adsorbed species suggest the adsorption geometries shown adjacent to each spectrum.
formers are present in liquid phase 1-bromopropane but only the trans conformer exists in the crystalline solid.” For a monolayer of 1-bromopropaneon Cu( 111) (Figure 4B), the C-Br stretch is at 595 cm-I. This frequency is -30 cm-’ higher than that for the gauche conformation and -50 cm-l below that for the trans. We assign this peak to a red-shifted trans conformation, consistent with the C-Br mode softening observed for bromomethane and bromoethane. The absence of a detectable peak at or below the gauche frequency of 563 cm-’implies that, analogous to crystalline 1-bromopropane, the trans conformation predominates in the monolayer. The conclusion that 1-bromopropane adsorbs as the trans conformer on Cu(ll1) is supported by the CHI rocking mode frequencies. As shown in Table 11, the CH2 rocking modes are split by 60 cm-I in the gauche conformation and by 110 cm-’ in the trans. It is reasonable, therefore, that these modes are un-
TABLE 11: Comparison of the Vibrational Frequencies in the Fingerprint Region for 1-Bromopropane Adsorbed on Cu(ll1) with Those for 1-Bromoprop.ne in the Cas Phase vibrational frequency: cm-’ symmetry gas phase“ valence coordinate type in C, multilayer/ monolayer/ ProPYlI mode description“ symmetry trans gauche Cu( 11 1)b Cu(ll1)c Cu( 1 1l ) d CHI d-deform 1435-1464 1435-1 464 1455 m 1460 m CH2 scissor CH, s-deform a‘ 1368 1382 1385 s CH2 wag 1213-1 330 1200-1 343 1285 sh 1240 vw 1165 w CH2 twist CH3 rock + skeletal stretch a‘ 1099 1082 1055 w 1050 w 1055 w CH; twist a” 1041 1065 skeletal stretch a‘ 1025 1032 skeletal stretch + CHI rock a’ 897 888 880 s 840 865 m CH2/ rock + CH, rock a” 850 778 770 s 755 m 720 vw CH2’ rock + CH2 rock a’‘ 740 595 sh a’ 648 563 575 sh C-Br stretch “Approximate mode description for trans conformer; ’ denotes carbon 1; from ref 17. 15 langmuirs at 120 K; this work. c4.5 langmuirs at 120 K; this work. d 4 langmuirs at 195 K this work. c w = weak; m = medium; s = strong; v = very; sh = shoulder.
Carbon-Halogen Bond Dissociation
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8533
(A) C3H,Br
E
5 3
I
220 K
I
( B ) C4H9Br 4L
'
I I
A
Dentene
I
150
a
iii a
-* a
--
fz
(C) CSHIIBr
210 K
> a a
W I-
I
1
I
I
I
I
200
250
300
350
400
(A) 15 L, 115 K
I(w
Il?
TEMPERATURE (K)
Figure 5. Temperatureprogrammed reaction spectra from Cu(111) after the indicated exposures of (A) l-C,H7Br, (B) 1-C4H9Br,and (C) 1CJHllBrto produce propylene, butene, and pentene, respectively. These
alkenes are formed by &hydride elimination from the alkyl groups generated by C-Br bond scission in the alkyl bromides.
resolved at 770 cm-l for the gauche conformation in the multilayer but partially resolved at 755 and 865 cm-' for the trans conformation in the monolayer. Because these CH2 rocking modes are totally symmetric in Cl symmetry but not C,,their observation in the specular HREELS spectrum implies, within the dipole scattering approximation,I8that the adsorption site symmetry is CI as opposed to C,. In addition, because the specular HREELS scattering intensity scales with the component of the dynamic dipole moment normal to the surface, the relatively large intensities of these rocking modes suggest that the plane of the alkyl chain lies parallel to the surface as shown schematically in Figure 4. When submonolayers of 1-bromopropaneare heated above 180 K, the C-Br bond dissociates to form propyl groups and bromine atoms. The HREELS spectrum for adsorbed propyl groups is shown in Figure 4C. Carbon-bromine bond dissociation is inferred based on the disappearance of the C-Br stretching frequency in the HREELS spectrum and the concomitant increase in the surface work function (see below), despite the absence of any desorption products over the same temperature range. C-H bond scission to form propylene (the eventual gas-phase product, see below) has not occurred on the basis of the significantly different HREELS spectrum in Figure 4D for the adsorbed propylene (the decomposition product). The vibrational frequencies for adsorbed propyl are assigned in Table I1 by comparison with those for 1-bromopropane. The most significant difference in the spectrum for adsorbed propyl groups is the softened C-H stretching mode frequency at 2740 cm-I. Deuterium isotope labeling studies reported elsewhere establish that this mode-softening is attributable to the C-H bonds on the rvcarbon (the one bonded to the surface) and that mode softening probably occurs as a result of charge donation from the metal to the C-H antibonding orbital localized on the a-carbon.19 The relative intensities of the methyl deformation modes at 1300-1450 cm-' suggest that the propyl groups stand upright on the surface with the C3axis of the methyl group oriented approximately along the surface normal. Note in particular the intense 6,(CH3) mode at 1385 cm-' and the relatively weak shoulder >1400 em-' for the antisymmetric methyl deformation and CH2 scissor modes. Because all of these modes are dipole active in the adsorption site symmetry (C,or CJ,the large relative intensity for the symmetric methyl deformation implicates a vertical orientation for the methyl groups. Consistent with this interpretation, the intense 880-cm-' peak is assigned to a skeletal stretching mode that would also have a dynamic dipole moment normal to the surface. Propyl groups are thermally stable on Cu( 11 1) up to 200 K. Above this temperature, they decompose to produce primarily
y
am8
I1
-
I \
0
1000
2000
9000
ENERGY LOSS (cm-1)
Figure 6. Specular HREEL spectra of Cu(111) after exposure to 1C4H9Br:(A) 15 langmuirs at 120 K to form a multilayer, (B) 4 langmuirs at 120 K to form a monolayer, and (C) 4 langmuirs at 120 K followed by annealing to 190 K to form C4H9(.)and Br(,). The carbon chains shown in the sketches adjacent to the figure merely identify the surface species; the molecular orientations are yet to be determined.
-
propylene (peak temperature 2 15 K, Figure SA) and adsorbed hydrogen atoms. Some propane is also evolved at 215 K for high coverages. The adsorbed hydrogen atoms that are not consumed in forming propane recombine and desorb at 320 K, the reported temperature for hydrogen recombination from Cu(l1 1).20 This decomposition chemistry is the same as that reported previously for 1-iodopropane on Cu( 1lo)?' Extensive mechanistic studies for that system have established that propylene is formed by cleavage for the &CH bond in adsorbed propyl groups, C-H bond scission is the rate-determining step in propylene evolution at 220 K, and the smaller peak for propylene evolution at 230-250 K is due to ratedetermining desorption of propylene from surface defects. We presume that the same reactions operate in forming propylene from propyl groups on Cu( 111). C4 and Cs Alkyl Bromides. Like 1-bromopropane, the straight-chain C4 and Csalkyl bromides dissociatively adsorb on Cu( 111) at temperatures above 180 K to form butyl and pentyl groups, respectively. HREELS spectra for the multilayer, monolayer, and dissociatively adsorbed alkyl groups of the C4 and C5 bromides are shown in Figures 6 and 7. Although the resolution of HREELS does not permit a detailed assignment of the vibrational peaks in these complex spectra, a comparison of the peak frequencies with those for the gas-phase molecules provides some information about the conformation and orientation of the adsorbed molecules. Specifically, the C-Br stretching mode at 555 cm-l in the C4 multilayer spectrum of Figure 6A indicates that a significant fraction of the molecules have a gauche conformation about the C I X 2bond; for comparison, the gas-phase frequency is 562 cm-l for the gauche conformation vs 643 cm-' for the trans conformation." The disappearance of this peak in Figure 6B suggests that the C4 bromide adopts a trans conformation about this bond in the monolayer. In addition, the relatively large intensity at 730 cm-' for CH2 rocking suggests that the hydrocarbon chain lies parallel to the surface. For the C5 bromide in Figure 7, the large relative intensity for the CH2 rocking mode at 725 cm-' in the monolayer suggests a flat-lying orientation; however, further experiments are necessary to confirm this inference. The spectra in Figures 6C and 7C for the C4 and Cs bromide monolayers after annealing to 190 and 180 K, respectively, are
Lin and Bent
8534 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
C-Br Bond Scission on Cu(ll1)
r
C u ( l l 1 ) / 1-CgHi1 Br
(c) 4
-
f
L, 120 K 4 180 K
w
0 0
(A) C&Br
1455
2.5 L
( 8 ) 4 L, 120 K 725
Br.CH2CHI.CHrCH2.CH,
777m7" (A) 30L, 120 K
I
I
' L -
0
1
1000
2000
3000
ENERGY LOSS (em-') Figure 7. Specular HREEL spectra of Cu(ll1) after exposure to 1C5HIIBr: (A) 30 langmuirs at 115 K to form a multilayer, (B) 4 langmuirs at 120 K to form a monolayer, and (C) 4 langmuirs at 120 K followed by annealing to 180 K to form C5Hll(r)and Br(a). The carbon chains shown in the sketches adjacent to the figure merely identify the surface species; the molecular orientations are yet to be determined.
attributed to alkyl groups. Dissociation of the C-Br bond is implicated for the same reasons discussed above for propyl bromide. The evidence against C-H bond scission comes from the temperature-programmed reaction studies shown in Figure 5B,C. The primary hydrocarbon product for both the C4 and C5 bromides is the componding olefin; as shown, butene and pentene are evolved at -220 K. Since both of these products desorb below 200 K when they are adsorbed separately onto Cu(l1 l), we can infer that their evolution at 220 K involves rate-determining decompositionof an adsorbed species. (Note, however, the small, higher temperature shoulders in each of the TPR of Figure 5. These peaks, as discussed elsewhere, are due to rate-determining desorption of olefins strongly bound at surface defects.ll In addition, rate-determining olefin desorption would presumably show a significant chain-length dependence. The logical conclusion, which is supported by studies on Cu(1 lo)," is that olefin evolution results from rate-determining &hydride elimination by surface alkyl groups.) C-Br Mssoeition Kinetics. The HREELS studies described above indicate that C-X bond dissociation for the C3-C5alkyl bromides on Cu( 1 11) occurs at 160-180 K. Consistent with this result, the surface work function for these alkyl bromide monolayers changes over the same temperature range, as illustrated for bromopropane in panel A of Figure 8. In both the HREELS and work-function studies, however, the surface is heated at 2.5 K/s to a desired temperature and then quenched to 120 K to record the surface vibrational spectrum or to measure the change in the surface work function. Such anneal and quench experiments are only approximate measures of the dissociation kinetics unless the heating and cooling sequence is extremely carefully controlled and analyzed. We have therefore utilized the elastic electron reflectivity from the surface22as a probe of the integrity of the C-Br bond during the heating ramp. In these experiments the HREELS spectrometer was used to measure the specular, elastic electron scattering intensity as a function of surface temperature. Although the scattering intensity is affected by the electric fields present when heating the sample, the smooth decrease in the elastic intensity because of these effects can be readily discriminated from
115
188
nr
zw
31s
TEMPERATURE (K)
Figure 8. (A) Surface work-function change as a function of annealing temperature for a 2.5-langmuir exposure of 1-C3H7Brto Cu(111) at 120 K, (B), (C) elastic electron scattering in the specular direction as a function of surface temperature as the Cu(ll1) surface is heated at 2.5 K/s after 4-langmuir exposures of 1-C3H7Br and 1-C,H9Br. The inflection points at 180 and 225 K reflect C-Br bond dissociation and decomposition of the surface alkyl groups, respectively.
the added effect of C-Br dissociation. The results for the C3 and C4 bromide are compared in panels B and C of Figure 8. In both cases, the inflection point in the elastic reflectivity due to C-Br dissociation is at 180 K. (The additional intensity drop at -225 K in Figure 8B,C as well as the 0.2 eV work function increase at this same temperature in Figure 8A correlates with alkyl decomposition and evolution of the alkene product from the surface.) This result indicates that, to within the resolution of a TPR-type experiment, the C-Br dissociation rate is u~ffectedby the increase in alkyl chain length. 3.2. Alkyl Iodidea Iodomethme. Unlike bromomethane, which desorbs molecularly intact from Cu(11l), iodomethane dissociates to form adsorbed methyl groups and iodine atoms. The bonding and chemistry of these methyl groups are discussed in detail elsewhere.23 The major conclusions are (1) C-I bond scission to form methyl groups occurs below 200 K,(2) at high coverages, methyl groups bind with C3, symmetry, (3) methyl groups are thermally stable up to 400 K, and (4) C-H bond scission to form CHZla)and H,,, is the rate-determining step in methyl decomposition above 400 K.23 C2 and C3 M y 1 Iodides. Iodoethane and iodopropane are dissociatively adsorbed at 120 K on Cu( 111). The result is evidenced by the HREELS spectra for monolayer exposures in Figure 9. The spectrum in Figure 9B for 1-iodopropane is virtually identical with that in Figure 4C for propyl groups formed by dissociatively adsorbing 1-bromopropaneat 195 K. We attribute the s p e c " in Figure 9A for iodoethane to adsorbed ethyl groups based on the disappearance of the C-I stretch and on the The remaining softened C-H stretching frequency at 2740 peaks in the ethyl spcctrum can be assigned by comparison with those for gas-phase CzH51.'3 The tilted configuration shown for the CH3 group in adsorbed ethyl and the upright orientation for CH3 in propyl is based on the relative intensities of the symmetric and antisymmetric methyl deformation modes. As with the alkyl bromides, alkyl groups formed by dissociation of the alkyl iodides decompose by 8-hydride elimination at tem-
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8535
Carbon-Halogen Bond Dissociation
[A) CzHsI 4 L, 120 K
I
o.o+
I
Cu(li1)
1
810
-0.84
, 120
,
I
,
140
180
1 BO
I
TEMPERATURE (K)
Figure 11. Surface work function change as a function of annealing temperature for (A) 3 langmuirs of CHJ and (B) 2.5 langmuirs of l-C3H,I on Cu( 111). The approximately constant value of -0.55 eV for 1-C3H71reflects the fact that the carbon-iodinebond is dissociated at the adsorption temperature of 120 K.
274s
0
1000
2000
3000
ENERGY LOSS (cm-1) Figure 9. Specular HREEL spectra of Cu( 111) after dissociation adsorption of (A) 4 langmuirs of C2HJ at 120 K to form C2HI(I)and I(a) and (B) 4 langmuirs of 1-C3H71at 120 K to form C3H1(a)and I(a).
[:j/-
cd100)/ 2 L Exposures
of n-Alkyl Chlorides p
-
13
Y
11
.. 250
z
K
i
i
s
Chloroheptane
Chloropentane Chlorobutane
ethylene
i
7
HO. or CARBONS
de-91
de-91
m/e=63
Chloropropane d e - 7 8 150
200
250
300
350
400
TEMPERATURE (K)
Figure 10. Temperature-programmedreaction spectra of (A) ethylene and (B) propylene from Cu(ll1) after the indicated exposures of C2HJ
and 1-C3H,I,respectively. peratures above 200 K. The olefin product evolution as a function of exposure is shown by the TPR spectra in Figure 10. Note, in particular, the similarity between the propylene TPR from iodopropane in Figure 10B and those for bromopropane in Figure 5A. The higher temperature for ethylene evolution from ethyl groups relative to those for propylene/butene evolution from propyl/butyl groups (247 vs -220 K) reflects a slower rate of @-hydrideelimination. This trend is typical of &hydride elimination rates at primary vs secondary carbons.’ C-I Bond Dissociation. Measurements of the surface work function change confim the conclusion from the HREELS studies above that C-I bond scission occurs at 140 K for iodomethane and upon adsorption at 120 K for the longer chain alkyl iodides on Cu( 11 1). These results are shown in Figure 11. The increase in the surface work function at 120-150 K for iodomethane is attributed to C-I bond scission, because this increase agrees with the disappearanceof the C-I stretching mode in HRJ2ELS23and no molecular desorption is observed for this exposure in TPD. By contrast, the surface work function for a 2.5-langmuir exposure of iodopropaneat 120 K is approximately constant (at -0.55 eV relative to the clean surface) up to 200 K. Although not shown here, the work function increases by 0.2 eV above 200 K as a result of the formation and evolution of propylene. This behavior is the same as that oherved in Figure 8A for propyl groups formed by
-
I
I
I
I
I
I
150
200
250
300
350
400
TEMPERATURE (K) Figure 12. Temperature-programmedmolecular desorption spectra for a series of straight-chainalkyl chlorides (C3C,) on Cu( 111). The inset
shows that the desorption activation energy, assuming first-order kinetics and preexponential factors of 10))sd, increases with carbon chain length by 1.3 f 0.2 (kcal/mol)/CHz group. dissociation of 1-bromopropane at 180 K. We conclude that 1-iodopropanedissociates at temperatures below 120 K to form adsorbed propyl groups and iodine atoms. 3.3. Alkyl chlorides. Unlike the alkyl iodides and bromides, studied on Cu( 11I), C-Cl bond dissociation in the alkyl chlorides was studied on Cu( 100). On the basis of comparative studies of bromopropane dissociation on Cu( 111) and Cu( loo), the effect of the more open (100) structure is to lower the carbon-halogen dissociation temperature by -20 K.24 For the alkyl chlorides on Cu(100).we find that C-Cl dissociation does not OcCuT until the chain length reaches seveal carbons. Molecular desorption for the straight-chain alkyl chlorides is shown by the TPD spectra in Figure 12. As shown in the inset, the increase in the molecular desorption temperature with alkyl chain length corresponds (assuming first-order preexponential factors of 10” s-’) to an increase in the heat of adsorption of 1.3 f 0.2 (kcal/mol)jCH2 group. When the alkyl chain length is increased beyond six carbons, dissociation and molecular desorption occur concurrently. A small amount of the C6chloride dissociates, presumably at surface defects. The 1chloroheptane decomposition product is heptene, as confirmed by monitoring the daughter ions at m / e = 27, 41,
:- --A n--& Lin anu mnr T
8536 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
TABLE IIk C o q " between C a r b o o - ~ e aBond Dlssochtioa Energies h tbe Gas PBUe a d on Cu(ll1)
Cu(100)/ I-Chloroheptane, 5 L
n-alkvl halide(s) .. C 2 C s iodides CH~I C l C 5 bromides C7 chlorided
I
I
I
I
I
150
200
250
300
350
54
56 68 81
EJBE
