9713
J. Phys. Chem. 1993,97, 9713-9718
Two Mechanisms for Formation of Methyl Radicals during the Thermal Decomposition of CHd on a Cu( 111) Surface Jong-Liang Lin and Brian E. Bent' Department of Chemistry, Columbia University, New York, New York 10027 Received: December 9, 1992; In Final Form: May 18, 1993'
When submonolayer quantities of iodomethane (CH31) are condensed on a Cu( 11 1) surface and the surface
is heated, methyl radicals are evolved at 140 and 470 K. These methyl radicals have been identified by mass spectrometry. The 140 K pathway for methyl radicals correlates with the temperature for carbon-iodine bond dissociation in CH31, and thermally-activated, dissociative electron transfer is shown to be a viable mechanism for reaction. The 470 K radical pathway results from homolysis of the Cu-CH3 bond for surface-bound methyl groups and corresponds to a Cu-CH3 bond strength of -29 kcal/mol. A competing reaction pathway at 470 K is methyl decomposition to produce methane, ethylene, and propylene. Coadsorbed iodine atoms promote methyl desorption over methyl decomposition, and the reaction kinetics suggest a site-blocking effect.
SCHEME I
1. Introduction A number of recent papers have reported the evolution of methyl radicals during the thermal decomposition of methyl-containing molecules on well-defined surfaces under vacuum conditions.14 A striking feature of the results is the different temperatures at which the methyl radicals are evolved. In the cases of Al(CH3)3 on Al,' CHSOHon oxygen-precoveredMo( 1 and Ga(CH3)3 on GaAs( loo),' methyl radical formation occurs at temperatures above 400 K. By contrast, for CH3Br on potassium-promoted Ag(lll), methyl radicals were detected at the adsorption temperature of 100 Ka4 These dramatically different temperatures for methyl radical ejection are indicative of different mechanisms. At high temperatures, the activation energies are on the order of the bond energies for the carbon-surface bonds broken. Radical ejection can thus be viewed as a simple bond fission reaction, as shown in Scheme I. Note, however, that decomposition of the parent molecule must occur at low temperature in order for the methylcontaining fragment to remain bound to the surface up to the high temperatures required for bond homolysis. For example, in the case of CH30H on Mo( 110)/0, 0-H bond scission occurs at 62 kcal/mol, respectively.
dramatically different temperatures (140 and 470 K) at which methyl radicals are detected during a TPR experiment. Surface vibrationalspectralOshowthat the 140K temperature for a radical evolution is the temperature at which C-I bond scission occurs to form adsorbed methyl groups. These adsorbed methyl groups remain intact until temperatures above 400 K, where they either desorb from the surface as methyl radicals or decompose and/or recombine to evolve methane (CH4), ethylene (C2H4), propylene (C3H6), and ethane (C2H6). The low- and high-temperature pathways for radical evolution are discussed separately below. 4.1. Methyl Radicals at 140 K. Assuming a standard firstorder preexponential factor of 10” s-l for methyl radical evolution at 140 K, the activation energy is 8.5 kcal/mol.15 Given the assumed value for the preexponential factor, the activationenergy hasanuncertaintyof f 3 kcal/mol. Whiletheactivationenergy is much less than the carbon-iodine bond energy of 56 kcal/mol, the reaction is thermodynamically allowed and kinetically facile because iodine bonds to the copper surface concurrently with C-I bond ~cission.~ A quantitative assessment of the reaction kinetics based on electron-transfer theory is presented below. Methyl ejection at 140 K during CH31decomposition on Cu(1 l l ) is formally analogous to the evolution of methyl radicals during the adsorption of CH3Br onto potassium-covered Ag(1 11) at 100 K.4 The surface temperature independence of this latter process led Zhou et al. to suggest that C-Br bond scission occurs by electron harpooning,16 Le., dissociative electron attachment via transfer of an electronfrom the Fermi level of K/Ag(1 11) to CH3Br.4 An analogous calculation for CH3I/Cu( 1 1 1) (see below) shows that electron harpooning is endothermic by -3 eV for physisorbed CH31. This result is not unexpected, because (1) methyl radical evolution on Cu(ll1) is thermally activated and (2) photochemical studies of alkyl halides on other metal surfaces with comparable work functions have shown that the threshold for photodissociation by dissociative electron attachment is 3-5 eV17J8depending on the adsorbate coverage. While electron harpooning cannot account for methyl evolution at 140 K during CH31 dissociation on Cu(l1 l), thermallyactiuated electron transfer from copper to CH31 is consistent with the measured activation energy. The energeticsfor electron transfer as a function of thermal excitation of the C-I bond can be approximated by combining a thermochemical cycle with harmonic potentialsconstrainedby the reaction thermodynamics. This approach is analogous to that developed by Marcus for electron-transfer reactions,*gand the application to CH3I/Cu(1 11) is described below. The energy for vertical electron transfer from the Fermi level of CH3I/Cu( 111) to adsorbed CHpI in its equilibrium geometry can be estimated from a thermochemical cycle. The difficulty is to choose a path that best accounts for the energy of electron transfer from the Fermi level into the dipole layer of the adsorbates using quantities that can be calculated or experimentally measured. The cycle we have chosen is shown in Figure 6. We take as our zero of energy the state with CH31adsorbed on Cu(1 l l ) . As a first step, an electron is transferred from the Fermi level to a position several hundred angstroms from the surface; the energy required is the work function for the adsorbate-covered surface, which is 4.3 eV for a 2.5-langmuir CH3I-covered Cu-
-
Figure 6. Thermochemical cycle for approximating the energy required for vertical electrontransfer from Cu( 111) to physisorbed CH31. Image charges within the metal are not explicitly shown in the labels for the
ionic states. Symbolsfor theenergydifferencesbetween states arcdefined in the text.
(1 11).8 Next, the electron is moved back into the adsorbed layer but not onto the adsorbate. The energy of the system decreases due to (1) the image charge stabilization, which is -1.2 eV assuming that the charge is 3 A from the image plane;” (2) the dielectric response of the surrounding adsorbed layer, which accounts for -0.6 eV for a monolayer of CH31;17 and (3) penetration into the surfacedipolelayer, which is ignored in zeroth order. (By using the work function for the adsorbed layer as opposed to the clean surface, we have at least partially accounted for this latter effect.) Finally, we can approximate the energy required to attach the electronto physisorbed CH31as thevertical electron affinity in the gas phase, VEA[CHJ(g)], which is -0.6 eV for the vibrational ground state.20 Note that the change in the affinity level of CH31as a result of its proximity to the surface21 has already been largely accounted for by considering the image charge and polarization stabilization of the electron. The energy required for dissociative electron transfer, ED,is thus given by
This equation is the same as that derived by Ukraintsev et al. using a similar thermodynamic ~ y c l e . 1Application ~ of thevalues given above yields 3.1 eV as the energy for vertical electron transfer from Cu( 111) to CH31. Although the calculated energy for dissociative electron attachment is much larger than the activation energy for C-I bond scission (3.1 eV vs. -0.37 eV), the energy required for electron transfer is strongly dependent on the C-I internuclear separation, and thermally-activated electron transfer is possible. This point is made quantitatively in Figure 7, which plots the energy for the CH3I/Cu and CH3I-/Cu systems as a function of C-I separation. The CH3I/Cu curve is a harmonic potential defined by the gas-phase bond energy of 2.4 eV.6 Since bonding of the iodine to the surface is neglected, this approximation provides an upper limit to the energy of the system. For CH3I-/Cu, we approximate the repulsive C-I potential as a harmonic function defined by the energy for vertical electron transfer (3.1 eV) and the enthalpy change for the reaction (which has an upper limit of 0.17 eV7). The reaction coordinate d(C-I) is taken as 1.6 A, which correspondsto the loss of the iodine and copper van der Waals radii during conversion of physisorbed CH31 into Cu-I and CH3 radical, as shown at the bottom of Figure 7.22 At the pointwhere theCH31/CuandCH31-/Cucurvesintersect in Figure 7, it is energetically neutral for an electron from the
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9717
Mechanisms for Methyl Radical Formation
f
I
3
0.8
m
* CH
DISPLACEMENT FROM C - I EQUUBRNM DISTANCE (A)
n
-Constant
1.6
M
A
Figure 7. Potential energy curves for the Cu( 111)/CHJ and Cu( 111)/ CH& systems as a function of C-I internuclear separation. The curves were constructed as described in the text. On the basis of the curve crossing at 0.7 eV and the measured barrier of -0.37 eV for C-I bond dissociation on Cu( 11 l), thermally-activated electron transfer appears to be a plausible mechanism.
metal to tunnelonto thermally-excited CH31. The thermal energy required to achieve this configuration is 0.7 eV. While the experimental value is about 0.3 eV less than that calculated, several approximations in the model should be noted. For example, iodine bonding to the surface has been neglected in constructing the neutral CH3I/Cu( 111) curve in Figure 7. Including this contribution would lower the crossing point. A related issue is the nuclear configuration at the curve-crossing point, which will differ somewhat for the neutral and ionic states as drawn. It should also be noted that provisions for partial electrontransfer throughout the reaction have not been accounted for, an effect that could also lower thecalculated barrier. Despite these deficiencies, we believe that the curve-crossing picture in Figure 7 captures the essence of thermally-activated electron transfer and shows that such a process is a viable mechanism for dissociation of the C-I bond in CH31. Such a process is to be contrasted with that for dissociative electron attachment by photochemical excitation of substrate electrons or by electron harpooning. In the latter, the surface work function must be sufficiently small that vertical electron transfer is energetically allowed,while in the former, sufficient photon energy is required to excite an electron from Cu to the average adsorbed CH31 molecule, which will bein its ground state. In thermally-activated electron transfer, only a small fraction of the adsorbed molecules (exp(-E/RT) = 6.5 X 10-14) for E = 8.5 kcal/mol and T = 140 K have sufficient energy for thermoneutral transfer of an electron from the Fermi level, but the attempt frequency (the Arrhenius prefactor) is 1013 s-1, so a measurable rate is achieved. 4.2. MethylRadicals at 470 K. The evolution of methyl radicals at 470 K upon heating submonolayersof CH31on Cu( 111) is due to homolytic (nonionic) cleavage of the Cu-CH3 bond. Assuming a first-order preexponential factor of 1013 s-1 for this process, the activation energy is 29 kcal/mol.ls This value is consistent with that of 34 f 6 kcal/mol determined previously for the C U - C ~ H ~ bond energy on a Cu(100) surface using a thermodynamic cycle.'l,23 The fact that the CH3-Cu bond energy is substantially smaller than that of 58 f 2 kcal/mol for Cu-CH3 in the gas phase24 may reflect, as indicated in the Introduction, increased
-
bonding within the surface upon methyl desorption, an effect that is not possible for an isolated copper atom. Similar explanations have been advanced previously to account for unexpectedly small methyl/surface bond energies.125 The metalcarbon energies obtained for CH,/Pt( 111)*6 by using a thermodynamic cycle and for CH3/Ni( 11l)*' by theoretical calculations are 30 and 39 kcal/mol, respectively. In the CH3I/Cu(lll) system, the potential effect of the coadsorbed iodine atom on Cu-CH3 bond energy must also be considered. From the results in Figure 4, it is clear that coadsorbed iodine has a dramatic effect on the branching between methyl decomposition and methyl desorption. The probable explanation is that coadsorbed iodine atoms block defect sites on the surface. Previous results on Cu( 110) have led us to suggest that defect sites favor methyl decomposition to methane, ethylene, and propylene,lI and studies of methyl radical adsorption on Cu(111) in the absence of iodine support this conclusion.28 Once these defect sites are blocked by iodine atoms (01= 0.015-0.03 in the studies here), methyl groups must bind to terrace sites where the desorptiondominates. It appears to be fortuitous that the methyl desorption and decomposition kinetics on Cu( 111) are so similar. 4.3. Methyl Radicals from Other Surfaces. Methyl evolution during thermal decomposition of CH31 on copper is unique in that it occurs by two mechanisms with very different kinetics. The activation energy for methyl ejection at 140 K during C-I bond scission is -8.5 kcal/mol, and the barrier for methyl desorption at 470 K by homolytic copper-carbon bond scission is -29 kcal/mol. While the CH3Br/K/Ag( 111) system shows similar low-temperature radical formation, the methyl groups that are trapped on the surface do not desorb as radical^.^ To our knowledge, copper is the only metal surface on which methyl groups are thermally stable to above 400 K. On Pt( 11l)?9 Pd(lOO),30 Fe(lOO),3I and Ni(lOO),32 methyl groups decompose by C-H bond scission at temperatures below 300 K. On Ag( 11 1)" and Au( 11l)?4 CH3 combines to form C2H6 at temperatures below 300 K. The lackof prior reports of methyl ejectionat low temperatures during methyl halide decomposition on transition-metal surfaces probably reflects lack of investigation as opposed to lack of reaction. On the basis of the analysis in section 4.1, methylejection during C-I bond scission is expected to be both thermodynamically and kinetically allowed on most transition-metal surfaces. In solution, radical formation during the reaction of alkyl halides with metal compounds and surfaces has been extensively doc: umented.35 The mechanism of these radical-forming reactions, however, remains in question. In most systems, the ionization potential (work function) of the metal center is sufficiently large that electron harpooning is not possible, and the reaction must involve thermally-activated electron and/or atom transfer. The parameters that determine the relative importance of these two pathways are the metal-halogen bond strength and the ionization potential (work function) of the metal-halogen system. Strong metal-halogen bonds favor atom transfer; low metal-halogen work functionsfavor electrontransfer. Provided these two factors do not show the same trend with variation of the metal, it should be possible to discriminate the relative importance of the atomand electron-transfer mechanisms by further studies of CH31 decomposition on atomically-clean transition-metalsurfacesunder ultra-high-vacuum conditions. 5. Conclusions
We have studied the thermal chemistry of submonolayer quantities of CH31 condensed on a Cu( 111) surface at 110 K under ultrahigh vacuum. CH31 decomposes at 140 K on the surface to eject methyl radicals into gas phase and to generate surface-trapped methyl groups and iodine atoms. The activation energy is -8.5 kcal/mol for this chemical process, assuming a preexponential factor of 1013s-l and first-order kinetics. When
9718 The Journal of Physical Chemistry, Vol. 97, No. 38, I993
the surface is heated up to 470 K, the surface methyl groups desorb into the gas phase and/or undergo C-H bond activation to give methane, ethylene, and propylene. Methyl ejection at 470 K demonstrates that the binding energy for methyl groups on Cu( 1 11) is -29 kcal/mol. The dramatic effect of coadsorbed iodine atoms in favoring methyl radical ejection is attributed to site blocking that inhibits C-H bond scission in adsorbed CH3.
Acknowledgment. Financial support from the National Science Foundation (CHE-90-22077) and the A.P. Sloan Foundation is gratefully acknowledged. We thank I. Harrison, R. M. Osgood, Q. Y. Yang, S.Jo, J. C. Tully, A. Gellman, V. Grassian, and F. Zaera for fruitful discussions in connection with various aspects of this work. References and Notes (1) Squire, D. W.; Dulcey, C. S.;Lin, M. C. Chem. Phys. Lett. 1985,116, 525.
(2) Serafin, J. G.; Friend, C. M. J . Am. Chem. SOC.1989, 111, 8967. (3) Creighton, J. R. Sur/. Sci. 1990, 234, 287. (4) Zhou, X.-L.; Coon, S.R.: White, J. M. J . Chem. Phys. 1991, 94, 1613. . (5) Jacko, M. G.; Price, S.J. W. Can. J . Chem. 1963,41, 1560. Long, L. H. Pure Appl. Chem. 1961, 2.61. (6) Sanderson, R. T. Chemical Bonds and Bond Energy, 2nd ed.; Academic: New York, 1976. (7) Lin, J.-L.; Bent, B. E. J. Am. Chem. SOC.1993, 115, 2849. (8) Lin, J.-L.; Bent, B. E. J. Phys. Chem., 1992, 96, 8529. (9) CO and HzO desorb by 190 K on a Cu(ll1) surface. (Lin and Bent, unpublished results). (10) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Technol. A 1992, 10, 2202. (11) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J . Phys. Chem. 1992, 96, 1836. (12) Xi, M.; Bent, B. E.Surf.Sci. 1992, 278, 19. (13) I. Harrison and co-workers have recently reported that angular distributions for methane formed by CHJ + H on Pt(ll1) are sharply peaked along thesurface normal. (American Chemical Society Meeting, Washington, DC, 1992).
Lin and Bent (14) Baiocchi, F. A.; Wetzel, R. C.; Freund, R. S.Phys. Rev. Lett. 1984, 53,771. Orient, 0.J.; Srivastava, S.K. J . Phys. B At Mol. Phys. 1987,20, 3923. Chatham, H.; Hils, D.; Robertson, R.; Gallagher, A. J. J. Phys. Chem. 1984, 81, 1770. (15) Redhead, P. A. Vacuum, 1962, 12, 203. (16) Electron harpooning is suggested based on a calculated distance of 8.2 A from the surface for electron transfer at 6~ 0.6.' Our calculations based on the equations in ref 4 indicate a different, but still reasonable,distance of 2.05 A at which the electron harpooning is energetically neutral. (17) Ukraintsev, V. A.; Long, T. J.; Harrison, I. J. Chem. Phys. 1992.96, 3957. Ukraintsev, V. A.; Long, T. J.; Gowl, T.; Harrison, I. J . Chem. Phys. 1992, 96, 9114. (18) March, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. Rev. Lett. 1988,61, 2725; J. Chem. Phys. 1990,92,2004. Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. (19) Albery, W. J. Ann. Rev. Phys. Chem. 1980,31, 227. (20) Moutinho, A. M. C.; Aten, J. A,; Los, J. Chem. Phys. 1974,5,84. (21) Holloway, S.;Gadzuk, J. W. J. Phys. Chem. 1985,82, 5203. (22) The value of 1.6 A is a typical van der Waals separation between unbonded atoms. (Pauling, L. The Nature ofthe Chemical Bond; Cornel1 University Press: New York, 1960.) (23) Jenks, C. J.; Xi, M.; Bent, B. E., manuscript in preparation. (24) Gcorgiadis, R.; Fisher, E. R.; Armentrout, P. B. J . Am. Chem. Soc. 1989, I l l , 4251. (25) Shiller, P.; Anderson, A. B. J . Phys. Chem. 1991, 95, 1396. (26) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (27) Yang, H.; Whitten, J. L. J . Am. Chem. Soc. 1991, 113, 6442. (28) Chiang, C.-M.; Bent, B. E. Surf.Sci. 1992, 279, 79, (29) Henderson, M. A.; Mitchel1,G. E.; White, J. M.Surf.Sci. 1987,184, L325. Zaera, F.; Hoffmann, H. J. Phys. Chem. 1991, 95, 6297. (30) Solymosi, F.; Rtvtsz, K. J . Am. Chem. SOC.1991, 113, 9145. (31) Benziger, J. B.; Madix, R. J. J . Catal. 1980, 65, 49. (32) Zhou, X.-L.; White, J. M. Surf.Sci. 1988, 194, 438. Tjandra, S.; Zaera F. J. Vac. Sci. Technol. A 1992, 10, 404. (33) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf.Sci. 1989, 219, 294. (34) Paul, A,; Bent, B. E., manuscript in preparation. (35) Collman, J. P.;Hegedus, L. S.;Norton, J. R.;Finke, R.G.Principles and Applications of Organotransition Metal Chemistry; University Science Books: California, 1987. Kochi, J. K. Organometallic Mechanisms and Catalysis; Academic Press: New York, 1978.
