Mechanism of Catalytic Decomposition of CH3I on the Cu(111

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Mechanism of Catalytic Decomposition of CH3I on the Cu(111) Surface: A UBI-QEP Approach S. Azizian and F. Gobal* Department of Chemistry, Sharif University of Technology, P. O. Box 11365-9516, Tehran, Iran Received March 13, 2000. In Final Form: July 24, 2000 The method of unity bond index-quadratic exponential potential (UBI-QEP) and the computer simulation of the temperature programmed desorption (TPD) patterns are employed to derive the kinetic and thermodynamic parameters associated with the steps of the pathway we propose for the catalytic decomposition of methyl iodide on the Cu(111) surface. Assuming a highly reactive “hot methyl” surface intermediate and on the basis of our calculations it is concluded that the desorption of a part of this species is responsible for the reported methyl radical TPD peak at 140 K, while a part of this surface species is trapped in the three-fold sites of the Cu(111) surface and desorbs to form the reported TPD peak at 470 K. It is also concluded that the rate-determining step of the formation of CH4, C2H4, C2H6, and C3H6, which are the products of the interaction of methyl iodide with the Cu(111) surface and all desorb at 470 K in a TPD experiment, is the surface dissociation of the adsorbed methyl groups. The effect of the surface coverage on the kinetic parameters of the reactions, appearing as changes of the products’ distributions, is quantitatively accounted for.

Introduction Alkyl radicals are the key intermediates in heterogeneous catalytic synthesis of higher hydrocarbons,1 Fischer-Tropech synthesis,2 and hydrogenolysis.3 Alkyl halides are highly reactive on the surfaces of transition metals undergoing dissociative adsorption to yield strongly adsorbed halogen species which remain intact in the course of the reactions of the remaining surface alkyl species.4-30 These species subsequently undergo desorption, coupling, dissociation, and reactions between the dissociated products on the surfaces to yield a broad range of products.4-30 We have successfully applied the method of bond order conservation-Morse potential (BOC-MP) to account for * Corresponding author. E-mail: [email protected]. (1) Zaera, F. Chem. Rev. 1995, 95, 2651. (2) Biloen, P.; Sachtler, W. H. M. Adv. Catal. 1981, 30, 203. (3) Coq, B.; Crabb, E.; Figueras, F. J. Mol. Catal. A.: Chem. 1995, 96, 35. (4) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836 and refs cited in this paper. (5) Tjandra, S.; Zaera, F. J. Am. Chem. Soc. 1992, 114, 10645. (6) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529. (7) Zaera, F. Surf. Sci. 1992, 262, 335. (8) Tjandra, S.; Zaera, F. J. Vac. Sci. Technol., A 1992, 10, 404. (9) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (10) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (11) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Technol. 1992, 10, 2202. (12) Lin, J.-L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 2849. (13) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1993, 97, 9713. (14) Lin, J.-L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 9643. (15) Tjandra, S.; Zaera, F. Surf. Sci. 1993, 289, 255. (16) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. J. Am. Chem. Soc. 1993, 115, 308. (17) Zaera, F.; Tjandra, S. J. Am. Chem. Soc. 1993, 115, 5851. (18) Tjandra, S.; Zaera, F. J. Catal. 1994, 147, 598. (19) Jenks, C. J.; Xi, M.; Yang, M. X.; Bent, B. E. J. Phys. Chem. 1994, 98, 2152. (20) Jenks, C. J.; Paul, A.; Smoliar, L. A.; Bent, B. E. J. Phys. Chem. 1994, 98, 572. (21) Paul, A.; Bent, B. E. J. Catal. 1994, 147, 164. (22) Lin, J.-L.; Chiang, C. M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 147, 250. (23) Zaera, F.; Tjandra, S. J. Phys. Chem. 1994, 98, 31044. (24) Zaera, F. J. Mol. Catal. 1994, 86, 221. (25) Tjandra, S.; Zaera, F. Surf. Sci. 1995, 322, 140. (26) Kash, P. W.; Sun, D.-H.; Xi, M.; Flynn, G. W.; Bent, B. E. J. Phys. Chem. 1996, 100, 16621.

the reaction pathways in a number of cases, including the decomposition of methyl iodide on a hydrogen-precovered Ni(111) surface.31-34 The purpose of the present work is to use the method of unity bond index-quadratic exponential potential (UBI-QEP), which is the improved version of the method of BOC-MP, to investigate the mechanism of the dissociative adsorption and reactions of methyl iodide on the Cu(111) surface. Also, the calculated energetics of the reactions are employed in a computer simulation of the temperature programmed desorption (TPD) patterns of methyl iodide and the desorption products in attempts to derive the Arrhenius constants. Methods The method of UBI-QEP35 is the improved version of the former BOC-MP36 method which has been used by us31-34 and others37-51 to successfully analyze the mechanism of a wide range of catalytic reactions. (27) Castro, M. E.; Chen, J. G.; Hall, R. B.; Mims, C. A. J. Phys. Chem. B. 1997, 101, 4060. (28) Scottjones, G.; Barteau, M. A.; Vohs, J. M. J. Phys. Chem. B. 1999, 103, 1144. (29) Zaera, F.; Gleason, N. R.; Klingenberg, B.; Ali, A. H. J. Mol. Catal. A: Chem. 1999, 146, 13. (30) Livneh, T.; Asscher, M. J. Phys. Chem. B. 1999, 103, 5665. (31) Gobal, F.; Azizian, S. Langmuir 1997, 13, 5999. (32) Gobal, F.; Azizian, S. J. Chem. Res., (Synop.) 1997, 324. (33) Gobal, F.; Azizian, S. J. Mol. Catal. A: Chem. 1998, 136, 169. (34) Azizian, S.; Gobal, F. J. Mol. Catal. A: Chem. 2000, 153, 191. (35) Shustorovich, E.; Sellers, H. Surf. Sci. Rep. 1998, 31, 1. (36) Shustorovich, E. Adv. Catal. 1990, 37, 101. (37) Shustorovich, E. Surf. Sci. 1991, 248, 359. (38) Shustorovich, E. Surf. Sci. 1991, 253, 386. (39) Shustorovich, E. Surf. Sci. 1991, 259, L791. (40) Shustorovich, E. Surf. Sci. 1992, 268, 397. (41) Shustorovich, E. Surf. Sci. 1992, 279, 355. (42) Sellers, H. J. Chem. Phys. 1993, 99, 650. (43) Sellers, H. J. Chem. Phys. 1993, 98, 627. (44) Shustorovich, E.; Bell, A. T. Surf. Sci. 1993, 289, 127. (45) Olivera, P. P.; Patrito, E. M.; Sellers, H. Surf. Sci. 1994, 313, 25. (46) Patrito, E. M.; Olivera, P. P.; Sellers, H. Surf. Sci. 1994, 306, 447. (47) Sellers, H. Surf. Sci. 1994, 310, 281. (48) Khanra, B. Indian J. Chem. 1995, 34A, 585. (49) Gislason, J.; Sellers, H. Surf. Sci. 1997, 385, 77.

10.1021/la0003847 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/20/2000

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The UBI-QEP method is the most general two-body interaction potential capable of providing heats of adsorption of molecules in various coordination modes, as well as the reaction activation barriers, with typical accuracies of 1-3 kcal/mol.35 The potential variable, bond index, is a general exponential function of the two-center bond distance, and the potential between the two interacting bodies is a quadratic function of the bond index with the bond indices of the interacting bodies assumed to be conserved at unity up to the dissociation point. The details of the derivations of the bond energies and activation barriers have been recently reviewed.35 Here only the equations used throughout this article are given.35 (1) Atomic heat of adsorption in an n-fold site (a site making n-bonds with the adsorbed atom) is

Qn ) Q0(2 - 1/n)

(1)

where Q0 is the heat of adsorption in the on-top position or, alternatively, the two-center bond energy. (2) For ad-molecule heats of adsorption, three cases, depending on the strength of bonding, can be visualized for an AB molecule adsorbed via the A atom: (2a) For the weak bonding typical of the adsorption of closed-shell molecules, the heat of adsorption, QAB,n, on the n-fold site is given by

QAB,n ) Q0A2/(Q0A/n + DAB)

(2)

where DAB is the gas-phase dissociation energy of the A-B bond. (2b) Strong bonding, which typically occurs upon the adsorption of molecular radicals, gives rise to

QAB ) QA2/(QA + DAB)

(3)

(2c) Bondings of medium strength, typical of the adsorption of alkyl radicals, are intermediate between the above two extreme cases, and their heats of adsorption are given by

QAB ) 0.5[(Q0,A2/(Q0A + DAB)) + (QA2/(QA + DAB))] (4) (3) For a dicoordinated adsorption of a homonuclear molecule occurring parallel to the surface, the heat of adsorption, QA2, is

QA2 ) 9Q0A2/(6Q0A + 16DAB)

(5)

(4) The enthalpy changes in the course of adsorption and reaction are accordingly given by

∆H ) -(

∑(Q + D)P - ∑(Q + D)R ) i

i

(6)

where Pi and Ri refer to the products and reactants, respectively. (5) The activation barrier for the dissociation of AB is

∆EAB ) 0.5[∆H + (QAQB/(QA + QB))]

(7)

∆Ea ) 0.5[∆H + (QABQC/(QAB + QC))]

(8)

where the direction of the reaction is in accord with DBC > DAB. In the above equations, both principal terms, Q0 and DAB, are of comparable magnitudes and neither could possibly be neglected in the subsequent calculations. Results and Discussions The adsorption and reactions of methyl iodide on Cu(111) surfaces are analyzed by the method of UBI-QEP in a broad range of coverage, extending from θ ) 0.2 to θ ) 0.8. Table 1 summarizes the gas-phase dissociation energies (D) and the heats of adsorption (Q) of all the envisaged species that may be possibly involved in the adsorption and reactions of methyl iodide. These are calculated on the basis of eqs 1-8. Bent et al.12 have estimated the heat of adsorption of iodine on the Cu(111) three-fold sites to be higher than 62 kcal/mol. Choosing the minimum of the experimentally determined53 values of the Cu-I bond energy, 42 kcal/mol, as the iodine surface bond energy at the on-top position and using eq 1, we estimated the heat of iodine adsorption on the three-fold position to be around 70 kcal/mol (Table 1). A. Decomposition of Methyl Iodide at Low Coverage (θ ) 0.2). Experiments carried out under 1 L exposure give rise to θ ) 0.2 ML or 2.5 × 1013 molecule/cm2. Under these conditions, CH3I adsorbed at 110 K undergoes surface dissociation at 145 K and leads to the evolution of gaseous methyl radicals.11-13 At around 470 K, gaseous methyl radicals, as well as the products of surface reactions (CH4, C2H4, C3H6, etc.), desorb.11,13 Bent et al.13 explained the desorptions of gaseous methyl radicals at two vastly different temperatures by parallel reactions, where, in one path, adsorbed iodine and gaseous methyl radicals form, while, in the second path, both constituents are adsorbed. On the other hand, recently, Mims et al.,27 in a study of methyl iodide adsorption on a previously hydrogen-covered Ni(111) surface, associated the desorption of methane at temperatures as low as 150 K with highly mobile “hot methyl” species. According to Mims, dissociative adsorption of methyl iodide gives rise to highly mobile surface methyl species that partly react with the preadsorbed hydrogen atoms to desorb as methane at 150 K and partly migrate to three-fold surface sites, partially stabilized, and react with hydrogen atoms at 250 K to desorb as methane. We applied a simple model to rationalize the mechanism by the energetics consideration.34 Here, on the Cu(111) surface, we assume the formation of hot methyl species and show that this nicely explains the entire findings. In the early 1990s, the formation of hot oxygen atoms on the Al(111) surface had been established.53,54 These species are highly mobile and have a lifetime of 1 ps that enables them to travel on the surface around 40 Å. In the present case, we assume the dissociation of the adsorbed CH3I to adsorbed iodine atom and CH3/ (hot methyl species). The heat of dissociation, -15 kcal/mol, is converted to the kinetic energy of the hot species which gains a speed of 2892 m/s. Assuming that these also would travel around 40 Å on the surface, a lifetime of 1.4 ps is expected. Using this in

t ) t0 exp(Q/RT)

(9)

(6) The activation barrier for the diproportionation reaction of type Aa + BCa f ABa + Ca, where the subscript a refers to the adsorbed state, is given through

with t0 ) h/kT, the heat of adsorption, Q, of CH3/ around

(50) Sellers, H.; Shustorovich, E. J. Mol. Catal. A: Chem. 1997, 119, 367. (51) Park, Y. K.; Aghalayan, P.; Vlachos, D. G. J. Phys. Chem. A. 1999, 103, 8101.

(52) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed. CRC Press: Boca Raton, FL, 1991; p (9-86)-(9-100). (53) Brune, H.; Wintterlin, J.; Behm, R. J.; Ertl, G. Phys. Rev. Lett. 1992, 68, 624.

Catalytic Decomposition of CH3I on Cu(111)

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Table 1. Total Bond Energies in the Gas Phase (D) and Heats of Adsorption (Q) at Low and High Surface Coverages

Scheme 1. Mechanism of the CH3I Decomposition on the Cu(111) Surface at Low Surface Coverage (θ ) 0.2), with ∆E and ∆H Values in kcal/mol

Q, kcal/mol species C H I CH CH2 CH3 CH3/ CH4 C2H4 C2H5 C2H6 C3H7 C3H6 CH3I

D,a kcal/mol

coverage ) 0.2

coverage ) 0.8

81 183 293 293 398 538 576 674 859 821 349

120 56 70 72 48 26 0.4 5 8 26 4 26 7 25

72 56 64b 43 29 16 0.4 3 5 16 3 16 4 15

a Reference 52. b Calculated on the basis of the simulated value of the activation energy of step 2.

0.4 kcal/mol is obtained. The value of Q is subsequently used in

Q ) Q0(2xn - x2n)

(10)

xn ) exp{-(r - r0)/b}

(11)

with

to derive r, the distance of the hot species-surface separation. Equations 10 and 11 are the basic UBI-QEP equations35 with Q0 being the heat of adsorption on the on-top position, r0 being the equilibrium adsorbateadsorbent separation in the on-top position, and b being a parameter of nearly 0.26 Å.55 The value xn is the bond order of adsorbate on an n-fold site.35 As for the magnitude of r0, the CH3-Cu bond lengths in the on-top position, no experimental value was available. However, the C-Cu bond lengths involving CHOH, CHO, and CH2OH adsorption on the Cu(111) on-top positions have been reported to vary in the range 1.96 to 2.11 Å, respectively.56 Assuming a value of 2.04 Å for the bond length of CuCH3 in eq 11, r ∼ 3.17 Å was obtained which is in accord with the instability (low lifetime) of the hot methyl group. Scheme 1 presents the mechanism we propose for methyl iodide decomposition on the Cu(111) surface at low coverage, θ ) 0.2. The enthalpies and activation energies associated with the steps calculated on the basis of UBI-QEP are also presented. According to the scheme, following the adsorption, methyl iodide dissociates with an activation energy of 10.6 kcal/mol to adsorbed iodine and the “hot methyl” group. The latter either desorbs to form the peak at 145 K in a TPD pattern (Figure 1 of ref 13) or is trapped in a surface three-fold site and remains stable up to 470 K. The activation energies for these processes are 0.4 and 0 kcal/mol, respectively. The methyl groups at the three-fold sites require an activation energy of 26 kcal/mol to desorb, which points to their relative stability and desorption at 470 K in TPD experiments.13 On the other hand, the methyl groups may couple on the surface and desorb as ethane. The activation energy of this process on Cu(110) has been experimentally determined4 and is 25 kcal/mol. Assuming roughly the same value for the Cu(111) surface, we would later show that the Arrhenius factor for this reaction is small, causing a (54) Brune, H.; Wintterlin, J.; Trost, J.; Ertl, G.; Wiechers, J.; Behm, R. J. J. Chem. Phys. 1993, 99, 2128. (55) Shustorovich, E. J. Am. Chem. Soc. 1984, 106, 6479. (56) Au, C.-T.; Zhou, T.-J.; Lai, W.-J. Catal. Lett. 1999, 62, 147.

small yield. As the third route, CH3a may further dissociate into CH2a and Ha with the activation energy of 32 kcal/ mol, with the subsequent formation of methane through CH3a and Ha combination and of ethene via CH3a and CH2a combination, followed by the loss of a β-H and other species. As the UBI-QEP calculations reveal, all of these reactions require activation energies < 32 kcal/mol, indicating that the processes following the dissociation of methyl groups all occur quickly. Thus, the latter is the rate-determining step. The argument is supported by the experimental findings that all of the products mentioned simultaneously desorb at 470 K.13 Having derived and partially analyzed the values of the activation energies, we turn our attention to the calculation of Arrhenius factors. For step 3, desorption of the “hot methyl” group, the Arrhenius factor, ν3, is the inverse of the lifetime of this species and is ∼7.2 × 1011 s-1. Assuming steady-state conditions for this species, one has for the rates of its formation and consumption

R2 ) R3 + R4

(12)

and at low coverage (θ ) 0.2), experimental evidence13 indicate that 28% of the methyl groups desorb at 145 K and the rest migrate to the mentioned three-fold sites. As these reactions are parallel (competitive), the ratio of their rates resembles the ratio of their yields, namely R4/R3 ) 0.72/0.28 ) 2.57. Having ∆E3, ∆E4, ν3, and this ratio, the Arrhenius factor for step 4, ν4, is found to be 4.6 × 1011 s-1. To calculate the Arrhenius factors for steps 2, 5, 6, and 7, computer simulation of TPD patterns has been employed. After using the energetics of the reactions derived on the basis of UBI-QEP criteria in the rate laws, these equations are solved to generate the experimental TPD patterns13 by tuning the Arrhenius factors until both TPD peaks’ positions and the ratios of the areas under the peaks are fitted best to the experimental values. The results are presented in the first column of Table 2. Figure 1 presents the simulated TPD patterns. As for steps 14 and 18, desorptions of ethene and propene, it has been claimed6,11 that the reaction proceeds via β-H eliminations from the adsorbed ethyl and propyl groups,

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Table 2. Variation of the Calculated Activation Energies (∆E) and Arrhenius Factors (ν) with Total Surface Coverage (θ) for Some of the Elementary Steps CH3a f CH3g ∆E (kcal/mol) ν (s-1) CH3/ f CH3g ∆E (kcal/mol) ν (s-1) CH3a f CH2a + Ha ∆E (kcal/mol) ν (s-1) CH3a + CH3a f C2H6g ∆E (kcal/mol) ν (cm2/molecule‚s) CH3Ia f CH3/ + Ia ∆E (kcal/mol) ν (s-1)

θ ) 0.2

θ ) 0.4

θ ) 0.6

θ ) 0.8

26 6.0 × 109

24 2.3 × 109

18 3.8 × 106

16 6.0 × 105

0.4 0.4 0.4 0.4 7.2 × 1011 7.2 × 1011 7.2 × 1011 7.2 × 1011 32 34 39 41 6.0 × 1012 7.0 × 1013 6.0 × 1015 3.0 × 1016 25 23 17 15 6.0 × 10-6 9.0 × 10-7 1.0 × 10-9 1.0 × 10-10 10.6 9 7.5 6.6 1.0 × 1014 1.0 × 1012 1.0 × 1010 5.0 × 108

Figure 2. Presentation of the simulated TPD patterns of the products of the surface reactions of methyl iodide on the Cu(111) surface at high coverage (θ ) 0.8): desorption of methyl radical, step 3; desorption of ethane, step 6; desorption of methyl radical, step 5; desorption of methane and higher hydrocarbons, step 7.

Figure 1. Presentation of the simulated TPD patterns of the products of the surface reactions of methyl iodide on the Cu(111) surface at low coverage (θ ) 0.2): desorption of methyl radical, step 3; desorption of ethane, step 6; desorption of methyl radical, step 5; desorption of methane and higher hydrocarbons, step 7. Scheme 2. Mechanism of the CH3I Decomposition on the Cu(111) Surface at High Surface Coverage (θ ) 0.8), with ∆E and ∆H Values in kcal/mol

respectively, with an activation energy of 4 kcal/mol, as calculated by the UBI-QEP method. Using this value and Redhead’s equation,57 the Arrhenius factors ν14 ) 286 s-1 and ν18 ) 764 s-1 have been estimated. B. Decomposition of Methyl Iodide at High Coverage (θ ) 0.8). Additionally, at high surface coverages, methyl iodide reactions on Cu(111) surfaces result in the desorption of methyl radicals at both 140 and 470 K, as well as desorption of methane and higher hydrocarbons at 470 K.11-13 Scheme 2 presents our proposed mechanism for the decomposition of methyl iodide on the Cu(111) surface along with the values of the enthalpies and activation energies of the steps calculated on the basis of (57) Redhead, P. A. Vacuum 1962, 12, 203.

the UBI-QEP method at θ ) 0.8 ML. Table 1 presents the heats of adsorptions. Increasing the coverage has influenced the energetics of the reactions by the introduction of the scale factor, σ, in the UBI-QEP formalism.35 Its value for the surface iodine atoms which act only as a site blocking entity is 0.9, while, for other surface species, it is 0.6 and signifies that the surface iodine atoms are least destablized upon crowding of the surface, probably because CuI-type species form.4,12,58 Also, the surface hot methyl groups are least influenced upon increasing the coverage, because of their relatively long bond length with the surface. Thus, the same values of the heat of adsorption, 0.4 kcal/mol, and the Arrhenius factor, ν3 ) 7.2 × 1011 s-1, are assumed. According to Scheme 2, the adsorbed methyl iodide on Cu(111) dissociates to a surface hot methyl group which subsequently either desorbs with a negligibly small (0.4 kcal/mol) activation energy, giving rise to the TPD peak appearing at 140 K,13 or migrates to three-fold sites which remain stable up to 470 K. This latter species may proceed along any of three paths: to desorb with an activation energy of 16 kcal/mol, forming a TPD peak at 470 K; to further dissociate slowly to CH2 and H, ∆E7 ) 41 kcal/ mol, with the ultimate formation of higher hydrocarbons; or to combine and desorb as ethane with an activation energy of 15 kcal/mol. Very interestingly, our calculations predict that, at high θ values, the activation energy of step 5 (the desorption of methyl radicals) decreases to 16 kcal/mol and that of the dissociation of adsorbed methyl radicals increases to 41 kcal/mol, both in perfect agreement with the experimental findings of changes in the product yields.13 To calculate the Arrhenius factors of the reactions of hot methyl radicals, parallel reactions of desorption at low temperature and migration to the three-fold sites are considered first. The rates (or yields) ratio is three.13 Using the values ∆E3, ∆E4, and ν3, one comes up with ν4 ) 5.1 × 1011 s-1. To derive the Arrhenius factors of steps 2, 5, 6, and 7, the computer simulation of the TPD patterns is used with the UBI-QEP-based values of the activation energies at θ ) 0.8 and the values of the Arrhenius factors (58) Di Cenzo, S. B.; Wertheim, G. K.; Buchanan, D. N. E. Appl. Phys. Lett. 1982, 40, 888.

Catalytic Decomposition of CH3I on Cu(111)

are tuned to achieve the best fit to the experimental values, as far as the peak positions and the ratio of the areas under the peaks are concerned. The results are collected in Table 3, while the simulated TPD patterns are presented in Figure 2. C. Decomposition of Methyl Iodide at the Intermediate Coverages, 0.2 < θ < 0.8. Following the procedures discussed in the Results and Discussions section A and B, the energetics and the Arrhenius factors are worked out and tabulated in Table 3. No inconsistencies have been observed. Interestingly, compensation effects have been detected for steps 2, 5, 6, and 7, where, upon increasing the surface coverage of methyl iodide, the activation energies of the mentioned steps change and the relevant Arrhenius factors also alter themselves correspondingly to compensate for the former, keeping the changes of the rate constants less sensitive to the surface coverage. Figure 3 presents the compensation plots, ∆E versus ln v, for steps 2, 5, and 7, where good linearities have been observed. Conclusions On the basis of this study, the following conclusions could be reached: The decomposition of methyl iodide on the Cu(111) surface is through the formation of highly mobile and loosely adsorbed methyl group species, hot methyl radicals, their subsequent desorption or surface reactions, and the desorption of the products. The method of UBI-QEP was applied for the calculations of the heats of adsorptions and the activation energies

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Figure 3. Presentation of the compensation effect involved in step 2 (hot methyl formation), step 5 (desorption of stablized adsorbed methyl radicals), and step 7 (desorption of methane and higher hydrocarbons).

of the surface reactions and the desorptions of the products, and it was found that the method is well capable of providing the energetics of the envisaged steps of the reaction, thus accounting for the mechanism. The method also correctly predicts the variations of the selectivities upon changes of the surface coverages. Fitting of the simulated TPD patterns to the experimental findings, as far as both the peak positions and ratios are concerned, revealed the Arrhenius factors for the steps involved in the overall reaction. LA0003847