2152
J. Phys. Chem. 1994,98, 2152-2157
Kinetics and Energetics of @-HydrideElimination on Cu( 100): Determining the Copper-Alkyl Bond Energy Cynthia J. Jenks,' Ming Xi,* Michael X. Yang, and Brian E. Bent'!$ Department of Chemistry, Columbia University, New York, New York 10027 Received: September 14, 1993; In Final Form: December 16, 1993"
Recent studies have shown that adsorbed alkyl groups can be generated and isolated on metal surfaces by a variety of methods, including the thermal dissociation of alkyl iodides. For copper surfaces, the primary reaction pathway is ,&hydride elimination a t -250 K to produce the corresponding olefin and adsorbed hydrogen atoms. We show here that, on a Cu(100) surface, 1-10% of the product olefin is rehydrogenated to the alkyl group. This reversibility has been detected and quantified by deuterium isotope labeling in temperature-programmed reaction experiments. In addition, partially deuterated ethyl groups have been synthesized on the surface by a novel hydrogenation reaction, and these selectively deuterated alkyls have been used to measure the deuterium isotope effect for &hydride elimination which is 10.3 f 0.7 a t 255 K. From these results, the enthalpy change for &hydride elimination and the metal-ethyl bond energy on Cu(100) have been determined to be 6.5 f 4 kcal/mol and 33 f 6 kcal/mol, respectively. These values are compared with those for alkyl groups on other metal surfaces, for alkyl groups in metal compounds, and for alkyl groups bonded to metal atoms in the gas phase.
1. Introduction An important issue for obtaining a predictive understanding of surface reactions is the determination of surface-adsorbate bond energies.' If the energy required for homolytic scission of a surface-adsorbate bond is known, then the heat of formation for the adsorbed species can be determined from eq 1, provided that the gas-phase valueof the heat of formation for the adsorbate is known. Clean Surface (S)
+ Adsorbate (A)
-
Surface-Adsorbate @-A)
AHr,, = - M a d s = AHf(S-A) - AHf(S) - AHf(A)
For Cu(100) : AH
7 = 14.5 f 2 kcalimol
AH
= 8 f 3 kcallmol
A H rxn
Here, the clean surface has been taken as the standard state, so that AH@) = 0. The relations above also follow the convention that M a d s is taken as the negative of the enthalpy change for the adsorption reaction, and the temperature dependence of the heat of formation has been ignored. Given the accuracy to which surface-adsorbate bond energies are currently known, this latter approximation is reasonable. Knowing the heats of formation for adsorbed species, one can calculate AH for surface reactions, even when the products do not desorb from the surface. This is an important capability, since direct measurement of the enthalpy change for surface reactions is difficult, given the small amounts of material involved. For adsorbates that can be thermally desorbed from surfaces, heats of adsorption and consequently the corresponding heats of formation can be determined either by studying the temperaturedependence of the adsorption/desorption equilibrium or by measuring the kinetics for adsorption and desorption. The latter t Present address: Department of Chemistry, Iowa State University, Ames, IA 50011. 8 Present address: Department of Chemical Engineering, MIT, Cambridge, MA 02139. 1 Presidential Young Investigator, A.P. Sloan Fellow, and Camille and Henry Dreyfus Teacher-Scholar. Abstract published in Advance ACS Abstracts, February 1, 1994.
0022-365419412098-2152$04.50/0
6.5 f 4 kcalimol
Figure 1. Energetics and kinetics for @-hydride elimination by ethyl groups on a Cu( 100) surface. Thevaluesgiven for theactivation enthalpies are approximations based on the experimental activation energies which were determined as described in the text.
approach is particularly useful for systems where the activation energy for adsorption is negligible. Under these conditions, the activation energy for desorption is equal to the heat of adsorption. For adsorbates that do not desorb from the surface molecularly intact, there is no good method for determining directly the heat of adsorption. As a result, the adsorbate-surface bond energy, the heat of formation for the adsorbate, and the enthalpy change for surface reactions involving these types of species (which are often those proposed as intermediates in catalytic reactions) are not generally known. In this paper, the procedure described above is inverted, and a kinetic approach is used to determine the enthalpy change for the catalytically-important surface reaction of @-hydrideelimination by adsorbed alkyl groups. This reaction is shown schematically in Figure 1 for the system studied in this work: ethyl groups on Cu( 100). The activation energies for both the @-hydride elimination step and the reverse hydrogenation of ethylene to ethyl have been measured, and from the difference in these values, the enthalpy change for @-hydrideelimination by 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2153
@-HydrideElimination on Cu( 100)
lEM-H
EUCl
spectra taken at an electron impact ionization energy of 15 eV to eliminate cracking and permit quantification of the isotopic purity showed a small amount of m / e = 31 (probably C2D3H), m / e = 30 (probably C ~ D Z H and ~ ) , m / e = 28 (probably C2H4 or N2). The latter two masses are not an issue for the studies here, but the possibility of CzD3H is significant for the kinetic isotope effect results presented in section 3.1. To confirm that m / e = 31 is due to an ethylene isotopomer, TPD studies of C2D4 adsorption/desorption from Cu( 100) using an ionization energy of 15 eV were performed. (If m / e = 31 is due to ethylene-dj, it should desorb from the surface at the same temperature as ethylene-d4.) The results show that both m / e = 3 1 and m / e = 3 1 are evolved at the same temperature (120 K) and the product ratio is the same as that detected in mass spectra of the dosing gas. This result strongly suggests that the m / e = 31 is due to ethylene-dj impurity in the ethylene-d4 source. On the basis of the ion intensity ratio, the level of impurity is 4.2% of the C2D4 C2D3H total.
+
I E M.C
33
* 6 kcallmol I
Figure 2. Thermochemical cycle for calculating the metal surface-alkyl
bond energy from the cnthalpy change for &hydride elimination, the product binding energies to the surface, and gas-phaseheats of formation. The values given for Cu(100) were determined as described in the text. ethyl groups on Cu(100) is determined to be 6.5 f 4 kcal/mol. This enthalpy change is then applied in a thermochemical cycle as shown in Figure 2 to determine the Cu( lOO)-ethyl bond energy, which is 33 f 6 kcal/mol. The heat of formation for ethyl groups on Cu(100) is therefore 5 f 6 kcal/mol.
2. Experimental Section The experiments were performed in two ultrahigh vacuum systems with base pressures below 2 X 10-lo Torr. Both systems have been previouslydescribed.2J Two Cu( 100) crystals (Monocrystals Inc. and Cornel1 Department of Materials Research) were used in these studies. Both were mounted on resistive heating elements that could be heated to 1 100 K and cooled to 1 10 K with liquid nitrogen. The surface temperatures were measured by chromel-alumel thermocouplejunctions wedged into holes spark eroded or drilled into the sides of the crystals. All reagents, except hydrogen and deuterium atoms, were adsorbed onto the Cu( 100) surfaces by back-filling the chambers. Exposures are reported in langmuirs where 1 langmuir = 1 W Toms; the pressures have not been corrected for differing ion gauge sensitivities of the compounds. H and D atoms were generated in situ using a hot (1800 K) tungsten filament to dissociate H2 and D2.4 The Cu(100) surface was held 3 cm from the filament during dosing, and exposures are reported as the H2/D2 exposure to the filament, since the atomic flux is not accurately known. For reference, a 100 langmuir exposure is required to saturate a clean copper surface with H / D atoms.4 During the temperature-programmed desorption/reaction (TPD/R) experiments, the adsorbate-covered surfaces were held 1-2 mm from 2 mm diameter apertures to shielded and differentlypumped quadrupole mass spectrometers. Experimental tests verified that only species evolved from the center of the Cu( 100) surfaces were detected in the TPD/R experiments. The surface heating rates in thesestudies were 2.5-4 K/s, and up to three ion intensities were monitored simultaneously by computer-controlled multiplexing of the mass spectrometer. Iodoethane and iodoethane-dJ (99 atm % D) were obtained from Aldrich; ethylene, H2, and D2 from Matheson; and ethylened4 (99 atom % ’ D) from MSD isotopes. All samples were used without further purification except for the iodoethanes which were filtered through an alumina column (basic pH) and taken through several freezepumpthaw cycles. Sample purities were verified in situ by mass spectrometry. In the case of C2D4, mass
3. Results and Discussion The thermodynamic values of interest in this work are those in the thermochemical cycle shown in Figure 1. In the case of the gas-phase ethyl ethylene H reaction, theenthalpy change is calculated to be 36.5 kcal/mol at 300 K from the literature values for the heats of formation of ethyl radicals (28.0 kcal/ mols), ethylene (12.5 kcal/moF), and hydrogen (52 kcallmol’). For the Cu(100)-H bond energy, we have chosen a value of 55 f 4 kcal/mol on the basis of measurements reported for Cu(100),8-11Cu(1 10),11-13andCu(l1 l)11J4surfaces. Thereason for considering data from all three copper surfaces is that the data for Cu(100) are probably less reliable. Specifically, since there is a barrier for H2 dissociation on copper surfaces, the heat of dissociative adsorption is given by the difference between the barriers for desorption and adsorption. For Cu(lOO), the desorption spectra are complex due to an H-induced surface reconstruction at high surface coverages,lO but for low coverages the activation energy for H2 desorption is -20 kcal/mol.11 The activation energy for adsorption was reported by Balooch et al. to be about 5 kcal/mol,8 but more recent measurements for Cu(1 10)13 and comparison of theoryI4 and experiment” on Cu(ll1) suggest that a value of 14-16 kcal/mol is more reasonable. If we chose an average value of 15 kcal/mol for Cu( loo), then the resulting Cu(100)-H bond energy is 55 kcal/ mol as mentioned above. The other value in Figure 2 that can be obtained from adsorption/desorption measurements is the binding energy of ethylene to Cu(100). We have determined this value by TPD experiments. The TPD spectra, not presented here, show that the activation energy,for ethylene desorption decreases significantly with increasing coverage. Similar results have been published for ethylene on Cu(llO).l5 For low coverages of ethylene on Cu(lOO), we measure a TPD peak temperature of 140 K for a surface heating rate of 2.5 K/s. A value of 190 K is reported in the literature16 for ethylene desorption from Cu( 100) for an unspecified heating rate. Assuming the value we have measured and a first-order preexponential factor of 1013 s-1, the activation energy for desorption (i.e. the heat of adsorption) is 8 kcal/mol. We estimate an uncertainty of f 2 kcal/mol (i.e. an uncertainty of lo3in the assumed value for the preexponential factor). The only remaining parameter to be determined in Figure 2 in order to determine the Cu( 100)-ethyl bond energy is the enthalpy change for @-hydrideelimination. As shown in Figure 1, thisvaluecan beobtained if theactivation energies for @-hydride elimination and the reverse partial hydrogenation of ethylene to ethyl groups can be determined. The remainder of this paper focuses on obtaining these two quantities. 3.1 &Hydride Elimination. To determine the kinetics of @-hydrideelimination, ethyl groups have been generated on a
-
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+
Jenks et ai.
2154 The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
Isotope Effect in P-Hydride Elimination
Cu(l00)liodoethane m/e = 27
250 K
/
I
5.0: \
1L
L
m/e = 34, CD ,H ,,
3
I
220
240
260 280 TEMPERATURE (K)
L J
Figure 3. Temperature-programmed reaction spectra of m / e = 27
evolution following the adsorption of iodoethane on Cu(100) at 115 K. Thesurfaceheatingratewas2.5 K/s. Asindicatedby theinsetschematics, the peaks at 24&250 K are due to ethylene evolution as a result of ,%hydride elimination by adsorbed ethyl groups.
Cu( 100) surface by the dissociative adsorption of iodoethane. As documented elsewhere,2JJ7 iodoalkanes thermally dissociate on copper surfaces below 200 K to form adsorbed alkyl groups and iodine atoms. The alkyl groups subsequently decompose to evolve the corresponding alkene of the same chain length at 230-250 K, and isotope labeling studies confirm that it is the &hydrogen that is abstracted by the surface to form this p r o d ~ c t . ~Figure J~ 3 shows TPR spectra of the ethylene formed by &hydride elimination from ethyl groups generated on Cu( 100) by the dissociative adsorption of iodoethane. Because the j3-hydride elimination reaction occurs above the 140 K temperature where ethylene desorbs from Cu(loo), the kinetics of ethylene evolution in Figure 3 reflect the kinetics of j3-hydrideelimination. Assuming a first-order preexponential factor of 1013s-1 for this reaction, the activation energy is 14.5 kcal/mol. Again, we believe that this value is only accurate to within f 2 kcal/mol, given the assumed value for the preexponential factor. A similar value of 12.6 kcal/ mol (preexponential factor = 6 X 1011 s-1) has been determined for @-hydrideelimination by propyl groups on Cu(l1I) by Forbes and Gellman,l* who applied a heating rate variation method to measure the activation energy.19 Several factors prohibit a significantly more accurate determination of the activation energy for @-hydrideelimination on Cu(100). One is the finding that site-blocking may play a role in the decomposition kinetics. In other words, if the &hydride elimination reaction requires an empty metal site adjacent to the alkylgroup to bind the product H atom and these sites are blocked by either other alkyl groups, the coadsorbed iodine atoms, or the product hydrogen atoms, then the 8-hydride elimination rate is not first order in adsorbed ethyl groups. This complication, which may be responsible for the peak shift to higher temperature with increasing coverage in Figure 3 and which has been discussed extensively for butyl groups on Cu(110),*0has been minimized here by working at low surface coverage. A second potential complicating factor is the presence of coadsorbed iodine atoms. This effect has been addressed by generating alkyl groups in the absence of iodine atoms using a novel H atom reaction as described below. Recent studies have shown that ethyl groups can be generated on a Cu( 11 1) surface by exposing a physisorbed monolayer of ethylene to a flux of hydrogen atoms generated by dissociation of Hz on a hot tungsten filament.4 The order of reagent addition
230
240
250
260
270
280
TEMPERATURE (K)
Figure 4. Temperature-programmed reaction spectra of m / e = 31 (CzDsH),m / e = 32 (C2D4), and m / e = 34 (C~D~HZ) after ethyl-1,1,2,2d4 groups have been formed on the surface by impinging 5 langmuirs of
H atomsonto a surface precovered with a 1.O langmuirexposure of C2D4. The spectra were obtained with a surface heating rate of 3 K/s and an electron impact ionization energy of 15 eK The inset schematics show the origin of the observed ion intensitis.
is particularly important. If a partial monolayer of H atoms is adsorbed on the surface prior to addition of ethylene, then no hydrogenation to form ethyl groups is observed. The ethylene desorbs unreacted at 120 K, and the hydrogen atoms subsequently recombine and desorb a t -300 K. If, however, the ethylene is adsorbed first on the surface, then subsequent D atom addition produces ethyl-d,, as evidenced by the evolution of ethylene41 a t the &hydride elimination temperature of 240 K in a subsequent TPR experiment? Based on these results, it was concluded that it is D atoms that have not thermally equilibrated with the Cu( 111) surface that react with adsorbed ethylene to form ethyl groupse4 The important point for the studies here it that ethyl groups and partially deuterated ethyl groups can be formed in the absence of coadsorbed iodine atoms on copper surfaces by this approach. The results of &hydride elimination by ethyl-1 ,I , 2 , 2 4 groups formed by H atom addition to adsorbed CzD4 are shown in Figure 4. In this experiment, the selectively-labeled ethyl groups were formed by exposing a Cu( 100) surface precovered with a partial monolayer of CzD4 (about 25% of monolayer saturation) to 5 langmuirs of H atoms. The hydrogen exposure unit here corresponds to the exposure of Hz to the tungsten filament used to generate the H atoms, so the actual H atom exposure to the copper surface is much less; an H exposure of about 100 langmuirs is required to saturate a clean Cu(100) surface with H atoms. The TPR experiments in Figure 4 were carried out using an electron impact ionization energy of 15 eV (as opposed to 70 eV in previous experiments) to detect the product ethylene evolved from the surface. Under these conditions, the ethylene mass spectrum is dominated by the molecular ion (the M - 1 ion intensity is less than 3% of the parent peak), so the ion intensities at .m/e = 32 and 3 1 in Figure4 accurately represent the yieldsof ethylened, and ethylene-dj, respectively. A TPR trace for m / e = 34 is also shown in Figure 4 to verify that none of the ethyl-d, groups are hydrogenated to ethane& For larger H exposures, this hydrogenation reaction is detectable. The point to note with respect to the effects of coadsorbed iodine on the &hydride elimination rate is that the TPR peak temperature of -258 K in Figure 4 is -20 K higher than that for a comparable ethyl coverage achieved with a low exposure of iodoethane in Figure 3. As has been discussed elsewhere, the coverage dependence to the TPR peak temperature in Figure 3 is probably due to site-
-
8-Hydride Elimination on Cu( 100) blocking effects.20 It should also be noted (see below) that there is a substantial deuterium kinetic isotope effect for 8-hydride elimination, which accounts, at least in part, for the -20 K higher peak temperature in Figure 4 relative to Figure 3. We conclude that the presence of coadsorbed iodine atoms does not have a substantial effect on the &hydride elimination kinetics. The TPR results in Figure 4 can also be used to determine the deuterium kinetic isotope effect for 8-hydride elimination, a quantity that will be important in the following section for quantifying the measurements of ethylene hydrogenation to ethyl groups. The deuterium isotope effect is determined from the results in Figure 4 simply by comparing the rates of ethylene-& and ethylene43 evolution. The relative rate constants for H and D elimination ( k H / k D ) are given by twice the rate of CzD4 evolution divided by the rate of C2D3H evolution, where the factor of 2 accounts for the D / H ratio in the 8-position of the ethyl group. The beauty of such an internal competition experiment is that the kinetic order and coverage dependence of the kinetic rate law need not be known.z1 In principle, the kinetic isotope effect can be determined as a function of temperature over the range of the TPR peak by the procedure described above. But because of the small surface ethyl coverage 0.05), the low signal/noise ratio in Figure 4, and possible effects of finite pumping speeds on the TPR peak shapes, the temperature-dependence of the isotope effect has been neglected, and an average value for k H / k D of 8.3 f 0.5 has been determined using the TPR peak areas. Here, the statistical uncertainty of 0.5 represents 95% confidence limits based on three experiments. In fact, however, a correction must also be made for the4.2% CzD3H impurity in the C2D4 source (see section 2). If we assume that H atom addition occurs with equal probability to either end of CzD3H and that k H / k D is the same for all ethyl isotopomers, then 89% of the impurity ethyl-dj (formed by H atom addition) decomposes to C Z D ~ Hand contributes to the m / e = 31 signal at 258 K. When this contribution is taken into account, the value for k H / k D is 10.3. Given the assumptions involved in this correction, we estimate an uncertainty of f0.7 for k H / k D . We note that the difference in the C-H/C-D stretching zero-point energy for the &hydrogens in ethyl groups on Cu(100) is about 4.3 kJ/mo1,2-ZZso the maximum semiclassical isotope effect expected for loss of all the zero-point energy difference in the transition state is 7.6 at 255 K. This value is quite sensitive to both temperature and zeropoint energy difference (the value is 5.6 at 300 K and 10.6 for a zero-point energy difference of 5 kJ/mol), but comparing the nominal value of 7.6 with the experimental value of 10.3 suggests that tunneling may play a significant role in the reaction. 3.2 Hydrogenation of Ethylene to Ethyl. Determining the activation energy for the hydrogenation of ethylene to ethyl groups on Cu( 100) is difficult because the product ethyl groups do not desorb from the surface and because the rate is small compared with that for ethylene desorption under even the most favorable conditions. For example, when D atoms are coadsorbed with ethylene on Cu( loo), all the ethylene desorbs below 150 K;there is no detectable reaction to form ethyl groups (which would be stable on the surface to above 200 K). As a result, we have taken two alternative approaches to induce and detect the hydrogenation of ethylene to ethyl groups. The first involves coadsorption of D atoms with ethyl groups on the surface. In this case, 8-hydride elimination by ethyl groups produces ethylene a t -250 K. The majority of the ethylene desorbs from the surface, but a small fraction reacts with the coadsorbed D atoms to form ethyl-dl on the surface. This reaction is detected by the subsequent @-hydride elimination from ethyl-dl to evolve ethylene-dl. Selected results of these studies are shown in Figure 5. In this case, 15 langmuirs of D atoms (-20% of saturation coverage) have been adsorbed on the Cu(100) surface a t 110 K followed by adsorption of 1.0 langmuir of CzH51 to generate adsorbed ethyl groups. The pri-
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The Journal of Physical Chemisrry, Vol. 98, No. 8, 1994 2155 Cu(lO0)
I
. m/e = 29 I
255 K
200 240 TEMPERAURE
9 1'6' E
15 L D atom
280
+ 1.0 L C,H,I
Ai I
II
m/e=2L/
I
mle= 29
1
160
I
I
I
I
I
I
180
200
220
240
260
280
2 '0
TEMPERATURE (K)
-
Figure 5. Evolution of C2H4 ( m / e = 28) and CzHdD ( m / e = 29) after the adsorptionof 15 langmuirsof D atoms ( 20%of saturationcoverage) followed by 1.O langmuir of C2H5I (-25% of monolayer saturation) on Cu(100) at 110 K. The inset compares the m / e = 29 TPR spectrumwith those for control experiments in which the reactants are adsorbed separately on Cu(100). The surface heating rates were 2.5 K/s.
mary product in the TPR experiment is ethylene (as monitored by m / e = 28), but a small amount of m / e = 29 is also observed concurrently. As shown in the inset by a direct comparison of the m / e = 29 spectrum with control experiments on CzHJ and D adsorbed separately on Cu(lOO), the amount of m / e = 29 exceeds the M 1 contribution from the 1% natural abundance of 13Cin the CzH4 product and there is no contamination at m / e = 29 from the D atom exposure. We attribute the small amount of m / e = 29 to formation of ethylene-dl by the hydrogenation/ 8-elimination pathway described above. This result is substantiated by the isotope crossover experiments shown in Figure 6. In this experiment, a mixture of ethyl and ethyl-d5 groups are generated on the surface by adsorbing a mixture of iodoethane and iodoethane-d5. By adsorbing a mixture, we ensure (barring phase separation of isotopes) that the ethyl and ethyl45 groups are intermixed on the surface. Thus, when 8-hydride elimination commences, D atoms (generated on the surfacevia @-eliminationfromethyl-d5) are intermixed with CzH4 (produced by 8-elimination from C2H5) and vice versa. This differs from the D CzH51experiment in Figure 5 where phase separation of the two reactants could occur because of the sequential adsorption. The results in Figure 6, however, suggest that phase separation is not an issue. The yield of ethylene-dl relative to ethylene is comparable to that in Figure 5 for the D atom + CzH5 experiment. Thevery different peakshapes for the ethylene and ethylene-& are probably due to differences in siteblocking as a result of the larger H / D atom coverage present on the surface when CzDs decomposes.20 (Both peak shapes are quite normal when the two reagents are adsorbed separately on Cu( loo).) Thecontrol experiment in theinset to Figure 6 confirms that m / e = 29 is a reaction product and not an impurity in CzH51 or C2DSI. Note in particular that while a small amount of m / e = 29 is detected for both C2H5I and C2D51adsorbed separately on Cu( loo), the signal from the mixed monolayer is far in excess of that due to the M + 1 ion of m / e = 28 or to contamination in the CzDsI source. We note also that in similar studies on Cu( 1lo), no formation of ethylene-dl is detected. In order to determine the rate of C2H4 + D C2H4D on the
+
+
-
2156
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
Jenks et al. Because of the complexity of the TPR peak shapes in Figures 5 and 6 (due to effects such as site-blocking20, we approximate
80
200 220 240 260 TEMPERATURE (K)
=
180
200
220
""\I\
240
260
TEMPERATURE (K) Figure 6. Evolution of C2D4 (monitored by m / e = 30, C2Dp+),C2H3D ( m / e =. 29), and C2H4 (monitored by m / e = 27, C2H3') after the adsorption of 2 langmuirs of a 5050 mixture of C2HJ and C2DsI on
Cu(100) at 115 K. The inset shows m / e = 29 (C2HpD) evolution after the indicatedexposures of CzHsI, C~DJI, and a 5050 mixture of the two. Thesurface heating rates were2.5 K/s. These results show that @-hydride elimination is at least partially reversible on Cu(100) under vacuum conditions.
SCHEME 1
J C2D4 (a)
+@ D a+ -
k.H
H (a)
I
kd
basis of the results in Figures 5 and 6, consider the reaction sequence shown in Scheme 1 for the CzHs + C2D5 experiment. Since the reaction is run under conditions where a t most 1 deuterium is incorporated into the product ethylene (Le. kd8cthylcnc >> k+D&&,ylcne) and the rates of CzH3D evolution ( R Q H ~ and D) C2H4 evolution ( R c ~ Hhave ~ ) been measured in the TPR experiments, k+D can be determined from eq 2:
where the first term on the right hand side is the fraction of ethylene that adds a deuterium to form C2H4D while the second term is the probability that CzH4D eliminates a hydrogen atom to form CzH3D. All of the variables in eq 2 are known from separate experiments except for Rc2H3D/Rc2~,, which is measured in the experiment described above, and k+D,which is determined from this measurement.
the R c ~ H ~ D / R cratio ~ H ,by the relative yield of these products which is -0.1. Using this value, a surface temperature of 230 K, an average surface D atom coverage of 0.1, and assumed pseudo-first-order preexponential factors of 1013s-l, the activation energy for the addition of deuterium atoms to C2H4 on Cu( 100) is 8 kcal/mol. Given the large number of approximations that go into this determination, it is reassuring to find that none of them affect the final value by more than 1 kcal/mol. The dominant factor in determining this value is the RC2H,D/RC2h ratio, and each order of magnitude change in this ratio alters the activation energy by -1 kcal/mol. In effect, the ethylene hydrogenation rate is clocked by the rate of ethylene desorption, i.e. the hydrogenation rate must be within about 1 order of magnitude of the desorption rate in order to be observed. 3.3 Energetics of &Hydride Eliminationand Metal-Alkyl Bond Energies. The measurements above show that the activation energies for @-hydride elimination and for hydrogenation of ethylene to ethyl groups are 14.5 f 2 kcal/mol and 8 f 3 kcal/ mol, respectively. Neglecting isotope effects, these values imply a AH of 6.5 f 4 kcal/mol for @-hydride elimination by ethyl groups on Cu( 100). (In calculating the uncertainty of f 4 kcal/ mol, the uncertainties in the contributing values were assumed to be random and were therefore added in quadrature, i.e. v'Z( 6 ~ where ~ ) ~ bxi is the uncertainty of measurement i . ) The Cu( 100)-ethyl bond energy has been determined from these results using the thermochemical cycle shown in Figure 2. The resulting value of 33 f 6 kcal/mol is consistent with the value of 29 f 2 kcal/mol determined for the methyl-Cu( 11 1) bond energy from TPD spectra of methyl radical desorption from Cu(ll1) in the presence of coadsorbed iodine.24 It should be noted, however, that on the basis of bond energies in molecules, metal-methyl bond energies are expected to be larger than metalethyl bond energies, and there may also be a significant difference in the binding energy of alkyl fragments on the (1 11) and (100) faces of copper (see below). The Cu( 100)-ethyl bond energy and energetics for &hydride elimination are compared with analogous values reported for Pt( 1 1l)?5 Al( 1 1l)/Al( 100),26,27 and Fe( 100)-H28 surfaces in Table 1. It is interesting to note that the transition-metal-alkyl bond energies are substantially smaller than the Al-isobutyl bond energy of -46 kcal/mol. This difference is reflected in the much larger reaction enthalpy and activation energy for &hydride elimination on aluminum surfaces. A similar trend is observed for the bond strengths and @-hydride elimination energetics/ kinetics in metal-alkyl compounds. @-Hydrideelimination by aluminum alkyl compounds is endothermic by 20-25 kcal/mol and has an activation energy of 25-30 kcal/mol.Z7 By contrast, @-elimination by transition-metal alkyls is only slightly endothermic and the activation energies are in the range of 10-20 kcal/m01.~~ In comparing the metal surface-alkyl bond energies in Table 1 with bond enthalpies reported for both saturated metal-alkyl compounds3~32and unsaturated gas-phase metal-methyl fragments,33-35 the values cover similar ranges of 20-50 kcal/mol, but more direct comparisons are difficult because the bond energies depend almost as much on coordination environment as on the identity of the metal. This effect is generally referred to as the reorganization energy associated with metal-carbon bond scission.30 In other words, metal-alkyl bond scission is accompanied by significant electronic and geometrical rearrangements within the metal and alkyl fragments p r o d u d , and this reorganization is specific to the coordination environment. In the case of methyl bonding to an isolated metal atom, the reorganization energy is electronic, and the electronic effects have been successfully correlated with the promotion energy to achieve a d*W configuration at the metal center.30.34p35Generalized valence bond
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2157
@-HydrideElimination on Cu( 100)
TABLE 1: Comparison of the Energetics and Kinetics of &Hydride Elimination on Metal Surfaces. All Values Are in kcal/mol 0-hydride elimination
Ea
surface/alkyl Cu(lOO)/ethyla Fe( 100)-H/ethylb Pt( 11l)/ethylc Al( 111),( 100)/isobutyld
-
14.5 31.2 6 30
2
AH
olefin AHa&
6.5 4 7&2 -7 -19
8*2 af2 11 -7
metal-H bond E 55 -60 -60 57
-
4
metal-alkyl bond E 33*6 -38 -38 -46
This work. Reference 28. Reference 25. References 26, 27.
calculations36 indicate that this correlation works because the singly occupied s-orbital plays the dominant role in metal-methyl bonding, particularly for the first row transition metals where the 3d orbitals have a much smaller spatial extent than the 4s. This reorganization effect is particularly evident when the bond enthalpies are compared for Cu-CH3 and Cu-CHs+, which are 55 f 4 and 30 f 3 kcal/mol, re~pectively.3~JSThe dramatic difference can be attributed to the ground-state electron configuration of 3d104sl for Cu vs 3d10 for C U + . The ~ ~former ~ configuration requires essentially no promotional energy to bond with CH3 while the latter requires -70 kcal/mol to achieve a 3d94sl configuration. Similar electronic and geometrical reorganization effects have been shown to be particularly important for bond scission in complexes involving metal-metal bonding,” and these effects are likely to be even more pronounced at metal surfaces. This assertion is supported by calculations of adsorbate bonding at surfaces which show that the heat of adsorption represents a compromise between the formation of the surface-adsorbate bond and the weakening of bonds within the surface and within the adsorbate.38 Both of these latter effects, rehybridization of the adsorbate and reconstruction of the surface, are well-documented experimentally. It is therefore not surprising that the copperalkyl bond energy of 30 kcal/mol should be substantially weaker than that of -55 kcal/mol for copper-methyl in the gas phase. One might also expect the surface-alkyl bond energy to vary significantly with surface geometry, an effect that is observed for other adsorbates such as carbon monoxide. Ultimately, one would hope to be able to define reorganization energies for surfaces analogous to the values that have been developed for gas-phase hydrocarbon fragments.30J9qM Regardless of the extent to which reorganization contributes to the relatively small metal surface-alkyl bond energies in Table 1, it is interesting that, in all cases except aluminum, the metalhydrogen bond energies are -20 kcal/mol larger than the metalcarbon values. This is a substantial and interesting difference, given the similar values for the bond energies of alkyl groups and hydrogen atoms to isolated metal atoms.3°.34,35 Halpern has suggested that similar differences in organometallic compounds are attributable to metal-alkyl bond weakening as a result of steric e f f e c t ~ . ~ ~Itq ~is*not clear whether a similar explanation would apply for smooth metal surfaces, particularly since alkyl and H bond energies on aluminum appear to be quite similar (see Table 1). Alternative explanations in terms of bond polarity may also be and in this regard, we note that in the case of copper surfaces there is evidence for metal-to-alkyl charge transfer.22
-
4. Conclusions The kinetics of 8-hydride elimination by adsorbed ethyl groups as well as the kinetics of the reverse hydrogenation of ethylene to ethyl groups have been studied on Cu( 100) by using deuterium isotope labeling in conjunction with temperature-programmed reaction experiments. The results show that @-hydrideelimination on this copper surface is endothermic by 6.5 f 4 kcal/mol and that the deuterium isotope effect for this reaction ( k ~ / kis~10.3 ) f 0.7 at 255 K. Through a combination of these values with the binding energy for ethylene and hydrogen on Cu( 100) and the gas-phase enthalpy change for @-hydrogen removal from ethyl
radicals, the Cu( lOO)-ethyl bond strength has been determined to be 33 f 6 kcal/mol. This value corresponds to a heat of formation for ethyl groups on Cu(100) of 5 f 6 kcal/mol.
Acknowledgment. Financial support from the National Science Foundation (Grants DMR-89-57236 and CHE-90-22077) and from the Camille and Henry Dreyfus Foundation is gratefully acknowledged. References and Notes (1) The term bond energy is used here, even though it is the bond enthalpy that is really desired, since the majority of surface-adsorbate values currently available correspond to activation energies and not enthalpics. (2) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529. (3) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. J. Am. Chem. Soc. 1993, 115, 308. (4) Xi, M.; Bent, B. E. J. Vac. Sci. Technol. B 1992, 10, 2440. (5) Castelhano, A. L.; Griller, D. J . Am. Chem. SOC.1982, 104, 3655. (6) Domalski, E. S.;Hearing, E. D. J. Phys. Chem. Ref. Data, NSRDS 1988, 17, 1637. (7) Weast, R. C., Ed. Handbook of Chemistry and Physics, 58th ed.; CRC Press: West Palm Beach, FL, 1977. ( 8 ) Balooch, M.; Cardillo, M.J.; Miller, D. R.;Stickney, R. E.Surf.Sci. 1974, 46, 358. (9) Comsa, G.; David, R. Surf.Sci. 1982, 117, 77. (10) Chorkendorff, I.; Rasmussen, P. B. Surf. Sci. 1991, 248, 35. (11) Anger, G.; Winkler, A.; Rendulic, K. D. Surf. Sci. 1989, 220, 1. (12) Spitzl, R.; Niehus, H.; Poelsema, B.; Comsa, G. Surf.Sci. 1990,239, 243. (13) Campbell, J. M.; Campbell, C. T. Surf. Sci. 1991, 259, 1. (14) Kiichenshoff, S.; Brenig, W. Surf. Sci. 1991, 258, 302. (15) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. Surf.Sci. Lett. 1992, 277, L89. (16) Amanitis, D.; Baberschke, K.; Wenzel, L.; DBbler, U. Phys. Rev. Lett. 1986, 57, 3175. (17) Jenks, C. J.; Chiang, C.-M.; Bent, B. E. J. Am. Chem. Soc. 1991, 113,6308. (18) Forbes, J. G.; Gellman, A. J. J . Am. Chem. Soc. 1993,115,6277. (19) The rate of @-hydrideelimination by propyl groups is measurably faster than that for ethyl groups on copper surfaces? and a slightly lower activation energy is expected on the basis of the hydride nature of the transition state.l*.*3 (20) Paul, A.; Jenks, C. J.; Bent, B. E. Surf. Sci. 1992, 261, 233. (21) Madix, R. J.; Telford, S . G. Surf.Sci. 1992, 277, 246. (22) Lin, J.-L.; Bent, B. E. Chem. Phys. Lett. 1992, 194, 208. (23) Jenks, C. J. Ph.D. Thesis, Columbia University, 1992. (24) Lin, J.-L.; Bent, B. E. J . Phys. Chem., in press. ( 2 5 ) Zaera, F. J. Phys. Chem. 1990, 94, 8350. (26) Bent, B. E.; Nuzzo, R. G.; Dubois, L. H. J . Am. Chem. Soc. 1989, 111, 1634. (27) Bent, B. E.; Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H.J. Am. Chem. SOC.1991, 113, 1137 and references therein. (28) Burke, M. L.; Madix, R. J. J . Am. Chem. Soc. 1991, 113, 3675. (29) Cross, R. J. In The Chemistry of the Metal-Carbon Bond, Hartley, F. R., Patai, S.,Eds.; Wiley: New York, 1985; Vol. 2, Chapter 8. (30) MartinhoSimiks, J. A.; Beauchamp, J. L. Chem. R w . 1990,90,629. (31) Martinho Sim&s, J. A., Ed. Energetics of Organometallic Species; Kluwer: Boston, MA, 1992. (32) Halpern, J. Acc. Chem. Res. 1982, 15, 238. (33) Marks, T. J., Ed. EondingEnergetics in Organometallic Compounds; ACS Symposium Series 428; American Chemical Society: Washington DC, 1990. (34) Armentrout, P. B.; Gcorgiadis, R. Polyhedron 1988, 7, 1573. (35) Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res. 1989,22,315. (36) Carter, E. A.; Goddard, W. A., 111. J. Phys. Chem. 1988,92,5679. (37) Pilcher, G.; Skinner, H. A. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S.,Eds.; Wiley: New York, 1982; Chapter 2. (38) Hoffmann, R. Solids and Surfaces: A Chemists View of Bonding in Extended Structures; VCH: New York, 1988. (39) Laidler, K. J. Can. J . Chem. 1965, 34, 626. (40) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic: London, 1970. (41) Halpern, J. Inorg. Chim. Acta 1985, 100, 41. (42) Labinger, J. A.; Bercaw, J. E. Organometallics 1988, 7, 926.
