Coverage Effects on the Kinetics of Benzene Formation from

Reported are investigations regarding the coverage dependence of the kinetic factors for acetylene cyclotrimerization on Pd(111). Laser-induced therma...
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J. Phys. Chem. 1996, 100, 10265-10268

10265

Coverage Effects on the Kinetics of Benzene Formation from Acetylene on Pd(111): A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Investigation Ihab M. Abdelrehim, Tracy E. Caldwell, and Donald P. Land* Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed: December 15, 1995; In Final Form: March 19, 1996X

Reported are investigations regarding the coverage dependence of the kinetic factors for acetylene cyclotrimerization on Pd(111). Laser-induced thermal desorption/Fourier transform mass spectrometry is used to monitor the rate of the surface reaction prior to conventional desorption. Surface benzene formation is rate limited by the addition of acetylene to C4H4, in contrast to gas-phase benzene production which is rate limited by subsequent benzene desorption. Increasing initial acetylene exposures lead to a substantial increase in both the activation energy and preexponential factor for benzene formation. For initial acetylene exposures from 1.0 to 2.5 langmuir, the activation energy varies from 24.4 ( 3.9 to 43.5 ( 3.9 kJ/mol (95% confidence). Fitting to pseudo-first-order kinetics, the preexponential factors vary from 103.1(0.1 to 109.8(0.1 s-1. A change in diffusion rate and steric blocking can both be ruled out as significant contributors to the coverage effect, since both of these processes would lead to the opposite trend in preexponential factors. The change in kinetic factors is most likely a consequence of an increased barrier to C-Pd bond breaking due to more tightly bound species at higher acetylene coverage. Supporting evidence for this is cited from the literature.

1. Introduction Acetylene cyclization studies on Pd(111) have been ongoing for the past 10 years, since temperature-programmed desorption (TPD) results showed efficient benzene production.1-13 Two distinct desorption states were observed. The low-temperature state (210-250 K) has been attributed to a tilted geometry, while high-temperature desorption (500 K) has been attributed to flatlying benzene.1,2,7 Acetylene cyclotrimerization is an important reaction, since similar products are formed under conditions of high-pressure catalysis and homogeneous catalysis and also in ultrahigh vacuum (UHV). Hence, reaction mechanisms in all three regimes can be compared, provided details for each regime are available. Previous UHV studies include isotopic-labeling studies indicating that no C-C or C-H bond scission occurs in the surface process,5 and various results have provided evidence that the reaction proceeds via a C4H4 intermediate.2,6 Since gas-phase benzene production in TPD appears to be desorption-rate limited, information about the mechanism of the surface process has been gleaned primarily by indirect methods such as observation of side products2 or through the use of alternative starting materials.7-9 Previous laser-induced thermal desorption/Fourier transform mass spectrometry (LITD/FTMS) results provided further evidence for a C4H4 intermediate via the direct observation of butadiene as a side product,6 but in addition, since these studies can probe adsorbate-adsorbate transformations, further information about the rate and mechanism of the surface processes can be obtained. For example, previous kinetics studies of a near-saturation acetylene exposure showed a one-to-one stoichiometry of acetylene consumed to benzene formed during the rate-determining process.6 Thus, not only is a stepwise process confirmed, but the rate-determining step can be identified as the addition of acetylene to C4H4. Reported here are kinetics investigations of benzene formation using acetylene exposures from 1.0 to 2.5 L (near saturation) on clean Pd(111). (L ) langmuir ) 10-6 Torr‚s) Increasing initial acetylene exposures * Author to whom correspondence should be addressed. Phone: (916) 752-5260. FAX: (916) 752-8995. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, May 15, 1996.

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lead to a substantial increase in both the activation energy and preexponential factor for benzene formation. The activation energy varies from 24.4 ( 3.9 kJ/mol (95% confidence) to 43.5 ( 3.9 kJ/mol for initial acetylene exposures from 1.0 to 2.5 L. Fitting to pseudo-first-order kinetics, the preexponential factors vary from 103.1(0.1 to 109.8(0.1 s-1. A change in diffusion rate and surface crowding can both be ruled out as significant contributors to the coverage effect, since both of these processes would lead to the opposite trend in preexponential factors. The change in kinetic factors is most likely a consequence of an increased barrier to C-Pd bond breaking due to more tightly bound species at higher acetylene coverage. Supporting evidence for this is cited from the literature. 2. Experimental Section Details of the apparatus were previously published by Land et al.14 Briefly, a Nd:YAG laser is focused onto a Pd(111) single crystal (5 ns pulse width and 1 mm diameter) at 1064 nm and ∼107 W/cm2 power density. This power density does not lead to substrate ablation; however, the temperature jump produced at the surface causes desorption of neutral adsorbates.15 Desorption is followed by postionization with an electron beam (70 eV) and detection by FTMS with a 0.5 T magnetic field. In the majority of cases the desorbed species have been shown to be representative of surface composition.16-19 FTMS allows observation of a complete mass spectrum, ranging from m/z ) 10 to 150, for each laser shot. The Pd sample is supported at the end of a long-motion manipulator in a UHV chamber (base pressure ) 2 × 10-10 Torr). Ar+ (500 eV) bombardment, followed by prolonged annealing, is used to remove tightly bound species, such as sulfur. Carbon is removed by heating to 600 °C in the presence of oxygen (partial pressures of 10-6 Torr). The elemental surface composition is monitored by Auger electron spectroscopy. In addition, carbon contamination was monitored by checking CO desorption with the mass spectrometer during and after oxygen treatments. Following cleaning, the surface is cooled to 80 K and dosed with acetylene. The acetylene is spectral grade (99.96% pure) © 1996 American Chemical Society

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Abdelrehim et al.

Figure 1. Polanyi-Wigner plot of benzene formation from acetylene (1.0 and 2.5 L exposures) using initial rates. The 1.0 L exposure is indicated by squares and the 2.5 L exposure by circles.

with an acetone stabilizer. The acetone impurity is removed by passing the acetylene through a dry-ice/acetone trap. (Purity of the acetylene is checked by FTMS during use.) A sapphiresealed variable leak valve is employed to dose the Pd surface with acetylene by back-filling. The sample is then positioned for LITD/FTMS. Acetylene cyclotrimerization kinetics studies are conducted using initial acetylene exposures between 1.0 and 2.5 L (near saturation)2 on clean Pd(111) at 80 K. The acetylene exposures are corrected using an ion gauge sensitivity factor of 1.66.20 The Pd is quickly heated to the desired temperature, and laser shots are taken at approximately 40 different positions on the surface (1 shot/position). Several temperatures ranging from 150 to 200 K are used to construct isothermal rate curves for each initial coverage.

Figure 2. Graphical depiction of the coverage dependence of the kinetic factors for benzene formation from acetylene. (a) Activation barriers for benzene formation as a function of initial acetylene exposure. (b) Preexponential factors for benzene formation as a function of initial acetylene exposure. Saturation of a x3 × x3 R 30° acetylene overlayer reportedly occurs near 2.6 L exposure. The curves are exponentials with an arbitrary offset at zero coverage used to guide the eye and not intended to imply the true functional dependence.

3. Results and Discussion 3.1. Use of Initial Rates to Calculate Activation Energies. Initial reaction rates can be used to construct a variation of a Polanyi-Wigner plot (see Figure 1) even without knowledge of the details of the reaction mechanism. A detailed explanation of data manipulation and kinetic interpretation has been published previously.6 In brief, if one assumes an irreversible reaction

xX + yY f Z the initial rate of reaction (r0) equals the initial rate of product formation and is merely the reaction rate constant multiplied by the initial concentrations of reactants (X and Y), each raised to some power. Substituting the Arrhenius equation for the rate constant and taking the natural logarithm of the initial rate give:

ln(r0) ) {ln(ν) + m ln([X]0) + n ln([Y]0)} - ((E/R)(1/T)) The initial concentrations of reactants are represented as [X]0 and [Y]0, n and m are dependent on the details of the reaction mechanism, ν is the Arrhenius preexponential factor, E is the activation energy, R is the gas constant, and T is the absolute temperature. The three terms in brackets on the right side of the equation should be independent of temperature for a small temperature range. Hence, a plot of ln(r0) vs 1/T should yield a slope of -E/R. However, the intercept can no longer be used to extract the preexponential factor unless the kinetic dependence and initial concentrations are known. This treatment is valid even when competing reactions for X and Y exist, provided the initial concentrations are identical and the extent of all reactions is sufficiently small. To estimate the initial rate of reaction, the benzene signal at T ) ∞ is estimated assuming pseudo-first-order kinetics, and then only data for

which the signal is less than 50% of this maximum value for benzene is used to estimate the initial rate. Even though this accounts for a significant fraction of the benzene formed at this temperature, the acetylene concentration changes by less than 10% at this temperature, so competition for acetylene is not a significant source of error, since the kinetics appear pseudozeroth order in acetylene under these conditions. This shows, in addition, that the formation of vinylidene, thought to occur near 300 K, is not competitive with benzene formation at 200 K.2,21 The C4H4 intermediate proposed as the other reactant has also been shown to be stable above 250 K.7,9,22 Additionally, no benzene desorption or decomposition takes place below 200 K.5 Initial reaction rates are estimated by fitting a straight line to the benzene signals at the beginning of each isothermal experiment. Figure 1 shows the natural logarithm of the initial rates plotted vs 1/T for initial acetylene coverages of 1.0 and 2.5 L. (Saturation of a x3 × x3 R 30° acetylene overlayer reportedly occurs near 2.6 L exposure.2) The offset of the two curves demonstrates an increase in the initial rate of benzene formation by a factor of 10-50 for the 2.5 L exposure relative to the 1.0 L exposure. Activation barriers for benzene formation are acquired using a weighted linear least-squares analysis. (The data are weighted using factors to account for both the differing uncertainties in the rates, as well as the logarithmic transformation.) The activation energies for benzene formation vary from 24.4 ( 3.9 kJ/mol (95% confidence) to 43.5 ( 3.9 kJ/mol, respectively.6 A near doubling in the activation energy is observed as the initial acetylene exposure is increased by a factor of ∼2, as shown in Figure 2. The total amount of benzene formed on the surface after warming to 310 K, as probed by LITD/FTMS, is approximately four times greater for the 2.5 L initial acetylene exposure than for the 1.0 L initial exposure.

Acetylene Cyclotrimerization on Pd(111) 3.2. The Reaction is Pseudo-First OrdersPreexponential Factors. If details of the reaction mechanism can be inferred, then a preexponential factor can be calculated. As mentioned previously, the reaction has been shown to follow pseudo-firstorder kinetics when acetylene is present in excess.6 Under the conditions of these experiments, acetylene consumption ceases leaving ∼85% of the initial acetylene concentration unreacted for the 2.5 L exposure of acetylene and ∼83% unreacted for the 1.0 L exposure. Hence, acetylene is not the limiting reactant, and since the acetylene concentration remains relatively unchanged, the reaction proceeds with pseudo-zeroth order with respect to acetylene (in large excess) and with first order with respect to the C4H4 intermediate.6 Using pseudo-first-order kinetics to model the isothermal data and incorporating the activation energy acquired from the initial rates, one can solve for the preexponential term at each temperature. All preexponential factors for each initial acetylene exposure are then averaged, yielding values ranging from 103.1(0.1 to 109.8(0.1 s-1 (95% confidence), for initial acetylene exposures between 1.0 and 2.5 L, respectively.6 The preexponential factors are also plotted vs initial exposure in Figure 2. 3.3. Source of the Coverage Dependence of Kinetic Factors. The rate-determining step of the surface reaction involves the addition of C2H2 to C4H4. The rate of this addition may be governed either by diffusion of the reactants, by bond breaking, or by rehybridization necessary to transform into the transition-state complex for conversion to adsorbed benzene. The adsorption of acetylene and its effects on the electronic structure of the surface prior to benzene formation have been examined by vibrational spectroscopy, ultraviolet photoelectron spectroscopy, and theoretical investigations.2,3,8,10-12 Adsorbed acetylene rehybridizes to sp2 with the CCH angle at 120-130°; however, the plane of the molecule is essentially perpendicular to the surface.2,3,8,10-12 Investigations have also been conducted to study the C4H4 surface species by adsorbing cis-3,4dichlorocyclobutene and dechlorinating at elevated temperatures.7-9 Near-edge X-ray absorption fine structure studies,9 supported by cluster calculations,10 suggest that adsorbed C4H4 is a metallocycle tilted by 20-30° from the surface normal. Note that while the hybridization of the carbon centers in each molecule is essentially sp2, thus correct for benzene, the adsorption geometries for C2H2 and C4H4 are not consistent with any C2 or C4 subunit of adsorbed benzene.10 Thus, it is likely that the reaction is rate limited by either diffusion or C-Pd bond breaking during reorientation. The trends observed in the kinetic factors can be used to elucidate the processes that are rate limiting. For example, surface crowding can be ruled out as the major contributor to the coverage dependence in the kinetic factors. Addition of an acetylene molecule to a C4H4 surface species may require a change in adsorption geometry.6,10 If the activation barrier for benzene formation involves the reorientation of the C4H4 species, then surface crowding at high acetylene exposures (2.5 L) may cause an increase in the activation barrier; however, one would also expect a decrease in the preexponential term under these conditions.23 In contrast, the results reported here show an increase in the preexponential factor for benzene formation, thus arguing against such a steric process. Therefore, either diffusion or bond breaking is responsible for the coverage dependence of the kinetic factors. For either of these processes, the increase in activation barrier with increasing acetylene coverage implies an increase in adsorbate-surface bond energy with increasing coverage. An increase in surface-adsorbate binding with increasing coverage can be rationalized for acetylene on Pd by considering

J. Phys. Chem., Vol. 100, No. 24, 1996 10267 the details of the electronic structure near the Fermi level. A previous study conducted by Hoffmann on the acetylene/Pt(111) system illustrates the nature of the orbitals involved in adsorbatemetal bonding.24,25 It was observed that most of the acetylene molecular orbitals were only slightly affected by adsorption, whereas the π* orbital was greatly stabilized. The acetylene π* orbital, then, is the major contributor in di-σ bonding between acetylene and Pt. The resulting states due to interaction of π* acetylene orbitals with Pt surface orbitals have a significant density near the Fermi level. Thus, an increase in the Fermi energy will result in increasing electron availability for di-σ bonding (back-bonding), leading to increased adsorbatesubstrate interaction. Previous results by Ormerod and Lambert illustrate a decrease in the work function of Pd(111) with increasing acetylene exposure, saturating at ∆φ ) -1.5 eV.13 The decrease in the work function is a consequence of net electron transfer from acetylene to the Pd surface and is consistent with an increase in the local Fermi energy. On both surfaces, the di-σ bonding also leads to sp2 character in the adsorbed acetylene. Carbon rehybridization is supported by vibrational and UPS studies conducted by several groups on the acetylene/Pd(111) system.2,3,8,10-12 Since rehybridization is observed for acetylene on Pd(111), a correlation with the Pt system may be inferred. The interaction of the C4H4 species with the surface may also be affected by acetylene coverage and thus affect the activation barrier to benzene formation, but information about the electronic structure of this species is unavailable. The bond strengthening resulting between the acetylene and Pd at higher acetylene coverages may cause an increase in the diffusion energy barrier. However, the preexponential factor for diffusion should decrease with increasing coverages, whereas the opposite trend is observed for benzene formation. Therefore, it seems unlikely that diffusion contributes significantly to the rate-limiting process in benzene formation. The increase in activation energy is most likely due to an increased barrier to C-Pd bond stretching or breaking to form a transition complex with the correct geometry for benzene formation. The corresponding change in preexponential factor with respect to the activation energy has previously been discussed and utilized in explaining the compensation effect.26,27 A more tightly bound reactant can be expected to lead to a greater entropy of activation, provided the transition state is not as greatly affected. 4. Conclusion The coverage dependence of the activation energy and preexponential factor for acetylene cyclotrimerization on Pd(111) were obtained from isothermal rate studies using LITD/FTMS. Increasing the initial acetylene exposure from 1.0 to 2.5 L caused an increase in the rate of benzene formation by a factor of 1050 and an increase in the relative yield of benzene by a factor of 4. The activation barriers for benzene formation increase from 24.4 ( 3.9 kJ/mol (95% confidence) for the 1.0 L exposure of acetylene to 43.5 ( 3.9 kJ/mol for the 2.5 L exposure. The preexponential factors, assuming pseudo-first order kinetics, increase from 103.1(0.1 s-1 for the 1.0 L exposure to 109.8(0.1 s-1 for the 2.5 L exposure. Since the activation barrier and preexponential factors both increase substantially with increasing acetylene exposure, effects due to crowding or changes in diffusion kinetics cannot account for the change in reaction kinetics observed. The most likely explanation is an increased barrier to C-Pd bond breaking due to stronger bonding at higher coverages. This is consistent with the calculated electronic structure of acetylene adsorbed on Pt(111).

10268 J. Phys. Chem., Vol. 100, No. 24, 1996 Acknowledgment. The authors thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. T.E.C. acknowledges support in the form of a Patricia Roberts Harris Graduate Fellowship. The authors also thank the Committee on Research, University of California, Davis, for additional support of this work. References and Notes (1) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. J. Chem. Soc., Chem. Commun. 1983, 623. (2) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. Surf. Sci. 1983, 135, 128. (3) Sesselmann, W.; Woratschek, B.; Ertl, G.; Kuppers, J.; Haberland, H. Surf. Sci. 1983, 130, 245. (4) Gentle, T. M.; Mutterties, E. L. J. Phys. Chem. 1983, 87, 2469. (5) Patterson, C. H.; Lambert, R. M. J. Phys. Chem. 1988, 92, 12661270. (6) Abdelrehim, I. M.; Thornburg, N. A.; Sloan, J. T.; Caldwell, T. E.; Land, D. P. J. Am. Chem. Soc. 1995, 117, 9509. (7) Patterson, C. H.; Mundenar, J. M.; Timbrell, P. Y.; Gellman, A. J.; Lambert, R. M. Surf. Sci. 1989, 208, 93. (8) Hoffman, H.; Zaera, F.; Ormerod, R. M.; Lambert, R. M.; Yao, J. M.; Saldin, D. K.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1992, 268, 1. (9) Ormerod, R. M.; Lambert, R. M.; Hoffman, H.; Zaera, F.; Yao, J. M.; Saldin, D. K.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1993, 295, 277. (10) Pacchioni, G.; Lambert, R. M. Surf. Sci. 1994, 304, 208. (11) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68.

Abdelrehim et al. (12) Kesmodel, L. L.; Waddill, G. D.; Gates, J. A. Surf. Sci. 1984, 138, 464. (13) Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. 1992, 96, 8111. (14) Land, D. P.; Abdelrehim, I. M.; Thornburg, N. A.; Sloan, J. T. Anal. Chim. Acta 1995, 307, 321-331. (15) George, S. M. In InVestigations of Surfaces and Interfaces, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Part A, Vol. IXA, pp 453-497. (16) Land, D. P.; Wang, D. T.-S.; Tai, T.-L.; Sherman, M. G.; Hemminger, J. C.; McIver, R. T., Jr. In Lasers in Mass Spectrometry; Lubman, D. M., Ed.; Oxford University Press: New York, 1990; p 157. (17) Land, D. P.; Pettiette-Hall, C. L.; Hemminger, J. C.; McIver, R. T., Jr. Acc. Chem. Res. 1991, 24, 42. (18) McIver, R. T., Jr.; Sherman, M. G.; Land, D. P.; Kingsley, J. R.; Hemminger, J. C. In Secondary Ion Mass Spectrometry: SIMS V; Benninghoven, A., Colton, R. J., Simons, D. S., Werner, H. W., Eds.; Springer-Verlag: New York, 1986; p 555. (19) Land, D. P.; Pettiette-Hall, C. L.; McIver, R. T., Jr.; Hemminger, J. C. J. Am. Chem. Soc. 1989, 111, 5970. (20) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149-153. (21) Ormerod, R. M.; Lambert, R. M.; Hoffman, H.; Zaera, F.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. J. Phys. Chem. 1994, 98, 2134. (22) Patterson, C. H.; Lambert, R. M. J. Am. Chem. Soc. 1988, 110, 6871. (23) Deckert, A. A.; Arena, M. V.; Brand, J. L.; George, S. M. Surf. Sci. 1990, 226, 42. (24) Hoffmann, R. M. O. In Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures; VCH Publishers: New York, 1988; p 40. (25) Silvestre, J.; Hoffmann, R. Langmuir 1985, 1, 621. (26) Galwey, A. K. AdV. Catal. 1977, 26, 247. (27) Conner, W. C., Jr. J. Catal. 1982, 78, 238.

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