Preparation and Kinetic Characterization of Hydrocarbon Fragments

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The Langmuir Lectures Preparation and Kinetic Characterization of Hydrocarbon Fragments on Transition Metals? J. M. White Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712 Received January 10, 1994. In Final Form: August 9, 1994@ The use of nonthermal activationmethods, particularly low-energyelectrons,to prepare spectroscopically sigmficant amounts of adsorbed hydrocarbon fragments on transition metal surfaces is reviewed. Generally, a weakly adsorbed molecule is exposed to a controlled fluence of low energy, (50 eV, electrons. Through impact ionization or resonant attachment, the neutral adsorbate becomes a short-lived ion and, during its lifetime, a C-H bond is activated and dissociates, providing the hydrocarbon fragment of interest. Once the alkyl fragment is formed, its transient kinetics, with and without coadsorbed species, can be investigated through thermal processing. In this paper, phenyl and vinyl groups on Ag(ll1) and methyl groups on Pt(ll1) are discussed. 1. Introduction Heterogeneous chemistry has been practiced commercially for many decades and continues to have a major impact on international commercial activity. One of its components, heterogeneous catalysis, is widely acknowledged as having a significant impact on the gross national product of many nations.’ Practitioners of heterogeneous catalysis, particularly chemical engineers, have made enormous advances at the microscopic level, using a mixture of empirical and molecular concepts. Thermodynamics, heat and mass transfer, and overall reaction rates have been explored, and rules for tuning the catalysts have evolved. But in many (if not most) cases, molecular level details have not been readily obtained. This is particularly true of the detailed mechanistic pathways by which reactants are converted to products. While many molecular level mechanistic explanations have been offered, they have been based for the most part on inferences rather than on direct identification and measurement of surface species, Le., key intermediates that control the reaction rates. This situation began to change with the advent of infrared spectroscopy and isotope labeling which, in the hands of expert catalytic practitioners, provided direct evidence for certain chemical species on high surface area technical catalysts.2 Even more significant changes were initiated by the development, about 3 decades ago, of vacuum equipment capable of routinely obtaining pressures at or below Torr and, slightly later, the development of electron spectroscopy as a tool for monitoring surface specie^.^ These and other technical adt Based on a Langmuir Lecture presented at the 206th National Meeting of the American Chemical Society, Chicago, IL, August 22-27, 1993. Abstract published in Advance ACS Abstracts, September 15, 1994. (1)National Research Council Report Catalysis Looks To Future; National Academy Press: Washington, DC, 1992. (2)Eichens, R. P.; Pliskin, W. A. Adv. Catal. 1958,10,1. (3)(a) Scheibner, E. J.; Germer, L. H.; Hartman, C. D. Rev. Sci. Ins. 1960,31,112.(b)Harris, L. A. J.Appl.Phys. 1968,39,1419.(c)Weber, R. E.;,Peria, W. T. J.Appl. Phys. 1967,38,4355. (d) Siegbahn, K.; Nordlmg, C. N.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bermark, T.; Karlsson, S. E.; Lindgren, I.; Lindberg, B. ESCA: Atomic, Molecular and Solid State Structure Studied by means of Electron Spectroscopy; Almqvist and Wiksells: Uppsala, 1967. (e) Brundle, C. R.; Roberts, M. W. Proc. R . SOC.London 1972,A311,383. @

vances made possible the detailed preparation and characterization of clean single crystal catalyst models. As a result, surface chemical science has developed as a subfield of ~ h e m i s t r y .Presently, ~ this subfield is making important contributions to catalysis as well as a number of other technologically relevant fields, e.g., electronic materials, polymers, and electr~chemistry.~ In many cases, surface science is a “johnny-come-lately” when it comes to heterogeneous catalysis. Multiple proposed reaction mechanisms, replete with intermediates, are already in the literature, many of them well supported by clever, but indirect, experimentse6 Often, all surface science measurements can do is distinguish among the existing plausible molecular level pathways. These distinctions are important and nontrivial; they form part of the basic molecular level detail we need to have in order to gain control of key steps in catalytic mechanisms. 2. Motivation One major area of catalytic research involves hydrocarbon processing over transition-metal catalysts. Hydrocarbon fragments are of central importance in all descriptions of the molecular level chemistry that occurs as reactants are converted to products over these metal catalysts. The unambiguous identification of key intermediates is extremely difficult, largely because they are, by their very nature, very reactive when the catalytic system is operating. As a result, spectroscopically significant, i.e., quantifiable, concentrations of these intermediates seldom accumulate. Thus, we are led to search for generalizable noncatalytic methods, typically nonthermal, to synthesize and maintain a single proposed intermediate of catalytic interest. Assuming we could prepare a single proposed intermediate in spectroscopically quantifiable concentrations under conditions where it is stable for extended periods, we then have at least two major opportunities. First, (4)Somorjai, G.A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1993. ( 5 ) Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Ed.; John Wiley & Sons: New York, 1983. (6)(a) Happel, J.; Suzuki, I.; Kokayeff, P.; Fthenakis V. J. Catal. 1980,66, 59. (b) Mims, C.A.; McCandlish, L. E. J. Am. Chem. Soc. 1985,107,696.

0 1994 American Chemical Society

The Langmuir Lectures structural characterization, e.g., vibrational and photoelectron spectra, provides unambiguous detail and serves as a key benchmark, just as it does for isolated molecules.' Second, and just as important, we have the opportunity to study transient thermal reactions ofthe fragment, with and without coadsorbed species; e.g., the reaction of phenyl with methyl to make toluene on Ag(ll1) has been investigated by preparing phenyls through the electron bombardment of benzene and subsequently adding methyl groups through the thermal decomposition of methyl iodide.8 While appreciating the differences between such transient kinetics experiments and commercially operating catalytic systems, this methodology nevertheless provides a clear route to a deepened quantitative understanding of the kinetic parameters that characterize the production of a product (e.g., toluene) from known concentrations of intermediates (e.g., phenyl and methyl fragments) in the presence of known amounts of other species (e.g., iodine and carbon). By measuring the kinetic parameters as a function of relative coverage and concentration of all the surface species, we can build a database that will guide the selection of the best of the proposed mechanisms for the technologically relevant operating catalyst. Even with our best efforts, this will remain a very complicated process. As one example of such a model system, perhaps irrelevant catalytically, the conversion of ethylene to ethylidyne on Pt(ll1) has been widely studied by surface chemists because this thermally activated conversion process leads cleanly to a stable product that remains a d s ~ r b e d . ~Thus, . ~ it is easy to accumulate concentrations of ethylidyne that are sufficient for quantification using the tools of modern surface science. While the reactants and the products are easy to characterize, the key intermediates lying between reactants and products are still a subject of wide debate. Included among them, and with some experimental justification for each ofthem, are vinyl (-CHCHz), vinylidene (-CCHz), ethyl (-CHzCH3), acetylene (CHCH), and ethylidene (-CHCH3).1° These observations illustrate our generally unsatisfactory understanding of the mechanistic surface reaction pathways and motivated the research described here. While we focus on electron activation methods, the next section briefly describes a range of thermal and nonthermal methods. 3. Thermal and Nonthermal Methods Thermal methods for preparing intermediates offer some promise. One method involves rapidly quenching an operating catalytic system to pressure and temperature conditions where firther reaction does not take place. The hope is that key intermediates will be retained for subsequent surface analysis. But this hope is seldom realized because the key intermediates are generally so reactive that their concentrations, in the first place, are relatively small and, in the second place, disappear during the quenching process. Moreover, it is generally not feasible to prepare, by this means, a single key intermediate. Rather, the surface is likely covered with numerous fragments, some of which are very reactive, some that are important in the reaction pathway, and others that are merely spectators. (7) Koestner, R. J.; VanHove, M. A.; Somojai, G. A. Surf. Sci. 1982, 121, 321. (8) Zhou, X.-L.; Schwaner, A. L.; White, J. M. J . Am. Chem. SOC. 1993,115,4309. (9) Creighton, J. R.; Ogle, K. R.; White, J. M. SurF Sci. 1984, 138, I*""

Id10 I .

(10) Zaera, F. Acc. Chem. Res. 1992,25, 260.

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A second thermal method involves the thermal decomposition of alkyl iodides on transition metals. The work of Zaera" and Bent,12 for example, has focused on small hydrocarbon fragments and their transient kinetic reactivity. The C-I bond is cleaved at temperatures well below room temperature and, generally, low enough to avoid further decomposition of the adsorbed alkyl group until the surface is heated. In many cases, the coadsorbediodine appears to have a minor effect on the kinetics and no effect on the reaction pathway. Nevertheless, this method, and others like it, suffers from the presence of a coadsorbed species. Moreover, it is not always the case that thermally activated C-I bond cleavage occurs at temperatures below those where additional C-C or C-H bond cleavage will occur. This re-emphasizes a major problem with thermal methods: they often lead to mixtures of hydrocarbon fragments. Recently, in another very interesting thermal approach, Trenary and Stair13 have prepared methyl fragments by the gas-phase pyrolysis of azomethane. While the importance of thermal activation is appreciated, nonthermal means, e.g., photons and electrons, of preparing interesting fragments offer certain potential advantages. These tools offer the promise of bond selective chemistry because they can, in principle, deposit large amounts ofenergy into very few localized modes of motion. If a subsequent reaction does occur, it can be very bond specific. Even ifthe energy input does not lead to reaction, the excitation does not raise the temperature significantly. In one nonthermal method, seeded molecular beam techniques have been used to increase the kinetic energy of gas-phase methane molecules.14J5 This work demonstrates that with sufficient energy methane can be activated and C-H bonds can be cleaved. While the method is certainly very interesting and provides a route to methyl fragments and, subsequently, to their surface chemical reactivity, it appears to be limited as a clean source of other hydrocarbon fragments. While larger precursors can be prepared and given translational activation, the result is likely to be a mixture of adsorbed fragments. Another readily applied method that does not suffer from multiple fragmentation is photochemical activation of organic halides. While small alkyl fragments have been studied most widely, for example by C O W ~ I IP01anyi,~~-~ ,~~J~ and our research g r ~ u p , ~we l -have ~ ~ extended the method to include phenyl fragments prepared from the haloben(11) Hoffian, H.; Griffiths,P. R.; Zaera, F. Surf.Sci. 1992,162,141. (12)Xi, M.; Bent, B. E. J.A m . Chem. SOC.1993,151, 7426. (13) Fairbrother, D. H.; Peng, X. D.; Viswanathan, R.; Stair, P. C.; Trenary, M.; Fan, J. J . Surf Sci. Lett. 1993,285, L460. (14) (a)Ceyer, S.T.Langmuir 1990,6,87. (b)Beckerle,J. D.;Johnson, A. D.;Ceyer,S . T.Phys.Rev.Lett. 1989,62,685. (c)Yang,Q.Y.;Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J . A m . Chem. Soc. 1989,111,8748. (15) Schoofs, G .R.; Arumainayagam,C. R.; McMaster,M. C.;Madix, R. J. Surf. Sci. 1989.215. 1. (16) Marsh, E.P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. Rev. Lett. 1988, 61, 2725. (17) (a) Marsh, E.P.; Schneider, M. R.; Gilton, T. L.; Tabares, P. L.; Meier, W.; Cowin, J. P. Phys. Rev. Lett. 1988,60,2551. (b) Marsh, E. P.; Tabares, P. L.; Schneider, M. R.; Gilton, T. L.; Meier, W.; Cowin, J. P. J. Chem. Phys. 1990,92, 2004. (18) Polanyi,J. C.;Relly, H. InDynamics of Gas-Surface Interactions; Rettner, C. T., Ashfold, M. N. R., Eds.; Royal Society of Chemistry: London, 1991; p 329. (19) Dixon-Warren, St.-J.;Jensen, E. T.; Polanyi, J. C. Phys. Rev. Lett. 1991, 67, 2395. (20) Dixon-Warren, St.4.; Jensen, E. T.; Polanyi, J. C.; Xu, G.-Q.; Yang, S. H.; Zeng, H. C. Faraday Discuss. Chem. SOC.1991,91, xxx. (21) Review article, Zhou, X.-L.;Zhu, X.-Y.;White, J. M. Surf.Sci. Rep. 1991, 13, 77. (22) Zhou, S.-L.;White, J. M. Laser Spectroscopy and Photochemistry on Metal Surfaces; Dai, Hai-Lung;Ho, Wilson, Eds.;World Scientific Publishers, in press. (23) Zhou, X.; White, J. M. J . Phys. Chem. 1991,95, 5575.

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Because these photochemical reactions can be driven at any substrate temperature, it is always possible to cool the substrate enough to make negligible any activated thermal process that occurs subsequent to 3 photodissociation. Thus, it appears that this method is quite general, cleanly producing a selected hydrocarbon fragment from its parent halide. Bromides and chlorides are of particular interest because the C-Br and C-C1 bonds are strong enough (and the adsorbate-substrate bonds weak enough)to preclude, on most surfaces,thermal decomposition when the photolyzed mixture is heated \ slightly to remove the remaining undissociated reactant. Parent removal is desirable as a prelude to spectroscopic I I . < , l I 0 and kinetic characterization. By contrast, the iodides are 0 1 2 3 4 readily thermally dissociated on most metal surfaces. ENERGY (eV) Consequently, thermal- and photon-driven decomposition Figure 1. Negative ion yield from CzHECl as a function of effects are more easily disentangled when using chlorides electron energy. "he ethyl chloride is gas phase and at 300 K. and bromides. As noted earlier, the major difficulty with From ref 31. this method has to do with the influence of the remaining two that can easily localize the required energy are halogens. described here-electron attachment and impact ionizaUnlike the alkyl halides, which are susceptible to tion. fragmentation by both photons and electrons, hydrocarAt low kinetic energies, < l o eV, the electron might bons often do not dissociate when irradiated with ultraattach to the fragment by populating the LUMO (lowest violet photons.21,26But low-energy electrons have been unoccupied molecular orbital). This is a resonant process; used successfully,in selected cases, to prepare hydrocarbon the energy of the electron must match that of the LUMO, fragments. For example, we have shown that benzene, and energies above or below this value will have no cross adsorbed on Ag(lll), can be controllably converted to section for a t t a ~ h m e n t . ~ ~ phenyl fragments, with no evidence for further decomAt high energies, i.e., those exceeding the ionization position of the phenyls.26 Similarly, we have shown that potential of the fragment, a nonresonant process is ethylene can be converted to vinyl fragments ~ n A g ( l l l ) . ~ ~ possible. In this case the incident electron transfers By careful heating, we have been able to remove coadenough energy to completely remove one or more of the sorbed atomic hydrogen leaving the vinyl and phenyl fragment electrons. Called impact ionization,this electron fragments. Other workz8examines electron-stimulated bombardment process is common in conventional gasreactions of adsorbed methane, ethane (Alberas), and phase mass spectrometry, where electrons with energies cyclohexane (Koel) on Pt(111).28In the latter, Koel has on "the order of 70 eV are used for ionization. It is shown that adsorbed cyclohexane can be converted to nonresonant because multiple unbound particles are cyclohexyl fragments on platinum surfaces through involved; thus, energy and momentum conservationrules electron-stimulated cleavage of the C-H bond. are easily satisfied for any incident energy. To bring this section to a close, we need only note that As an interesting example of electron attachment, there are now available to surface chemists a number of consider Figure 1, which plots the ion current versus complementary tools for the preparation of hydrocarbon incident electron energy for gas-phase ethyl chloride at fragments. In many cases, these methods are capable of 300 K.31 Clearly, electrons with energies below 1eV and producing a single hydrocarbon fragment, albeit in the above 3 eV are ineffectivein producing ions; i.e., the process presence of a coadsorbate. In a few cases, it is possible is resonant. The interpretation is that a 1.6-eV electron by careful annealing to remove the coadsorbate and leave is resonantly attached to ethyl chloride by occupying the a spectroscopically significant coverage of the single LUMO. hydrocarbon fragment. This situation is, of course, Interestingly, we have found, for ethyl chloride weakly desirable as a starting condition for other experiments; held on Pt(lll),that the cross section for breaking the we want to spectroscopicallycharacterize these fragments C-C1 bond shows significant structure in the region and study their transient thermal reaction behavior among between 1 and 50 eV.32 As shown in Figure 2, 1-eV themselves and with intentionally added fragments. electrons have a reasonably high cross section for bond cleavage, whereas 3-eV electrons do not. Near 8 eV the 4. How Electrons Drive Surface Chemistry cross section begins to rise again and reaches a plateau near 20 eV. A reasonable interpretation is relatively Since electron-driven activation is our focus,we consider straightforward. At low energiesthe resonant attachment how electrons can drive surface chemistry. As we all know, process occurs using the LUMO of ethyl chloride, which the mass of an electron is small compared to that of any is perturbed slightlyby the presence of the metal substrate. atom, molecule, or fragment. Consequently, we cannot The resulting temporary anion is characterized by strong expect energetic electrons to directlytransfer a significant repulsive forces along the C-C1 bond direction, i.e., the amount of momentum to such particles. Rather, the dissociation ensues. Beginning at 8 eV, and extending incident electron interacts with the bound ele&rons of toward higher energies, the process becomes dominated the system. While there are numerous scattering paths, by impact ionization, in which the incident electron essentially ionizes the ethyl chloride and, just as in the (24)Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Acc. Chem. Res. 1990,23, gas phase, there is a relatively high probability of 327. subsequent C-C1 cleavage. In this context, it is important (25)Zhou, X.-L.; White, J . M. J. Chem. Phys. 1990,92 (91,5612. I I I I . l l l l . .

(26)Zhou, X.-L.; Castro, M. E.; White, J. M. Surf. Sci. 1990,238,215. (27)Zhou,X.-L.; White, J. M. J.Phys. Chem. 1992,96,7703. (28)Alberas, D.A.;White, J. M. To be submitted for publication. Xu, C.; Koel, B. E. Surf. Sci. 1993,292,L803. (29)Ramsier, R. D.;Yates; J. T., Jr. Surf. Sci. Rep. 1991,12,243.

(30)Jordan, K.D.;Burrow,P.D.Chem. Rev. 1987,87,557. (31)Pearl, D.M.;Burrow, P.D.Chem. Phys. Lett. 1995,206,483. (32)Liu, Z.-M.; Zhou,X.-L.;White, J. M. Chem.Phys.Lett.1992,198 (6),615.

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I

!I

I

,

I 0

'

I

Before e-beam irradiation

I 10

20

30

40

50

Electron Energy (eV)

Figure 2. As a function of electron energy, semilogarithmic plot of the relative cross section for electron-drivenloss of ethyl chloride adsorbed on Ag(ll1). From ref 32.

to note that, while bond selectivity may be realized, it cannot be controlled easily; intrinsic properties of the orbitals and the surroundings control. Compared to the gas phase, there are other interesting effects on Pt(ll1). Specifically, the incident electrons excite not only adsorbate electrons but substrate electrons as well. Since electron scattering cross sections are very high, if an electron enters the substrate it will be scattered in a very short distance, losing energy and momentum to the surrounding electronic structure. Insofar as the substrate is concerned, this excitation leads to excited electrons (secondaries) and holes above and below the Fermi level, respectively, and a n adsorbate will certainly take note of these. For the case a t hand, the hot electrons are important because they can contribute to the electron attachment process. Put in other terms, even though the horizontal axis of Figure 2 is properly thought of as the incident energy, the process involves a distribution of electrons made up of the incident electrons plus the secondary distribution excited in the substrate. With this background in mind, we turn to some examples that employ incident electrons with energies sufficient to cause impact ionization of selected adsorbed hydrocarbons.

5. Interaction of Electrons with Benzene Adsorbed on Ag(ll1) The interaction of electrons with C6H6 adsorbed on Ag(ll1) illustrates the feasibility of bond selective chemistry.6,26C6H6 molecules adsorb weaklyonAg(ll1) at 100 K and desorb molecularly a t -160 K (physisorbed) and 208-220 K (chemisorbed) with no decomposition.26 The adsorption lowers the work function ofAg(ll1) by 0.7 eV at monolayer and 0.9 eV at multilayer coverages. Even though benzene adsorbs photons,26no photoeffects were observed with ultraviolet light from a high-pressure Hg arc lamp (5.3 eV, maximum photon energy). This is attributed to the rapid and efficient substrate quenching of the photoexcited CGHG(~). ARer exposure to electrons, ClzHlo and HZwere observed in TPD, indicating decomposition of C6H6 (Figure 3). Interestingly, no species other than C6H6 (210 K), ClzHlo ( r 3 7 5 K), and HZ(e200 K) were detectable. After TPD,

Temperature (K)

Figure 3. TPD spectra (3.5 Ws) taken after exposing 1ML of benzendAg(ll1) to 6 x 10I6electrons (50 eV) at 90 K. The broken curve is the C6H6 TPD spectrum before electron irradiation. From ref 8.

r----

0

2 4 Electron Fluence

6

a

io

e'/cm2)

Figure 4. TPD areas of C6D6, Dz, and ClzDlo (from Figure 3) as afunctionofeledronfluenceforlMLofCsDdAg(1ll)exposed to 50-eV electrons. From ref 26.

surface carbons were below the detection limits of AES and XPS. In experiments using C6D6, with increasing electron fluence, the C6D6 TPD area decreases (Figure 4) and the DZand ClzDlo peak areas increase. As for CzH5 (Figure 21, the incident electron energy is important. For electron energies below 11 eV, no decomposition is detected, but it is readily detected when 16-eV electrons are used. The decomposition cross section is of order 10-17 cm2 between 30 and 50 eV. To confirm that phenyl fragments, not biphenyls, are formed and retained at 100 K during electron irradiation, we prepared C & / A g ( l l l )by irradiating 1monolayer (ML) C6DdAg(lll) at -100 K with 5 x 10l6electrons/cm2 (50 eV) and then heating to 300 K to remove parent benzene, presumably leaving C&(a). After cooling to 100 K, iodobenzene (C6H51) was dosed. The resulting TPD not ~ ,CIzD12. spectrum (Figure 5) was dominated by C I Z H ~ D

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Figure 5. TPD spectra of ClzDsHs,ClzDlo, and ClzHlofollowing adsorption of > 1ML of iodobenzene on C&/Ag(lll)at -100 K. The C~D5/Ag(lll)surface was prepared by first irradiating 1ML of C&dAg(lll)at 100 K with 5 x 10l6electrons/cm2(50 eV) and then heating it to 300 K. The dotted curve is ClzDlo desorption from CadAg(ll1)surface without dosing iodobenzene. The temperature ramping rate was 4 Ws. From ref 26.

The latter would dominate if it formed either at 100 K or upon heating to 300 K. We take this as strong evidence for the presence of phenyl groups up to at least 300 K. As the electron fluence increases, the increased C12H10 TPD area and decreased C6H6 area are linearly related (Figure 41, indicating that the fraction of CGH5 retained is constant. Since electron stimulated desorption (ESD) of surface hydrogen is w e l l - k n o ~ dand ~ has been reported in the EID of C6H5C1on Ag(lll),34we believe that atomic His both retained and desorbed. For irradiation exceeding 5 x 10l6electron cm-2 (50 eV), some C,H,(a) (x and y 5) may form. Since the EID threshold is > 11eV, we propose that the EID involves an electron impact ionization process, i.e., CsHs(a) CsHs+(a) CsHs(a) H(a). Electron impact ionization occurs above 9 eV.35,36In the gas-phase mass spectrum of benzene, ions of C6Hs+, C6H5+, C&+ (x = 2-4) and C3H3+are seen. Electron-benzene interactions also produce CsH5- and H.37 It is striking that the decomposition of C6H6 on Ag(lll), induced by