J. Phys. Chem. 1992, 96, 5045-5048 Ann Arbor Science: Ann Arbor, MI, 1981;pp 183-217. (h) Westall, J. C. In Geochemical Processes at Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.;ACS Symposium Series 323;American Chemical Society: Washington, DC, 1986;pp 54-78. (i) Schindler, P. W.; Stumm, W. In Aquatic Suqace Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; pp 83-110. 6 ) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling, Wiley: New York, 1990. (13)Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, 0.Anal. Chem. 1989,61, 132. (14) For reviews see: (a) Bard, A. J.; Fan, F.-R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. 0.;Zhou, F. Science 1991,254,68.(b) Bard, A. J.; Denault, G.; Lee, C.; Mandler, D.; Wipf, D. 0. Acc. Chem. Res. 1990, 23, 357. (15) See,for example: (a) Visvanathan, R.; Burgess, D. R., Jr.; Stuir, P. C.; Weitz, E. J. Vac. Sci. Technol. 1982,20, 605. (b) George, S . M.; DeSantolo, A. M.; Hall, R. B. Surf. Sci. 1985, L428, 1985. (16) Kwak, J.; Bard, A. J. Anal. Chem. 1989,61, 1221. (17) Unwin, P. R.; Bard, A. J. J. Phys. Chem. 1991,95,7814. (18) See,for example, ref 12h and (a) Westall, J. C. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987;pp 3-32. (b) van Riemsdjik, W. H.; Bolt, G. H.; Koopal, L. K.; Blaakmeer, J. J. Colloid Interface Sci. 1986,109,219. (c) Hiemstra, T.; van Riemsdjik, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989,133,91. (d) Hiemstra, T.; de Wit, J. C. M.; van Riemsdjik, W. H. J. Colloid Interface Sci. 1989, 133, 105. (19) Yates, D. E.; Levine, S.; Healy, T. W. J. Chem. Soc.,Faraday Trans. 1 1974,70, 1807. (20) Huang, C. P.; Stumm, W. J. Colloid Interface Sci. 1973,43,409. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 507.
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(22)Saito, Y. Rev. Polarogr. Jpn. 1968,15, 177. (23)Peaceman, D. W.; Rachford, H. H. J. Soc. Indust. Appl. Math. 1955, 3,28. (24) Bard, A. J.; Mirkin, M. V.; Unwin, P. R.; Wipf, D. 0.J. Phys. Chem. 1992,96, 1861. (25) Bard, A. J.; Denault, G.; Friesner, R.A.; Dornblaser, B. C.; Tuckerman, L. S.Anal. Chem. 1991, 63, 1282. (26)Gilbert, S.E.; Kennedy, J. H. J. Electrochem. Soc. 1988,135,2385. (27) (a) Fmklea, H. 0.J. Electrochem. Soc. 1982,129,2003.(b) Cooper, G.; Turner, J. A.; Nozik, A. J. J. Electrochem. SOC.1982,129, 1973. (28)Compton, R. G.;Unwin, P. R. Phil. Trans. R . SOC.London 1990, A330, 1, and references therein. (29) (a) Berube, Y. G.; de Bruyn, P. L. J. Colloid Interface Sci. 1968,27, 305; 1968,28,92. (b) Ahmed, S.M.; Maksimov, D. J. Colloid Interface Sci. 1969.29.97. (c) Yates. D. E.: Healv. T. W. J. Chem. Soc.. Faradav Trans. 1 1g0, 76,9.'(d) Fokknk, L.G. J:;'De Keizer, A.; Lyklema, J. J.'Colloid Interface Sci. 1989, 127,116. (30)Yates, D. E.Ph.D. Thesis, University of Melbourne, Australia, 1975. (31) Yates. D. E.: James. R. 0.:. Healv. .. T. W. J. Chem. Soc.. Faradav Trans.' I 1980, 76, 1; (32) Blum, A.; Lasaga, A. Nature (London) 1988,331,431. (33) (a) Petit, J. C.; Mea, G. D.; Dran, J. C.; Schott, J.; Berner, R. A. Nature (London) 1987,325,705.(b) Scott, J.; Petit, J. C. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987;pp 293-315. (c) Casey, W. H.; Westrich, H. R.; Arnold, G. W. Geochim. Cosmochim. Acta 1988,52,2795. (34)Hellmann, R.;Eggleston, C. M.; Hochella, M. F.; Crerar, D. A. Geochim. Cosmochim. Acta 1990,54, 1267.
Platinum-Proton Coupling in the NMR Spectrum of Benzene on an Alumina-Supported Platinum Catalyst Charles F. Tirendi>*tC. Alex MiUs,s Cecil Dybowski,*.* Department of Chemistry and Biochemistry, Department of Chemical Engineering, and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716
and Ciinther Neue Physical Chemistry, University of Dortmund, Dortmund, Germany (Received: December 3, 1991; In Final Form: January 31, 1992)
The homonuclear decoupled proton NMR spectrum of benzene associativelychemisorbed on a 5.0% wt/wt Pt/A1203catalyst is reported. The structure of the resonance is attributable to heteronuclear dipolar or pseudodipolar couplings to platinum-195 nuclei in the surface of the particle. Comparison of simulated spectra for benzene molecules in various sites coupled by the heteronucleardipolar interaction to an array of platinum atoms indicatesthat only benzene in the "on top" geometry is consistent with the observed spectrum. If the coupling is attributed solely to direct dipolar couplings, the predicted Pt-benzene distance is 1.56 f 0.02 A. Considering both dipolar and pseudodipolar couplings leads to the conclusion that this value is a minimum for the Pt-benzene distance.
Introduction
Benzene chemisorbed on a Pt/A1203catalyst ?r bonds to surface platinum atom.'" In this state, the benzene molecule may be in the 'on-top", "three-fold-hollow", or "bridge" site, as shown in Figure 1. In the on-top site, the molecule's centroid lies above a single platinum atom, whereas it lies above the hole formed by three platinum atoms in the three-fold-hollow geometry. In the bridge site the centroid lies over the midpoint of the vector connecting two adjacent platinum atom. Lehwald et a1.6 have shown that the HREELS spectrum of benzene on singlecrystal Pt( 111) is consistent with a *-bonded complex having the plane of the benzene ring parallel to the plane of the platinum surface. Similarly, Jobic and Renouprez' found that neutron inelastic scattering from benzene adsorbed on Raney platinum indicates the benzene is bound to one metal atom. To whom correspondence should be addressed. Present address: AKZO Chemical Company, Livingstone Ave., Dobbs Ferry, NY 10522. *Department of Chemistry and Biochemistry. 8 Department of Chemical Engineering.
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In other preparations, a variety of geometries may be adopted. When CO is wadsorbed with benzene on Pt( 11l), LEED studies indicate that an ordered structure is formed, in which the benzene molecule is distorted to C2, symmetry.8 Dissociatively chemisorbed u-bonded benzene complexes have also been reported to form on catalysts containing carbon residues.' LEED investigations of this material suggest that the plane of the u-bonded benzene is inclined relative to the plane of the Pt( 111) surface?Jo Trace desorption experiments on materials containing both *-and u-bonded benzene indicate that some adsorbed benzene exchanges when put in contact with labeled benzene, but a portion can only be removed by reaction with hydrogen." Thermal desorptionFTIR studies indicate that both the *-and a-bonded benzenes exchange hydrogen with surface hydroxyl groups of the alumina support, exchange from the a-bonded benzene being much more facile than from the *-bonded material.' In this paper, we examine the solid-state nuclear magnetic resonance (NMR) spectrum of protons of benzene chemisorbed on a Pt/A1203catalyst as the T complex. Conventional singlepulse. solid-state NMR spectroscopy has previously been reported to give a featureless resonance dominated by proton-proton dipolar 0 1992 American Chemical Society
Tirendi et al.
5046 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
order, the Hamiltonian for a particular proton I subject to direct heteronuclear dipolar couplings is given byI6 His = 27nsh Cr;3D#(o,ei,o)I+9zi I
Figure 1. Schematic diagram of the various sites for the sorption of benzene on the (1 11) plane of platinum. (a) on top; (b) three-fold hollow with an alignment of the CH bond parallel to a chain of platinum atoms; (c) bridge site; (d) three-fold hollow with the CH bond aligned at 30' to this axis.
couplings at low temperatures.I2 Suppression of homonuclear dipole-dipole couplings by use of a multiple-pulse sequence yields a spectrum affected by weaker interactions such as the direct heteronuclear dipolar and pseudodipolar couplings to platinum-195 nuclei. The observed spectral features are consistent with a model in which benzene rotates rapidly about its hexad axis and lies with its plane parallel to the plane of the platinum surface, with its centroid above a platinum atom.
Experimental Section The 5.0% wt/wt Pt/Al,O, catalyst was synthesized by the incipient-wetness technique. The amount of platinum in the catalyst was determined by atomic absorption spectrometry of the digested catalyst, using a Perkin-Elmer 503 atomic absorption spectrometer. The q-alumina on which the platinum was dispersed was produced from p-A1203(Air Products and Chemicals) by calcination in air at 863 K for 4 h, a procedure described by Garten.', The nitrogen BET surface area of the catalyst was determined to be 8 1 f 2 m2 g-I, and the platinum surface area was determined by hydrogen adsorption to be 2.0 f 0.2 m2 g-I. A 0.15-g sample of catalyst was calcined in a glass reactor at 648 f 2 K for 3 h under flowing oxygen, followed by outgassing at 623 f 2 K for 2 h to a final pressure of 2 X Torr. This procedure removed most hydroxyl groups from the catalyst. (Under the conditions of examination of the benzene on this catalyst, no detectable signal could be obtained from a sample prepared identically, but without benzene.) Prior to adsorption, benzene was subjected to a series of freezepumpthaw cycles to remove molecular oxygen and other dissolved gases. The catalyst was equilibrated with benzene vapor (Fisher spectroscopic grade) in a glass, grease-free manifold. Subsequently the sample was outgassed at 295 f 2 K, thereby leaving only chemisorbed benzene on the surface of the catalyst. NMR spectra were obtained using a spectrometer designed and built in this laboratory, which operates at a frequency of 56.403 MHz for the proton resonance. Multiple-pulse line narrowing was accomplished by applying the eight-pulse cycle of Rhim, Elleman, and Vaughan14 with a cycle time of 48 KS and a relaxation delay of 15 s. Phase adjustments were performed using the procedure described by Haubenreisser and S~hnabe1.l~The chemical-shift scaling factor was checked on a liquid sample and found to be constant at 2.03, a value within experimental error of the theoretical value calculated using a r / 2 pulse length of 1.5 ws and the cycle time.14 No substantial detuning of the circuit occurred when the sample was inserted into the NMR spectrometer. The resonance of the aromatic protons contribute to the dipole-dipole-decoupled spectrum at a chemical shift not strongly shifted from its value in the bulk solid or liquid state. Theoretical Considerations Proton spectra obtained with homonuclear decoupling show direct heteronuclear direct dipolar and pseudodipolar couplings of protons with nearby '95Ptspins. These couplings depend on the relative geometries and molecular dynamics of the adsorbed molecules with respect to the platinum surface layers. To first
(1)
The summation runs over all nearby 195Pt spins. We neglect any coupling among platinum spins. Bi is the angle between the external magnetic field and the vector ri joining a proton and the ith platinum spin. Due to homonuclear decoupling, proton-proton interactions enter only through pulse imperfections and small nonsecular parts of the Hamiltonian, which we neglect. For practical calculations, it is convenient to introduce an intermediate frame fixed with respect to the crystal lattice: +2
HIS = 2 y ~ C ~ Db~(.,B,r)Cr;,D~(sl(ri))l2S,i h (2) m=-2
i
The set of Euler angles (a,&r)describes the orientation of a crystal grain with respect to the external magnetic field. sl(ri) defines the direction of ri in the crystal frame. From eq 2, it follows that allowed transitions occur at frequencies N @k
= Yr/ShC(*)ikf; 1=1
(3)
where +2
f;: =
C f;:mDb2(.Ar) m=-2
and N is the number of neighboring 195Pt spins. The symbol (f)ik takes on a values of +1 or -1 depending on i and k. Physically, this operation is determined by the spin states for the various configurationsof the spins Si. The whole spectrum is thus given by a symmetric multiplet of 2N lines at frequencies wk. Each of these lines shows a different variation with respect to rotation of the crystal. Taking into account the 0.338 natural abundance of 195Pt, even a strictly periodic overlayer will exhibit many different surroundings of magnetically active and inactive Pt nuclei for each adsorbed benzene molecule. As a practical consideration,the calculations are restricted to possible sites of ISsPtatoms close enough to benzene protons to result in splittings greater than the observed line width of the central line in the observed spectrum. As will be explained in more detail below, one has to assume fast six-fold jumps of the benzene molecules about their hexad axes. Therefore, all sites have to be included in the calculation that are close enough to any proton positions of the same molecule. In Figure 1 are four commonly discussed adsorption site models, showing the relationship of benzene to the surrounding Pt atoms. Parts b and d of Figure 1 differ only in the preferred directions of the CH bond relative to the underlying platinum lattice at the three-fold hollow site. A significant reduction of wmputational effort results from the fact that configurations with more than half the positions occupied by magnetidly active Pt atoms together contributeonly approximately 5% to the total signal. In addition, the signals from molecules in these configurations are spread over large spectral regions. Both arguments show that these configurations are of minor importance for line-shape simulations and, hence, are neglected. Despite these simplifications,calculation of the spectrum of benzene in the bridge position (Figure IC) results in the computation and addition of almost 360000 different lines wk, each line being an average over the six equivalent proton positions necessary to account for fast jumps about the hexad axis. According to eq 3, each line is a function of (a,&y), the orientation of the crystalline, and gives rise to an individual powder spectrum. To obtain a complete spectrum requires between a few hours and 1 week of CPU time on an APOLLO DN 4000 or IBM RISC 6000 workstation.
Results and Discussion The homonuclear dipole-dipole-decoupled proton spectrum of benzene chemisorbed on Pt/A1203 is shown in Figure 2. Since we have shown that the preparation produces a catalyst that gives
Benzene on an Alumina-Supported Platinum Catalyst
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Frequency (Hz) Figure 2. Homonuclear dipolar-decoupled proton spectrum of benzene adsorbed on 5.0% wt/wt Pt/A1203. The solid line is a simulated spectrum for the on-top site with an optimized platinum-benzene spacing of 1.56 A. no proton NMR signal (under the conditions of data acquisition) before adsorption of benzene,12 this resonance arises from the aromatic protons deposited in the adsorption step. The spectrum consists of a broad structure with a particularly sharp central feature flanked by at least a doublet. The appearance is reminiscent of a superposition of Pake patterns but cannot be interpreted in terms of a single two-spin coupling. The average shift of +6 f 1 ppm relative to an external sample of tetramethylsilane is similar to that of benzene in the bulk. The central feature has a line width of about roughly 2-3 ppm, of the order of the reported chemical shift anisotropy (-5.3 ppm) of polycrystallinebenzene undergoing rotation about the hexad axis in the rotator phase.17 The broad-line (single-pulse) NMR spectrum [not shown] of this chemisorbed benzeneI2exhibits a resonance with a full width at half-maximum of approximately 11 kHz, somewhat narrower than that of polycrystalline benzene in the rotator phase1* but nevertheless indicative of strong interproton dipole-dipole couplings. Thus,these chemisorbed molecules are similar to benzene molecules in the rotator phase of the solid, in the sense that the dipole-dipole interactions among protons are partially averaged by motion. We conclude that the benzene is bound to the platinum particles and that the proton resonance we detect is from spins in these molecules, which are likely to be jumping rapidly about their hexad axes, similar to the motion of benzene in its rotator phase. The proton resonance obtained with homonuclear decoupling obviously shows the effects of coupling to other nuclei. The spacing of the first singularities is 1092 f 40 Hz, after correction for scaling of interactions linear in I by the multiple-pulse sequence. To analyze this spectrum more nearly completely, we simulated spectra for cases in which the plane of the benzene molecule is parallel to the plane of the surface layer of platinum atoms, as has been proposed from LEED and infrared studies (Figure 3). We had to assume rapid jumps about the hexad axis to be consistent with the observed line width of the single-pulse proton spectrum. We interpret the line shape in these spectra in terms of direct 1H-'95Pt dipolar couplings. Because of the substantial natural abundance of '95Ptand the relatively large number of platinum sites where potential coupling partners for the protons may reside, each spectrum is the superposition of many subspectra. (Vide supra.) The spectra of Figure 3 are thus averages over the various possible random distributions of 195Ptamong the nearby platinum sites and over a random distribution of crystallite axes relative to the magnetic field. Although this calculation is extremely time consuming, the resultant line shapes depend only on a limited number of parameters-the platinum-platinum distance, the intramolecular geometry of benzene, and the distance between the center of the benzene and the nearest platinum atom. In the simulationsof Figure 3, we have considered benzene to be adsorbed predominantly on (1 1 1) faces, using an interplatinum spacing of 2.775 A. One of us has shownLgthat the l3C-I3C bond length in a similar chemisorbed benzene remains close to that of
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Fi&e 3. Simulated proton spectra for benzene coupled to an array of platinum atoms containing randomly placed ISsPt nuclei, assuming rapid six-fold jumps about the hexad axis of benzene. The labels a-d correspond to the structures of Figure l a d , respectively.
solid benzene, with only a single bond length observed. Thus, we take the structure of benzene to be planar and identical to that of solid benzene. With these assumptions, the only variable parameter is the distance from the plane of the benzene ring to the first la er of platinum atoms, which has been reported to be 2.0 f 0.5 for benzene chemisorbed on Raney p l a t i n ~ m .The ~ interplanar spacing was adjusted to values typically reported for these complexes. It is clear from inspection of these spectra that the on-top site is most closely consistent with the observed spectrum. In particular, benzene at other characteristic sites does not reproduce the signature experimental line shape. Having determined this geometry to be optimal, we varied the benzenelayer-platinum-layer spacing for this configuration to obtain the most consistent fit to the experimental data-a spacing of 1.56 f 0.02 A. Although these results suggest that the ?r-bonded benzene r e sides in the on-top site, heteronuclear dipoledipole couplings may not be the only source of coupling in the NMR spectrum. Pseudodipolar couplings contribute in a similar manner. The exact magnitude of such couplings is not known. However, values of the isorropic J coupling between protons and platinum-195 for platinum hydrides in solution can be as high as 700-1300 Hz20 and are dominated by the Fermi contact In contrast, reports in the literature2*indicate that isotropic platinum-proton J couplings involving n-bonded organic ligands are generally below about 40 Hz. To address the contribution of pseudodipolar couplings qualitatively, we simulated the spectrum with only direct dipolar coupling to the platinum, assuming the benzene ring is fixed directly above a platinum atom at a distance of 2.0 A, consistent with the distances observed in typical arene-transition-metal com~lexes.2~Comparison of the spectrum for this geometry with the best fit case (zR-bcnzcne = 1.56 A) is made in Figure 4. The first major splitting is only 580 Hz, as compared to the observed splitting of 1092 Hz. Direct dipolar couplings, therefore, cannot account for the observed spectrum if benzene is at this distance from the platinum surface. To consider all the possible pseudodipolar couplings is not possible; however, we can qualitatively model the effect by considering the effect of a single coupling expressed as a traceless tensor with axes parallel to the direct dipolar tensor. [The isotropic values are typically less than the observed splitting by at least l.order of magnitude.] Then the difference between the observed major splitting (1092 Hz) and the predicted major splitting (580 Hz) is a measure of the anisotropic pseudodipolar coupling, 512 Hz. Under these conditions, it would appear that the pseudodipolar and dipolar couplings are of similar magnitudes. If the benzene ring is located at a closer
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that the Pt-benzene distance is similar to that in transitionmetal-arene complexes leads to the conclusion that the pseudodipolar coupling may have an anisotropy of the order of 500 Hz.
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Acknowledgment. This work was supported, in part, by the Sponson of the Center for Catalytic Science and Technology and by a Grant-in-Aid from Hercules, Inc. C.D. acknowledgessupport by the National Science Foundation under Grants CPE 82- 17890 and CHEM-9013926. G.N. acknowledgesa grant from the Fonds der Chemischen Industrie. RWhy NO. 19'Pt, 14191-88-9; Pt, 7440-06-4; A1203, 1344-28-1; benzene, 71-43-2.
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Frequency (Hz) Figure 4. Simulated spectra for benzene in the on-top site at 2.0 and 1.56 A above the surface, respectively.
distance, the pseudodipolar coupling would be smaller; if it were at a greater distance, the pseudodipolar coupling would have to be larger to account for the observed spectrum.
Conclusions The NMR spectrum of protons on benzene associatively chemisorbed on Pt/A120s, measured with suppression of homonuclear dipolar couplings, indicates that it interacts with platinum- 195 nuclei in the surface of the metal particle. The observed powder pattern can be interpreted in terms of coupling between protons and nuclei. From comparison with simulated spectra for various geometries, one concludes that the benzene is in the on-top site with the plane of the ring parallel to the surface platinum layer. The benzene is not static on the NMR time scale; to simulate the spectrum requires one to assume it rapidly jumps about the hexad axis. The value of the platinum-benzene interplanar spacing can be determined to be greater than 1.56 f 0.02 A. The effects of pseudodipolar couplingscannot be excluded, which would cause one to underestimate the distance by the assumption that only direct dipole-dipole couplings affect the proton spins. In particular, if one examines simulations of benzene at a typical Pt-benzene distance of 2 A, one finds that the major splitting may have a substantial contribution from pseudodipolar coupling. Thus, 1.56 A must be viewed as a lower limit for the benzeneplatinum distance. Making the reasonable asssumption
References and Notes (1) (a) Haaland, D. H. SwJ Sci. 1981,102,405. (b) Haaland, D. H. Ibid. 1981, 111, 555. (2) Primet, M.; Basset, J. M.; Mathieu, M. V.; Prettre, M. J. Caral. 1973, 29, 213. (3) Moyes, R. B.; Wells, P. B. Adv. Catal. 1973, 23, 121. (4) Palazov, A. J. Catal. 1973, 30, 13. (5) Horsley, J. A,; Stoehr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099, (6) Lehwald, S.; Ibach, H.; Dehmuth, J. E. S u r - Sci. 1978, 78, 577. (7) Jobic, H.; Renouprez, A. Surf. Sci. 1981, I l l , 53. (8) Ogletree, D. F.; van Hove, M. A.; Somorjai, G. A. SurJ Sci. 1987,183, 1. (9) Gland, J. L.; Somorjai, G. A. Surf Sci. 1973, 38, 157. (10) Stair, P. C.; Somorjai, G. A. J . Chem. Phys. 1977,67,4361. (11) Tetenyi, P Babernics, L. J. Catal. 1967, 8, 215. (12) Tirendi, C. F.; Mills, G. A.; Dybowski, C. J . Phys. Chem. 1984,88,
5765.
(13) Garten, R. L. J. Catal. 1976, 43, 18. (14) Rhim, W.-K.; Elleman, D. D.; Vaughan, R. W. J . Chem. Phys. 1973, 58, 1772. (15) Haubenreisser, U.; Schnabel, B. J. Magn. Reson. 1979, 35, 175. (16) Gerstein, B. C.; Dybowski, C. Transienr Techniques in NMR of Solids; Academic Press: Orlando, FL, 1985. (17) Ryan, L. M.; Wilson, R. C.; Gerstein, B. C. J . Chem. Phys. 1977,67, 43 10. (18) Andrew, E. R.; Eades, R. G. Proc. R . Soc. London 1953, A218,537. (19) Englesberg, M.; Yannoni, C. S.; Jacintha, M. A.; Dybowski, C. J . Phys. Chem., submitted for publication. (20) (a) Jesson, J. P. In Transition Meral Hydrides; Muetterties, E. L., Ed.; Dekker: New York, 1971; p 75. (b) Appleton, T. 0.;Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,10, 335. (c) Powell, J.; Gregg, M. R.; Sawyer, J. F. Inorg. Chem. 1989, 28, 4451. (21) Pople, J. A.; Santry, D. P. Mol. Phys. 1964, 8, 1. (22) (a) Anklin, C. G.; Pregosin, P. Magn. Reson. Chem. 1985, 23,671. (b) Albinati, A.; Pregosin, P.; Wombacher, F. Inorg. Chem. 1990,29, 1812. (23) See, for example: Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;Pergamon: Oxford, U.K.,
1982.
Carbon Filament Growth on Platinum Catalysts W. T. Owens, N. M. Rodriguez, and R. T. K. Baker* Chemical Engineering Department, Auburn University, Auburn, Alabama 36849 (Received: October 18, 1991) A combinationof experimental techniques including controlled atmosphere electron microscopy, thermogravimetry,and flow reactor studies have been used to study the formation of carbon deposits resulting from the interaction of platinum with ethylene and acetylene. It was found that in order to produce significant amounts of carbon on the metal it was necessary to perform the reaction in the presence of added hydrogen. In this investigation we have focused attention on all of the steps involved in the reaction, including those which occur at the metal-gas interface: diffusion of carbon through the metal particles and eventual precipitation at the metal-carbon interface to form a carbon filament. In both CBSCS as the amount of hydrogen in the reactant gas was increased, there was a corresponding increase in the degree of crystalline perfection of the carbon filaments which were produced on the platinum particles. This effect was more pronounced for acetylene than ethylene, and if the metal was treated in a mixture of acetylene containing excess hydrogen, it was possible to produce filaments which had identical oxidation characteristics to those of graphite.
