Dimerization of 1,3-Butadiene on Highly Characterized Hydroxylated Surfaces of Ultrathin Films of γ-Al2O3

We describe here the dimerization of 1,3-butadiene to form 4-vinyl-cyclohexene on highly ordered hydroxylated ultrathin films of γ-Al2O3. High surface area, powdered γ-Al2O3 is widely used as a catalyst and catalyst support. In addition, there is interest in the chemistry of Al2O3 particulates which exist in the troposphere. In contrast to the less reactive and more stable R-Al2O3, which is available in macroscopic single crystal form, γ-Al2O3 is usually only available in powdered form. Thus, it has been difficult to study the details of the surface chemistry of this chemically important phase of Al2O3 using the powerful methods of modern surface sensitive spectroscopy. There are many examples of careful surface science studies of thin films of Al2O3 grown on aluminum and other metal substrates.1-7 Surface science techniques have also been used to characterize the reactive sites and the chemistry on powdered samples.8-15 Recently, Libuda et al.16 and Gassmann et al.17 have shown that ultrathin films of highly ordered γ-Al2O3 can be grown on substrates of NiAl(110) and NiAl(100). Using their methods, essentially single crystalline films of γ-Al2O3 can be prepared which are 10-15 A in thickness. Such ultrathin films are sufficiently thin to allow characterization by conventional surface electron spectroscopies and electron diffraction without complications due to charging which occur when working with bulk samples of such electrically insulating materials. We have extended the methods of Libuda et al.,16 and Gassmann et al.17 to prepare not only highly characterized surfaces of γ-Al2O3, but also to prepare hydroxylated γ-Al2O3 ultrathin films. These films can be characterized by low energy electron diffraction (LEED) for structure, Auger electron spectroscopy (AES) for elemental composition, and high-resolution electron energy loss spectroscopy (HREELS) for vibrational spectra. We report here the use of laser desorption Fourier transform mass spectrometry (LD-FTMS) to study the chemistry of 1,3-butadiene adsorbed on these ultrathin films of non-hydroxylated and hydroxylated γ-Al2O3. To our knowledge, this is the first demonstration of the generation of highly characterized hydroxyl species on structurally characterized ultrathin films of γ-Al2O3 and the first observation of the dramatic differences in surface chemistry which can be caused by surface hydroxyls on these ultrathin films. 1,3-Butadiene has recently been added to the California Air Toxics list as the cause of increases in leukemia and certain types of cancers which can result from chronic exposure.18,19 The chemistry of 1,3-butadiene on the surfaces of Al2O3 particulates, which are known to exist in the troposphere, is one piece of the development of a complete picture of the eventual fate of butadiene which is emitted by industrial sources. Figure 1 shows the vibrational spectrum (HREELS) of a γ-Al2O3 ultrathin film with a hydroxylated surface which was grown on a NiAl(100) substrate. This spectrum was obtained with an LK 2000 HREELS spectrometer in an ion-pumped ultrahigh vacuum chamber with a base pressure of 1 × 10-10 Torr. The vibrational modes at 899, 702, 592, and 416 cm-1 are the γ-Al2O3 phonons. The mode at 1797 cm-1, and the low intensity modes at ∼1500 cm-1 and ∼1100 cm-1 are combination modes of the phonons. The narrow widths of the phonon peaks are limited by the spectrometer resolution in these experiments and are indicative of the highly ordered nature of the oxide film. The mode at 3675 cm-1 shown in the inset of Figure 1 is the OH stretching mode of the surface hydroxyl groups. This mode is extremely narrow when compared to the corresponding OH stretching mode seen for hydroxyl groups on Al2O3 powdered samples.13 The OH stretching mode frequency which we observe is characteristic of isolated OH groups on the surface which are bonded to three Al sites.20 Identical spectra can be obtained by (1) Crowell, J. E.; Chen, J. G.; Yates, J. T., Jr. Surf. Sci. 1986, 165, 37. (2) Chen, J. G.; Crowell, J. E.; Yates, J. T., Jr. Phys. ReV. B 1986, 33, 1436. (3) Street, S. C.; Guo, Q.; Xu, C.; Goodman, D. W. J. Phys. Chem. 1996, 100, 17599. (4) Wu, M.-C.; Goodman, D. W. J. Phys. Chem. 1994, 98, 9874. (5) Goodman, D. W. J. Vac. Sci. Technol. A 1996, 14, 1526. (6) Wu, Y.; Garfunkel, E.; Madey, T. E. Surf. Sci. 1996, 365, 337. (7) Wu, Y.; Garfunkel, E.; Madey, T. E. J. Vac. Sci. Technol. A 1996, 14, 2554. (8) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Chem. Phys. 1990, 92, 5519. (9) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041. (10) Ballinger, T. H.; Smith, R. S.; Colson, S. D.; Yates, J. T., Jr. Langmuir 1992, 8, 2473. (11) Coustet, V.; Jupille, J. Surf. Sci. 1994, 307-309, 1161. (12) Knozinger, H. Angew. Chem., Int. Ed. Engl. 1968, 7, 791. (13) Liu, X.; Truitt, R. E. J. Am. Chem. Soc. 1997, 119, 9856. (14) Fleisch, T. H.; Meyers, B. L.; Hall, J. B.; Ott, G. L. J. Catal. 1984, 86, 147. (15) Baker, B. G.; Jasieniak, M. In Surface Science. Principles and current applications.; MacDonald, R. J., Taglauer, E. C., Eds.; Springer-Verlag: Berlin, 1996. (16) Libuda, J.; Winkelmann, F.; Baumer, M.; Freund, H.-J.; Bertrams, T.; Neddermeyer, H.; Muller, K. Surf. Sci. 1994, 318, 61. (17) Gassmann, P.; Franchy, R.; Ibach, H. Surf. Sci. 1994, 319, 95. (18) Ward, E. M.; Fajen, J. M.; Ruder, A. M.; Rinsky, R. A.; Halperin, W. E.; Fessler-Flesch, C. A. Toxicology 1996, 113, 157. (19) Macaluso, M.; Larson, R.; Delzell, E.; Sathiakumar, N.; Hovinga, M.; Julian, J.; Muir, D.; Cole, P. Toxicology 1996, 113, 190. (20) Knozinger, H.; Ratnasamy, P. Catal. ReV.-Sci. Eng. 1978, 17, 31. Figure 1. HREELS vibrational spectrum of a hydroxylated γ-Al2O3 ultrathin film which was grown on a NiAl(100) surface by a 100 L exposure of H2O to the surface as it was held at 900 K. The inset shows a blow-up of the O-H stretching region of the spectrum. 10980 J. Am. Chem. Soc. 1998, 120, 10980-10981