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Langmuir 2001, 17, 798-806
Thermal Activation of tert-Butyl Nitrite on Pt(111): tert-Butoxy Dehydrogenation and Oxametallacycle Formation H. Ihm,†,§ J. W. Medlin,‡ M. A. Barteau,‡ and J. M. White*,† Department of Chemistry & Biochemistry, Texas Materials Institute and Center for Materials Chemistry, University of Texas at Austin, Austin, Texas 78712, and Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received September 12, 2000. In Final Form: November 15, 2000 The adsorption and thermal reactions of an alkyl nitrite, t-C4H9ONO, on Pt(111) are reported. Dissociative chemisorption at the weak (171 kJ/mol) ROsNO bond accompanies adsorption at 115 K, forming adsorbed t-C4H9O and NO. During heating to 200 K, some t-C4H9O dehydrogenates at the γ-carbon (methyl group) to form a proposed oxametallacycle species that dehydrogenates further upon heating to 250 K. tert-Butyl alcohol, t-C4H9OH, desorbs in three coverage-dependent peaks (200, 250, and 300 K) attributable to hydrogenation of both t-C4H9O and the oxametallacycle. The yields and reaction paths depend on the initial dose of t-C4H9ONO. Vibrational modes of the oxametallacycle were compared for several plausible structures, with modes calculated using density functional theory. Among these structures, a four-membered oxametallacycle ring (containing only one Pt atom in the ring) gave the best agreement with the experimental data. Finally, a reaction path potential energy diagram was constructed.
Introduction Adsorbed alkoxy species are of interest because of their roles in heterogeneously catalyzed partial oxidation reactions. Methoxy, CH3O, is the smallest and exhibits different reactivity on various surfaces. For example, CH3O on Cu1 dehydrogenates partially to formaldehyde, H2CO(g), whereas CH3O on Pt, Ru, and Ni2 dehydrogenates fully to form H2(g) and CO(g). However, regardless of the surface the initial CH3O dissociation step is dehydrogenation of the β-C-H bond, not O-C cleavage. t-C4H9O is of interest because it has the same (C3v) symmetry as CH3O but no β-C-H bonds. Every β-H is replaced by CH3, and the γ-C-H bonds in t-C4H9O are stronger by 17 kJ mol-1 than the β-C-H bonds in CH3O. Because of this and electronic structure differences, adsorbed t-C4H9O typically survives to much higher temperatures than CH3O. For example, on Cu(100) CH3O dehydrogenates and desorbs as H2CO at 350 K, but t-C4H9O is stable up to 500 K.3 The situation is somewhat different on O-covered surfaces; on O-covered Ag(110), the first reaction products of t-C4H9O desorb at 440 K,4 but on O-covered Rh(111), t-C4H9O is stable only up to 300 K.5 * Corresponding author. E-mail:
[email protected]. Fax: 512-471-9495. Phone: 512-471-3704. † University of Texas at Austin. ‡ University of Delaware. § Current address: Department of Chemistry, University of Washington, Seattle, WA 98195. (1) (a) Sexton, B. A. Surf. Sci. 1979, 88, 299. (b) Andersson, S.; Persson, M. Phys. Rev. B 1981, 24, 3659. (c) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. (2) (a) Sexton, B. A Surf. Sci. 1981, 102, 271. (b) Rubloff, G. W.; Demuth, J. E. J. Vac. Sci. Technol. 1977, 14, 419. (c) Erskine, J. L.; Bradshaw, A. M. Chem. Phys. Lett. 1980, 72, 260. (d) Demuth, J. E.; Ibach, H. Chem. Phys. Lett. 1979, 60, 395. (3) Ihm, H.; Scheer, K.; Celio, H.; White, J. M. Langmuir, in press. See also ref 28. (4) Brainard, R. L.; Madix, R. J. J. Am. Chem. Soc. 1989, 111, 38263835. (5) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779-6785.
t-C4H9O is also an attractive precursor to stable cyclic species containing O and metal atoms, that is, surface oxametallacycles. Oxametallacycle formation from t-C4H9O requires only one, probably concerted, reaction to cleave a C-H and form a C-Pt bond. No additional skeletal rearrangement is necessary. For circumstantial reasons, such oxametallacycles have been suggested as intermediates for reactions of t-C4H9O on Ag(110)4 and Rh(111).5 They have also been invoked as contributors to reactions of ethylene oxide and propylene oxide on Rh(111)6 and of ICH2CH2OH on Ag(110).7 The latter provides vibrational evidence from high-resolution electron energy loss spectroscopy (HREELS) that is compared with a theoretical IR spectrum calculated by density functional theory (DFT). No spectroscopic evidence has been reported of an oxametallacycle with two CH3 groups on its β-position with respect to the O-metal bond. Reactions of oxametallacycles are also of interest. For example, on O-covered Ag(110) three reaction pathways are evident in temperature-programmed desorption (TPD) of tert-butoxy: (1) isobutene oxide, water, isobutene, and carbon dioxide desorb at 440 K; (2) isobutene oxide, tertbutyl alcohol, isobutene, water, and carbon dioxide desorb at 510 K; (3) acetone desorbs at 590 K.4 The 440 K reaction is typical in that coadsorbed O plays an important role.4,5 Surface reactions of oxametallacycles in the absence of coadsorbed O have not been widely studied. In this paper, we discuss the preparation of t-C4H9O from t-C4H9ONO and the thermally activated reaction path followed by t-C4H9O. Some dehydrogenation of the strong C-H bonds (∼410 kJ mol-1) occurs as low as 180 K, leading to tert-butyl alcohol formation and a species identified as an oxametallacycle. At 250 K, the dominant surface species is the oxametallacycle, and its HREEL spectrum is compared with those of other oxametallacycles7,8 and with (6) (a) Brown, N. F.; Barteau, M. A. J. Phys. Chem. 1996, 100, 22692278. (b) Brown, N. F.; Barteau, M. A. Surf. Sci. 1993, 298, 6-17. (7) Jones, G. S.; Mavrikakis, M.; Barteau, M. A.; Vohs, J. M. J. Am. Chem. Soc. 1998, 120, 3196-3204.
10.1021/la001315v CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000
Thermal Activation of tert-Butyl Nitrite
Langmuir, Vol. 17, No. 3, 2001 799
vibrational modes calculated for four plausible oxametallacycle structures using DFT. Of the four, a fourmembered oxametallacycle gives the best fit. The major TPD products are t-C4H9OH, with three desorption peaks, H2 in a broad distribution, NO, and CO. The absence of isobutene and isobutene oxide desorption indicates favorable activation of C-H bonds compared to C-O or C-C bonds. The three t-C4H9OH desorption peaks indicate three different reaction pathways. Finally, on the basis of bond energies and the peak desorption temperatures of the products, we constructed a semiquantitative reaction path potential energy diagram. Experimental Section The experiments were carried out in an ultrahigh vacuum chamber with a base pressure of 3 × 10-10 Torr. The system was equipped with TPD, HREELS, X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) capabilities. Further details are available elsewhere.9 The Pt(111) crystal was 8 mm in diameter and 1.5 mm thick. The sample was cleaned twice by sputtering with 3 keV Ar+ at 500 K for 5 min and annealing at 1150 K for 5 min. Then the sample was examined with LEED, XPS, and HREELS to assess surface crystal order and surface cleanliness. The substrate temperature was monitored by a chromelalumel thermocouple spot-welded to the back of the crystal. Cooled by thermal contact to liquid nitrogen, the substrate had a base temperature between 110 and 120 K. The sample was heated resistively and the TPD ramp rate was 3 K s-1. HREELS measurements were taken with primary beam energies of 1, 3, and 5 eV, and typically the elastic peak full width at halfmaximum (fwhm) was 100 cm-1. XPS data were taken with Al KR photons (1486.6 eV) and a hemispherical analyzer with a step size of 0.01 eV. For C1s and Pt4f measurements, 50 eV pass energy was applied, but for O1s 80 eV was used to increase an otherwise weak signal. The N1s spectra were not accumulated because the low surface concentrations and low excitation cross sections led to unacceptable signal-to-noise ratios. All binding energies are referenced to a Pt4f7/2 binding energy of 70.9 eV.18 t-C4H9ONO (90%, Aldrich) and t-C4H9OH (99.0%, EM Scientific) were used after several freeze-pump-thaw purifications. The major impurity in t-C4H9ONO was t-C4H9OH, and it remained in small amounts (40), t-C4H9OH (41), and NO (92) are estimated from TPD peak temperatures using conventional Redhead analysis. The activation energy of t-C4H9ONO(p) f t-C4H9O(a) + N(a)O(a) is estimated at
