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 reac- tions. Methoxy, CH 3 O, is the smallest and exhibits different reactivity on various surfaces. For example, CH 3 O on Cu 1 dehydrogenates partially to formaldehyde, H 2 CO(g), whereas CH 3 O on Pt, Ru, and Ni 2 dehydroge- nates fully to form H 2 (g) and CO(g). However, regardless of the surface the initial CH 3 O dissociation step is dehydrogenation of the -C-H bond, not O-C cleavage. t-C 4 H 9 O is of interest because it has the same (C 3v ) symmetry as CH 3 O but no -C-H bonds. Every -H is replaced by CH 3 , and the γ-C-H bonds in t-C 4 H 9 O are stronger by 17 kJ mol -1 than the -C-H bonds in CH 3 O. Because of this and electronic structure differences, adsorbed t-C 4 H 9 O typically survives to much higher temperatures than CH 3 O. For example, on Cu(100) CH 3 O dehydrogenates and desorbs as H 2 CO at 350 K, but t-C 4 H 9 O 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-C 4 H 9 O desorb at 440 K, 4 but on O-covered Rh(111), t-C 4 H 9 O is stable only up to 300 K. 5 t-C 4 H 9 O is also an attractive precursor to stable cyclic species containing O and metal atoms, that is, surface oxametallacycles. Oxametallacycle formation from t-C 4 H 9 O requires only one, probably concerted, reaction to cleave aC-H and form a C-Pt bond. No additional skeletal rearrangement is necessary. For circumstantial reasons, such oxametallacycles have been suggested as intermedi- ates for reactions of t-C 4 H 9 O 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 ICH 2 CH 2 OH on Ag(110). 7 The latter provides vibrational evidence from high-resolution electron energy loss spec- troscopy (HREELS) that is compared with a theoretical IR spectrum calculated by density functional theory (DFT). No spectroscopic evidence has been reported of an oxa- metallacycle with two CH 3 groups on its -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, tert- butyl 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-C 4 H 9 O from t-C 4 H 9 ONO and the thermally activated reaction path followed by t-C 4 H 9 O. 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 oxametallacycles 7,8 and with * Corresponding author. E-mail: jmwhite@mail.utexas.edu. 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, 3826- 3835. (5) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779-6785. (6) (a) Brown, N. F.; Barteau, M. A. J. Phys. Chem. 1996, 100, 2269- 2278. (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. 798 Langmuir 2001, 17, 798-806 10.1021/la001315v CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000