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Influence of Surface Modifiers on the Thermal Decomposition of Methanethiol on Fe(110) J. D. Batteas,† T. S. Rufael,‡ and C. M. Friend* Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 Received December 2, 1997. In Final Form: February 8, 1999 The reactions of methanethiol (CH3SH) on clean, sulfur-covered, and oxygen-covered Fe(110) have been investigated by X-ray photoelectron spectroscopy, temperature-programmed reaction spectroscopy, and low-energy electron diffraction. On clean Fe(110), the S-H bond in methanethiol breaks below 100 K, affording methylthiolate (CH3Sa) and Ha. Heating to 600 K results in C-S bond cleavage near 290 K, leading to the evolution of methane and dihydrogen, and a p(2 × 2) sulfur overlayer. On the oxygen-covered surface (θO ) 0.25 ML), adsorbed methylthiolate and hydroxyl are formed. The amount of irreversible thiol reaction is essentially the same on the O-covered and clean Fe(110) surfaces. The amount of irreversibly bound thiol is reduced by approximately half on the (2 × 2)-S overlayer (θS ) 0.25 ML). The C-S bond in methylthiolate is stabilized more by surface sulfur than by surface oxygen, based on the methane formation temperature shifts of +55 and +20 K, respectively. Oxygen coverages greater than 1 monolayer form an FeO thin film, weakening the interaction of methanethiol with the surface, further reducing the methane yield which now reacts above 400 K.
Introduction Methanethiol has been studied extensively on a range of single-crystal metal surfaces, including Fe(100),1-3 Ni(100),4,5 Ni(110),6 Ni(111),7,8 Pt(111),9,10 Mo(110),11 W(211),12 W(100),13 Ru(0001),14 Cu(100),15 and Cu(111).16-18 In general, facile S-H bond scission is observed on these transition metal surfaces resulting in adsorbed methylthiolate (CH3Sa) and hydrogen (Ha). The temperature and mechanism for methylthiolate decomposition at higher temperatures depend on the coverage and the nature of the surface. On clean Fe(100), a saturation * Corresponding author. † Present address: Department of Chemistry, CUNYsStaten Island, 2800 Victory Boulevard, Staten Island, NY 10314. ‡ Present address: Texaco, Inc., Advanced Technology Section, P.O. Box 509, Beacon, NY 12508. (1) Albert, M. R.; Lu, J. P.; Bernasek, S. L.; Cameron, S. D.; Gland, J. L. Surf. Sci. 1988, 206, 348-364. (2) Cheng, L.; Bocarsly, A. B.; Bernasek, S. L. Langmuir 1994, 10, 4542-4550. (3) Cheng, L.; Bocarsly, A. B.; Bernasek, S. L. Langmuir 1996, 12, 392-401. (4) Parker, B.; Gellman, A. J. Surf. Sci. 1993, 292, 223-234. (5) Mullins, D. R.; Tang, T.; Chen, X.; Shneerson, V.; Saldin, D. K.; Tysoe, W. T. Surf. Sci. 1997, 372, 193-201. (6) Huntley, D. R. J. Phys. Chem. 1989, 93, 6156-6164. (7) Castro, M. E.; White, J. M. Surf. Sci. 1991, 257, 22-32. (8) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472-11480. (9) Rufael, T. S.; Koestner, R. J.; Kollin, E. B.; Salmeron, M.; Gland, J. L. Surf. Sci. 1993, 297, 272-285. (10) Koestner, R. J.; Stohr, J.; Gland, J. L.; Kollin, E. B.; Sette, F. Chem. Phys. Lett. 1985, 120, 285-291. (11) Wiegand, B. C.; Uvdal, P.; Friend, C. M. Surf. Sci. 1992, 279, 105-112. (12) Benziger, J. B.; Preston, R. E. J. Phys. Chem. 1985, 89, 50025010. (13) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 92269232. (14) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 1200812013. (15) Sexton, B. A.; Nyberg, G. L. Surf. Sci. 1986, 165, 251-267. (16) Bao, S.; McConville, C. F.; Woodruff, D. P. Surf. Sci. 1987, 187, 133-143. (17) Prince, N. P.; Seymour, D. L.; Woodruff, D. P.; Jones, R. G.; Walter, W. Surf. Sci. 1989, 215, 566-576. (18) Seymour, D. L.; Bao, S.; McConville, C. F.; Crapper, M. D.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1987, 189/190, 529-534.
coverage of adsorbed methylthiolate reacts at ∼300 K to form methane; adsorbed methyl ((CH3)a) is proposed to be an intermediate. Gaseous methane, hydrogen, and a c(2 × 2) sulfur structure are the ultimate products. Only trace amounts of methane are observed when methanethiol reacts with sulfur-covered Fe(100) (Θs ) 0.5 mL), similar to our observation on the 0.4 monolayer (ML) S/Fe(110). In contrast, there is a significant amount of methanethiol adsorption and reaction on sulfur-covered W(211),12 including the formation of a disulfide intermediate. Similar results are also observed on Ni(110)6 in that methane yield increases initially until the S coverage approaches saturation. There are only a few studies of thiol reactions with oxygen-covered metal surfaces. Oxygen on W(211)12 inhibited methanethiol decomposition and stabilized molecular intermediates which ultimately react to form gaseous methane and Sa. A decrease in methanethiol adsorption was also observed with increasing oxygen coverages (up to 1.5 ML), even though the methane yield remained nearly constant. Surprisingly, the yield of methane increased with oxygen coverage on Ni(110).6 Methane and dihydrogen formation are accompanied by water on oxygen-covered Ni(110). Ethanethiol has also been examined on the oxygen-covered Fe(100),3 where water becomes a reaction product; however, the yield of hydrocarbon products decreases with oxygen coverage. In this investigation, methanethiol has been examined on clean, sulfur-covered, and oxygen-covered Fe(110) surfaces. Two distinctly different oxygen layers were investigated in order to probe for differences in chemisorbed oxygen and a thin-film FeO surface. The chemisorbed oxygen is characterized by a sharp p(2 × 2)-O lowenergy electron diffraction (LEED) pattern, corresponding to an oxygen coverage of 0.25 ML, and occupies the longbridge site. At higher coverages, ∼1.3 ML, the surface consists mainly of FeO particles.19,20 (19) Pirug, G.; Broden, G.; Bonzel, H. P. Surf. Sci. 1980, 94, 323338. (20) Wight, A.; Condon, N. G.; Leibsle, F. M.; Worthy, G.; Hodgson, A. Surf. Sci. 1995, 331-333, 133-137.
10.1021/la971313j CCC: $18.00 © 1999 American Chemical Society Published on Web 03/12/1999
2392 Langmuir, Vol. 15, No. 7, 1999
Batteas et al.
Figure 1. Temperature-programmed reaction following saturation exposure of methanethiol on (a) Fe(110), (b) Fe(110)-p(2 × 2)-S, and (c) Fe(110)-0.40 ML S at 100 K. The crosshatched area in the methane curve indicates contribution from fragmentation of methanethiol and desorption from the supporting/heating elements. The heating rate was approximately 5 K/s.
Unlike W(211) and Ni(110), the reactivity of methanethiol on sulfur-covered Fe(110) appears to be associated with the availability of open iron sites, since the methane yield is inversely proportional to the amount of sulfur present on the surface. Chemisorbed oxygen, however, reacts to form water; thus, vacancies are created and methanethiol decomposition is sustained. In contrast, no water is formed on the oxidized, FeO, surface and the minor amount of methanethiol reaction is attributed to defects in the FeO film which are filled by sulfur. Experimental Section All experiments were performed in an ultrahigh vacuum chamber with a typical base pressure of
