Langmuir 1994,10, 4542-4550
4542
Interaction of Alkanethiols with Single Crystal Iron: The Low-Temperature Decomposition of Ethanethiol on the Fe(100) Surface Longchun Cheng, Andrew B. Bocarsly,*and Steven L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544
T. A. Ramanarayanan* Corporate Research Laboratories, Emon Research and Engineering Company, Annandale, New Jersey 08801 Received March 21, 1994. In Final Form: September 22, 1994@ The decomposition mechanism of ethanethiol (CzHsSH)on the Fe(100)surface under ultrahigh vacuum has been investigated using temperature programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). Upon adsorption at 100 K, ethanethiol undergoes S-H bond scission to form a surface ethanethiolate (-SCzHs). The ethanethiolate species starts to decompose below 253 K via C-S bond cleavagewhich is identified as the rate-determiningstep. HREELS data suggest different mechanisms for the ethanethiolate decomposition at different coverages. On the unsaturated surface, the C-S bond cleavage is followed by p-hydrogen elimination leading to the formation of ethylene and surface hydrogen. Part of the ethylene molecules interact with reactive iron sites and further decompose to surface CH, CCH, and surface carbon, while the rest of the ethylene evolves to the gas phase. For the saturated surface, ethane is also observed while ethylene remains the major reaction product. Further decomposition of the hydrocarbons on the surface was prohibited by the passivation effect of the coadsorbed species. The decomposition of the saturated ethanethiolate overlayer leaves a c(2 x2) sulfur overlayer on the Fe(100) surface.
1. Introduction The decomposition ofthe alkanethiols and the formation of stable alkanethiol adlayers on various transition metals have drawn wide attention during the past decade. These studies have been motivated by two issues of considerable industrial importance. The first issue involves the selective C-S bond activation associated with the catalytic hydrodesulfurization processes,1while the second is aimed at the preparation of well-defined model organic films as metal surface coatings2 and corrosion inhibitors. It is well-known that alkanethiol molecules can form well-ordered adlayers on noble transition metals such as A u . ~ -Increasing ~ the length of the hydrocarbon backbone has been suggested to significantly enhance the stability and ordering of the overlayer due to the hydrophobic interchain attractions. Such systems, which can be viewed as two-dimensional crystals, are referred to as selfassembled monolayers (SAMs).It has been reported that interfaces of this type serve as tunneling barriers for electron transport.'^^ The suggestion has also been made that these structures may have an important role in the suppression of corrosion reactions. I t has also been
* Authors t o whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, November 15, 1994. (1) Schuman, S.C.; Shalit, H. Catal. Rev.-Sci. Eng. 1970,4 , 245318. (2)Swallen, J. D.; Allara, D. L.; h d r a d e , J. D.; Chandross, E. A.; Garoff, S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Hu, H. Langmuir 1987,3,932-950. (3)Strong, L.; Whitesides, G. M. Langmuir 1988,4 , 546-558. (4)Whitesides, G. M.;Laibinis, P. E. Langmuir 1990,6,87-96. ( 5 ) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J.Am. Chem. SOC. 1989,111,321-335. (6)Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Chem. Phys. 1993, 98.678-688. (7)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D.J.Am. Chem. SOC.1987,109,3559-3568. (8)Chidsey, C.E.D.; Loiacono, D. L. Langmuir 1990,682-691. @
reported that a chemical bond between iron and thiols can be formed ifthe substrate is free of oxide.g In addition, it has been noted that thiols are effective in inhibiting the corrosion of stainless steel in acidic media.1° Studies of the dissociation of thiols on reactive metal surfaces are therefore critical in providing insights into overlayer formation and film breakdown mechanisms. Very few investigations are available, however, on the dissociation of alkanethiols on reactive metal surfaces. Although the detailed reactivity of methanethiol has been studied on a variety of metal surfaces including Pt(lll),11J2C U ( ~ O O )W(211),14 ,~~ Fe(100),15Ni(l1l),l6Ni(loo)," Ni(110),l8 and Au(lll),19 little work is available on the interaction of reactive metal surfaces with higher molecular weight alkanethiols. For methanethiol on most of these metal surfaces, cleavage of the S-H bond a t low temperature results in the formation of a methanethiolate film, except for Au(ll1) on which methanethiol adsorbs and desorbs molecularly. Further decomposition of the methanethiolate film depends on the metal substrate as well as the methanethiol coverage. Roberts and Friend (9)Volmer-Uebing, M.; Stratmann, M.AppZ. Surf. Sci. 1992,55,1935. (10)Saleh, J. M.;Al-Haidari, Y. K. Bull. Chem. SOC.Jpn. 1989,62, 1237-1245. (11)Koestner, R. J.; Stohr, J.; Gland, J. L.; Kollin, E. B.; Sette, F. Chem. Phys. Lett. 1986,120,285-291. (12)Rufael, T.S.;Koestner, R. J.;Kollin, E. B.; Salmeron, M.; Gland, J. L. Surf. Sci. 1993,297,272-285. (13)Sexton, B. A.; Nyberg, G. L. Surf. Sci. 1986,165,251-267. (14)Benziger, J.;Preston, R. E. J. Phys. Chem. 1986,89,50025010. (15)Albert, M. R.;Lu, J. P.; Bernasek, S.L.; Cameron, S. D.; Gland, J. L. Surf. Sci. 1988,206,348-364. (16)Castro, M. E.; White, J. M. Surf. Sci. 1991,22-32. (17)Castro, M. E.; Ahkter, S.; Golchet, A.; White, J. M. Langmuir 1991,7,126-133. (18)Huntley, D. R. J. Phys. Chem. 1989,93,6156-6164. (19)Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC. 1987,109,733-740.
0 1994 American Chemical Society
Interaction of Alkanethiols with Iron (a)
Langmuir, Vol. 10,No. 12, 1994 4543
Mass 2 (x3)
Mass 28
L
Mass 30 (x3)
(C)
L
0.72 L
0.72 L
L
L 0.50 L
0.50 L
&.-
. .
0.36L
0.36 L
0.IBL
L 0
200
400
600
-
0.15L
OI5L
0.IOL
0 IOL
800
Temperature (K)
IO00 0
200
400
600
800
Temperature (K)
0.15 L
0.IOL
1000 0
200
400
600
800
1000
Temperature (K)
Figure 1. TPRS of ethanethiol on the Fe(100) surface as a functionof ethanethiol exposure: (a)hydrogen; (b)ethylene;(c) ethane. Vertical scale expansion factors of mass 2 and mass 30 are referenced t o mass 28.
have investigated ethanethiolZ0and other higher molecular weight sulfur-containing molecules21~22 on the Mo(110) surface. It was found that the ethanethiol underwent dissociative adsorption via S-H bond scission to form surface ethanethiolate (-SCHzCHs) which further decomposed to surface sulfur, surface hydrocarbon fragments, and gas phase hydrocarbons (ethane and ethylene). In the present investigation, the adsorption and decomposition of ethanethiol on the Fe(100) surface has been studied using temperature-programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES),low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). Reaction mechanisms are proposed to elucidate the decomposition processes. We also report preliminary results for the adsorption and decomposition of higher molecular weight alkanethiol molecules on the Fe(100) surface. These results indicate that the reactivity of the substrate is critical to the stability of the interface, affecting the reactivity of the thiol C-S bond. In contrast to the reported interactions between alkanethiols and gold surfaces, increasing the hydrocarbon chain length above CZis not effective in enhancing the stability of alkanethiol films on reactive substrates such as iron. Thus, no special stability can be expected from the generation of alkanethiol SAMs on iron surfaces.
2. Experimental Section The experiments were performed in an ion pumped stainless steelultrahighvacuum chamber equippedwith facilitiesfor AES, LEED, TPRS, and HREELS measurements. The base pressure Torr. of the system was kept below 2 x The Fe crystalspot-weldedon two tantalum wires was mounted on an off axis manipulator. The temperature of the sample could be adjustedfrom 100to 950 K by a combinationof liquid nitrogen cooling and electricalresistiveheating. The Fe(100)surface was cleaned by repeated argon ion sputtering followed by annealing (20)Roberts, J. T.;Friend, C. M. J. Phys. Chem. 1988,92, 52055213. (21)Roberts, J.T.;Friend, C. M. J.Am.Chem. SOC.1986,108,72047210. (22)Roberts, J.T.;Friend, C. M. J.Am. Chem. SOC.1987,109,38723882.
under vacuum. The surface cleanliness was checked by AES and HREELS, and the crystallographic order was verified by LEED. Gases were admitted into the vacuum chamberthrough a doser attached to a Varian adjustableleak valve. The exposures were Torrs) using an ion measured in langmuirs (1langmuir = gauge to monitor the pressure in the chamber. Ethanethiol (Aldrich 97%) was dried over type 4A molecular sieves and degassed by several freeze-pump-thaw cycles prior t o use. Hydrogen (99.9995%)and deuterium (CP) were obtained from Matheson and used without further purification. The TPRS measurements were carried out at an initial adsorption temperature of 100 K. The TPRS spectra were recorded by placing the crystal in front of the quadrupole mass spectrometer and linearly ramping the crystal temperature at 10Ws. The mass spectrometerionizer is enclosedina thin metal cylinder (2 in. in diameter)with a coaxial entrance aperture 1/4 in. in diameter. This shield serves to reduce the signal from background gas desorption and to prevent electron impact on the surface. The HREELS monochromator and analyzer were both 127" cylindrical sectors. The vibrational spectra were collected under an incident electron beam energy of 5.0 eV. The full width at half maximum (fwhm)of the elastic peak from the clean Fe(100) was typically 70 cm-l. All the HREELS spectra were recorded at 100 K. Data acquisition for HREELS, TPRS, and AES was accomplished with an IBM-PC-AT interfaced to the spectrometers. The software used has been described e1se~here.l~ 3. Results TPRS, AES, LEED, and HREELS have been employed to investigate the adsorption and decomposition of ethanethiol on the Fe(100) surface. The results are presented in the following sections. Section 3.1 summarizes the results ofTPRS measurements of ethanethiol on the clean Fe(100) surface as well as on hydrogen and deuterium precovered Fe(100) surfaces, followed by the AES and LEED information in section 3.2. Section 3.3 presents a detailed description of HREELS data and assignments. On the basis of this spectroscopic information, decomposition products and reaction intermediates are characterized and mechanistic conclusions are then discussed. 3.1. Temperature-Programmed Reaction Spectroscopy. Figure 1shows the temperature-programmed
Cheng et al.
4544 Langmuir, Vol. 10, No. 12, 1994
L Mass 30 (x3)
Mass 31(x3)
L
Mass 30(x3)
Mass 30 (x3)
L Mass 2 (x3)
0
200
400
600
Temperature (K)
800
1000 0
200
400
600
800
Temperature (K)
1000 0
200
400
600
800
1000
Temperature (K)
Figure 2. TPRS of ethanethiol on (a) the clean Fe(100) surface, with ethanethiol exposure of 0.72 langmuir, (b)Hz precovered Fe(100) surface, Hz exposure is 20 langmuirs, ethanethiol exposure is 0.68 langmuir, and (c) D2 precovered Fe(100) surface, Dz exposure is 20 langmuirs, ethanethiol exposure is 0.70 langmuir. Vertical scale expansion factors are referenced t o mass 28.
reaction spectra of ethanethiol on the Fe(100) surface as a function of ethanethiol exposure from 0.10 to 0.72 langmuir. The initial adsorption temperature is 100 K. Upon heating, three products, hydrogen (mass 2), ethylene (mass 28), and ethane (mass 30) were detected in the gas phase. No sulfur-containing species or other hydrocarbon species were detected below 0.50 langmuir exposure. In the case of multilayer adsorption (0.72 langmuir), the ethanethiol parent molecule (mass 62) was observed to desorb at 136 K with a shape and behavior consistent with zero-order desorption kinetics. The saturation coverage of the first ethanethiol monolayer on the Fe(100) surface was shown to correspond to 0.5 langmuir by AES measurements (see section 3.2). Ethylene (Figure lb) is the major hydrocarbon product observed in the gas phase during the temperatureprogrammed reaction process. The TPRS spectra of mass 28 show that the intensity increases with increasing ethanethiol exposure. The peak maximum is a t 260 K. When ethanethiol exposure reached 0.72 langmuir, a small peak appeared at 136 K which results from multilayer desorption noted above. At 0.50 langmuir and higher, ethane evolution is clearly evident (Figure IC).The ratio of the intensity of mass 28 and mass 30 was determined to be 2.6:l for authentic ethane using the experimental setup in this work. When the peak at mass 28 is corrected for the contribution due to the cracking of ethane, ethylene still remains the major product formed during the temperature-programmed reaction. Hydrogen (Figure l a ) desorbs a t 300 K a t low coverage with a broad peak a t 360 K tailing up to 450 K. The peak maximum slightly shifts to lower temperature as the ethanethiol exposure is increased. The desorption temperatures are close to the temperature of the /31 and /3z states assigned to surface hydrogen recombination and desorption on the clean Fe(100) surface.23 However, the peak shape and behavior with exposure are quite similar to hydrogen desorption from the Fe(100) surface covered with adsorbates such as (23) Bozso, F.;Ertl, G.;Grunze,M.; Weiss, M.Appl. SUI$ Sci. 1977, 1, 103-108.
methanethi~l,'~ water,24and ethylene.25 It has also been reported that surface adsorbates containing oxygen and sulfur can deviate the iron surface and reduce the binding energy for hydrogen.26 Rufael et a1.12demonstrated a low temperature hydrogen desorption from methanethiol decomposition on the Pt(ll1)surface due to the repulsive interaction between surface atomic hydrogen and surface coadsorbates. A similar process is likely to occur here. While the mass 2 intensity increases with increasing ethanethiol exposure in the range from 0.10 to 0.36 langmuir, the change in intensity is not directly proportional to the exposure. At 0.50 langmuir exposure or higher, the peak intensity is actually attenuated. This observation suggests different decomposition mechanisms for ethanethiol decomposition at different exposures (different enthanethiol coverages). To determine the mechanistic effect of the surface hydrogen on ethane and ethylene formation, TPRS measurements of ethanethiol were carried out on hydrogen and deuterium precovered Fe(100) surfaces. The Fe(100) surface was predosed with 20 langmuirs of hydrogen or deuterium at 100K. The hydrogen or deuterium coverage is estimated to be 0.5 monolayer (ML).23t26Parts b and c of Figure 2 show the TPRS offully saturated ethanethiol (-0.7 langmuir) on the hydrogen and deuterium precovered Fe(100) surface. The TPRS of ethanethiol on clean Fe(100) is shown in Figure 2a for comparison. On the hydrogen precovered surface, the hydrogen peak intensity (mass 2) is dramatically increased, while the desorption temperature decreases to 281 K. The TPRS data of ethylene (mass 28) and ethane (mass 30), however, do not seem to be altered in either intensity or peak maximum temperature (260 K for ethylene and 256 K for ethane). Compared to the TPRS ofthe hydrogen precovered surface, the major hydrocarbon product is still ethylene at mass 28 for the deuterium precovered surface. However, surface deuterium results in the formation of several deuterium incorporated species. Mass 3 and mass 4 correspond to (24) Hung, W. H.; Schwartz, J.;Bernasek, S. L. Surf. Sci. 1991,248, 332-342. (25) Hung, W.-H. Ph.D. Thesis, Princeton University, 1992. (26) Benziger, J.; Madix, R. J. Surf. Sci. 1980,94, 119-153.
Interaction of Alkanethiols with Iron
Langmuir, Vol. 10,No. 12, 1994 4545 xzoo
00
1
1
I
\
00
04
08
1437
n
12
Exposure (L)
I / 1 / , 1 1 , , 1 1 1 1 1 1 1 / 1 1 1 1 1 1 1 1 ,
2909
I
Electron Kinetic Energy (eV)
Figure 3. Auger spectra of clean (bottom)sulfur covered (top) Fe(100)surface. Inset: Auger peak-to-peakratio of sulfur and iron as a function of the ethanethiol exposure.
deuterated hydrogen (H-D) and deuterium (D-D). Mass 31 corresponding to CzH6D is also seen in this case, but not mass 32 or higher. The weak peak intensity of mass 4 in Figure 2c is due to the low sensitivity of our mass spectrometer to Dz. For the hydrogen and deuterium precovered Fe(100) surface, the intensity of the major decomposition product (mass 28) is nearly equal to that for the clean Fe(100) surface. After TPRS, similar A E S and LEED were also observed, suggesting that the adsorption of ethanethiol is not affected by preexposure of the surface to hydrogen or deuterium. 3.2. Auger Electron Spectroscopy and Low Energy Electron Diffraction. Figure 3 (bottom) shows the AES of a clean Fe(100) surface. After the TPRS run, the saturated ethanethiol overlayer is found to completely decompose. The sulfur (KLL) transition is seen at 154 eV in Figure 3 (top). The Auger spectrum yields no indication of organic fragments on the iron surface. This sulfurcovered surface shows a clear ~ ( 2 x 2 LEED ) pattern. Similar results have been reported for methanethiol decomposition on the Fe(100) surface.15 This observation is also in agreement with the TPRS results showing that the ethanethiol overlayer decomposes to gas-phase hydrocarbons, hydrogen, and surface-bound sulfur. The relative sulfur coverage is determined by the Auger peakto-peak height ratio (APPHR) of the sulfur (KT,L) line to the iron (LMM) line a t 662 eV. The inset in Figure 4 shows the sulfur to iron ratio as a function of ethanethiol exposure. Saturation is observed at -0.5 langmuir ethanethiol exposures. 3.3. High Resolution Electron Energy Loss Spectroscopy. The HREELS spectra for ethanethiol adsorption are shown in Figure 4. These spectra were collected for ethanethiol overlayers on a Fe(100) surface a t 100 K as a function of ethanethiol exposure. The vibrational frequencies of the energy loss peaks and the peak assignments are summarized in Table 1. The peak
1
0
,
,
,
'
1
,
1000
,
'
'
1
1
1
,
2000
1
1
3000
'
1
,
1
1
1
~
4000
Energy Loss (cm")
Figure 4. HREELS spectra ofethanethiol at various coverages at 100 K. Vibrational energies are summarized in Table 1. Table 1. HREELS Vibrational Energies (in cm-l) for Ethanethiol on Fe(100)Surface: Ethanethiol Coverage Dependence ethanethiol exposure (langmuir) 0.15 0.35 0.45 0.60 0.80 liquid 353 594 962 1189 1415
382 580 693 991 1189 1415
382 552 736 991 1217 1415
2909
2909
2923
382 580 523 743 736 1005 1040 1246 1267 1444 1437 2520 2513 2945 2938
332 660 740 970 1246 1450 2570 2967
assignments are based on the reported IR spectra ofliquid phase e t h a n e t h i ~ l . ~ ~ An intense energy loss peak is observed between 2909 and 2938 cm-l. This peak is assigned to v(C-H), the C-H stretching model. The S-H stretching mode which is at 2650 cm-' for liquid phase ethanethiol is absent a t ethanethiol exposures below 0.45 langmuir. At 0.60 langmuir or higher, the energy loss near this frequency is clearly seen at 2520 and 2513 cm-', indicating multilayer adsorption of the intact molecule. Another strong peak a t 1415-1444 cm-l is assigned to d(CH3) and d(CHz), an (27)McCullough, J. P.; Scott, D. W.; Finke, H. L.; Gross, M. E.; Williamson, K. D.; Pennington, R. E.; Waddington, G.; Huffman, H. M. J.Am. Chem. SOC.1962,74, 2801-2804.
4546 Langmuir, Vol. 10,No.12, 1994
0.IOL Ethanelhiol/Fe( 100)
x200\ 269
x200
Cheng et al.
j
9
0
Mass 28
0
K
I
3 . . ~
507
1026 1231
x200
396
3015
12101423
,
323KI
4-
181 K
/
,
Figure 5. HREELS spectra (left panel) as a function of annealingtemperature as indicated and thermal desorption spectrum (right panel) after the adsorption of 0.10 langmuir of ethanethiol on Fe(100) at 100 K. Vibrational energies are summarized in Table 2.
overlap of the CH3 and CH2 bending modes, which are not well resolved. The loss peak around 1200 cm-l is due to CH2 wagging and twisting. The peak at 962-1040 cm-l is assigned to the C-C stretch. The CH3 rocking mode at 1100 cm-l in the liquid phase is not observed. This is likely to be due to its low intensity and the broad nature of the nearby C-C stretching mode. The peak at 552594 cm-l is assigned to the C-S stretching mode. Although the Fe-C stretching mode could also be present in this region, the assignment to the C-S stretch is supported by the presence of the C-C-S skeleton (discussed below). This peak is shifted more than 50 cm-' compared to that in the liquid phase. This is presumably due to the strong interaction between the sulfur atom and the iron surface. A similar shift has been reported in the HREELS study of methanethiol on the Fe(100) surface.15 The peak at 736 cm-' emerges when the surface is covered by multilayer ethanethiol. We assign this loss peak to d(CHz), the CH2 bending mode. There are two possible assignments for the energy loss at 353-382 cm-l. Compared to liquid phase ethanethiol, it is close to the C-C-S bendingmode. It could also be the Fe-S stretching, since the Fe-S stretching falls in this region in some iron complexes.28 However, no loss peak was observed in this region for methanethiol on the Fe(100)surface, suggesting assignment here as the C-C-S bending. Figures 5 and 6 depict the HREELS spectra for ethanethiol at 100 K, subsequently heated to the indicated temperatures. The indicated annealing temperatures were chosen according to the TPRS data which are shown in the right panel next to the HREELS spectra. Vibrational energies and mode assignments are summarized in Table 2 and Table 3. Figure 5 shows the HREELS for 0.1langmuir exposure of ethanethiol. As discussed above, the absence of the (28) Nakamoto, Kazuo Infrared and Raman Spectra of Inorganic ana' Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986.
S-H stretch a t -2650 cm-' indicates that ethanethiol dissociativelyadsorbs on the Fe(100) surface via S-H bond cleavage. The loss peaks a t 559, 757,1012,1201,1423, and 2916 cm-' correspond to the vibrational modes of ethanethiolate. Following annealing at 181 K, there is no significant change in the frequencies or intensities. Upon heating to 253 K, the onset temperature of the ethylene desorption, the loss peak at 2916 cm-' is broadened and shifted to 2987 cm-' suggesting more s-character in the C-Hbond. Energy losses at 1005,1210, and 1423 cm-' are attenuated. Meanwhile, new loss peaks at 502 and 800 cm-l were observed, suggesting the formation of new surface hydrocarbon species. Upon heating to 323 K, the vibrational feature of these species can be clearly seen. The C-H stretch is shifted to higher frequency, 3015 cm-l. Loss peaks at 507 and 800 cm-' become more intense. The peak at 1423 cm-l is totally attenuated, while two new peaks at 1026 and 1231 cm-l are observable. The observed energy losses at 502, 800, and 3015 cm-' suggest the formation of surface methylidyne, -C-H, and are assigned to the Fe-C stretching, -C-H bending, and C-H stretching modes. The losses a t 1026 and 1231 cm-l indicate the presence of CCH on the Fe(100) surface, corresponding to the -C-H bending and C-C stretching modes. These assignments are made based on the HREELS studies of ethylene and acetylene decomposition on the Fe( 100) surface.25At 473 K, surface CH and CCH completely decompose. The loss at 509 cm-', which remains, is attributed to the carbon adatom. Annealing the surface at 900 K, the Fe-S stretch can be clearly seen at 269 cm-l but the Fe-C stretch is absent. The absence of surface carbon can be explained by carbon diffusion into the bulk as has been previously reported on the Fe(100) surface.15 Figure 6 shows the HREELS data for 0.8 langmuir exposure of ethanethiol at 100 K followed by annealing at the indicated temperatures. The 0.8 langmuir exposure results in multilayer adsorption of ethanethiol on the Fe-
Langmuir, Vol. 10, No. 12, 1994 4547
Interaction of Alkanethiols with Iron
0.80L Ethanethiol/Fe( 100)
Mass 28
x4w
0
2000
1000
3000
Energy Loss (cm")
4000
0
200
400
600
800
IO00
Temperature (K)
Figure 6. HREELS spectra (leftpanel)as a function of annealingtemperature as indicated and thermal desorption spectrum (right panel) after the adsorption of 0.80 langmuir of ethanethiol on Fe(100) at 100 K. Vibrational energies are summarized in Table 3.
Table 2. HREELS Vibrational Energies (in cm-l) for Ethanethiol on Fe(100) Surface: Temperature Dependencea temperature (K) vibrationalmode 100 181 253 323 473 900 liquid v(M-S) 4M-C) B(C-C-S) v(C-S) 559 e(CHd 722 B(-C-H) v(C-C) e(CH3) 1012 r(CH2) w(CH2) 1210 B(CH3) B(CH2) 1423 v(S-H) v(C-H) 2916
+ + +
Table 3. HREELS Vibrational Energies (incm-') for Ethanethiol on Fe(100) Surface: Temperature Dependencea temperature (K) vibrationalmode 100 181 253 323 473 liquid
269
254
269
269
502 507 509 396 559 757 1005 1210 1423
332 660 740 800 800 1005 1026 1210 1231 1423
2916 2909(2987) 3015
970 1246 1450 2570 2967
Ethanethiol exposure is 0.10 langmuir at 100 K. All spectra were recorded at 100 K after flashing to the indicated temperature. (100) surface as observed in the TPRS measurements and the coverage dependence of the HREELS experiments. The S-H stretching mode at 2513 cm-' is evident for the ethanethiol multilayer adsorbed at 100 K. According to the TPRS measurement, annealing the surface at 181 K causes desorption of the ethanethiol multilayers. The absence of an S-H stretching in the HREELS spectrum aRer annealing the surface at this temperature is consistent with the TPRS result. The spectrum after the multilayer desorption is quite similar to that for the unsaturated surface (0.45 langmuir exposure in Figure 4). The C-S stretch is intense, indicating the formation of ethanethiolate on the surface. At 253 K, the ethanethiolate surface starts to decompose and hydrocarbon products are detected in the gas phase. After annealing at this temperature, the most notable feature in the HREELS spectrum is the attenuation ofthe C-S stretch at 566 cm-'. The C-C bond stretch (1005 cm-') and the CH2 wagging and CH2 twisting (1231 cm-') modes shiR to higher energies, while the CH vibrational band at 2916
523 736 1040 1267 1437 2513 2938
382 566
382 594
969 1203 1415
1005 1231 1401
1005
2916
2916
2930
1401
332 660 740 970 1246 1450 2570 2967
a Ethanethiol exposure is 0.80 langmuir at 100 K. All spectra were recorded at 100K after flashing to the indicated temperature.
cm-I remains unchanged. These peaks are attenuated, as well. Upon heating to 323 K, peak intensity further decreases but no dramatic change was observed. At 473 K, the hydrocarbon peaks have completely disappeared leaving a single peak at 269 cm-'. This is due to the absorbed sulfur on the Fe(100) surface and is in good agreement with both TPRS and AES results. Interestingly, the metal carbon stretch at -510 cm-' was not observed in this case. The existence of surface hydrocarbon fragments can be ruled out. The ethanethiolate, -SC2H5, does not fragment but just decreases in loss peak intensity.
4. Discussion 4.1. Adsorption of Ethanethiol on the Fe(100) Surface. As indicated by the spectroscopic data shown in the previous section, ethanethiol dissociatively adsorbs on the Fe(100) surface at temperatures as low as 100 K. It is known that the S-H bond cleavage has a low activation energy.20The S-H bond scission was observed
Cheng et al.
4548 Langmuir, Vol. 10, No. 12, 1994 previously for methanethiol on the Fe(100) surface. Similar behavior on various transition-metal surfaces has also been reported. Similar to ethanethiol on the Mo(110) surface, the scission of the S-H bond results in the formation of an ethanethiolate (-SCH2CH3) overlayer on Fe(100). This is demonstrated in the HREELS data which show the unperturbed vibrational bands of the hydrocarbon group, the existence of the C-S vibrational band, and the notable lack of a S-H stretch. Compared with the C-S vibrational band in the liquid phase ethanethiol (660 cm-l), the ethanethiolate C-S band is shifted to lower frequency. This indicates strong bonding between the sulfur and the Fe(100) surface. The ethanethiolate remains intact over the temperature range of 100-181 K. It is the precursor to further surface decomposition. As determined by TPRS, AES, LEED, and HREELS, the maximum coverage of the ethanethiolate overlayer is 0.5 ML, where 1ML is defined as the number of iron atoms in the topmost layer of the Fe(100) surface. This saturation coverage is equivalent to that of methanethiol adsorption on the Fe(100) surface.15 4.2. Thermal Decomposition of Ethanethiolate. Decomposition at Low Coverage. At low coverage, TPRS shows that ethylene is the only hydrocarbon product observed in the gas phase. This desorption temperature is higher than for ethylene from the Fe(100) surface. Thus, ethylene evolution is limited by the ethanethiolate decomposition. In the HREELS experiments, the observation of the Fe-C stretch at 502 cm-' indicates the C-S bond dissociation and formation of surface hydrocarbon fragments at 253 K. At 323 K, surface hydrocarbon fragments can be clearly identified as CH and CCH, identical to the products identified for the decomposition of ethylene on the Fe(100) surface.25 Since the ethylene evolution temperature is close to the decomposition temperature of ethylene on the Fe(100) surface,29it is reasonable to hypothesize that part of the ethylene decomposes on the surface. A decomposition mechanism can be proposed as follows: C2H5SH(g)
C2H5S(ad) C2H4(g)
-
C2H5S(ad)
+ 'ZH4(ad)+
+ H(ad)
'(a)
loo
253
+ H(ad)
> 323
K
> 473
K
-
300 K (&-state)
After the C-S bond cleavage, /?-hydrogen elimination to form surface hydrogen and ethylene is favored. Two competing reaction pathways are involved in the ethanethiolate decomposition. One is the direct evolution of ethylene to the gas phase following/?-hydrogenelimination without being adsorbed to the metal surface. The other is the total decomposition of ethylene on the iron surface. Decomposition at Saturation Coverage. Similar to the decomposition at low coverage, the saturated ethanethiolate overlayer starts to decompose at -253 K. The detailed mechanism of the decomposition at high coverage is different, however. At saturated coverage (0.5 ML (29) Bemasek, S. L. Annu. Rev. Phys. Chem. 1993,44, 265-298.
ethanethiolate on the Fe(100) surface), ethane is also detected in the gas phase although ethylene remains the major product. The attenuation of the C-S stretching mode is clearly seen in the HREELS spectrum (Figure 6). The Fe-S stretch is observed as the only vibrational mode after the decomposition is completed at 473 K. This indicates that the C-S bond cleavage is the most important step in the ethanethiolatedecomposition. No new surface hydrocarbon species are observed during the decomposition process, although all the vibrational bands are attenuated, suggesting ethanethiolate retains its identity. This behavior is very different from the low coverage decomposition. It is also different from the ethanethiol decomposition on the Mo(l10) surface where other hydrocarbon fragments were observed.20 The decomposition of ethanethiolate at saturation coverage is summarized below:
100 K 253 K C2H5S(ad)
+ H(ad)
-
C2H6(g)
-
+ '(ad)
-
253 K
300 K @,-state) 360 K (&-state)
Since no further reaction intermediates are observed at high coverages, it can be concluded that the reaction is simply limited by the C-S bond dissociation. Dehydrogenation at the /%carbon position is the preferred reaction pathway compared to hydrogen abstraction from the surface to form ethane. The surface adsorbed hydrogen generated by the S-H scission does not affect the kinetics of ethanethiolate decomposition. This is supported by the TPRS of ethanethiol decomposition on the hydrogen precovered Fe(100) surface which indicates that the hydrocarbon formation is not the rate-determining step during the decomposition process. Instead, the decomposition is controlled by C-S bond cleavage. After the C-S bond is activated, the reaction kinetics favor the formation of ethylene via /?-hydrogenelimination rather than ethane by hydrogen abstraction. The TPRS of ethanethiol in the presence of surface deuterium atoms indicated the formation of CH3CHzD confirming that surface hydrogen participates in ethane formation. The ethylene evolution is less affected by surface hydrogen and surface deuterium, indicating that ,&hydrogen elimination is an irreversible step. Another interesting aspect of these results is the relative amounts of ethylene and ethane observed in the reaction. On the Fe(100) surface, ethane is observed only if the surface coverage by ethanethiol is high. At this stage, the LEED pattern indicates that the surface is covered by c(2x 2) sulfur after the decomposition. The coadsorbed surface species (S or -SCzH5) may affect neighboring sites by altering their reactivity. It was found that surface coadsorbates affect the interaction between the surface hydrogen atom and the iron surface. Benziger et ~ 1 . ~ 1 6 reported that the binding energy between hydrogen and iron can be greatly reduced by sulfur adatoms. Similarly, Rufael et all2observed that the high coverage of surface coadsorbed species causes a low desorption temperature for hydrogen. Investigations of methanethiol on the Fe(100) surface also indicated this type of dea~tivati0n.l~ It is possible that surface coadsorbates weaken the hydrogen-iron bond allowing for the recombination of surface hydrogen and the ethyl group.
Langmuir, Vol. 10, No. 12, 1994 4549
Interaction of Alkanethiols with Iron Comparison of T w o Proposed Mechanisms. On the basis of the discussion above, C-S bond cleavage is the ratedetermining step in the decomposition af ethanethiolate at both low and saturated coverages. This step is followed by three reaction pathways: ethylene formation, ethylene decomposition and ethane evolution. Among these, ethylene formation is the predominant reaction pathway. This process, via j3-hydrogen elimination, is kinetically favorable. Ethylene decomposition and ethane evolution, however, are dependent on the ethanethiolate coverage. This is evident in the hydrogen TPRS (Figure l a ) where the hydrogen peak intensity does not change dramatically as the ethanethiol exposure increases. In fact, the decomposition of ethylene results in a relatively higher surface hydrogen concentration at lower ethanethiol exposure. Huntley'* demonstrated that surface electronegative modifiers such as S and 0 could alter the reaction pathway. In that investigation of methanethiol decomposition on the Ni(ll0) surface, S or 0 caused a stabilization of the thiolate species with respect to decomposition and an increase in the methane yield. It is reasonable to propose that the surface coadsorbates could stabilize the neighboring surface ethanethiolate group. This is likely to occur here based on the HREELS data. At less than saturation coverage, there are "open" iron sites which may serve as a reaction catalyst. These reactive sites are effectively blocked by the coadsorbates such as ethanethiolate at saturation coverage. Therefore, the iron surface is passivated and the formation of surface hydrocarbon fragments is prohibited. The passivation effect opens the pathway for the ethyl group to combine with surface hydrogen to produce ethane. 4.3. DecompositionTemperature. The decomposition temperature of ethanethiol on the Fe(100) surface (260 K) is lower than that found for methanethiol on various transition-metal surfaces such as Fe(100) (320 K),15W(211) (410 K),14Cu(100) (370 K),13Ni(100) (270 K)," and Pt(ll1) (>300 K)lZsurfaces. It is also lower than that for the same molecule on the Mo(ll0) surfacez0 (300 K for ethane evolution and 340 K for ethylene evolution). The vibrational frequency of the C-S stretch is 566 and 601 cm-l for ethanethiolate and methanethiolate on Fe(100),respectively. The C-S bond strength of the ethanethiolate is slightly weaker than that of methanethiolate. This results in C-S bond cleavage below 260 K for the ethanethiolate film but above 273 K for methanethiolate. On the other hand, the difference in the decomposition temperature could also result from kinetic effects. While the ethanethiolate undergoes j3-hydrogen elimination, the methanethiolate undergoes methyl group and surface hydrogen recombination. Although long chain alkanethiols form stable layers at room temperature on some of the transition-metal surfaces, this is not the case for iron, likely due to the reactivity of iron. This conclusion is in disagreement with that proposed by other authors. Blyholder and Cagle30studied ethanethiol adsorption on Fe and concluded that the C-C-S chain remains intact on the Fe surface at 298 K. However, their infrared spectrum does not cover the frequency region in which the C-S stretch can possibly be seen. The assignment of the peak at 965 cm-' to a C-C-S stretch is quite arbitrary. Our HREELS results along with the TPRS results suggest that the C-C-S skeleton could not remain intact above 253 K. 4.4. Heavy Molecular Weight Alkanethiols. In order to determine the effect of the hydrocarbon chain length on the stability of alkanethiols on the Fe(100) (30)Blyholder, G . D.;Cagle, G. W. Enuiron. Sci. Technol. 1971,5, 158-161.
350
I a. Methanethiol b. Ethanethiol c. 1-Butanethiol
325 -
B .-$
300
-
0 .*
8
E
8
B
215 b
250.
'
-
c
"
-
e
d
"
0 '
I
'
I
'
4550 Langmuir, Vol. 10, No. 12, 1994 composition, via C-S bond cleavage and P-hydrogen elimination. At less than saturation coverage, ethylene decomposition on the iron surface competes with ethylene desorption due to the availability of reactive iron sites. The formations of surface CH, CCH, and then surface carbon at higher temperatures are observed. Surface carbon was found to diffuse into the bulk at a temperature > 473 K. At saturation coverage, coadsorbates on the Fe(100) surface passivate the iron surface and stabilize the hydrocarbon group. Decomposition of hydrocarbons on the surface is greatly restricted. In addition, the bond strength between adsorbed hydrogen and the substrate is reduced. These effects favor ethane formation near saturation exposure. Strong iron-sulfur interaction weakens the C-S bond, leading to the decomposition of
Cheng et al. the ethanethiolate at a lower temperature on iron than on many other transition metals. At the same time, the elimination ofp-hydrogen provides a kinetically favorable pathway for ethanethiolate dissociation on iron. Due to the strong interaction between the substrate and alkanethiol molecules as well as the &elimination process, longer hydrocarbon chains do not enhance the stability of alkanethiols on the Fe(100) surface, in contrast to what has been proposed for these self-assembled monolayers on noble metal surfaces such as g ~ l d . ~ - ~
Acknowledgment. Partial support of this research by Exxon Research and EngineeringCompany is gratefully acknowledged.
