Langmuir 1991, 7, 126-133
126
Methanethiol Decomposition on Ni( 100) M. E. Castro, S. Ahkter, A. Golchet, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712
T . Sahin Department of Chemical Engineering, Montana State University, Bozeman, Montana 5971 7 Received February 23, 1990. In Final Form: June 21, 1990 Static secondary ion mass spectroscopy (SSIMS), temperature programmed desorption (TPD), and Au er electron spectroscopy (AES)were used under ultrahigh vacuum conditionsto study the decomposition of 6H3SH on Ni(100). Only methane, hydrogen, and the parent molecule are observed in TPD. Complete decomposition to C(a), S(a) and desorbing H2 is the preferred reaction pathway for low exposures, while desorption of methane is observed at higher coverages. Preadsorbed hydrogen promoted methane desorption. Upon adsorption, and for low coverages, SSIMS evidence indicates S-H bond cleavage into CH3S and surface hydrogen. S-H bond cleavage is inhibited for high coverages. The TP-SSIMS data are consistent with an activated C-S bond cleavage in CH&, with an activation energy of 8.81 kcal/mol and preexponential factor of 106.5s-l. The low preexponential factor is taken as indicating a complex decomposition pathway. A mechanism consistent with the observed data is discussed.
1. Introduction We now briefly summarize the work on Ni surfaces. On Ni(loo), UPS results indicate that the thiol decomposes Hetereogeneous sulfur chemistry is an important aspect a t temperatures lower than 110 K6 to form a thiomethoxy of several technologically important chemical processes intermediate. On Ni(llO), HREELS and X-ray photoinvolving fuels, e.g., coal, oil, and natural gas, petrochemical electron spectroscopy (XPS) results indicate a similar processes, and combustion. Thus, studying the surface process a t 125 K.4 On Ni(lll), the thiol adsorbs molecchemistry of intrinsically interesting model sulfur-single ularly for initial coverages higher than 0.10 ML and S-H crystal metal systems also helps one understand these bond cleavage occurs below 115 K.7 On all the above complex chemical processing reactions. Methanethiol is surfaces, the thiomethoxy intermediate is stable to above an attractive candidate; it is the simplest thiol, the surface room temperature. Methane and ethane desorptions have chemistry of which can provide insight into C-S and S-H been found on Ni(lll)7 and Ni(l10).4 bond activation. In this paper we present results on the decomposition Methanethiol adsorption and decomposition have been kinetics of methanethiolon Ni(100) as revealed by SSIMS, studied on W(211),lPt(111),2Fe(100),3Ni(110),4C~(100),~ TPD, and AES measurements. A simple mechanism for Cu(ll1) and Ni(100): Ni(lll),7and rutile and anatase.8 the decomposition of methanethiol is proposed. In general, CH3SH adsorbs through the sulfur atom with the methyl group pointing outward from the ~ u r f a c e . ~ + ~ 2. Experimental Section The product distribution observed during thermal decomThe ultrahigh vacuum (UHV) chamber used in this work has position varies with the surface under study; evolutions been described elsewhere? Briefly, the chamber was equipped of methane, ethane, dimethyl mercaptan, and hydrogen with an argon ion gun and quadrupole mass spectrometer for have been reported. secondary ion mass spectroscopy (SIMS) and temperature Low-temperature S-H bond cleavage of the thiol has programmed desorption (TPD)and a double pass cylindrical been reported on Ni(100),6 Fe(100),2CU(~OO),~ Ni(111),7 mirror analyzer for Auger electron spectroscopy (AES). and Ni(l10).4 Thiomethoxy (CH3S) species have been The working base pressure during the experiments reported identified by high-resolution energy electron loss specin this work was4.2 X 10-10Torr. The Ni(100)samplewas cleaned troscopy (HREELS) on Fe(100),3Pt(111),2CU(~OO),~ and by using a cycle of Ar+ ion bombardment and heating to 1200 K, Ni(l10)4and by ultraviolet photoelectron spectroscopy 15 min of annealing at this temperature, followed by oxidation (02, P = 2 X 10-8Torr, 1000 K) and reduction (Hz, P = 5 X 10-7 (UPS)on Ni(100).6 On Fe(100), evidence has been Torr, 1150 K) until no impurities were detected by AES. The reported for a surface methyl group formed upon C-S temperature was measured with a chromel-alumel thermocouple bond cleavage of the thiomethoxy. Methyl groups have spot welded onto the back of the crystal. The crystal could be also been proposed as intermediates on Cu(lOO)5and Nicooled to 95 K with liquid nitrogen and resistively heated to 1250 (lll).7On Pt(111),2 thioformaldehyde (CH2S) has also K. been identified by angular resolved ultraviolet photoelecMethyl mercaptan (Union Carbide Co., Linde Division,99.5% tron spectroscopy (ARUPS). pure) and methyl-& mercaptan (MerckSharp and Dohme, 70% pure) were degassed at 77 K and dosed with a directed capillary (1) Benziger, J. B.; Preston, R. E. J. Phys. Chem. 1985,89, 5002. array doser. To minimize extensive H-for-D isotope exchange (2) Koestner, R. J.;Stohr, J.; Gland, J. L.; Kollin, E. B.;Sette, F. Chem. into the acidic S-D bond, the gas handling system and chamber Phys. Lett. 1985, 120,285. were conditioned with the labeled thiol before the experimental (3) Albert, M. R.;Jiong, P. L.; Bernasek, S.L.; Cameron, S. D.; Gland, work. Gas purity was verified by mass spectrometry. J. L.Surf. Sci. 1988, 206, 348. During exposure, the sample was placed between 0.50 and (4) Huntley, D. R. J. Phys. Chem. 1989,6256,93. (5) Sexton, B. A.; Nyberg, G . L. Surf. Sci. 1987, 165, 251. 0.80 cm from the end of the doser. Compared to backfilling, this (6)Bao, S.;McConville, C. F.; Woodruff, D. P. Surf. Sci. 1987, 287, gave an enhancement factor of approximately 100. Unless 133. otherwise stated, all doses were carried out at 100 K and partial (7) Castro, M. E.;White, J. M. Paper in preparation. (8) Beck, D.; White, J. M.; Ratcliffe, C. T. J.Phys. Chem. 1986, 90, 3137.
~. ..,I O. 0743-7463,I91,12407-0126SO2.50 ~
~
~~
~
(9) Akhter, S.;White, J. M. Surf. Sci. 1986, 267, 101.
0 1991 American Chemical Society
Methanethiol Decomposition on Ni(100) 1.3
1 .o
r-7
"0
Langmuir, Vol. 7,No.1, 1991 127 ,
2
A
P
2 Y
x
\
t 81
"
Y
c
2 150 200 250 Temperature ( K )
0
10 20 Dote (1)
Figure 1. Left-hand panel: TPD of molecular methanethiol on Ni(100). The exposures (L)are from bottom to top, (a) 0.30, (b) 0.50, (c) 0.75, (d) 5, (e) 2.5, (f) 10,and (g) 20. Right panel: sulfur coverage (closedsymbols)and the normalized methanethiol TPD area (open symbols) as a function of dose. Temperature ramp is 3.3 K/s. pressures ranging between 4 X lo-" and 1.5 X 1O-g Torr. The sample was placed in line-of-sight with the mass spectrometer for SSIMS measurements. The primary Ar+ ion current ranged from 2.3 to 4 nA and the energy was 1keV. For TPD, the same geometry was used but the Bessel box prevented line-of-sight with the ionizer. Each TPD and TP-SSIMS experiment was carried out between 100 and 650 K. Linear temperature ramps were used: 3.3 K/s for TPD and 8 K / s for SSIMS. The accumulated coverage of sulfur was determir-ed by comparing the S/Ni AES ratio after each experiment with that measured for saturation sulfur on Ni(100). The latter, taken to be 0.50 ML, where 1 ML = 1 sulfur for each nickel atom, was prepared by dosing HzS at 300 K, flashing the sample to 600 K, and annealing at 400 K for 30 s.l0 The estimated uncertainty in the sulfur coverage is f9 ?%,the maximum deviation observed for three equal exposures. Desorbing hydrogen was calibrated by assuming that a saturation dose of Hz on clean Ni(100) corresponds to 1 ML." The main desorption products observed during CH3SH decompositionwere methane,hydrogen,and the parent molecule. Low-temperaturepeaks correspondingto C&, Hz,H a , CH3SCH3, and CH3CH3 were observed during desorption of the parent molecule. These are attributed to decomposition of nonsample surfaces and/or reactions in the ion source of the mass spectrometer and not to any reaction on the sample.12In the following sections, these low-temperature artifacts are not discussed. 3. Results 3.1. TPD. 3.1.1. CH3SH/Ni(lOO). The left panel of Figure 1 shows the methanethiol TPD spectrum as a function of gas exposure. No molecular desorption is observed for exposures lower than 0.50 L. There is no appreciable shift in the peak temperature or leading edge with increasing gas exposure up to 20 L, where the peak is centered at 199 K. Multilayer desorption, following with the exposure a t 85 K, has been reported on Cu( peak temperature a t 115 K. The right side of Figure 1 shows the sulfur coverage (ML), after TPD, as a function of gas exposure (L). The normalized CH3SH TPD area is also shown. On the basis of residual sulfur, the maximum amount of decomposed methanethiol is 0.38 f 0.04 ML. A negligible amount of CH3SH, less than 0.01 % as compared with a 20-L dose, ~~
(10)Hardegree, Eric. Dissertation Thesis, University of Texas, 1985. (11) Zhu, X. Y.; Caetro, M. E.; Akhter, S.;White, J. M. Surf. Sci. 1988, 195, L146.
(12)Zhou, X. L.; White, J. M. Surf. Sci. 1988, 194,438.
0 200
300 400
500 200 300 Temperature ( K )
400
500
Figure 2. Hydrogen (leftpanel) and methane (rightpanel)TPD profiles for methanethiol adsorbed on Ni(100). Exposures (L) are, from bottom to top, (a) 0.04, (b) 0.06, (c) 0.08, (d) 0.20 (e) 0.25, (f) 0.30, (9) 0.50, (h) 0.75, (i) 2.5, (i) 5, (k) 10, and (1) 20. Temperature ramp is 3.3 K/s. desorbs for a 0.75-L dose. Substantial desorption is observed for exposures higher than 5 L, and the amount of sulfur left is, within experimental error, saturated. Figure 2 shows the H2 (left panel) and CH4 (right panel) TPD profiles as a function of gas exposure. For a 0.04-L dose, HPdesorbs in a single peak centered at 346 K. This peak temperature shifts down slightly with increasing exposure and a shoulder starts to develop between 200 and 300 K for exposures between 0.08 and 0.30 L. A new structure in the H2 TPD spectra appears for a 0.50-L dose. THe TPD shows two peaks centered at 225 and 340 K and a high-temperature tail that extends to 500 K. With increasing exposure to 2.5 L, the two low-temperature peaks shift slightly downward to 220 and 320 K. There is no shift with further exposure. Between 2.50 and 5 L, the high-temperature tail develops additional peaks. This is more obvious for a 20-L exposure, where three high temperature peaks centered at 330, 440 and 540 K are observed. Turning to CH4, it is observed only for doses exceeding 0.20 L, where it desorbs with a peak a t 240K and ashoulder a t 270 K that extends to 360 K. The main peak and shoulder shift toward higher temperatures with increasing dose up to 2.50 L. There is no change in this part of the spectrum for doses above 2.50 L; the peak temperature is centered at 270 K and the high-temperature shoulder a t 300 K. For a 5-L dose, a high-temperature tail above 360 K starts to develop, which is more pronounced for a 20-L dose. Figure 3 shows the H2 (in ML) and CHd TPD area (normalized to the methane peak area for a 20-L dose) as a function of sulfur left after TPD. Up to about 0.20 ML of sulfur, the amount of Ha desorption follows the sulfur. Above this coverage, the amount of hydrogen desorption decreases, but themethane TPD area increases. The trend in the methane yield with increasing amounts of decomposed thiol is very similar to that found on clean and sulfur precovered Ni(l10)4andNi(1111.' Whereas smallamounta of ethane have been found on Ni(ll0) and Ni(lll), neither ethane nor any other hydrocarbon desorption is observed on Ni(100). On the basis of the difference between the actual and expected amount of hydrogen (assuming complete decomposition of the thiol) roughly 0.28 ML of methane desorbs for a 20-L exposure. Thus, for this dose, about
Castro et al.
128 Langmuir, Vol. 7,No. 1, 1991
1 .o
t
9'0 0
I c3 9
n
-I
E 0 0)
0.5
.
D
iii0
0
0
200 300 400 500 600 Temperature ( K )
200 300 400 500 600
n
i!
4
a
t:
a
I/ P
0.0 ~1 0.0
0.1
c 2. H " 0.2
0.3
0.4
Sulfur Left (ML)
Figure 3. Amount of Hz and CHh desorption from methanethiol decomposition, after exposure at 100 K, as a function of the final sulfur coverage. For corresponding CH3SH exposures, see Figure 1. 5.0. x4 0
. 0 X
2
3
8 Y
2.5
'
t
5 ! I c
0
. '. ' 500 200 300 Temperature ( K )
-
. . . . . . . . . . . . . . . . . . . . . . . .
200
300
400
400
500
Figure 4. TPD of isotopically labeled methane and hydrogen
after a 20-L exposure of CH3SD (70% deuterated) on Ni(100) at 100 K. The dotted curve shows the CH4 TPD after a 20-L dose of CH3SH. The right panel has been multiplied by a factor of 4, compared to the left panel. The temperature ramp is 3.3 K/s.
75 5% of the thiol produces methane and the sulfur coverage is 0.38 f 0.04 ML. 3.1.2. CHsSD/Ni( 100). Figure4showstheTPDresults for a 20-L dose of labeled methanethiol (70% CH3SD). Both CH3SH and CH3SD (not shown) desorb with the peak temperature at 119 K. DZand HD (right panel) desorb with peaks at 220 K and a high-temperature shoulder between 280 and 350 K. Dihydrogen, on the other hand, showed, at most, a small shoulder at 220 K. The Hz TPD was dominated by three high-temperature peaks at 340, 440, and 540 K. As indicated by the 17 amu signal, CH3D desorbed in a peak centered around 270 K. Mass 16, CH4 (corrected for CH3D fragmentation), desorbed in the same temperature region, but with a noticeable shoulder at 300 K and a high-temperature tail extending to 500 K. Only a trace of CHzDz desorption was observed, barely above the noise level of the experiment,and in the same temperature region
Figure 5. Dz (left side) and CH3D (right side) TPD for 1 ML of preadsorbed D and thiol postexposures of (a) 0.04 L and (b) 20 L. The dotted curve is Dz TPD of D(a) alone. Spectrum c is the Hz TPD after a 20-L CH3SH postdose on 1 ML of D(a). Spectrum d is CHI TPD after same CH3SH dose on 1 ML of H(a). Temperature ramp is 3.3 K/s. The closed circles in the inset show the total methane yield (CH3D + CH4, normalized to a 20-L dose on the clean surface) as a function of sulfur left after TPD. The open circles show the CH4 yields after similar experiments with preadsorbed hydrogen. as CH3D desorption. The area of the CH2Dz peak was less than 5% of the CH3D area, indicating that replacement of H by D in C-H bonds was negligible. 3.1.3. Effect of Preadsorbed Dz. The effect on thiol decompositionof a saturation amount of deuterium preadsorption is summarized in Figure 5. CH3SH(D) desorbed with the peak temperature at 119 K (not shown). Preadsorbed Dz reduces the maximum amount of thiol that decomposes from 0.38 f 0.04 to 0.29 f 0.03 ML, based on residual sulfur. Preadsorbed deuterium also promoted methane desorption (right panel) at low coverages of the thiol, as evidenced by the CH3D desorption peak at 230 K for a 0.04-L dose. At this dose, in the absence of D(a), no methane desorbs (Figure 2). For a 20-L postdose, the CH3D TPD area is higher and the peak temperature shifts to 260 K. When hydrogen is preadsorbed, no isotope effect is observed in the methane peak (curve d). The D2 desorption changes when the thiol is dosed. As shown in Figure 5a, dosing the thiol on D(a)/Ni(100) enhances Dz desorption at 200 K (solid curve) compared to D2 desorption from clean Ni(100) (dotted curve). The area under the low-temperature peak increases and the peak shifts to 220 K with increasing thiol postexposure to 20 L (curve b). Furthermore, the onset in the Dz and CH3D desorption coincides at around 160 K. We will come back to this point in the discussion section. 3.2. SIMS. The dominant secondary ions observed on clean Ni(100) are at m / e = 23 (Na+),39 (K+),19 (H30+), 76 (NiHzO+),94 (Ni(HzO)z+),117 (NizH+),and 58 (Ni+). For those ions containing Ni, the 60Ni(isotopicallylabeled) components are also observed. The SSIMS spectrum of CH3SH on Ni(100) is shown in Figure 6. For a 0.04-L dose at 100 K, the Ni+ (58) and NizH+ (117) intensities increase, and CH3+(15)is observed. With TP-SSIMS, Figure 7a shows that the CH3+/Ni+ decreases with increasing sample temperature whereas the NizH+/Ni+ ratio increases at least up to r300 K. No new ions were observed during temperature programming. At 550 K, the dominant ion is Ni+. For a 0.50-L dose at 100 K (not shown) the dominant ions are 106 (NiCH3SH+),15 (CH3+),and 58 (Ni+). Ions
Langmuir, Vol. 7, No. 1, 1991 129
Methanethiol Decomposition on Ni(100) SSIMS of CH1SH/Ni(100) 1001
I5
I
50
A
r 3 e
e g c a v
r e -
t
9) c
C
118
v)
I
z i v)
20
40
60
80 m/e
100 120 14t
Figure 6. SSIMS spectra of CH3SH for a (a) 0.04- and (b) 20-L exposure. The top spectrum (c) shows the SSIMS spectrum of CH3SD after a 20-L dose. Sample temperature is 100 K throughout. 6 0
P 0
I
300 K
a
I
a
180I K\
0 0
SSIMS spectrum from CH3SD on Ni(100) is shown in Figure 6c. Protonation is now, as expected, largely D+ and the methyl ion region is almost all CH3+. After the sample was flashed to 270 K to completely desorb physisorbed CH3SH, ions appeared at m / e = 60 (CH2SCH2+or (CHz)S+),77 ((CH3)3S+),91 (NiHS+),and 148 amu (NizS+); ions observed at 100 K that are also observed at 270 K include CH3+, CH3S+, CHS+, and Ni+. No ions are observed at m / e = 60, 77, and 91 amu for a 0.50-L dose heated to 270 K. After the sample was flashed to 420 K, ions are observed at 15 (CH3+),58 (Ni+), 148 (NizS+), and 71 amu (NiCH+). After the sample was flashed 550 K, only 58 (Ni+)and 144 (NiZS+)amu ions are observed. Experiments with preadsorbed deuterium (not shown) indicated: (1)a decrease in the intensity of the NizD+ as the NiCH3SH+ and CH3SH2+ intensities increase, (2) no incorporation of D into the CH3+ ion at 100 K, and (3) only a small amount of deuterium exchange into the sulfhydryl bond. Previous SSIMS studies of the formation and decomposition of ethylidyne on Pt(lll)'3 and methyl halides on Ni( 100)12J4have indicated that the CH3+ secondary ion is predominantly due to surface R-CH3 groups (with R being either carbon or a halide),while methyl groups bound to Ni give a NizCH3+ secondary ion cluster. We attribute the CH3+ secondary ion observed here mainly to surface thiomethoxy species, CH3S-Ni, not to surface methyls, H3C-Ni. Questions may be raised concerning the validity of this assignment, and it is possible to argue that the parent methanethiol and/or surface methyl groups could contribute to the secondary ion yield. However, CH3+ is observed up to 450 K, well above the desorption and decomposition temperature of CH3SH. Furthermore, CH3S species have been identified on Ni(100)s and Ni(110): where no evidence for surface methyl groups was found. Extensive SSIMS work describing the decomposition of ethylene and acetylene on Ni(100) and Ni(lll)l5l7 and methyl halides on Ni(100)1zJ3and in the adsorption and desorption of hydrogen from clean and carbon-~overed~~J~J* and sulfur-~overed~~ Ni( 100) have established the use of the NizH+/Ni+secondary ion ratio as a monitor of surface hydrogen concentration. In this study, we use the Ni*H+/ Ni+ ratio to measure surface hydrogen during thiol decomposition. The observation of the CH3+ and NizH+ secondary ions for a 0.04-L exposure and the complete absence of any secondary ion containing the parent molecule is attributed to decomposition upon adsorption of the thiol at 100 K on Ni( 100). This low-temperaturedecomposition was also verified by experiments with CHsSD, summarized in the inset of Figure 5. The appearance of the NiCH3SH+and CH3SHz+ions for a 0.50-L dose is attributed to the presence of molecularly adsorbed methanethiol. 3.3. Temperature-Programmed SSIMS. The structure of this section is as follows. First, we establish a stoichiometricrelationship between the decomposition of (13)Ogle, K.M.Dissertation Thesis, University of Texas, 1984. (14)Zhou, X.L.;White, J. M. Chem. Phys. Lett. 1987, 142, 376. (15) Zhu, X.Y.; Castro, M. E.; Akhter, S.; White, J. M.; Houston, J. E.Surf. Sci. 1988,207, 1. (1G)Zhu, X.Y.;Castro, M. E.; Akhter, S.; White, J. M.; Houston, J. E.J. Vac. Sci. Technol., A 1989, 1991, 7. (17)Zhu, X.Y.;White, J. M. J. Phys. Chem. 1988,92, 3970. (18)Zhu, X.Y.; Castro, M. E.; White, J. M. J. Chem. Phys. 1989,90, 7442. (19)Castro, M.E.;Zhu, X. Y.; Golchet, A,; Akhter, S.; White, J. M.; Sahin, T. Paper in preparation.
Castro et al.
130 Langmuir, Vol. 7, No. I, 1991 Table I. Relative Change in Ion Ratios with Temperature
exposure
T,K
IP
0.04 0.04 0.04 0.04 0.04
260 270 280 300 290
0.08 0.08 0.08 0.08
260 280 300 340
I2b
1 2 1I 1
0.15 0.20 0.28 0.47 0.34
0.48 0..56 0.77 1.22 0.88
3.2 2.8 2.8 2.6 2.6
0.125 0.27 0.34 0..68
0.36 0.79 1.20 0.36
2.9 2.9 3.5 0.5
I112 K
a Relative decrease of CH3+!Ni+ signal, compared to 95 K. * Relative increase of NiZH+/Ni+ signal, compared to 95 K.
the thiomethoxy, as monitored by the CH3+/Ni+ ratio, and the accumulation of surface hydrogen. Second, the temperature dependence of the secondary ion yield is interpreted in terms of the species that are present on the surface. In order to be consistent with previous work,11112,14,15,18120the secondary ion yields have been normalized to the Ni+ ion signal. To a first approximation, division by Ni+ accounts for changes in ion yields with work function changes.'5 3.3.1. Accumulation of Surface Hydrogen. Figure 7 shows the NizH+/Ni+ (left panel) and CH3+/Ni+ (right panel) TP-SSIMS profile as a function of gas exposure. For a 0.04-L dose (no methane), the CH3+/Ni+secondary ion ratio begins to decrease at about 180 K. This is taken as the onset of CH3S decomposition. Consistent with this, the NizH+/Ni+ secondary ion ratio starts to increase, indicating the accumulation of surface hydrogen. The latter starts to decrease around 320 K, when hydrogen desorbs. The only source of surface hydrogen is from C-H or S-H bond cleavage. Since no evidence was found, at this coverage and temperature region, for molecularly adsorbed methanethiol, we conclude that the hydrogen signal grows because C-H bonds cleave (i.e., for this dose all the S-H bonds are broken during adsorption and give the NiZH+/Ni+ signal at low temperatures). Between 260 and 300 K, the NizH+/Ni+increases 3-fold, relative to the initial intensity. This parallels the decrease in the CH3+/Ni+ signal. By 270 K, the CH3+/Ni+ ratio has decreased to 20% of its original intensity, and the NizH+/Ni+ signal ratio has increased by 56%, a ratio of 2.80:l. This comparison is also observed for a0.08-Ldose, and, as shown in Table I, is maintained, within experimental error, up to 300 K, where hydrogen desorption is important. In this table, I1is the relative decrease of CH3+/ Ni+ and IZ is the relative increase of Ni2H+/Ni+. For 0.04- and 0.08-L doses, the ion signals, normalized to Ni+, are independent of temperature between 100 and 180 K. Comparing the two doses, we find the following ratios, Zx(0.08L)/Ix(0.04L) = 2.6 (for x = CH3+/Ni+)and 2.1 (for x = NizH+/Ni+). After TP-SSIMS, the sulfur AES signal ratio is 2.4. This indicates that, in this low coverage region, where no molecularly adsorbed methanethiol is observed,the SSIMS yield ratios, to a reasonable approximation, represent the concentrations of surface hydrogen and thiomethoxy species and are consistent with our previous statement that for low coverages all the sulfhydryl bonds are broken upon adsorption at 100 K. At higher coverages of the thiol, two effects limit a quantitative interpretation of the experimental results: (1) the effect of molecularly adsorbed thiol on the secondary ion emission of the NiZH+ ion, and (2) the fact (20) Zhu,X. Y.; 1442.
Castro, M. E.; White, J. M. J. Chem. Phys. 1989,90,
J. ._ . / a Temperature ( K )
Figure 8. TP-SSIMS profile of the NiCHaSH+/Ni+ and CH&HZ+/Ni+ratio as a function of gas exposure a t 100 K. Exposures are (a and c) 0.50 L and (b and d) 20 L. The temperature ramp is 8 K/s.
that hydrogen desorption is, as evidenced in the TPDdata, mostly reaction limited. 3.3.2. Thiomethoxy Decomposition. For a 0.04-L dose, the thiomethoxy decomposition sets in around 180 K, but most of the decomposition takes place between 200 and 340 K. Assuming a first-order decomposition reaction for thiomethoxy species on Ni(100),and using the method developed by Ogle,'3 an activation energy of 8.81 kcal/ mol and a preexponential factor of 106.5s-' is estimated for a 0.04-L dose of methanethiol (AT = 60 K, T p = 312 K). The low preexponential factor is taken as evidence that the decompositionrate coefficient is not characteristic of an elementary step; rather, it probably involves several steps, some of which are in equilibrium with each other. The decomposition of the thiomethoxy shifts to slightly higher temperatures with increasing thiol exposure. This is evidenced by the shift in the onset of the decay in the CH3+/Ni+TP-SSIMS maximum from 180 to 210 K for a 0.04- and 0.08-L dose, respectively. Comparing 0.50- and 20-L exposuresof CH3SH, the maximum in the TP-SSIMS shifts from 220 to 260 K. The decomposition rate of CH3S above 350 K is the same for the two coverages being compared, indicating that changes in the structure of the high-temperature tail in the hydrogen TPD spectra are not related to changes in CH3S decomposition. 3.3.3. Methanethiol Decomposition. Figure 8 shows the TP-SSIMS profile of the CH3SH*+/Ni+ (top panel) and NiCHsSH+/Ni+(bottom panel) ion ratio. For a 0.50-L dose (molecular desorption), the NiCH3SH+/Ni+ secondary ion yield ratio (Figure 8a) starts to decrease around 110 K. With the dose increased to 20 L (Figure 8b), the onset in the decrease in the NiCH&H+/Ni+ SSIMS signal shifts to slightly higher temperature and the decomposition reaction extends above 250 K. Following the decrease in the NiCH&H+/Ni+ secondary ion ratio, the CH3+/Ni+ ratio starts to increase around 110 K (Figure 7c) and keeps increasing up to 265 K, where it starts to decrease, tailing up to 500 K. This is consistent with a mechanism in which molecularly adsorbed meth-
Methanethiol Decomposition on Ni(100) anethiol decomposes by S-H bond cleavage at low temperatures to form thiomethoxy species and surface hydrogen, in excellent agreement with recent UPS studies of CH3SH on Ni(100L6 For both 0.50- and 20-L exposures (Figure 8c,d), the CHaSHz+/Ni+ratio starts to decrease around 110 K. On the basis of the decrease in the CH3SHz+/Ni+signal ratio for a 20-L exposure, the peak temperature where the rate maximizes is 120 K (AT = 11 K). This is in excellent agreement with the molecular desorption peak, centered at 119 K. The other ions at m / e = 61, 63, 79, and 97 followed the sharp decrease in CH3SHz+. It is clear from the data that the CH3SHz+/Ni+ ratio does not contribute to the increase in the CH3+/Ni+above 200 K, as it disappears completely by 150 K. 4. Discussion 4.1. TPD. On the basis of the residual sulfur, the maximum amount of decomposed methanethiol found in this study is 0.38 f0.04 ML. The only desorption products observed in the thiol decomposition are Hz, CHI, and CH3SH. Molecular desorption starts for exposures above 0.50 L, and there is no appreciable shift in its peak temperature with increasing exposure up to 20 L. We conclude that molecular desorption is from weakly held species and occurs only when the Ni(100) surface is passivated with Hand CH3S. The molecular desorption peak at 119K is consistent with literature results for multilayerlike desorptionon CU(~OO)~ (115 K) and Ni(ll1)' (119 K), although on Pt(111)2and Ni(l10)4the peak is centered at significantly higher temperatures (130 K). This could be due to the higher adsorption temperature, 110 K on Pt(111)z and 120 K on Ni(llO),I where part of the lowtemperature edge of the TPD peak may have been missed. Adsorption below 100 K may be necessary to obtain the full multilayer desorption peak. 4.2. SSIMS. The SSIMS on methanethiol on Ni(100) is very similar to that observed on Ni(lll).7 No ions containing the parent molecule are observed for low exposures of the thiol. This is attributed to dissociative adsorption. For 0.50-L and higher exposures, secondary ions containing the parent molecule are observed, Le., CH~SHZ+, (CH&SH+, and NiCH3SH+,and are attributed to molecularly adsorbed methanethiol. The CH3SHz+ion (protonation) suggests short-range intermolecular interactions between thiol molecules, probably via hydrogen bonding. The CH3SH2+ secondary ion cannot be accounted for by a recombination mechanism between surface hydrogen and the thiol, as verified by deuterium preadsorption experiments, which showed negligible CH3SHD+ and CH3SD2+. We suppose this ion arises from protons formed during Ar+ collisions with CH3SH(a). The SSIMS spectra for a 20-L dose are very interesting. The appearance of cluster ions derived from more than one parent molecule is suggestive of methanethiol multilayers, at least locally, consistent with the substantial amount of CH3SH desorption at 119 K. 4.3. Decomposition Mechanism. Here we propose and discuss a mechanism for methanethiol decomposition on Ni(100). 4.3.1. Methanethiol Decomposition. For low coverages, CH3SH decomposes upon adsorption, by S-H bond cleavage, forming adsorbed H and CH3S. This is evidenced by (1)the absence of any CH~SHZ+ or NiCH3SH+ secondary ions, (2) the ratio of the initial intensities of the NizH+/Ni+ and CH3+/Ni+ yield for 0.04 and 0.08 L, and (3) the isotope labeling experiments with CH3SD.
Langmuir, Vol. 7,No. 1, 1991 131
At higher exposures (>0.50 L), CH3SH also adsorbs molecularly, as evidenced by the appearance of the CH3SH2+and NiCH3SH+ secondary ions for a 0.50-L dose. Additional low-temperature S-H bond cleavagetakes place around 110 K. Low-temperature S-H(D) bond scission is also suggested by the TPD results, involving CH3SD and Dz preadsorption, which indicate that the desorption peak at 220 K is desorption limited hydrogen (deuterium). The inhibition of S-H bond cleavage, at higher total coverages, is also reflected in the tailing of the NiCH&H+/ Ni+ratio toward higher temperature, with increasing thiol exposure. This accounts for the smaller maximum in the CH3+/Ni+ratio for a 20-L compared to a 0.50-L dose; Le., CH3SH decomposition extends to temperatures where CH3S decomposes. This interpretation might be too simple, as the factors that are involved in the secondary ion emissionare not fully understood. However, our results are consistent with, and extend, literature results for H-S bond cleavage for HzSand CH3SH on Ni(100). The SSIMS results are clearly consistent with a CH3+ signal derived mainly from CH3S-Ni. Contributions from CH3SH(a)and NiCH3 appear to be, at most, small. The chemistry of the thiol sulfhydryl bond is similar to that of H2S on Ni(100). Baca et al. have reported, based on HREELS, S-Hbond cleavage for low coverages of HzS on Ni(100) at 110 K.21This decomposition is inhibited at higher HzS coverages, in excellent agreement with the results obtained in this work. On Ni(100), Bao et al.,'j reported CH3SH dissociation below 110K (adsorption temperature) to form CH$, which was stable to room temperature. Our results are consistent with their interpretation, as evidenced by the decrease in the NiCH3SH+/Ni+ ratio and the increase in the CH3+/ Ni+ ratio around 110 K for 0.50- and 20-L exposures of CH3SH. 4.2.2. Thiomethoxy Decomposition Pathway. There are two plausible pathways for the decomposition of the thiomethoxy intermediate: it can undergo C-H bond scission or it can decompose by C-S bond cleavage. Clearly, one might follow promptly, or occur simultaneously with, the other. On the basis of bond energies, we expect the C-S bond to break, but steric factors could play a major role and lead to preferential C-H bond cleavage. Cleavage of C-S first is consistent with the TPSSIMS data. The 3-fold increase in the accumulation of surface hydrogen is not consistent with C-H bond cleavage preceding C-S bond scission. The first species expected upon C-H bond scission of CH3S is thioformaldehyde, CHzS, which is stable on Pt(ll1) to 400 K.2 The major increase in the NizH+/Ni+ signal ratio occurs below 300 K. Assuming Ni and Pt can be compared, at least roughly, if CH3S decomposition occurs via C-H bond cleavage, then one would have expected a 1:l relation between the increase in NizH+/Ni+and the decrease in CH3+/Ni+signal ratio. We find 2.8:l and 3.1:l between 260 and 300 K for 0.04and 0.08-L exposures of methanethiol, respectively. That C-S bond cleavagedominates is further supported by lack of evidence in the literature for a CHzS on Ni surface^.^^^^^ Other decomposition mechanisms for the thiomethoxy are possible, for example, CH3S 3H + S + C. However, the negligible amount of CHzD2 desorption during CH3SD decomposition and the CH3D desorption peak at 230 K after a 0.04-L postdose of CH3SH on the deuterium-covered surface indicate that this possibility is less important than C-S bond cleavage.
-
(21) Baca, A. G.; Schulz, M. A.; Shierly, D. A. J . Chem. Phys. 1984,81, 15.
132 Langmuir, Vol. 7, No. 1, 1991 4.3.3. Methane Desorption. Methane does not adsorb on Ni(100) under our conditions. Thus, the observed methane desorption is reaction limited. No isotope effect is observed in the CH3D peak temperature from CH3SD decomposition or deuterium preadsorption experiments. This indicates that formation of a C-H(D) bond is not the rate-limiting step; rather, some step($ leading to the C-H(D) formation step must control. Two mechanisms can be proposed for methane desorption: a direct one-step-concerted hydrogenolysis of thiomethoxy species or hydrogenation of a short-lived and labile surface methyl group formed by C-S bond cleavage. As noted above, the TP-SSIMS results are consistent with C-S bond cleavage before C-H bond cleavage. This process leaves, at least transiently, surface methyl groups. That the lifetime of the methyl groups is very short is evidenced by the nearly 3-fold greater increase, between 260 and 300 K, in Ni:!H+/Ni+ than the decrease in the CH3+/Ni+ signal ratio between 260 and 300 K for 0.04and 0.08-L thiol exposures. This indicates that for low coverages, where little surface hydrogen is available, CH3. (a)undergoes rapid and multiple C-H bond cleavage when it is formed. However, if the hydrogen concentration is high, CH3(a) readily hydrogenates, as evidenced by the appearance of CHsD (CH4) desorption for a 0.04-L thiol postdose on the deuterium (hydrogen)-covered surface. Moreover, at higher thiol exposures, more surface hydrogen is available due to low-temperature S-H bond cleavage. This facilitates the formation of methane. The methane peak at 270 K involved mostly hydrogen that accumulates during thiol decomposition (i.e., not C-H bond cleavage). This is evidenced by the absence of a high-temperature tail in CH3D desorption during CH3SD decomposition. The shift in the CHI peak toward higher desorption temperatures with increasing thiol coverage can be accounted for by a shift, due to crowding, in the decomposition reaction of the thiol and thiomethoxy. Questions may be raised regarding the thermodynamic stability of surface methyl groups. On Ni(100), CH3(a) decomposition occurs below 250 K.12J4 However, the literature indicates that the decomposition reaction is likely to be different in the presence of coadsorbed sulfur. On Ni(lll),' Ni(110),4 and Fe(100),3preadsorbed sulfur deactivates the surface toward C-H bond cleavage. It is likely that a similar deactivation occurs on "00). This is reflected in our results by (1) the methane yield dependence on the amount of decomposed thiol, which is similar to that observed on Ni(ll1)' and Ni(110),4 (2) appearance of reaction limited hydrogen, and (3) changes in the structure of the hydrogen thermal desorption spectra. 4.4. Hydrogen Desorption. 4.4.1. DesorptionLimited Hydrogen. For low coverages (& < 0.20 ML) hydrogen desorption is mostly desorption limited, as evidenced by the desorption temperature region, and compares favorably with clean and sulfur-covered Ni.10122 The peak at 220 K at high thiol exposures is due to hydrogen that accumulates on the surface after the lowtemperature S-H bond cleavage,as evidenced by the TPD results with CH3SD and deuterium (hydrogen)preadsorption experiments. Zhou et a1.,22have attributed this lowtemperature hydrogen desorption peak to the effect of sulfur in the local electronic structure of Ni. The results obtained in the Dz (H2) preadsorption experiments are consistent with this interpretation. The correspondence of the onsets of the deuterium and methane-dl (hydrogen (22) Zhou, Y.; White, J. M. Surf. Sci. 1987, 183, 363.
Castro et al.
and methane) desorption indicates that the peak shift is caused by sulfur atoms formed during C-S bond cleavage and not by surface CH3S. Another plausible explanation for the shift of the hydrogen desorption peak toward lower temperatures is a coverage effect. Sulfur and hydrogen are believed to occupy 4-fold hollow sites on Ni(lOO).1° When C-S bond cleavage occurs, the sulfur atoms can force surface hydrogen to occupy sites on the surface with lower binding energy, and lead to a shift in the onset and peak temperature of hydrogen (deuterium) desorption. 4.4.2. Reaction-LimitedHydrogen. Reaction-limited hydrogen is readily identified by hydrogen desorption above the normal desorption temperature of dosed dihydrogen. For 0s < 0.20 ML, only a very small amount of reaction-limited hydrogen was observed. For 0s > 0.20 ML, the hydrogen TPD includes a substantial amount of reaction-limited hydrogen desorption. The identity of the species that lead to the hightemperature tail and the changes in the structure of the hydrogen TPD spectra on Ni(100) at high thiol exposures have not been determined in this study. However, these peaks are not due to the decomposition of CH& as evidenced by the overlap in the CH3+/Ni+ratio above 350 K for a 0.50- and 20-L dose of methanethiol. We suppose that, at high doses, CH3 bound to S-modified Ni(100) is stabilized. The TPD area ratio of the three high-temperature peaks for a saturation amount of decomposed methanethiol is 1.5:l:l. This suggests a stepwise decomposition mechanism for C-H bond cleavage of CHS(a). We speculate that CHda) species are formed upon a single C-H bond scission of a CH3(a) group between 300 and 400 K. Methylene fragments decomposeto CH(a)species above 400K, giving rise to the H2 TPD peak at 440 K. Decomposition of CH(a) takes placed above 500 K, giving rise to the H:! TPD peak centered at 540 K. Stepwise decomposition on Ni surfaces has been reported previously in the literature. On the basis of XPS and UPS data, Steinbach has reported a stepwise decomposition of CH3(a) on polycrystalline Ni.23 In that study, CH:!(a) species are reported to decompose around 400 K to form CH(a), which decomposes above 500 K. This is consistent with our assignment of the hightemperature hydrogen peaks to a stepwise decomposition of CH, species (x = 3-1). Vibrational spectroscopy is needed to definitively characterize the hydrocarbon species above 300 K during methanethiol decomposition on Ni(100). Furthermore, it would be of great interest to examine C-C bond formation. On the basis of the arguments presented above, a simple mechanism that is consistent with the observed data is summarized in Figure 9. For low exposures, CH3SH decomposes upon adsorption on Ni(100) at 100 K to form CH3S and surface hydrogen. At high exposures, a significant amount of CH3SH adsorbs molecularly and S-H bond cleavage occurs around 110 K during heating. CH3S decomposes by C-S bond scission. The short-lived methyl groups readily decompose at low coverages into carbon and surface hydrogen. The latter desorbs in desorptionlimited peaks between 200 and 450 K. A t higher coverages, CH3(a) is probably involved in a reaction that leads to reaction-limited methane desorption, although the data do not allow a definitive identification of the methane precursor. C-H bonds are stabilized through steric (23)Steinbach, F.; Kiss,J.; Krall, R. Surf. Sci. 1985, 157, 401.
Langmuir, Vol. 7, No. 1, 1991 133
Methanethiol Decomposition on Ni(100)
I
Upon low coverages adsorp+ion\
carbon, probably assisted by S, into the bulk of nickel, which has been reported to occur on Ni(100) above 600
K.25 T= 110- 120K
s
(0)
300 K Low coverages T>2W K
CH, + H ‘4)
L
T >400K
i
T>500K
Figure 9. Proposed mechanism for methanethiol decomposition on Ni(100). The temperature regions of desorption products are obtained from the thermal desorption data and that of surface intermediates, from the TP-SSIMS spectra.
interactions, at high exposures, and the decomposition of CH3(a) appears to be stepwise for a 20-L dose of methanethiol. Although this decomposition mechanism predicts residual carbon on the surface after TPD and TPSSIMS, particularly at low thiol exposures, AES spectra obtained after TPD showed predominantly sulfur, with no appreciable accumulation of carbon. A similar result has also been found after methanethiol decomposition on Ni(l10)24and Ni(lll).’ We attribute this to diffusion of (24) Huntley, D. R. Personal communication.
5. Summary The results presented in this paper can be summarized as follows: (1)The main products of methanethiol decomposition on Ni(100) are Hz and CH4. The maximum amount of methanethiol that decomposes is 0.38 f 0.04 ML. The remainder desorbs molecularly. (2) For low coverages, all the S-H bonds are broken upon adsorption at 100 K. This low-temperature decomposition becomes inhibited as the methanethiol exposure increases. (3) Thiomethoxy, CH3S, is the stable intermediate in the decomposition of methanethiol on Ni( 100) below 200 K. The data are consistent with CH3S decomposition through a complex mechanism initiated by C-S bond cleavage between 200 and 400 K. The activation energy is 8.81 kcal/mol and the preexponential factor is 106.5s-l. This process involves, in part, the formation and hydrogenation to methane of a short-lived methyl group. (4)Complete decomposition of the thiol is the preferred reaction pathway for coverages below 0.10 ML. Formation of reaction-limited methane becomes an important reaction pathway at higher coverages. Preadsorbed hydrogen promotes methane formation. There is no isotope effect in CH4 (CHsD) desorption. (5) At low thiol exposures,hydrogen desorption is mostly desorption limited. Reaction-limitedhydrogen is observed at higher thiol exposures. Sulfur displaces surface hydrogen desorption toward lower temperatures. (6) A simple mechanism that is consistent with the observed data is proposed to account for the thermal decomposition of methanethiol on Ni(100). Acknowledgment. M. C. thanks Dr. X.-L. Zhou and Dr. Y. Zhou for valuable discussions and advice. Support of this work by the U S . Army Research Office and by the Texas Advanced Research Program is gratefully acknowledged. Registry No. Ni, 7440-02-0; methanethiol, 74-93-1. (25)KO,E.J.; Maddix, R. J. Appl. Surface Sci. 1976, 3, 236.
