J. Phys. Chem. 1993, 97, 3584-3590
3584
Effect of Sulfur on Desulfurization Kinetics and Selectivity: 2,5Dihydrothiophene on Mo(110)-(4Xl)-S Han Xu and C. M. Friend' Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 Received: September 23, 1992; In Final Form: January 5, 1993
The bonding and reactions of 2,5-dihydrothiophene on Mo( 1 l0)-(4X 1)-S have been studied under ultrahighvacuum conditions using temperature-programmed reaction, X-ray photoelectron, and high-resolution electron energy loss spectroscopies. Unlike clean Mo( 1IO), the first layer of 2,5-dihydrothiophene is molecularly bound on Mo( 110)-(4X 1)-Sat 120 K so that structural characterization is possible. The ring of the 2,5-dihydrothiophene is tilted away from the surface normal for coverages below saturation but reorients to a nearly perpendicular geometry for coverages near saturation. Upon heating the surface, about 70% of the 2,5-dihydrothiophene desorbs molecularly. Most of the remaining 2,5-dihydrothiophene undergoes intramolecular elimination to form 1,3-butadiene below 350 K. There is also a minor amount of nonselective decomposition, ultimately producing surface carbon, sulfur, and dihydrogen. While the reactions are similar to those on the clean surface, the selectivity for butadiene elimination is increased from 67% on the clean surface to 83% on Mo(l10)(4X 1)-S. The overall reactivity is decreased, however, to approximately one-third that of clean surface. Site blocking of sulfur probably accounts for the decreased reactivity since the initial sulfur coverage is already high (ds = 0.5)on the Mo(l10)-(4Xl)-S surface. The increased selectivity is attributed to inhibition of C-H bond activation which leads to nonselective reaction. No C-H bonds are broken or formed along the pathway to butadiene elimination.
Introduction Hydrodesulfurization(HDS) is an important industrial process which removes sulfur from fossil fuel feedstocks. Although molybdenum disulfide promoted with cobalt or nickel is the most commonly used catalyst, there is still considerable controversy regarding the roles of sulfur and cobalt. Due to the complexity of the working catalyst, a detailed understanding of the reactivity of specific sites is not possible on the actual catalytic material. Hence, model studies on single-crystal surfaces are important in determining the molecular-level effects of surface modifiers. In this paper, we have studied the effect of sulfur on the reactivity of 2,5-dihydrothiophene. 2,5-Dihydrothiophene has been previously proposed to be an intermediate in thiophene hydrodesulfurizationI.2 and has been previously studied on clean Mo( 1lo).' The effect of sulfur can, therefore, be determined by performing parallel investigations on Mo( 110)-(4X l)-S. The Mo( 110)-(4X 1)-S surface has a coverage of 0.5 monolayer and is proposed to have the structure shown in Figure l e 4 On clean Mo( 1 lo), intramolecularelimination of 1,3-butadiene is the primary pathway, accounting for -67% of the 2,5dihydrothiophene that reacts at saturation coverage. The relatively high selectivity for butadiene elimination from 2,5dihydrothiophene is attributed to the fact that no C-H bond formation or cleavage is required and that only a minor reorganization of the hydrocarbon framework is necessary along thereaction path. The remaining 33%of the 2,Sdihydrothiophene nonselectively decomposes to surface carbon, sulfur, and gaseous dihydrogen. A significant amount of decomposition occurs upon adsorption of 2,5-dihydrothiopheneon clean Mo( 110) at 120 K, based on X-ray photoelectron data. The low-temperaturereaction deposits sulfur on the surface, which is thought to stabilize the 2,5dihydrothiophene with respect to decomposition. Hence, some molecular 23-dihydrothiophene is present on the surface at low temperatures and high coverages. Unfortunately, the molecular structure of 2,5-dihydrothiophenecould not be determined dug to the low-temperature decomposition pathway. This investigationwas undertaken in order to specifically test the effect of sulfur on reaction kinetics and selectivity. Indeed, 0022.3654193 f 2097-3584304.00f 0
MO( 1 1 0 ) - ( 4 ~)S 1
Figure 1. Proposed Mo( 110)-(4X 1)-S structure, after Bauer and wit^^
23-dihydrothiopheneremains molecularly bound to the Mo( 1 10)(4x1)s surface at low temperature. As a result, detailed vibrational studies could be performed so as to determine the orientation of the 2,Sdihydrothiophene ring with respect to the surface. Our studies show that both C-H and C S bond activation are inhibited by surface sulfur. Dehydrogenationis differentially suppressed by the sulfur, however, resulting in increased selectivity for butadiene elimination on the sulfur-covered surface. The total amount of reaction is also smaller on Mo(l10)-(4Xl)-S, reflecting an overall inhibition of desulfurization.
Experimental Section Experiments were performed in two stainless steel ultrahighvacuum chambers with a base pressure of -1 X 1 0 - l O Torr, 0 1993 American Chemical Society
2,s-Dihydrothiophene on Mo( 1 10)-(4X I)-S described in detail elsewhere.5.6 Both chambers were equipped with a computer-controlled quadrupole mass spectrometer for temperature-programmed reaction studies,' low-energy electron diffraction optics, and a retarding field Auger electron spectrometer. One chamber was equipped with an X-ray photoelectron spectrometer (Perkin Elmer PHI-5000) and the other with a high-resolution electron energy loss spectrometer (LK2000). The heating rate used for temperature-programmed reaction studies was nearly constant a t -10 K/s. A broad search for reaction products was performed by monitoring -90 masses during a single experiment.' Subsequently, moredetailed studies were performed in which up to 10 masses were monitored in a single experiment to achieve better signal-to-noise ratios and temperature resolution. X-ray photoelectron spectra were acquired using 1253.6-eV photons from a Mg Ka anode (15 kV, 300 W). Data were accumulated simultaneously for the Mo(3d) (1 min), C( 1s) (1 1 min), and S(2p) (7 min) energy regions. Each region was 8-eV wide. Absolute binding energies were calibrated against the Mo 3d5p photoemission peak a t 227.7 eV. High-resolution electron energy loss spectra were acquired with a primary electron energy of 3 eV and a resolution of about 70 cm-I. Typically, a single spectrum was obtained by averaging four 10-min scans. Since vibrational data were only acquired for relatively high coverages and the base pressure was maintained in the 1O-Iorange, background adsorption did not interfere with our experiments. Specifically, no CO adsorption was detected in the vibrational data. Sulfur overlayers were prepared by dosing hydrogen sulfide onto the Mo(l10) surface maintained at a fixed temperature, followed by annealing. The specific conditions used depended on the nature of the overlayer to be studied. Several different sulfur overlayers were formed on Mo( 110): p(2X2), c(2X2), (4X1), or (1XS). All of the structures have been previously reported.4~8 The specific structure formed depends on the hydrogen sulfide exposure, adsorption temperature, and annealing temperature. The Mo( 110)-(4X I)-S structure was formed by adsorbing H2S at 1300 K for 1 min and subsequently annealing to 850 K for 30 s. The S:Mo Auger ratio9 was 1.4 f 0.1 for the sharpest (4x1) LEED pattern. This ratio was used as a standard for a sulfur coverage of 0.5 monolayer,'O based on earlier work.4.8 A reversible transition between the (4X 1) and the ~ ( 2 x 2 phases ) occurs at 500 K, with the Mo( 110)-(4X 1)-S phase being stable below the transition temperature. The phase transition occurs well above the temperature where 2,5-dihydrothiophene reacts, however.
-
Results Temperature-Programmed Reaction Spectroscopy. 1,3-Butadiene is the major product evolved during temperature-programmed reaction of 2,s-dihydrothiophene on Mo( 110)-(4X 1)-S (Figure 2). The broad peak in the m / e 54 signal between 250 and 300 K is identified as the reaction product, 1,3-butadiene. based on the mass spectrometer fragmentation pattern." The two narrow 54-amu peaks a t 180 and 210 K are attributed to fragmentation of 2,s-dihydrothiophene. These peaks are aligned perfectly with those for the 2.5-dihydrothiophene multilayer and monolayer desorption peaks, respectively, and their intensities relative to the 2,s-dihydrothiophene peaks are constant as a function of exposure. A small amount of dihydrogen ( m / e 2) is evolved between 350 and 650 K. It is the only other volatile product. The maximum yield of H2 produced during temperatureprogrammed reaction on the Mo( 110)-(4X 1)-Soverlayer is about 0.2 of that on the clean surface. Butadiene is eliminated in an intramolecular process based on isotopic exchange experiments. A thiolate intermediate, 1, is explicitly ruled out by these experiments. Formation of a thiolate requires addition of one H or D to one of the carbons adjacent
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3585
100
300
500
100
700
300
500
700
900
Temperature (K) Figure2. Temperature-programmed reaction spectra obtained following adsorption of 23-dihydrothiophene on (a) Mo( 110)-(4xl)-S and (b) clean Mo(l IO) at 120 K. Spectra for the most intense ions a r e shown for each product: (i) 2,5-dihydrothiophene, (ii) butadiene, and (iii) Hz. The data are uncorrected for fragmentation. The contribution of 2,5dihydrothiophene fragmentation to the butadiene peak is thecross-hatched area.
H-C
\\
C-H
1 tosulfur. Nodeuterium is incorporated into either the butadiene or the 2,s-dihydrothiophene when deuterium is present on the surface. Hence, a thiolate is ruled out as an intermediate to butadiene. The presence of adsorbed hydrogen or deuterium does not significantly affect the reactionsof 2,s-dihydrothiophene. N o new products are formed when either deuterium or hydrogen is adsorbed prior to 2,s-dihydrothiophene adsorption and reaction. Hydrogen and deuterium were adsorbed by exposing the Mo(llO)-p(4Xl)-S surface to -1 X IO-* Torr of H2 and Dl, respectively, for 60 s. Furthermore, the yields, peak temperatures, and peak shapes for both 2,s-dihydrothiophene and butadiene evolution are essentially unchanged when hydrogen is present. Approximately 0.1 monolayer of sulfur is deposited during reaction of 2,s-dihydrothiophene on Mo( 110)-(4Xl)-S at saturation exposures. The S:Mo Auger ratio is measured to be 1.6 f 0.1 followingreaction compared to 1.4 f 0.1 for freshly prepared Mo( 110)-(4x1)-S. Only a minor amount of nonselective decomposition occurs during 2,s-dihydrothiophene reaction on Mo( 110)-(4X 1)-S. The level of carbon remaining after temperature-programmed reaction is below the limits of detectability in our Auger electron spectrometer. Residual carbon is present, however. Carbon monoxide is produced from temperature-programmed reaction of oxygen with the residual carbon at -900 K. The amount of 2,s-dihydrothiophenc decomposition on Mo( 110)-(4XI)-S is approximately one-sixth that on the clean surface based on the relative yields of CO from oxidation of the carbon deposited on the two surfaces. The selectivity for 1,3-butadiene elimination vs nonselective decomposition during 2,s-dihydrothiophene reaction on Mo( 110)(4Xl)-S is estimated to be about 83%. The selectivity for
Xu and Friend
3586 The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 TABLE I: Curve Fit Parameters for C(ls) and S(2p) X-ray Pbotoelectron Spectra of (4Xl)-S Overlayer and 2.S-Dibvdrotbio~bene(Relative Exposure 1.14) at 130 K and after Annealing to the Indicated Temperatures ~~~
binding energy, eV (4XI)-S
I30 K
I90 K
220 K
350 K
c Is
S 2pl,?
S 2pl,?
S 2plI2
S 2~1,:
284.6
163.9
162.7
162.4
161.3
0.92 I538 35.42
0.92 2804 64.58
0.014
F W H M , eV area, counts eV/s % of total area
area ratio (S(atomic)/Mo 3d)
F W H M , eV area, counts eV/s %of total area
1.35 5764 100
1.08 1208 17.49
I .08 2096 30.35
0.92 1 I93 17.28
0.92 2408 34.87
0.0 14
FWHM, eV area, counts eV/s % of total area
1.35 4915 100
1.04 960 14.99
1.04 1737 27.13
0.92 1155 18.04
0.92 255 I 39.84
0.014
F W H M , eV area, counts eV/s 9% of total area
1.35 799 100
I .04 I47 2.89
1.04 294 5.78
I .04 I560 30.65 1.03 1874 36.53
1.04 3088 60.68
0.0 I5
1.03 3256 63.47
0.01 7
FWHM, eV area, counts eV/s % of total area
butadiene elimination on clean Mo( 110) was previously estimated to be -67%, while the total amount of sulfur deposited on the clean surface was 0.3 m~nolayer.~J* The value of 83% for the Mo( 110)-(4X 1)-S surface was estimated by using the fact that approximately one-sixth as much carbon and approximately onethird as much sulfur remain after reaction on the sulfur-covered surface relative to the clean surface. The amount of residual carbon reflects the amount of nonselectivedecomposition,whereas the total amount of sulfur deposited is a measure of the total amount of reaction. The reactivity of 2,5-dihydrothiophene is relatively insensitive to coverage on the (4x1)-S overlayer. At the lowest coverage studied, 0.14 of saturation, both 1,3-butadiene and dihydrogen are evolved. The 1,3-butadiene yield increases rapidly as a function of 2,5-dihydrothiophene exposure and levels off a t relative coverage of 0.4 of saturation. The dihydrogen peak area remains essentially constant up to this coverage and increases slightly for higher coverages. The temperature-programmed reaction peak temperature for 1,3-butadiene does not change with 2,5-dihydrothiophene coverage, consistent with a unimolecular decomposition.') For exposures between 0.4 and 1.O of saturation, the amount of 2,5-dihydrothiophene desorption increases. The maximum selectivity for butadiene elimination over nonselective reaction is observed for a sulfur coverage of -0.4, for which a ~ ( 2 x 2LEED ) pattern is observed. The selectivity for butadiene elimination is -90% for 0s = 0.4. For sulfur coverages above 0.5, there is only a small amount of nonselective decomposition and no 1,3-butadiene elimination. The small amount of nonselective reaction is probably due to defects.
X-ray Photoelectron Spectroscopy. Sulfur(2p) X-ray photoelectron data clearly show that the C-S bond remains intact upon adsorption of 2,5-dihydrothiophene at 130 K. The data obtained for a saturation coverage of 2,5-dihydrothiophene adsorbed on Mo( 110)-(4X I)-S at 130 K is best fit with two sets of peaks. Each set of peaks corresponds to a single sulfur environment (Figure 3a and Table I). There are two peaks, which differ in energy by 1.2 eV and have a relative intensity of 1.8, for each state due to spin-orbit interaction. The set of peaks at 161.3 and 162.5 eV is attributed to atomic sulfur and is quantitatively accounted for by the (4X 1)-S layer. The peaks at 163.9 and 162.7 eV are ascribed to intact 2,5-dihydrothiophene. The S(2p) peaks corresponding to intact C S bonds are 1.2 eV lower in energy than for molecular 2,s-dihydrothiopheneadsorbed onclean Mo( 1 TheS(2p) bindingenergyofthechemisorbed state is the same as those measured for multilayers of 2,5dihydrothiophene on the Mo( 1 10)-(4X 1)-S surface, suggesting that the difference in energy compared to the clean surface is due to a change in the work function of the surface. Although these
-
,(i), 168
164
162
Binding energy (eV)
160
1
286
284
I I2
Binding energy (eV)
Figure 3. S(2p) and C(1s) X-ray photoelectron spectra for 2,sdihydrothiophene adsorbed on Mo(llO)-(4Xl)-S a t (i) 130 K and annealed to (ii) 220 K and (iii) 350 K. All spectra were collected a t a surface temperature of 130 K.
energies are similar to S(2p) binding energies measured for thiolateson Mo( 1lO),I4itis ruled out as themajority intermediate based on the isotopic exchange experiments. Carbon( Is) X-ray photoelectron data are also consistent with the presence of intact 2,Sdihydrothiophene at 130 K. A single peak with a FWHM of 1.35 eV is observed at 284.6 eV after adsorption of a saturation amount of 23-dihydrothiophene (Figure 3b). This C(1s) peak is 0.6 eV lower than for 2,5-dihydrothiophene on the clean Mo( 110) surface.' X-ray photoelectron data demonstrate that sulfur elimination from 2,5-dihydrothiophenecommencesat190KI5andiscomplete by 350 K (Figure 3 and Table I). As desulfurization begins, the C Is intensity decreases due to evolution of gaseous butadiene and 2,5dihydrothiophene desorption. As the 2,5dihydrothiophene layer is heated, loss of sulfur, due to 2,Sdihydrothiophene desorption, is also detected. Approximately 72% of the 2,5dihydrothiophene desorbs from the surface between 190 and 220 K, based on the relative intensities of all sulfur 2p peaks. These calculations take into account the attenuation in the intensity derived from the (4Xl)-S layer due to the presence of 2,5dihydrothiophene.I6 No additional loss of sulfur is detected in the range 220-700 K. However, the intensities of the sulfur peaks redistribute in the range 190-350 K, such that the peaks at 162.7 and 163.9 eV, due to intact C-S bonds, attenuate and those characteristic of atomic sulfur at 161.3 and 162.5 eV increase. By 220 K, the C(1 s) signal has decreased to 16% of its original value. Since 72% of the loss is due to 2,5-dihydrothiophene
2,5-Dihydrothiophene on Mo( 110)-(4X 1)-S
1500
=
5. v)
e l 000
-
h
--c
t\\
675cm-1 2900cm-1 Elastic/l 00 3050cm-1
al
-c
L.
500
r 2
0
4
8 1 0 1 2 1 4 Angle
6
The Journal of Physical Chemistry, Vo1. 97, No. 14, 1993 3587
TABLE 11: Vibrational Assignments for 2,5-Dihydrothiophene energy,cm gas phase
2000
1000
0
3000
Energy Loss (cm')
-
Figure 4. High-resolution electron energy loss spectra for 2,5-dihydrothiopheneadsorbedon Mo(l10)-(4Xl)-Sat 120K. (a)Condensed 2,5-dihydrothiophene multilayer, (b) one monolayer,2x and (c) -0.3 monolayer. The angular dependences of the elastic (triangle), C-H stretch (solid and empty circles), and =C-H bend (square) losses for 0.3 monolayer of 2,5-dihydrothiophene (spectrum c) are shown in the inset.
-
desorption, the remaining 12% must have left the surface as butadiene. Only 11% of the carbon remains on the surface at 250 K. The sulfur signal is unchanged in this range so that the 5% of the carbon lost must be due to butadiene elimination based on the difference in the C( 1s) signals for annealing temperatures of 220 and 250 K. By 350 K, no carbon is detected on the surface, indicating that very little nonselective reaction occurs. There are no noticeable changes in the X-ray photoelectron spectra above 350 K. The intensity of the atomic sulfur peaks following reaction relative to that of fresh (4X 1)-Slayer is 1.2,suggesting that 0.1 monolayer of sulfur is deposited during reaction. The kinetics for C-S bond breaking are more rapid at lower 2,5-dihydrothiophene coverages based on X-ray photoelectron data (not shown). For example, S(2p) peaks characteristic of intact C-S bonds are detected a t 162.7 and 163.9 eV along with atomic sulfur for a 2,5-dihydrothiophene coverage of 0.29 of saturation. However, -20% of the 2,5-dihydrothiophene desulfurizes at 130 K based on the intensity of the atomic S peaks relative to the (4xl)-S overlayer. High-Resolution Electron Energy Loss Spectroscopy (HREEIS). Vibrational data indicate that most 2S-dihydrothiophene is molecularly bound to the Mo( 110)-(4X 1)-S surface at 120 K (Figure 4 and Table 11). The electron energy loss spectrum obtained for 1 monolayer of 2,5-dihydrothiophene (Figure 4b) is similar to the reference spectrum of condensed
-
mode
S M stretching S M stretching
400 515 675
=C-H
800
C-S-C
950
C-C
out-of-plane bending
64 I 669 716 824 953 96 1 1112
1100
=C-H in-plane bending and CH2 twisting
1114 1119
I225
=C-H in-plane bending and C H ? wagging
1325
C H ? wagging
1425 1625 2900
CH2 deformation C-C stretching -C-H stretching
3050
=C-H
1227 1273 1343 1451 I647 2866 2936 3065
x600
adsor bed
stretching stretching
stretching
2,5-dihydrothiophene (Figure 4a). The same mode structure is present at lower coverages of 2,5-dihydrothiophene (Figure 4c). The vibrational modes are assigned based on comparison to gasphase IR data for 2,5-dihydr0thiophene~~ and the vibrational data for a similar molecule, thiophene, adsorbed on Ru(001) and Mo( 100).18J9 Unfortunately, isotopically-labeled 2.5-dihydrothiophene is not available. The primary difference between the gas-phase and surface spectra for 2,5-dihydrothiophene is the appearance of modes a t 400 and 575 cm-1 for the surface case. These modes are assigned as Mo-sulfur stretch modes, since the electron energy loss spectrum for Mo(l10)-(4Xl)-S comprises two modes at the same energy (data not shown). The main features of the vibrational spectrum do not change significantlyuponannealingto 210 K (Figure Sb),whichsuggests that the ring structure is preserved. Molecular 1,3-butadiene can be ruled out as the majority intermediate at 210 K since the vibrational spectrum for 1,3-butadiene on the Mo( 110)-(4X I)-S surface is qualitatively different than the spectrumobserved after heating 2,5-dihydrothiophene to 210 K (Figure 5). Specifically, there are intense losses at 925 and 1020 cm-I in the spectrum of 1,3-butadiene that are absent in the spectrum derived from 2,sdihydrothiophene heated to 210 K. The vibrational data also provide a means of evaluating the orientation of the 23-dihydrothiophene ring with respect to the surface. Specifically, the intensity of the out-of-plane methylene bend at 675 cm-l is related to the disposition of the ring. The assignment of the loss at 675 cm-l as the C-H out-of-plane bend is based on comparison with gas-phase IR data for 2,sdihydrothiophene as well as for vibrational data obtained for thiophene adsorbed on metal surfacesI7-I9and tetrahydrothiophene adsorbed on Mo( 1 The vibrational spectra of thiophene adsorbed on Ru(0001)18and M 0 ( 1 0 0 ) ~contain ~ a loss near 675 cm-1, which is assigned to the=CH bend based on isotopic labeling experiments. The C-S stretch would also lie in the same energy range, but it is weak for tetrahydrothiophene on Mo(1 lo), for example (data not shown).2' The orientation of 2,s-dihydrothiophene depends on its coverage, based on changes in the intensity of the =C-H out-ofplane bend. Free 2,5-dihydrothiophene is planar22 according to infrared ~tudies23.2~ so that the dynamic dipole moment of the =C-H out-of-plane bend vibration is perpendicular to the ring plane, 2. Hence, the orientation of the ring is derived from the intensity of the C-H out-of-plane bend with the caveat that dipole scattering is predominant. Only the component of the dipole moment perpendicular to the surface plane is allowed by the
3588
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
W\I I
I
0
120K
I
I
I
1000 2000 3000 Energy Loss (cm-1)
Figure 5. High-resolution electron energy loss spectra for 2,5dihydrothiophene:(a) 1 monolayerat 120 K and multilayersannealed to (b) 210 K, (c) 350 K, and (d) 700 K. Spectrum e is for 1,3-butadiene adsorbed on Mo(1 lO)-(SXI)-S at 120 K.
2 dipole selection rule. In dipole scattering, the intensity of the 4 - H out-of-plane bend will increase as the ring plane tilts toward the surface, 2. The C-H out-of-plane bend is primarily dipole scatteredbased on the dependence of its intensity on scattering angle (Figure 4 inset). The intensity of the C-H bending mode (out-of-plane) at 675 cm-1 dramatically decreases as the scattering angle is moved off-specular. In contrast, theolefinicC-H stretchingmode at 3050 cm-1 is relatively insensitive to the scattering angle and hence mainly impact scattered, as expected. The reorientation of the ring is clearly evidence based on the coverage dependence of the out-of-plane bend. As the coverage of 2,s-dihydrothiophene increases, the intensity of the 4 - H out-of-plane bend decreases relative to the olefinic C-H stretch. The relative intensity of these two modes can be used as a measure of the dipole scattering of the bend, independent of coverage since the v(C-H) mode is relatively insensitive to molecular orientation. In the limit of the multilayer, the orientation of 2,s-dihydrothiophene is assumed to be random, offering a reference state. For dipole scattering, the intensity of a mode is proportional to the component of the dynamic dipole moment perpendicular to the surface. Hence, the intensity of the out-
Xu and Friend of-plane bend relative to the C-H stretch is an indication of the ring tilt, increasing as the ring is inclined toward the surface. The increased relative intensity of the C-H bend for lower 2,s-dihydrothiophenecoveragesindicates that the ring tilts toward the surface at low coverage. At coverages near saturation, the ring assumes a nearly perpendicular disposition based on the decreased relative intensity of the 4 - H bend. Unfortunately, the tilt anglescannot bequantified due to the potential contribution of impact scattering to the peak intensity and the lack of information regarding the dipole moment for the 4 - H bend for 2,5-dihydrothiophene in the adsorbed state. It is possible that a mixture of perpendicular and parallel orientations coexists at coverages below saturation. If this were the case, the apparent tilting at low coverage may be instead due to a superposition of the spectra for a parallel and perpendicular state. A parallel-bound species is highly unlikely on the Mo( 110)-p(4X 1)-S surface, however, since thering wouldoverlap at least one sulfur atom in such a configuration. The presence of sulfur would inhibit metal-ring bonding and would probably lead to S-ring repulsions. Therefore, we attribute the increased intensity at lower coverages to a uniform tilting of the molecules in the layer. The similarity of the peak positions for low and saturation coverages suggests that there is not a substantial change in the bonding of the 2,s-dihydrothiophene as a function of coverage. The absence of s p t r a l shifts argues against a parallel state at low coverage. A parallel orientation would generally lead to a larger surface-adsorbate interaction that should lead to perturbation of the intramolecular potential and hence shifts in vibrational energies. Indeed, vibrational energiesare very similar to the gas-phase data, suggesting that there is not a significant amount of C-H or C-C bond weakening in the chemisorbed 2,5-dihydrothiophene. The extent of C-S bond weakening cannot be evaluated because of the low intensity of the C S - C stretch. According to the temperature-programmed reaction data, most 2,s-dihydrothiophene desorbs molecularly at saturation coverage, while it desulfurizes readily at low coverages. This suggests that a more tilted orientation may play a role in C-S bond activation. Theoretical studies are necessary to address this point. Carbon-sulfur bond cleavage precedes complete dehydrogenation along the pathway for nonselective decomposition. There are still intact C-H bonds in the species that remain on the surface after annealing to 350 K, the temperature where all gaseous butadiene has been evolved and all C-S bonds cleaved. This hydrocarbon intermediate reacts further to produce surfacecarbon and H2. Carbon-hydrogen stretch modes are clearly detected at 350 K (Figure 5c). There are indications of C = C and C - C stretches and CH2 deformation modes as well. However, the signal-to-noise level is too low to make a reliable assignment as a specific intermediate. These modes disappear, and only the M o S stretch modes are detected after heating to 700 K (Figure 5d). The strong M0-S stretch mode from the (4Xl)-S overlayer prevents the detection of the M0-S mode of 2,Sdihydrothiophene.
2,s-Dihydrothiophene reacts very selectively on Mo( 110)(4X 1)-S. Of the 2,5-dihydrothiophene that reacts, 83%eliminates 1,3-butadiene in an intramolecular process (Figure 6). The scheme shown is consistent with all of our data. A significant amount of molecular desorption also occurs,accounting for 70% of the 2,Sdihydrothiophene adsorbed at saturation coverage. Overall, the reactivity of 2,5-dihydrothiophene is similar on clean Mo( 110) and Mo( 110)-(4X 1)-S, however. The primary differences are that the selectivityfor butadiene elimination is higher on the sulfur-covered surface, that the rates of desulfurization and dehydrogenationare slower on the Mo( 110)-(4X1)-Ssurface, and that less 2,Sdihydrothiophene reacts on the (4x1)-S overlayer.
-
2,s-Dihydrothiophene on Mo( 110)-(4x1)-S
8 -B
120K4
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3589
Desomtion r
21OK
D
Reaction
H, (9) +
sscss
Figure 6. Proposed reaction scheme for 2,5-dihydrothiophene on the Mo( 110)-(4XI)-S surface.
Sulfur inhibits the cleavage of the C-S bonds of 2,sdihydrothiophene so that it remains molecularly bound at 130 K. This accounts for the increase in the amount,of desorption relative to reaction. The total amount of 2,5-dihydrothiophenethat reacts on the sulfur-coveredsurface is approximately one-third as much as on the clean surface. Site blocking probably plays a major role in decreasing the reactivity since there is already relatively dense packing of the sulfur in the (4Xl)-S overlayer (Figure 1). The sulfur in the layer may force the 2,s-dihydrothiophene into a different coordination site. In addition, the heat of adsorption decreases for sulfur at high coverages25so that the thermodynamic driving force for desulfurization is less than for the clean Mo(110) surface. Electronic effects may also play a role, but they cannot be evaluated on the basis of this work. The reactivity of 2,s-dihydrothiophene on Mo( 110)-(4X 1)-S correlates with the orientation of the ring. The layer of molecular 2,s-dihydrothiophene undergoes a structural transition as the coverage is increased. The ring orients more closely to the surface normal at coverages near saturation, whereas it is inclined with respect to the surfaceatlow coverage. At high coverage, molecular desorption occurs below 200 K, whereas at low coverages, butadiene is eliminated. These correlationssuggest that the C-S bond is more activated in the tilted configuration, although other factors such as coverage dependence of adsorption energies may also contribute to this effect. The bonding factors that underly this correlation are not known and require theoretical treatment. The difference in ring orientation does not appear to substantially alter the C-C and C-H bonds in 2,s-dihydrothiophene. There is a close correspondence between the vibrational energies for 2,5dihydrothiophene in the gas phase and those on the Mo(110)-(4X 1)-Ssurface (Table 11). In particular, there is not a significant perturbation of the C 4 bond at any coverage. The C==C stretch is at 1625 cm-I in the adsorbed state compared to 1647 cm-I in the gas phase. The lack of bonding between the C = C bond and the Mo atoms in the surface is not surprising given the density of sulfur atoms on the surface. The sulfur in the Mo( 110)-(4Xl)-S surface apparently blocks metal- bonding. For example, benzene desorbs below 200 K from Mo( 110)(4Xl)-S, suggesting that 'K bonding to the surface is blocked or at least severely weakened.26 The extent of 'K bonding between 2,Sdihydrothiopheneand molybdenum atomson the clean surface is unknown since the molecular state could not be isolated and spectroscopically characterized. Several 2,5-dihydrothiophenemetal complexes have been synthesized and can be compared to the structure and reactivity of 2,s-dihydrothiophene on Mo(110)-(4X 1)-S.27 In all of the known complexes, 2,5-dihydrothiophene bonds to the metal through the sulfur atom. The structure of [C~(PMe3)~Ru(2,5dihydrothiophene)]PF6 determined by X-ray diffractionindicates
that the S-Ru bond does not lie in the C S - C plane of the 2,sdihydrothiophent, and it is tilted by ~ 6 0 In~ (P~-H)RU~(CO)~. (p3-S,2,3,4-q4-2,5-dihydrothiophene),the sulfur is bonded to one Ru and the olefinic moiety is bonded to another Ru. These structures establish a precedent for the tilted configuration observed for low 2,s-dihydrothiophene coverage on Mo( 110)(4Xl)-S. Such a tilted geometry is consistent with bonding to a single metal center as might be expected on the sulfur-covered surface. At high coverages, adsorbateadsorbate interactions become more important and 2,s-dihydrothiophene is apparently forced into a more upright position. The reactions of 2,s-dihydrothiophene-metal complexes are similar to that on the Mo( 110) and Mo( 110)-(4Xl)-S surfaces. Upon heating W(C0)5(2,5-dihydrothiophene) and Re2(C0)9(2,Sdihydrothiophene) to 283 K, butadiene and free 2,sdihydrothiopheneare released in reactions analogous to the surface case.27 In contrast, MC12(2,5-dihydrothiophene)2(M = Pd, Pt) complexes produce mainly thiophene and free 2,s-dihydrothiophene upon heating to 453 K. Unfortunately, there are no reports on the reactions of two structurally characterized complexes, [Cp(PMe3)2Ru(2,5-dihydrothiophenel)]PF6 and
(~2-H)R~~(CO)9(~3-S,2,3,4-q~-2,5-dihydrothiophene). The nonselective decomposition pathway is not well-defined, although C-H bonds clearly persist after all C-S bonds are cleaved. One or more hydrocarbon species are present on the surface in the range 350-700 K based on the vibrational data. X-ray photoelectron data indicate that all C-S bonds are broken by 350 K. As the intermediates are heated above 350 K, they decompose completely to form surface carbon and gaseous dihydrogen. The nonselective reactions probably occur at defects in the sulfur overlayer since they persist even for sulfur coverages above 0.5 albeit representing a minor pathway. The sulfur apparently inhibits all bond breaking reactions. Theoretical investigations are needed to understand this effect.
Conclusions The reactions of 2,Sdihydrothiophene are similar on the Mo( 110)-(4Xl)-S and clean surfaces, although the selectivity for butadiene elimination is increased from 67% on the clean surface to 83% on Mo( 110)-(4Xl)-S. The overall reactivity is decreased, however, to approximately one-third of the clean surface. Site blocking of sulfur probably accounts for the decreased reactivity. The increased selectivity is mainly attributed to inhibition of C-H bond activation which leads to nonselective reaction. The first layer of 2,Sdihydrothiophene is molecularly bound on Mo(llO)-(4Xl)-S at 120 K. The ring of the 2,5dihydrothiophene is tilted away from the surface normal for low coverages but is nearly perpendicular to the surface for coverages near saturation. The tilted geometry and selective elimination of butadiene are examples of structural and reactive parallels on surfaces and in organometallic complexes.
Acknowledgment. We gratefully acknowledge the support of the U.S. Department of Energy, Basic Energy Sciences, under Grant DE-FG02-84ER13289. We also thank Dr. Moon-Gun Choi and Prof. R. J. Angelici of Iowa State University for providing the 2,s-dihydrothiophene. References and Notes (1) Sauer, N. N.; Markel, E. J.; Schrader, G. L.; Angelici, R.J. J. Cutuf. 1909,117,295. (2) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387. (3) Liu, A. C.; Friend, C. M. J . Am. Chem. Soc. 1991,113, 820. (4) Witt, W.; Bauer, E. Ber. Bunsenges. Phys. Chem. 1986, 90,248. (5) Wiegand, B. C.; Friend, C. M.; Roberts, J. T. Lungmuir 1989, 5, 1292. (6) Wiegand, B. C.; Uvdal, P. G.; Friend, C. M. J . Phys. Chem. 1992, 96,4527. (7) Liu, A. C.; Friend, C. M. Reo. Sci. Insrrum. 1986, 57, 1519.
3590 The Journal of Physical Chemistry. Vol. 97, No. 14, 1993 (8) Sanchez, A.; de Miguel, J. J.; Martinez, E.: Miranda, R. SurJ Sci. 1986. 171, 157.
(9) The S:Mo ratio is defined as [S(152 e v ) + Mo(148 eW]/[Mo(186 + Mo(22I eW1. (IO) Our previous coverage calibration' ws in error due to the misidentification of thec(2x2) pattern as the p( 2x2) structure. Hence, the S coverage of thec(2X2) structure previously ascribed toa coverageof 0.25 had an actual coverage of 0.35 monolayer. ( I 1 ) TheDeakratiosform/e54and 53 reoorted in the EPA/NIH handbook for 'I ,3-butadiene and 1,2-buiadiene are I .45 and 2.07, respectively. A ratio of 1.36 is measured for authentic I ,3-butadiene in our mass spectrometer. The same peak ratio of 1.36 is observed during temperature-programmed reaction of 2.5-dihydrothiophene on the surface. ( 1 2) The total amount of S deposited on clean Mo( I IO) was recalibrated as0.3 monolayer. Given theselectivityof67%. -0. I monolayerofirreversibly adsorbed 2,5-dihydrothiophene decomposes to carbon, H., and sulfur. Since onlyone-sixthasmuchdecomposeson Mo( 1 IO)-(4XI)-Sandthe totalamount of S deposited is 0.1 monolayer, the selectivity for butadiene elimination on the sulfur overlayer is calculated to be 1 - (0.1 X 1/6)/0.1 = 83%. ( I 3) King, D. A. SurJ S i . 1975, 47, 384. (14) Roberts, J. T.: Friend, C. M. J . Phys. Chem. 1988, 92. 5205. ( I S ) The atomic S(2p) signal is attenuated by multiple layers of 2,5dihydrothiophene. Hence, theratioofatomicS(2p) intensity tothatof Mo(3d) (Table I) was used to monitor changes in the amount of atomic sulfur on the surface at low temperature and high coverages. eV) I
I ,
Xu and Friend (16) Desulfurization does not commence until above 190 K. Therefore, the atomics( 2p) intensity of freshly prepared (4X I )-Soverlayer wassubstituted asthecorrectedatomicS(2p)intensityinthepresenceof2,5-dihydrothiophene at I90 K. (17) Green, W. H.; Harvey, A. B. Spectrochim. Acta 1969. 25A. 723. (18) Heise, W. H.: Tatarchuk. B. J. Surf. Sei. 1989, 207, 297. (19) Zaera, F.: Kollin. E. B.; Gland, J. L. Sur!. Sci. 1987, 184, 75. (20) Xu, H.: Friend, C. M. Unpublished results. ,-,I\ groups. so the weak mode near ( L 1 1 Tetrahydrothiophene has no =CH 660 cm ' must be due to the C-S stretches. (22) X-ray structure analysis of [Cp(PMei)~Ru(2,5-dihydrothiophene)]PF,, shows that 2,s-dihydrothiophene is slightly nonplanar.!' The dihedral anglebetweenc-C-CandC-S-Cis26.30. However,in thispaper,weassume that 25dihydrothiophene is planar. The tilted geometry at low coverage may also be explained by a nonplanarity of the ring. (23) Veda, T.: Shimanouchi*T. J. Chem. Phys. 1967,47, 5018. (24) Green, W. H.: Harvey, A. B. J . Chem. Phys. 1968, 49, 177. (25) Peralta, L.: Berthier, Y.: Oudar, J. SurJ Sci. 1976, 55, 199. (26) Wiegand. B. C.; Friend, C. M. Unpublished results. (27) Choi, M.-G.: Daniels, L. M.; Angelici, R. J . Inorg. Chem. 1991. 30, 3647. (28) One monolayer is defined as the coverage for which the higher temperature molecular desorption peak at 210 K reaches the maximum intensity-programmed reaction.
