990
Langmuir 1989,5, 990-997
tercept of this linear segment is equal to iiki = 1.70 mmol/g. Because ri:ef = 0.40 mmol/g for spheron, the slope of the linear n / x segment yields a value of fike for charcoal equal to 0.26 mmol/g. Thus, the total surfacephase capacity for charcoal is iie = iiLi + iiLe = 1.96 mmol/g. This value agrees with the 1.92 mmol/g obtained by the Everett method.I4 This work presents for the first time a comparative method for assessing the microporosity of porous solids (14) Dabrowski, A.; Jaroniec, M. J. Colloid Interface Sci. 1980, 73,475.
on the basis of the liquid-solid adsorption isotherms. An experimental illustration shows that the excess adsorption isotherms measured for the liquid-solid interface may be used for characterizing microporous solids. This work presents also a new approach toward a quantitative description of adsorption from solutions on microporous solids.
Acknowledgment. This work was supported in part by the National Science Foundation under Grant no. CBT-872195. We thank Mr. X. Lu for providing the figures and a reviewer of this paper for suggesting eq 13.
Adsorption of Elemental Sulfur on Pt(100) and Pt(ll1) Faces: Investigation of Weakly Bound States H. Gutleben and E. Bechtold" Institut fur Physikalische Chemie, Uniuersitat Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received November 20, 1988. I n Final Form: March 31, 1989
The weakly adsorbed @ states of sulfur which are formed in addition to the dissociatively adsorbed a states have been investigated on Pt(100) and Pt(ll1) single-crystal faces. The experiments were done in a UHV system with LEED, AES, and thermal desorption mass spectrometry. Two well-resolvedp desorption peaks occur at 400 K (& state) and 700 K (pl state). The & adsorption state corresponds to a physisorbed adlayer. Its thermal desorption produces mainly sg, S7,and S8molecules. Their relative amounts and the shape and the positions of desorption peaks depend on the pretreatment. Desorption of the p1state yields exclusively Sz molecules. On Pt(100) the amount of adsorbable &-state sulfur is determined by defects ) structure of the preadsorbed a-state sulfur. Slight Ar+ bombardment renews the in the ~ ( 2 x 2surface capability of the already saturated PI adlayer to adsorb sulfur. Alternating ion bombardment and sulfur dosing causes the amount of adsorbed P1-state sulfur to be increased up to several monolayer contents. On the basis of the body of observations, the PI state is interpreted to consist of chains of sulfur atom which are chemisorbed at their ends and, in addition, bound by van der Waals interactions along the chains. The adlayer does not attain internal equilibrium, neither after dosing nor on heating up to the desorption temperature.
A. Introduction Most reported studies on adsorption of sulfur on platinum concentrate on dissociatively adsorbed which can be formed from elemental sulfur as well as from various sulfur compounds, such as hydrogen sulfide or mercaptans. Apart from general aspects, this interest stems from technical implications to heterogeneous catalysis, corrosion, etc. The present work is concerned with more weakly bound adsorbed states which are formed in addition to dissociatively adsorbed sulfur when elemental sulfur is dosed onto platinum.2
B. Experimental Section The experiments were done in a stainless steel UHV system capable of a base pressure of 2 X mbar which was equipped with a LEED/AESsystem and a quadrupole mass spectrometer.46 The mass spectrometer was housed in a titanium getter pumped and liquid nitrogen cooled compartment, which enabled lineof-sight detection of species desorbing from the central part of the specimen into directions near to the surface normals4 The platinum crystals with (100) and (111) orientation, respectively, were mounted on a rotatable L-shaped holder.* The (1) Berthier, Y.; Perdereau, M.; Oudar, J. Surf. Sci. 1973, 36, 225. (2) Heegemann, W.; Meister, K. H.; Bechtold, E.; Hayek, K. Surf. Sci. 1975, 49, 161. (3) Fischer, T. E.; Kelemen, S. R. Surf. Sci. 1977, 69, 1. (4) Gutleben, H.; Bechtold, E. Surf. Sci. 1987, 191, 157. (5) Gutleben, H. Dissertation, Innsbruck, 1988.
0743-7463/89/2405-0990501.50/0
temperature was regulated by a programmable controller using a Pt/PtRh thermocouple s e n s ~ r . ~After ? ~ the usual cleaning mbar O2at 1300 K, procedures (alternating heating in 5 X argon ion bombardment, and heating to 1500 K), the crystals and Ptexhibited the known Pt(100)-(5x20) or Pt(lOO)-he~~~~ (111)-(1x 1) structures, respectively. A molecular beam of elemental sulfur was produced inside the UHV chamber by decomposition of silver sulfide in a solid-state electrochemical cell Pt/Ag/AgI/Ag#/Pt.w The cell was operated at temperatures between 460 and 500 K and with various voltages Thereby, the relative below the standard EMF of the amounts of different sulfur molecules in the emanating sulfur beam could be changed from nearly pure S2up to predominanting S, molecules with x > 2.9J0
C. Results and Discussion 1. General Characterization of the Adsorption Systems. On the initially clean Pt(100)-hex and Pt(Ill)-( 1x1) surfaces, sulfur molecules were adsorbed with high sticking probability.2 On Pt(100), sulfur adsorption resulted in the formation of streaked LEED patterns.2 Wherl the saturation of the dissociatively adsorbed a state (6) Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Biberian, J. P.; Kesmodel, L. L.; Bartos, I.; Somorjai, G. A. Surf. Sci. 1981, 103, 189. (7) Heinz, K.; Lang, E.; Strauss, K.; Maller K. Surf. Sci. 1982, 120, L401. (8) Wagner, C. J . Chem. Phys. 1953, 21, 1819. (9) Rickert, H. Ber. Bunsen-Ges. Phys. Chem. 1961,65, 463.
(IO) Detry, D.; Drowart, J.; Goldfinger, P.; Keller, H.; Rickert, H. 2. Phys. Chem. NF 1967,55, 314.
0 1989 American Chemical Societv
Langmuir, Vol. 5, No. 4, 1989 991
Adsorption of Sulfur on Pt(100) and P t ( l l 1 )
of the complete
was attained (6.5 X 1014S atoms/cm2),a c(2X2)-S structure with broadened spots was present. This LEED pattern persisted during further dosing, which led to the formation of the p adlayer, but it was weakened, and the background intensity increased. When sulfur dosipg was interrupted in the concentration range up to the saturation of the cy state, subsequent heating at 830 K resulted in LEED patterns of (2X2), (!l and 42x2) structures, depending on coverage. On Pt(lll), the adsorption of sulfur produced successively a (2x2) (3.8 X 1014 S atoms/cm2) and a (d3X43)R3Oo structure (5.0 x l O I 4 S atoms/cm2) of dissociatively adsorbed sulfur (cy state).'S2 Sulfur dosing beyond the saturation of the ( 4 3 x 4 3 ) R30"-S structure led to a metastable structure and various low-order structures,2but eventually fractional order spots disappeared in the increased background intensity when the p1 adlayer was near completion.2 Figure 1 shows a desorption spectrum obtained on heating a complete sulfur adlayer adsorbed on Pt (100). The peaks are designated according to the nomenclature introduced previously.2 With the exception of the peak near 400 K (&), MS signals occurred only at 32 and 64 amu, which was also true for Pt(ll1). Hence, desorption rates of Sa molecules for the P1 and cy peaks were obtained directly from the amu 64 signal, whereas S atom desorption rates were derived from the amu 32 signal by subtracting the contribution from S2fragmentation in the ion source? Desorption of the cy state produces S atoms and Sz molec u l e ~ the ; ~ & state yields only S2 molecules. The S2 & desorption peak in Figure 1 results mainly from fragmentation of larger sulfur molecules in the ion source (see section C.5) and does not represent the S2desorption rate. Desorption of the dissociatively adsorbed cy state from Pt(100) has been described in a previous paper.4 The present work is concerned with the more weakly bound (3 states, which do not form ordered surface structures and whose molecular composition is unknown. 2. Pt(100): Adsorption and Desorption of the p1 State. a. Surface Concentration and Calibration of the Desorption Signals. Surface concentrations were
8
10
12
14
x l e l 4 [S/cm2]
Figure 2. Pt(100): sulfur AES signals as a function of the total sulfur concentration derived from the peak areas of the TPD
spectra. derived from the areas of S2desorption peaks. The calibration of the MS signals was based on the known concentration of the cy state as described previously! Possible calibration errors resulting from different velocities and angular distributions of S2molecules desorbing from the CY and /3 adlayers, respectively, were considered negligible. For the adsorption of the cy state, the present data are in agreement with the reported adsorption curve: which was determined from AES measurements. But for the PI state, a distinctly higher saturation concentration resulted, i.e., 11.1X 1014S atoms/cm2 as compared to the reported 5.5 X 1014 S atoms/cm2.2 The reason for this discrepancy becomes clear from Figure 2, where sulfur AES signals (corrected for the contribution of the overlapping Pt signal) are plotted as a function of the corresponding sulfur concentration derived from the TPD spectra for various exposures. A t first the AES signal increases in proportion to the sulfur coverage up to completion of the a adlayer (6.5 X 1014 S atoms/cm2). At higher surface concentrations, when the p1adlayer is formed the AES signals grow further, approximately linearly with the desorption peak areas, but with a reduced slope. The high total sulfur concentration present at the saturation of the PI adlayer
Gutleben and Bechtold
992 Langmuir, Vol. 5, No. 4 , 1989 dn/dt
/le14
8)
SI
[S/cm2
K K
310 K 360 K
300 I
400 I
I
600
500 ,
/
,
I
700 /
000
I
,
I
IK1
T
exposure
')
d)
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,
Figure 3. Pt(100): & and & desorption spectra obtained after dosing sulfur onto the Pt(100)-hex surface. p1 coverages: 1.3, 4.6, 7.5,9.0,9.4, 10.5, and 11.1 X 1014 S atoms/cm*. Crystal temperature, 310 K; heating rate, 8.1 K/s. The scale of the ordinate refers only to the desorption of the &-state sulfur. This is also true for Figures 4, 6, 7 , and 8.
10
20
30
40
50
290
300
400
500 xle14
Figure 5. Pt(100): adsorption of pl-state on differently pretreated Pt/a sulfur substrates: (a) substrate is saturated a adlayer (dosed onto Pt(hex); (b) substrate is saturated a adlayer (dosed onto Pt(lx1); (c) substrate is semisaturated CY layer annealed at 830 K (2x2), saturated after cooling to 360 K; (d) substrate is saturated N adlayer annealed at 830 K. Temperaturesshown are the crystal temperatures during adsorption of the p sulfur
dn/dt
/le14 [ S / c m 2 SI 1.5
1.0
0.5
400
300 l
~
500 ~
,
700
600 l
,
000 l
*
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l
,
pigure 4. Pt(100): Bz and & desorption spectra obtained after dosing sulfur onto the Pt(100)-(lXl) surface. p1coverages: 4.0, 5.5,5.8, and 7.3 X 1014 S atoms/cm2. Crystal temperature, 320 K; heating rate, 7.9 K/s. Dashed line shows the desorption spectrum obtained after dosing sulfur onto the Pt(lOO)-hex surface, for comparison (pl coverage: 11.4 X 1014S atoms/cm2). (17.6 X 1014 S atoms/cm2) indicates that the p1 state is adsorbed in a second layer. The lower total AES sensitivity in this concentration range appears to be caused by attenuation of electron beams in the p1 adlayer, which affect the intensity of the primary beam intensity for the a adlayer as well as the intensity of Auger electrons emitted from the a adlayer. In addition, different Auger sensitivities of a- and @-statesulfur and a change of electron reflectivity could influence the observed Auger signals. b. Adsorption on Differently Pretreated Surfaces. Desorption spectra obtained after various dosages onto the initially clean reconstructed hex and the unreconstructed (1x1)surface are shown in Figures 3 and 4, respectively. The metastable Pt(loo)-(1X 1)was prepared by adsorption of NO followed by reaction with hydrogen and heating up to 390 K." The resulting surface exhibited a reasonably sharp (1x1) LEED pattern. After an exposure of approximately 110 X 1014 S atoms/cm2, the amount of sulfur adsorbed on this Pt(lOO)-(lxl)surface was only 7.3 ~~
(11)Bonzel, H. P.; Broden, G.; Pirug, G. J. Catal. 1978, 53,96.
X 1014S atoms/cm2, i.e., about 70% of that obtained with the initially reconstructed Pt(100)-hex surface for the same sulfur exposure. As can be seen from Figure 5, true saturation of the pi sulfur adlayer is not obtained even for very large exposures, a fact which can be explained by the model proposed in section C.4. However, for sulfur exposures greater than approximately 30 X 1014S atoms/cm2, the sticking coefficient for p1 adsorption is extremely low and nearly identical for any Pt/a sulfur substrate. Therefore, P1-state coverages obtained by sulfur exposures greater than =lo0 X 1014S atoms/cm2 are called saturation values in the following. As can be seen from Figure 5a, 5c, and 5d, respectively, the maximum adsorbable amount of &-state sulfur was further decreased when sulfur dosing onto the initially l ~ reconstructed hex surface was interrupted in the range of the a adlayer population and was continued after annealing at 830 K. In general, the &-state saturation concentration decreases with increasing a-state precoverage at the heating stop, but not continuously (Figure 6 ) . A steep decrease of the saturation concentration from 11.4 X 1014to 4.6 X 1014S atoms/cm2 at an a-state precoverage of 8, = 0.4 (relative to saturation of the a adlayer), where in the LEED pattern the features of the hex structure have completely disappeared, is followed by a slow decrease to 3.9 x 1014S atoms/cm2 at an a-state precoverage of 8, = 0.8, as can be seen from the inset of Figure 6. Finally, with the complete predosed a adlayer corresponding to a sharp 42x2) LEED pattern, the &-state saturation concentration is decreased to 1.9 x 1014 S atoms/cm2. It should be remembered that in any case the a adlayer is completed on further dosing after the heating stop, which results in a ~ ( 2 x 2 structure. ) The ~ ( 2 x 2 LEED ) pattern persisted during the adsorption of the p1state, but its quality was strongly influenced by the pretreatment. The pattern obtained with the initially reconstructed hex surface exhibited distinctly broader spots and higher background intensity than that obtained with the (1x1) surface or with the predosed and heated surface. This indicated low order in the ~ ( 2 x 2structure ) for the surface with high & adsorption capacity and good order for that with small capacity. Thus disorder in the ~ ( 2 x 2structure ) seems to be decisive for the formation of the p1 adlayer.
Langmuir, Vol. 5, No. 4, 1989 993
Adsorption of Sulfur on Pt(100) and Pt(111) dn/dt
/ie14 [S/cm2
SI
300
400
500
600
700
B@O
T
[KI
Figure 6. Pt(100): p2 and PI desorption spectra obtained after dosing sulfur onto differently precovered and annealed surfaces. After different amounts of a sulfur had been adsorbed onto the Pt(lOO)-hex surface at 320 K, the crystal was heated to 850 K. The crystal was then cooled down to 315 K, the a sulfur adlayer saturated in any case and a further 115 X 1014S atoms/cm2 dosed finally. Heating rate is 7.9 K/s: (a) 8, = 0.04, Pt(hex), ng,= 11.4 X 1014S atoms/cm2; (b) 8, = 0.24, Pt(hex)+(BXS), ngl = 7.9 X 1014 S atoms/cm2; (c) 8, = 0.42, (2X2), ng = 4.6 X l O I 4 S atoms/cm2; (d) 8, = 0.60, (!, ;), ng = 4.2 X 1614S atoms/cm2; (e) 8, = 0.79, c(2X2), na = 3.9 X ld14 S atoms/cm2; (f) 8, = 1.00, c ( 2 ~ 2 )ng , = 1.9 X S atoms/cm2. 8, is the relative amount of primarely adsorbed a sulfur, from which the indicated surface structures resulted after heating. Inset shows the amount of @,-state sulfur adsorbed as a function of the relative a precoverage.
Different basic structures of the differently pretreated ~ ( 2 x 2surfaces ) as another possible cause for the different adsorption properties could be excluded by measuring LEED intensities as a function of electron energy.5 The integral and fractional order spots exhibited intensity maxima and minima at the same electron energies for all surfaces. Disorder in the c(2X2)-S structure may have its origin only in the sulfur adlayer (mainly caused by domain boundaries), or it may be induced by substrate defects. The influence of the former type of disorder becomes evident when the p1adsorption on an annealed c(2x2)-S structure is compared with that on a c(2x2)-S structure prepared by dosing sulfur onto the metastable unreconstructed (1x1)surface. Sharp integral-order LEED spots indicated good substrate order in both cases. But for the unannealed c(2X2)-S structure, distinctly broader halforder spots pointed to considerable disorder in the sulfur adlayer. In fact, the p1adsorption capacity of this structure was about 3 times higher than that of the annealed c(2X 2)-S surface (Figure 5b and 5d, respectively). Some substrate disorder, which necessarily accomplishes defects in the c(2x2)-S structure, was caused in the present case by the imperfect surface orientation. From splitting of LEED spots which occurred when the (2X2)-S structure was formed, a deviaton of 3.5’ with respect to the (100) orientation was derived. Spot splitting disappeared on heating at higher sulfur concentrations, probably as a consequence of formation of larger terraces, accompanied by higher than monatomic steps. These remaining steps, however, will certainly have exhibited some influence on the formation of the p1state. Another cause for substrate defects, which is inherent to an (100)-oriented platinum surface, is the removal of reconstruction, which occurs during adsorption. This substrate rearrangement produces various surface defects, in particular steps. Since the topmost substrate layer of the hex surface is more
densely packed than that of the (1x1)surface?’ the surface atoms becoming supernumerary build up islands of an additional layer.12 Annealing the precovered surface effects the aggregation of these islands and a concomitant decrease of the length of steps. The strong decrease of the p1saturation coverage (Figure 6), which occurs up to the complete removal of the hex structure, suggests that defects connected with these steps favor the adsorption of the p1 state. On considering the strong influence of defects in the substrate structure on the formation of the p1adlayer, it seems that the state cannot be formed on a perfect Pt(lOO)-c(2X2)-S surface at all. c. Desorption. Desorption spectra of the P states are shown in Figures 3, 4, and 6 for adlayers differing in dosages and in pretreatment, respectively. They exhibit not only different &-state saturation coverages, as discussed in C 2.2, but also different structures of desorption traces. A comparison of p1desorption rates obtained from differently prepared adlayers shows that desorption kinetics depends on the history of the adlayer. Thus, the usual kinetic evaluations, which are based on internal quasi-equilibrium in the adlayer, cannot be applied. A tentative evaluation of isosteric desorption rates obtained from a series of desorption experiments with different heating rates and with initially saturated p1adlayers prepared by dosing sulfur onto the hex surface yielded a desorption energy of about 40 kcal/mol. After desorption of the p1adlayer by heating up to 800 K, the surface exhibited a sharp c(2X2)-S LEED pattern in any case. d. Adsorption on an Ar+ Ion Bombarded p1 Adlayer. In an attempt to get additional information on the p1state, a nearly saturated p1adlayer prepared by dosing sulfur onto the Pt(100)-hex surface was bombarded with small doses of Ar+ ions (1 keV). During the ion bombardment, all of the pz adlayer and about 10% of the p1 adlayer were desorbed. When sulfur dosing was continued after the ion bombardment, further sulfur adsorption occurred up to about the 1.6-fold amount (23.3 X 1014S/cm2, Figure 7b) of the p1 sulfur concentration obtained by further sulfur dosing without Ar+ bombardment (Figure 7a). Repeated Ar+ ion bombardment and sulfur dosing increased the amount of adsorbed &-state sulfur further (e.g., to 30 X 1014S/cm2 after two cycles, Figure 7c). The capability to adsorb sulfur beyond “normal” saturation was lost when the Ar+ ion bombarded adlayer was heated to 500 K prior to sulfur dosing (Figure 7d). The effect caused by the Ar+ bombardment seems to have its origin in the P1 adlayer since Ar+ bombardment of the clean surface or of the surface covered with the annealed ~ ( 2 x 2adlayer ) enabled subsequent p1adsorption only up to the same or an even smaller amount (Figure 7e) as found for the initially reconstructed surface without Ar+ bombardment. In view of the reported influence of light on the composition of elemental sulfur and of the possibility of UV photodissociation of S-S bonds,13 similar effects as observed after Ar+ ion bombardment were suspected to be caused by UV irradiation. But irradiation of the p1adlayer with light of wavelengths 254 and 350 nm through a sapphire window did not induce any change either of the further sulfur uptake during subsequent dosing or of the desorption kinetics. (12) Hosler, W.; Ritter, E.; Behm, R. J. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 205. (13) Schmidt, M.; Siebert, W. In Comprehensive Inorganic Chemistry;
Trotman-Dickerson, A. F., Ed.; Pergamon Press: New York, 1973; Vol. 2, p 795.
994 Langmuir, Vol. 5, No. 4, 1989
Gutleben and Bechtold
dn/dt / l e 14 [ S / c m 2 SI
A I!
I!
I
6.0
~
d 8/d t
1
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0.05 640 K i
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I
500
400
300 ,
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700
600 ,
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.
Figure 7. Pt(100): p2 and desorption spectra obtained after dosing sulfur onto a nearly saturated (exposure: 115 x 1014S atoms/cm2) and then Art-sputtered p1 adlayer. Crystal temperature, 320 K heating rate, 7.9 K/s: (a) without Ar' sputtering and a further 115 x 1014S atoms/cm2dosed, ng = 14.7 x 1014 S atoms/cm2;(b) 1.5-min Ar' sputtering + 115 X S atoms/cm2 dosed, ng = 23.3 x 1014S atoms/cm2;(c) 1.5-minArt sputtering + 115 X S atoms/cm2dosed + 1.5-min Ar+ sputtering + 115 X l O I 4 S atoms/cm2 dosed, ng, = 30.2 X 1014S atoms/cm2; (d) l.5-minArt sputtering,heating to 500 K + 115 X 1014S atoms/cm2 dosed, ng = 14.7 x 10" S atoms/cm2; (e) annealed (830 K) CY adlayer (ct2X2)) 2.5-min Ar' sputtered + 230 X 1014S atoms/cm2 dosed, ng, = 12.2 X loi4 s atoms/cm2.
1.5
0
dntdt 11014 [ S k i n 2 s]
20
40
trsi
60
I
Figure 8. Pt(ll1): p2 and p1 desorption spectra obtained after dosin sulfur at 310 K. p1 coverages: 1.5,4.9,6.8, 9.2, and 11.5 X 10' f S atoms/cm2. Heating rate: 6.8 K/s. 3. Pt(ll1): Adsorption and Desorption of the p1 State. B1desorption traces for Pt(ll1) are shown in Figure 8 for various dosages. The p1 desorption peaks were practically independent of the initial surfaces conditions, i.e., clean surface or predosed and annealed a adlayer. On Pt(lll), the p1 saturation concentration was about the same as that found with the initially reconstructed Pt(100)-hex surface (11.5 X 1014S atoms/cm2). The shapes of the p1peaks in Figure 8 point to zero- or fractional-order kinetics. But several observations indicate that the desorption rate depends not only on the surface concentration and temperature but also on the history of the adlayer. This becomes evident when the desorption experiments are evaluated in more detail. In Figure 9, desorption rates for constant temperatures are shown as a function of the relative sulfur concentration e,, of the p1adsorption state. The data were determined from temperature-programmed experiments with different initial concentrations and with different heating rates, respectively, as well as from isothermal experiments. Heating to temperatures near the
0.5
_-/
I
I
!
1.o
Figure 9. Pt(ll1): isothermal desorption rates (monolayers/s) as a function of the relative PI sulfur coverage at two different temperatures: (a) data from TPD experiments with different initial coverages; (b) data from TPD experiments with different heating rates, initial coverage e,, = 1.0; (c) data from isothermal experiments, initial coverage 8 = 1.0; (d) data from isothermal experiments, initial coverage 8g1 = 0.41. onset of desorption or staying at 300 K for some hours had only minor effects on the subsequent desorption. The experiments clearly indicate that the adlayer does not attain internal quasi-equilibrium during desorption. After desorption of the state by heating to 700 K, the surface exhibited the (d3Xd3)R30° LEED pattern of the a adlayer. 4. Summarizing Discussion of the p1 State. The high total sulfur concentration which is present when the p1 adlayer has attained saturation ( ~ 1 7 . 6X 1014 S atoms/cm2 for normal dosing conditions and far more after Ar+ bombardment; for comparison, a close-packed layer of S atoms with van der Waals radii 1.85 %, would result in 8.45 X 1014S atoms/cm2) indicates that the p1state is bound in a second layer. The packing density suggests the presence of sulfur-sulfur bonds in the adlayer. Evidence for lateral attractive interactions in the p1adlayer comes from investigations with a field electron microscope,14 which have shown that adjacent regions covered with the saturated a adlayer and with additionally adsorbed &state sulfur, respectively, persist up to the desorption temperature of the p1state. Experiments in which the composition of the sulfur beam emanating from the electrochemical cell was varied showed that the state can be formed from impinging S2as well as from larger sulfur molecules. This indicates that in general the adsorbing molecules do not retain their molecular identity. In view of the binding properties of sulfur, the formation of ring- or chainlike molecules appears plausible. Sulfur rings bound to the substrate only by van der Waals interactions would represent physisorbed molecules. Their desorption occurs already at lower temperatures (near 400 K, pz state, see section C.5). Therefore, the P1-state sulfur seems to consist either of chainlike molecules, which are anchored at their ends by chemisorption bonds, or rings with hypervalent sulfur and bonding to surface via a side chain or directly. The presence of hypervalent sulfur, however, appears less probable for energetic reason^.^^^^^ For detailed conclu(14) Bechtold, E.; Block, J. H. Z. Phys. Chem. NF 1974,90,135. (15) Lenain, P.;Picquenard, E.; Corset, J.; Jensen, D.; Steudel, R. Ber. Bunsen-Ges. Phys. Chem. 1988,92,859.
Langmuir, Vol. 5, No. 4, 1989 995
Adsorption of Sulfur on Pt(100) and P t ( l l 1 )
sions on the binding state, one has to await further experimental information, e.g., from HREELS. Additional van der Waals interaction with the underlying a adlayer may occur along the sulfur chain. In any case, the adsorption of the 0, state involves breaking and formation of sulfur-sulfur bonds. In the gas and in the condensed phases, the processes governing the interconversion of sulfur molecules have attracted interest for a long time. The mechanism, however, is still controversial. Basically, radical chain mechanisms1' and mechanisms with special intermediates as supervalent sulfur have been proposed.16 For the formation of the 0, adlayer, a radical mechanism appears to be favored by several observations (see below). Accordingly, it is assumed that during sulfur dosing the growth of the sulfur chains starts a t free surface sites or defects in the a adlayer and continues at the open chain end until this is deactivated by recombination with the surface or with another chain end. Desorption occurs in a reverse process and yields exclusively S2 molecules, which correspond to the main equilibrium component in sulfur vapor at the desorption temperature and the density of the desorbing beam.l0 In view of the proposed model, most of the experimental results shown in the sections above can be rationalized: 1. The low reactivity of sulfur-sulfur single bonds in the chainlike molecules causes the adlayer to remain in the primarily formed state and not to rearrange to the equilibrium state. This can be concluded from the observation that desorption kinetics depend not only on the surface concentration and temperature but also on the initial concentration, on the heating program, and on the history of the adlayer. This difficulty to establish internal equilibrium parallels effects observed with liquid sulfur, whose molecular composition strongly depends on the pretreatment.ls 2. The diminished amount of @,-statesulfur (only about one-fifth of the "normal" saturation coverage) which can be adsorbed on the annealed Pt(100)-c(2x2)-S structure indicates that defects in the adlayer are a prerequisite for the adsorption. The formation of the 0, state starts a t defects, but the further growth of the adlayer extends into ordered regions. This follows from the observation that the attainable @,-statesulfur concentration surpasses any reasonable defect concentration. The difference of adsorbable @,-statesulfur on the initially reconstructed hex and (1x1)surface, respectively, is due to the higher defect concentration on the former, which results from the removal of surface reconstruction during the formation of the a adlayer. 3. On Pt(lll),in contrast to Pt(100), the chemisorbed sulfur atom can be displaced from their positions in the (v'3xv'3)R3O0 structure by slight heating or additional sulfur doses. Thereby, nucleation sites for the formation of the 0, adlayer can be formed easily, and no defects in the Pt/a sulfur substrate are necessary for p1 adsorption. 4. The renewed capability to adsorb sulfur, which is induced in the saturated P1 adlayer by Ar+ bombardment, is caused by defects produced in the 0, adlayer, probably by breaking sulfur chains. These open ends enable further adsorption until they are deactivated by recombination. 5. Adsorption and Desorption of the pz State on Pt( 100) and Pt(ll1). Series of 0-state desorption spectra with various initial sulfur coverages are shown in Figures (16) Laitinen, R. S.: Pakkanen, T. A.: Steudel, R. J. Am. Chem. SOC. 1987, 109,710. (17) Gardner, D. M.; Fraenkel, G. F. J. Am. Chem. SOC.1956,78,3279. (18)Steudel, R.Z.Anorg. Allg. Chem. 1981, 478, 139. (19)Auer, M.;Leonhard, H.; Hayek, K. Appl. Surf. Sci. 1983,17,70.
1I
20
0
400
500
t [a' T
E
Figure 10. pz desorption from Pt(100): mass filter intensity at various amu after dosing 50 X 1014S atoms/cm2 onto the unannealed c(2x2)-a adlayer. Heating rate: 11 K/s.
3 and 4 for the Pt(lOO)-hex and the Pt(100)-(lXl) surface as well as in Figure 8 for the Pt(ll1) surface. On Pt(lll), the p2 state starts to be formed during dosing when the 0, adlayer is nearly completed as can be seen from the evolution of the 0, and P2 peaks in Figure 8, whereas on Pt(100) the population of both states overlaps depending on the substrate conditions (Figures 3 and 4). With the substrate a t temperatures >300 K, the saturation of the p2 adlayer was not obtained. Slow desorption occurred even at room temperature. The desorption peaks in Figures 3,4, and 8 correspond to the 64 amu (S2+) signals, but only the 0, peaks represent S2desorption rates. The P2 signal at 64 amu results mainly from fragmentation of larger sulfur molecules in the ion source. Thermal desorption of the p2 state produces S,+ signals with x = 1-8 (Figures 10 and 11) for both faces. The search for S,+ with x > 8 was prevented by the restricted mass range of the quadrupole mass spectrometer. The desorption spectra in Figures 10 and 11 are not corrected for contribution from fragmentation in the ion source. The largest observed S,+ signal, Le., s8+(which appears at higher initial concentration with high intensity), is certainly due to desorbing s8 molecules because noticeable desorption of S, molecules with x > 8, which could produce sa+fragments, appears improbable when the equilibrium composition of sulfur vapor is c ~ n s i d e r e d . ' ~ ~ ~ Two further types of S, molecules can be identified as desorbing species for both faces on comparing the various peaks. The different shapes and positions of the &+,S7+, and S8+ signals (Figures 10 and 11)show also that s6 and S7 are desorbing molecules. Whether the S,+ signals with x = 2-5, shown in Figures 10 and 11,are caused exclusively (20) Drowart, J.; Goldfinger, P.; Detry, D.; Rickert, H.; Keller, H. Adu. Mass Spectrom. 1967, 4, 506.
996 Langmuir, Vol. 5, No. 4, 1989
Gutleben and Bechtold
I
0
I
40
I I
400
20
500
t
w
TrK'
Figure 11. p2 desorption from Pt(100): mass filter intensity at various amu after dosing 150 X 1014S atoms/cm2 onto the unannealed c(2X2)-a adlayer. Heating rate: 11 K/s.
by fragmentation cannot be decided on the basis of present experiments. Considering the equilibrium composition of sulfur corresponding to the desorption temperature and to the densities of the desorbing beams, sulfur molecules S, with x C 6 should be only minor components in the desorbing sulfur beam. On P t ( l l l ) , at low initial &-state concentration only a small, broad S2+peak (and its S+ fragmentation signal) occurs, which can be ascribed to desorbing S2 molecules. On Pt(100), the shapes of the pz desorption traces and the amount of the various desorbing species were found to be influenced by the molecular composition of the dosing beam and by the amount of adsorbed &-state sulfur, which in turn is determined by the perfection of the primarily formed c(2X2)-S structure (section C.2). p2 desorption spectra of S8 molecules obtained after dosing preferentially s6, s,, and S8 molecules onto the annealed (830 K) and the unannealed Pt(100)-S-c(2X2) surface are shown in parts a and b of Figure 12, respectively, for various exposures. On both surfaces, two peaks occur at 370 K (11) and 410 K (I), but on the annealed 42x2) structure, which exhibits only a small capacity for the adsorption of the & state, the peak at 370 K is much more pronounced. On the unannealed surface, a t first peak I1 evolves a t 370 K with increasing exposure, but at higher exposures peak I (410 K) grows faster and peak I1 even decreases. Since a calibration of the mass filter sensitivity for the different sulfur molecules S, was not done, the quantitative composition of the beam desorbing a t 370 K (11) and that desorbing a t 410 K (I) is unknown. Comparing the areas of the S8 peak and the peak with integral mass range (amu L 96) for the same sulfur exposure (peaks 4 and 7 and peaks 6 and 8, respectively, of Figure 12b), one can conclude that the desorbing sulfur beam at 410
Figure 12. Pt(100): p2 desorption peaks at amu 256 (S8+,solid lines) and integral intensity 96 amu 5 m 5 256 amu (dotted lines) after dosing predominantly S, molecules with 6 5 x 5 8 onto a (a) c(2X2)-a adlayer, annealed at 830 K, sulfur exposure for peaks 1-5 (30,49,115,180, and 180 X 1014S atoms/cm2) (b) c(2X2)-a adlayer, not annealed, sulfur exposure for peaks 1-8 (14, 21,29, 49,120,170,48, and 170 X lo1*S atoms/cm2). Crystal temperature during sulfur adsorption, 310 K; heating rate, 11 K/s.
0 6
10 I
400 1
20
t
500
[SI
T [K]
1
Figure 13. Pt(100): pz desorption peaks at 256 amu (Sg+, solid lines) and integral intensity 96 amu 5 m 5 256 amu (dotted lines) after dosing predominantly Sz molecules onto a (a) c(2X2)-a adlayer, annealed at 830 K, sulfur exposure for peaks 1-4 (115, 180,360, and 180 X 1014Satoms/cm2) and (b) c(2X2)-a adlayer, not annealed, sulfur exposure for peaks 1-6 (29,49,110,170,49, and 170 X l O I 4 S atoms/cm2). Crystal temperature during sulfur adsorption, 310 K, heating rate, 11 K/s.
K (I) mainly contains S, molecules with 3c # 8, whereas at 370 K (11) mainly Ss molecules desorb. With the assumption that the s,+signals x C 6 in Figures 10 and 11 are exclusively caused by fragmentation, it even seems as if sulfur desorbing at 370 K solely consists of S8molecules when the shape of the s 6 , S7,and S8 peaks are considered. The comparison of the peak series in Figure 12 with that in Figure 13, where a dosing beam containing preferentially
Adsorption of Sulfur on Pt(100) and P t ( l l 1 )
S2molecules was used, shows that only peak I1 a t 370 K for the annealed surface is influenced markedly by the type of dosed molecules. For the annealed c(2X2)-S structure, which can be covered only to a limited extent with the p1adlayer, the large S8peak (11) obtained a t 370 K (Figure 12a) seems to result from S8 molecules adsorbed directly on the a adlayer, whereas peak I should correspond to P2-state sulfur molecules adsorbed on surface regions covered with the p1adlayer. The decrease of peak I1 on the unannealed surface at higher dosages (Figures 12b and 13b) seems to be caused by the growth of the P1 adlayer into regions already covered with &-state sulfur. The simultaneous population of the p1and P2 states during dosing (Figures 3 and 4) can be interpreted by a patchy surface structure which is formed because the growth of the p1adlayer starts only on defects in the c(2X2)-S structure. In contrast, on Pt(ll1) the p1and & states are formed successively (Figure 8), since the p1 state can be formed everywhere on the surface. The kinetic evaluation of the p2 desorption traces cannot be done in a simple way. It would require calibration and correction of the MS signals for contributions from fragmentation in the ion source as well as the consideration of rearrangements and of interconversions of sulfur molecules occurring in the adlayer during the desorption experiment. Neglecting the latter, the Sa+ peak was evaluated under the assumption that the signal corresponds to desorbing Sa molecules. For Pt(100), the evaluation of the zero-order leading peak edge (Figure 12a) yielded a desorption energy of 22 kcal/mol. An analogous evaluation for Pt(ll1) resulted in 19 kcal/mol. Comparison with the heat of vaporization of the S8 molecules (26 f 2.7 kcal/ mol)'O supports the identification of the p2 adlayer as a physisorbed state. Sulfur adlayers with thermal stabilities similar to that of the p2 state are formed also on tungsten.21*22 For tungsten, the analysis of the adlayer composition by field desorption mass spectrometry22yielded S, molecules with x = 2-22 but signals for x > 8 with low intensity, which indicated a physisorbed layer with liquidlike composition.n The comparison with tungsten suggests that the molecular species found in the P2 thermal desorption peaks for platinum are at least in part present already in the adlayer and are not formed during desorption. Hence the P2 state (21) Davis, P. R.; Bechtold, E.; Block, J. H. Surf.Sci. 1974,45, 585. (22) Cocke, D.L.; Abend, G.; Block, J. H. J. Phys. Chem. 1976,80,524.
Langmuir, Vol. 5, No. 4, 1989 997
appears to consist mainly of various cyclic sulfur molecules which are known to occur also in saturated sulfur vapor.20 The presence of small amounts of larger sulfur molecules (S, with x > 8) in the adlayer cannot be excluded by the present experiments. The observed adlayer composition does not correspond to equilibrium, as can be seen from its dependence on the composition of the dosing beam, but it represents an intermediate reaction stage of the impinging sulfur molecules toward the equilibrium adlayer.
D. Summary On Pt(100) as well as on P t ( l l l ) , sulfur forms two weakly bound adsorption states (8, and P2) in addition to an atomically adsorbed state. The p1 and p2 states are clearly distinguishable from each other by their desorption temperatures and by the types of desorbing molecules. The p2 state is desorbed already near 400 K. S6,S, and Sa molecules were identified in the desorbing beams. On account of the desorption temperature and the types of desorbing molecules, the p2 adlayer is considered to represent a physisorbed state, consisting of molecules similar to that present in saturated sulfur vapor. The p1state is desorbed near 700 K, yielding exclusively S2molecules. A number of observations indicated that the p1adlayers do not attain internal equilibrium, neither after dosing at 300 K nor during desorption. On Pt(100), defects in the initially formed c(2X2)-S structure are prerequisite for the adsorption of sulfur in the p1state. An annealed, well-ordered Pt(lOO)-c(2X2)-S surface exhibits only a small adsorption capacity for the p1state. In contrast, on Pt(ll1) the weak lateral localization of chemisorbed sulfur atoms enables the formation of the p1 state also on the perfect Pt(lll)-(d3Xd3)R3O0-S surface. Alternating sulfur dosing and Ar+ ion bombardment allows high sulfur concentrations to build up, which correspond to several monolayer contents. On the basis of these observations, it is proposed that sulfur-sulfur bonds are present in the p1adlayer leading to chainlike molecules which are chemisorbed at their ends. van der Waals interactions along the sulfur chains with the underlying dissociatively adsorbed a-state sulfur are supposed to enhance the adsorption strength. Acknowledgment. This work was supported by the Fonds zur Forderung der Wissenschaftlichen Forschung, Austria. We are indebted to Dr. H. Leonhard for the construction of the temperature controller. Registry No. S, 7704-34-9; Pt, 7440-06-4.
