J . Phys. Chem. 1989, 93, 815-826
815
Ptoblng Ensemble Effects In Surface Reactions. 2. Benzene Adsorption on Clean and Bismuth-Covered Pt(ll1) J. M. Campbell, S. Seimanides, and C. T. Campbell* Chemistry Department, Indiana University, Bloomington, Indiana 47405 (Received: December 17, 1987; In Final Form: August 4, 1988)
The interactions of benzene with the clean and Bi-dosed Pt( 111) surface have been studied between 110 and 850 K with a combination of thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Below -350 K, benzene adsorbs molecularly. The first monolayer saturates at a coverage of 0.16 (molecules per Pt atom). About 55% of this dehydrogenates upon heating to liberate H2in a series of steps between 450 and 800 K. This leaves residual carbon on the surface in a graphitic overlayer. The remaining benzene desorbs molecularly at -505 K (Ed N 30.8 kcal/mol) and 350 K (Ed N 21 kcal/mol). Substantial isotopic scrambling is seen in TDS from coadsorbed mixtures of C6H6 and C6D6. The activation energies for dehydrogenation of perdeuterated benzene are 1.2 kcal/mol larger than for C6H6 When benzene is coadsorbed with bismuth adatoms, the competition between dehydrogenation and molecular desorption is strongly influenced. Dehydrogenation is almost completely suppressed by Oei = 0.15, with a corresponding increase in molecular desorption. Since the activation energies for desorption and dehydrogenation are not strongly influenced by such low Bi coverages, this result is attributed to the steric blocking by Bi of free Pt sites needed for dehydrogenation. On the basis of these results and kinetic modeling, it is estimated that an ensemble of 1 6 free Pt atoms is required for the dehydrogenation of an adsorbed benzene molecule (in addition to the -6 Pt atoms needed to accommodate the benzene molecule itself). At higher Bi coverages, steric and electronic effects of Bi manifest themselves as the desorption temperature of benzene shifts to lower temperature, and its C(1s) XPS peak shifts to higher binding energy.
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I. Introduction The interaction of benzene with metal surfaces has been a subject of ongoing research for quite some time. It is the simplest aromatic compound and is involved in many important reactive processes on Pt catalysts. Of particular interest have been questions of surface bonding and geometry, electronic and structural influences upon chemisorption, surface reaction mechanisms, and catalytic reaction kinetics. The interaction of benzene with a clean Pt( 111) surface has been previously studied by a number of authors.'-'4 In general, benzene appears to be molecularly adsorbed up to 300 Desorption of both H2 and benzene are observed above 400 K at low benzene coverages's2 suggesting a competition in adsorbed benzene between dehydrogenation and desorption. Benzene is thought to chemisorb parallel to the P t ( l l 1 ) surface with little distortion of the molecular geometry (in the absence of coadsorbed CO).4911-i4The present study helps to further clarify the interaction of benzene with Pt( 111) with the aid of quantitative XPS and TDS measurements. Ensemble effects are thought to play a major role in controlling the selectivity of bimetallic catalysts (ref 16 and references therein). A major objective of the present study was to determine the ensemble (site-size) requirements for the chemisorption and dissociation of benzene on the Pt( 111) surface. For that purpose we have employed Bi as a site blocker in coadsorption experiments with benzene. It has previously been shown that Bi has great potential as a steric site blocker since it spreads uniformly across the Pt( 11 1) surface, is itself very inert in chemisorption, and has the same electronegativity as Pt and should therefore have a minimum of electronic effects on the chemisorption properties of the surface (ref 15 and references therein). The present study shows that adsorbed Bi is indeed an excellent choice since low coverages of Bi inhibit the dissociation of benzene by steric site blocking, without causing significant electronic effects on the Pt sites that chemisorb and dehydrogenate benzene. This present study is part of a series of papersI6l8 where the influence of bismuth on the chemisorption and dehydrogenation of several cyclic hydrocarbons has been studied on the Pt( 111) surface. 11. Experimental Section The experiments were performed in an ultrahigh vacuum (UHV) chamber pumped by a turbomolecular pump and a titaK . 1 9 2 9 5
Alfred P.Sloan Research Fellow. To whom correspondence should be addressed.
0022-3654/89/2093-08 15$01.50/0
nium sublimation pump. The system routinely had a base pressure at the start of the day of 3 X lo-" Torr. After several doses of Torr. benzene the vacuum later in the day was typically 2 X The chamber contained a Leybold-Heraeus LH- 10 hemispherical electron energy analyzer for Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS). It contained an electron gun and a dual-anode (Mg and Al) X-ray source, both incident 60° from the surface normal. It contained a Leybold-Heraeus noble-gas ion sputter gun, also incident 60' from the surface normal, which was modified so that the argon-ion flux was 30-fold higher for a given argon pressure rise in the chamber. This enhancement also enabled us to perform ISS with this inexpensive ion gun. All XPS spectra were collected with the Mg X-ray source. The spectrometer energy was calibrated by using XPS and XAES transitions for pure Ag.I9 Our XPS binding energy scale was
(1) Tsai, M. C.; Muetterties, E. L.J. Am. Chem. SOC.1982, 104, 2534. (2) Abon, M.; Bertolini, J. C.; Billy, J.; Massardier, J.; Tardy, B. Surf.Sci. 1985, 162, 395. (3) Ogletree, D. F.; Van Hove, M. A,; Somorjai, G. A. Surf. Sci. 1987, 183, 1. (4) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf.Sci. 1978, 78, 577. (5) Garfunkel, E. L.; Farias, M. H.; Somorjai, G. A. J. Am. Chem. SOC. 1985, 107, 349. (6) Abon, M.; Billy, J.; Bertolini, J. C.; Tardy, B. Surf.Sci. 1986, 167, L187. (7) Gland, J. L.; Somorjai, G. A. Surf.Sci. 1973, 38, 157. (8) Stair, P. C.; Somorjai, G. A. J . Chem. Phys. 1977, 67, 4361. (9) Mate, C. M.; Somorjai, G. A. Surf.Sci. 1985, 160, 542. (10) Johnson, A. L.; Muetterties, E. L.; Stohr, J. J . Am. Chem. Soc. 1983, 105. 7183. (11) Horsley, J. A.; Stohr, J.; Hitchcock, A. P.; Newburg, D. C.; Johnson, A. L.; Sette, F. J . Chem. Phys. 1985, 83(12), 6099. (12) Anderson, A. B.; McDevitt, M. R.; Urbach, F. L. Surf. Sci. 1984, 146, 80. (13) Somers, J.; Bridge, M. E.; Lloyd, D. R.; McCabe, T. Surf. Sci. 1987, 181, L167. (14) Netzer, F. P.; Matthew, J. A. D. Solid State Commun. 1979,29,209. (15) Paffett, M. T.; Campbell, C. T.; Taylor, T. N. J . Chem. Phys. 1986, 85, (IO),6176. (16) Campbell, C. T.; Campbell, J. M.; Dalton, P. J.; Hem, F. C.; Rodriguez, J. A,; Seimanides, S . G. J . Phys. Chem., first of four papers in this issue. (17) Rodriguez, J. A.; Campbell, C. J. J. Phys. Chem., third of four papers in this issue. (18) Hem, F. C.; Dalton, P.J.; Campbell, C. T. J. Phys. Chem., fourth of four papers in this issue.
0 1989 American Chemical Society
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referenced to the Pt(4f7/2)peak for clean Pt( 11l ) , which we set at 70.90 f 0.04 eV binding energy.I9 At the pass energy (100 eV) which we used for the XPS spectra reported here, the narrowest peaks showed a full width at half-maximum (fwhm) of 1.65 f 0.05 eV, which we take as the overall instrumental resolution. All reported XPS peak positions were determined from the center of the fwhm and are accurate to within f0.04 eV (95% confidence limits). A linear background between the limits of a fixed window was subtracted from the XPS peaks in determining these areas or integrated XPS intensities reported here. The approximate window values used for the C(ls), Pt(4f), and Bi(4f) peaks were 287.5-279.75, 78.5-67.5, and 165.5-154.5 eV of binding energy (BE), respectively. These Pt and Bi windows cover both the 4f5/2and the 4f712peaks. All XPS, AES, and ISS spectra were collected with detection normal to the surface. Since the XPS spectra are not angular-integrated, there may be some error in the quantitative coverages determined here by XPS. The magnitude of this error is generally small, and as a consequence fixed-angle XPS intensities are frequently used by many laboratories for quantitative elemental analysis. Favorable comparisons between the absolute coverages of hydrocarbons determined here and in our accompanying papers1'J8 with results in the literature help substantiate our values. The system also contained a quadrupole mass spectrometer with an ion source located line-of-sight -4 cm from the surface at 60' from the surface normal. This was used for thermal desorption mass spectroscopy (TDS). Six masses could be followed simultaneously with a computer interfaced to the mass spectrometer. Two of these masses were always followed simultaneously with X-Y recorders as well. The TDS experiments were performed using a feedback circuit in which the heating rate was held constant at 0.286 mV s-l on the chromel-alumel thermocouple. This leads to a temperature rate of 11 to 7 K s-l between 120 and 250 K and to a constant value of 7 K s-l above 250 K. The typical temperature range monitored during the TDS experiments was between 100 and 860 K. The TDS spectra presented here are presented in raw form, with no background subtraction. The background intensity from the backs and edges of the crystal and sample holder was almost insignificant (see below). The system also contained a Bi vapor deposition source constructed as described previously.20 We found that the Bi in the resistively heated Ta boat could be mounted vertically and, upon melting at 544 K, the Bi did not drop off the Ta. We used a piece of 99.99% pure Bi, initially 6 mm X 5 mm X 2 mm in size, which was larger than in our previous designs. This gave a dose rate of 9.4 X 1013 Bi atoms cm-2 min-' onto the Pt( 11 1) surface at a doser temperature of 71 1 K, which was reproducible to within -2% over a period of several months. For dosing benzene, we generally used a cosine-emitting pinhole doser such as described previously.21 This was located -8 cm from the sample and gave an enhancement factor of 2.8 compared to dosing only from the background pressure. This enhancement factor was taken into consideration in reporting the exposures here. The exposures reported here do not take into consideration the ion gauge sensitivity of benzene, which is approximately 6 times that of N2.22 This correction was, however, used in calculating the sticking probabilities reported here. The benzene was of research grade purity, further purified by freeze/pump/thaw cycles. Its purity was proven in situ with mass spectroscopy. The Pt single crystal was cut and mechanically ground to within 0.5O of the (1 11) surface, mechanically polished to a mirror finish, and finally briefly (-3 s) etched in boiling aqua regia. The UHV cleaning procedure consisted of many (- 100) cycles of heating (19) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook
of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: M N , 1978. (20) Paffett, M. T.; Campbell, C. T.; Taylor, T. N. J . Vac. Sci. Technol. A 1985, 3, 812. (21) Campbell, C. T.; Valone, S.M. J . Vac. Sci. Technol. A 1985,3,408. (22) Summers, R.L. NASA Technical Note TN-D-5285; National Aeronautics and Space Administration: Washington, D.C., June, 1969.
Campbell et al. r , Bi(4f
712
)
I
40
I
1.98
1.M l.W
0.83
166
I&
162
1W
158
156
154
Bindug Enugy/eV
Figure 1. Bi(4f512) and Bi(4f7/,) XPS spectra for various Bi doses (coverages) on Pt(ll1) at 100-120 K.
in 2 X lod Torr of O2at 750-1000 K for -2 min followed by Ar+ ion bombardment (3 kV) at temperatures up to 1300 K. The sample initially showed very large amounts of nondesorable oxygen on the surface after heating in 02,which indicated the oxidation-induced surface segregation of unusually large concentrations ~ - ~found ~ that this subsurface impurity of bulk Si i m p ~ r i t y . ~We could most rapidly be segregated to the surface for sputter removal by heating in O2 at 1000 K, which is considerably hotter than normally ~ r e s c r i b e d .Eventually, ~~ the buildup of nondesorbable oxygen during an O2dose was reduced to a negligible level, and the Pt(ll1) surface showed reproducible CO, 02,and H2 thermal desorption spectra that matched those expected from the literaThe very small size of the high-temperature (520 K) shoulder on the CO TDS spectra indicated a very high quality Pt( 111) surface with very minimal (12%) contributions from defect sites such as steps.26 Bismuth was vapor-deposited only on the front face of the Pt( 111) sample, and a coverage BBi = 0.56 (which corresponds to a closely packed monolayer15)was sufficient to suppress the saturation CO TDS peak area to .o
OW
00
0.2
0.4
0.6
0.8
1.0
'Ei
0.0
Figure 8. Summary of the influence of Bi precoverage on the areas of
the TDS spectra following various benzene doses to Pt(ll1) at 110 K (A) 0.34langmuir (same as in B); (B) 0.74langmuir: (a) H2 (A),(b) Pt-bonded benzene, desorbing ,asthe a and 0 states between 250 and 570 K (a), (c) Bi-bonded benzene, desorbing at -190 K (0). (Scaled to absolute benzene coverage units as described in text.) The H2 curve
0.2
0.1
'Bi
Figure 9. Effect of
&* on the dissociation probability of benzene and
reflects the probability for dissociation of adsorbed benzene during the TDS, as indicated on the right vertical axis; (C) properly scaled sum of H2 and Pt-bonded benzene TDS peak areas. This reflects the total chemisorption capacity of the surface for molecularly chemisorbed benzene on Pt sites at 110 K since both curves are for benzene exposures (0.74and 1.16 langmuir) which saturate the first monolayer. The solid line through these data represents a model described in the text which requires five free Pt atoms to adsorb each benzene molecule.
perdeuterated benzene in the limit of low C6H6 and C,D6 exposures (0.12 langmuir) to Bi/Pt(l 1 1) at 110 K. The dissociation probability at 8Bi* = 0, Po,was 10.95 for C6H6and ~ 0 . 9 for 0 C6D6. We have subtracted from these data the constant H2 background seen at high eBi* and attributed to the sample holder as described above. The solid lines are theoretical fits to the data, indicating that a minimum of six free Pt atoms are required for the dehydrogenation of adsorbed benzene (see ref 16 for details). Also included here is the influence of Bi upon the probability that adsorbed benzene will dehydrogenate only in the temperature region marked A in Figure 10. (This is simply the properly scaled integrated intensity of the A region in the H2 or D2 TDS spectra used here.)
as OBi* increases to 0.35. Thereafter, benzene adsorption on Pt sites is suppressed a t the expense of adsorption on Bi sites. The H2 peak area here can be directly taken as proportional to the dehydrogenation probability for adsorbed benzene during TDS. Since there is no measurable molecular desorption at this exposure in the absence of Bi this probability is 1.0 for OBi = 0. The TDS spectra of benzene and H2 at an exposure of 0.74 langmuir shown in Figure 7 as a function of OBi* are very similar to those of Figure 6. Here, however, the molecular desorption peak CY appears already even in the absence of Bi. Again dehydrogenation (H,) dies off quickly with increasing OBi*, and by OBi* = 0.25 the m / e = 2 intensity is completely suppressed to a broad, structureless background that is due to not the Pt( 111) surface but to the sample holder. As dehydrogenation decays with Bi addition, there is a corresponding increase in molecular desorption of benzene, first from the Pt-bonded sites. Again, chemisorption at the Pt-bonded sites is blocked by Bi at the expense of adsorption on Bi sites. These changes with Bi precoverage in the TDS peak areas after a 0.74-langmuir dose are again quantitatively summarized in Figure 8 (center). The changes in the TDS peak areas here are qualtitatively quite similar to those discussed previously at 0.34 langmuir, except that the changes occur a t slightly lower Bi coverage for this higher benzene coverage. This difference can be attributed to the fact that the additional adsorbed benzene here blocks Pt sites in a way similar to that of Bi itself and can be considered as an additional increment of "effective" Bi coverage. The influence of Bi upon the total adsorption capacity of Pt(111) for chemisorbing (Pt-bonded) benzene at 100-120 K is shown in the top panel of Figure 8. This curve reflects the scaled sum of both dehydrogenated benzene (the H2TDS peak area) and benzene which desorbs molecularly above 250 K (i.e., from Pt-bonded sites). Both exposures used here (0.9 and 1.16 lang-
muir) were sufficient to saturate the chemisorbed monolayer on clean P t ( l l 1 ) as can be seen by comparison with Figure 4. The benzene chemisorption capacity dies smoothly with OBi* and is almost completely suppressed by OBi* = 0.6-0.7. We should point out that in all the desorption spectra of Figures 6 and 7, the desorption of H2 and benzene was completed before any Bi itself desorbed from the surface. These spectra therefore represent constant Bi coverages. At a coverage OBi* = 1 , Bi desorption begins above 740 K, and at lower OBi*, Bi desorption starts only considerably above 800 K.lS The experiments of Figures 6-8 were reproduced at lower benzene exposures to observe the influence of Bi upon dehydrogenation in the absence of any self-poisoning by coadsorbed benzene molecules. Figure 9 shows the dependence of the dehydrogenation probability upon OBi* for TDS following 0.12langmuir benzene exposures to Bi/Pt( 111) at 110 K. As can be seen by comparison to Figure 8A, this result is almost identical with that at 0.34-langmuir exposure, which verifies that the result of Figure 9 is truly for the limit of low benzene coverage, where self-poisoning is negligible. For comparison, we include in Figure 9 the result for an identical (0.12 langmuir) dose of perdeuterated benzene (C&), where the dehydrogenation probability was determined as in Figure 8, except here by using the D2 TDS peak areas instead of the H2 areas. Representative D2 TDS spectra for this experiment are shown in Figure 10. As can be seen in Figure 9, the dehydrogenation of perdeuterated benzene is poisoned much more rapidly with Bi than is that of normal benzene. A major result of Figures 6-10 is the fact that benzene dehydrogenation is almost completely suppressed by OBi* = 0.25, before any significant electronic influences of Bi are manifested in, for example, the activation energies for benzene desorption or dehydrogenation. Notice in particular that the dehydrogenation spectra ( m / e = 2) are suppressed uniformly in intensity without
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
822
A
Campbell et al.
1I
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C(l8) XPS
(\ 0.74L C6H6/Bi/Pt( 11 1) ,220K
2:
4 Dz/O 1 2 L C g D 6 / B i / P t ( l l l )
1
I
0 00 3 2: 0 4.2 0 52
18 -
104 287
288
286
285
204
283
202
281
280
278
Binding Energy/eV
Figure 12. C(1s) XPS spectra for various Bi precoverages after heating the 110 K, 0.74-langmuir benzene exposures of Figure 10 to 220 K to remove all multilayer and Bi-bonded benzene. The spectra therefore represent only the Pt-bonded benzene. TemperaturelK
Figure 10. Representative spectra showing the effect of Bi coverage upon the D2TDS spectra follow a 0.12-langmuir exposure of CsD6to Bi-dosed Pt(ll1) at l l O K . C(ls) XPS
22
I
74L C6H6/Bi/Pt(lll),l10 K
-13
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+
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.
0 52 0 42 0 21
288
-~ 287
28e
. 285
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280
Z ~ Q
Binding Energy/eV
Figure 11. C(1s) XPS spectra following a benzene exposure of 0.74 langmuir to F’t( 11 1) at 100-120 K containing various precoverages of Bi. substantial (nonbackground related) changes in their line shape or temperature. The major H2 desorption regions also do not change their relative area ratios A:B:C very much from that of the clean surface as Bi is added. (An exception to this is a noticeable decrease in the A:B ratio, to be discussed below.) Similarly, the molecular desorption spectra for OBI* C 0.25 are almost identical with those in Figure 3 for clean Pt( 111) at benzene exposures that gave equivalent extents of molecular desorption. Compare, for example, the m / e = 78 curves in Figure 6 at &I* = 0.05, 0.10, and 0.21 with the curves of Figure 3 at 0.38, 0.46, and 0.7 1 langmuir, respectively. By directly comparing many curves of this type, we have determined that the TDS peak temperatures for desorption and dehydrogenation shifted by I 1 5 K in the coverage range below OBI* = 0.25. These results highlight the dominant importance of ensemble effects compared to electronic influences in the mechanism by which Bi poisons the dehydrogenation of adsorbed benzene. That is, the major role of Bi in this coverage range is to sterically block free Pt sites, which are necessary for abstracting hydrogen atoms from adsorbed benzene during its dehydrogenation. We wish to emphasize that the lack of strong electronic effects outlined above are limted entirely to the coverage range below OBI* = 0.25. When OBI* > 0.25, electronic or geometric effects due to Bi appear as the benzene desorption peaks shift substantially to lower temperature. These are discussed below. 2. X-ray Photoelectron Spectroscopy. Figure 11 presents a series of C(1s) XPS spectra taken following a benzene exposure of 0.74 langmuir to P t ( l l 1 ) at 100-120 K with various Bi pre-
coverages. The C(1s) peak area is independent of e&*, proving that the sticking probability is independent of bismuth precoverage. The C ( 1s) peak shifts to higher binding energies with increasing Bi coverage when OBi* exceeds 0.2. This is an indication of the difference between the Pt-bound and the Bi-bound benzene. In particular Bi-bound benzene has a C ( 1s) binding energy that is approximately 1.1 eV higher than Pt-bound benzene. Similar shifts in the C( 1s) spectra have been seen in all of our other studies of coadsorption of cyclic hydrocarbons with Bi.17J8 These shifts are explained in a separate paperI7 by using a model similar to that used by Wandelt to explain the photoemission of adsorbed xenon (PAX).33 The C( 1s) XPS spectra in Figure 12 are for the same exposures as shown in Figure 11, but after a flash to 220 K to remove all Bi-bound adsorbed benzene. The remaining C ( 1s) spectra show an area that decreases with OBi* very similarly to that for total Pt-bonded adsorption as measured by TDS and reported in the top part of Figure 8. The spectra for Pt-bound benzene show only slight shifts of binding energy with OBi. The small magnitude of these shifts attests to the fact that there are only small electronic influences of coadsorbed Bi upon the Pt sites that chemisorb benzene. This fact was perhaps more sensitively proven by the TDS spectra above. Since Bi induces a much larger work function decrease on Pt( 11 1),l5these small shifts also show that, like Xe in PAX,33the C( 1s) of the adsorbed benzene tracks the “local” work function. D. Bismuth Postdosing Experiments. In an effort to characterize the species of stoichiometry C6H3(or 3C2H) formed at 550 K from adsorbed benzene on clean Pt(l1 l ) , we performed experiments where various Bi coverages were postdosed to the surface at 120 K after the adsorbed benzene had been flashed to 550 K to form this species. Independent of the Bi postdose coverage in the range explored (OBi* I0.31), no hydrocarbon species were observed desorbing in a subsequent TDS. The evolution of H2 above 550 K was hardly effected by the Bi postdose. These results show that either the effective ensemble requirement for the dehydrogenation of this species is considerably smaller than that for benzene itself, or that the species has no accessible competing desorption products. This result contrasts with similar experiments with the partially dehydrogenated adsorbates formed from cyclohexane and cyclopentene on Pt( 11 l ) , where the Bi postdose suppressed dehydrogenation and led to desorbing hydrocarbons characteristic of the adsorbed intermediates.I7Js
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IV. Discussion Several researchers have investigated the chemisorption of benzene on Pt( 111) using TDS.’**v5 The line shapes of our H2 (31) Redhead, P.A. Vacuum 1962.12, 203. (32) Koel, B. E.;Crowell, J. E.; Bent, B. E.; Mate, C. M.;Somorjai, G. A. J . Phys. Chem. 1986, 90, 2949. (33) Wandelt, K.J . Vac. Sci. Technol. A 1984, 2, 802.
Benzene Adsorption on Bi/Pt( 11 1)
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 823
evolution spectra of Figure 3 are in good agreement with the previous studies.',* Tsai and Muetterties' in a room-temperature benzene adsorption study observed no molecular C6H6 desorption at low coverages and two benzene maxima at -385 and -485 K as the coverage increased, consistent with our results of Figure 3. Other authors have also reported similar molecular TDS peak temperatures but somewhat different relative intensities of the two statess2SsWith standard, first-order Redhead analysis31 and our desorption peak temperatures (505 and 350 K), the activation energies for desorption of the two molecular chemisorption (aand 0) peaks were found to be 30.8 and 21.0 kcal/mol, respectively (assuming a preexponential factor for desorption of lOI3 Garfunkel et aL5 published a series of TDS spectra for lowtemperature benzene adsorption that showed strong molecular desorption maxima at -280 and 190 K after large exposures. While the 190 K peak compares well with our multilayer peak, we do not report any peak at -280 K. We could generate such a peak with very large C6H6 exposures (>3 langmuirs) at 110 K. However, our C(1s) XPS measurements proved that this desorption peak did not originate from the Pt(1 l l ) surface, since the surface showed no additional C(1s) intensity (after flashing to 210 K to desorb the multilayer) above that seen for the 1.37langmuir exposure result of Figure 3. W e felt that this peak at -280 K was due to desorption of multilayer benzene from the sample holder as it warmed up during TDS heating. This anomalous peak did not appear to saturate with exposure, also consistent with this assignment. Our results indicate that benzene is molecularly adsorbed on clean Pt( 111) up to -400 K, above which dehydrogenation begins. By -570 K, each benzene molecule that did not desorb had lost an average of three hydrogen atoms. The final three hydrogen atoms are lost in a broad H2 evolution peak between 570 and 810 K. The thermal stability of adsorbed benzene is consistent with the conclusion that no dehydrogenation occurs bel(,w 320 K by Lehwald et al.4 based on vibrational spectroscopy. The partially dehydrogenated species remaining on the surface after the sharp 540 K H2TDS peak has a stoichiometry of two carbon atoms per hydrogen. This is consistent with the reinterpretation of vibrational spectra on Pt(ll1) offered by Koel et ahs2 who suggest that CCH units are the dominant adsorbed species remaining after heating adsorbed ethylene to 470 K on Pt( 111)34and other surface^.^^-^' The thermal decomposition of adsorbed benzene on Rh( 11 l ) , which in terms of TDS is similar to Pt(l1 l), has been studied with vibrational ~pectroscopy.~~ The results were interpreted to indicate that decomposition begins at -400 K by C-C bond cleavage to form three acetylenes, which rapidly dehydrogenate to a mixture of C2H, and CH, species. These fragments are then thought to dehydrogenate to C,", polymers above 500 K. The stoichiometry we observe at -570 K would certainly be consistent with the decomposition of benzene on Pt( 1 11) to produce three C2H, species. As benzene exposure increases, our results (Figures 3 and 4) show the growth of molecular benzene TDS peaks at 505 and then at 350 K. It appears that at these high coverages desorption begins to compete more effectively with dehydrogenation, perhaps due to the absence of free Pt sites necessary for the abstraction of H atoms in C-H bond cleavage. Our Bi poisoning results are also consistent with a considerable requirement of free Pt sites in dehydrogenating adsorbed benzene. It is interesting to note that molecular desorption occurs up to 525 K, which is considerably warmer than the temperature required to initiate benzene dehydrogenation (-470 K). Thus, the dehydrogenation appears not to be limited at high coverages by insufficient energy of activation but rather by the inavailability of sites. The H2 evolution peaks in TDS occur at temperatures of -460, -540 and 600-700 K. These reflect the kinetics of dehydrogenation and as such can be treated with a first-order Redhead
-
~~
(34) (35) (36) 464. (37)
~~~
Baro, A. M.;Ibach, H. J . Chem. Phys. 1981, 74, 4194. Demuth, J. E.;Ibach, H. Surf.Sci. 1978, 78, L238. Kesmcdel, L. L.;Waddill, G. D.; Gates, J. A. Surf.Sci. 1984, 138, Stroscio, J. A,; Bare, S.R.; Ho, W. Surf.Sci. 1984, 148, 499.
analysis.31 Assuming a preexponential factor of lOI3 s-I, these temperatures yield activation energies for dehydrogenation of 28, 33, and 37-43 kcal/mol, respectively. The lowest value can be compared to the activation energy for desorption of benzene for the a! state, which is 30.8 kkal/mol. At -500 K, where desorption and dehydrogenation are in strong competition, this difference in activation energies implies a ratio in the rate constants for dehydrogenation:desorptionof -20. This is consistent with the lower limit of 10 we placed on this ratio based on the uptake curves of Figure 4 (see above). For perdeuterated benzene, the D2 evolution peaks were shifted by -20 K to higher temperature than the corresponding H2peaks in TDS from C6H6. This increase implies an increase in the activation energies for dehydrogenation of 1.2 kcal/mol for C6D6 compared to those reported above for C6H6, using the same analysis as used there. (Here we have ignored possible differences in the preexponential factors.) This compares very well with the difference in zero-point energies of 1.1 kcal/mol of C-D compared to C-H stretching frequencies reported for C6D6 and C6H6 on Pt( 11l).4 This favorable comparison implies that C-H (or C-D) bond cleavage is occurring via elongation of a single C-H (or C-D) bond as part of the rate-limiting step in the dehydrogenation of adsorbed benzene. The above values suggest a primary kinetic isotope effect" in this dehydrogenation, whereby C6D6 dehydrogenation should be approximately 3.3-fold slower than that Of C6H6 at 500 K. Another result that can be understood qualitatively in terms of a primary kinetic isotope effect is the fact that the integrated intensity ratio A:B for the regions of Hz evolution was 1.5 times larger for C6H6 than for C6D6. Since the A region covers a lower temperature than the B region, its rate will be more strongly influenced by the difference in zero-point energies between C-H and C-D. As noted above, this difference favors C-H over C-D bond cleavage, so more H2than D2 is liberated in region A relative to region B. Tsai and Muetterties' found nothing but C6H6 within the molecular benzene TDS peak after dosing clean Pt( 1 11) at 293 K first with a low coverage of perdeuterated benzene (c6D6) followed by a much larger dose of normal benzene (C6H6). They interpreted this to indicate that there was no hydrogen exchange in adsorbed benzene. Abon et a1.2challenged this conclusion, since they found that C6H6 easily displaces C6D6,a on Pt( 111) even at 300 K. Our TDS results following the adsorption of an equimolar mixture of C6H6 and C6D6 on clean Pt( 111) show clearly that there is substantial isotopic scrambling in adsorbed benzene and that this mostly occurs after the onset of dehydrogenation (>420 K). Surnam et al.39saw rapid H-D exchange in the vibrational spectrum of benzene (C&) adsorbed on Pt( 110) when exposed to Dz below 350 K. Our XPS value for the saturation coverage of chemisorbed = benzene on clean Pt( 111) of 2.4 X lOI4 molecules/cm2 0.16) indicates that every adsorbed benzene molecule requires the equivalent area of about six Pt atoms on the surface. This value is in good agreement with published saturation coverages on Pt Tsai and Muetterties' reported a saturation coverage on the P t ( l l 1 ) surface based on AES results of about 2.3 X lOI4 molecules/cm2 for room-temperature benzene adsorption. Fisher et alSa have reported a saturation coverage of 2.8 X l O I 4 molecules/cmz for a Pt(100) surface held at 333 K during C6H6 adsorption; their result was also based on AES measurements. Vibrational spectroscopic studies have indicated that while C6H6 added to clean Pt/Al2O3 forms only 7r-bonded benzene, C6H6 added to Pt/A1203with ordered carbon residues forms both ?Fbonded and a-bonded benzenes.41 There is general agreement that benzene chemisorbs parallel to the Pt( 111) surface with little distortion of the molecular g e ~ m e t r y ! J ~ J ' J ~ -The ~ ~ C-C bond
-
-
(38) Gardiner, W. C.,Jr. Rates and Mechanisms of Chemical Reactions; W. A. Benjamin: CA, 1972; p 56. (39) Surnam, M.;Bare, S. R.; Hoffmann, P.; King, D. P.Surf. Sci. 1983,
-
126. . - -, 349 ...
(40) Fischer, T.E.;Kelemen, S.R.; Bonzel, H. P. Surf.Sci. 1977,64, 157. (41) Haaland, D. M.Surf.Sci. 1981, 1 1 1 , 555.
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The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
lengths were found to be within f0.02 A of the gas-phase value." (This result is apparently different when benzene is coadsorbed with C0.3) The maximum packing density of benzene molecules in a single plane using van der Waals radii of 1.2 8, for the hydrogen atoms42is 2.84 X lOI4 molecules/cm2. Our observed saturation coverage is slightly lower than this. The highest possible coverage where benzene molecules can be packed parallel to the surface without overlapping van der Waals radii but with the benzene molecules all sitting in identical adsorption sites on the This corresponds to the Pt( 11 1) surface occurs at 8C6H6= ( d 7 X d 7 ) R19O surface net, where each benzene molecule requires a hexagonal array of seven Pt surface atoms. Interestingly, our observed benzene coverage at an exposure that just saturates the a-TDS peak but does not yet populate the /3 peak corresponds to almost exactly this coverage. Thus, the transition from filling the a to the fi states, which results in a 30% decrease in the activation energy for molecular desorption, correlates with the compression of the benzene adlayer from a situation where all the molecules can find identical chemisorption sites on the surface, into an overlayer that must be incommensurate with the substrate sites. The resulting 10 kcal/mol decrease in the heat of adsorption thus may be attributed to this loss of registry between the molecules and favorable substrate sites and the corresponding repulsive lateral interactions that arise as the adlayer is compressed. No distinct LEED pattern has been confirmed for pure benzene adlayers on Pt( 11I), although detailed studies of this coverage transition have not appeared. The lack of observation of the d 7 structure suggested here may also be due to long-range disorder in the adlayer. Note that this net has three equivalent rotational domains, each of which has several possible origin sites, which would contribute to disorder. This system might provide an interesting test case for new crystallographic techniques that do not require long-range order. From XPS measurements we found that 55% of the saturated adlayer of chemisorbed benzene dissociates upon heating in TDS. This corresponds to 1.32 X lOI4 molecules/cm2. This value is close to the value of 1.7 X lOI4 molecules/cm2 determined for the "irreversible" saturation coverage (or dissociated coverage) of benzene on P t ( l l 1 ) by Abon et ala2using quantitative TDS and AES. From Figure 4, the C(273 eV)/Pt(237 eV) AES peakto-peak ratio is 0.45 for this saturated quantity of dissociated benzene, measured after flashing the full monolayer to 830 K. As noted above, this corresponds to 1.32 X loi4 molecules/cm2 or 0.526 carbon atoms per Pt surface atom (8, = 0.526). These results indicate that the relationship between Oc and the C/Pt AES ratio is given in the first monolayer by BC = 1.17[C(273 eV)/ Pt(237 eV)]. Thecalibration constant here differs by almost a factor of 2 from that determined by Davis et al.43using radiotracer techniques. This discrepancy is probably not due to differences in the transmission functions of the AES spectrometers since the transitions involved here are so close together in energy. The very good agreement between our values for the saturation coverage of molecularly adsorbed benzene (and other hydrocarbon^'^^^^) with those in the literature attests to the accuracy of the quantitative XPS method we use here for carbon coverage calibration. Since the AES method probes a carbon transition involving valence electrons, its line shape is expected to vary with the chemical nature of the molecule. Such line-shape changes can indeed lead to gross errors in using the peak-to-peak intensity in the derivative spectrum as a quantitative estimate of concentration. Differences in Auger detection angle may also contribute to the differences observed here. Our XPS results and, in particular, Figure 5 indicate that the progression from multilayer (e), to chemisorbed (b), to partially dehydrogenated (c), and finally to fully dehydrogenated (d) states of adsorbed benzene shows a small but distinct stepwise shift of the C( Is) peak to lower binding energy. The final carbon residue (42) C.R.C.Handbook ofchemistry and Physics; Weast, R. C., Ed.,61st ed.; CRC Press: Boca Raton, FL, 1980. (43) Davis, S.M.; Gordon, B. E.; Press, M.; Somorjai, G. A. J . Vuc. Sci. Technol. 1981, 19, 231.
Campbell et al. after complete dehydrogenation shows a BE of 283.9 eV, which is very similar to that of bulk graphite (284.3 eVI9) and identical with that following complete dehydrogenation of other hydrocarbons on Pt(l1 l).17*18*51 It has been shown that this residue is indeed islands of a thin layer or even monolayer of graphitic carbon (ref 18 and references therein). Calculations by Anderson et aLi2 indicate that benzene should bond to Pt by electron donation from the r(el,) orbitals of benzene into empty Pt d orbitals. This bonding mechanism would be expected to be quite similar for a graphitic monolayer. The 0.7 eV lower BE observed for graphitic residue could be due to more efficient stabilization of the final-state core-hole by the more extensive r-electron system of graphite. If indeed the partially dehydrogenated species found at 570 K is CCH, as suggested above, nonequivalent carbon atoms are indicated. This is consistent with the extra width of this XPS peak (see above). One carbon atom of this species does not bond to Pt directly and therefore does not donate electron density to Pt as suggested for the carbon atoms of benzene. Thus, an initial-state chemical shift simply related to the charge on the carbon atoms can explain the higher C( 1s) binding energy for benzene as compared to its partially dehydrogenated product of stoichiometry C2H,. Although our C ( 1s) XPS peak positions for adsorbed hydrocarbons in this series of studies are consistent with the structural models proposed for the various adsorbates, they cannot in themselves be used to determine structures because the peak shifts are so small compared to the peak widths. Perhaps the most dramatic difference between species occurs when all of the carbons in an adsorbate are either sp2 or sp3 hybridized. In the former case (e.g., C-CsH6 and c-C5H5,I8the c ( 1s) peak appears at 284.2 f 0.1 eV, and in the latter case (e.g., c-C6HI2,l7d i - a - ~ - C ~ H ~ , ~ ~ and C - C ~ H ~ , ,it~ ~appears ) at 283.6 f 0.2 eV. Even here, the difference (0.7 eV) is rather small compared to the peak width, but it is large enough to be of potential use in future analyses. However, the main utility of XPS here has clearly been in quantitative analysis. A major conclusion of our results is that the dehydrogenation of adsorbed benzene is poisoned by lowcoverage Bi addition according to a mechanism dominated by ensemble effects whereby the Bi atoms free Pt sites required for C-H bond cleavage and accommodation of the resulting hydrogen adatoms. Thus, a relative Bi coverage of 0.21 is sufficient to decrease the amount of dehydrogenation of adsorbed benzene by a factor of at least 3 (Figures 8 and 9), with only minor shifts (I 15 K) in the TDS peak temperatures for molecular desorption or dehydrogenation from the clean surface values (see Figures 6 and 7). Thus, we can estimate that the activation energies for desorption and dehydrogenation of benzene change by less than -0.9 kcal/mol from their clean surface values when 8Bi* = 0.21 (using a Redhead analysis such as that used above to determine desorption energies). It is hard to imagine any real physical system showing a more subtle electronic influence than the type we have seen for Bi when 8 ~ i *< 0.21. At higher Bi coverages, a clear shift to lower temperatures is observed in the benzene molecular desorption peaks (beyond that expected from the sequential filling of states as seen on the clean surface). Ultimately, as the Bi monolayer completes, the benzene desorbs in a single peak at 190 K, which is attributed to benzene molecules bonded only to the Bi adatoms, similar to that from benzene adsorbed on pure Bi. Due to the large size of the benzene adsorbate (-6 Pt atoms in area), relatively low coverages of Bi (OBi* 1 0.3) may already prevent the benzene molecule from lying flat on the surface due to the presence of Bi adatoms. This may lead to weaker bonding geometries, manifested in a substantially lower desorption temperature in this coverage region. In these geometries, the benzene may have bonding interactions with both Pt and Bi atoms, leading to the intermediate desorption temperatures observed. The TDS peak shift at intermediate Bi coverages could also be attributed to repulsive lateral interactions between the Bi adatoms and the benzene molecules chemisorbed in their usual geometry to Pt sites. This could be due to dipole-dipole repulsions since both benzene2,' and BiI5 adsorption on Pt( 1 1 1 ) result in work-function decreases, indicative of
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 825
Benzene Adsorption on Bi/Pt(l 11) downward-pointing dipoles on the surface. This would be reminiscent of results by Garfunkel et aL5 for the coadsorption of potassium and benzene on P t ( l 1 l ) , where the dipole is 5-fold larger. The D2 TDS spectra of Figure 10 reflect the dehydrogenation kinetics of a very low coverage of perdeuterated benzene. Since there is much less D2 than H2in the background gas, these spectra are not complicated by desorption in the range 300-410 K due to background adsorption, as was the case with C6H6. This makes inspection of the A region of the spectrum very instructive. Note that, even for these low coverages, the A region makes only a very minor contribution compared to total dehydrogenation of C6D6 (A B region). The A region intensity is attenuated significantly faster by very small amounts of Bi than is the total. This can be seen in Figure 9, where the contribution of the A region to the total dehydrogenation probability of C6D6 is plotted as a function of 8Bi*. The very rapid initial decay might be tentatively interpreted in terms of a low concentration of defect sites being responsible for at least some of the dehydrogenation in the A region. If Bi then selectively decorates these defects, it could inhibit their dehydrogenation activity at lower coverage. A similar but more obvious effect was seen in the dehydrogenation of c - C ~ Hon ~,~ Pt( 11 1) in the accompanying paper.'* Blass et al.52 attribute similar structure in their H2TDS spectra from benzene on Ni( 100) to defect sites. Tsai and Muetterties reported the H2 TDS spectra from benzene on a Pt [6( 111) X (1 1l ) ] surface.' The amount of H2 evolution in the A region (420-490 K) was equal to that in the B region (490-570 K) for this highly stepped Pt( 111) surface, which further supports a model whereby the B region is related to step or defect sites. The independence of the integrated A:B intensity ratio upon benzene coverage (see Figure 3) is, however, hard to rationalize if the A region is dominated by dehydrogenation at defects, unless the defect sites are not preferentially filled. Notice in Figure 9 that the fraction of dehydrogenation that decays extra rapidly with 8Bi* in region A is very small relative to the total dehydrogenation probability (-6% of A B for C6D6 and 10% for C&). Thus, only a very small fraction (-&lo%) of the total dehydrogenation probability could be clearly attributed to such a defect-related mechanism. The benzene coverage is also E 0.024), so that a trivial concentration of very low here defect Pt atoms (
