Langmuir 1991, 7, 134-139
134
Coadsorption of Benzene and Hydrogen on Ru(001) P. Jakob and D. Menzel* Institut fur Festkorperphysik E 20, Technische Universitat Munchen, 0-8046 Garching b. Munchen, Federal Republic of Germany Received March 6, 1990. In Final Form: June 18, 1990 Coadsorption of benzene and hydrogen on the close-packed plane of Ru has been investigated. Using HREELS and TDS, we show that desorptionof hydrogen off the benzene-coveredsurfaceis indeed governed by the dehydrogenation process, as proposed earlier. The signal in question can be easily separated from the corresponding one of coadsorbed hydrogen, which desorbs at clearly lower temperatures. Only slight mutual influences exist in the vibrational modes of both coadsorbate species at 250 K. Clear evidence for isotopic exchange between Hadand at temperatures considerably below dehydrogenationis found by TPD as well as by vibrational spectroscopy. These exchange reactions are found to be fully reversible, but only a fraction of the hydrogen can be exchanged. Reaction mechanismsas well as possible explanations for this particular behavior are discussed.
I. Introduction The motivation for this study of the coadsorptionsystem benzene + hydrogen on Ru(001) was twofold. First, a proper understanding of the coverage-dependent dehydrogenation behavior of pure benzene/Ru(OOl)l requires knowledge about the mutual influences of coadsorbed benzene and hydrogen, in particular about the desorption kinetics of both molecules. Second, the study of exchange reactions between these (isotopically labeled) species is of interest in itself, as it yields information on the reactivity of these species in a range where there is not yet a gross reaction. In our earlier work,l a strong downward shift of the temperature of incipient Hz desorption with increasing benzene coverage was found. Two possible models were discussed for this effect. (i) Dehydrogenation couid be the rate-limiting step for desorption. Then its temperature could be strongly dependent on the initial coverage; possibly different reaction paths for dissociation could exist for low and high coverages. (ii) If the dehydrogenation temperature of adsorbed benzene were lower than the observed desorption temperature, then liberated hydrogen would first accumulate on the surface, until progressing saturation would lead to a sufficient desorption rate. The peak shift mentioned above would then result from a lower number of possible adsorption sites for Hadsa t high dbenzene, which would lead to high local H density even at low overall H coverage. In addition, the desorption kinetics could be influenced by the interaction of Hadsand the benzene fragments formed. In order to obtain a decision, coadsorption experiments (TDS and HREELS) of H2 and C6D6 have been carried out on the Ru(001) surface. The use of the two hydrogen isotopes serves to separate the signals originating from adsorbed hydrogen atoms and from the benzene species. Experiments at about half and close to full coverage of preadsorbed benzene (Obenzene 0.07 and 0.13 monolayer (ML);saturation corresponds to 0.15 ML) were performed at 130-250 K (the adsorption temperature is not crucial as long as it is lower than 350 K to avoid dissociation of benzene and higher than 170 K to prevent multilayer growth).2 These layers were then exposed to hydrogen at different temperatures. All vibrational spectra were
obtained after recooling to 115 K. Apparatus and techniques were identical with those used previously.lI2 11. Results a n d Interpretation The H + CsD6 coadsorbate layer on Ru(001) was prepared by exposure of the clean Ru(001) sample at T = 130 K to 0.6 X 1014cm+ C6D6; subsequent heating to T = 250 K led to a well-ordered ( 2 d 3 X 2v' 3)R3OoLEED pattern. This corresponds to about one-half of the saturation coverage of the chemisorbed layer.' Then a dose of 1000 X 1014 cm-2 Hz was added to saturate the layer. The work function change A@, which was determined to be -1.10 eV1 for the pure benzene layer, was reduced slightly by 15 meV with exposure to hydrogen, in agreement with the effect of the pure hydrogen on Ru(OOl).3p4 At the same time, the 2 4 3 LEED spots became diffuse. Figure 1 shows the vibrational spectra of such a halfsaturated layer of c~,
[email protected](001) postsaturated with hydrogen, measured at 115 K. According to our earlier work,l benzene is oriented parallel to the surface, and the molecular distortion induced by adsorption is only weak. The strong mode a t 540 cm-l has been assigned to the C-D bending mode perpendicular to the molecular plane. The weaker ones a t 840 and 1335 cm-l are correlated to the ring breathing mode and other C-C stretch modes of the aromatic ring. No serious modification of the spectra by the added hydrogen can be stated. The loss intensities as well as their frequencies remain essentially unchanged, indicating the lack of noticeable chemical interactions between the coadsorbate species under these conditions. The shift of the weak benzene mode a t 840 cm-l to 815 cm-' is believed to be due to a mode of H/Ru(001) located a t 800 cm-' in the pure layer.5-7 As for the pure H/Ru(001) layer, its intensity is weak, and no dipole enhancement in the specular direction is found. Figure 2 shows the thermal desorption spectra of this layer, obtained in parallel by multiplexing all possible signals. No signal of desorbing C6D6 or of any partially deuterated species was found even if a high amplification factor (X100) was used; this also holds for ethene. This (3) Feulner, P. Ph.D. Thesis, TU Mtinchen, 1980. (4) Feulner, P.; Menzel, D. Surf. Sci. 1985, 154, 465. (5) Stengl, R. Diplom Thesis, TU Miinchen, 1983. Menzel, D. Surf. Sci. 1983,133, (6) Barteau, M. A.; Broughton, J. Q.; 44.7. .._
(1) Jakob, P.; Menzel, D. Surf. Sci. 1988, 201, 503. (2) Jakob, P.; Menzel, D. Surf. Sci. 1989, 220, 70.
0743-7463/91/2407-0134$02.50/0
(7) Conrad, H.; Scala, R.; Stenzel, W.; Unwin, R. J. J. Chem. Phys. 1984,81, 6371.
0 1991 American Chemical Society
Coadsorption of Benzene and H2 on Ru(001)
Langmuir, Vol. 7 , No. 1 , 1991 135
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c
VI
cc 2
P1
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-e L
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*
t
Ln
z w
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f
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2000 AE Icm-'1
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Figure 1. HREEL spectra (speculardirection)of the pure C&/ Ru(001) layer (Obnzsne = 0.07 ML) and after adsorption of 1000 x 10" cm-* Hz at Tab= 250 K (OH = 0.5 ML).
H2
1
T D - Spectra
Temperature [ K l
Figure 3. TD spectra (masses 2 and 4) of pure CeDe/Ru(001) (Obnzene = 0.05 ML) and of H/Ru(001) at different coverages (from refs 3 and 4; note: the ordinates of the two sets of data are different). Heating rate dT/dt = 5 K/s.
0 07 ML C6D6/RU(001] + 1000 EX H-,
338K
H2 + C ~ D I ~ / R U ( O O ~ ) dt
L
E
5Kls
365
AH '2 HD
300
300
700
500 Temperature
[K]
Figure 2. TD spectra (masses 2, 3, and 4) of the CeDe coadsorption layer on Ru(001) shown in Figure 1.
+H
is in agreement with the results for the pure benzene layer at similar coverages. We conclude that stepwise dehydrogenation as known from the pure benzene layer is the only reaction path for dissociating benzene in the coadsorbate layer described and that no new, hydrogen-induced reaction intermediates are formed. Interesting details are found in the TD spectra of masses 2,3, and 4. Roughly, all three show similar structures, but they differ in detail, both in terms of their relative intensities and peak temperatures. For comparison, spectra of the pure hydrogen- or benzene-covered surface (Obnzene = 0.05 ML;OH is variable) are shown in Figure 3.314 For the mass 4 signal, we find an additional peak at T = 372 K for the coadsorbate layer; apart from that, the spectrum agrees well with that of the pure benzene layer. On the other hand, the observed series of peaks at T > 400 K for masses 2 and 3 are substantially different from the spectrum of the pure hydrogen layer; they cannot be explained as desorption from H/Ru(001). Their resemblance to the structures originating from the dehydrogenation of the pure benzene layer leads us to the conclusion that they originate from this process too. We propose this to be direct evidence for hydrogen-exchange reactions between adsorbed hydrogen and benzene which occurs before or during dehydrogenation of the benzene; in the following, this effect will be discussed more closely. Apparently, the TD spectra of desorbing Hz, HD, and
LOO 500 600 Temperature 1 K I
700
800
Figure 4. TD spectra (masses 2,3, and 4) of a C6De + H coadsorption layer on Ru(001): Obnzsne = 0.13 ML; OH = 0.1 ML; Tab = 250 K.
Dz can be divided into two regions in terms of peak shapes and temperatures, roughly below and above 400 K, which are connected with (i) desorption of coadsorbed hydrogen and (ii) dehydrogenation of benzene. While we observe a considerable irifluence of coadsorbed benzene on the desorption kinetics of hydrogen, the desorption due to decomposition of benzene is almost unchanged compared to the pure benzene layer. This is mainly due to the fact that most of the hydrogen (adsorbed as such) has already desorbed by T N 400 K, i.e., in the temperature range of incipient dehydrogenation of benzene. The peak of the mass 2 signal at T = 338 K can therefore be safely assigned to the associative desorption of adsorbed hydrogen. Although only half of the maximum H coverage is adsorbed, the peak temperature corresponds to the high coverage peak of the saturated pure hydrogen layer on Ru(001). This must be a consequence of the reduced number of free sites available for the hydrogen atoms in the presence of benzene and consequent high local H density. The two peaks of the masses 3 and 4 at T = 365 and 372 K can be interpreted as recombination of adsorbed H and D atoms and associative desorption of HD and D2. They therefore do not originate from dehydrogenation of C6D6 in the presence of adsorbed hydrogen (which would mean that coadsorbed hydrogen would reduce the dehydrogenation temperature of benzene) but must be the result of exchange reactions between Hadsand Dbenzene, followed by associative desorption. Their des-
Jakob and Menzel
136 Langmuir, Vol. 7, No. 1, 1991 orption kinetics are then mainly determined by the associative recombination of adsorbed hydrogen (which is slightly modified by the presence of benzene molecules) and the relative amounts of Had8 and Dads. The different peak temperatures of the mass 2,3, and 4 signals (at T < 400 K) can be understood if one assumes an increasing number of adsorbed D atoms, i.e., the degree of exchange increases with temperature. For the desorption peaks around 450 K, we observe slight differences (a few degress Kelvin) for the masses 2,3, and 4 in the peak temperatures which were already observed for the pure benzene layers.' In agreement with ref 1, these deviations are assigned to a kinetic isotope effect in the dehydrogenation of benzene. If exchange precedes dehydrogenation, the number of H atoms, N H ,incorporated into the benzene molecules should be the same as that of D atoms, N D ,which desorb below 410 K. For quantitative comparison, the relative sensitivity factors for H2, HD, and D2 have to be taken into account (H2, Xl; HD, X1.2; D2, X1.4, for the instruments used in this work?. For these corrected values, good agreement between N H and ND is found (rt5 $; ); the total amount of H D exchange is about 30% of the total amount of H or D species originally present = 0.07 ML) On the as Hads(OH 0.5 ML) or C6D6 (obnzene surface. This value is likely to depend on the ratio of H and C6D6 moleculesand need not represent a definite limit for exchange reactions. As we will see below, a higher value ( ~ 5 0 )%is obtained for Hz exposure at about 350 K. To summarize, we find that addition of H2 to a submonolayer of C6D~,/Ru(001)leads to separate peaks of the mass 2, 3, and 4 signals in TDS around 350 K which can be assigned to associative desorption of adsorbed H and D species following isotopic exchange of intact benzene. The peaks originating from the dehydrogenation of benzene (which are found at clearly higher temperatures, i.e., above 430 K) are only slightly modified. The combined observation of masses 2,3, and 4 throughout the entire TD spectra shows that mixing of the two isotopic species of hydrogen (originally present as Had8 and c~Ds/RU(001)) occurs at temperatures below those of dehydrogenation. Further, it gives strong support to our suggestion that coadsorption of hydrogen does not lead to new reaction paths in the dissociation pattern of benzene. We also investigated coadsorbate layers at benzene coverages close to saturation of the monolayer (Obnzene = 0.13 ML). Subsequent exposure to hydrogen (-100 X 1014cm-2at Tad8 = 115 K) leads to the adsorption of =lo% of a monolayer. The vibrational spectra were virtually identical with the corresponding pure benzene layer; they are not shown therefore. Figure 4 shows the T D spectra of desorbing H2, HD, and D2. Nonvanishingsignals of masses 2 and 3 above 500 K are found, which indicates that H D exchange reactions occur for this layer too. The onset of the signals of both masses 3 and 4 occurs at the same temperature. This shows that the overlap of dehydrogenation of benzene with exchange reactions is considerably enlarged at higher benzene coverages. The relative ratio of the mass 3 and 4 peak intensities at higher temperatures (T< 420 K)is found to stay constant within a few percent. For desorbing H2, an additional peak is found at T = 300 K which indicates desorption of Hads. The exchangereactions between coadsorbed Hand C6Ds have also been studied in detail by HREELS. Figure 5 shows a sequence of spectra of a H + C6D6 coadsorbate layer on Ru(001) (@benzene = 0.08 M k A@ = -1.395 ev; Tad8 = 120 K). Short heating to T = 370 K causes only minor changes in the vibrational spectra, namely, a slight
570
d
Ill
a
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.two bi heated to
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cl t4+2W Ex HI
Ill
J
1
370K
0
l\i3
0
x
1000
AElcm-lI
2000
3MM
c
3000
Figure 5. HREELspectra (speculardirection)of a C,& + H/Ru(001)coadsorption layer: (a) adsorption of about half saturation of C a s , postsaturated with HZ at T = 120 K;(b)layer in (a)heated to 370 K; (c) additional exposure (=200 x 1014 cm-2, PH = 5 x lo-* mbar) to Hz at T = 370 K; (d) additional exposure to Hz mbar) at T = 370 K. (=lo00 X l O I 4 cm-2, PH = 5 X
broadening of the loss at 550 cm-1. Exposure to H2 at this temperature leads to a further broadening (the loss at 500 cm-l could contain contributions of small amounts of coadsorbed CO from residual gas (==3% in spectra d of Figure 5 ) , but see below), the increase of a new mode at 780 cm-l and peaks shifts on the order of 10-30 cm-l. The losses at 780 and 570 cm-' are again attributed to the out-ofplane C-H (C-D) bending modes perpendicular to the surface. Both modes are considerably broadened and shifted in frequency as compared to pure benzene layers. The appearance of the 780-cm-l mode thus indicates isotopic exchange. Further experiments have been carried out at T = 350 K (Figure 6) to investigate the revenibility of the exchange. = 0.07 ML; For this, a C6D6 + H/Ru(001) layer (Obnzene A@ = -1.180 eV; Tad8 = 250 K) was first exposed to H2 (1000 X 1014 cm-2 at Tad8 = 350 K; see Figure 6b) and subsequently exposed to 300 X 1014cm-2 D2 at the same temperature (Figure 6c and 6d). The vibrational features shown in Figure 5d could be reproduced in every respect (Figure 6b). The low CO contamination in this series assures that the peak at 500 cm-l in Figure 5d really stems from the hydrocarbon species and not mainly from coadsorbed CO, as previously suspected. Besides that, we find that all vibrational features indicative of partially deuterated benzene molecules (and therefore indicative for H D exchange reactions) disappear reversibly when Dz is offered. The spectrum in the off-specular direction which accentuates the parallel modes (like the C-C stretch mode of the aromatic ring) agrees excellently with the data obtained for the pure benzene layer on Ru(001).' We therefore are confident that no new dissociation reactions did occur; this emphasizes that the H D exchange reactions really proceed reversibly between adsorbed hydrogen and benzene before dehydrogenation is possible. In our experiments, we could not replace all D atoms of
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Coadsorption of Benzene and HZon Ru(001)
t
Langmuir, Vol. 7, No. 1, 1991 137
dI
el in off sDecular
most of the H part of the benzene layer present in Figure 6b has been removed by subsequent exposure to D2 at 350 K. It is interesting to note that the exchange of C6D6 with H is slower than that of C6H6 with D (compare Figure 6b and 6 4 , which parallels the easier dehydrogenation of CsH6.198 This effect has not been investigated in detail, however. 111. Discussion
I
v \
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0
aP
bl a b 1000 E x H, Tad, = 350K
A855
.loo0
555
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1000
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'
?D 02
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'
3h
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560
600
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Temperature [ K l Figure 7. T D spectra (masses 2, 3, and 4) of the CgDg + H + D coadsorption layer on Ru(001) shown in Figure 6 (Obnzene = 0.07 ML t9,,42). T h e signals of Hz and H D are enlarged by a factor of 5 with respect to De; heating rate d T / d t = 5 K/s.
by H atoms; rather it appears that a ratio of 3:3 can be obtained at most. However, we are not sure whether this value represents a kinetic limit (meaning much higher doses would be necessary for a further increase of the H:D ratio) or whether there is an intrinsic limit. Also, it is not clear whether the 3:3 ratio is a consequence of positionselective exchange. Figure 7 shows the TD spectra (masses 2, 3, and 4) of the layer in Figure 6c and 6d. The curve of desorbing Dz deviates only in a minor respect from that for the pure benzene layer (at the corresponding coverage). The peak at T = 350 K can be attributed to remnants of adsorbed D/Ru(001) which, as we now know, desorbs at distinctly lower temperatures than Dz originating from the dehydrogenation of the benzene species. The signals of the masses 2 and 3 have both been enlarged by a factor of 5; their low contribution is obvious. This shows that indeed
One of the most important results of this work is that hydrogen desorption originating from coadsorbed hydrogen and from dehydrogenation of benzene can be clearly separated into two regimes, the former occurring at lower temperatures. Applied to the TD spectra of pure benzene/ Ru(001), this means that the evolution of H2 (D2) is definitely reaction-limited, so that the shift of incipient H2 (D2) desorption to lower temperatures with increasing benzene coverage cannot simply be explained by the coverage dependence of H2 desorption from adsorbed hydrogen atoms (see Introduction). This is in agreement with the cofklusion of Benziger9from a thermodynamic analysis that benzene is thermodynamically stable a t low temperatures on Ni (which is not too different from Ru in terms of carbide and hydride thermodynamics) but becomes unstable above about 495 K (on Ni and for a dissociation pressure of mbar). From this analysis, Benziger draws the conclusion that the evolution of hydrogen from benzene is determined thermodynamically, not kinetically, i.e., that there is essentially no barrier for dissociation. This approximate treatment, which only considers full dissociation to carbide and hydrogen, can of course not catch the full reality. The graded evolution of hydrogen with its strong coverage dependence, both for pure benzene layers and benzene + hydrogen coadsorbate layers, suggests strongly that intermediate, partly dehydrogenated species play an important role. Also, the existence of additional barriers (which could be strongly coverage-dependent)cannot be excluded. Arguments for such barrier effects are the very strong difference in the branching between molecular desorption and disintegration of benzene on Ru(001)' and on Ni(111)'O and the strong shift of dissociation temperature at high coverages (which led us to the proposition of a new easier reaction path accessible only at high coverages, compatible with all present findings). On the other hand, the ease of isotopic exchange at temperatures considerably below those necessary for dissociation could be taken as indication that barriers are not important in dissociation, compared to thermodynamic factors. So, we agree generally with Benziger's analysis, even if it cannot explain the full richness of details of the dehydrogenation reactions because the nature of the various dehydrogenation products and their thermodynamic properties are not known. A second important point concerns the vibrational spectra of the H + C6D~/Ru(001)layer which has undergone H D exchange reactions. Surprisingly, two separate peaks are found at 570 and 780 cm-' which could be attributed to YC-D and YC-H modes, respectively. Intuitively, one would expect a progressing broadening to the high-frequency side of the 550-cm-' peak if arbitrary exchange were to take place. In fact, a broad band from 500 to 850 cm-I has been found for C6H6 + D / P d ( l l l ) at
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(8)Jakob, P. Ph.D. Thesis, TU Mirnchen, 1989. (9) Schoofs, G. R.; Benziger, J. B. Langmuir 1988,4, 526. Benziger, J. B. J. Catal. Submitted. (10)Steinrirck, H.-P.; Huber, W.; Pache,T.; Menze1,D. Surf. Sci. 1989, 218, 293.
Jakob and Menzel
138 Langmuir, Vol. 7,No.1, 1991
T = 450 K." A comparison with the vibrational spectra of partially deuterated benzene molecules (gas phase)12 led these authors to the conclusion that arbitrary exchange occurs. However, their discussion is not conclusive for a number of reasons: Many more species are possible besides those which were considered in ref 11. The molecular symmetry of the partly deuterated species is lowered, so that in addition to the v4 mode others can be dipole enhanced and have to be included into the considerations. The frequency shift (induced by adsorption) of the different modes from the gas-phase values need not be equal for the modes of interest of the different benzene species (as has been suggested in ref 11). We conclude that it is not clear which kinds of partially deuterated benzene species were formed in that work (arbitrary exchange vs formation of species with 'high' symmetry: C3u,CzU,CJ. This arises from the fact that the vibrational features of a given mixture of partially deuterated adsorbed benzene species are not definitely known. We therefore let the matter rest there and Jimply state that different kinds of vibrational spectra (one singlebroad peak vs two separate peaks) can be observed on the Pd(111)and the Ru(001) surfaces following isotopic exchange reactions in the benzene + hydrogen coadsorption layer. This might be an effect of the dissimilar nature of the two metals featuring different surface chemistry in terms of hydrogen-exchange reactions of hydrocarbon species. However, the observation of distinct losses in our vibrational spectra together with the finding of incomplete exchange indicates that the variety of isotopically mixed species is not high and suggests selective exchange. The temperature which is necessary to produce hydrogen exchange can be estimated from the temperature of the onset of HD or Dz desorption as H2 desorption starts a t clearly lower T; this gives T = 320-340 K for Ru(001). This value is considerably lower than the temperature of dehydrogenation of adsorbed benzene/Ru(001) at ebenzene = 0.07 ML. However, it roughly corresponds to the temperature which is found for the saturated benzene layer. We conclude that the reaction rate for H D exchange is considerably higher than that for the proper dehydrogenation process of benzene (at low coverages). As mentioned above, this could be taken as an argument for the negligibility of barriers and the predominance of thermodynamic factors in dissociation; a particular low activation path at high coverages cannot be excluded, however. In general, one can set up a number of possible reaction D exchange: (i) hydroschemes for the observed H genation of benzene followed by dehydrogenation
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(ii) formation of a shortlived (DH)* intermediate state C6D6+ Hads
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C6D5(DH)* c6D5(HD)* C,D,H
+ Dads
(iii) dehydrogenation of benzene followed by hydrogenation C6D6
+ Hads
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+ Dads + Hads
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C6D5H
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+ Dads
Reaction iii appears improbable since dehydrogenation of benzene should be negligible at T I350 K. In our (11) Kesmodel, L. L.; Waddill, G . D.;Gates, J. A. Surf. Sci. 1984,238,
464.
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opinion, such a process could explain H D exchange of small quantities but not of 30% of the total amount of adsorbed H during a heating rate of 5 K/s. For processes i and ii, the decision is more difficult since the reaction mechanisms to be considered are not well-known. In the case of reaction i, the primary step is hydrogenation of the benzene molecule; it therefore can be described as a first step of the hydrogenation of benzene to cyclohexane (CGH12). In the C6D6H intermediate state, both hydrogen isotopes are assumed to be chemically bonded to the carbon ring as CHD. Such a hydrogenation reaction, however, would lead to the formation of a-benzene (these molecules have lost their aromaticity and form covalent a-bonds to the metal atoms) or a carbenium species which certainly are energetically less favored than the aromatic compound. We therefore consider this possibility as unlikely. It is interesting that in our mixed layers no indication of benzene hydrogenation to stable aliphatic rings has been observed, although this reaction does occur under catalytic conditions on a variety of metals.13J4 This is in agreement with the thermodynamic analysis of Benziger et a1.: who have shown that cyclohexene becomes more stable than benzene on Ni(100) only below 130 K under UHV conditions. A t our lowest temperature, hydrogenation is then prevented by an activation barrier, even in coadsorption layers with high hydrogen coverages. At higher temperatures, hydrogenation is prevented by thermodynamics. Reaction ii proposes the existence of a CeDdDH)* intermediate state to explain the observed H D exchange reactions. One of the hydrogen atoms to be considered is bonded chemically to the benzene molecule, the other is adsorbed in the very close neighborhood of it. Hydrogenation of benzene does not occur, and full aromaticity of the carbon ring is given for the period of exchange. The principal difference between reactions i and ii lies in the D exchange occurs: solely after moment when H complete hydrogenation or already during the approach of both coadsorbate species. In contrast to reaction iii, which requires complete loss of a D atom, reaction ii merely needs a weakening of the C-D bond. An alternative description of (ii) starts with the idea that dehydrogenation instantly followed by hydrogenation, i.e., in a concerted fashion, should possess lower activation energies for the complete reaction than two single-step reactions. To our knowledge, such a reaction mechanism via a short-lived intermediate state has not yet been observed in this context, so its existence must be denoted as hypothetical. However, (ii) might be describable as the limiting case of very short-lived hydrogenated (dehydrogenated) benzene molecules; by this the differences between (i) and (ii) (as well as between (iii) and (ii)) disappear so that the discrimination between the three processes becomes more or less meaningless. H-D exchange reactions of benzene (and other hydrocarbon species) have been studied extensively in the field of catalytic re~earch.13-l~ In general, two different reaction schemes, the dissociative and the associative mechanism, were considered by the various authors (the underlying concepts were originally introduced by M. Polanyi et al." and Farkas et a1.9. On the basis of these works, a number
(12) Sverdlov,L. M.; Kovner,M. A,; Krainov,E. P. Vibrational Spectra of Polyatomic Molecules; Wiley: New York, 1974; p 324.
(13) Ozaki, A. Isotopic Studies of Heterogeneous Catalysis;Kodansha: Tokyo, 1977. (14) Bartok, M. Stereochemistry of Heterogeneous Metal Catalysis; Wiley: Chichester, 1985. (15) Kemball, C. Ado. Cntal. 1959, 22, 223. (16) Redey, A.; Hall, W. K. J. Catal. 1987, 108, 185. (17) Horiuti, J.; Ogden,G.;Polanyi, M. Trans. Faraday. Soc. 1934,30, 663.
Langmuir, Vol. 7, No. 1,1991 139
Coadsorption of Benzene and Hz on Ru(001)
of possible reaction paths have been proposed. The idea of phenyl or phenylene formation as a first step in the H-D exchange reaction of benzene was proposed by Kemball et al.19 In later work, however, this group favored an associate reaction mechanismzO~zl similar to our process (i). The formation of a a-bonded phenyl radical as an intermediate species was suggested by Garnett et al.,2z-z4 who also proposed detailed models for both reaction mechanismsz5which were later modified slightly by van Meerten et a1.z6 In particular, the maintenance of the attractive deuterium-metal interaction for the deuterated benzene species is proposed (whichnevertheless leads to a breakdown of full aromaticity). This reaction mechanism can be considered as intermediate between our models (i) and (ii).
IV. Conclusions The coadsorption system C&6 + H/Ru(001) has been investigated by HREELS, LEED, TEDS, and A@ measurements. Apart from a site-blocking effect for hydrogen by the benzene molecules, we find only marginal mutual (18) Farkas, A.; Farkas, L. Trans. Faraday. SOC.1937, 33,678. (19) Anderson, J. R.; Kemball, C. A d a Catal. 1957,9, 51. (20) Crawford, E.; Kemball, C. Trans. Faraday. SOC.1962,58,2452. (21) Harper, R. J.; Kemball, C. Proc. 3rd Znt. Congr. Catal. 1964,11, 1145. (22) Garnett, J. L.; Sollich, W. A. J. Catal. 1963, 2, 350. (23) Garnett, J. L.; Sollich-Baumgartner, W. A. J.Phys. Chem. 1964, 68,3177. (24) Garnett, J. L.; Sollich-Baumgartner, W. A. Ibid. 1965,69, 1850. (25) Garnett, J. L.; Sollich-Baumgartner, W. A. Ado. Catal. 1966,16, 95. (26) van Meerten, R. Z. C.; Morales, A.; Barbier, J.; Maurel, R. J. J. Catal. 1979,58,43.
influences of both species on static properties (dipole moment, vibrational spectra, long-range ordering). As an effect of the reduced number of free sites and possibly by slight reduction of the H binding energy by coadsorbed benzene, the desorption temperature of adsorbed hydrogen is lowered and roughly matches the value for the pure saturated hydrogen layer. The features in TDS originating from desorption of adsorbed hydrogen and from dehydrogenation of benzene are well separated in temperature. This leads us to the conclusion that the observed desorption of hydrogen from the pure benzene layer on Ru(001) is determined by the thermodynamics and kinetics of the dehydrogenation process itself rather than by the recombination of hydrogen released by dissociation of benzene followed by associative desorption. We find direct evidence for hydrogen-exchange reactions between adsorbed hydrogen and hydrogen chemically bonded to the benzene molecules. This exchange reaction occurs at considerably lower temperatures than dehydrogenation of benzene. In order to account for this, a CsD5(DH)* transition state is proposed. Two well-separated loss peaks a t 780 and 570 cm-l are observed in the vibrational spectra after the H D exchange is accomplished. Also, at most 50% of the hydrogen could be exchanged. These observations of incomplete exchange point to a certain selectivity. Under no conditions of mixed layers and temperature has any indication of hydrogenation of benzene been observed.
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Acknowledgment. This work was financially supported by the Deutache Forschungsgemeinschaft through SFB 128 and SFB 338.
