J. Phys. Chem. C 2007, 111, 18367-18375
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Thermal Chemistry of 1-Methyl-1-cyclohexene and Methylene Cyclohexane on Pt(111) Single-Crystal Surfaces Ricardo Morales and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, RiVerside, California 92521 ReceiVed: September 10, 2007; In Final Form: September 25, 2007
The reactivity of 1-methyl-1-cyclohexene (1MC6)) and methylene cyclohexane (MeC6) on a Pt(111) singlecrystal surface was investigated by means of temperature-programmed desorption (TPD) and reflectionabsorption infrared spectroscopy (RAIRS). Dehydrogenation of both compounds occurs via the formation of a common 1-methyl cyclohexenyl species, as corroborated by supporting data from cyclohexene dehydrogenation and 3-bromo cyclohexene thermal activation on Pt(111), and leads to the ultimate formation of toluene, which desorbs at 430 K. On the other hand, hydrogenation, isomerization, and H-D exchange reactions all take place by following the so-called Horiuti-Polanyi mechanism and involve a common 1-methyl cyclohexyl surface intermediate. 1MC6) and MeC6 display different reactivities, with MeC6 isomerizing to 1MC6), but both hydrogenate to methyl cyclohexane (MC6). These reactions are enhanced by the presence of predosed hydrogen on the surface and undergo extensive early H-D exchange if deuterium is used instead.
1. Introduction Hydrocarbon conversion reactions on transition metals, although extensively studied, are not yet fully understood from a mechanistic point of view. Hydrogenation and dehydrogenation steps in alkanes and alkenes in particular are central to many catalytic processes associated with the oil and petrochemical industry, and significant finesse is required to control their selectivity. The basic reaction scheme for these systems is based on sequential single-hydrogen-atom incorporation or removal steps along the alkane-alkyl-alkene reaction coordinate, following the so-called Horiuti-Polanyi mechanism.1,2 However, additional subtleties may arise because of specific regio- or stereospecificities in those steps.3-7 Indeed, we have found in our laboratory that both thermodynamic and kinetic factors may control the selectivity in double bond isomerization reactions. 8,9 In one case, we determined that trans-2-butene isomerizes to its cis counterpart on a Pt(111) surface, despite the fact that this is the opposite to what is expected thermodynamically.8 In another, we found that 1-methyl-1-cyclopentyl species, a common intermediate in the conversion of 1-methyl-1-cylopentene and methylene cyclopentane, undergoes selective β-hydride elimination at a position in the ring to form 1-methyl-1cyclopentene.9 More generally, it has been established that β-hydride elimination reactions are favored in more substituted carbons and, therefore, lead to the preferential desorption of alkenes with a short chain length or a large number of substitutions. Hydrogenation steps, on the other hand, tend to occur on the less hindered groups. These kinetic trends do not always lead to the formation of the most stable compounds; therefore, the balance between the kinetics and thermodynamics of these reactions needs to be understood in order to be able to predict selectivities. To complicate matters further, another type of mechanism has also been proposed to compete with the Horiuti-Polanyi mechanism involving an initial dehydrogenation step at an allylic * To whom correspondence should be addressed. Email: zaera@ ucr.edu.
position.10-12 Indeed, surface allyls have been proposed as intermediates in some catalytic hydrogenations as well as in skeletal isomerization reactions10 and may be particularly favorable in processes with cyclic species.13-17 In surface science studies, cyclohexenyl (C6H9) species have been proposed as intermediates for the formation of benzene from cyclohexene and cyclohexane on Pt(111).14,18-23 This species is quite stable on the surface, surviving until temperatures of up to 250 K.23 In the present study, we have embarked on the characterization of the surface chemistry of 1-methyl-1-cyclohexene (1MC6)) and methylene cyclohexane (MeC6) on Pt(111) because of their promise in forming allylic species. Indeed, these reactants exhibit labile allylic hydrogens, lack many hydrogens at β-positions, and offer steric constraints for their hydrogenation imposed by their cyclic nature. The surface chemistry of 1-methyl-1cyclopentene and methylene cyclopentane was studied by us previously with the same purpose, but no evidence of allylic species was found there.9 However, previous reports on allyl intermediates during the dehydrogenation of C6-membered rings to the corresponding aromatic molecules17,18,22,24-26 suggested to us that the molecules chosen for this study may perhaps be better candidates for allyl formation. At the end, though, it was found that allylic species dehydrogenate to produce toluene but do not appear to participate in any hydrogenation, isomerization, or H-D exchange reactions, which still take place via the Horiuti-Polanyi mechanism. It was also learned that the expected common 1-methyl cyclohexenyl intermediate forms upon thermal activation of both 1MC6) and MeC6 on Pt(111) and that such a species is the one that participates in these reactions. The details are discussed below. 2. Experimental Section All temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) experiments were performed in a two-level ultrahigh vacuum (UHV) chamber cryopumped to a base pressure of less than 3 × 10-10 Torr.9 The main stage of this chamber is used to carry out the
10.1021/jp077240m CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2007
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Figure 1. Temperature-programmed desorption (TPD) data for 1-methyl-1-cyclohexene (1MC6)) adsorbed on Pt(111) at 90 K, showing the profiles for molecular desorption (left, as followed by the signal for the C6H9+ cyclohexene carbocation at 81 amu) and hydrogen production (right) at different exposures.
sample cleaning and TPD experiments, which are performed by means of a UTI 100C quadrupole mass spectrometer retrofitted with a retractable nose cone ended in a 5 mm diameter aperture. That aperture can be placed within 1 mm of the single crystal for the selective detection of molecules desorbing from the front surface of the crystal. The mass spectrometer is interfaced to a personal computer capable of monitoring the time evolution of up to 15 different masses in a single TPD experiment. A constant heating rate of 10 K/s was used in all TPD runs by employing homemade electronics, and a bias of -100 V was applied to the crystal in order to avoid any chemistry induced by stray electrons from the ionizer of the ion gauge or the mass spectrometer.27 The partial pressures in the TPD data are reported in arbitrary units, but scales are provided in the figures to allow for relative comparisons. The second level of our chamber, accessible by using a longtravel manipulator, is dedicated to the RAIRS experiments. The IR beam from a Bruker Equinox 55 FT-IR spectrometer is passed through a polarizer and focused through a NaCl window onto the sample at grazing incidence (∼85°). The reflected beam is then passed through a second NaCl window and focused onto a mercury-cadmium-telluride (MCT) detector. The entire beam path is enclosed in a sealed box purged with dry air, purified using a scrubber (Balston 75-60) for CO2 and water removal. All spectra presented in this paper correspond to averages of 2000 scans taken with 4 cm-1 resolution and ratioed against similarly obtained spectra for the clean surface prior to any gas dosing. The Pt(111) single crystal, a disc 8 mm in diameter and 2 mm in thickness, is mounted onto the sample holder via spotwelding to a pair of tantalum wires attached to corresponding copper electrical feedthroughs. This arrangement allows for the Pt crystal to be cooled to approximately 80 K by using a continuous flow of liquid nitrogen through the air side of the feedthroughs and to be heated resistively to up to 1100 K. The temperature is measured with a chromel-alumel thermocouple spot-welded to the side of the crystal. The sample is routinely cleaned by cycles of oxidation in 2 × 10-6 Torr of oxygen at 700 K and annealing in vacuum at 1100 K and, occasionally, by Ar+ sputtering, although this latter procedure was minimized
to avoid the creation of surface defects. The 1-methyl-1cyclohexene (1MC6), 97% purity), methylene cyclohexane (MeC6, 98% purity), cyclohexene (>98% purity), and 3-bromo cyclohexene (>98% purity) liquid samples were obtained from Aldrich and treated via a series of freeze-pump-thaw cycles before use; their purity was checked periodically by mass spectrometry. Gas exposures were performed by backfilling of the vacuum chamber and are reported in Langmuirs (1 Langmuir ≡ 10-6 Torr‚s), not corrected for differences in ion gauge sensitivities. All dosings were done at temperatures below 100 K unless otherwise indicated. 3. Results 3.a. Thermal Chemistry on Clean Pt(111). The thermal chemistry of 1-methyl-1-cyclohexene (1MC6)) and methylene cyclohexane (MeC6) on the clean Pt(111) single crystal was studied by means of temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Figure 1 shows the exposure-dependent molecular and hydrogen TPD spectra for the adsorption of 1MC6) on Pt(111). The signal for 81 amu was used to follow the molecular desorption because of its larger intensity, but the identity of the desorbing product was corroborated by simultaneously recording other significant fragments of 1MC6), including the molecular ion. The main observations here in terms of molecular desorption are: (1) it starts at an exposure of 4.0 L and initially occurs at 283 K; and (2) a second desorption peak appears at 230 K after 6.0 L, and two more appear at 160 and 190 K after 8.0 L. All but the 160 K peak saturate by 9.0 L. Similar desorption features have been reported for cyclohexene on Pt(111)22,28 and assigned to, from low to high temperature, desorption from the multilayer, second layer, and two di-σ-bonded cyclohexene states in chair and boat configurations in the first layer.18 Similar assignments may be made here for the 1MC6), although it is worth pointing out that this desorption pattern was not observed in our study of 1-methyl-1-cyclopentene (1MCp)) on Pt(111).9 This suggests that the additional carbon in the ring may provide additional conformations for adsorption not available with the C5 ring. In reference to the dehydrogenation of 1MC6), reflected by the hydrogen TPD data in the right panel of Figure 1, that occurs
1MC6) and MeC6 on Pt(111) Single Crystal Surfaces
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Figure 2. TPD data for methylene cyclohexane (MeC6) adsorbed on Pt(111) at 90 K, showing the profiles for molecular desorption (left, as followed by the 67 amu signal) and hydrogen production (right) at different exposures.
in a stepwise manner, with two main desorption peaks at 300 and 500 K and a third broad desorption region between 580 and 800 K. The relative areas of the H2 TPD peaks follow a relation of approximately 4:3:5, suggesting the formation of at least two partially dehydrogenated surface intermediates with C7H8 and C7H5 average stoichiometries before total dehydrogenation to surface carbon is reached above 800 K. The first could, in fact, be adsorbed toluene, as suggested by the results from Figure 8 (see below), but since these stoichiometries only represent average values, more than one species could be present on the surface in each case. The desorption peak at 300 K may be desorption-limited; therefore, the initial dehydrogenation steps may occur at rather low temperatures. Figure 2 reports the TPD data for methylene cyclohexane (MeC6). Molecular desorption, followed in this case by using the signal at 67 amu, shows features at 160 (multilayer), 170 (second layer), and 270 K (monolayer) starting at 9.0, 7.0, and 5.0 L, respectively (Figure 2, left panel). The separation between the second layer and the multilayer peaks is not large, but it is reproducible and evident in the evolution of the spectra as a function of exposure. Clearly, the molecular desorption profile for this molecule is much simpler than the one observed for 1MC6) and closer to those observed for C5 and methyl-C5 cycloalkenes.9,15,29 The dehydrogenation profile also differs from that obtained for 1MC6) and resembles more closely the one recorded for methylene cyclopentane (MeCp),9 with three main hydrogen desorption peaks at 310, 410, and 510 K and a broad desorption region between 600 and 800 K. Notice, in particular, that the second hydrogen desorption feature seems to be unique to the decomposition of methylene cycloalkanes. The relative peak areas in this case follow ratios of 3:2:2:5, suggesting intermediates with C7H9, C7H7, and C7H5 average stoichiometries on this surface at 340, 440, and 550 K, respectively. These do not correspond to any obvious surface species and, again, could reflect an average over several different species. In addition to molecular desorption and dehydrogenation, these molecules are also capable of self-hydrogenation and, in the case of MeC6, double-bond migration. These reactions are promoted by coadsorbed hydrogen (see below) but can also occur on the clean surface. The bottom traces in Figure 3
correspond to spectra for 9.0 L of each of these hydrocarbons on clean Pt(111) obtained after TPD deconvolution of the raw data for 82, 67, and 55 amu into contributions for 1MC6), MeC6, and MC6.30 Self-hydrogenation is easily seen at 275 K with MeC6 but does not occur to any appreciable extent with 1MC6). Since MC6 on Pt(111) is reported to desorb between 240 and 255 K,17,31 the desorption of this molecule here is likely to be rate-limited by the dehydrogenation reaction that provides the required surface hydrogen. Regarding double-bond isomerization reactions, they are also observed only with MeC6, as indicated by the small but reproducible 1MC6) feature seen at about 280 K. Figures 4 and 5 show the RAIR spectra for the uptake of 1MC6) and MeC6 on Pt(111) at 90 K, respectively, and Tables 1 and 2 summarize these vibrational data and provide our mode assignments, made by comparison with reported liquid- and gasphase data for cyclohexane,21 cyclohexene,21 1MCp), MeCp and methyl cyclopentane,31-33 and also for methyl cyclopentenes adsorbed on Pt(111).9 Our low-temperature IR data are quite similar to those obtained in the gas phase,34 indicating molecular adsorption. Also, for both molecules and after all exposures, the most intense vibrational modes are those due to the CHx stretchings (2800-3000 cm-1 region), followed by the CdC stretching, CH2 scissoring, and CH2 wagging at ∼1650-1700, ∼1400, and ∼1000 cm-1, respectively. Particularly significant is the fact that in both molecules, the CdC and dC-H stretching modes are seen even at low exposures (0.1 L) and that the intensity of their signal increases linearly with exposure. This indicates that even at low coverages, adsorption of these molecules does not occur by means of their double bond. In addition, it is interesting to note the high relative intensities of the peaks associated with the asymmetric deformation and -CH stretching modes of the methyl and methylene groups in the spectra for the 0.05 L exposure; since these have dynamic dipoles perpendicular to the molecular plane, their high intensity indicates a flat adsorption geometry at low coverages. This applies to both 1MC6) and MeC6 molecules, in contrast with what was seen previously with 1MCp) and MeCp on Pt(111), which adopt different adsorption geometries at low exposures (1MCp) adsorbs by means of its carbon-carbon double bond).9
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Figure 3. Deconvoluted TPD spectra for the hydrogenation and isomerization of 9.0 L of 1MC6) (left panel) and MeC6 (right panel) adsorbed on clean (bottom spectra) and 5.0 L hydrogen-predosed (top spectra) Pt(111) surfaces.
Figure 4. Reflection-absorption infrared spectroscopy (RAIRS) data for 1MC6) adsorbed on Pt(111) at 90 K as a function of exposure.
This difference may be due to an increase in steric hindrance in the 1MC6) case or perhaps to an increase in the potential interaction of the CH2 groups with the surface. Temperature-dependent RAIRS traces for 1MC6) and MeC6 on Pt(111) in the range of 90 to 350 K, obtained by dosing ∼10 L of the corresponding chemical at the indicated temperatures, are shown in Figure 6. Again, the most intense features observed in all of those spectra are those belonging to the C-H stretching modes (2800-3000 cm-1). With 1MC6) (left panel), the spectra obtained between 130 and 180 K are quite similar to those obtained at 90 K at submonolayer exposures, even if the intensity of the γ(CH2) peak at ∼1440 cm-1 is somewhat higher; this indicates multilayer desorption below 180 K. However, a new mode appears at 1060 cm-1 in the 180 K spectrum, the threshold for desorption from the monolayer (Figure 1). The vibrational mode observed at ∼1440 cm-1, assigned to methyl deformations, suggests that the moiety remains intact at this temperature. In fact, the same C-H stretching features are observed at 210 K, even though the C-H deformation region only shows weak features. Similar thermal chemistry is observed in the case of MeC6 (Figure 6, right panel), except that the C-H stretching modes are more intense
Figure 5. RAIRS data for MeC6 adsorbed on Pt(111) at 90 K as a function of exposure.
than those in 1MC6) and are retained at all temperatures; only the νa(CH2) mode shifts from 2929 to 2935 cm-1 at 350 K. Also, C-H deformation modes are observed only up to 180 K, and the ν(CdC) is evident only up to 130 K. Interestingly, the 210 K spectrum for both 1MC6) and MeC6 is quite similar, suggesting the presence of the same species on the surface in both cases at this temperature. Then, at 245 K, a new shoulder grows at 2944 cm-1, and the γ(CH2) mode at 1440 cm-1 is still present, events consistent with the formation of a π-allylic species, 1-methyl cyclohexenyl. Such a species has, in fact, been reported to form via dehydrogenation of 1-methyl-1-cyclohexene on Pt(111) by Xu et al.17 and by Yang and Somorjai.35 Additional RAIRS data were acquired for cyclohexene and 3-bromo cyclohexene at different temperatures to further test the possible formation of allyl intermediates in these systems; past studies have indicated that cyclohexene dehydrogenates to benzene through the formation of a cyclohexenyl (C6H9) surface species.15,18,19,22 Ihm and White have also studied the thermal chemistry of 3-bromo cyclohexene on Pt(111) by means of TPD and HREELS,23 but the low resolution of the HREELS data precluded us from being able to compare them with our results obtained for 1MC6) and MeC6. The spectra for cyclohexene (Figure 7, left) are similar to those reported by Manner et al.15
1MC6) and MeC6 on Pt(111) Single Crystal Surfaces TABLE 1: Vibrational Assignment of the Spectra for 1-Methyl-1-cyclohexene (1MC6)) on Pt(111) at 90 K as a Function of Exposure (Figure 4) on Pt(111), 90 Kb vibrational modea ν(CC), F(CH2) ν(C-C) F(CH3) ν(CC), F(CH2) ω(CH2) τ(CH2) δs(CH3) δa(CH3) γ(CH2) ν(CdC) νs(CH2) νs(CH2) νs(CH3) νa(CH2) νa(CH3) ν(dCH)
>0.2 L
0.0-0.2 L 916 (vw) 1237 (vw) 1367 (w) 1437 (sh), 1447 (m) 1456 (m) 1700 (m), 1714 (sh) 2861 (s) 2878 (m) 2930 (vs)
894 (w) 916 (w) 999 (vw), 1089 (m) 1022 (w) 1140 (m), 1239 (m) 1158 (m) 1365 (s), 1377 (s) 1437 (sh), 1447 (s) 1457 (s) 1706 (s) 2834 (s) 2854 (s) 2872 (m) 2926 (vs) 2964 (s) 3005 (w)
a Nomenclature: δ, deformation; ν, stretching; τ, twisting; F, rocking; ω, wagging; γ, scissoring. Subindices: s, symmetric; a, asymmetric. b Relative peak intensities provided in parentheses: w, weak; m, medium; s, strong; v, very; sh, shoulder.
TABLE 2: Vibrational Assignment of the Spectra for Methylene Cyclohexane (MeC6) on Pt(111) at 90 K as a Function of Exposure (Figure 5) on Pt(111), 90 Kb vibrational modea ν(CC), F(CH2) ω(dCH2) τ(dCH2) ν(CC), F(CH2) δip(dCH2) γ(dCH2) γ(CH2) ν(CdC) νs(R-CH2) νs(CH2) νa(CH2) νs(dCH2), νa(β-CH2) νa(dCH2)
0.0-0.2 L 888 (m)
1448 (w) 1650 (w) 2834 (sh) 2854 (m) 2930 (s)
>0.2 L 855 (s) 890 (vs) 986 (m) 1025 (w) 1343 (w) 1402 (w) 1448 (s) 1650 (s) 2831 (sh) 2854 (s) 2933 (vs) 2978 (m) 3070 (m)
a Nomenclature: δ, deformation; ν, stretching; τ, twisting; F, rocking; ω, wagging; γ, scissoring. Subindices: s, symmetric; a, asymmetric; ip, in-phase. b Relative peak intensities provided in parentheses: w, weak; m, medium; s, strong; v, very; sh, shoulder.
At 200 K, the data display vibrational bands at 1051, 1183, 1225, 1455, 2860, 2900, and 2931 cm-1, but annealing to 225 K leads to a reduction to only two major vibrational bands at 2842 and 2923 cm-1, which then shift slightly to 2853 and 2930 cm-1 above 250 K. A similar behavior is seen with 3-bromo cyclohexene, where the CH stretching region shows two features at 2840 and 2926 cm-1 at T < 250 K and at 2860 and 2936 cm-1 at T > 250 K. Because the C-Br bond in this molecule is expected to break easily and to lead to the formation of a cyclohexenyl species adsorbed on the Pt(111) surface,36 the match between the vibrational bands observed for cyclohexene and 3-bromo cyclohexene above 225 K indicates that they must be characteristic of the allyl species on the surface. According to the data in Figure 7, this cyclohexenyl species is stable on the Pt(111) surface between 250 and 400 K. In addition, the fact that the vibrational bands seen with these molecules are similar to those obtained with 1MC6) and MeC6 in the same range of temperatures supports the idea of the formation of allylic species in those cases as well.
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18371 Finally, our TPD experiments also identify some dehydrogenation in both 1MC6) and MeC6 to toluene (Figure 8). No appreciable toluene desorption is seen when the molecules are adsorbed at 90 K, but that reaction becomes evident if the dosing is carried out at temperatures above 300 K, at which point dehydrogenation competes favorably with molecular desorption. The small yields observed here can also be explained by competition with further dehydrogenation to hydrogen and surface carbon.37 3.b. Experiments with Coadsorbed Hydrogen/Deuterium. The hydrogenation and isomerization of 1MC6) and MeC6 on Pt(111) were probed further by experiments on surfaces predosed with hydrogen or deuterium. The top traces in Figure 3 show the results for the hydrogenation of both molecules, obtained by deconvolution of the 82, 67, and 55 amu raw TPD data for 9.0 L of each olefin (C7H12) dosed on 5.0 L hydrogenpredosed Pt(111) surfaces. Significant hydrogenation to methyl cyclohexane (MC6) is seen in both cases, in two stages between approximately 210 and 260 K. In addition, a small amount of isomerization of MeC6 to 1MC6) is seen at about 220 K. This appears to indicate that a double bond in the endo position is more stable than that in the exo position, a fact that kinetically favors the formation of 1MC6) from MeC6 on Pt(111). Similar reactivities (for both self-hydrogenation and isomerization) were observed in the study of 1MCp) and MeCp on Pt(111).9 The TPD results from experiments with 6.0 L of 1MC6) and MeC6 on 10 L deuterium-predosed surfaces are shown in Figures 9 and 10, respectively. Analysis of the data in the case of 1MC6) (Figure 9) leads to the identification of three desorption features at ∼170, ∼225, and ∼250 K, related to multilayer and monolayer molecular desorption of the olefin and to cycloalkane (MC6) production, respectively. The peaks in the 81 and 96 amu traces correspond to desorption of normal 1MC6) desorption, and those for higher masses, both in the C6+ fragment and the molecular ion, correspond to desorption of deuterated species. Also, the features at ∼220 K correspond mostly to deuterated olefins (although a component from MC6-dx is seen in the traces for g100 amu), while those at ∼250 K are associated with desorption of normal and deuterated MC6. In general, multiple H-D exchange is evident, with an approximately exponential decay in intensity with increasing deuterium content, as typically seen in cases where the steps involved display constant probabilities. Peaks for the C6 cycle in the exchanged olefin are seen only up to 85 amu, suggesting a maximum of four substitutions, but in fact, the peak for that mass (85 amu) appears at significantly lower temperatures and is most likely due to the formation of MC6-d2; three H-D exchanges is what would be expected if substitution only occurred in the carbon atoms adjacent to that bonded to the methyl moiety. Notice, however, that extensive H-D exchange is seen in the molecular peaks, suggesting exchange of hydrogen atoms in the methyl moiety as well. If so, this seems to take place at slightly higher temperatures. In terms of hydrogenation (deuteration) at higher temperatures, a maximum of five deuteriums is seen in the C6 ring, as expected from deuteration of the triply exchanged cyclohexene ring. Molecular signals are seen for masses up to 106 amu (the highest recorded here), again pointing to full exchange in the methyl group before deuteration. However, at these higher (∼250 K) temperatures, exchange in the methyl fragment seems to take precedence over substitutions at the ring because similar yields are seen for MeC6 with up to six H-D exchanges. At low temperatures, in contrast, the exchange appears to occur at the ring first.
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Figure 6. RAIRS data for 1MC6) (left) and MeC6 (right) as a function of adsorption temperature. Exposures of ∼10 L were used in all cases, sufficient to saturate the monolayer. Several surface species can be identified as a function of temperature, including a 1-methyl cyclohexenyl species.
Figure 7. RAIRS reference data for cyclohexene (left) and 3-bromo cyclohexene (right) as a function of adsorption temperature. Notice that after 225 K, the spectra for both molecules are similar, with vibrational bands assignable to cyclohexenyl species.
An analogous analysis of the data can be carried out for the case of MeC6 (Figure 10). The ∼220 K peak there shows extensive H-D exchange up to three and four deuteriums in the ring and the overall molecule, respectively. In fact, a maximum in yield is seen for those species, suggesting fast conversion up to that point. This is easily explained by an initial deuterium incorporation in the methylene group of MeC6 followed by β-hydride elimination from a position in the ring to produce 1MC6)-d1. That species subsequently exchanges the three accessible hydrogens in the ring rapidly. Also, the potential rate-limiting nature of the methylene deuteration step is suggested by the slightly higher peak temperatures seen in the 82 and 98 amu traces. Finally, the ∼255 K peaks in Figure 10 can be used to explore the mechanism of the hydrogenation (deuteration) of MeC6. Here, the maximum intensity is seen for MC6-d2 with one deuterium in the ring (that is, for 84 and 100 amu), as expected from direct deuteration of the original double bond. Then, a monotonic semilogarithmic decrease is observed in both the C6+
cycle and molecular TPD sequences. Also, as with 1MC6), only a maximum of five deuterium atoms are taken by the ring, presumably at the three carbon atoms closer to the methyl moiety. Moreover , again, extensive exchange in the methyl group is seen in the molecular signals, pointing to multiple 1MC6) T MeC6 interconversion at these temperatures. Overall, all of these observations are consistent with the Horiuti-Polanyi mechanism, and indicate (1) a fast initial hydrogenation at the least sterically hindered carbon of the double bond, (2) a subsequent preferential β-hydride elimination at a position in the ring to form 1MC6) regardless of the starting material, and (3) rapid multiple exchange at the two carbon atoms adjacent to that bonded to the methyl but no migration of the double bond down the other positions in the cycle. In addition, and in contrast with the case of the C5 cyclic compounds characterized before,9 the H-D exchange is competitive with hydrogenation to the alkane. Perhaps, more puzzling is the fact that both exchange and hydrogenation reactions are seen to occur in two distinct coverage ranges,
1MC6) and MeC6 on Pt(111) Single Crystal Surfaces
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18373 predosing, in which case the yield becomes larger and desorption of MC6 is also observed. MeC6 is also produced on the hydrogen-predosed surfaces, in peaks narrower and at slightly lower temperatures than those seen for 1MC6), suggesting a possible close competition in the β-hydride elimination step from the methyl group versus that from the ring. The former seems to be slightly more favorable in kinetic terms, but the latter leads to the more thermodynamically stable product. The small difference seen between the 1MC6) and MeC6 traces may explain the ease with which the interchange occurs, at least at ∼255 K. 4. Discussion
Figure 8. TPD spectra for the formation of toluene (92 amu) from MeC6 (top) or 1MC6) (bottom) on Pt(111), dosed at 90 (bottom) and 310 (top) K. Aromatization of the olefins occurs only at the higher temperatures.
Figure 9. TPD spectra for the cyclohexyl (C6+) (left) and molecular (right) ions detected by the mass spectrometer in experiments with 6.0 L of 1MC6) on a 10 L deuterium-predosed surface. The data show extensive H-D exchange, with the first D atom incorporating at a ring position.
possibly because of the changes in adsorption geometry as some products desorb and the surface coverage decreases. An interesting consequence of this is the fact that the H-D exchange at higher (∼255 K) temperatures involves fast interconversion between 1MC6) and MeC6, even if the latter is both thermodynamically less stable and kinetically harder to make than the former. None of this was seen with methylene cyclopentane or 1-methyl-1-cyclopentene.9 3.c. Thermal Chemistry of 1-Bromo-1-methyl Cyclohexane. Finally, the thermal chemistry of 1MC6-Pt, prepared by thermal activation of adsorbed 1-bromo-1-methyl cyclohexane (1BrMC6), was briefly characterized on both clean and hydrogenpreadsorbed Pt(111). This alkyl halide was used as the precursor for the alkyl species because of the ease with which the scission of the C-Br bond can be activated thermally on metal surfaces.10,38,39 The deconvoluted 1MC6), MC6, and MeC6 TPD spectra for 11 L of 1BrMC6 adsorbed on Pt(111) after different hydrogen exposures are shown in Figure 11. The main product here is 1MC6), the product of β-hydride elimination from the alkyl at the C2 position in the C6 ring. A broad and weak peak centered at 290 K is seen on the clean surface, but this desorption shifts down in temperature with increasing hydrogen
The results obtained from our temperature-programmed desorption and reflection-absorption infrared studies provide some useful information about the thermal chemistry of 1-methyl-1-cyclohexene (1MC6)) and methylene cyclohexane (MeC6) on Pt(111) surfaces. From the RAIRS data, it can be seen that both molecules adsorb molecularly at 90 K by means of interactions of the hydrogens in the ring and not by their carbon-carbon double bond, since the ν(CdC) is clearly visible at very low exposures. Those data also provide evidence for a rehybridization to a di-σ-bonded configuration above 180 K, a change indicated mainly by the disappearance of the ν(CdC) mode. The surface chemistry of the molecules studied here resembles that of cyclohexene on Pt(111).18,21,22 In particular, dehydrogenation of both C6 ring molecules leads to the formation of an allylic surface intermediate, something that, incidentally, was not seen with C5 cycles.9 Moreover, all evidence points to an allylic moiety only involving carbon atoms in the ring, without participation of the methyl group. Allyl formation starts at temperatures above 210 K, where both 1MC6) and MeC6 form the same surface cyclohexenyl species as that obtained by thermal activation of 3-bromo cyclohexene, and remains on the surface until at least 350 K. This species is proposed to be the precursor for the formation of toluene, which desorbs from the surface at ∼410 K, but to not participate in any other hydrogenation-dehydrogenation steps. The main argument against the allylic intermediate being involved in hydrogenation, double-bond migration, or H-D exchange reactions is that those reactions are promoted by coadsorbed hydrogen, but the product distributions observed in the H-D exchange are also difficult to explain with such a mechanism. In terms of the hydrogenation, H-D exchange, and isomerization reactions, they all can be explained by using a classic Horiuti-Polanyi mechanism.1 This starts with the halfhydrogenation of either 1MC6) or MeC6 to form a 1-methyl cyclohexyl surface species, followed by β-hydride elimination from that intermediate at a preferred position in the molecule. Elimination would be expected to take place at the methyl group first because this position is less hindered than those in the C6 ring, but in fact, the kinetics are so close in this case that the first elimination appears to occur in the ring; this is highlighted by the distribution of isotopomers obtained in the conversion of MeC6 with deuterium (Figure 10). The issue is complicated further by the fact that two hydrogenation/H-D exchange regimes are observed at ∼220 and 255 K. In fact, extensive deuterium substitution in the methyl moiety is seen in the high-T state, indicating fast 1MC6) T MeC6 interconversion. It seems that coverage effects complicate the energetics of this system. Nevertheless, a crude energy diagram can be developed by using the TPD data for 1MC6), MeC6, and MC6 on Pt(111) obtained here (Figure 12). We start with the
18374 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Morales and Zaera
Figure 10. TPD spectra for the cyclohexyl (C6+) (left) and molecular (right) ions detected by the mass spectrometer in experiments with 6.0 L of MeC6 on a 10 L deuterium-predosed surface. The data show extensive H-D exchange, with the first D atom incorporating in the methyl moiety but the first hydrogen elimination taking place from a carbon at the ring.
Figure 11. TPD spectra for 11.0 L of 1-bromo-1-methyl cyclohexane (1BrMC6) adsorbed on both clean and hydrogen-predosed Pt(111) surfaces. Each spectrum was deconvoluted in order to show the possible contributions of 1MC6), MeC6, and MC6 to the gas products. Preferential formation of 1MC6) is seen in all cases, but a competitive hydrogenation reaction of the alkyl to produce MC6 is also observed in the presence of hydrogen.
enthalpies of formation of these molecules in the gas phase, which have been reported to be -19.4, -17.7, and -37.0 kcal/ mol, respectively.40-42 The energetics of the adsorbed reactants can then be calculated by taking into account the energy of the dissociative adsorption of H2(g) to 2H(ad) on Pt(111), ∆Eads(H2) ) -10.4 kcal/mol,43 and the energies of adsorption of each compound estimated from their temperature of desorption from their first layer (as seen in the TPD traces in Figures 1 and 2) by using Redhead’s equation.44 By using preexponential factors of 1 × 1015 s-1, values of -13.0 and -11.8 kcal/mol for 1MC6) and MeC6 are obtained at low coverages, respectively. Finally, all of those adsorbates can be connected to the common 1MC6Pt intermediate by means of the desorption temperatures seen for MeC6 and 1MC6) from the dehydrogenation of 1MC6-Pt (Figure 11); activation barriers of 12.6 and 12.9 kcal/mol are estimated for the β-hydride elimination from the methyl and ring positions to form MeC6(ad) and 1MC6)(ad) species, respectively. In terms of the reverse olefin hydrogenation steps to produce the alkyl, those are likely to exhibit higher activation
Figure 12. Energy diagram for the thermal chemistry of 1MC6), MeC6, and MC6 on clean Pt(111) surfaces.
1MC6) and MeC6 on Pt(111) Single Crystal Surfaces energy barriers because both olefins are made at significantly lower temperatures from 1BrMC6 (Figure 11) than what is seen in the TPDs of 1MC6) and MeC6 either on clean (Figures 1 and 2) or in the presence of coadsorbed hydrogen or deuterium (Figures 3, 9, and 10). However, the differences cannot be too large, and the 1MC6-Pt species is likely to be less stable than the MeC6(ad) or 1MC6)(ad) species by only a few kcal/mol. In fact, from the TPD results for the isomerization of MeC6 to 1MC6) (where the limiting step is the formation of 1MC6-Pt), we find that the energy of the alkyl species must be at least 3.4 kcal/mol above that of MeC6(ad). Nevertheless, all of this needs to be taken as rough estimates because the energy values are obtained from TPD results on the clean platinum surface and are likely to decrease in the presence of coadsorbed hydrogen or deuterium. Next, the barrier for the alkyl hydrogenation was estimated from the TPD data for 1BrMC6 at 13.9 kcal/mol, even though that may be an overestimation because of potential problems with the mobility of the adsorbed hydrogen (which is hindered by the coadsorbed olefin) in this bimolecular step. A value of ∼5 kcal/mol is more in line with previous reports, but the higher estimate derived from Figure 11 was used in this scheme to reflect the relative higher rates for hydrogenation versus dehydrogenation.45 The high activation energy barrier for the activation of a C-H bond in MC6(ad) to produce 1MC6Pt of approximately 32 kcal/mol estimated by this diagram explains why no detectable amount of dehydrogenation or H-D exchange is seen with MC6.31 5. Conclusions The TPD and RAIRS data reported above indicate that the thermal chemistry of 1-methyl-1-cyclohexene (1MC6)) and methylene cyclohexane (MeC6) on Pt(111) surfaces is dominated by individual hydrogenation and dehydrogenation steps similar to those observed in other cycloalkanes and cycloalkenes on this and other metal surfaces. By means of RAIRS, it was determined that adsorption at 90 K occurs molecularly but does not involve the CdC bond. Once the temperature is raised to ∼180 K, however, the geometry of the adsorbates changes, and a new di-σ state is produced. Aromatization to toluene is possible, but only in significant quantities when the molecules are adsorbed at temperatures above 300 K; the formation of a π-allyl intermediate followed by dehydrogenation to a benzyl species is proposed to explain that reaction. At lower temperatures, additional thermal chemistry is observed involving hydrogenation, H-D exchange, and doublebond migration reactions. Indeed, even on the clean surface, MeC6 can self-hydrogenate and isomerize to form methyl cyclohexane (MC6) and 1MC6), respectively. The presence of coadsorbed hydrogen enhances both reactions and opens new reaction channels for 1MC6). By means of H-D exchange experiments, it was found that both MeC6 and 1MC6) share a common alkyl intermediate, namely, 1-methyl-1-cyclohexyl. This species undergoes β-hydride elimination according to the Horiuti-Polanyi mechanism, preferentially at positions in the ring, even though the methyl moiety is less hindered and expected to be more reactive. H-D exchange is fast in these systems and competes favorably with hydrogenation (deuteration) reactions. The same steps also promote an initial doublebond migration in MeC6 to produce the more stable 1MC6),
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18375 but fast MeC6 T 1MC6) interchange is possible at higher temperatures. A diagram explaining the kinetics and energetics of those processes is provided in Figure 12. Acknowledgment. This research was funded by the U.S. National Science Foundation. References and Notes (1) Horiuti, J.; Polanyi, M. Trans. Faraday Soc. 1934, 30, 1164. (2) Clarke, J. K. A.; Rooney, J. J. AdV. Catal. 1976, 25, 125. (3) Bent, B. E.; Nuzzo, R. G.; Dubois, B. R. Z. L. H. J. Am. Chem. Soc. 1991, 113, 1143. (4) Chrysostomou, D.; Chou, A.; Zaera, F. J. Phys. Chem. B 2001, 105, 5968. (5) Chrysostomou, D.; French, C.; Zaera, F. Catal. Lett. 2000, 69, 117. (6) Chrysostomou, D.; Zaera, F. J. Phys. Chem. B 2001, 105, 1003. (7) Zaera, F.; Tjandra, S.; Janssens, T. V. W. Langmuir 1998, 14, 1320. (8) Lee, I.; Zaera, F. J. Am. Chem. Soc. 2005, 127, 12174. (9) Morales, R.; Zaera, F. J. Phys. Chem. B 2006, 110, 9650. (10) Bent, B. E. Chem. ReV. 1996, 96, 1361. (11) Bond, G. C. Metal-Catalysed Reactions of Hydrocarbons; Springer Science: New York, 2005. (12) Bond, G. C.; Wells, P. B. AdV. Catal. 1964, 15, 91. (13) Henn, F. C.; Dalton, P. J.; Campbell, C. T. J. Phys. Chem. 1989, 93, 836. (14) Land, D. P.; Erley, W.; Ibach, H. Surf. Sci. 1993, 289, 237. (15) Manner, W. L.; Girolami, G. S.; Nuzzo, R. G. J. Phys. Chem. B 1998, 102, 10295. (16) Su, X.; Kung, K.; Lahtinen, J.; Shen, R. Y.; Somorjai, G. A. Catal. Lett. 1998, 54, 9. (17) Xu, C.; Koel, B. E.; Newton, M. A.; Frei, N. A.; Campbell, C. T. J. Phys. Chem. 1995, 99, 16670. (18) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.; Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965. (19) Land, D. P.; Pettiette-Hall, C. L.; McIver, R. T. J.; Hemminger, J. C. J. Am. Chem. Soc. 1989, 111, 5970. (20) Koel, B. E.; Blank, D. A.; Carter, E. A. J. Mol. Catal. A: Chem. 1998, 131, 39. (21) Lamont, C. L. A.; Borbach, M.; Martin, R.; Gardner, P.; Jones, T. S.; Conrad, H.; Bradshaw, A. M. Surf. Sci. 1997, 374, 215. (22) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500. (23) Ihm, H.; White, J. M. Langmuir 1998, 14, 1398. (24) Bratlie, K. M.; Flores, L. D.; Somorjai, G. A. J. Phys. Chem. B 2006, 110, 10051. (25) Pansoy-Hjelvik, M. E.; Schnabel, P.; Hemminger, J. C. J. Phys. Chem. B 2000, 104, 6554. (26) Yang, M. C.; Chou, K. C.; Somorjai, G. A. J. Phys. Chem. B 2004, 108, 14766. (27) Zaera, F.; Chrysostomou, D. Surf. Sci. 2000, 457, 89. (28) Xu, C.; Koel, B. E. Surf. Sci. 1994, 304, 249. (29) Avery, N. R. Surf. Sci. 1984, 146, 363. (30) Wilson, J.; Guo, H.; Morales, R.; Podgornov, E.; Lee, I.; Zaera, F. Phys. Chem. Chem. Phys. 2007, 9, 3830. (31) Morales, R.; Zaera, F. Unpublished results. (32) Durig, J. R.; Shing, A. C.; Bucy, W. E.; Wurrey, C. J. Spectrochim. Acta 1978, 34, 525. (33) Malloy, T. B., Jr.; Fisher, F.; Laane, J.; Hedges, R. M. J. Mol. Spectrosc. 1971, 40, 239. (34) NIST Webbook. http://webbook.nist.gov/chemistry/. (35) Yang, M.; Somorjai, G. A. J. Phys. Chem. B 2004, 108, 4405. (36) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (37) Tsai, M. C.; Muetterties, E. L. J. Am. Chem. Soc. 1982, 104, 2534. (38) Zaera, F. Chem. ReV. 1995, 95, 2651. (39) Zaera, F. Acc. Chem. Res. 2002, 35, 129. (40) Labbauf, A.; Rossini, F. D. J. Phys. Chem. 1961, 65, 476. (41) Yursha, I. A.; Kabo, G. Y. Zh. Fiz. Khim. 1975, 49, 1302. (42) Prosen, E. J.; Johnson, W. H.; Rossini, F. D. J. Res. Natl. Bur. Stand. (U.S.) 1946, 37, 51. (43) Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365. (44) Redhead, P. A. Vacuum 1962, 12, 203. (45) Zaera, F. J. Phys. Chem. 1990, 94, 5090.
