Infrared Spectroscopy Characterization of the Chemistry of C4

Jun 19, 2007 - The uptake and thermal chemistry of a number of C4 hydrocarbons on Pt(111) single-crystal surfaces were characterized by ...
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J. Phys. Chem. C 2007, 111, 10062-10072

Infrared Spectroscopy Characterization of the Chemistry of C4 Hydrocarbons on Pt(111) Single-Crystal Surfaces Ilkeun Lee and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: March 29, 2007; In Final Form: April 25, 2007

The uptake and thermal chemistry of a number of C4 hydrocarbons on Pt(111) single-crystal surfaces were characterized by reflection-absorption infrared spectroscopy (RAIRS). The compounds studied include butane, 1-butene, cis-2-butene, trans-2-butene, and 1,3-butadiene as well as 1-iodobutane, 2-iodobutane, 1-bromo2-butene, and 1-bromo-3-butene, precursors for the formation of corresponding alkyl and alkenyl intermediates. Much of the chemistry reported here agrees with previous work on similar systems, and conforms with the general idea of hydrogenation, dehydrogenation, and H-D exchange reactions occurring by single hydrogen incorporation and elimination steps according to the so-called Horiuti Polanyi mechanism. In addition, though, a couple of surprising observations are presented of particular relevance to the mechanism of carbon-carbon double bond isomerizations. In particular, it was seen that although the isomerization of trans- to cis-2butene is more favorable and occurs at lower temperatures than the reverse cis-to-trans conversion, β-hydride elimination from the common 2-alkyl intermediate produces trans-2-butene preferentially. This indicates that it is the relative adsorption energy of the alkene what determines selectivity in these reactions.

1. Introduction Although the conversion of hydrocarbons on transition metals is one of the oldest and most widely used family of reactions in catalytic processes,1-3 some fundamental questions about the surface chemistry involved remain unanswered still. In the case of alkenes, hydrogenation, dehydrogenation, and H-D reactions are believed to all follow the so-called Horiuti Polanyi mechanism, which consists of a sequential series of interconversion steps among alkanes, alkyls, and alkenes.3-6 However, that deceivingly simple reaction scheme hides some important subtleties associated with the regio- and stereoselectivity in the hydrogen abstraction step from the β position of the alkyl surface intermediate:7 the removal of hydrogen atoms from different β carbons results in the migration of double bonds, whereas the abstraction of a different hydrogen atom from the same carbon leads to a cis-trans interconversion of the original double bond. Extensive surface-science work has been carried out on the conversion of ethylene on transition metal surfaces, a great deal on Pt(111) single crystals.8-11 This work has by and large corroborated the general ideas advance by previous catalytic work, including the rehybridization of the carbon atoms in ethylene to a sp3 configuration to facilitate di-σ bonding to the metal upon adsorption,5,12-14 the rapid interconversion between ethylene and ethyl adsorbed species as a way of exchanging hydrogens for deuteriums,15,16 and the stepwise hydrogenation of the alkene to the alkane.17,18 A particularly important development in these studies was the ability to isolate the facile β-hydride elimination step from ethyl surface moieties back to adsorbed ethylene.5,7 However, some of the surface chemistry of alkenes relevant to catalysis could not be probed with such a simple C2 hydrocarbon. For one, the activation of allyl hydrogens as a competitive pathway for alkene conversion requires at least one additional carbon atom in the hydrocarbon chain.19 In that vein, recent studies using propylene and other * Corresponding author. E-mail: [email protected].

alkenes have indicated that, at least under vacuum, no allylic dehydrogenation steps are ever significant in the hydrogenation and dehydrogenation of hydrocarbons.20,21 The absence of allylic activation has also been recently corroborated in our laboratory in studies with butenes on Pt(111).6,22 Extension of our work to C4 hydrocarbons has been driven by the additional flexibility offered by those molecules to probe double-bond isomerization reactions. The selectivity between cis and trans alkene formation in particular has been difficult to test in the past using a surface-science approach,23 because many analytical techniques are not suitable to discriminate between cis and trans isomers.24 Specifically, both isomers show essentially the same cracking pattern in mass spectrometry,25 making their separation in temperature-programmed desorption (TPD) surface-science studies nearly impossible. We have recently figured out a way to work around this limitation by using H-D isotope exchange reactions as proxies for cistrans isomerizations,6 a development that has allowed us to investigate the mechanism of the latter on model catalytic surfaces directly. What we have found is that on Pt(111) single-crystal surfaces the isomerization of trans-2-butene to its cis conformer is easier than the opposite cis-to-trans conversion.6 This is not what is expected based on thermodynamic grounds nor what is seen in most catalytic processes.26-30 In an attempt to better understand the fundamental kinetic driving force behind this unexpected result, here we expand on our past studies by characterizing the surface chemistry of a number of C4 hydrocarbons using reflection-absorption infrared spectroscopy (RAIRS). Most of the results from this work are consistent with the chemistry expected based on our prior knowledge of these processes, and also agree with previous research on the conversion of butenes on Pt(111).31-34 However, in addition, new and important aspects of this surface chemistry were identified. Specifically, it was found that β-hydride elimination from 2-butyl adsorbed intermediates leads to the preferential formation of trans-2-

10.1021/jp0724830 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

Chemistry of C4 Hydrocarbons butene. This suggests that the facile trans-to-cis conversion reported before may be due, at least in part, to an increase in relative stability of the cis isomer upon adsorption. The structure of the surface and the presence of coadsorbed hydrogen have both been identified as potential reasons for this unique selectivity on Pt(111), but those ideas need to be tested further. 2. Experimental Section All of the reflection-absorption infrared spectroscopy (RAIRS) experiments were performed in an ultrahigh vacuum (UHV) chamber cryopumped to a base pressure below 8 × 10-11 Torr.35,36 This chamber consists of two volumes: a main surface cleaning and characterization stage and a second, smaller vessel accessible via a long-travel manipulator used for the RAIRS sample characterization. In connection with the latter, the infrared beam from a FT-IR spectrometer (Bruker Equinox 55) is passed through a polarizer and focused through a NaCl window onto the platinum crystal at a grazing (∼85°) incidence, and the reflected beam escaping from the UHV chamber through a second NaCl window refocused onto a narrow band mercurycadmium-telluride (MCT) detector. The entire beam path is enclosed in a sealed box purged with an air scrubber (Balston 75-60) for CO2 and water removal. All spectra were taken at ∼80 K by averaging over 2000 scans at a resolution of 4 cm-1, a process that takes about 4 min per experiment, and ratioed against spectra from the clean sample similarly acquired before gas dosing. The sample condition and the infrared beam alignment were routinely checked by contrasting infrared spectra for a saturation coverage of CO to those reported in the literature.37-39 The platinum (111) single crystal, a disk 8 mm in diameter and 2 mm in thickness, was mounted on the sample holder via spot-welding to two tantalum wires attached to copper electrical feedthroughs. This arrangement allows the sample to be cooled to below 85 K by using a continuous flow of liquid nitrogen through hollow tubes connected to the nonvacuum side of the copper feedthroughs and to heat it resistively up to 1100 K. The surface temperature is measured with a chromel-alumel thermocouple spotwelded to the side of the crystal and controlled using homemade feedback electronics. The sample was regularly cleaned by cycles of oxidation in 5 × 10-7 Torr oxygen at 700 K and annealing in vacuum at 1100 K; Ar+ ion sputtering followed by annealing at 1100 K was also used on occasion but sparingly to avoid the creation of surface detects. 1,3Butadiene (>99% purity), cis-2-butene (>95% purity), trans2-butene, (>95% purity), 1-butene (>99% purity), butane (>99% purity), and deuterium (>99.5% atom purity) were all purchased from Matheson, and hydrogen (>99.995% purity) was obtained from Liquid Carbonic. All of the gases were used as supplied, but their purities were frequently checked by mass spectrometry. 1-Iodobutane (99% purity), 2-iodobutane (99% purity), 1-bromo-2-butene (85% purity, mixture of cis and trans (1:5), main impurity: 3-bromo-1-butene), and 1-bromo-3-butene (97% purity) were obtained from Aldrich and distilled by a series of freeze-pump-thaw cycles before dosing; their purities were also checked daily using the mass spectrometer in our UHV chamber. Dosing of the liquids was achieved by backfilling of the vacuum chamber with their vapors using leak valves and is reported in langmuir (1 L ) 10-6 Torr s-1), not corrected for differences in ion-gauge sensitivities. 3. Results and Discussion 3.1. Low-Temperature Uptake. 3.1.1. 2-Butenes. The adsorption of the C4 hydrocarbons on the Pt(111) surface at

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10063 low (80 K) temperatures was first characterized as a function of exposure. The corresponding RAIRS traces for cis- and trans2-butenes, together with a detailed assignment of the vibrational features observed, is provided in the Supporting Information. In general, three distinct regimes were identified. The first, observed for exposures below 1.0 L, involves the sp3 rehybridization of the CdC bond upon adsorption and the formation of a di-σ bonded species on the surface seen in other systems.5,10,12,40,41 In this case, the adsorption is characterized by IR peaks for the symmetric C-H stretching of the terminal methyl groups (νs(CH3), at 2868 and 2884 cm-1 for cis-2-butene and at 2870 cm-1 for the trans), the asymmetric methyl deformation (δas(CH3), at 1442 and 1452 cm-1 for the cis isomer and at 1436 cm-1 for the trans), the C-CH3 stretching (ν(C-C), 1093 and 1106 cm-1 for the cis and 1097 cm-1 for the trans) and the methyl rocking (F(CH3), 995 and 1016 cm-1 for the cis and 1022 and 1039 cm-1 for the trans), modes with dynamic dipoles in the molecular plane that would be silent in the IR spectra if the molecule were adsorbed flat and nonrehybridized because of the surface selection rule that applies to RAIRS on metals.42-45 In addition, the frequencies measured in this coverage regime are somewhat shifted from those both in the isolated molecules and at higher coverages (see below). Finally, no signals are seen for the asymmetric C-H stretching or CdC stretching modes. The second adsorption regime was seen for exposures between 1.0 and 4.0 L, a range in which, according to TPD results, some hydrocarbon conversion and high-temperature (255 K) molecular desorption takes place but no low-temperature (140 K) molecular desorption occurs.22,46 This state is likely to be associated with a π-bonded species: new IR bands develop for the asymmetric C-H stretching (νas(CH3), 2937 and 2974 cm-1 for cis-2-butene and 2937 and 2966 cm-1 for trans2-butene), the CdC stretching (ν(CdC), 1661 cm-1 for the cis, too weak to be seen for the trans), the symmetric (umbrella) methyl deformation (δs(CH3), 1374 and 1373 cm-1 for the cis and trans isomers, respectively), and a new C-C stretching (ν(C-C), at 972 cm-1 for the cis and at 966 and 976 cm-1 for the trans) vibrations, all with frequencies close to those seen for the pure butenes. Also, the asymmetric methyl deformation peak splits, probably because of crystal field effects on the ordered adsorbed layer. A third state appears after exposures above 4.0 L. In general, no new peaks are seen at this stage, but some old ones grow preferentially into intense and sharp features, suggesting a different adsorption geometry. Indeed, the predominance of the peaks for the in-phase asymmetric C-H stretching (νas(C-H), 2937 cm-1 in both cases) and methyl deformation (δas(CH3), 1439 and 1444 cm-1 for the cis and trans isomers, respectively) and for the low-frequency C-C stretching (ν(C-C), 972 and 966 cm-1, respectively) argues for a tilted arrangement on the surface at these high coverages. In our past studies on this system, we have identified a different reactivity for cis- and trans-2-butenes due, at least in part, to the stronger adsorption of the former on the Pt(111) surface.6,22 It would be useful to be able to identify the differences in bonding of the two species on the surface responsible for such a difference. Unfortunately, on this point, the RAIRS data is not unequivocal. Nevertheless, a few subtle differences are evident from the spectra: (1) The inner C-H stretching (ν(dC-H), 3011-3012 cm-1) mode is seen much earlier in the cis case. (2) The symmetric methyl deformation mode (δs(CH3)) is clearly visible in all of the IR traces for the cis isomer but only appears with the build up of the π-bonded species in the case of the trans. (3) The methyl rocking modes

10064 J. Phys. Chem. C, Vol. 111, No. 27, 2007 (F(CH3)) are red-shifted by about 25 cm-1 in the case of the cis but almost unaltered in the adsorbed trans. All of those observations point to a higher degree of sp3 rehybridization for cis-2-butene upon adsorption in the di-σ state. That would be consistent with a stronger adsorption and also with a previous claim of a longer C-C bond for that molecule based on photoelectron spectroscopy measurements.47 3.1.2. Reference C4 Hydrocarbons. For reference, RAIRS spectra for the uptake of butane, 1-butene, and 1,3-butadiene on Pt(111) at 80 K were also studied; the spectra are provided in the Supporting Information. The adsorption of 1-butene goes through the same sequential population of di-σ and π adsorption states as the 2-butenes. Indeed, at low coverages, the main features seen in the spectra are those corresponding to the stretching and deformation modes of the ethyl group, at νas(CH3) ) 2958 cm-1, νas(CH2)et ) 2931 cm-1, νs(CH3) ) 2872 cm-1, and δas(CH3) ) 1457 cm-1, an indication of bonding with the carbon-carbon double bond parallel to the surface. Surprisingly, though, clear features are also seen for the asymmetric stretching and deformation modes of the terminal methylene group (νas(dCH2) ) 3070 cm-1, γ(dCH2) ) 1444 cm-1), both of which have dynamic dipoles within the main plane of the molecule, suggesting limited rehybridization but nonplanarity of that methylene group with the surface. Also unique to this case is the clear development of an intermediate adsorption regime between 0.5 and 1.0 L exposures characterized mainly by a sharp methyl symmetric deformation peak at δs(CH3) ) 1369 cm-1. It is quite possible that, as the surface becomes more crowded, the proximity of the butene molecules forces this terminal methyl to stand up in order to minimize its footprint on the surface. In any case, the growth of the π state is manifested above 1.0 L by the growth of new vibrational peaks for the CdC stretching (ν(CdC) ) 1639 cm-1), C-H in-plane deformation (δ(CH)ip ) 1418 cm-1), and C-H and terminal methylene out-of-plane wagging modes (ω(C-H)oop ) 995 cm-1, ω(dCH2)oop ) 913 cm-1). In the case of butane, a clear transition is observed at exposures of around 2.0 L, the limit for monolayer saturation and the threshold for multilayer condensation according to our TPD results (data not shown). Also clear from the RAIRS data is the fact that adsorption in the monolayer is flat, with the molecular plane parallel to the surface; that is why only the modes with perpendicular dynamic dipole moments, the asymmetric C-H stretchings (νas(CH3) at 2930 and 2946 cm-1 and νas(CH2) at 2906 cm-1) and the methylene and asymmetric methyl deformations (δ(CH2) and δas(CH3), at 1439 and 1446 cm-1 respectively), are seen in the spectra.42-45 It is also worth noticing that all of those modes are red-shifted by about 20-30 cm-1 from their values in the pure molecule (and the condensed multilayer), an indication of a slight mode softening upon interaction with the surface.48,49 All this is consistent with a previous report on this system.50 Monolayer molecular desorption occurs at about 175 K, which, using Redhead’s analysis, is estimated to correspond to an adsorption energy of ∼12 kcal/mol. The adsorption of 1,3-butadiene also shares some common features with that of the butenes. Again, the low coverage regime is characterized by a flat adsorption geometry, as indicated by the appearance of vibrational modes in the RAIRS spectra at 959 and 896 cm-1 for the out-of-plane C-H and CH2 wagging modes, respectively. A peak is also observed for the scissoring of the methylene groups at 1409 cm-1, implying a slight nonplanarity, but there is no significant rehybridization upon adsorption, because the RAIRS peaks positions here are all

Lee and Zaera similar to those of pure butadiene51-53 and far from the ones obtained for a di-σ iron-butadiene complex.54 Finally, above about 2.0 L, new features appear in the vibrational traces for the asymmetric C-H and methylene stretchings (νas(CH2) ) 3083 cm-1, ν(CH) ) 3043 cm-1), the CdC stretching (ν(Cd C) ) 1589 cm-1), the out-of-phase methylene deformation (δ(CH2) ) 1371 cm-1), and a number of rocking and wagging modes for the C-H (1023 cm-1) and methylene (995, 922, 905, and 890 cm-1) groups, suggesting a preferential trans conformation for the adsorbed butadiene.55 The results reported here are consistent with previous studies on this system.32,56 3.1.3. Intermediate Precursors. In order to probe the chemistry of alkenes on Pt(111) surfaces further, additional RAIRS characterization studies were also carried out with a number of halohydrocarbons. Because of the lability of most carbonhalogen bonds, such halohydrocarbons have proven excellent precursors for the preparation of key surface hydrocarbon intermediates,5,23,57,58 including alkyls59-62 and alkenyls.20,21,63,64 Here we report on the RAIRS data obtained during the uptake of 1- and 2-iodobutanes (Figure 1) and of 1-bromo-2- and 1-bromo-3-butenes (Figure 2) on Pt(111) at 80 K. The evolution of the adsorbed species formed during the adsorption of the iodobutanes is by and large similar to that seen in analogous systems,44,61,65 with low-coverage adsorption characterized by flat-lying geometries and a collective layer rearrangement to a more upright molecular configuration at higher coverages. Interestingly, the asymmetric methyl C-H stretching and deformation modes are better seen at low coverages with 1-iodobutane (νas(CH3) ) 2938 cm-1, δas(CH3) ) 1450 cm-1) than with 2-iodobutane, most likely because of the alternating orientation that the terminal methyl adopts as a function of chain length from the surface. Also to note are the unique peaks at 1158 and 1120 cm-1 seen for the 1- and 2-iodobutanes after low exposures, respectively, which we assign here to a C-C stretching mode. Spectra more typical of these compounds in the liquid or solid phases are then seen after doses of 1.0 L or more, indicating a new molecular adsorption state. Since all of the features seen at saturation can be correlated to the spectra of the pure compounds,66-68 it is concluded that no molecular dissociation occurs on the surface at this temperature. A full assignment of the vibrational features reported in Figure 1 is provided in Table 1. In the case of the bromoalkenes, three states are clearly distinguishable in the RAIRS spectra as a function of exposure, perhaps even four in the case of 1-bromo-2-butene. Below 1.0 L only a few vibrational modes are visible, notably those associated with the methylene group close to the halogen (τ(CH2)CH2Br ) 1164 and 1063 cm-1 for the 1-Br-2- and 1-Br3-butenes, respectively, ω(CH2)CH2Br ) 1188 cm-1 for 1-Br-2butene, and νas(CH2)CH2Br ) 2976 and 2983 cm-1 for 1-Br-2and 1-Br-3-butene, respectively), possibly because the Br-C bond standing up from the surface. This suggests that bonding must be predominantly through the halogen, not via a π interaction of the double bonds with the metal, although the CdC double bond is still likely to be oriented in the plane of the surface because of the significant signal detected for the vinyl C-H out-of-plane deformation modes (ω(dCH)oop ) 967 cm-1 for 1-Br-2-butene and ω(dCH2)oop ) 923 cm-1 for 1-Br-3-butene). Some in-plane dCsH deformations (δ(dCH)ip ) 1401 and 1425 cm-1 for 1-Br-2-butene, γ(dCH2)ip ) 1441 cm-1 for 1-Br-3-butene) do become visible after low exposures and quite intense by 2.0 L, most likely because of a plane realignment to allow for better packing within the monolayer. The remaining vibrations only become evident at

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J. Phys. Chem. C, Vol. 111, No. 27, 2007 10065

Figure 1. Reflection-absorption infrared spectroscopy (RAIRS) traces for the uptake of 1-iodobutane (left) and 2-iodobutane (right) on Pt(111) at 80 K.

Figure 2. RAIRS traces for the uptake of 1-bromo-2-butene (left) and 1-bromo-3-butene (right) on Pt(111) at 80 K.

higher coverages. A summary of these assignments is provided in Table 2.69-71 3.2. Thermal Chemistry. 3.2.1. 2-Butenes. The RAIRS data for the evolution of the surface species that form upon thermal activation of adsorbed 2-butenes on Pt(111) indicate that lowtemperature molecular desorption22 is followed by at least two additional transformations at around 250 K and between 350 and 400 K (see the Supporting Information). First, there is a change in molecular adsorption geometry by 180 K, and perhaps even in the type of bonding to the surface (toward higher sp3 hybridization and di-σ bonding). In the case of cis-2-butene this is indicated by the lack of signal for the methyl rocking (995 cm-1) and C-C stretching (1093 and 1106 cm-1) peaks observed after doses below 1.0 L at 80 K, together with the appearance of an additional feature at 1242 cm-1 that we ascribe to a C-H wagging. For trans-2-butene the story is somewhat different, since all the same methyl rocking and C-C stretching modes are seen in both cases but new peaks are also observed at 180 K due to the wagging of the C-H moiety

(981 and 1025 cm-1). Also, the symmetric (1364 cm-1) rather than the asymmetric (1436 cm-1) methyl deformation mode is evident in the spectra at the higher temperature. Nevertheless, these observations can still be interpreted as being due to methyl moieties in a more perpendicular configuration because of a higher degree of sp3 hybridization of the central carbon atoms. The next surface species seen in these systems forms by heating to 280 K and can be identified in both cases, that is, with both the cis- and trans-2-butenes, as an acetylenic surface species.32 That species display only a few peaks in the RAIRS traces, mostly due to the terminal methyl groups (F(CH3) ) 1036 cm-1, δs(CH3) ) 1355 cm-1, and νs(CH3) ) 2888 cm-1 for cis-2-butene, F(CH3) ) 1036 cm-1, δs(CH3) ) 1356 cm-1, νs(CH3) ) 2888 cm-1 for trans-2-butene), suggesting the formation of a simple and symmetric intermediate; no modes are seen for the inner C-H fragments. There are some subtle differences between the results obtained with the cis versus trans isomers, though, in particular the different values of the C-C stretching modes (ν(C-C) ) 1074 cm-1 for the cis isomer and

10066 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Lee and Zaera

TABLE 1: Vibrational Assignment of the Features Seen in the RAIRS Spectra of 1- and 2-Iodobutanes Adsorbed on Pt(111) at 80 Ka 1-iodobutane66-68 modeb ν(C-C), F(CH3) ω(CH) τ(CH2) ω(CH2) ω(CH) δs(CH3) γ(CH2) δas(CH3) νs(CH3) νs(CH2) νas(CH2) νas(CH3)

low coverage

2-iodobutane66

high coverage

1158 (s)

1092 (w)

1243 (m) 1191 (w), 1224 (m), 1335 (m)

1177 (s), 1287 (w) 1193 (w), 1246 (s), 1332 (m), 1342 (m)

1370 (m) 1416 (m) 1450 (m)

1370 (m), 1378 (m) 1423 (s) 1462 (s)

2878 (w) 2902 (w) 2915 (w), 2938 (s)

2871 (s) 2928 (s), 2938 (s) 2958 (vs)

low coverage

high coverage

991 (w), 1021 (m) 1120 (w) 1184 (w) 1254 (w), 1265 (w)

953 (m), 993 (m), 1146 (vs) 1191 (s) 1270 (m) 1291 (m)

1356 (w) 1377 (w) 1437 (w)

1358 (sh) 1378 (s) 1443 (sh) 1453 (s) 2856 (sh), 2871 (m) 2917 (m) 2933 (m) 2965 (vs)

2849 (w), 2871 (w) 2910 (w) 2967 (w), 2985 (m) 2956 (m)

a Two coverage regimes are reported, for exposures below 1.0 L and at close to monolayer saturation. Frequencies in cm-1. Relative intensities in parenthesis: w ) weak, m ) medium, s ) strong, vs ) very strong, sh ) shoulder. b ν ) stretching, δ ) deformation, γ ) scissoring τ ) twisting, F ) rocking, ω ) wagging; s ) symmetric, as ) asymmetric.

TABLE 2: Vibrational Assignment of the Features Seen in the RAIRS Spectra of 1-Bromo-2- and 1-Bromo-3-butenes Adsorbed on Pt(111) at 80 Ka 1-Br-2-butene71 modeb ω()CH2)oop F(CH3) ω()CH)oop F()CH2) F(CH3) ν(C-C) τ(CH2)CH2Br ω(CH2)CH2Br ω(CH2) ω(CH2)CH2Br δs(CH3) δ()CH)ip γ()CH2) δas(CH3) ν(C)C) νs(CH3) ν()CH) νs(CH2) νas(CH3) ν ()CH) νas(CH2)CH2Br νs()CH2) ν()CH) νas(CH2)

low coverage

967 (w) 1012 (w), 1039 (vw)

1-Br-3-butene69,70 high coverage

low coverage

924 (vs)

1030 (m)

998 (s) 1032 (m)

928 (s) 969 (vs)

1164 (w) 1188 (w)

1020 (m), 1040 (w) 1081 (w) 1165 (m) 1206 (vs)

1374 (sh), 1386 (m) 1401 (m), 1425 (w)

1305 (w) 1377 (s) 1401 (w), 1425 (sh)

1063 (w)

1441 (w) 1432 (sh), 1452 (w) 2884 (m), 2897 (sh)

1434 (s), 1449 (vs) 1665 (m) 2885 (m), 2898 (sh) 2938 (m) 2967 (vs) 2976 (w)

1063 (m) 1198 (sh), 1215 (s) 1269 (s) 1305 (w) 1413 (sh), 1427 (vs) 1444 (s) 1642(m)

2910 (s) 2919 (m) 2928 (sh), 2938 (m) 2969 (m) 2976 (w)

high coverage

923 (w)

2960 (m) 2983 (sh)

2921 (m) 2962 (m) 2982 (s) 3003 (m)

3016 (s), 3028 (m) 3070 (w)

Two coverage regimes are reported, for exposures below 1.0 L and at close to monolayer saturation. Frequencies in cm-1. Relative intensities in parenthesis: w ) weak, m ) medium, s ) strong, vs ) very strong, sh ) shoulder. b ν ) stretching, δ ) deformation, γ ) scissoring, ω ) wagging, τ ) twisting, F ) rocking; s ) symmetric, as ) asymmetric, ip ) inplane, opp ) out of plane. a

980 and 1095 cm-1 for the trans) and the small feature visible in the spectra with the trans adsorbate for the methyl asymmetric deformation mode (δas(CH3) ) 1430 cm-1), but those can be ascribed to slight differences in adsorption geometries. Perhaps the intermediate in the case of the trans-2-butene is somewhat twisted, with the methyl groups in opposite sides of a bisecting perpendicular plane (that is, with some remanent of the initial configuration of the trans alkene). The acetylenic species dehydrogenates further by 400 K, at which point only a small feature for a C-C stretching is seen in the case of the trans2-butene. Since the stoichiometry at this point is approximately C4H2, the surface species are highly dehydrogenated. 3.2.2. 1-Butene and 1,3-Butadiene. Like in the case of the two 2-butenes, the spectrum obtained with 5.0 L of 1-butene at 160 K resembles that of the species that form after low exposures (below 1.0 L) at 80 K (data in Supporting Informa-

tion). Particularly noteworthy are the sharp and intense peaks seen for the symmetric methyl deformation and C-H stretching modes (δs(CH3) ) 1369 cm-1 and νs(CH3) ) 2866 cm-1) and for the asymmetric methylene C-H stretching (νas(CH2) ) 2828 cm-1) that imply a standing-up adsorption geometry with the terminal methyl group perpendicular to the surface. A small change in the RAIRS traces is observed at 210 K, as indicated by the appearance of a weak peak at 1106 cm-1 due to a C-C stretch, but since the rest of the spectra remains unaltered, this must be interpreted as due to a minor change in adsorption geometry. The next significant conversion occurs at around 270 K, and is associated with the formation of butylidyne (Ptt C-CH2-CH2-CH3).31 Additional peaks appear at this point at 1110, 1331, and 1380 cm-1 due to C-C stretching, methylene wagging, and symmetric methyl deformation modes, respectively, and the methyl group is only minimally disturbed even

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Figure 3. RAIRS traces from 5.0 L of 1-bromo-2-butene (left) and 1-bromo-3-butene (right) on Pt(111) as a function of annealing temperature.

though its orientation changes to a more tilted configuration. This species survives heating to temperatures of up to 400 K. With 1,3-butadiene the strongest features in the RAIRS data obtained at low temperatures (below 150 K) are the out-ofplane, out-of-phase deformation modes for the terminal methylene (ω(CH2) ) 892 and 909 cm-1) and inner C-H (ω(CH) ) 1014 cm-1) moieties, all with dynamic dipoles of Au symmetry and vectors perpendicular to the molecular plane, indicating flat adsorption on the platinum surface plane. A transformation is seen around 160 K, at which point only the first methylene deformation mode survives together with the peaks for the methylene scissoring at 1419 cm-1 and a combination band at 2903 cm-1. An additional weak feature at 966 cm-1 is also observed at this stage, most likely due to an out-of-plane C-H deformation, but none of the sp2 C-H or CH2 stretching modes at frequencies above 3000 cm-1 seen at lower temperatures survive this heat treatment. All of this is consistent with extensive rehybridization toward sp3 bonding, perhaps with the entire molecule adopting a double di-σ bonding configuration while retaining its planarity (with the inner C-H bonds at an angle from the surface plane). It is worth remembering that no dehydrogenation is seen with this molecule until temperatures above 320 K.22 A somewhat similar interpretation of this chemistry has been advanced before.32,56 3.2.3. Bromobutenes. The potential formation of allyl and other alkenyl intermediates during alkene thermal activation on Pt(111) surfaces was tested by investigating the thermal conversion of 1-bromo-2-butene and 1-bromo-3-butene. The appropriate vibrational spectra are provided in Figure 3. No similarities were seen between the spectra of these compounds and those from the butenes at any temperature, a clear indication that the two groups of molecules follow entirely different thermal chemistry and that no allyl hydrogen activation is possible from the adsorbed butenes. It is also worth pointing out that, although 1-bromo-3-butene looses (on average) one hydrogen atom by 250 K (with a TPD profile quite similar to that of 1-butene), 1-bromo-2-butene is more resilient and only starts to dehydrogenate above 300 K.22 With 1-bromo-2-butene, a subtle transformation is observed around 190-200 K where the weak modes due to the C-H out-of-plane deformation (ω(dC-H)oop ) 967 cm-1), methyl rocking (F(CH3) ) 1039 cm-1), and C-C stretching (ν(C-C)

) 1200 cm-1) all disappear together with the methyl asymmetric deformation (δas(CH3) ) 1434 cm-1); all that is left in the lowfrequency region of the spectra are two overlapping peaks at 1374 and 1386 cm-1 corresponding to the symmetric (umbrella) methyl deformation mode. These changes are most likely due to the scission of the C-Br bond and the formation of a C4 allylic surface intermediate. A weak and broad additional feature is seen between 190 and 220 K around 2990 cm-1 that disappears at higher temperatures, suggesting a change in adsorption geometry from an initial η1, σ coordination to η3, π bonding above 220 K. It is worth noticing the simplicity of the spectra in all cases, with features only due to a terminal and tilted methyl group. No additional conversions are seen until 300 K, at which point one hydrogen atom is lost from the allyl moiety (according to TPD results, data not shown) but the methyl symmetric deformation and C-H stretching modes remain in the RAIRS data. The spectra for 1-bromo-3-butene can be interpreted as follows. Because below 180 K the data are similar to those obtained after 1.0 L exposures at 80 K, they suggest preservation of the molecular character of the surface species. Only around 190 K a transformation takes place, as manifested by the growth of vibrational peaks for the twisting of the methylene group near the bromine atom (τ(CH2)CH2Br ) 1060 and 1067 cm-1) and the disappearance of the signal for the asymmetric C-H stretching of that moiety (νas(CH2)CH2Br ) 2976 cm-1). That indicates a subtle reorientation, likely due to the scission of the C-Br bond and the formation of a 3-buten-1-yl surface species. Note that both the out-of-plane wagging and scissoring modes of the terminal methylene moiety (ω(dCH2)oop ) 922 cm-1 and γ(dCH2) ) 1442 cm-1) persist throughout this conversion, suggesting that the carbon-carbon double bond is not involved in this chemistry. A second minor conversion takes place at about 240 K, perhaps a reorientation toward a flatter CdC bond as indicated by the suppression of the γ(dCH2) vibrational mode. The next set of changes is seen at about 280 K, at which point the RAIRS traces point to the potential rehybridization of the terminal methylene moiety (since the peak for the ω(d CH2)oop is no longer visible and that for γ(dCH2) is shifted to 1422 cm-1). Two more peaks are seen in the deformation range of the spectra, broad features around 1056 and 1103 cm-1, but none of the lower frequency signals detected at lower temper-

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Lee and Zaera

Figure 4. RAIRS traces obtained as a function of annealing temperature from 5.0 L of cis-2-butene (left) and trans-2-butene (right) on Pt(111) surfaces predosed with 50.0 L of H2, all at 80 K.

atures, and the TPD data indicates the average loss of one hydrogen atom.22 It would be tempting to assume that the alkenyl intermediate undergoes β-hydride elimination from the alkyl moiety to form a butadiene-like intermediate, but the RAIRS data do not resemble those reported from adsorbed butadiene. The most likely assignment of the data for this temperature is to a metallacycle with bonding at both alkyl and carbon-carbon double bond ends, but other possibilities include 1,2-di-σ bonded but-3-ene-1,2-diyl56 or but-2-ene-1,4-diyl species.72 What is clear is that a new conversion starts by 320 K and is completed by 400 K, at which point the main species on the surface is butylidyne (with its RAIRS features for ν(C-C) ) 1110 cm-1, δs(CH3) ) 1378 cm-1, νs(CH3) ) 2873 cm-1, and νas(CH3) ) 2927 cm-1). This requires not only isomerization (hydrogen shift) but also hydrogenation steps, so at least one other more dehydrogenated species is likely to exist on the surface at this stage. 3.3. Reaction with Coadsorbed Hydrogen. 3.3.1. 2-Butenes on Hydrogen-Predosed Pt(111). One of the main goals of this work has been to identify the intermediates involved in the interconversion between the cis and trans isomers of 2-butene. Since that reaction is believed to follow the Horiuti-Polanyi mechanism,3-6 that is, to involve an initial half hydrogenation to a surface 2-butyl intermediate followed by β-hydride elimination from that intermediate back to 2-butene, the chemistry of the adsorbed butenes with coadsorbed hydrogen was explored. The relevant temperature-dependent RAIRS data, for 5.0 L of either cis- or trans-2-butene adsorbed on Pt(111) surfaces previously exposed to 50.0 L of hydrogen, are shown in Figure 4. The first thing to note is that slight differences are seen for both butenes on the hydrogen-predosed surface versus on the clean Pt(111), mostly because the adsorption is weaken by the presence of the coadsorbed hydrogen. This has in fact been suggested before for other alkenes,17,58,73,74 although no evidence for the π bonding generally believed to dominate on H-Pt(111) was observed in this case. The next conversion, at around 200 K, is only seen with trans2-butene. The changes are quite subtle, manifested mainly by the decrease in the signal for the dC-H stretching RAIRS peak at 2910 cm-1, a small additional signal from the νas(CH3) mode,

and the growth of weak peaks at about 998, 1020, 1376, and 1460 cm-1,6 the same frequencies detected in the spectra for cis-2-butene at the same temperature (and due to two rocking and the symmetric and asymmetric deformation modes of the terminal methyl groups, respectively). This corroborates the trans-to-cis isomerization detected indirectly by TPD before.6,22 Further changes are then seen for both isomers between 250 and 270 K. In the case of cis-2-butene, first the peak for one of the methyl rocking modes at 995 cm-1 disappears, and then new features grow at 1032 and 1069 cm-1; the spectra at this point resemble those obtained for cis-2-butene on clean Pt(111) between 280 and 350 K, which were assigned to an acetylenic surface species (see section 3.2.1). With trans-2-butene, a peak at 1033 cm-1 is also seen after heating to 280 K, but that is not the dominant feature in the spectra. The shift of the asymmetric methyl deformation from 1440 cm-1 at low temperatures to 1455 cm-1 under these conditions suggests the survival of some cis-like structure in coexistence with the acetylenic hightemperature product. Certainly, the methyl deformation region of the RAIRS of trans-2-butene on the hydrogen-predosed surface seen above 270 K resembles that for cis-2-butene at 200 K. Therefore, the RAIRS data are supportive of the transto-cis isomerization reported in our earlier papers.6,22 3.3.2. 2-Butenes on Deuterium-Predosed Pt(111). Figure 5 displays the equivalent RAIRS data obtained for the thermal chemistry of the 2-butenes on deuterium-predosed Pt(111) surfaces, used here not only to corroborate the isomerization steps reported before but also to identify the temperatures at which H-D exchange takes place. The important observation here is that while for trans-2-butene the first H-D exchange, as followed by the appearance of new C-D stretching vibrational modes around 2050-2080 cm-1, becomes evident between 190 and 200 K, for cis-2-butene it only occurs around 220 K. The earlier H-D exchange seen with the trans isomer attests to its lower stability and higher reactivity on the Pt(111) surface and to the easier trans-to-cis than cis-to-trans conversion seen in TPD experiments.6,22 Additional slight changes in position and shape in that first ν(C-D) peak are seen for both isomers between 230 and 250 K, and the incorporation of a second deuterium is observed at about 270 K. The fact that the

Chemistry of C4 Hydrocarbons

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10069

Figure 5. RAIRS traces obtained as a function of annealing temperature from 5.0 L of cis-2-butene (left) and trans-2-butene (right) on Pt(111) surfaces predosed with 50.0 L of D2, all at 80 K.

H-D exchange takes place at the inner carbons is the most evident in the RAIRS obtained above 270 K for trans-2-butene, because the ν(dC-H) peak at ∼2900 cm-1 in the spectra with normal hydrogen (Figure 4) is absent in those with deuterium (Figure 5). The rest of the spectra in Figure 5 change little until heating to ∼270 K. In the case of cis-2-butene in particular, the only changes seen are in the peak around 1070 cm-1, which first sharpens and then disappears above 190 K, and in the methyl symmetric deformation feature (δs(CH3) ) 1372 cm-1), which grows and then broadens, probably because of an increase in sp3 hybridization of the central carbon atoms that places the terminal methyl groups in a more upright orientation. This is understandable, since most of the detectable modes correspond to vibrations associated with the terminal methyl groups of the 2-butenes, and those do not participate directly in the H-D exchange or isomerization reactions discussed here. However, more extensive changes are seen in the RAIRS of trans-2butene, including a decrease in intensity for the out-of-plane C-H wagging mode at 976 cm-1 (because one of those is replaced by a C-D), a more clear splitting of the methyl rocking and C-C stretching modes (F(CH3) ) 1024 and 1036 cm-1 and ν(C-C) ) 1095 and 1107 cm-1 between 190 and 230 K), and the growth of a new methyl asymmetric deformation peak at about 210 K (δas(CH3) ) 1423 cm-1). As with the RAIRS for trans-2-butene adsorbed on H-predosed surfaces, the increased complexity of the data points to low-symmetry adsorbed species with nonequivalent methyl groups. It is somewhat puzzling that the spectra obtained with trans-2-butene never indicates the formation of the same species made with cis-2butene even though a trans-to-cis conversion takes place around 190 K, but the differences may be just due to different coordinations on the surface, probably driven by differences in monolayer packing during the initial butene adsorption. The larger relative intensities typically seen with trans-2-butene for its ω(C-H)oop and δas(CH3) modes suggest a lesser degree of sp3 hybridization and a more planar coordination to the surface, although the increased complexity of the spectra also indicates lower symmetry and some inequality between the two methyl groups.

Figure 6. RAIRS for the thermal chemistry of 1-iodobutane on Pt(111).

3.3.2. Thermal Chemistry of Iodobutanes. Since, according to the Horiuti-Polanyi mechanism, the first intermediate likely to be made on the surface during any carbon-carbon double bond migration or cis-trans isomerization is an alkyl species, the chemistry of 1- and 2-butyl moieties on Pt(111) was characterized by preparing those using the corresponding iodobutanes.5,23,61 The RAIRS data for the case of 1-iodobutane indicates that the first change due to molecular desorption from a condensed multilayer occurs between 160 and 170 K (Figure 6). This takes place with a molecular rearrangement of the monolayer, since the traces seen here between 170 and 190 K resemble those reported in Figure 1 for exposures below 1.0 L. The next transformation occurs at 200 K, by which point the RAIRS spectra become fairly simple, with peaks at τ(CH2) ) 1173 cm-1, ω(CH2) ) 1193 cm-1, δs(CH3) ) 1369 cm-1, δas(CH3) ) 1456 cm-1, νs(CH2) ) 2867 cm-1, νas(CH2) ) 2927 cm-1, and νas(CH3) ) 2958 cm-1; all these features are easily assigned to a 1-butyl surface species produced by the scission of the C-I bond in 1-iodobutane. The surprising result is the apparent stability of this 1-butyl species, which would have been expected to undergo a facile β-hydride elimination step to 1-butene.5,7,58,61 In fact, a conversion is seen at 270 K,

10070 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Lee and Zaera though, and undergoes β-hydride elimination by 170 K. The interesting observation here is that that conversion produces only trans-2-butene: the spectra between 170 and 240 K, with peaks at F(CH3) ) 1025 cm-1, ν(C-C) ) 1103 cm-1, δas(CH3) ) 1440 cm-1, νs(CH3) ) 2877 cm-1, and νas(CH3) ) 2928 cm-1, are similar to those obtained for trans-2-butene in the presence of coadsorbed hydrogen (Figure 4); the only differences are the absence of a peak at 976 cm-1 and some additional intensity around 2960-2980 cm-1. Subsequent conversions are seen about 250 K, where all RAIRS intensity above 2900 cm-1 disappears, and around 280 K, to form acetylenic surface species.

Figure 7. RAIRS traces from 5.0 L of 2-iodobutane on Pt(111) as a function of annealing temperature.

but not to 1-butene or butylidyne but rather to a dehydrogenated species with average C4H6 stoichiometry,22 at least one remaining methylene group (hence the sharp peak for ω(CH2) ) 1193 cm-1), and some unsaturated C-H bonds (because of the ν(C-H) signal around 3000 cm-1). Perhaps the butene produced in this case all desorbs immediately (significant butene desorption is clearly seen in TPD experiments), and what is left behind on the surface is unreacted butyl species. Finally, we turn our attention to the thermal chemistry of 2-iodobutane on Pt(111). Molecular desorption seems to be accompanied by a direct conversion of the remaining adsorbates a 2-butyl group species at around 160 K, as indicated by the transitional RAIRS trace seen at that point, with only one strong and broad peak in the deformation region of the spectrum around 1380 cm-1 possibly due to a wagging of the C-H bond in the second carbon and a complex C-H stretching region that includes some intensity around 2900 cm-1 assignable to the same second carbon (Figure 7). The butyl species is unstable,

4. Conclusions The thermal chemistry of adsorbed butane, 1- and cis and trans 2-butenes, and 1,3-butadiene and 1- and 2- iodobutanes, 1-bromo-2-butene, and 1-bromo-3-butene on a Pt(111) singlecrystal surface was characterized by reflection-absorption infrared spectroscopy (RAIRS). Many of the observations and conclusions deriving from these studies agree or are consistent with results from previous work on this or similar molecules, including the following: •The adsorbed alkenes can adopt two distinct bonding configurations on the surface, an initial di-σ bonding at low coverages and a second more weakly bound π complex at close to monolayer saturation. This is a typical behavior for alkenes on transition metal surfaces of great relevance for catalysis3,14,17,58,75 and has been reported extensively in the literature already.5,8,10,12,41,47,76 Several combinations of these two types of bonding are possible for 1,3-butadiene.56 •The uptake of most of the halohydrocarbons at low temperatures is molecular but involves a sequence of geometries starting with flat adsorption at low coverages and a collective rearrangement to a more perpendicular configuration as the surface coverage is increased. Similar reorientations have been reported before for haloalkanes on platinum23,35,44,61 as well as on other metals.77

Figure 8. Energy diagram for the cis-trans isomerization of 2-butene on Pt(111).

Chemistry of C4 Hydrocarbons •Thermal activation of adsorbed haloalkanes on transition metal surfaces leads to the formation of the corresponding surface alkyl species.5,23,57,78 This is a facile reaction, and typically occurs below 200 K. Further thermal activation of the alkyl moieties results in β-hydride elimination, typically from the inner carbon positions, to yield adsorbed alkenes.7,60,62 •Similar carbon-halogen activation can be used to produce other surface intermediates, including metallacycles79-82 and alkenyls.20,63,81 The allylic species formed by activation of 1-halo-2-alkenes on Pt(111) is likely to undergo a reorientation from η1 to η3 bonding upon thermal activation.64,81,83,84 However, no allylic intermediates appear to form upon thermal treatment of adsorbed alkenes.20,21 •Coadsorption of alkenes with hydrogen leads to a competition between hydrogenation and dehydrogenation steps,10,11,85 and if deuterium is used instead, this is also manifested by extensive H-D exchange.16,21,74,86 All evidence from this and other reports points to the validity of the so-called HoriutiPolanyi mechanism3,4,58,87 and to the nonparticipation of allylic intermediates in these catalytic processes.88,89 •Further heating of adsorbed alkenes leads to the formation of a number of partially dehydrogenated surface species. For terminal alkenes the most stable intermediate is an alkylidyne moiety, butylidyne for 1-butene.31,40,74,90-96 For alkenes with internal double bonds, however, this is not always feasible. In the case of the 2-butenes, a 1,3-dimethyl acetylenic species forms instead.32,33,97 In addition, some new observations are reported here as well. For instance, the surface chemistry of the bromobutenes described above, including the formation of unsaturated metallacycles, is by and large new. Also, the uptake of 1-butene on Pt(111) seen in this study is more complex than previously realized. An additional surprising result is the apparent stability of 1-butyl species, which do not produce surface 1-butene species but remain intact to temperatures of up 270 K. But perhaps the most important new observations from this study pertain the surface chemistry of H-D exchange and doublebond isomerization with adsorbed 2-butenes. In connection with that, here we report the following: •A weakening of the interaction of the 2-butenes with the surface by the presence of hydrogen. This has been reported before for other alkenes17,58,73,74 but, in this case, appears to apply to different extents for the cis and trans isomers. The consequence of this is a possible inversion in the relative stability of the adsorbed butenes as a function of surface coverages. •A direct observation of the conversion of trans-2-butene to cis-2-butene on Pt(111) at about 200 K. This is indicated by the changes seen in the RAIRS spectra reported in Figure 4 for the Pt(111) surface predosed with hydrogen. •An earlier H-D exchange in trans-2-butene (190 K) than in cis-2-butene (220 K). This is clearly seen in Figure 5 by the appearance of the new vibrational bands for the C-D stretching modes around 2050-2080 cm-1. •The selective thermal conversion of 2-butyl surface species, prepared via thermal activation of 2-iodobutane, to trans-2butene. This, which is manifested by the similarity of the RAIRS spectra obtained at around 200 K with 2-iodobutane (Figure 7) and trans-2-butene plus hydrogen (Figure 4), is quite significant, because it indicates that it must be the relative adsorption energies of the alkenes and not the activation barriers for the β-hydride elimination steps from the alkyl intermediate what determines the selectivity of the cis-trans interconversion. Overall, these studies enhance our understanding of the mechanistic details that define selectivity in carbon-carbon

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10071 double bond isomerizations. Our current hypothesis is that this is defined by the relative adsorption energies of the cis versus trans alkenes and that those are affected by surface coverages and by surface structure. An energy diagram with the relevant steps in this Horiuti-Polanyi mechanism that includes the information obtained in these studies is provided in Figure 8. Both theoretical calculations and studies on other surface planes are currently under way to test our ideas further. Acknowledgment. Funding for this project was provided by the U.S. National Science Foundation. Supporting Information Available: RAIRS sequences for the low temperature uptake and thermal chemistry of butane, 1-butene, cis- and trans-2-butene, and 1,3-butadiene adsorbed on Pt(111), and the peak assignment for the RAIRS of cis-2-, trans-2-, and 1-butenes, are provided. References and Notes (1) Thomas, J. M.; Thomas, W. J. Introduction to the Principles of Heterogeneous Catalysis; Academic Press: London, 1967. (2) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (3) Bond, G. C. Metal-Catalysed Reactions of Hydrocarbons; Springer: New York, 2005. (4) Polanyi, M.; Horiuti, J. Trans. Faraday Soc. 1934, 30, 1164. (5) Zaera, F. Chem. ReV. 1995, 95, 2651. (6) Lee, I.; Zaera, F. J. Am. Chem. Soc. 2005, 127, 12174. (7) Zaera, F. J. Am. Chem. Soc. 1989, 111, 8744. (8) Bertolini, J. C.; Massardier, J. Hydrocarbons on metals. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984; Vol. 3B (Chemisorption Systems); pp 107-136. (9) Cremer, P. S.; Somorjai, G. A. J. Chem. Soc. Faraday Trans. 1995, 91, 3671. (10) Zaera, F. Langmuir 1996, 12, 88. (11) Zaera, F. Catal. Lett. 2003, 91, 1. (12) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985, 89, 3183. (13) Cassuto, A.; Mane, M.; Jupille, J. Surf. Sci. 1991, 249, 8. (14) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (15) Loaiza, A.; Xu, M.; Zaera, F. J. Catal. 1996, 159, 127. (16) Janssens, T. V. W.; Stone, D.; Hemminger, J. C.; Zaera, F. J. Catal. 1998, 177, 284. (17) O ¨ fner, H.; Zaera, F. J. Phys. Chem. 1997, 101, 396. (18) Neurock, M.; Pallassana, V.; van Santen, R. A. J. Am. Chem. Soc. 2000, 122, 1150. (19) Bond, G. C.; Turkevich, J. Trans. Faraday Soc. 1953, 49, 281. (20) Chrysostomou, D.; Zaera, F. J. Phys. Chem. B 2001, 105, 1003. (21) Morales, R.; Zaera, F. J. Phys. Chem. B 2006, 110, 9650. (22) Lee, I.; Zaera, F. J. Phys. Chem. B 2005, 109, 2745. (23) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (24) Burwell, R. L., Jr. Acc. Chem. Res. 1969, 2, 289. (25) NIST Standard Reference Database; National Institute of Standards and Technology: Gaithersburg, MD, 2003; http://webbook.nist.gov/ chemistry/. (26) Veldsink, J. W.; Bouma, M. J.; Schoon, N.-H.; Beenackers, A. A. C. M. Catal. ReV.-Sci. Eng. 1997, 39, 253. (27) Koetsier, W. T. Lipid Technol. Appl. 1997, 265. (28) Balakos, M. W.; Hernandez, E. E. Catal. Today 1997, 35, 415. (29) Pushpinder, S. P. J. Am. Oil Chem. Soc. 1980, 57, 850A. (30) Gonzalez-Marcos, M. P.; Gutierrez-Ortiz, J. I.; De Elguea, C. G.O.; Alvarez, J. I.; Gonzalez-Velasco, J. R. Can. J. Chem. Eng. 1998, 76, 927. (31) Avery, N. R.; Sheppard, N. Proc. R. Soc. A 1986, 405, 1. (32) Avery, N. R.; Sheppard, N. Proc. R. Soc. London A 1986, 405, 27. (33) Avery, N. R.; Sheppard, N. Surf. Sci. 1986, 169, L367. (34) Chesters, M. A.; De La Cruz, C.; Gardner, P.; McCash, E. M.; Pudney, P.; Shahid, G.; Sheppard, N. J. Chem. Soc. Faraday Trans. 1990, 86, 2757. (35) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (36) Janssens, T. V. W.; Zaera, F. J. Catal. 2002, 208, 345. (37) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (38) Zaera, F. Langmuir 1991, 7, 1188. (39) Zaera, F.; Liu, J.; Xu, M. J. Chem. Phys. 1997, 106, 4204. (40) Sheppard, N.; de la Cruz, C. AdV. Catal. 1996, 41, 1. (41) Zaera, F.; Chrysostomou, D. Surf. Sci. 2000, 457, 71.

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