Effects of Coadsorbed Hydrogen (or D) on the Dehydrogenation of

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J. Phys. Chem. B 2000, 104, 6554-6561

Effects of Coadsorbed Hydrogen (or D) on the Dehydrogenation of Cyclohexane on Pt(111): Observation of the Production of Adsorbed Cyclohexyl (C6H11) M. E. Pansoy-Hjelvik, P. Schnabel, and John C. Hemminger* The Department of Chemistry, UniVersity of California, IrVine, California 92697 ReceiVed: August 4, 1999; In Final Form: March 26, 2000

The effects of coadsorbed hydrogen (or deuterium) on the dehydrogenation of cyclohexane on Pt(111) has been studied using laser induced thermal desorption-Fourier transform mass spectrometry (LITD-FTMS), Auger electron spectroscopy (AES), and thermal desorption spectroscopy (TDS). With 0.25 of saturation monolayer coverage of cyclohexane on the crystal surface, the onset for the competing processes of desorption and dehydrogenation occurs at approximately 205 K. Our experiments show that coadsorbed hydrogen causes a shift to lower temperature of the onset of both cyclohexane dehydrogenation and molecular desorption. In the presence of saturation amounts of coadsorbed hydrogen, the onset for the two processes occurs at approximately 165 K. In the presence of coadsorbed hydrogen or deuterium, LITD-FTMS experiments show that adsorbed cyclohexyl (C6H11) is produced in the first step of the dehydrogenation of cylclohexane. These experiments provide the first observation of cyclohexyl as a product of the thermal dehydrogenation of cyclohexane on Pt. The cyclohexyl species is stable on the Pt(111) surface at temperatures up to 205 K. In addition, H coadsorption thermal desorption spectra of cyclohexane show two desorption maxima, at 195 and 235 K. In the presence of coadsorbed H the total amount of C6H12 that desorbs is a factor of 2 more than in the noncoadsorption case. Deuterium coadsorption experiments indicate that the higher temperature desorption process is the result of the rehydrogenation reaction: C6H11 + H (D) f C6H12 (C6H11D). In the case of deuterium coadsorption with C6H12, both C6H12 and C6H11D are observed as rehydrogenation products, the C6H12 resulting from surface hydrogen generated in the initial dehydrogenation process. In this case, both rehydrogenation products (C6H12 and C6H11D) are observed at 255 K. This observation is discussed in terms of a kinetic isotope effect for the rehydrogenation of C6H11.

Introduction The dehydrogenation of cyclohexane on transition metal surfaces has been extensively studied. Despite the large amount of research that has gone into cyclohexane dehydrogenation, there still remains some speculation on the exact surface chemistry involved and the factors which may affect the dehydrogenation process. In this paper, effects of hydrogen (or deuterium) coadsorption on the reactivity of cyclohexane on Pt(111) are presented. Previous studies by Madey and Yates have shown that cyclohexane on Ru(001) is in a stable chair conformation, with the molecular plane essentially parallel to the metal surface and the three lower axial -CH bonds pointing downward toward the crystal surface.1 Similar conclusions have been reached for the cyclohexane/Pt(111) system by Land et al.7 In HREELS studies by Demuth, et al., an electronic interaction between the three axial hydrogen atoms and the metal surface was postulated to be directly related to the mechanism of dehydrogenation.2 Other theoretical and experimental (EELS, TDS, IR, ESDIAD) research has concentrated on determining the specific surface sites which are involved in the cyclohexane axial hydrogens surface interaction.3-7 To this date, there is no absolute concensus on whether the one-, two-, or 3-fold site is involved in the interaction. Overall, there is accord that the mechanism of cyclohexane dehydrogenation is related to the interaction of the three axial CH bonds with the metal surface. * To whom correspondence should be addressed. Phone (949) 824-6020. E-mail: [email protected].

Other earlier studies have speculated about the possible intermediates which exist as products of cyclohexane dehydrogenation.8-12 TDS studies showed that for submonolayer coverages of cyclohexane on Pt(111) only intact molecular cyclohexane and hydrogen desorption occurred; the latter was understood as being a dehydrogenation product.13 The presence of benzene was deduced through high-temperature hydrogen desorption peaks in TD spectra and vibrational spectroscopy.2,13 It was also postulated that the formation of cyclohexene, C6H10, was the rate limiting step in the formation of benzene in cyclohexane dehydrogenation, though that intermediate was not directly observed in those experiments.12 Recently, several studies have directly identified cyclohexene and benzene as definite products of the dehydrogenation process, and there is strong evidence for another intermediate C6H9 from which benzene is formed.14-18 Parker, et al. have studied the kinetics underlying the first step in the dehydrogenation of cyclohexane.19 The results from the Parker, et al. studies show that a compensation effect is displayed as a simultaneous increase in the activation energy and preexponential with increasing cyclohexane coverage. The absolute dehydrogenation rate does, however, decrease with increased coverage.19 A recent review by Koel et al. provides a good summary of the experimental observations regarding cyclohexane dehydrogenation on Pt and discusses the thermochemistry of possible intermediates.26 They point out that a stepwise mechanism for the dehydrogenation of cyclohexane involving the formation of cyclohexyl (C6H11) in the first step is likely. Xu et al. have

10.1021/jp992766v CCC: $19.00 © 2000 American Chemical Society Published on Web 06/20/2000

Dehydrogenation of Cyclohexane on Pt(111) shown that cyclohexyl can be formed by electron-induced dissociation of cyclohexane multilayers on Pt. However, prior to the experiments described here, there has been no direct experimental evidence for the formation of cyclohexyl in the thermal dehydrogenation of cyclohexane on Pt. The current study explores how the reactivity of cyclohexane and dehydrogenation products may be affected by coadsorbed H (or D) on Pt(111). The interest in this coadsorption system stems from the fact that during catalytic reforming, most hydrocarbon catalyst reactions are performed in an excess of hydrogen. Earlier work by Gillespie, et al. studied cyclohexane dehydrogenation to benzene over Pt at high reactant pressures and high crystal temperatures.20 The results of those studies showed that the rates of dehydrogenation and hydrogenolysis of cyclohexane are dependent on the Pt surface structure.20 In the work presented here, a submonolayer coverage of cyclohexane is followed by a saturation exposure of hydrogen (or deuterium) on the Pt(111) surface held at 150 K. The effects on the first step in cyclohexane dehydrogenation are followed by a combination of AES, TDS, and LITD-FTMS. The major results of this study are 3-fold. First, the results indicate that H (or D) coadsorption shifts the onset of the initial C-H bond breaking reaction and molecular desorption to lower temperature while enhancing the desorption yield slightly. When only cyclohexane is on the surface, the onset occurs at 205 K. In the case of H coadsorption, the onset occurs at 165 K. Second, in the presence of hydrogen, a C6H11 surface species is found to be a stable dehydrogenation product of cyclohexane in the temperature range of 165-205 K (providing the first direct observation of cyclohexyl as the first step in the thermal dehydrogenation of cyclohexane on Pt(111)). Finally, a kinetic isotope effect is observed for the rehydrogenation of the C6H11 species. Experimental Section Details of the experimental setup have already been described.21 Briefly, the UHV chamber is equipped with an ion gun for Ar+ sputter cleaning of the sample, low energy electron diffraction optics (LEED), and an Auger electron spectrometer (AES) with a single-pass cylindrical mirror analyzer. By the controlled horizontal movement of a long z-motion manipulator, the Pt crystal can be moved from the sample preparation part of the UHV chamber, through a sidearm extension, to a position in front of a hole in the FTMS analyzer cell which is centered in the middle of an electromagnet (0.6 T). This point in the chamber is considered the LITD-FTMS position. A second hole on the opposite side of the FTMS analyzer cell and a MgF2 window at the end of the sidearm extension allow passage of laser light to the single-crystal surface in order to perform laser induced thermal desorption (LITD) experiments. The laser light is aligned so that the reflected beam exits through the MgF2 window. The LITD-FTMS experiment is controlled by an OMEGA data acquisition system (Ion Spec Corporation). The base pressure in the chamber is 1 × 10-10 Torr. The inlet of gases can be accomplished through various sapphire sealed leak valves. A directional dosing tube is attached to one of the leak valves to concentrate the pressure of a sample gas at the crystal surface versus the chamber walls. The temperature of the Pt single crystal can be varied between 130 and 1300 K by a combination of resistive heating and thermal connection to a liquid nitrogen reservoir. A chromelalumel thermocouple spot-welded to the bottom edge of the crystal monitors the crystal temperature. Thermocouple voltages are referenced to 273 K via an ice bath.

J. Phys. Chem. B, Vol. 104, No. 28, 2000 6555 To clean the crystal surface, oxygen treatments are performed for up to 7 min at an oxygen partial pressure of 1 × 10-7 Torr and a crystal temperature of 900 K. After each oxygen treatment, the crystal is flash annealed to approximately 1200 K. Cycled Ar+ sputtering is performed at room temperature and 900 K, to remove calcium or high levels of carbon when necessary, with subsequent crystal annealing at 1200 K. The cleanliness of the crystal surface is determined by AES. The cyclohexane used in the experiments is obtained from Aldrich Chemicals and has a purity of 99.995%. The liquid sample is further purified by several freeze-pump-thaw cycles. The purity of the cyclohexane is checked daily by FTMS. Hydrogen with a purity of 99.95% is obtained from Matheson; deuterium with a purity of 99.95% is obtained from Cambridge. After the crystal surface is cleaned and cooled to approximately 150 K, cyclohexane is dosed onto the surface using the directional doser, which provides an enhancement in the molecular flux at the surface of a factor of about 50 times that obtained by simply backfilling the chamber to the same pressure. Reproducible coverages are ascertained by AES and TDS. The dosing is performed by leaking cyclohexane into the chamber through the directional doser for 10-240 s. From a series of TDS experiments with variable dosing times, it was determined that a 100 sec dose leads to a saturation monolayer coverage of cyclohexane. This corresponds to 0.21 cyclohexane molecules per Pt atom.13 In the hydrogen or deuterium coadsorption experiments, the crystal temperature is at 150 K during gas exposures and the chamber is backfilled with either of these gases after cyclohexane adsorption. From a series of TDS experiments with variable exposures of molecular hydrogen, it was determined that a 100 L exposure saturates the clean Pt(111) surface at 150 K. The desorption of molecules from the surface can be accomplished either by (1) thermal desorption (TDS) during which resistive heating of the crystal at a heating rate of 4.0 K/s results in the thermal desorption of neutrals from the entire crystal surface; or by (2) laser induced thermal desorption (LITD) in which desorption occurs only from a small area on the crystal surface probed by a laser pulse. In both kinds of experiments, with the crystal in the LITD-FTMS position, neutral molecules which desorb from the crystal surface into the gas phase of the analyzer cell are ionized by impact with a 70 eV electron beam. The ions formed are trapped in the analyzer cell and detected using impulse excitation methods.22,23 Ions over a broad mass range, typically between 15 and 100 amu in these experiments, that are formed by the ionization process can all be detected simultaneously operating the electromagnet at 0.6 T. In these experiments, the detection of H2+ required that the magnetic field strength be reduced to about 0.1 T in order to shift the H2+ cyclotron frequency below 1 MHz which is the upper limit for frequency measurements with the version of the OMEGA data system used in these experiments. Under these low magnetic field conditions, it is not possible to efficiently trap and detect heavier ions in the FTMS analyzer cell. During TDS experiments, mass spectra are taken in one to two second intervals. The LITD experiments use an excimer laser operating at a wavelength of 248 nm (KrF*) with a pulse width of 20 nanoseconds. Before any LITD experiments are carried out, it is necessary to determine the appropriate incident power density of a laser pulse to be used during the LITD experiment. The power density of the laser pulse is adjusted as follows. With the Pt crystal at 140 K, 0.25 saturation monolayer of cyclohexane is adsorbed. Previous studies have shown that at this

6556 J. Phys. Chem. B, Vol. 104, No. 28, 2000

Figure 1. Laser desorbed cyclohexane as a function of incident laser pulse power density. The (m/z) ) 56 ion intensity is used to represent the cyclohexane. The power density used during the LITD experiments described in the rest of this manuscript (25 MW/cm2) is in the range where the curve has a plateau. At power densities above 35 MW/cm2, Pt ablation occurs.

temperature, cyclohexane exists in its molecular form.13 The spot size area of the laser beam incident on the surface is kept constant, at approximately 10-3 cm2, as each laser pulse probes different spots on the Pt surface. With each laser pulse, we obtain a complete mass spectrum of the desorbed cyclohexane. In Figure 1 is a plot of the intensity of the (m/z) ) 56 positive ions resulting from the electron beam ionization of desorbed cyclohexane versus the incident laser power density. The mass 56 ion is used in this plot because it is the most intense peak in the electron ionization mass spectrum fragmentation pattern of cyclohexane. Identical behavior is observed for the (m/z) ) 84 parent ion signal. The power density used during a LITD experiment is in a range where the depicted curve has a plateau. This ensures that fluctuations of the laser energy output that can be up to ( 20% have a limited effect on the amount of desorbed cyclohexane. Furthermore, it has been verified that, at these power densities, no laser ionization of cyclohexane occurs and that the fragmentation pattern resulting from the electron beam ionization of the cyclohexane desorbed does not deviate from the fragmentation pattern of gas-phase cyclohexane using 70 eV electrons. An increase of the power density to values above 35 MW/cm2 leads to ablation of platinum atoms which are detected as Pt ions. The details of the temperature jump at the surface due to the laser pulse allow for the intact desorption of intermediates that normally would react on the surface during normal heating as in TDS. As a result, LITD can be used to monitor the relative surface concentration of cyclohexane and resulting surface reaction products as a function of surface temperature. Experiments of this type are referred to as survey experiments and are performed as follows. With the Pt crystal at 150 K, 0.25 saturation monolayer cyclohexane is adsorbed; in the coadsorption experiments, the crystal is subsequently exposed to H2 or D2. In the FTMS position, the crystal is heated at a heating rate of 10 K/s to consecutively higher flash temperatures then cooled back to 150 K. After each flash/cooling cycle, the surface adsorbate mixture is desorbed from a different spot of the surface by LITD and a complete mass spectrum of all the desorbed species is generated. Performing a series of such flash/cooling

Pansoy-Hjelvik et al.

Figure 2. (a) Cyclohexane desorption from TDS (temperature ramp of 4 K/s) of 0.25 monolayer cyclohexane coverage showing only one desorption peak maximum which occurs at 235 K. (b) LITD survey experiment of 0.25 monolayer cyclohexane. The onset for cyclohexane decay in the survey experiment commences at 195 K. The leading edge of the TD trace (Figure 2a) and the survey data overlap in the region of 195-210 K which suggests that the decay of cyclohexane in the survey is partly due to molecular desorption in addition to its dehydrogenation on the surface.

cycles with increasing flash temperature and probing the adsorbate mixture after each cycle, always from a different spot, leads to a series of mass spectra which allows for the determination of the relative surface concentrations of adsorbates as a function of temperature. Results In this work we report about differences that are observed in TD spectra of cyclohexane adsorbed on a Pt(111) surface with and without coadsorbed hydrogen or deuterium. Furthermore, we report LITD survey experiments in which the relative surface concentration of cyclohexane with and without coadsorbed hydrogen or deuterium is monitored as a function of surface temperature. Figure 2a shows a TD spectrum of cyclohexane from a coverage of 0.25 of saturation monolayer. The heating rate in this experiment was 4.0 K/s. The intensity of the (m/z) ) 56 ion is plotted as a measure of the cyclohexane desorption from the surface. The single desorption peak with a maximum at 235 K is consistent with TD spectra obtained for this coverage by other researchers.13,19 Since our data system does not allow us to take mass spectra faster than every second, the uncertainty in the peak maxima is about 4 K. Figure 2b shows data from the LITD survey experiment at this coverage. The data are displayed in a plot of the (m/z) ) 56 ion intensity versus surface flash temperature. As shown, the relative surface concentration of cyclohexane starts to decrease at 195 K and reaches zero at about 220 K. The low-temperature onset of the TDS peak, shown in Figure 2a, overlaps with the decrease observed in the LITD survey experiment in the temperature region of 200220 K. This suggests that in the survey, the decay of cyclohexane in this temperature range is partially due to molecular

Dehydrogenation of Cyclohexane on Pt(111) desorption. This is consistent with work done by other researchers that indicates that desorption is a simultaneous competing process with decomposition of the molecule at these temperatures and coverages.18,19 In our work described here, it was determined by Auger electron spectroscopy that the branching ratio between surface reaction and desorption at a 0.25 monolayer cyclohexane coverage is approximately 3:1 (favoring surface reaction). This was determined by comparing the ratio of the C(272 eV)/Pt(237 eV) Auger transitions at approximately 170 K, before cyclohexane reacted on the surface, and at approximately 270 K after cyclohexane desorption was complete. The TD spectrum, shown in Figure 2a, indicates that the cyclohexane desorption rate peaks at 235 K with total desorption occurring over the 220-260 K temperature range. However, in the survey experiment at 220 K, the surface concentration of cyclohexane is shown to be zero. These two seemingly contradictory observations can be resolved by considering the way the survey experiment is performed, and the time scales involved in the two experiments. During the survey experiment, the crystal temperature is flashed to the temperature of interest, then cooled to 150 K, before LITD is used to probe the surface reaction mixture. The cooling rate is approximately a factor of 10 slower than the heating rate. A flash temperature of 220 K is near the maximum rate in cyclohexane desorption. The cooling of the sample is not instantaneous, thus, the cyclohexane will continue to desorb or react during the cooling segment of a single point of the survey experiment. By the time the LITD survey experiment probes temperatures of ∼ 220 K, the cyclohexane has completely reacted. The hydrogen coadsorption experiments require a quantification of the amount of coadsorbed hydrogen. We have done this by thermal desorption experiment. Monitoring the integrated area of the mass 2 desorption peak allows us to generate a hydrogen uptake curve for various H2 exposures. The hydrogen uptake curve we obtain for H2 exposure to clean Pt(111) is in good agreement with the hydrogen uptake measurements published by Christmann et al.24 In the coadsorption case, the H2-TD spectrum was generated by first adsorbing 0.3 monolayer cyclohexane and then coadsorbing 200 L exposure of hydrogen to the 150 K Pt surface. The shape of the desorption signal and its position does not deviate from the TD spectra obtained for a saturation exposure of hydrogen without coadsorbed cyclohexane. Although some of the total detected hydrogen in the TD spectra is coming from the dehydrogenation of cyclohexane, we can deduce from the integrated mass 2 intensities in these coadsorption TDS experiments that a 0.3 monolayer coverage of cyclohexane does not significantly change the sticking probability of hydrogen on the uncovered regions of the Pt(111) single-crystal surface. The onset of the hydrogen desorption occurs under these conditions at about 210 K. Figure 3a shows a TD spectrum of cyclohexane that is obtained from the adsorption of 0.25 monolayer cyclohexane followed by a 200 L exposure of H2 to the 150 K Pt(111) crystal surface. The TD spectrum shows two desorption maxima for cylcohexane. One is centered at 195 K, the second at 235 K. From these data, it is clear that in the presence of hydrogen, there are two distinct mechanisms which lead to cyclohexane desorption. At this point, we assign the desorption at 195 K as due to the desorption of unreacted, molecular cyclohexane, and the peak at 235 K is due to another desorption mechanism. As will be described later in this paper. the 235 K desorption is due to rehydrogenation of a C6H11 species and desorption of the resulting cyclohexane. These processes will be discussed

J. Phys. Chem. B, Vol. 104, No. 28, 2000 6557

Figure 3. (a) Cyclohexane desorption from TDS (temperature ramp of 4K/s) of 0.25 monolayer cyclohexane coadsorbed with a saturation coverage of hydrogen which shows two desorption maxima at 195 and 235 K in contrast to the single desorption peak at 235 K for no hydrogen coadsorption. (b) LITD survey experiment of the same mixture as in 4a which shows that the onset for cyclohexane reaction has shifted to 165 K, 40 K lower than the onset for no hydrogen coadsorption.

further in the discussion section later in this paper. Figure 3b is the LITD survey experiment of cyclohexane coadsorbed with a saturation coverage of hydrogen. The decrease of the cyclohexane surface concentration starts at approximately 165 K, 40 degrees lower than the onset shown in the LITD survey in Figure 2b (no H coadsorption). At 175 K the decay slows down and reaches a plateau at 180 K, at 25% of the initial cyclohexane surface concentration, which is evident in the survey as the start of a shoulder. The plateau extends over a temperature range of about 20 K and further cyclohexane decay starts at about 205 K with a markedly lower slope compared to the slope between 165 and 175 K. At 220 K, the cyclohexane signal in the LITD experiment is zero. The appearance of the high-temperature desorption maximum in the TD spectrum and the high temperature shoulder in the survey could result from a rehydrogenation of known dehydrogenation intermediates, such as cyclohexene or C6H9, which have been shown to be stable intermediates in the dehydrogenation of cyclohexane on Pt(111) in the absence of coadsorbed hydrogen.15-16,18 Therefore, TDS and survey experiments were performed in which cyclohexene or C6H9 had been coadsorbed with hydrogen. In the cyclohexene experiments, the cylcohexene was adsorbed onto the surface from the gas phase. Since C6H9 is not a stable gas-phase molecule, we generated the C6H9 species on the surface by dehydrogenating cyclohexane at a temperature of 250 K,14,15,19 lowering the crystal temperature to 150 K, coadsorbing hydrogen, then performing the TDS experiment. In both cases, no cyclohexane was observed. Thus, the rehydrogenation of cyclohexene and C6H9 is negligible under our conditions. To aid in the diagnosis of which process leads to the high temperature shoulder in the H coadsorption LITD survey experiment, we have carried out isotopically labeled experiments involving the coadsorption of C6H12 with D2. The coadsorption of a saturation coverage of deuterium following 0.25 monolayer

6558 J. Phys. Chem. B, Vol. 104, No. 28, 2000

Figure 4. (a) Cyclohexane desorption from TDS (temperature ramp of 4 K/s) of 0.25 monolayer cyclohexane coadsorbed with a saturation coverage of deuterium. C6H12 (monitored here as (m/z) ) 84) is observed at 195 K, and C6H11D (monitored here as (m/z) ) 85) is observed at 255 K. (b) LITD survey experiment of the same mixture as in 5a, which shows that C6H11D is observed in the temperature range of 165 K to 205 K. No indication of any multiply deuterated species were observed in either experiment.

cyclohexane adsorption results in the observation of only monodeuterated cyclohexane in the high temperature shoulder region of the LITD survey experiment data. The associated TD spectra for C6H12 and C6H11D are shown in Figure 4a. The (m/ z) ) 84 and (m/z) ) 85 ion intensities have been chosen to monitor C6H12 and C6H11D desorption from the surface. The TD spectra in Figure 5a show that the high temperature desorption signal of cyclohexane consists in this case primarily of C6H11D, though there is some small contribution from C6H12, and the signal maximum is shifted to about 255 K. The C6H12 at this temperature presumably arises from rehydrogenation of C6H11 with surface H which was generated from the dehydrogenation of the initially adsorbed C6H12. The C6H11 species is a product of the dehydrogenation of C6H12 at lower temperature. A kinetic isotope effect is apparent for the desorption process since the peak desorption temperature in the case of hydrogen coadsorption is 20 K lower (235 K). It is interesting to note that, in the case of deuterium coadsorption, both of the rehydrogenation products (C6H11D and C6H12) desorb at the higher temperature (255 K). This will be discussed in detail later. The LITD survey results for both (m/z) ) 84 and (m/z) ) 85 ions are shown in Figure 4b. The LITD survey reveals that the high temperature shoulder in the decline of C6H12 observed under conditions of coadsorption of hydrogen has disappeared. Instead, the C6H12 signal decreases monotonically and rapidly to zero and C6H11D is detected in the temperature range between 165 and 205 K. A comparison of the TD spectra and the LITD survey experiments indicates that the fast part of the C6H12 decay between 165 and 180 K is due to a combination of cyclohexane desorbing and dehydrogenating on the surface, similar to what we observe in the absence of coadsorbed H. At the onset of this decay, the C6H11D is first observed. At the point where the decay of C6H12 is completed, the amount of C6H11D plateaus. The C6H11D is clearly related to a dehydrogenation product of

Pansoy-Hjelvik et al.

Figure 5. Series of TDS experiments (temperature ramp 4 K/s) for 0.25 monolayer cyclohexane coadsorbed with various exposures of deuterium (C6H12 is plotted here as the (m/z) ) 84 ion intensity and the C6H11D is plotted as the (m/z) ) 85 ion intensity).

cyclohexane. At approximately 205 K, the concentration of this species starts to decay and it is completely gone by 215 K. Figure 5 shows a series of TD spectra of C6H12 and C6H11D that were taken for variable deuterium exposures after 0.25 monolayer cyclohexane was adsorbed. At zero or low coverages of coadsorded D, molecular desorption of C6H12 is observed. As the surface coverage of coadsorbed D is increased, the desorption of molecular C6H12 moves to lower temperature. At higher concentrations of coadsorbed D a new peak is observed at higher temperatures due to the recombination of a surface C6H11 (vide infra) species (predominantly as C6H11D). As is evident from this figure, the low-temperature desorption feature (desorption of the molecular C6H12 species) shifts to lower temperature with increasing deuterium exposure. The high temperature recombination-desorption signal is observed only after high deuterium exposures. A comparison of the total area under the cyclohexane desorption peaks for a zero and a 200 L exposure of hydrogen reveals that the coadsorption of deuterium (or hydrogen) enhances the desorption of cyclohexane by a factor of approximately 2. It is possible that some or all of the C6H11D that we observe in the experiments leading to Figures 4 and 5 was produced via H/D exchange to produce C6H11D as a surface species. To investigate this possibility further, we performed an experiment involving coadsorption of C6H12 and D2 and held the sample temperature at 190 K, in the middle of the plateau of the C6H11D survey shown in Figure 4b. The surface adsorbate mixture was then probed in 1 min intervals by LITD. Even after 15 min, only monodeuterated cyclohexane was observed (no multiply deuterated species were observed). Figure 6 shows the LITD FTMS spectrum obtained after 15 min at 190 K in this experiment. This spectrum is consistent with only C6H11D (monodeuterated cyclohexane) in the gas phase. The parent mass 85 is the major peak as expected. The small peak at mass 86 is due to 13C12C5H11D as expected. The small peak at mass 78 is due to a small amount of benzene that is formed by dehydrogenation of the surface cyclohexyl species during the laser

Dehydrogenation of Cyclohexane on Pt(111)

J. Phys. Chem. B, Vol. 104, No. 28, 2000 6559 The major results in these studies are as follows: (1) We provide the first direct experimental evidence for C6H11 (cyclohexyl) as the product of the first step in the dehydrogenation of cyclohexane, and it is stable on the Pt(111) surface between 165 and 205 K. (2) Coadsorption of hydrogen with cyclohexane on Pt(111) shifts the onset for both cyclohexane molecular desorption and dehydrogenation to lower temperature. (3) A kinetic isotope effect is apparent for the rehydrogenation of C6H11. Coadsorption with hydrogen, in slow heating TDS experiments, results in C6H12 desorption due to recombination at 235 K; coadsorption with deuterium, in slow heating TDS experiments, results in C6H11D and C6H12 desorption due to recombination at 255 K. Discussion

Figure 6. LITD-FT mass spectrum obtained from an experiment in which C6H12 was coadsorbed with 200L D2 on Pt(111) followed by heating to 190 K. Spectra were taken at one minute intervals while the sample was held at 190 K. This spectrum was obtained after 15 min at 190 K. Mass 85 is the parent peak for the monodeuterated cyclohexane (C6H11D) that is formed from rehydrogenation of surface cylcohexyl (C6H11) as a result of the laser induced temperature jump of the LITD experiment. The other peaks in the spectrum are consistent with electron beam ionization of the monodeuterated cyclohexane. As discussed in the text, we do not observe multiply deuterated species in this experiment.

induced temperature jump of the LITD experiment.15,19,21 No significant contributions from multiply deuterated species are observed. Based on the signal-to-noise ratio of these experiments, we would be able to detect the doubly deuterated species if it were in the gas phase in concentrations above a few percent of the concentration of the monodeuterated species. Thus, simple H/D exchange on the surface can be ruled out as the source of the C6H11D signal which we observe. The most likely explanation for the C6H11D is that it arises from the thermally activated rehydrogenation of a surface cyclohexyl (C6H11) species during the LITD temperature jump. In the LITD experiment this happens as a result of the laser induced temperature jump which is used to desorb the reaction mixture from a spot on the surface. In the TDS experiments the C6H11D arises from a reaction ratelimited rehydrogenation of the cyclohexyl species. (rate limiting)

C6H11(a) + D(a) 98 C6H11D(a) f C6H11D(gas) Xu, et al.25 have previously observed cyclohexyl formed as a product of the electron bombardment of multilayers of cyclohexane on Pt(111). For thermally activated adsorbates which have access to one of several competitive reaction pathways, the partitioning among each pathway is often dependent on the heating rate. High heating rates occur when a metal surface is heated by a laser pulse, and adsorbates will undergo channels not available under normal heating conditions. This is the process by which LITD can lead to the intact desorption of surface species when slower heating would lead only to reaction. However, in the case of a species which is strongly bonded to the surface (such as the cyclohexyl species proposed here), the laser induced temperature jump can lead to the thermally activated production of a species with a lower desorption activation energy, followed by its desorption. Thus, the C6H11D detected between 165 and 205 K in the LITD survey experiment is the result of a laser-heating driven process which includes the recombination of C6H11 with surface D, and the subsequent desorption of C6H11D. In the absence of laser heating this recombination occurs at the slightly higher temperature of 235 K (H coadsorbed), and 255 K (D coadsorbed).

Our experiments establish cyclohexyl (C6H11) as a stable surface species in the thermally activated dehydrogenation of cyclohexane on Pt(111). In the presence of coadsorbed hydrogen or deuterium the cyclohexyl species is stable on the Pt(111) at surface temperatures up to 205 K. Coupling of Desorption and Reaction. The most dramatic effect that hydrogen coadsorption has on this reaction system is that the reaction and molecular desorption are both shifted to lower temperatures in the presence of surface hydrogen. The onset of reaction (Figures 2b, 3b, 4b) shifts from ∼205 K in the absence of hydrogen to ∼165 K in the presence of saturation amounts of hydrogen (a shift of 40 K). As one might expect, an identical temperature shift is observed for deuterium coadsorption. The peak in the molecular desorption feature in the TDS experiments shifts from 235 to 195 K without and with coadsorbed hydrogen (also a shift of 40 K). It should be remembered that, as pointed out previously, the fact that the peak temperature for molecular desorption is above the temperature for completion of reaction in the LITD survey experiment is a result of the differences in time scales between the TDS experiment (4 K/sec temperature ramp) and the LITD survey experiment. Since the initial dehydrogenation reaction rate in the absence of hydrogen at 205 K is approximately the same as the reaction rate at 165 K with saturation coverage of hydrogen (Figures 2b and 4b), we can calculate the change in reaction activation energy caused by surface hydrogen if it is assumed that only the activation energy is effected by the coadsorbed hydrogen.

(

A exp -

)

(

)

E(no hydrogen) E(with hydrogen) ) A exp R 205 R 165

This leads to a result that the coadsorption of saturation coverages of hydrogen leads to a ∼20% reduction in the activation energy.

E(no hydrogen) ) 1.24 E(with hydrogen) We have previously measured the activation energy for this reaction in the absence of coadsorbed hydrogen. At a cyclohexane coverage of 0.25 of saturation monolayer the activation energy is 13 kcal/mol.19 This leads to an activation energy in the presence of saturation coverage of coadsorbed hydrogen of 10.5 kcal/mol. Clearly, it is important to measure the effect of surface hydrogen on the activation energy and preexponential factors of the kinetics independently. Quantitative, isothermal measurements as a function of reaction temperature are planned to accomplish this. Previous work has indicated that the temperatures for the onset of dehydrogenation and molecular desorption for cyclo-

6560 J. Phys. Chem. B, Vol. 104, No. 28, 2000 hexane on clean Pt(111) surfaces are the same.18,19 The identical temperature shifts that we observe for the onset of reaction and the molecular desorption as a result of hydrogen coadsorption would indicate that the fact that both processes occur at the same temperature is not simply coincidental. Previous work on the coverage dependence of this reaction in the absence of hydrogen has indicated that a substantial ensemble of open platinum sites is required for the reaction.18,19 One could imagine that the coupling of the molecular desorption and reaction even at the low coverages used here was due to islanding of the cyclohexane on adsorption which then required desorption to provide open Pt sites. The hydrogen coadsorption effect could then be explained by the hydrogen crowding the surface leading to higher local concentrations of cyclohexane, thus destabilizing the bonding of the cyclohexane to the Pt resulting in desorption and thus reaction at a lower temperature. While this explanation is likely to be part of the story, the effect of coadsorbed hydrogen on this system cannot be completely explained by simple surface crowding arguments. In cyclohexane dehydrogenation coverage dependent experiments, the molecular desorption of cyclohexane does shift slightly to lower temperatures as a function of crowding the surface with cyclohexane. However, the desorption temperature shifts by only ∼5 K (from ∼235 K to ∼230 K) as the cyclohexane coverage is increased from ∼0.25 monolayer to full monolayer coverage.19 Thus it is clear that while surface crowding may occur, the 40 K shift to lower temperature of the desorption and reaction processes with hydrogen coadsorption must involve more than simple surface crowding. Indeed, it seems that the hydrogen must modify the Pt surface in a manner which destablizes the cyclohexane-platinum interaction. While it is possible that the cyclohexane does island under the hydrogen coadsorption conditions used here, the islands must be relatively small since the molecular desorption peak (Figure 5) shifts uniformly to lower temperature as a function of surface hydrogen coverage and there are no indications of any different behavior between molecules at the edges of islands (and presumably closer to coadsorbed hydrogens) and molecules in the interior of islands. Stability of C6H11 on the Surface. The data in Figures 3b and 4b indicate that the cyclohexyl species is stable on the Pt surface at temperatures up to 205 K. Indeed, under typical temperature ramping conditions as in TDS(Figures 4a and 5a) it is still on the surface to even higher temperatures. At first observation, it might appear that our data indicate an enhanced stability of the cyclohexyl species in the presence of surface hydrogen. However, this is not really substantiated. In the presence of coadsorbed hydrogen or deuterium (Figure 3b or 4b), the cyclohexyl species decomposes at 205 K to give cyclohexene and eventually C6H9. In the absence of surface hydrogen, the cyclohexyl species is not formed until this temperature, and we have no experimental observation of it under those conditions (e.g., it reacts further to form cyclohexene immediately on formation). Thus, the simplest interpretation of our data is that the subsequent dehydrogenation chemistry of C6H11 is not affected by coadsorbed hydrogen and that at 205 K it converts to cyclohexene which then converts to C6H9, both in the absence and presence of coadsorbed hydrogen. The overall effect of surface hydrogen coadsorption is to open a window of temperature (165-205 K) in which we can form and observe cyclohexyl on the surface. This happens as a result of the destabilization of the cyclohexane in favor of desorption and dehydrogenation occurring at a lower temperature. H/D Isotope Effect on the Rehydrogenation of Cyclohexyl. An interesting H/D isotope effect can be observed on the

Pansoy-Hjelvik et al. rehydrogenation of the cyclohexyl species. Figure 3a indicates that the cyclohexyl is hydrogenated with coadsorbed H to form C6H12 with a peak temperature of ∼235 K. Under the same heating rate conditions, the peak for the rehydrogenation of cyclohexyl with D to form C6H11D (Figure 4a) is ∼255 K. This observation is consistent with the hydrogenation of cyclohexyl being rate limited by surface hydrogen mobility, leading to a measurably higher required temperature in the case of deuterium coadsorption. One can also see from Figure 4a that in the case of deuterium coadsorption, we still observe a small amount of C6H12 as the hydrogenation product. This is presumably due to surface hydrogen produced by the dehydrogenation of the original cyclohexane since we take care to avoid surface hydrogen from the chamber background. In the deuterium coadsorption experiments, the C6H12 resulting from the hydrogenation of cyclohexyl is observed at the same (higher) temperature as the C6H11D. This can be understood if the mobility of the small amount of hydrogen is determined by the mobility of the sea of adsorbed deuterium in which it finds itself. We plan to look at this effect more carefully as a function of hydrogen/deuterium ratio in future experiments. Summary From TDS, AES, and LITD-FTMS results, we have shown that hydrogen (or deuterium) coadsorbed with cyclohexane on Pt(111) changes the reactivity of the metal surface for cyclohexane dehydrogenation. Our experiments show that with coadsorbed hydrogen (or deuterium) the onset for cyclohexane dehydrogenation and molecular desorption occur at a surface temperature which is 40 K lower than in the absence of surface hydrogen. This corresponds to a ∼20% reduction in the activation energy for dehydrogenation in the presence of coadsorbed hydrogen. The initial product of cyclohexane dehydrogenation is seen to be cyclohexyl (C6H11) which is stable on the surface at temperatures up to 205 K. The cyclohexyl species can be detected and monitored by following its rehydrogenation to form C6H12 or C6H11D (for experiments with coadsorbed deuterium) which then rapidly desorbs. On the basis of this and previous work,14-19 the dehydrogenation of cyclohexane on Pt(111) can be seen to occur in the following stepwise manner. The rehydrogenation of the cyclohexyl species shows

an H/D isotope effect that leads to the deuteration of C6H11 to form C6H11D at 255 K, whereas coadsorption with H leads to C6H12 from rehydrogenation at 240 K. Interestingly, the small amount of C6H12 that is formed in the deuterium coadsorption experiments (from surface H formed in the initial dehydrogenation of the cyclohexane) occurs at 255 Ksthe same temperature as the C6H11D. The fact that C6H12 is produced at this higher temperature in the presence of large amounts of surface deuterium is presumably due to the surface hydrogen mobility

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