Molecular Mechanisms of Propyne Oxidation on the Pt(111) Surface

Adriaan J. M. Mackus , Noémi Leick , Layton Baker , and Wilhelmus M. M. ... Daniel J. Burnett, Aaron M. Gabelnick, Daniel A. Fischer, Anderson L. Mar...
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7748

J. Phys. Chem. B 2001, 105, 7748-7754

Molecular Mechanisms of Propyne Oxidation on the Pt(111) Surface: In Situ Soft X-ray Studies in Pressures of Oxygen Aaron M. Gabelnick,† Daniel J. Burnett,‡ and John L. Gland*,† Departments of Chemistry and of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109

Daniel A. Fischer National Institute of Standards and Technology, Gaithersburg, Maryland ReceiVed: March 28, 2001

Oxidation of preadsorbed propyne has been characterized on the Pt(111) surface in oxygen at pressures up to 0.009 Torr using fluorescence yield ultrasoft X-ray adsorption methods above the carbon K edge. A combination of temperature-programmed reaction spectroscopy (TPRS) and temperature-programmed fluorescence yield near edge spectroscopy (TP-FYNES) experiments indicating that similar oxidation pathways occur both for coadsorbed oxygen and pressures of oxygen. Soft X-ray spectroscopy indicates that propyne’s π system adsorbs nearly parallel to this surface with a saturation coverage of 1.45 × 1015 C atoms/cm2. Oxidation of small propyne coverages with coadsorbed oxygen results in simultaneous CO2 and H2O peaks at 320 and 420 K, as seen in TPRS. Oxidation of higher propyne coverages with coadsorbed oxygen results in a broad oxidation peak over the 350-420 K temperature range. Oxidation of a saturated propyne monolayer in oxygen pressures (TP-FYNES) results in a rapid decrease in carbon coverage over the same temperature range, suggesting similar mechanisms. Isothermal oxidation in oxygen atmospheres indicates that propyne oxidation is first-order in propyne coverage and has an activation energy of 17 kcal/mol for high coverages. Deviations from first-order behavior suggest that a second process may become important at lower coverages in oxygen atmospheres. Regardless of coverage and initial conditions, both TPRS and TP-FYNES indicate oxydehydrogenation and skeletal oxidation occur simultaneously and the oxidation proceeds with a fixed C3H4 stoichiometry. Taken together, these results give a molecular picture of propyne oxidation on the Pt(111) surface.

Introduction Deep oxidation of simple hydrocarbons using metal catalysts is used for calorimetric sensing of hydrocarbons and for the removal of hydrocarbons from combustion exhaust streams. The molecular mechanisms of deep oxidation processes on metal surfaces are just beginning to emerge, despite their importance.1-6 This paper is part of a series of in situ studies of deep oxidation on a range of Pt surfaces. Studies of C3 oxidation mechanisms have recently been completed on Pt single crystals, foils, and thin films.7-9 Recently, we reported that propylene oxidation on the Pt(111) surface with coadsorbed oxygen proceeds via a C3H5 intermediate with removal of the vinyl hydrogen based on TPRS studies in UHV.7 TP-FYNES studies in pressures of oxygen indicate that this same intermediate is dominant for oxygen pressures up to 0.02 Torr.9 Spectroscopic identification of the C3H5 intermediate showed that the structure of the intermediate is tri-σ-1-methylvinyl adsorbed with the dehydrogenated C-C bond parallel to the surface. Similar sequential oxydehydrogenation results were obtained during studies of cyclopropane oxidation on the Pt(111) surface.10 Using a combination of HREELS, TPD, and isotope studies, Madix et al. have recently conducted mechanistic studies of alkene oxidation with coadsorbed oxygen on the Pd(100) surface.3,4 Sequential oxidation † ‡

Department of Chemistry. Department of Chemical Engineering.

is also observed on the Pd surface where the vinyl hydrogen is removed in the first step followed by skeletal oxidation and the removal of methyl hydrogens. However, TPD studies of acetylene oxidation on the Pt(111) surface indicate that oxydehydrogenation does not precede skeletal oxidation for this highly unsaturated molecule with very strong C-H bonds.11 The mechanism for acetylene oxidation was suggested to proceed via a CH(a) intermediate, since the C-H stoichiometry remained constant during the two-step oxidation process.11 These studies clearly indicate that surface properties and reactant properties both play major roles in controlling oxidation reactions. Recently a TPD study of propyne adsorption and decomposition has been reported on Pt(111) and Sn/Pt(111) surface alloys.12 In contrast to several other surfaces including Pd(111), propyne does not cyclotrimerize on the Pt(111) surface. Molecular desorption is the dominant path on the alloy surfaces, whereas dehydrogenation is the major reaction pathway on the Pt(111) surface. Although the bonding and orientation of propyne on the Pt(111) surface has not been studied, several analogous molecules have been studied using a combination of techniques. The bonding and orientation of propylene to the Pt(111) surface has been studied using NEXAFS.13 Spectra for ethylene and acetylene on the Pt(111) surface have also been published.14,15 Both propylene and ethylene bond to the Pt(111) surface in a di-σ configuration. The CsC bond is parallel to the surface in

10.1021/jp011167x CCC: $20.00 © 2001 American Chemical Society Published on Web 07/19/2001

Propyne Oxidation on the Pt(111) Surface

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these adsorbed species as indicated by the relative intensities of the π* and CdC σ* resonances with normal and glancing incidence. The geometry of adsorbed acetylene on the Pt(111) surface remains unclear. NEXAFS studies suggest that acetylene lies nearly parallel to the surface upon adsorption at 90 K;15 however, recent SFG studies of acetylene adsorption on the Pt(111) surface suggest that acetylene adsorbs in a tilted η2µ3-vinylidene configuration at 125 K.16 However, both techniques indicate that the triple bond in acetylene is reduced to a double bond through interaction with the surface. Experimental Section TP-FYNES and FYNES experiments were conducted on the U7A beamline at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory using a fluorescence yield detector optimized for carbon fluorescence.17 The U7A beamline surface science endstation is optimized for the investigation of catalytic reactions and characterization of important complex surface intermediates and has been described in detail elsewhere.7 The Pt(111) crystal was mounted on Ta wire supports on the end of a 6 ft liquid nitrogen cooled re-entrant manipulator. Temperature was measured with a 0.01 mm chromel-alumel (type K) thermocouple spot-welded to the back of the crystal and controlled with a RHK temperature controller. For all TPFYNES experiments, a linear heating rate of 0.5 K/s was used. The crystal was cleaned by initial Ar+ ion sputtering followed by annealing to 1000 K. During experimental runs, the sample was cleaned by annealing the crystal to 600 K in 0.002 Torr of oxygen for 1 min, followed by a 20 s anneal at 1000 K. Reactive gases were admitted to the background through leak valves. Flow was maintained throughout reactivity studies using a throttled turbo pump. Oxygen pressures were measured with a capacitance manometer. Spectra with 150 µm/150 µm slits gave an overall resolution of 0.4 eV. Spectra of adsorbed species were divided by a clean spectrum taken on the same ring fill to minimize the effects of scattered light. During experiments involving pressures of reactants, the chamber was separated from the synchrotron using a 2000 Å Al window valve. The in situ kinetic experiments were performed with 450 µm/450 µm slits which results in a resolution of 1.2 eV and an intensity of 10 000 cts/s for a CO monolayer with the window in. The heating rate was 0.5 K/s. Data were averaged over a 4 s interval, which resulted in a signal-to-noise ratio of about 4/1 for a CO-saturated monolayer. This combination of experimental considerations limits the temperature resolution for the temperature-programmed experiments to approximately 2-3 K. Repeated experiments indicate thermal transitions are reproducible to 2 K. CO TP-FYNES desorption spectra were used to confirm the performance of this system and reproduce published results. Absolute carbon coverages were determined by comparing the carbon continuum levels observed for adsorbed propyne with a saturated monolayer of CO. Since the absolute carbon coverage for a CO monolayer is known,18 it is possible to determine absolute surface carbon concentrations. Daily calibration using a saturated CO monolayer was used to ensure the consistency of experiments. TP-FYNES experiments were conducted in the following manner. After the crystal was cleaned and cooled to 150 K, a monolayer of propyne was dosed while coverage was verified by monitoring the surface carbon concentration using continuum fluorescence. When the surface carbon concentration reached the saturation level, the leak valve was closed. Oxygen was introduced through a leak valve, the ion gauge was turned off,

Figure 1. TPRS of propyne with excess coadsorbed oxygen. Two reaction channels are present for this low coverage of propyne, yielding the complete oxidation products: CO2 and H2O.

and the reaction pressure was achieved by using a combination of gate valve throttling and leak valve control while monitoring the pressure with a 1 Torr capacitance manometer. For TP-FYNES experiments, the crystal was heated at 0.5 K/s to 600 K. The carbon edge function was placed at the energy of the XPS maximum of C 1s of propyne on Pt(111) of 285.0 eV plus the work function of Pt(111) with an adsorbed layer of propylene of 4.4 eV,19 yielding a placement of 289.4 eV. Since the work function of propyne on the Pt(111) surface is not known, the propylene value was used. Several adsorbed hydrocarbons (acetylene, ethylene, and propylene) yield similar work functions of the Pt(111) surface, giving an approximate error of the step function placement at (0.2 eV. Spectra were fitted after subtraction of the carbon step edge using Gaussian functions. The functional form of the step edge, location of the step edge, determination of resonance angle, and FYNES peak fitting in general is described in considerable detail by Outka and Stohr in refs 20 and 21. TPRS experiments were performed at the University of Michigan in an ultrahigh vacuum chamber equipped with turbomolecular, ion, and TSP pumps, which combined to give a base pressure of 1 × 10-10 Torr.9 The system was equipped with a quadrapole mass spectrometer for TPRS and auger electron spectroscopy (AES) to verify surface cleanliness. Details of this experimental system and TPRS methods are available in the literature.9,22 All TPRS experiments were conducted with a 5 K/s heating rate. Propyne (Aldrich, 98%) and oxygen (Matheson, 99.999%) were dosed into the chambers using a differentially pumped gas manifold system. Propyne was purified using a series of freeze-pump-thaw cycles prior to dosing. Results The TPRS spectrum resulting from propyne coadsorbed with excess atomic oxygen is shown in Figure 1. The major desorbing species from the surface are oxygen, water, and carbon dioxide. The two major carbon dioxide peaks are centered at 320 and

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Figure 2. TPRS spectra demonstrating the transition from rich to lean surface stoichiometry in propyne oxidation. As the stoichiometry becomes richer, incomplete oxidation and dehydrogenation products become dominant.

420 K. Water desorbs in two peaks corresponding to the carbon dioxide desorption peaks. Complete propyne oxidation is observed in excess oxygen. Carbon monoxide is not formed, the slight increases in the m/e ) 28 trace can be attributed to the fragmentation of carbon dioxide in the mass spectrometer. No H2 desorbs from the surface (data not shown). Above 700 K, the recombination of excess atomic oxygen results in oxygen desorption. This oxygen desorption clearly indicates that excess oxygen is present on the surface throughout the oxidation process. The transition from rich to lean surface stoichiometry for coadsorbed propyne and oxygen can be seen in Figure 2, a series of desorption traces where a saturation coverage of atomic oxygen is postdosed with increasing amounts of propyne. The transition can be seen in the desorption traces of O2, H2, H2O, CO2, and CO. Partial oxidation and decomposition products are observed for excess propyne. With increasing propyne coverage, the amount of oxygen desorbing decreases abruptly and only the smallest dose of propyne results in excess oxygen. The H2 desorption trace shows increased intensity with increased propyne exposure. For the smallest propyne dose, no H2 desorbs. However, the amount of

H2 desorbing at 400 K shows a sharp increase as the propyne postdose increases. The hydrogen desorption spectrum is similar to the spectrum resulting from propyne dehydrogenation on Pt(111). The amount of CO desorbing also increases with increasing propyne postexposure. For the smallest propyne dose, no CO desorbs from the surface. As propyne postexposure increases, CO desorption peaks near 500 K increase, with an additional feature developing near 800 K. With the largest postexposure of propyne, a large CO desorption peak develops at 435 K. The desorption spectra for the complete combustion products, CO2 and H2O, show interesting propyne coverage dependencies. With increasing propyne postexposure, the higher temperature CO2 desorption peak shifts down in temperature, from 420 to 340 K for the highest propyne postexposure. The low-temperature CO2 desorption peak at 320 K remains at constant temperature and intensity over the entire range of propyne exposures. The behavior of the H2O desorption trace mirrors the CO2 desorption trace, except for the lower temperature shoulder that develops from background H2 in the vacuum chamber. This has been verified in oxidation experiments of propyne-d4 (data not shown).

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Figure 4. Comparison of the carbon continuum concentrations for monolayer propyne oxidation and monolayer propylene oxidation in 0.002 Torr of oxygen using TP-FYNES. Skeletal oxidation of both reactants begins at 330 K. Figure 3. FYNES spectra at normal and glancing incidence of a saturated propyne monolayer at 150 K.

The FYNES spectrum of an adsorbed propyne monolayer for both normal and glancing incidence at 150 K is shown in Figure 3. The spectrum has been fit based by procedures outlined in the Experimental Section. The peaks in the propyne spectrum have been assigned on the basis of previous NEXAFS studies of related hydrocarbons and are as follows. The peak at 284 eV corresponds to the π* orbital of the C-C triple bond. The peak at 288 eV is the C-H σ* resonance. The two peaks at 293 and 298 eV correspond to C-C σ* resonances. The lower energy 293 eV resonance corresponds to the longer C-C bond, namely the C-C single bond to the methyl group. The higher energy 298 eV resonance corresponds to the shorter, unsaturated C-C bond. The 310 eV resonance has been seen in a number of adsorbed hydrocarbons on surfaces and previously has been assigned as a multiple scattering resonance. In this case, it is simply an artifact resulting from normalization.23 The peak fit has been included for figure clarity. The angular dependence of the 298 eV C-C σ* resonances associated with the unsaturated C-C bond indicates it is nearly parallel to the surface. This resonance is considerably larger in normal incidence than in glancing incidence. Since the resonance is above the carbon step edge, determination of the exact angular orientation of the resonance is not possible. The angular dependence of the small π resonance in these normalized spectra are consistent with a near parallel orientation, as expected for an unsaturated bond. However, limited signal-to-noise and the possibility of multiple π resonances make it difficult to determine surface orientation directly from the π system. The strong interaction between the unsaturated bond in propyne and the surface is reflected in a significant decrease of the 284 eV π* resonance intensity. Compared to the multilayer spectrum (data not shown) the relative intensity of the π* resonance to the carbon continuum has decreased significantly. This decrease is indicative of bonding through the π orbitals to the surface, as seen in the di-σ bonding of propylene on Pt(111). The orientation of the methyl group in propyne can be determined by examining the relative intensities of the C-C

single bond resonance at 293 eV. The intensity of the resonance is slightly larger in glancing incidence than normal incidence, indicating the resonance is at least 55° up from the surface plane. This orientation of the methyl group is consistent with the unsaturated C-C bond being nearly parallel to the surface and the methyl group rising off the surface plane. The TP-FYNES of the carbon continuum for oxidation of saturated monolayers of propyne (solid line) and propylene (dashed line) in 0.002 Torr of oxygen are directly compared in Figure 4. The propylene TP-FYNES oxidation spectrum (dashed line) has two distinct features, as indicated in a previous publication.7 Molecular desorption and initial oxydehydrogenation of propylene occurs in the 210-270 K temperature range. The second drop in surface carbon concentration results from skeletal oxidation of a 1-methylvinyl intermediate in the 330420 K temperature range. The propyne TP-FYNES oxidation spectrum (solid line, Figure 4) is quite different than the propylene spectrum. The coverage for a saturated layer of propyne is 1.45 × 1015 C atoms/cm2, compared to 0.90 × 1015 C atoms/cm2 for a saturated layer of propylene. The propyne level remains constant until 330 K, where oxidation begins and continues until 420 K, where all carbon-containing species have been removed. This decrease is the result of skeletal oxidation and is similar to skeletal oxidation of propylene. This conclusion is based on the similarity of the TPRS data (Figure 1) and the TP-FYNES data (Figure 4) and a common temperature for removal of carbon for both propylene and propyne. No molecular desorption of propyne is observed in the TP-FYNES or TPRS studies. C-H stoichiometries for the surface species can be determined by comparing the intensity of the C-H resonance at 288 eV with the carbon continuum intensity at 330 eV at the magic angle.7 Calibration of the intensity ratio is based on known adsorbed molecular species. A direct comparison of two TPFYNES spectra for propyne oxidation is shown in Figure 5. The solid trace is the 288 eV C-H σ* resonance and the dashed trace is the 330 eV carbon continuum. Both traces are on an absolute fluorescence count scale, enabling the determination

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Figure 5. C-H stoichiometry determination at the magic angle during propyne oxidation indicates constant C-H stoichiometry throughout the entire oxidation process. No dehydrogenated intermediates are detected in the oxidation process.

Figure 6. TP-FYNES for oxidation of saturated monolayers of propyne in three different oxygen pressures. In higher oxygen pressures, propyne oxidation occurs at a lower temperature.

of surface stoichiometry. Since these two traces mirror each other exactly, the stoichiometry of surface species remains constant at C3H4 through the entire oxidation process. This establishes that oxydehydrogenation and skeletal oxidation of propyne on the Pt(111) surface occur simultaneously. The TP-FYNES taken during oxidation of saturated monolayers of propyne in three different oxygen pressures can be seen in Figure 6. All three TP-FYNES traces begin with the same initial surface carbon concentration. In all three pressures of oxygen the surface carbon concentration begins to drop around 330 K, with the highest pressure of oxygen resulting in a slightly lower removal temperature. The temperature of the surface carbon concentration drop decreases by 30 K as the oxygen pressure increases from 0.0005 to 0.009 Torr of oxygen

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Figure 7. Propyne TP-FYNES with gas phase oxygen only and propyne TP-FYNES with both preadsorbed atomic oxygen and gasphase oxygen indicates that atomic oxygen is the oxidizing agent in both cases.

for a given surface coverage. The surface is clean of carbon for all three traces above 420 K. The effect of coadsorbed oxygen on the oxidation of propyne is examined in Figure 7. The solid trace is the TP-FYNES of a monolayer of propyne in 0.002 Torr of oxygen, as described in Figure 4. The dashed trace represents a TP-FYNES where the Pt(111) surface has been predosed with 0.25 monolayer (ML) of O(a). The oxygen predose inhibits propyne adsorption on the Pt(111) surface, yielding an overall surface carbon concentration of 1.2 × 1015 C atoms/cm2 with coadsorbed atomic oxygen. Upon heating of the coadsorbed O(a) + propyne surface, the overall surface carbon concentration remains constant until nearly 350 K. Above 350 K, the carbon concentration decreases and all carbon is removed by 420 K. The decrease in surface carbon for the O(a) + propyne case (dashed trace) mirrors the TP-FYNES of a fully propynesaturated case (solid line) at temperatures of 350 K and above. The initial drop in concentration observed for the full monolayer of propyne at 330 K is not observed for the partial monolayer of propyne. Instead, the oxidation process begins at a higher temperature and essentially meets up with the full monolayer oxidation at 350 K. The activation energy of oxidation was determined through a series of isothermal experiments shown in Figure 8. In all data presented, a full monolayer of propyne was heated in 0.002 Torr of oxygen from 150 K at 3 K/s to 10 K below the desired reaction temperature. The surface was then heated at 1 K/s to the final reaction temperature. This method of heating reduces temperature overshoot. At the end of each isothermal experiment, the surface was heated to 600 K in oxygen to establish an internal zero carbon level. The decays were fit with firstorder exponential fits (upper panel). Below coverages of ca. 0.5 ML, the decay rate decreased. On the basis of the firstorder region at high coverage an Arrhenius plot was constructed. The activation energy was determined to be 17 kcal/mol or 71 kJ/mol for the initial oxidation process, with a preexponential factor of 1.4 × 108 s-1.

Propyne Oxidation on the Pt(111) Surface

Figure 8. Isothermal kinetic experiments of the propyne oxidation at high coverage (upper panel) for four different temperatures used for determination of the activation energy of propyne oxidation in 0.002 Torr of oxygen via Arrhenius methods (lower panel). The activation energy is 17 kcal/mol.

Discussion The combination of TPRS, FYNES, TP-FYNES, and isothermal kinetic experiments gives a detailed molecular picture of the mechanism and energetics of propyne oxidation on the Pt(111) surface. Spectroscopy of adsorbed propyne elucidates the bonding of propyne to the Pt(111) surface. The oxidation mechanism has been studied for a wide range of conditions using a combination of in situ temperature-programmed techniques and isothermal studies. The bonding of propyne to the surface at low temperature is elucidated using FYNES spectroscopy. Stoichiometric analysis indicates propyne adsorbs molecularly with a C3H4 stoichiometry at 150 K. Analysis of the π* resonance and corresponding CsC σ* resonances indicates that the π system is diminished upon adsorption and the π system is adsorbed nearly parallel to the surface. Exact orientation determination is not possible due to the splitting of the π* orbital upon adsorption and low S/N. However, the angular dependence of the CtC σ* resonances indicate the π system is nearly parallel to the surface with the methyl group up from the surface plane. FYNES spectra alone cannot determine if a similar rearrangement to methyl-

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7753 vinylidene occurs for propyne adsorption or whether a di-σ adsorption is favored. SFG experiments suggest that upon adsorption at 125 K, acetylene rearranges into a η2-µ3-vinylidene configuration. If the π system tilt angle is small, this arrangement is also consistent with the FYNES results reported here. The overall carbon concentration for a saturated propyne monolayer is increased by a factor of 1.6 compared to a saturated carbon coverage for propylene (see Figure 4) and is also increased by a factor of 1.6 compared to the saturated coverage of acetylene on the Pt(111) surface.24 The increased packing density of propyne compared to propylene is consistent with the upright orientation of the methyl group, which reduces the surface footprint. The methyl group is nearly parallel to the surface for propylene adsorption on the Pt(111) surface;7 however, FYNES spectra indicate the methyl group is considerably up off the surface plane for propyne. The surface carbon coverage of propyne is 1.6 times greater than that of acetylene, which leads us to believe the packing of the π system is similar for both molecules. If the surface footprint of propyne and acetylene were exactly the same, the carbon coverage would increase by a factor of 1.5, with an identical molecular coverage. The extra surface carbon concentration in propyne is due to the methyl group, and since it is up off the surface, the overall molecular packing density is not affected. In excess-oxygen conditions from UHV to 0.009 Torr of oxygen, both oxydehydrogenation and skeletal oxidation proceed at the same rate. This is true for all cases studied, from coadsorbed excess oxygen in UHV to saturated propyne layers in pressures of oxygen. This is also the case for acetylene oxidation on the Pt(111) surface, indicating that the π functionality and increased C-H bond strength limits the oxydehydrogenation process on Pt(111). For the oxidation of propylene, oxydehydrogenation precedes skeletal oxidation; however, it is the vinyl hydrogen that is removed first in propylene oxidation, forming the stable 1-methylvinyl intermediate. The oxidation temperature for 1-methylvinyl and propyne is nearly identical, suggesting a common rate-limiting step. Considering 1-methylvinyl has one more hydrogen than propyne and does not undergo oxydehydrogenation prior to skeletal oxidation, it is not surprising that propyne does not either. In excess oxygen, oxydehydrogenation and skeletal oxidation occur simultaneously throughout the oxidation process. When the oxidation process begins near 320 K, both H2O and CO2 desorb from the surface, as indicated by TPD experiments. An identical mechanism is demonstrated by the in situ stoichiometric measurements shown in Figure 5. Throughout the oxidation process, the reaction proceeds with a fixed C-H stoichiometry. Fixed stoichiometry is demonstrated by the comparison of the C-H σ* and C continuum resonances (Figure 5) and by integration of the CO2 and H2O desorption peaks from TPD experiments (Figure 1). Acetylene is also known to oxidize with a fixed C-H stoichiometry, with adsorbed CH proposed as the oxidation intermediate.11 TPRS of propyne oxidation with coadsorbed oxygen indicates that two reaction channels exist for low coverage propyne oxidation, with CO2 desorption peak maxima at 320 and 420 K. Integration of both the CO2 and H2O desorption peaks indicates that the first desorption peak represents half of the total desorption. This suggests the possibility of a surface intermediate above 320 K for low coverages; unfortunately, FYNES experiments for small propyne coverages are not currently feasible. With higher propyne coverages, the CO2 TPRS desorption peaks grow together (Figure 2), undoubtedly why no intermediate is observed in the TP-FYNES experiments.

7754 J. Phys. Chem. B, Vol. 105, No. 32, 2001 TP-FYNES experiments of propyne in pressures of oxygen with and without coadsorbed atomic oxygen illustrate two mechanistic details (Figure 7). Since the removal of carbon from the surface occurs in the same temperature range and in combination with TPRS results that indicate similar temperatures of oxidation, we can conclude the oxidizing agent is adsorbed atomic oxygen even when adsorption is from the gas phase. The initial oxidation temperature does shift slightly higher for reduced coverages of propyne seen in the case of coadsorbed oxygen. This coverage dependence indicates that propynepropyne interactions decrease the initiation temperature for higher propyne coverages. In terms of activation energy, this suggests increasing activation energies with decreasing coverage. These results are consistent with the TPD results. Using Arrhenius methods shown in Figure 8, the activation energy of propyne oxidation was determined to be 17 kcal/mol, with a prefactor of 1.4 × 108 s-1. Using first-order Redhead analysis,22 the activation energy for the first CO2 desorption peak was estimated at 19 kcal/mol. We believe the Arrhenius method is more representative of the actual mechanism, since no preexponential factor needs to be assumed to determine activation energy, whereas a standard prefactor was assumed in our Redhead analysis. The Arrhenius activation energy value is slightly higher than the energy reported for acetylene oxidation with coadsorbed oxygen in a vacuum (∼12.8 kcal/mol);11 however, preexponential factors were assumed in the published study. Our activation energy matches well with the 17.0 kcal/ mol activation energy reported for propane oxidation on a platinum wire.25 Kinetic modeling based on the mechanism demonstrated here is currently underway.26 Initial efforts suggest increasing activation energies from 17 to 23 kcal/mol with decreasing coverage. The change in activation energy reflects an oxidation process in which it becomes more difficult to oxidize surface species as coverage decreases. The activation energy reported in this study reflects the initial activation energy for a saturated propyne coverage; however, as TPD experiments show, oxidation peaks at higher temperature exist for lower propyne coverages. By combining kinetic modeling with these powerful in situ methods, mechanisms for increasingly complex reactions will become accessible. Conclusions The oxidation of propyne on the Pt(111) surface has been characterized from conditions of coadsorbed oxygen to 0.009 Torr of oxygen. Propyne bonds to the Pt(111) surface through its π system, which is diminished upon adsorption. Skeletal

Gabelnick et al. oxidation and oxydehydrogenation occur simultaneously for a wide range of oxygen-rich conditions on the Pt(111) surface with an initial activation energy of 17 kcal/mol. The oxidation proceeds with a fixed C-H stoichiometry for all cases studies. Multiple techniques indicate that the rate of the oxidation processes decrease with decreasing coverage, resulting in broad temperature-programmed peaks and tailing in isothermal experiments. With a preadsorbed atomic oxygen layer, propyne adsorption decreases by 20%; however, the oxidation process proceeds in a similar manner. Acknowledgment. Financial support was provided by DOE grant number DE-FG02-91ER1490. NSLS is a DOE facility. References and Notes (1) Aryafar, M.; Zaera, F. Catal. Lett. 1997, 48, 173. (2) Veser, G.; Schmidt, L. D. AIChE J. 1996, 42, 1077. (3) Guo, X. C.; Madix, R. J. J. Am. Chem. Soc. 1995, 117, 5523. (4) Guo, X. C.; Madix, R. J. Surf. Sci. 1997, 391, L1165. (5) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (6) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779. (7) Gabelnick, A. M.; Capitano, A. T.; Kane, S. M.; Fischer, D. A.; Gland, J. L. J. Am. Chem. Soc. 2000, 122, 143. (8) Walton, R. W.; Gland, J. L. Manuscript in preparation. (9) Gabelnick, A. M.; Gland, J. L. Surf. Sci. 1999, 440, 340. (10) Franz, A. J.; Ranney, J. T.; Gland, J. L.; Bare, S. R. Surf. Sci. 1997, 374, 162. (11) Megiris, C. E.; Berlowitz, P.; Butt, J. B.; Kung, H. H. Surf. Sci. 1985, 159, 184. (12) Peck, J. W.; Mahon, D. I.; Koel, B. E. Surf. Sci. 1998, 410, 200. (13) Cassuto, A.; Mane, M.; Tourillon, G.; Parent, P.; Jupille, J. Surf. Sci. 1993, 287/288, 460. (14) Horsley, J. A.; Stohr, J.; Koestner, R. J. J. Chem. Phys. 1985, 83, 3146. (15) Stohr, J.; Sette, F.; Johnson, A. L. Phys. ReV. Lett. 1984, 53, 1684. (16) Cremer, P. S.; Su, X.; Shen, R.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 6474. (17) Fischer, D. A.; Colbert, J.; Gland, J. L. ReV. Sci. Instrum. 1989, 60 (7), 1596. (18) Norton, P. R.; Davies, J. A.; Jackman, T. E. Surf. Sci. 1982, 122, L593. (19) Gland, J. L.; Somorjai, G. A. AdV. Colloid Interface Sci. 1976, 5, 205. (20) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992. (21) Outka, D. A.; Stohr, J. J. Chem. Phys. 1988, 88 (6), 3539. (22) Redhead, P. A. Vacuum 1962, 12, 203. (23) Two normalization methods are used in interpretation of spectra. Division by a clean spectrum is used in cases of low S/N, whereas subtraction by clean is used is cases of high S/N. Comparison of these two methods allows for the determination of which peaks are artifacts. If a peak is present only in one normalization method, it is called into question and examined further. (24) Burnett, D. J.; Gabelnick, A. M.; Fischer, D. A.; Gland, J. L., in press. (25) Hiam, L.; Wise, H.; Chaikin, S. J. Catal. 1968, 10, 272. (26) Miletic, M.; Gabelnick, A. M.; Gland, J. L., in preparation.