240
J. Phys. Chem. 1994,98, 240-244
Desorption Kinetics and Adlayer Structure of mPentane on A1203(0001) C. M. Aubuchon, B. S. Davison, A. M. Nishimura, and N. J. Tro' Department of Chemistry, Westmont College, 955 La Paz Road, Santa Barbara, California 93108 Received: June 24, 1993; In Final Form: October 22, 1993'
Temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the desorption kinetics and adlayer structure of n-pentane adsorbed on A1203(0001) in ultrahigh vacuum, At low coverages, T P D traces displayed one peak that saturated with increasing exposure time and was attributed to monolayer desorption. A second, multilayer peak was observed a t a lower temperature for coverages exceeding 1 ML. At coverages below one monolayer, pentane was found to display first-order desorption kinetics with an activation barrier for desorption of Ed = 13.3 f 0.8 kcal/mol and a first-order preexponential of u1 = 3 X lO20*0.5 s-l. The multilayer displayed zero-order desorption kinetics with an activation barrier of Ed = 9.2 f 0.6 kcal/mol and a zero-order preexponential of vo = 1 X 1030*0.3 molecules/(cm2 s). FTIR spectra revealed that pentane molecules orient primarily parallel to the surface a t submonolayer coverages and perpendicular to the surface a t multilayer coverages. Independent coverage measurements using laser interference techniques to calibrate infrared absorption cross sections revealed that the surface number density a t one monolayer was (2.8 f 0.5) X 1014 molecules/cm2, which is in agreement with molecules laying flat separated by their van der Waals radii.
Introduction
SPECTROMETER
Aluminum oxide surfaces play a significant role in the catalysis of many hydrocarbon reactions.14 Not only do these surfaces act as supports for catalytically active metals, they often play a direct role in the catalytic process itse1f.I~~Consequently, an understanding of fundamental surface processes, such as adlayer orientation and desorption, in these systems is important. Although these processes have been well studied on metal surfaces,s*6relatively few experiments have been performed on single-crystal oxide surfaces. In this paper, we will utilize TPD to examine the desorption kinetics of n-pentane from A1203(000 1) in the submonolayer and multilayer regimes. Molecular orientation of n-pentane on the surface will be determined using FTIR and polarized FTIR spectroscopy. The desorption kinetics for n-pentane from Ru(001) have been measured and orientation of the pentane backbone parallel to the surface has been postulated.6 Long-chain hydrocarbons in Langmuir-Blodgett films are known to orient with the hydrocarbon chains perpendicular to the surface at all while the examination of ethane on graphite revealed that the ethane backbone orients parallel to the graphite surface at low coverage and perpendicular to the surface at high coverage.1b13 Crystallization of the long-chain alkanes, Clg, C ~ Oand , C24, was observed on the surface of the neat liquid; the carbon backbone was found to orient perpendicular to the liquid-air interface.14 Surface orientations are determined by the relative strengths of molecule-molecule and molecule-surface interactions. In the case of Ru(001) the strong interactions between pentane and the metal surfacecause surfaceparallel orientation. In the LangmuirBlodgett films one end of the molecule is anchored onto the surface while the other end is free. The free ends align parallel to each other in order to maximize their intermolecular interactions. For graphite, the intermolecular interactions become important at higher coverages and cause the phase transition to perpendicular orientation. The system under study here is unique in that the molecules do not anchor to the surface as in a Langmuir-Blodgett film, nor are they expected to interact strongly with the surface as on a metal surface. Orientations and desorption energies will depend on both molecule-molecule and moleculesurface interactions. The measurement of the desorption kinetics in combi-
* Abstract published in Aduunce ACS Absrrucrs, December 15,
1993.
0022.3654194 f 2098-0240%04.50/0
W T V Figure 1. Schematic diagram of theUHV chamber used for temperatureprogrammed desorption and infrared absorbance measurements.
nation with surface orientation should allow the unraveling of the relative importance of these two interactions. Experimental Section Figure 1 shows a schematic diagram of the UHV chamber used for these experiments. The UHV chamber was pumped by tandem turbomolecular pumps with pumping speeds of 170 and 110 LIS. The chamber was quipped with an ion gauge and a UTI Model lOOC mass spectrometer. Background pressures of 5 X lO-'O Torr were maintained during the course of the investigation with the background gas being predominantly hydrogen. The chamber was also equipped with CaF2 windows for infrared transmission measurements. A single crystal of A1203(0OO1) with dimensions of 2 cm X 1.S cm X 0.75 mm was purchased from Crystal Systems. A niobium film 0.50 pm thick was evaporated on the backside of the crystal followed by a copper film 0.10 Mm thick for resistive heating. During the evaporation of the film, a mask with a width of 0.5 cm prepared a clear window at the center of the crystal for infrared 0 1994 American Chemical Society
n-Pentane on A1203(0001) transmission measurements. A chromel-alumel thermocouple was attached directly to the sample using a high-temperature alumina-based ceramic adhesive. The crystal was then cleaned with acetone and methanol and mounted a t the bottom of a liquid nitrogen cooled cryostat on a differentially pumped rotary feed through. The A1203(OOOl) surface was cleaned in vacuum by exposure to an oxygen plasma discharge with the crystal at 373 K. Auger analysis has demonstrated that this procedure produces clean AI203 surface^.^^ A temperature range of 90-700 K was attainable using liquid nitrogen cooling and resistive heating. The n-pentane was introduced into a gas-handling line which was attached to the inlet of a variable leak valve. The outlet of the variable leak valve was connected to a stainless steel tube with l/a-in. i.d. which was directed toward the A1203 crystal surface for dosing. The distance between the end of the stainless steel tube and the crystal surface was approximately 1 cm. Infrared absorption measurements were made with a modified Mattson Polaris FTIR spectrometer. The infrared light was directed out of the external part of the spectrometer and focused onto the A1203surface at normal incidence using a 90° off-axis parabolic mirror with a focal length of 10 in. The transmitted light was then focused ontoa liquid nitrogen cooled MCT detector. Acquisition times of approximately 10 min were adequate to achieve good signal-to-noise ratios with pentane coverages in the 2-3-monolayer range. For polarized FTIR measurements, the infrared beam was incident upon the crystal a t a 60' angle with respect to the surface normal. Infrared spectra were then obtained as a function of the polarizer angle with respect to the plane of incidence. Surface coverage measurements were made by measuring the infrared integrated absorbance for a 1200-&thick pentane film on A1203(0001)a t 90 K. The thickness of the film was measured using the interference pattern from a helium neon laser at an incident angleof 22S0 with respect to thesurface normal. Using the index of refraction of 1.35 for pentane, the first minimum in the reflectance of the light occurs at a film thickness of 1200 ~4.16917 Using a solid number density of 7.3 X 1021 molecules/ cm3,1*a coverage of 8.9 X 10l6molecules/cm2 is obtained for the 1200-A film. Thecoverageis related to theintegrated absorbance according to
S A dv = aN where A is the absorbance, Y is the frequency in cm-I, u is the integrated absorptivity in cm/molecule, and N is the coverage in molecules/cm.2 Several infrared spectra in this coverage range were used to measure the integrated absorptivity (a) of the surfaceadsorbed film, and a value of u = 2.3 X 1O-I' cm/molecule was measured between 2830 and 2980 cm-I. TPD areas could then be calibrated with respect to surface coverage by obtaining TPD traces with corresponding FTIR spectra. Since the integrated absorptivity a t very low coverages may be different than for a thick film, this calibration was performed a t coverages in the 8.0-12.0-monolayer range. All TPD areas and FTIR integrated absorbances were proportional to exposure time in this coverage range. R€dts Figure 2 displays the TPD traces for various n-pentane coveragesonA1203(0001) at mass 57. Thecoveragesareobtained from the integrated TPD traces by assigning the first appearance of the lower temperature peak to a coverage of 1.0 monolayer (ML). This assignment is justified by the independent coverage measurements discussed below. The traces were acquired by linearly ramping the temperature of the crystal at a heating rate of 1 K/s while digitizing the ion current at mass 57. The TPD traces display one peak at low coverages, and the appearance of a second lower temperature peak at higher coverages. The hightemperature peak is attributed to monolayer desorption while the
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1991 241
3.1 ML
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2
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...... ","I
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160
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TEMPERATURE (K)
Figure 2. Temperature-programmed desorption mass spectrometric signals at mass 57 for pentane on AI2o3(0001)as a function of surface coverage for a heating rate of 1 K/s. Mass intensity units are arbitrary.
7
A
7-
6-
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Figure 3. Temperature-programmed desorption mass spectrometric signals at mass 57 for pentane on A1203(0001)as a function of surface coverage for a heating rate of 1 K/s. Mass intensity units are arbitrary.
low temperature peak is attributed to pentane multilayer desorption. The apparent increase in intensity of the monolayer peak past coverages of 1 ML is probably due to the increase in the baseline produced by the growing multilayer peak as well as somedesorption from the backsideof the A1203crystal. However, it is also possible that the multilayer begins to form before the monolayer completely fills. The calibration of the TPD using FTIR as described in the Experimental Section indicates that the multilayer peak appears at a surface coverage of (2.8 f 0.5) X 1014 molecules/cm2. Figure 3 displays TPD traces for various multilayer pentane coverages. The shape of the peak changes from that of the monolayer and appears zero-order. The peak intensity increases with increasing coverage, and no other peaks are observed. Figure 4 displays TPD traces at coverages of approximately 0.7 ML at different ramp rates ranging from 0.1 to 4.0 K/s. The peak temperature shifts from 133.6 K at the slowest ramp rate to 142.6 K at the highest ramp rate. These data will be used in a Redhead analysis in the discussion section. Figure 5 shows infrared absorption spectra in the C-H stretching region for various pentane coverages adsorbed on A1203(0001) at 90 K taken with a resolution of 4 cm-I. The assignments of the peaks along with the orientation of the transition dipole moments with respect to the carbon backbone aredisplayed in Table l.I9320 Table 2 displays the relative integrated areas of peaks with parallel and perpendicular orientation as a function of coverage. The peaks a t 285 1 and 2874 cm-l were deconvoluted in order to construct this table. As the coverage increases, the relative intensities of those peaks with transition dipole moments
242
Aubuchon et al.
The Journal of Physical Chemistry, Vol. 98, No. I, 1994
4.0 Ws 2.0ws 1.0 ws 0.5 Ws
0.2ws
i3.
21-
FREQUENCY (wavenumbers)
Figure 4. Temperature-programmed desorption mass spectrometric
signals at mass 57 for pentane on A1203(0001) as a function of heating rate for a submonolayer (0.7 ML) coverage. Mass intensity units are arbitrary.
Figure 6. Polarized FTIR absorbance spectra of a multilayer coverage
of pentane adsorbed on A1203(0001). The infrared beam was incident at 60° with respect to surface normal. Indicated polarization angles are with respect to the plane of incidence.
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Figure 5. Normalized FTIR absorbance spectra of various coverages of pentaneadsorbtdonAl~O~(OOO1). Theinfrared beam was incident normal to the surface and the resolution was 4 cm-I.
Figure 7. Redhead analysis of the TPD vs ramp rate data in Figure 5. Solid line shows a least-squares fit to the data.
TABLE 1: Assignment of Infrared Spectral Peaks from Refs 19 and 20 and Associated Transition Dipole Moment Polarizations' freauencv. cm-I vibrational mode dipole moment polarization 2851 sym CH2 perpendicular 2874 sym CH3 parallel 2918 asym CHI perpendicular 2937 sym CH3 parallel 2954 asym CH3 perpendicular 2962 asym CHp parallel a All polarizations are with respect to the carbon backbone.
AI203(OOOl). As the polarization angle changes from 90° to Oo the intensity of those transitions with dipole moments aligned parallel to the carbon backbone increase in intensity with respect to those transitions with dipole moment alignedperpendicular to the carbon backbone.
~
~~~~~
TABLE 2 Relative Int ated Areas of Infrared Peaks from Figure 5 with Parallel an$Perpendicular Orientation as a Function of Coverage coverage, ML AIIIAI 1.5 2.5
1.30
0.90 9.6 0.72 C- A i / A l representsthe ratio of the sum of the areas for all peaks with parallel orientation to the sum of the areas for all peaks with parallel orientation. The peaks at 2851 and 2874 cm-I were deconvoluted to construct this table. oriented parallel to the carbon backbone decrease with respect to the intensities of those peaks with transition dipole moments oriented perpendicular to the carbon backbone. Figure 6 displays polarized infrared absorption measurements taken a t a multilayer coverage of pentane adsorbed on
Discussion The TPD spectra shown in Figure 3 reveal a monolayer desorption peak at low coverage which saturates, accompanied by the growth of a multilayer peak. The relative intensities for all peaks in the mass spectrometer were identical to the cracking pattern for indicating molecular desorption. The desorption kinetics of the monolayer and multilayer were measured separately. Figure 7 displays a Redhead analysisz2of the submonolayer TPD vs heating rate data shown in Figure 4. A plot of In(@/Tp2)versus 1/ Tp will be linear with a slope of -Ed/R and an intercept of h(Rv/&), where is the heating rate in K/s, Tp is the temperature at the peak of the TPD curve in kelvin, R is the gas constant, vis the preexponential for desorption, and Ed is the activation barrier for desorption. The solid line shows a least squares fit to thedata that corresponds to a desorption activation barrier for the monolayer of Ed = 13.3 f 0.8 kcal/mol and a first-order preexponential of VI = 3 X 102NO.5 s-1. The activation barrier is consistent with pentane physisorbing on the A1203surface. In the multilayer range, the desorption kinetics can be extracted from the initial rate of desorption.23 Figure 8 displays an Arrhenius plot obtained from the leading edge of the highest coverage (6.9 ML) TPD trace shown in Figure 3. The solid line
n-Pentane on A1203(0001)
274 8
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 243
1 8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
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Figure 8. Arrhenius plot obtained from the leading edge of the highest coverage (6.9 ML) TPD trace in Figure 3. Solid line shows a least-
5.3 X 1014 molecules/cm2 is predicted for pentane molecules oriented perpendicular to the surface. The observed orientation at low coverage and the activation bamer of 13.3 kcal/mol for desorption areconsistent with pentane molecules interacting with the surface via van der Waals interactions. These interactions would be maximized by the pentane molecule laying flat. However, as the coverageincreases beyond one monolayer, pentane molecules can no longer interact directly with the surface. They can, however, interact with other pentane molecules, and they maximize this interactionby orienting perpendicular to the surface and parallel to each other. This orientation minimizes each molecule's exposure to the vacuum in analogy to the longer hydrocarbons on the surface of neat liquids14where the outer layer orients orthogonal to the interface. The persistence of the monolayer peak upon the growth of the multilayer peak suggests that those molecules in contact with the surface do not reorient with increasing coverage but remain flat on the surface.
squares fit to the data. shows a least-squares fit to the data which corresponds to an activation barrier of Ed = 9.2 f 0.6 kcal/mole and a zero-order preexponential of YO = 1 X lO30fOJ molecules/(cm2 s). In comparison, the heat of sublimation for pentane is 8.32 kcal/ m0I.2~ The multilayer displays a lower activation barrier for desorption than the monolayer because pentane molecules cannot interact directly with the A1203 surface. The relative intensities of the C-H stretching peaks in the infrared spectrum displayed in Figure 5 display significant changes as the coverage increases. The correlation between the changing intensities for all peaks with similarly polarized transition dipole moments suggests that the changes observed in the infrared spectra are primarily attributable to differences in molecular orientation between the monolayer and the multilayer. As the coverage increases, the relative intensities of all peaks with transition dipole moments orientedparallel to the carbon backbone show a relative decrease in intensity. This suggeststhat pentane molecules orient primarily perpendicular to the surface at multilayer coverages. All transition dipoles with polarizations parallel to the carbon backbone would then be oriented in the direction of infrared light propagation and would therefore show little absorbance. At low coverages the infrared spectra show significantintensity for both perpendicular and parallel polarized transitions. This would be consistent with the molecules oriented primarily parallel to the surface at low coverage. All transition dipole moments could then be oriented orthogonal to the direction of light propagation and therefore show significant absorption intensity. Although the infrared spectra as a function of coverage are consistent with pentane oriented parallel to the surface in the monolayer regime and perpendicular to the surface in the multilayer regime, some additional evidence is necessary to confirm these orientations. The polarized infrared spectra taken at multilayer coverage displayed in Figure 6 show an increase in the intensity of the parallel peaks as the polarization of the light is rotated into the plane of incidence (OO). Since the incident angle of the infrared light is 6O0with respect to the surface normal, light of Oo polarization would have a significant component polarized normal to the surface and parallel to the carbon backbone. The observed increase in absorbance from transition dipole moments oriented parallel to carbon backbone confirms that pentane is oriented perpendicular to the surface in the multilayer regime. The monolayer orientation is confirmed by the appearance of the multilayer TPD peak at a coverage of (2.8 f 0.5) X 1014 molecules/cm2. Using van der Waals radii of 1.2 A25 for the hydrogen atoms. C-H bond lengths of 1.1 A, and a C1-CS separation of 5.08 A,'* a coverage of 2.4 X 1014 molecules/cm2 is predicted for pentane molecules laying flat on the surface. In comparison, using the same radii and bond lengths a coverage of
Conclusions Temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the desorption kinetics and the adlayer structure of n-pentane adsorbed on A1203(OOOl) in ultrahigh vacuum. Pentane was found to orient parallel to the surface at submonolayer coverages and desorb molecularly according to first-order kinetics. The activation barrier for desorption was Ed = 13.3 f 0.8 kcal/mol with a first order preexponential of V I = 3 X 102ofo.ss-1 consistent with van der Waals interactions with the surface. Independent coverage measurements revealed that the surface number density at one monolayer was (2.8 f 0.5) X l O I 4 molecules/cm2, which is in agreement with n-pentane molecules laying first separated by their van der Waals radii. Molecules in the multilayer oriented normal to the surface and displayed zero-order desorptionkinetics with an activation barrier of Ed = 9.2 f 0.6 kcal/mol and a zero-order preexponential of YO = 1 X 1030f0.3 molecules/(cm2 s). This orientation allows pentane molecules to maximize their intermolecular interactions and minimize their exposure to thevacuum. The lower activation barrier for desorptionsuggeststhat molecule-surface interactions are greater than molecule-molecule interactions, perhaps due to the greater polarizability of the A1203surface.
Acknowledgment. This research was supported in part by the Petroleum Research Fund, administered by the American Chemical Society and by a grant from Research Corporation. N.J.T. gratefully acknowledges Tamara L. Camis for her involvement in the latter stages of this research effort. References and Notes Tereshchenko, A. D.; Veselov, V. V. Khim. Tekhnol. 1987,29, 17. (2) McCabe, R. W.; Mitchell, P. S. J . Catal., 1987, 103, 419. (3) Yomaoa, M.; Yasumaro, J.; Hovalia, M.; Hercules, D. M. J . Phys. Chem. 1991, 95, 7037. (4) Kakhniashuili. G. N.: Mischenko. Yu A.: Dulin. D. A,: Isaeva.. E.0.: . Gelibshrein, A. I. K i n k Katal. 1985, 26; 134. (5) King, D. A. Surf. Sci. 1975, 47, 384. (6) Brand, J. L.; Arena, M. V.; Deckert, A. A.; George, S.M. J. Chem. Phys. 1990.92, 5136. (7) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (8) Rabolt, J. F Burns, F. C.; Schlotter, N. E.;Swalen, J. D. J . Chem. Phys. 1983, 78, 946. (9) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y . R.Phys. Reu. k r t . 1987, (1)
'
59, 1597. (10) Coulomb, J. P.; Bienfait, M.Discuss. Faraday Soc. 1985, 80, 79. (11) Moller, M. A.; Klein, M. L. J . Chem. Phys. 1989, 90,1960. (12) Suzanne, J.; Gay, J. M.; Wang, R.Surf. Sci. 1985, 162, 439. (13) Osen. J. W.: Fain, Jr., S . C. Phys. Reo. B 1987, 36. 4074. ( 1 4 Wu, X.2.; Sirota, E.B.;Sinha, S. K.; Ocko, B. M.Drsch. Phys. Rev. Lett. 1993, 70, 958.
244 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 ( 15) Poppa. H.; Moorehead, D.; Heineman, K. Thin Solid Films 1985, 128, 252. (16) Rosetti, R.;Brus, L.E. J. Chem. Phys. 1980, 73, 572. (17) Olsen, G.L.; Kokorowski, S.A.; McFarlane, R. A.; Hm,L. D. Appl. Phys. Lcrr. 1980, 37, 1019. (18) Mathisen,H.;Norman, N.; Pedersen, E.F. Acra. Chem.Scand. 1%7, 21, 127. (19) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acra 1%3,19, 85. (20) Snyder, R. G.; Hsu, S.L.; Krimm, S.Specrrochim. Acra 1978,34A, 395.
Aubuchon et al. (21) Eighr Peak Index of Mass Spectra, 3rd ed.; Compiled by the Mass Spectrometry Data Center, Nottingham, UK, 1983. (22) Redhead, P. A. Vacuum 1962, 12, 203. (23) King, D. A.; Madey, T. E.;Yatca. Jr., J. T. J. Chem. Phys. 1971,55, 3247' (24) Rossini, F. D.; Pitzer, K. S.;Amett, R. L.;Braun, R. M.;Pimentel,
Selected values Of physical and ThermodyMmic PrOPrrieS Of Hydrocarbom and Related Compounds;Camegie Prm: Pittsburg, PA, 1953. (25) Pauling, L. The Nature of the Chemical B o d , Comell University Press: Ithaca, NY, 1967.
