2151
J. Phys. Chem. 1995,99, 2151-2154
Desorption Kinetics and Adlayer Sticking Model of It-Butane, It-Hexane, and It-Octane on A1203(0001) R. M. Slayton, C. M. Aubuchon, T. L. Camis, A. R. Noble, and N. J. Tro* Department of Chemistry, Westmont College, 955 La Paz Road, Santa Barbara, California 93108 Received: September 22, 1994; In Final Form: November 18, 1994@
Temperature-programmed desorption (TPD) was used to investigate the desorption of butane, hexane, and octane from A1203(0001) in ultrahigh vacuum. At low coverages, TPD traces for butane and hexane displayed one peak which was attributed to monolayer desorption. A second, multilayer peak, was observed at a lower temperature as the coverage was increased. However, the multilayer peak appeared at coverages well below the saturation coverage of the monolayer peak implying that the multilayer was forming before the monolayer was completely full. A simple statistical adlayer sticking model was used to simulate the relative number of molecules in the monolayer and multilayer as a function of total coverage giving good agreement with the TPD data. In addition, the variation of ramp rates method was used to measure the desorption kinetics at coverages well below one monolayer for each of the alkanes. All three alkanes displayed first-order desorption kinetics with activation barriers of butane Ed = 8.4 f 1.2 kcaYmol; hexane Ed = 10.4 f 0.8 kcaYmol; octane Ed = 14.6 f 0.8 kcaYmo1. The first-order preexponentials were butane v1 = 4 x 1010i2 s-l., hexane v1 = 5.4 x 109*1.5 s-l; octane v1 = 1.6 x 1012*2s-l. The comparison of these desorption barriers to the bulk heats of sublimation along with the separation between monolayer and multilayer peaks in the TPD as a function of chain length suggest that the relative magnitude of molecule-surface interactions compared to moleculemolecule interactions decreases with increased chain length.
10
Introduction
Aluminum oxide surfaces play a significant role in the catalysis of many hydrocarbon reaction^."^ Not only do these surfaces act as supports for catalytically active metals, they often play a direct role in the catalytic process it~e1f.l.~ Consequently, an understanding of fundamental surface processes such as adlayer formation and desorption in these systems is important. Although these processes have been well studied on metal surfaces5v6and graphite surfaces,7-1° relatively few experiments have been performed on single-crystal oxide surfaces." In this paper, we utilize TPD to examine the desorption of n-butane, n-hexane, and n-octane from A1203(0001) in the submonolayer and multilayer regimes. By comparing the desorption activation barriers to the heats of sublimation, the relative strengths of molecule-molecule and molecule-surface interactions can be determined as a function of chain length.
Experimental Section The UHV chamber used in these experiments was pumped by tandem turbomolecular pumps with pumping speeds of 170 and 110 Us. The chamber was equipped with an ion gauge and a UTI Model lOOC mass spectrometer. Background pressures of 5 x Torr were maintained during the course of the investigation with the background gas being predominantly hydrogen. A single crystal of A1203(0001) with dimensions of 2 cm x 1.5 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 pm thick for resistive heating. 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 at the bottom of a liquid nitrogen cooled @
Abstract published in Advance ACS Abstracts, January 1, 1995.
0022-365419512099-2151$09.00/0
,
I
524 0 0
75
100
125
150 175 200 225 TEMPERATURE (K)
250
275
Figure 1. Temperature-programmed desorption mass spectrometric signals at mass 43 for butane on Al~O~(OOO1) as a function of surface coverage for a heating rate of 5 Ws. Mass intensity units are arbitrary.
cryostat on a differentially pumped rotary feed through. The A1203(0001)surface was cleaned in vacuum by exposure to an oxygen plasma discharge with the crystal at 373 K. Auger analysis by other groups has demonstrated that this procedure produces consistently clean A1203 surfaces.l 2 A temperature range of 90-700 K was attainable using liquid nitrogen cooling and resistive heating. The n-alkanes were 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 '/g 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. Results
Figures 1 and 2 display the TPD traces for various n-butane and n-hexane coverages on A1203(0001)at masses 43 and 57, respectively. The mass spectrum of each desorbing n-alkane was identical to the mass spectrum observed when that n-alkane was leaked into the chamber, indicating molecular desorption. 0 1995 American Chemical Society
Slayton et al.
2152 J. Phys. Chem., Vol. 99, No. 7, 1995 7
4 -
I
1
c6 5 5
0.39ML
E4
t
a3 ;2
v)
=1
0
75
100
125
150
175
200
225
250
75
275
100
125
150
175
200
225
TEMPERATURE (K)
TEMPERATURE (K)
Figure 2. Temperature-programmed desorption mass spectrometric signals at mass 57 for hexane on A1203(0001)as a function of surface coverage for a heating rate of 5 Ws. Mass intensity units are arbitrary.
Figure 4. Temperature-programmed desorption mass spectrometric signals at mass 43 for butane on A1203(0001)as a function of surface heating rate for a 0.10 ML coverage. Mass intensity units are arbitrary. 7
7 ,
6
s 5
2
P 4
z
k3 0 v)
22 1
75
100
125
150
175
200
225
250
275
TEMPERATURE
Figure 3. Temperature-programmed desorption mass spectrometric signals at mass 57 for octane on A1203(0001) as a function of surface coverage for a heating rate of 5 Us. Mass intensity units are arbitrary.
The coverages were obtained from the integrated TPD traces which were calibrated using the adlayer sticking model presented in the discussion section of this paper. The traces were acquired by linearly ramping the temperature of the crystal at a heating rate of 5 K/s while digitizing the ion current at mass 43 for butane and mass 57 for hexane. The TPD traces for both butane and hexane display one peak at low coverages, and the appearance of a second lower temperature peak at higher coverages. As the coverage continues to increase, the lower temperature peak continues to grow and no other peaks are observed. The high-temperature peak is attributed to monolayer desorption, while the lowtemperature peak is attributed to multilayer desorption. The appearance of the multilayer peak before saturation of the monolayer peak implies that the multilayer is forming before the monolayer fills. Although the butane and hexane TPD traces look qualitatively similar there are two important differences. First, hexane desorbs about 35 K higher than butane, indicating a greater interaction with the surface. Second, the monolayer and multilayer peaks are closer together for hexane than for butane, indicating less of a difference between the magnitudes of molecule- surface interactions relative to molecule-molecule interactions. Figure 3 displays TPD traces for various octane coverages on A1203(0001) at mass 57. The traces were acquired by linearly ramping the temperature of the crystal at a heating rate of 5 K/s while digitizing the ion current at mass 57. Octane desorbs about 30 K higher than hexane and does not show distinct monolayer and multilayer peaks indicating that moleculesurface interactions are similar in magnitude to moleculemolecule interactions. Because the monolayer and multilayer peaks could not be distinguished, the adlayer sticking model could not be used to determine coverage. The coverages were estimated based on comparing TPD areas to hexane and
0
100
125
150
175 200 225 TEMPERATURE (K)
250
275
300
Figure 5. Temperature-programmed desorption mass spectrometric signals at mass 57 for hexane on A1203(0001) as a function of surface heating rate for a 0.10 ML coverage. Mass intensity units are arbitrary.
125
150
175
200
225
250
275
300
325
TEMPERATURE (K)
Figure 6. Temperature-programmed desorption mass spectrometric signals at mass 57 for octane on A1~03(OOal)as a function of surface heating rate for an estimated coverage of 0.05 ML. Mass intensity
units are arbitrary. correcting for the difference in mass spectral intensity between the two molecules at mass 57. Figures 4-6 display TPD traces at coverages below 0.10 monolayer (ML) for each alkane at different ramp rates ranging from 0.2 to 20 Us. The peak temperature shifts toward a higher value as the ramp rate increases. These data will be used in a Redhead analysis in the discussion section of this paper.
Discussion The TPD spectra shown in Figures 1 and 2 reveal a monolayer desorption peak at low coverage and the growth of a multilayer peak before the monolayer peak saturates. This implies that the multilayer begins to form before the monolayer is completely full. The relative areas of the monolayer and multilayer peaks as a function of coverage were simulated with a simple statistical sticking model. The model utilized 3000 surface sites and allowed them to be randomly occupied. As the sites filled, no preference was given to empty sites over occupied ones; the
J. Phys. Chem., Vol. 99, No. 7, 1995 2153
Desorption Kinetics and Adlayer Sticking Model
t '
v)
w 2.0
Multilayer coverage
A
? First layer coverage
1.0
f y
a
0.5
v)
0.0 0
0.5
1
1.5
2
2.5
0
0.5
1
COVERAGE (ML)
Figure 7. Comparison of adlayer sticking model to the relative areas obtained from TPD deconvolution for butane. The solid line represents the model and the points represent the TPD deconvolutions.
0.8
4
1.5
2
2.5
COVERAGE (ML)
Figure 9. Comparison of adlayer sticking model to the relative areas obtained from TPD deconvolution for hexane. The solid line represents the model and the points represent the TPD deconvolutions.
I\
\
-12
0
0 -
75
100
125
150
175
200
225
250
275
0.004
TEMPERATURE (K)
Figure 8. Sample TPD deconvolution for butane. The dotted lines represent the multilayer and monolayer contributions to the overall curve.
probability of occupying any one site was the same. By performing the calculation at different total coverages, the relative number of molecules in the first layer and the multilayer could be calculated. The results are shown by the solid line in Figure 7. The first layer or monolayer does not saturate until the equivalent of over 2.5 ML of total coverage is adsorbed. The multilayer begins to fill at coverages as low as 0.1 ML. The coverages in the monolayer and multilayer are equal at a total coverage of approximately 1.5 ML. The results of this model were compared to the experiment by determining the relative areas in the monolayer and multilayer peaks in the experimental TPD traces. Since these peaks overlapped substantially, a convolute-and-compare routine was employed to extract the relative areas. In this analysis, the TPD curves were simulated using zero-order (multilayer) and firstorder (monolayer) desorption kinetic expressions with Gaussian broadening. The two curves were then summed, and the contribution of each to the overall simulated TPD spectrum was adjusted until the best fit was obtained with the experimental TPD spectrum. A sample convolution for butane is shown in Figure 8. By carrying out this analysis for a number of TPD traces, the relative number of molecules in the monolayer and multilayer could be obtained as a function of total coverage. The data were then compared to the sticking model by scaling the data with a single parameter. The results are shown in Figure 7 for butane and Figure 9 for hexane. In both cases the model was in excellent agreement with the experimental data. The variation of ramp rates method was used to determine the activation barrier and pre-exponential for desorption of each alkane. In this analysis13 a plot of ln@/T;) versus l/Tp will be linear with a slope of -EdR and an intercept of ln(Rd&), where @ is the heating rate in Ws,Tp is the temperature at the peak of the TPD curve in K,R is the gas constant, Y is the preexponential for desorption, and Ed is the activation barrier for desorption.
0.005
0.006
0.007
0.008
0.009
0.01
Figure 10. Redhead analysis of the TPD vs ramp rate data in Figures 5-7.
Solid line shows a least-squares fit to the data.
TABLE 1: Activation Barriers and Preexponential Factors for the Desorption of Each Alkane. Also Included Are the Bulk Heats of Sublimation Obtained from Ref 14 alkane
E&s (kcallmol)
n-butane n-hexane n-octane
8.4f 1.4 10.4f0.8 14.6 f0.8
(s-')
AHsub(kcaymol)
4 x 101M2.5 5.4 x 109*'.5 1.6 x lO'**'.'
6.14 10.65 14.86
vdes
Figure 10 shows the results of this analysis for the TPD ramprate data shown in Figures 4-6. The lines show least-squares fits to the data and result in the kinetic parameters listed in Table 1. These activation barriers are consistent with the alkanes physisorbing on the A1203 surface. Table 1 also lists the bulk heats of sublimation for each alkane.14 The heats of sublimation for hexane and octane are similar to the desorption activation barriers measured for monolayer desorption, indicating little difference between the overall magnitude of molecule-molecule and molecule-surface interactions. However, the monolayer desorption activation barrier for n-butane is slightly higher than the heat of sublimation, indicating a slightly stronger interaction with the surface. A trend is clearly seen in the TPD data in the progression from butane to octane. The peak temperature differences between the monolayer and multilayer peaks are greatest for butane and diminish as the length of the carbon chain increases. Since these are all even alkanes, the only difference between the alkanes is chain length. Changes in monolayer melting behavior in n-alkanes as a function of chain length has been attributed to the role of molecular flexibility15 by molecular dynamics simulations. In these studies,15the melting of hexane monolayers was preceded by a trans-to-gauche transformation in some of the adsorbed molecules. Butane, however, melted in the trans conformation, because, at its melting point, there was insufficient thermal energy to convert to the gauche conformation. Similarly, butane
Slayton et al.
. 2154 J. Phys. Chem., Vol. 99, No. 7, 1995
may be desorbing in a trans conformation, while the thermal energies required to desorb hexane and octane are sufficient to induce a trans to gauche transformation on the surface. Such a change in conformation would lower the footprint of the molecule on the surface and consequently lower the activation barrier for desorption relative to A H s u b . This idea is also consistent with the higher preexponential observed in the case of octane. The less restricted gauche conformation prior to desorption should lead to a higher preexponential. Conclusions
Temperature-programmed desorption was used to investigate the desorption of butane, hexane, and octane on A1203(0001) in ultrahigh vacuum. At low coverages, TPD traces for butane and hexane displayed one peak which was attributed to monolayer desorption. A second, multilayer peak was observed at a lower temperature as the coverage was increased. However, the multilayer peak appeared at coverages well below the saturation coverage of the monolayer peak, implying that the multilayer was forming before the monolayer was completely full. The relative areas of the monolayer and multilayer peaks as a function of coverage were simulated with a simple statistical adlayer sticking model. Using this model, the relative coverages in the first layer and the multilayer were calculated as a function of total coverage. The results of the simulation were in excellent agreement with the relative areas of the monolayer and multilayer peaks in the TPD traces. In addition, the variation of ramp rates method was used to measure the desorption kinetics at coverages well below one monolayer for each of the alkanes. All three alkanes displayed first-order desorption kinetics with activation barriers as follows: butane Ed = 8.4 & 1.2 kcaymol; hexane Ed = 10.4 f 0.8 kcaymol; octane Ed = 14.6 f 0.8 kcaumol. The first-order preexponentials were as follows: butane v1 = 4 x s-l; hexane v1 = 5.4 x s-l; octane v1 = 1.6 x 1012*2s-l. The separation between
monolayer and multilayer peaks in the TPD as a function of chain length as well as the comparison of the desorption barriers to the bulk heats of sublimation suggest that the relative magnitude of molecule-surface interactions compared to molecule-molecule interactions decreases with increased chain length. 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 David F. Marten for may helpful discussions. References and Notes (1) 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. G.; Gelibshrein, A. I. Kiner. Karal. 1985, 26, 134. (5) King, D. A. Surf. Sei. 1975, 47, 384. (6) Brand, J. L.; Arena, M. V.; Deckert, A. A.; George, S. M. J. Chem. Phys. 1990, 92, 5136. (7) Coulomb, J. P.; Bienfait, M. Discuss. Faraday SOC.1985, 80, 79. (8) Moller, M. A,; Klein, M. L. J . Chem. Phys. 1989, 90, 1960. (9) Suzanne, J.; Gay, J. M.; Wang, R. Surf. Sci. 1985, 162, 439. (10) Osen, J. W.; Fain, Jr., S. C. Phys. Rev. B. 1987, 36, 4074. (11) Arthur, D. A,; Meixner, D. L.; Terry, J. H.; George, S. M. Private communication. (12) Poppa, H.; Moorehead, D.; Heineman, K. Thin Solid Films 1985, 128, 252. (13) Redhead, P. A. Vacuum 1962, 12, 203. (14) Rossini, F. D.; Pitzer, K. S.; Amett, R. L.; Braun, R. M.; Pimentel, G. C. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds; Camegie Press: Pittsburg, PA, 1953. (15) Hansen, F. Y.; Newton, J. C.; Taub, H. J. Chem. Phys. 1993, 98, 4128. JP942563 1
