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Langmuir 1987, 3, 1178-1180
hydrogen-rich compound. These unsaturated hydrocarbon species are the spiltover carbon reactive intermediates in our particular case.
This paper is dedicated to the late Drofessor Paul Biloen. The untimelv death of Dr. Biloen late 1986 deprived me of a coauthoi who had an intimate
and incisive knowledge of the subject. The finished product would, of course, have been different in style and content had Dr. Biloen shared more in its authorship. Further, the opinions in the paper reflect only my own points of view, for which 1 am solely responsible. Registry No. CO, 630-08-0; H2,1333-74-0; Co, 7440-48-4.
Identification of Metastable Adsorption States in Thermal Desorption Spectroscopy J. A. Polta, P. J. Schmitz, and P. A. Thiel*f Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011 Received January 20, 1987. In Final Form: May 18, 1987 Variation of heating rate in thermal desorption spectroscopy can change the distribution of molecules in separate desorption states. Two examples are provided: benzene and water, both desorbing from Ru(OO1). This simple experiment allows identification of metastable adsorbed species under favorable conditions.
During the past 25 years, thermal desorption spectroscopy (TDS) has proven to be the workhorse of surface science. It is a relatively simple experiment that provides a wealth of basic information, and more sophisticated spectroscopies must usually be used in concert with TDS to obtain interpretable data. It can be used to measure relative coverages and desorption rate parameters and to identify gas-phase products of surface reactions. The applications of TDS in surface science have been reviewed extensively by King1 and Schmidt.2 However, one type of information easily obtained from the thermal desorption experiment has been largely overlooked the identification of desorption states which represent metastable adsorbed species by variation of heating rate. Gorte and Schmidt discussed this possibility theoretically in 1979: but concrete examples have been absent from the literature. In this paper we provide examples of this simple application of TDS and discuss its usefulness. We define a metastable adsorbed species as one which can be converted to a more stable adsorbed form but which is kinetically trapped in the less stable state at low temperature. In other words, it is not the equilibrium state. As temperature is raised during a thermal desorption experiment, conversion can begin as the kinetic barrier is overcome. Desorption from the metastable state can also begin. As temperature is raised still further, desorption from the more stable state (formed in the conversion step) can take place. These ideas are illustrated in Figure 1. We are particularly interested in those cases where the rate of conversion (with an associated rate constant k,) is comparable to the rate of desorption (with an associated rate constant k2) from the metastable state. In those cases, a slight variation in heating rate (p) may drastically change the amounts of metastable species which desorb and which convert. As a result, the relative intensities of the two desorption features which represent the metastable and stable species will also change. The rate constant for a reaction, k, can be expressed in the form proposed by Arrhenius: 'National Science Foundation Presidential Young Investigator (1985-1989), Alfred P. Sloan Research Fellow (1984-1986), and Camille and Henry Dreyfus Foundation Teacher-Scholar (1986-1990).
0743-746318712403-1178$01.50/0
k = Y exp(-E,/kT)
(1)
where u is the preexponential factor, E, is the activation barrier for reaction, k is Boltzmann's constant, and T is the temperature of reaction (T, is the surface temperature in a surface reaction). The rate constant carries a strong dependence on temperature through the exponential term. We are interested in the ratio of the rate of conversion to the rate of desorption from the metastable state, R1/R2. This ratio can be written as
where
It is easy to show that, at fixed surface coverage during a desorption experiment, the value of T,is higher for a larger value of 8, regardless of the order of the desorption reaction (e.g., ref 4). Equation 2 shows that, if AE, > 0, then the exponential term increases as T,increases; that is, R1/R2increases as 0 increases. Conversely, if AE, C 0, the exponential term in eq 2 becomes smaller as T, increases; in other words, R,/R2 decreases as increases at a given coverage, 6. This ratio, R1/R2,determines the relative numbers of molecules which desorb from the metastable and stable states as a function of the heating rate, 0. Figure 2 illustrates the qualitative changes in the thermal desorption spectra that may be observed in these two cases. These qualitative changes are independent of the orders of the desorption and conversion reactions (for orders not less than zero), based both on the arguments presented above and on simple computer simulations that we have carried out in our laboratory. These simulations confirm that the ratio of the number of molecules which desorb from the low-temperature and high-temperature states increases as @ increases if AE, > 0, and the ratio (1)King, D.A. Surf. Sci 1975, 47, 384. (2) Schmidt, L. D. Catal. Rev. Sci. Eng. 1974, 9,115. (3) G o d , R.;Schmidt, L. D. Appl. Surf. Sci. 1979, 3, 381. (4) Taylor, J. L.;Weinberg, W. H.Surf. Sci. 1978, 78, 259.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 6, 1987 1179
Letters Gas Phase 0
e=oi
AEa
e=02 0 .
e=o3
- Model
T
Metastable Slate
E
A
Stable State
Reaction Coordinate
d
Figure 1. Energy diagram of three phases between which con-
version can occur during a thermal desorption experiment: a metastable adsorbed phase, a stable adsorbed phase, and the gas phase. The k's represent rate constants.
Stable 7 MeiastableT
I 71
01
I
I
I
0
5
IO
15
Metastable
I 20
I 25
Heating R a t e , K/s
A 2 A1
P 25 10 4 2 Tf /
3
Temperature
-
150
Tf
the initial temperature, and Tfis the final temperature. Effecta of @ on peak positions are ignored.
0
5
10
K/s 20
15 I
Enc;
r
I
1
I
120
160
250
Temperature, K
Figure 2. Schematicthermal desorption spectra illustrating the effect of changing j3 on the spectral shapes for the two cases hEa > 0 (left) and AE, < 0 (right). is defined in Figure 1. Ti is
Heating rate,
200
200
Temperature, K
Figure 3. Thermal desorption of benzene from Ru(001) at
constant initial coverage but different heating rates. The bottom panel (A) illustratesthe spedra themselves. In order of increasing temperature, the three features are denoted az,a3,and al. The metastable state is az,which can convert to the a3state. The top panel (B) shows the peak height of each of these three features (normalized to the s u m of all three peak heights) as a function of heating rate. Reprinted with permission from ref 5. Copyright 1986 American Chemical Society.
Figure 4. Thermal desorption of HzO from Ru(OO1)at constant
initial coverage (e& but different heating rates. The bottom panel (A) illustrates the desorption spectra of HzO at 0, = 0.2 monolayers. In order of increasing temperature, the two main features are denoted Az and Al. The metastable state is Az, which can convert to AI. The spectra are labeled with the values of the heating rate, p. The top panel (B) shows the ratio, Az:AI,of peak heighb as a function of p for both HzO and D20at three different coverages. The solid lines represent the results of a kinetic model described in ref 6. Reprinted with permission from ref 6. Copyright 1987 North-Holland Publishing.
decreases as j3 increases if hE, < 0, regardless of the values of x and y in eq 2. We have recently encountered two esamples of metastable states in thermal desorption spectra?*6 The first case is benzene on Ru(OO~).~ Thermal desorption spectra are reproduced in Figure 3A. In order of appearance with increasing exposure, the three states are denoted q ,a2, and q p 5 The peak height of each feature, normalized to the sum of heights of all three features, is shown as a function of heating rate in Figure 3B. I t is clear that variation of heating rate strongly influences the distribution of molecules that desorb from the three states. (Decompositiondoes not compete with de~orption.~) The a2and a3 states are most strongly affected, with the a2 state enhanced and the as state suppressed by faster heating rates. We interpret this to mean that the azstate is a metastable adsorbed species which can convert to the cy3 state. For this system, the value of AEa in eq 2 must be less than zero, since conversion from a2to a3slows down (relative to azdesorption) as /3 increases. We are unable to identify the a2state except to describe it as a metastable (6) Polta, J. A,; Thiel, P. A. J. Am. Chem. SOC.1986, 108,7560. (6) Schmitz, P. J.; Polta, J. A.; Chang, S.-L.; Thiel, P. A. Surf.Sei. 1987,186,219.
Langmuir 1987, 3, 1180-1181
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adsorbed state that exhibits zero-order desorption kinetics and a heat of sublimation equal to 7.1 f 1.4 kcal/mol.6 The a3state represents sublimation of the bulldike benzene m~ltilayer.~ The second example is water on Ru(001)? Madey and Yates first reported the desorption spectra in 1977,' and since then there have been many studies of this system (e.g., ref 8-11). We are the f i i t , however, to syatematidy study changes in the spectra which take place as the heating rate is variedeeRepresentative desorption spectra are illustrated in Figure 4A, where the initial HzO coverage is held constant but each curve is taken at a different value of /3. In order of increasing temperature, the two main states are denoted Az and Al. Previous work has shown that these states are due to desorption of water from hydrogen-bonded clusters of water molecules arranged in a three-dimensional, chemisorbed bilayer."ll In Figure 4B, the variations of relative peak heights of the Az and AI states are shown as functions of increasing 6 for both HzO and D20.6 These data show that the Az state is a metastable state which can convert to the AI state; conversion is enhanced by slower heating rates. As for benzene, this behavior again indicates that AE, in eq 2 is less than zero. We believe that conversion between the two states actually consists of a structural rearrangement which requires rotation of one or two water molecules within a hydrogenbonded cluster; the difference in rotational zero-point energies for HzO and D20 makes this process much slower for the heavier isotope? By quantitatively modeling the data of Figure 4B, we determine that the rate of conversion is 3-8 times slower for D20 than for H20a6 (7) Madey, T. E.; Yates, J. T., Jr. Chem. Phys. Lett. 1977,51, 77. (8)Thiel, P. A.; Hoffmann, F. M.; Weinberg, W. H. J. Chem. Phys.
1981,75,5556. (9) Doering, D. L.; Madey, T. E. Surf. Sci. 1982,123,305. (10)Thiel, P. A.; DePaola, R. A.; Hoffmann, F. M. J. Chem. Phys. 1984,80,5326. (11)Williams, E. D.; Doering, D. L. J. Vac. Sci. Technol. 1983,i23, 305.
Variation of heating rate has been used only seldom in thermal desorption spectroscopy, with the primary purpose of measuring desorption rate parameters (e.g., ref 4 and 12). In those cases the heating rate must be varied by at least 2 orders of magnitude, which is sometimes experimentally difficult. In the examples described here, metastable desorption states have been identified by varying the heating rate as little as 1order of magnitude. In other words, useful information potentially can be obtained by changing /3 over a relatively narrow range, and this is easily done in most laboratories. In summary, we find that simple variation of heating rate in thermal desorption spectroscopy can be used to identify metastable adsorbed species. Two examples are discussed: benzene on Ru(001)6 and HzO on the same surface.6 We suggest that it may be useful to apply this technique more frequently in studies of surface chemistry. Acknowledgment. This research has been supported in part by a Cottrell Research Grant from the Research Corp. and in part by the Director for Energy Research, Office of Basic Energy Sciences. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-ENG-82. Note Added in Proof. A recent publication describes the heating rates necessary to obtain a 1:l product distribution when desorption competes with a surface reaction (such as conversion) during a thermal ramp, but it emphasizes the very large heating rates accessible by using lasers. (See: Deckert, A. A.; George, S. M. Surf. Sci. 1987, 182, L215.) Of course, some situations may require laser heating to vary the ratio of desorption to conversion products significantly, but others, such as those described in this paper, do not. Registry NO. R U , 7440-18-8;C&s, 71-43-2; H20,7732-18-5. (12) Redhead, P. A. Vacuum 1962,12,203.
News and Announcements News 1987 Langmuir Award Lecturers: Prof. Kamil Klier and Prof. D. Fennell Evans (Award Lectures presented at the 194th National Meeting of the American Chemical Society, New Orleans, LA, Aug 30-Sept 4,1987). (Prof. Evans is a member of the Langmuir Editorial Advisory Board.) 1988 ACS National Award winners include the following: ACS Award for Computers in Chemistry, Prof. William A. Goddard, California Institute of Technology (former Langmuir Advisory Board member); ACS Award in Applied Polymer Science, Dr. David S. Breslow, Hercules Inc. (retired);ACS Award in Chromatography, Prof. Milton L. Lee, Birgham Young University; Garvan Medal, Prof. Marye Anne Fox, University of Texas; ACS Kendall Award in Colloid or Surface Chemistry, Prof. Howard Brenner, Massachusetts Institute of Technology. Prof. Arthur W. Adamson received a Certificate of Appreciation from the ACS Division of Colloid and Surface Chemistry. A new department, Notes and Comments, will be started 0743-7463/87/2403-ll80$01.50/0
with the January/February issue of Langmuir. Notes will comprise short, original research papers not sufficiently complete for extensive introduction or discussion; Comments will comprise short technical commentaries on subjects in the surface/colloid area. See the Editorial in the January/February issue. Some recently established journals in the surface/colloid area are the following: Journal of Adhesion Science and Technology; K. L. Mittal and W. J. van Ooij, Eds.; VNU Science Press (P.O. Box 2093, 3500 GB Utrecht, The Netherlands). Liquid Crystals; G. R. Luckhurst and E. T. Samulski, Eds.; Taylor and Francis (242 Cherry St., Philadelphia, PA 19106-1906, or Rankine Rd., Basingstoke, Hanta, RG24 (PR, U.K.). Gas Separation & Purification; W. Baldus, T. Lavin, and R. M. Thorogood, Eds; Butterworths (P.O. Box 63, Westbury House, Bury St., Guildford, Surrey GU2 5BH, U.K.). Aduances in Colloids and Interface Science; A. C. Zettlemoyer and J. T. G. Overbeek, Managing and Associate Eds.; Elsevier (P.O. Box 211, 100 AE Amsterdam, The Netherlands). 0 1987 American Chemical Society
