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(3) (4)
(5)
(6) (7) (8) (9) (10) (11)
The Journal of Physical Chemistry, Vol. 83, No. 9,
1979
163 (1977); J. Silver, N. C. Blais, and G. Kwei, J . Chem. fhys., 67, 839 (1977). M. F. Golde, "Gas Kinetics Energy Transfer", Vol. 2, Specialist Periodical Report, Chemical Society, London, 1977. J. W. McGowan, Ed., "The Excited State in Chemical Physics", Wiley-Interscience, N.Y., 1975; J. Steinfeid, Ed., "Electronic Transition Lasers", MIT Press, Cambridge, 1976. Abstracts of the symposium "Energy Transfer and Chemical Reactions of Electronically Excited Atoms and Small Molecules", American Chemical Society/Chemicai Society of Japan Chemical Congress, Honolulu, Hawaii, 1979. W. H. Breckenridge, R. J. Donovan, and 0. Kim Malmin, Chem. fhys. Lett., in press. W. H. Breckenrldge, R. P. Biickensderfer, J. Fitzpatrick, and D. Oba, J . Chem. fhys., in press. W. H. Breckenridge and A. M. Reniund, J . fhys. Chem. in press. W. H. Breckenridge, 0. Kim Malmin, W. L. Nikobi, and D. Oba, Chem. fhys. Lett., 59, 38 (1978). W. H. Breckenridge and A. M. Renlund, J. fhys. Chem., 82, 1484 (1978). W. H. Breckenridge and A. M. Reniund, J . fhys. Chem., 82, 1474 119781
(12) W: HI'Breckenridge and J. Fitzpatrick, J . fhys. Chem., 80, 1955 (1976). (13) W. H. 'Breckenridge, R. P. Biickensderfer, and J. Fitzpatrick, J. fhys. Chem., 80, 1963 (1976). (14) R. P. Biickensderfer, W. H. Breckenridge,and D. S.Moore, J. Chem. fhys., 63, 3681 (1975). (15) W. H. Breckenridge, T. W. Broadbent, and D. S . Moore, J . fhys. Chem., 79, 1233 (1975). (16) V. Mahaven, N. N. Lichtin, and M. 2 . Hoffman, J . fhys. Chem., 77, 875 (1973). (17) K. Schofield, J . fhotochem., 9, 55 (1978). (18) J. C. Tuily, J . Chem. fhys., 62, 1893 (1975). (19) G. E. Zahr, R. K. Preston, and W. H. Miiier, J. Chem. Phys., 62, 1127 (1975).
W. H. Breckenridge, G. T. Bida, and W. S. Kolln (20) G. V. Van Volkenburgh and T. Carrington, J . Quant. Spectrosc. Radiat. Transfer. 11, 1181 (1971). (21) M. J. Boxaii, C. J. Chapman,'and R. P. Wayne, J . fhotochem., 4, 281, 435 (1975). (22) T. Holstein, Phys. Rev., 72, 1212 (1947); 83, 1159 (1951). (23) P. J. Walsh, fhys. Rev., 118, 511 (1959). (24) The upper-state %,- 3P, radiative lifetimes have been determined to be 14 f 3 and 15 k 2 ns for the transitions of interest for Zn and Cd, respectively: J. Landais, M. Chantepie, and B. Laniepce, Opt. Commun., 23, 80 (1977); A. R. Shaefer, J. Quant. Spectrosc. Radiat. Transfer, 11, 197 (1971). (25) I f collisional equilibration between the Zn(3P,), ZII(~P,!, and Z~I(~P,) states is always rapid compared to net collisional deactivation of any of the J states and to radiative decay of the ZII(~P,) level, then it can be shown" that the quenching rate-constant k , is actually a popuiation-weighted sum of deactivation rate constants for each ZI(~P,) level for Stern-Volmer-like measurements. (26) S. Yamamoto, T. Takei, N. Nishimura, and S.Hsegawa, Chem. Lett., 1413 (1976). (27) R. P. Blickensderfer, W. H. Breckenridge, and J. Simons, J . Phys. Chem., 80, 653 (1976). (28) J. E. Velazco, J. H. Koits, and D. W. Setser, J . Chem. Phys., 69, 4357 (1978), and references therein. (29) P. B. Foreman, T. P. Parr, and R. M. Martin, J. Chem. fhys., 67, 5591 (1977); T. P. Parr and R. M. Martin, J. fhys. Chem., 82, 2226 (1978). (30) M.E. Gersh and E. E. Muschlitz, Jr., J. Chem. phys., 59, 3538 (1973). (31) 8. L. Earl and R. R. Herm, J . Chem. fhys., 60, 4568 (1974); J. R. Barker and R. Weston, Jr., J. Chem. fhys., 65, 1427 (1976). (32) W. J. Balfour and B. Lindaren, Can. J . fhvs.. 56. 767 (1978). (33) A. B. Callear and J. McGufk, J . Chem. Soc., Faraday Trans. 2 , 68, 289 (1972). (34) WaH. Breckenridaeand W. L. Nikolai. to be submitted for Publication. (35) N. Adams, W. H.Breckenridge, andJ. Simons, to be summitted for publication.
Catalysis by Iron Pentacarbonyl of the Polymerization of Carbon Monosulfide in a Flow Tube at Low Pressures W. H. Breckenridge," Gerald T. Bida, and Werner S. Kolln Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112 (Received November 20, 1978) Publication costs assisted by the Petroleum Research Fund
The presence of very small pressures of iron carbonyl, Fe(CO)6,in a gas flow tube at -70 "C catalyzes the polymerization of carbon monosulfide (CS) on the walls of the reactor. Unusual kinetic behavior is observed in which the decay of CS is zero order under all conditions studied, even when the CS concentration is more than ten times greater than that of Fe(CO)+ The rate of CS decay was also independent of Fe(COISconcentrations at PFe(C0)6 2 0.02 torr, and decreased markedly only at PFe(CO)j < 0.001 torr. It is shown that a mechanism in which catalytic sites are created from Fe(CO)5adsorbed via a Langmuir-type adsorption isotherm on the growing CS polymer can successfully rationalize all the experimental observations. Introduction A flow tube method for studying the reactions of the short-lived diatomic molecule carbon monosulfide, CS, has been developed in our laboratories. Studies of the reactions of CS with O2 and with the ground-state oxygen atom, O(3PJ),have been published We report here a novel process in which extremely small concentrations of Fe(CO)6in the gas phase are able to accelerate the polymerization of CS, apparently through the efficient production of active catalytic sites on the walls of the flow tube. The catalysis occurs with other metal carbonyls as well, and has prevented us from pursuing the original goal of the research, to study a ligand substitution reaction of a neutral coordination complex in the gas phase in the absence of solvent. Many thiocarbonyl metal complexes
* Camille and Henry Dreyfus Foundation Teacher-Scholar. 0022-3654/79/2083-1150$01 .OO/O
have been prepared and c h a r a ~ t e r i z e d ,and ~ ~ ~the demonstrated strength of metal-CS bonds led us to believe that a ligand replacement process of CS for CO could proceed readily through a dissociative kinetic m e ~ h a n i s m . ~It- ~now appears, however, that the presence of wall sites introduces a parallel catalytic polymerization mechanism which masks any homogeneous ligand substitution processes. Thus, gas-phase substitutions of CS for metal complex ligands will be difficult to study by conventional techniques. We were able to characterize the wall-catalyzed polymerization process in sufficient detail to warrant the following report of the rather unusual kinetic conditions encountered. Experimental Section A schematic diagram of the flow tube apparatus is shown in Figure 1. It is a typical fast flow system with a moveable injector, a fixed detector, and a variable temperature reaction region.* Some aspects of the apparatus 0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
Fe(CO)5Catalyzed Polymerization of CS Monochroma tor High Vacuum 0-Ring
Sliding Seals
Ar +Fe(CO), \ iniet
-
F!!, Pressure
0 0 0
Rotatable
''
c Detection
M'irowave i'ischarge
C:oli Trap
Variable Temperature Region
o
7 Removable Product Trap
1151
0
0
'\Metal Cold
(-130'C I
0
Figure 1. A schematic diagram of the experimental apparatus.
have been described Carbon monosulfide is produced in the main flow tube by low-power electrodeless discharge of a dilute CS2/Ar stream. Just downstream, a cold trap at -130 "Ci condenses any remaining CS2as well as other possible discharge products while alIowing passage of CS. Efficient production of CS from CS2 requires that the power dissipated in the microwave cavity be as low as possible for a stable discharge. A 6-dB attenuator was used for better control of power. Typical dissipated microwave power in the discharge was only -5 W. The optimum power for production of CS appeared to increase as the flow rate and CS2 concentration increased, as one might expect. Detection of CS is accomplished1Z2by multipath absorption measurements a t 2576 A, using a modified Beer-Lambert expression: log ( I o / l ) = a(c1)" where c is the concentration of CS, 1 the path length, and a = 0.71 f 0.05, determined empirically by varying 1 (i.e-, the number of optical passes) at constant c. Several experiments were performed using the process O(3P) CS2.+ CS -t SO as an alternate source of CS (for details, see ref 9), the results of which provided a check on the value of a, since c could be varied at constant 1. There was considerable difficulty with rapid deposition of sulfur on the windows under the high [CS2]/[O(3P)l conditions necessary to prevent rapid secondary reaction of O(3P) with product CS, but a consistent value of a = 0.66 f 0.11 was derived from the data. The sulfur undoubtedly resulted from the following minor exit channels in the O(3P)-CS2reaction: OCS S(3P);CO + SDl0 These experiments also provided absolute rather than relative concentration measurements of CS, and showed that under optimum discharge conditions greater than 90% of the CS2 could be converted to CS. Finally, a second series of measurements was performed in which 1 was again varied at constant c, the results of which gave a = 0.72 f 0.05. The final value adopted for the measurements reported here was a := 0.71. All the uncertainties listed above correspond to two s5tandard deviations of the mean of several determinations. The argon carrier gas was Matheson Ultra High Purity Grade and was used without further purification. The Fe(CO)5 (Alpha products, 99.5%) was filtered, freezepumped thoroughly, distilled from 0 to -78 "C, and stored in the dark at -78 "C. The Cr(CO)6 and W(CO& (Strem Chemicals) were sublimed before use. Fe(CO)5 was delivered to the injector system as an Fe(CO),/Ar mixture. Purified Fe(C0)5 was freeze-pumped at -196 "C, distilled again from 0 to -78 "C, then allowed to warm slowly and fill a previously evacuated darkened bulb to the desired pressure. Argon was then added for the desired dilution, and the gases were allowed to mix for a t least 2 h. Flow rates in the main flow tube in this study were normally 200-800 crnl/s, and total pressures were 1-2 torr. The flow tube inside diameter was 22 mm.
+
+
0
IO
20
30
40
50
Reaction Time (msec)
Figure 2. A typical first-order kinetic plot of the decay of CS in the presence of Fe(CO), at 70 "C: [Fe(CO),] = 0.027 torr.
Elemental analyses were performed by M-H-W Laboratories, Phoenix, Ariz.
Results a n d Discussion When an Fe(CO)5/Ar mixture is added to the flowing CS/Ar stream through the injector tip, the CS concentration at the fixed detector region decreases, indicating that the presence of Fe(CO)5somehow causes CS to decay much more rapidly than the very slow heterogeneous decomposition on the flow-tube walls at moderate temperatures which has been characterized When the injector tip is moved back, increasing the distance between injection and detection (and thus lengthening the time over which CS is in the presence of Fe(C0)5 before being detected) the CS concentration drops off smoothly when the temperature of the heated region of the flow tube is -70 "C. If the Fe(CO)5concentration is much greater than the CS concentration, one might expect typical flow-tube kinetic conditions of pseudo-first-order decay of CS. Shown in Figure 2 is a representative first-order decay plot of In [CS] vs. time in contact with Fe(CO)5 (i.e., time is directly proportional to injectorto-detector distance at constant velocity). As can be seen, there is severe curvature in the plot, showing that whatever the process responsible for the decay of CS in the presence of Fe(C0)5, a simple bimolecular collision process is not a rate-determining step. Ligand substitution reactions of metal-carbonyl-like compounds often follow dissociative mechani~ms,5-~ however, whereby the rate-determining step is the dissociation of the leaving ligand, followed by the rapid association of the new ligand with the resulting unsaturated fragment. Under conditions where back reaction of the leaving ligand is negligible, a dissociative mechanism would produce a zero-order (linear) decay of CS, the rate of which depends only on the Fe(C0)5 concentration (at constant temperature). Shown in Figure 3 is a zero-order decay plot, i.e., [CS] vs. time. Obviously, a mechanism which predicts initial zeroth-order decay of CS is required to explain the kinetic data. Unfortunately a simple gas-phase dissociative substitution mechanism (Le., such as that which obtains in the exchange of CO with Ni(C0)4)7is not consistent with the following observations: (i) Over a certain range of Fe(C0)5 concentrations, the zeroth order decay rate of CS is independent of Fe(CO)6 concentration, although there is some scatter in the data; (ii) In the heated region of the flow tube where rapid CS
1152
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
W. H. Breckenridge, G. T. Bida, and W.
S.Kolln
TABLE I: Elemental Analysis of the Black Film Formed When CS Decays in the Presence of Fe(CO),
c:s
wt
C:S modifieda mole mole
%
C
S
Fe
26.8 27.2 26.4 25.9 26.3
55.9 59.0 54.8 57.5 57.9
9.47 6.09 8.28 6.46 4.97b
0
ratio
1.28 3.47 1.23 6.44 1.28 8.26 1.20 1.21 av 1.24
ratio
1.13 1.07 0.93 1.04
Assuming each 0 is bound to one C. Atomic absorption, all other Fe determinations from ash, a
IO
20
30
40
50
Reaction Time (msec) Figure 3. A typical zero-order kinetic plot of the decay of CS in the presence of Fe(CO), at 70 OC: [Fe(CO),] = 0.027 torr. A A 0
I 01
02
03
04
05
Fe(CO)5 Pressure (torr) Figure 4. A plot of the variation of the zero-order decay rates of CS with Fe(CO), pressure, T = 70 OC: (e)normal experiments; (A) experiments in which the flow tube was lined with a thin layer of quartz wool.
decay is occurring, a brown-black film forms readily; (iii) Careful analysis of the condensibles in the product trap revealed nothing but unreacted Fe(C0)5. A mechanism involving some kind of gas-solid interaction was suspected, and several experiments were carried out to determine the possible cause for such unusual kinetic conditions. First, the CS concentration at the detection window was measured with a clean flow tube (heated region a t 70 “C). Iron carbonyl was added until the black film had deposited. The iron carbonyl flow was then turned off, and the CS concentration was observed to return to exactly the same value as that measured with clean walls. This showed that the black film itself does not cause CS to decay; Fe(C0)5 must be present. Next, the Fe(C0)5concentration was lowered in a series of experiments to determine the form of the CS decay falloff (which must occur at sufficiently low concentrations of Fe(CO)& and to see if zero-order CS decay was still maintained under those conditions. Shown in Figure 4 (circles) is the CS decay rate as a function of Fe(CO)5 pressure. The CS decay rate remains independent of Fe(C0)5concentration down to at least PFe(C0)6 0.02 torr, and does not decrease markedly until PFe(co)6 I 0.001 torr. Even a t the lowest Fe(COIS pressure investigated, zeroorder kinetic plots were always linear. For the low Fe(C0I5 pressures, this was true even when the concentration of CS was ten times as great as that of Fe(COI5. A catalytic process is therefore indicated. Finally, the brown-black material which formed when CS was decaying in the heated portion of the flow tube had
-
characteristics similar to a CS “polymer” described by several ~ o r k e r s . ’ l - The ~ ~ film was difficult to remove from the flow tube and tends finally to come off in flakes. Klabunde et have characterized such a material as “a brown-black polymer which adheres tenaciously to metal and glass”. Several elemental analyses were performed on samples of the material, with the results shown in Table I. The measured amounts of carbon and sulfur are fairly reproducible, with an average C:S ratio of 1.24:l.OO. The small amounts of iron and oxygen which varied considerably are undoubtedly due to Fe(CO)5 and Fe(CO)x fragments ( X I 4) imbedded in the polymer. If all the oxygen is assumed to be bound as CO, then the true C:S average mole ratio would be 1.04:1.00, consistent with polymeric CS. This ratio compares well with that obtained for CS polymer by Dewar and Jones12 (1:l) and Steudel13 (1.l:l.O) (Hogg and Spice,14 however, found a ratio of 2.8:l.O.) The black polymer observed in our experiments was quite inert chemically. Oxygen atoms from discharged O2 would not readily oxidize it, and it was stable in the presence of strong acids and bases. We were also unable to find a solvent which would dissolve the substance. Confirmation of CS polymerization is strong evidence that catalytic gassolid interactions are responsible for the kinetics observed, but two further experiments were performed to see if the rate did in fact depend on the surface area available to the reactants. A thin layer of quartz wool was used to line the heated section of the flow tube without changing the flow characteristics. As expected, faster decay rates were observed (the triangular data points in Figure 4). The mechanism for CS decay must therefore involve a surface reaction which results in the production of CS polymer. As we now show, if some form of adsorbed Fe(C0)5 catalyzes CS polymerization, it is possible to explain the zero-order decay of CS and the variation of the CS decay rate with Fe(CO)5pressure. The mechanism consists of three steps: (i) Adsorption of Fe(C0)5 on the walls: Fe(C0)5 + wall ~1Fe(C0)5.wall (1) (ii) Transformation of the adsorbed Fe(CO)5into a form which can catalyze CS polymerization: Fe(CO),.wall
k2 e [activated site] k-2
-
(2)
(iii) Catalytic formation of CS polymer: CS + [activated site] (l/X)(CS), + [activated site] (34 (l/X)(CS), + destruction of activated site (3b) The time between data points at different injector positions was always at least 1or 2 min, so that the adsorption equilibrium was always readily established on the growing polymer surface by the time a measurement was made.
-
Fe(CO)5Catalyzed Polymerization of CS
::I {-;:E, ,,l i /
01
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
*.
02
a
03
04
05
Fe(C0)5 Pressure (torr)
Flgure 5. Comparison of Langmuir adsorption isotherms with the experimental CS decay rate data. Experimental points correspond to values of 6’ which are CS decay rates normalized to an assumed maximum decay rate at high Fe(CO), pressure. Solid lines are the functional form of the Langmuir adsorption isotherm with various values of b (see text).
Assuming a steady-state in “activated site” surface concentration is established, and that kT2> h3b, since at low Fe(C0)5it is known experimentally that many CS decays occur for each Fe(CO)5present. In simple terms, catalytic sites are produced at a rate proportional to [Fe(CO)5.wall], but they polymerize several CS molecules ( - k 3 / h 3 b molecules of CS, on average) before being destroyed. All that is required for a consistent mechanism is an adsorption process in which [Fe(CO)5.wall]is the correct function of gas-phase [Fe(C0)5]. A simple Langmuir adsorption isotherm appears to do just that. The Langmuir adsorption isotherm15J6assumes that the surface contains a fixed number of adsorption sites each capable of holding one molecule and that each site is independent of the others. If 6’ is the fraction of sites occupied and P is the pressure of the molecule being adsorbed, the Langmuir adsorption isotherm equation is 0 = bP/(1 bP)
+
where b is the adsorption coefficient. The adsorption coefficient is the ratio of the rate constants for adsorption and desorption. Since the CS decay rate should be proportional to the amount of adsorbed Fe(C0)5 (Le., [Fe(CO)5.wall]), the decay rates in Figure 4 were converted to 8’s. It was assumed that the average of the relatively constant CS decay rates a t high [Fe(C0)5] corresponded to 6’ = 1. Values of 6’ were then calculated at each Fe(C0)5pressure from the CS decay rates. These values of 0 are plotted vs. Fe(C0)5pressure in Figure 5. Values of 6’ calculated for different values of b using the adsorption equation are also shown in Figure 5. The data points exhibit some scatter, presumably due to the changing nature of the surface from day to day, but a Langmuir adsorption isotherm will give a reasonable qualitative fit of the experimental points. No attempt has been made to fit the data to more sophisticated isothennsl5J6since the main interest was to establish the viability of the proposed mechanism. The value of b necessary to fit the data (-500 torr-l) is rather large, but this is obviously an unusual case in that a growing polymer is the adsorbing surface. Also,
1153
chemisorption rather than physical adsorption is probably involved, and it is well known that the high heats of adsorption (strong chemical bonds) which are often observed in chemisorption processes can facilitate saturation of surface sites a t comparably low pressures of adsorbent gas.15J6 A brief summary of the kinetic mechanism is appropriate. At higher pressures of Fe(C0)5virtually all of the adsorption sites are occupied, so that the decay rate of CS (controlled by the rate of formation of “active sites” from adsorbed Fe(CO)5) is essentially constant. At lower Fe(CO)5 pressures the amount of adsorbed Fe(CO)5 decreases according to a Langmuir-like adsorption isotherm and the CS decay rate slows proportionately. Over the whole range of Fe(COI5pressures, however, the CS decay is zero order in CS, depending only on the steady-state concentration of active sites. It is tempting to speculate about the actual molecular mechanism of the catalysis. One possibility is that the initial chemisorption involves dissociation of Fe(C0)5 and the resultant surface complexation of Fe(C0)4. The activation step could then be dissociation of further CO molecules leaving unsaturated Fe(CO)x fragments (bound to the growing polymer) as catalytic sites for CS polymerization. Such a process would essentially be irreversible (k2 very small), consistent with the kinetics. Spoiling of catalytic sites (reaction 3b) could correspond merely to the trapping of Fe(CO)x fragments by the growing layers of CS polymer (the elemental analyses are consistent with Fe(CO)x sites, where X = 3 f 1). From the S:Fe mole ratio in the polymer (14 f 3), each catalytic site appears to be able to polymerize -15 molecules of CS before being covered. Experiments with other volatile metal carbonyls (Cr(CO), and W(CO),) were also attempted, and production of polymeric CS was again observed. Auxiliary preparative experiments were conducted in which gaseous W(CO)6and Cr(CO), were photolyzed with UV light in the presence of CS, and CS polymer was also observed on the walls of the vessel. A careful search of the product traps in each case gave no indication of the known compounds17Cr(C0)5CS and W(CO)5CS. Thus, catalysis of CS polymerization by adsorbed metal carbonyl species may be generally efficient. It would be interesting to determine whether polymerizations of other gaseous monomers by very low pressures of metal-carbonyl-like complexes are also efficient. Such processes could prove to be important from a practical as well as a basic point of view.
Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Research support by the University of Utah Research Committee is also gratefully acknowledged. We thank the Phillips Petroleum Company for the Phillips Research Fellowship held by W. Kolln during the early stages of this research. References and Notes (1) W. H. Breckenridge, W. S.Kolln, and D. S. Moore, Chem. mys. Left., 32, 290 (1975). (2) G.T. Bida, W. H. Breckenridge, and W. S. Kolln, J . Chem. Phys., 64, 3296 (1976). (3) 1. S.Butler, Acc. Chem. Res., 10,359 (1977), and references therein. (4) P. V. Yanett, Coord. Chem. Rev., 23, 183 (1977), and references therein. (5) G. R. Dobson, Acc. Chem. Res., 9, 300 (1976). (6) F. Basolo and R. G. Pearson, “Mechanlsms of Inorganic Reactions”, 2nd ed, Wlley, New York, 1967. (7) J. P. Day, R. G. Pearson, and F. Basolo, J. Am. Chem. Soc., 90, 6927, 6933 (1968). (8) A. A. Westenberg, Science, 164, 381 (1969).
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The Journal of Physical Chemistry, Vo/. 83,
No. 9, 1979
(9) W. S. Kolln, Ph.D. Thesis, University of Utah, 1978. (10) I. R. Slagle, J. Gilbert, and D. Gutman, J . Chem. Phys., 84, 3296 (1976). (11) K. J. Klabunde, C. M. White, and H. F. Efner, Inorg. Chem., 13, 1778 (1974). (12) J. Dewar and H. Jones, R o c . R . SOC. London, Ser. A , 85, 574 (1911).
C.-2. Wan and G. L. Haller (13) R. Steudel, 2.Nafurforsch. 8 , 21, 1106 (1966). (14) M. A. P. Hogg and J. E. Spice, J . Chem. SOC.,4196 (1958). (15) S.J. Gregg, "The Surface Chemistry of Solids", Chapman and Hall, London, 1961. (16) J. H. deBoer, "The Dynamical Character of Adsorption", Clarendon Press, Oxford, 1968. (17) B. D. Dombeck and R. J. Angelici, Inorg. Chem., 15, 1089 (1976).
Effect of Coadsorbed Water and Alcohol on the Surface Transport of Stearic Acid on a-Al,O, Chung-Zong Want and Gary
L. Haller"
Department of Engineering and Applied Science, Yale University, New Haven, Connecticut 06520 (Received September 7, 1978; Revised Manuscript Received February 7, 1979) Publication costs assisted by Yale University
The effect of coadsorbed water, methanol, and 2-propanol on the surface diffusion of stearic acid on a-A1203 has been investigated. The structure of the surface was varied by choosing two crystal planes, (0001) and (4150), which have site densities which differ by more than a factor of 4. Changes in coverage of coadsorbed water or alcohol cause the surface diffusion coefficient to vary over two orders of magnitude and to pass through a maximum at a co-coverage of about a monolayer. The activation energy for this process is approximately 30 kcal/mol in the region of monolayer coverage. This indicates that diffusion is an activated random walk or hopping between sites. The decrease in the diffusion rate as coverage of co-adsorbed species moves toward zero is attributed to heterogeneity in the surface, i.e., the heat of adsorption of stearic acid increases as total coverage decreases. The decrease in the diffusion rate above a monolayer is associated with activated hole formation in the second layer and is less pronounced if stearic acid has a high solubility in the coadsorbed species, i.e., at a given co-coverage the diffusion coefficient decreases in the order 2-propanol > methanol > water.
Introduction Surface transport is a well-recognized, if not well-studied, phenomenon.lI2 In most systems where surface diffusion has been investigated, the diffusing molecules are weakly bound to the surface, but even when molecules are only physically adsorbed, surface transport may often contribute the major part of the flux through a porous material.3-s Most previous investigations of surface migration of chemisorbed species involved inorganic molecules (e.g., hydrogen, oxygen, and carbon monoxide) and have used field emission or ionization micros~opies.~ There are only a few examples where chemisorbed hydrocarbons were studied.1°-12 Chemisorbed hydrocarbons are the dominant surface species under the temperature and pressure conditions of most industrial catalytic systems and, because surface transport may play an important role in the overall rate of some of these catalytic reactions, they deserve further attention. A situation where surface diffusion may be of great importance involves a very active catalytic material not well dispersed on a support. The isolated reaction sites deplete the concentration of adsorbed reactant in the immediate vicinity and "draw" additional reactant from the surrounding surface by surface diffusion. The surface concentration is replenished by adsorption and the adsorbed reactant is in equilibrium with gas phase only on the surface far removed from the active sites. This problem was treated theoretically for an irreversible, first-order reaction by Aris13 and identified experimentally.l4-l6 More recently Mestdagh et al.17 have documented the importance of surface diffusion in the hydrogenation of di-tert-butyl nitroxide on a silica supported +Departmentof Chemical Engineering, University of Mississippi, University, Miss. 38677. 0022-3654/79/2083-1154$01 .OO/O
platinum catalyst and Thomson and co-workers18 have observed a similar phenomena for the hydrogenation of chlorobenzene on silica supported palladium. While in both cases the authors prefer to interpret their results as surface diffusion of the reactant hydrocarbon, they might alternatively be considered examples of hydrogen spilloverlgwhere the reactant is stationary on the silica and the hydrogen moves out from the metal surface. The measurement of surface transport is not always straightforward. It has been measured by electron spin r e s ~ n a n c e , 'nuclear ~ magnetic resonance,20and neutron scattering spectroscopy,21but is most often measured in porous materials using either a Wicke-Kallenback apparatus, e.g., Reed and Butt,6 or a permeability experiment in the Knudsen-flow regime, e.g., Lee and O'Connell.8 In either case, it is necessary to separate the gas and surface phase contributions to the total mass transport. This is done by first studying a nonadsorbing gas. The transport of a nonadsorbing gas is inversely proportional to the square root of molecular weight when the temperature, total pressure, and pressure drop are equal. Thus, the gas phase transport of the adsorbed gas is calculated by multiplying the flux of the nonadsorbing gas by the inverse ratio of the square root of the two gases' molecular weights. Field et aLZ2cautioned that this method may be unreliable because it ignores gas-adsorbed phase interactions. Bell and B r o ~ nhave ~ ~ shown , ~ ~ experimentally and theoretically that, in fact, this method of measurement is not always reliable. In a recent reanalysis of old data taking gas-adsorbed phase interactions into account, we find that errors are often as large as 5070.'~ One approach to avoid the problem mentioned above is to arrange conditions such that the bulk or gas phase flux is a small part of the total flux, e.g., Lee and 0'ConnelP have found conditions where the bulk flux is less 0 1979 American Chemical
Society
