CHARLES P. NASHAND RONALD L. MUSSELMAN
2166 latices (Table IV) show a marked increase in molecular weight as the particle size increases. Tentatively, the decline of air-solid surface tension of latices seems to be associated with the increase in average molecular weight of the polymer chains forming the particles. For No. 4 Saran I latex (T, -32") only a part of the original specific surface is maintained during lyophilization. Using the determined surface of the lyophilized latex the calculated total surface energy is high (80 dyn/cm at 40°), and reproducibility is poor. It is possible this composition crystallizes above the T , and shows excess heat evolution. Kumber 5 Type Saran I1 latex has a high degree of crystallinity as determined by X-ray diffraction measurements. Its film formation under ordinary circumstances is poor and the contours of the individual
latex particles on the electromicrographs of deposited films is clearly distinguishable. In agreement with this neither the CCM nor the dta counterpart shows signs of change up to -180" where a well-defined maximum occurs. The interpretation of this is cloudy since the material starts to decompose around this temperature. The energies measured, therefore, can be the combined effect of surface loss, melting, and decomposition. The dta and CCM methods, described here, are a sensitive analytical tool for observing sintering and film formation of colloidal materials. Acknowledgments. I would like to express my thanks to Dr. F. P. Gay of Du Pont, Film Department, Experimental Station, and to Professor W. H. Stockmayer for help and encouragement.
Hydrogenation of Ethylene over Exploded Palladium Wire by Charles P. Nash and Ronald L. Musselman Department of Chemistrv, Unizersity of California, Davis, California 95616
(Received October 27, 1969)
Metal dispersions produced by exploding palladium wire in argon near atmospheric pressure catalyze the hydrogenation of ethylene at room temperature. The fastest reaction follows a Rideal mechanism with hydrogen as the adsorbed species. The kinetic data also suggest that ethylene adsorbed on these surfaces may be hydrogenated, but at a relatively slow rate.
Introduction In recent years a few reports have appeared documenting attempts to utilize the metal dispersions produced when a wire is exploded in a gaseous atmosphere. Karioris, Fish, and Royster1a2 have established that under the proper conditions an aerosol of spherical metal particles having diameters of a few hundred angstroms may be produced by this method. Nash and De Sieno3 have shown that in some particularly favorable cases the infrared spectra of molecules absorbed on these surfaces may be recorded. A recent paper by Thomson and Webb4 reports that nickel, palladium, and platinum, when exploded in argon, failed to catalyze the hydrogenation of ethylene, whereas palladium wires exploded in hydrogen atmospheres gave catalytically active surfaces. Because of the possible mechanistic implications of this result, we have undertaken a reexamination of the catalytic activity of palladium exploded in an argon atmosphere.
Experimental Section Materials. The 0.25-mm diameter palladium wire used in these experiments was obtained from the Metals The Journal of Phgeical Chemistry
and Controls subsidiary of Texas Instruments, Attleboro, Mass. The argon (purity > 99.99%) and ethylene (CP grade) were supplied by the Matheson Co. The argon was used without further treatment while the ethylene was distilled three times by trap-to-trap transfer methods and stored frozen in a liquid nitrogen trap. The mass spectrum of the ethylene prepared in this manner showed a trace of contamination by butane, with no other impurities. The hydrogen (minimum 99.99%, from Liquid Carbonic Co) was passed through a liquid nitrogen trap packed with glass beads. No impurity could be detected in its final mass spectrum. Reaction Vessel. The mires were exploded and the catalytic reactions were studied in a chamber consisting of three parts. The uppermost part vias an inverted 160mm desiccator bottom with the flange polished smooth (1) F. G. Karioris, B. R. Fish, and G. W. Royster, "Exploding Wires," Vol. 2, W. G. Chace and H. K. Moore, Ed., Plenum Publishing Gorp., New York, N. Y., 1962, p 299. (2) F. G. Karioris and B. R. Fish, J . CoZZoki Sci., 17, 155 (1962). (3) C. P. Nash and R.P. De Sieno, J . Phys. Chem., 69, 2139 (1965). (4) S. J. Thomson and G. Webb, Chem. Commun., 473 (1965).
HYDROGENATION OF ETHYLENE with cerium oxide. The center part mas made from a 20 cm diameter X 3.7 cm thick Pyrex telescope blank. A 12-cm diameter hole was cut in the center of the blank. Electrodes fashioned from 12-mm steel bolt stock were installed in diametrically opposed holes drilled through the edges of the disk. The electrodes were sealed into the disk by filling the entire channel with epoxy resin and using Viton 0 rings between the nuts and the glass on both ends of the bolt stock, further to ensure vacuum tightness. When a wire was fastened between the two electrodes across the hole in the disk its length measured 11 cm. The plane faces of the disk were polished smooth and flat n-ith cerium oxide. The bottom part of the chamber was constructed entirely of heavy-wall Pyrex tubing. One end of a 10cm length of 15-cm 0.d. tubing was flanged and polished in a fashion identical with the lip of a desiccator. The other end was joined with a gradual taper to a 20-cm length of 8-cm 0.d. tubing, the bottom end of which terminated in a hemispherical dome. Near the bottom of the 8-cm section, across its diameter, flanged holes were provided to accept two sodium chloride windows. The windows themselves were made from salt plates secured with epoxy resin to Lucite disks in which 0ring grooves had been machined to match the flanges on the openings into the chamber. The salt plates and the grooves were both on the same side of the plastic disk. Thus the windows were supported inside the chamber and were forced against the backing disk by the shock wave from the explosion. When the windows were on the outside of the disk they were invariably blown off , usually through failure of the salt itself rather than the adhesive joint. Two Viton 0 rings were used to provide vacuum seals when the three sections of the chamber were assembled in sandLYich fashion. Clamping rings and bolts were used to secure the sections together, and the bolts were tightened continually as the chamber was evacuated and the 0 rings were compressed. Viton 0 rings and clamping plates also secured the windows to the chamber. The chamber was connected to the vacuum system by a 10-mm Teflon stopcock, also fitted with Viton 0 rings, mounted near the top of the 15-cm 0.d. section just below the electrode disk. The chamber was isolated from the remainder of the vacuum system by a Dry Ice-acetone trap. When the chamber was properly assembled it was capable of maintaining a vacuum of at least lop4 Torr for at least a week. The volume of the entire chamber asisembly was 6 1. Experimental Procedure. Preliminary experiments quickly established that palladium exploded in argon gave catalytically active materials for which reaction times were typically a few minutes. We therefore have used the infrared spectrum of the product ethane to monitor the course of the reaction. A typical experimental sequence involved the installation of a wire weighing 70 mg in the chamber, after which it was
2167 outgassed by heating it electrically at a dull red glow
in vacuo for at least 4 hr. The chamber was filled with 700 Torr of argon, and the wire was exploded with a 28-pF condenser bank charged to 11 kV. The resulting aerosol was allowed to settle for a t least 2 hr, at the end of which a heavy, soot-like deposit was found, principally in three places: on the hemispherical bottom, on the tapering section of the wall, and on top of the electrode disk. Thus there are roughly three well-separated zones in which the catalytic reaction occurs in our “static” system. After the aerosol had been deposited, the chamber was evacuated for about 6 hr at a pressure below Torr. One of the reactant gases was then introduced into the chamber at a known presmre. The other gas was stored in a 1 1. side-bulb, branching off from the main stopcock, at a known overpressure such that when the main stopcock was opened for 10-15 sec and then closed again, pressure equalization would yield the desired composition of reactant gases in the chamber. Experiments in which ethane was pulsed into a chamber filled with hydrogen showed that within 10 sec pressures were equalized and homogeneous mixing was obtained. The chamber was mounted in the sample beam of a Beckman IR-12 infrared spectrophotometer set to monitor the absorbance of the ethane peak at 2881.5 cm-’ as a function of time. The usual rate of chartdrive was 1.8 in./min. The spectrophotometer was adjusted to read zero absorbance, the main stopcock was cracked briefly, and the desired reaction ensued immediately. The monitor peak of ethane at 2881.5 cm-l was shown in separate calibration experiments to span the absorbance range 0-1 when the ethane pressure in the chamber ranged from 0 to 77 Torr, irrespective of the partial pressure of hydrogen which was present. The raw data for any experiment comprised a plot of the absorbance (pressure) of ethane vs. time. Instantaneous rates of reaction wereobtainedby constructing tangents to the curve by the usual mirror method. The instantaneous compositions could be calculated from the known stoichiometry of the hydrogenation reaction, which mass spectrometer analysis showed to be the only process occurring in the system.
Results The first result of these experiments, of a purely qualitative nature, is the observation that, irrespective of the order of addition of the reactant gases, palladium exploded in 700 Torr of argon produces a functioning catalyst for the hydrogenation of ethylene. This result contrasts sharply with that of Thomson and Webb, who reported the system to be inactive. A freshly prepared dispersion gave a catalyst which displayed extremely high, but erratic, activity. After six or seven runs on a given preparation, however, Volume 74, Number 10 May 1.4, 1070
2168 the surface stabilized and thereafter consistently reproducible behavior was obtained. Laidler and Townshend5have noted a similar effect with nickel films. A series of ten experiments with the most active stable surface which we obtained served to establish the nature of the dominant mechanism for reaction mixtures in which hydrogen, in excess, was introduced into the reaction vessel first. When the data gathered over the course of each of these reactions was plotted in first order fashion, Le., -log (fraction of ethylene remaining) vs. time, consistently good straight lines were obtained to over 90% conversion. The slopes of these lines, however, showed slight but real increases as the initial hydrogen :ethylene ratio was increased, For example, a series for which the initial ethylene pressures vere all 20 Torr and the initial hydrogen pressures were 60, 120, and 240 Torr, gave apparent first-order rate constants of 1.1, 1.6, and 1.8 min-’, respectively. When the data were plotted as log rate us. log ethylene pressure, good straight lines were obtained with slopes in the range 1.0-1.3. By taking vertical sections through these curves it was possible to construct a new family of curves depicting reaction rate vs. hydrogen pressure, the ethylene pressure being a constant on each curve. Figure 1 shows this family of constructs. While the data for the various ethylene isobars show considerable scatter, the drawn curves provide a reasonable fit to the points. The four curves have been drawn in accordance with a first-order ethylene dependence. The inclined straight lines in Figure 1, whose slopes are the same as the several apparent first-order rate constants, join the points obtained from any single run. With few exceptions, the data points scatter from the drawn curves in a systematic fashion. Those points from runs for which the initial ethylene pressure was high lie below the average curve, while those for which the initial pressure of ethylene was low lie above it. Since our mechanistic conclusions for this surface will be bawd on E’igure 1, which was constructed from a number of discrete experiments, it is essential to demonstrate both the stability of the “seasoned” surface and to shorn that it is instantaneously responsive to the changing conditions in the gas phase. Fulfillment of the stability condition was assured by periodic check runs using a standard mixture. The responsive character was demonstrated in a series of four experiments in which premixed samples of 80 Torr of H, 40 Torr of ethylene was allowed to react in the chamber until half the ethylene was hydrogenated. In two experiments the hydrogen pressure was then suddenly increased from 60 Torr to 120 Torr with a pulse from a side storage bulb. I n two other experiments 60 Torr of helium rather than hydrogen was injected. I n either case the system was subjected to an appreciable disturbance, the relaxation time of which is nonnegligible and indeterminate. Thus the apparent rates of ethane production measured immediately after the
+
The Journal of Physical Chemistry
CHARLES P. NASHAND RONALD L. MUSSELMAN
Hydrogen Pressure, Torr
..
Figure 1. The hydrogen dependence of the rate of production of ethane. The ethylene dependence is first order.
injection of the two different gases were not quite those which one would have expected on the basis of the 20 Torr ethylene isobar in Figure 1 for hydrogen pressures of 60 and 120 Torr. The diference in the apparent rates, however, was nearly the 7 Torr/min value which this isobar yields for those hydrogen pressures. As the reactions proceeded, both the rates and the rate differences tended to conform to the predictions of Figure 1. As further substantiation of the stability of the surface, all four of the runs superimposed up to the time of gas injection and each like pair superimposed thereafter. In contrast to the fairly straightforward kinetic behavior found in runs over a catalyst having high intrinsic activity, studies of a preparation whose activity was, for no obvious reason, only about half as great, gave very complex results. When pseudo-first-order plots were made for reaction mixtures having 120 Torr of hydrogen plus 11, 22, 41, and 75 Torr of ethylene, rate constants of 2.6, 1.2, 0.7, and 0.4 min-’ were obtained. That is, the apparent rate constants are nearly inversely proportional to the initial ethylene pressure. Qualitatively identical behavior was found for mixtures in which the ethylene was introduced first, but the magnitude of the rate constants was diminished by -15%. When the data from a number of these same runs was plotted as log rate us. log ethylene, the “apparent order’’ in ethylene changed continuously from -1.5 at the beginning of the reaction to -0.5 near the end.
Discussion We shall discuss the kinetic results in terms of the three principal mechanisms which have been proposed for this kind of process.8 In the so-called Rideal Type I mechanism, reaction occurs between an ethylene ad(5) K. J. Laidler and R. E. Townshend, Trans. Faraday Soc., 57, 1590 (1961). (6) K. J. Laidler, “Chemical Kinetics,” 2nd ed, McGraw-Hill Publications, New York, N. Y . , 1965.
HYDROGENATION OF ETHYLENE sorbed on the surface and hydrogen from the gas phase. The rate law for this process has the form
where K is a measure of the intrinsic activity of the catalyst, L is the ratio of the rate constants for the adsorption and desorption of ethylene, and the bracketed quantities are pressures of the species in the gas phase. I n the alternative Rideal Type I1 mechanism, adsorbed hydrogen molecules react with gaseous ethylene in accordance with the rate law
The third mechanism, that of Langmuir and Hinshelwood, describes the reaction between adsorbed hydrogen molecules and adsorbed ethylene. The rate law is
The single mechanism which fits nearly all the facts depicted in Figure 1, and the discussion preceding it, is the Rideal Type 11. The drawn curves in this figure were calculated from eq 2 using the parameters A = 4.1 X Torr-' min-', B = 2.1 X Torr-'. By taking the total derivative of eq 2 and using the known stoichiometry of the reaction t o equate d [C2H4] = d [Hz], one may write
2169 adsorbed ethylene must also be possible, and indeed are quite important on the least active surfaces. Equation 5 embodies the functional form of the log (Rate) vs. log [C2H4]plots to be expected for surfaces on which only the Rideal Type I1 mechanism is operating. From this equation one might reasonably expect apparent orders in ethylene which are between 1.0 and 1.3, but never less than 1.0. This prediction is contradicted by the behavior cited for the inactive surfaces, on which fractional orders were commonly observed toward the end of the reaction. Neither a pure Rideal Type I nor a pure LangmuirHinshelwood mechanism reproduces this trend. Total differentiation of eq 1 and 3 gives d log (Rate) d log [Cz&l
respectively. Both of these equations allow fractional order in ethylene but neither permits a given run to change from >1 to
