T H E COHERER METHOD O F DETERMIXIKG HEATS OF AD SORPTION * BY SAMUEL LESHER AND G . HARVEY CAYEROS
Introduction The direct determination of heats of adsorption of gases on solids is of importance in the field of contact catalysis. Heats of adsorption can be calculated from the Clausius-Clapeyron equation applied to adsorption isotherms. This method, however, has all the disadvantages of an indirect calculation. Calorimetric determinations of heats of adsorption have been made by several investigators' using finely divided catalysts. While the calorimetric method yields valuable results it is so cumbersome that its application has been limited. Amethod of great experimentalsimplicity for measuring heats of adsorption has recently been described by Palmer.2 Two loosely touching metallic (or carbon) filaments are caused to cohere by applying a small potential difference across the junction, the resistance of which decreases markedly as a result of the act. Palmer assumes that the action of the coherer is due to the evaporation of an adsorbed film of the gas in which the coherer is immersed and that the heat of adsorption of the gas on the filaments can be computed from the minimum voltage required t o cause coherence. The use of the coherer should present distinct advantages for the investigation of heats of adsorption, for the technique allows only true surface films to play a part in the measurements. Adsorption is assumed to take place on the relatively non-porous surface of a fine metal filament which can be easily heat-treated and which should remain with an unchanged surface activity throughout a series of measurements. The cohering test in vacuum is taken as a test of the cleanness of the surface from the gas studied or from contamination. Experiment Details The apparatus (see Fig. I ) was similar to that described by Palmer.' The coherer consisted, in most of the work, of tungsten filaments 0.02 mm. in diameter welded to heavier tungsten leads. One of these was sealed into the bulb, F; the other was sealed through a ground glass stopper which permitted adjustment of the contact, The potential was applied by charging the con* Contribution S o . 69 from the Experimental Station, E. I. du Pant de h'emoiirs and Company, Wilmington, Delaware. 1 Foresti: Gam., 53, 487 (1923); Beebe and Taylor: J. Am. Chem. SOC.,46, 43 (1924); Garner and McKie: J. Chem. Sac., 1927, 2451; H. S. Taylor and Kistiakowsky: Z. physik. Chem., 125, 341 (1927). G. B. Taylor, Kistiakowsky and Perry: J. Phys. Chem., 34, 748 (1930); Flosdorf and Klstiakowskky: 39, 1907 (1930); Keyes and Marshall: J. Am. Chem. Sac., 49, 156 (1927); Garner and Ingman: S a t u r e , 126, 352 (1930). Palmer: R o c . Roy. Sac., 106.4, 55 (1924);110, 133 (1926); 115, 227 (1927); 122, 487 (1929).
COHERER METHOD FOR HEATS OF ADSORPTION
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denser, C, of o.orgz microfarads capacity by the potentiometer P and connecting it a t once t o the coherer. The insulation of the condenser was tested with a Dolezalek electrometer; the loss of charge in one minute was 0.24 per cent at one volt. The chromel-alumel thermocouple, T, a t 100-200~C. mpplied a small potential to test the junction for coherence. The bulb was heated with a flame several times while evacuating to remove volatile impurities, and the filaments were heated electrically before each series of measurements. All switches were of the mercury cup type. The apparatus was sufficiently removed from X-ray machines and other sources of electrical disturbance to avoid coherence from effects of this kind.
FIG.I Coherer Circuit
The bulb was evacuated with a mercury pump; pressures were read on a RlcLeod gauge and a mercury manometer. The gases used, oxygen, nitrogen] and hydrogen, were taken from commercial cylinders and stored in evacuated bulbs in contact with anhydrous magnesium perchlorate. Nitrogen and hydrogen were freed from oxygen by passing over hot reduced nickel. In some of the experiments a liquid-air trap was attached immediately in front of the bulb F to remove water and mercury vapor. I n spite of careful attention to all the precautions mentioned by Palmer, reproducible cohering voltages could not be obtained. The cohering voltages in vacuum, and at various pressures of nitrogen, hydrogen] and oxygen were equally erratic. For example, in one of a large number of experiments the apparatus was evacuated and hydrogen admitted. The filaments were glowed at a bright red heat. At a pressure of 1 1 5 mm. the filaments cohered once out of six times at 1.0volts and four out of five times at 1.5 volts. The following day, under the same conditions, out of six trials at each voltage they cohered twice at 0.5 volts, once a t 2 . 0 , once a t 3.0, three times at 6.0, and
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SAMUEL LESHER AKD G . HARVEY CAMERON
five times at 7.0 volts. S e w filaments were put in and glowed out in hydrogen; out of 30 attempts the filaments cohered only three times at voltages between 0.5 and 4.0 volts after which coherence could not be obtained a t all even a t 16 volts. It was observed several times that a pair of filaments would cohere a t a comparatively low voltage but after tapping off would not cohere again until a higher voltage applied. Although a great variety of conditions was tried no reason for the erratic behavior could be found. Discussion Even under the most carefully controlled experimental conditions i t has been found impossible to obtain reproducible effects, and consequently Palmer's results have not been confirmed. If Palmer's assumed mechanism for the action of the coherer were correct, i t should be possible to repeat the operation at a given voltage any number of times provided the surface was not altered by the cohering act. K e found, on the other hand, that some factors other than the voltage were instrumental in determining whether coherence occurred or not. In other words, the filaments rarely cohered every time the potential was applied at any voltage and no more frequcntly a t five volts than at one volt in many cases. Furthermore, the lapse of time between applications of the potential seemed to play a part in determining the subsequent behavior. \Ye observed also in many cases a weak coherence: Le., the resistance of the junction as indicated by thermocouple and g d vanometcr did not drop to the usual low value. Palmer's assumed mechanism was bmed on the work of Branly3 who experimented with a coherer consisting of two plane metallic discs separated by a thin sheet of a solid dielectric. Branly believed that the action of this coherer was to be attributed to a sudden incrcase in the conductivity of the dielectric, and consequently that in the ordinary form of coherer the contacts must be separated by an insulating layer which becomes conducting under the influence of a sinall potential and regains its insulating power when the contact is tapped. Shaw and Garrett4 measured the force required to break a coherer contact and found it to be of the order of magnitude of the tensile strength of the metal. They calculated that there was sufficient heat developed t o weld the minute areas in contact and concluded that this was the correct explanation of the effect. On this basis it seems probable that the gaseous atmosphere would influence the potential required to actuate the coherer by altering the character of the surface, especially after the filaments had cohered a few times. This might account qualitatively for some of the effects observed by Palmer. The heat treatment of the tungsten filaments used by Palmer was not sufficient to remove the oxygen from the stable film of tungsten oxide on a tungsten surface according to the work of Langmuir.j 3
Branly: Compt. rend., 155, 933 I 19121. Shaw and Garrett: Phil. hlag., 8, r6j fr904). Langmuir: Trans. Faraday Soc., 17, 608 (1922;.
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3085
In a recent theoretical paper based on the quantum mechanics Ehrenberg and H6n16 have shown that metallic contact is not essential to coherence. Their work indicates that the resistance of a contact has a1re:dy dropped to a very low figure when the surfaces are still of the order of I O Angstroms apart. Heats of adsorption calculated by Palmer from “crytical” cohering voltages do not agree even in order of magnitude with the findings of other investigators. Table I shows the striking disagreement between Palmer’s
TABLE I Heat of .4dsorption in Calories’gram mol. Palmer Taylor, Kistiakowsky and Perry
Hydrogen Sulfur dioxide Oxygen Carbon dioxide
I 60 5340
121000
~~~t of liquefaction of the gas in calories 2 00
j,ooo
5800
I200
I20,OOO
1950
7 50
30,000
1440
2
values and those of Taylor, Kistiakowsky and Perry’ for the heats of adsorption of several gases on platinum. The latter measurements were made calorimetrically on a powdered catalyst.
Summary The repetition of some of Palmer’s experiments on the coherer method of determining heats of adsorption has given very erratic values for the cohering voltages of tungsten filaments in hydrogen, oxygen, nitrogen, and in vacuum. Objections are raised to the mechanism of the cohering act assumed by Palmer which are based on the present experiments and on the work of previous investigators. Ehrenherg and Honl: Z. Physik, 68, 289 (1931). Taylor, Kistiakowsky and Perry: J. Phys. Chem., 34, 799 (1930)