1398
E.L.PACEA N D A. R.SIEBERT
Vol. 63
HEAT OF ADSORPTION OF PARAHYDROGEN AND ORTHODEUTERIUM ON GRAPHON BY E. L. PACEAND A. R. SIEBERT illorley Chemical Laboratory, Western Reserve University, Cleveland, Ohio Received January 16, 1969
An experimental investigation of the differential and isosteric heats of adsorption for parahydrogen and orthodeuterium adsorbed on graphon has been carried out in the neighborhood of the normal boiling points of the gases by the use of conventional low temperature calorimetric procedures. Assuming the interaction of an isolated molecule of the gas with a surface having the structure and parameters of graphite, good agreement is obtained between the computed and experimental values of the heat of adsorption a t low coverages. The larger heat of adsorption of deuterium as compared to that of hydrogen is reasonably explained by the difference in the zero point energies associated with the degree of freedom perpendicular to the adsorbent surface.
Introduction The present investigation is one of a series which is being carried out which involves the adsorption of low boiling gases on Graphon. The interest in the two isotopic species of hydrogen arises from the fact that the relative masses should enter significantly into the energetics of the adsorption process. Qualitative confirmation of this fact has come from studies with hydrogen and deuterium'J adsorbed on carbon surfaces which report the stronger adsorpt,ion of deuterium. It is the purpose of the present investigation to reach some semi-quantitative conclusions on this matter. The use of the forms of hydrogen and deuterium stable a t low temperature is governed by the necessity of minimizing the uncertainty in the heat of adsorption coming from conversion effects on the adsorbent surface. The choice of Graphon as an adsorbent is based on evidence3 that it is a material closely approaching the characteristics of true graphite while retaining a surface area large enough to make calorimetric adsorption studies possible. Recent work by Graham4has confirmed this fact by showing that Graphon has a very small percentage of non-uniform sites. Experimental The adsorbent used in the investigation was a 31.3-g. portion of a sample received from Godfrey L. Cabot, Inc. Laboratories. X-Ray data provided with the sample specified a C value of 6.96 A. as compared to 6.70 A. for true graphite. The BET area with nitrogen was 85.9 m.2 g.-l. The sample of Graphon was activated by heating at 220' for five days and simultaneous evacuation until a pressure 1~ than mm. was obtained. The hydrogen was obtained from the Ohio Chemical Company, Cleveland, Ohio. The purity was stated to be 99.5%. Deuterium of 99.5% purity was purchased from the Stuart Oxygen Company, San Francisco, California. Each of the gases was passed through a trap immersed in liquid nitrogen to remove traces of water. Prior to their use in the absorption experiments, the hydrogen and deuterium were converted to approximately 95y0 parahydrogen and orthodeuterium by contact with act,ivated charcoal in successive baths of liquid .nitrogen and liquid helium. The calorimetric apparatus and experiment,al procedure for determining heats of adsorption and adsorption isotherms have been previously described .616
-
(1) R. M . Barrer and E. K . Rideal, Proc. Roy. SOC.(London), 8149, 231 (1935). (21 W. van Dingener and A. van Itterbeck, Physica, 6, 49 (1939). (3) W. D. Schaeffer, W. R . Smith and 14.H. Polley. Ind. Eng. Chem., 45, 1721 (1953). (4) D. Graham, THTS J O U R U A L61, . 1310 ( 1 9 5 7 ) . (5) E. L. Pace, L. Piercc and Ii. S. Dennis, lieu. Sa. I n s ( ! . . , 26, 20 (195;7).
Even though the hydrogen and deuterium were largely converted to the stable low temperature forms, the measurement of the heat of adsorption was complicated by the presence of small scale heat effects from orthc-para conversion associated with changes in temperature of the calorimeter during the measurement. Including the uncertainty in the conversion process, the error in the experimental values is estimated as being not more than k 2 0 cal. per mole. The heats of adsorption as a function of coverage were measured calorimetrically at average temperatures of 20.4"K. for parahydrogen and 23.5"K. for orthodeuterium. Equilibrium pressures in the neighborhood of the boiling points were obtained either as a function of coverage a t two temperatures or a t several essentially constant coverages over a range of temperatures. It was possible to derive values of the isosteric heat of adsorption from conventional methods employing these data either in plots of In p us. 1/T or in the integrated form of the Clausius-Clapeyron equation.
Results and Discussion The results of measurements are presented in Fig. 1 for parahydrogen and Fig. 2 for orthodeuterium. The calorimetrically determined differential heats of adsorption are represented by open circles while the isosteric heats of adsorption determined from equilibrium pressure data are represented by solid or half-solid circles. Since the isosteric heat of adsorption is identified thermodynamically with the differential heat of adsorption, the values obtained by the two different experimental methods appear to be internally consistent with this fact. The shape of the curves is of interest. By analogy with the case for argon adsorbed on Graphon,' the sharp break in the curve can be reasonably interpreted as the result of the completion of the first layer and incipient occupation of second and third layer sites by molecules of the adsorbed gas. We wish to remark here without further amplification a t this time since the matter will be discussed in a later paper that the break in the curve corresponds to a coverage which gives approximately the BET area with nitrogen if liquid packing of the hydrogen and deuterium on the surface is assumed. A considerable difference exists between this value and the values of 169.3 and 174.3 m.2~ . - l for hydrogen and deuterium, respectively, obtained by the use of isotherm data with molecular areas from liquid packing in conventional BET plots. The curveE:are also capable of interpretation with respect to (1) order of magnitude of the observed ( 6 ) E. L. Pnce, E. L. Heric and I
