HAROLD C. BEACHELL AND HAROLD S. VELORIC
102
Vol. 60
ADSORPTION ISOTHERMS, ISOBARS AND ISOSTERES OF DIBORANE ON BORON NITRIDE AND PALLADIUM ON CHARCOAL BY HAROLD C. BEACHELL AND HAROLD S. VELORIC' Department of Chemistry, University of Delaware, Newark, Delaware Received June 29. 1966
Adsorption isotherms, isobars and isosteres of diborane on boron nitride and palladium on charcoal were obtained in the pressure range of 200 to 750 mm. and temperature range of 180 t o 300°K. The sorption isotherms were found to be Langmuir, Type I, with negligible hysteresis even at the lowest temperatures. Fit of the Langmuir low pressure adsorption isotherm to the data was found in the case of palladium on charcoal. Calculations of the isosteric heat of adsorption as a function of the quantity of gas adsorbed were made. It was concluded that the adsorption process represents pure Van der Waals adsorption which fits the Langmuir hypothesis for low pressure adsorption. No evidence was found for chemisorption.
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Introduction In view of the numerous gas phase reactions of diborane a systematic investigation of the sorption properties of the compound was initiated. A complete study of the adsorption phenomena includes isotherms, isobars and isosteres. The Langmuir2 adsorption isotherm is frequently employed t o express the variation of adsorbent, at constant temperature, with pressure. X =- abp -
1+ap
m
At low pressure the isotherm simplifies to
--
A great deal of additional work3 has shown that in many cases when the surface is clean and the pressure is low curious relationships frequently are found4 between the total amount of gas adsorbed and the temperature; at very low temperature the amount decreases with increasing temperature and then rises as the temperature increases. At the Iower temperatures the adsorption is entirely van der Waals type, a t higher temperatures the rate of chemisorption increases giving an increase in the total amount of adsorption. By this criterion the adsorption isobar is a good indication of the physical or chemical nature of adsorption. Brunauer5 has developed by thermodynamic means an indirect method of calculating the isosteric heat of adsorption.
The heats of van der Waals adsorption are of the same order of magnitude as the heats of liquefaction of gases. Ordinarily the heats of adsorption vary considerably with the amounts of gas adsorbed. Experimental Materials.-Boron nitride was of the highest available commercial purity. The palladium on charcoal was a cominercislly nvnilnble surface active agent purchased from Unlier and Adnmson. Diborane was prepared by the method described by Shnpiro and Smith.6 The vapor pres( 1 ) H. s. Veloric, Ph.D. Dissertation, University of Delaware, Newark, Delaware, June 1955. (2) I. Langmuir, J . Am. Chem. Soc., SS, 2207 (1916). (3) J . W. McBain and Britton, ibid., 59, 2198 (1930). (4) A. W. Gauger and H. S. Taylor, ibid., 45, 1920 (1923). (5) 8. Brunauer, "The Adsorption of Gasee and Vapors," Princeton, New Jersey, 1943, p. 222. (6) I. Shapiro and G. B. Smith, J . Am. Chem. Soc., 74, 901 (1952).
sure of the BzH6 at 185°K. was 22.5 mm. The purity of the gas was checked by infrared analysis. Apparatus and Procedure.-The high vacuum system used was similar to the adsorption apparatus developed by Pease.' It consists of a high vacuum side, McLeod gage, closed end manometer with a leveling bulb attachment, a series of catalysts bulbs and a water cooled precision gas buret. The dead space was measured by introducing "Matheson" helium from the gas buret at known temperature and pressure. The samples of boron nitride and palladium on carbon were prepared according to the method of Bischoff and Adkkmb The temperatur: of the water-cooled gas buret was maintained to f l . The pressure was read on the closed end manometer to f l mm. The temperatures of the sample tube were obtained by placing about the tubes Dewar flasks containing ice-water, liquid nitrogen-carbon tetrachloride mixtures, liquid nitrogen-chloroform, liquid nitrogen-acetone, and liquid nitrogen-methanol mixtures. The temperatures were measured with a calibrated copperconstantan thermocouple. Diborane was passed from the gas buret into a catalyst bulb after the catalyst sample had been exposed to prolonged evacuation a t mm. The pressure of the system was varied at constant volume and temperature. The purity of the diborane was monitored with infrared analysis to make certain no chemical changes had taken place.
Results and Discussion For the palladium on charcoal the adsorption was studied at pressures of 30 t o 70 em. and at temperatures varying from 200 to 300°K. I n the case of the adsorption of diborane on boron nitride the temperature range was 180 to 315°K. and pressures varying from 18 to 75 cm. Each recorded point is the average of two measurements. Hysteresis effects even a t the lowest temperatures were small enough to be neglected. I n this range of temperature and pressure all of the palladium on charcoal isotherms showed a linear variation with pressure. K Odid not vary with temperature and was equal t o 0.24 cc./g. cm. The diborane on charcoal isobar also showed a linear variation, the slope of each line being equal to - 1.3 cc./g. KO. Lower temperatures could not be conveniently utilized since the gas liquefies at 181"K., higher temperatures were not investigated because the rate of decomposition becomes too large.gJO The isotherms for diborane on boron nitride approximated the Langmuir type isotherm as the temperature was lowered. No break in the adsorption isobar was observed for either adsorbent. The adsorption isotherms are shown in Figs. 1and 2. The (7) R. N. Pease, ibid., 45, 1196 (1923). (8) J. Bischoff and H. Adkins, ibid., 47,823 (1925). (9) R . P. Clarke and R. N. Pease, ;bid., 73, 2132 (1951). (10) J . K. Brag&and L. V. McCerty, ibid., 73, 2134 (1951).
ADSORPTION OF DIBORANE ON BORON NITRIDE AND PALLADIUMQN-CHARCOAL 103
Jan., 1956
equal to 10 kcal./mole. The ratio of the heat of adsorption to the heat of liquefaction for this adsorbent is (10 X 108)/(3.8 X lo3) s 2.6." This value is in good agreement, with the value of 2.5 obtained for other gases in this boiling range with various absorbents.6
18
g
I
14 a-12 $10 86-
4.2
4.0
$ 3.8
4-
3
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+---+-+-+-t-t
3.6
Fig. 1.-Adsorption isotherm BzHe on BN: 0,T = 178°K.; 0 , 210; A, 251; v, 273. 296; 314.
x,
+,
3.2 2.6 3.0
3.4 3.8 4.2 4.6 5.0 5.4 5.8 1/T , .x. 10-8. Fig. 3.-Isosteres B2Hson Pd on charcoal: 0,X / M = 50; 0 , 80; A, 100; v, 120; X - 140; 170.
220 200
+,
180 160
4.2
--. S 140 hi
4.0
5 120
3.8
%
-
100
E
80
3.6 3.4
60 40 40 50 60 70 80 P, cm. Fig. 2.-Adsorption isotherms diborane on Pd on charcoal: 0,T = 203°K.; 0 , 246; A, 273; v, 297.
30
3.0 3.2
3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 1/T x 10-8. Fig. $.-Adsorption isostere8 of B ~ H on I BN: 0,X / M = 2; 0 , 4; A , 6 ; V, 8; X, 10.
diborane on charcoal isoster showed a good fit to a straight line when In P was plotted versus 1/T. The expected decrease of Qiso with increased adsorption The data indicate that the surface area of the was observed. The calculated isosteric heats of charcoal catalyst is much larger than that of the adsorption corresponding to various values of X/m boron nitride. Although the values of X / m are are given in Table I. large for the palladium on charcoal no leveling off of the heat of adsorption is observed. TABLE I ISOSTERICHEATS OF ADSORPTIONCALCULATED FROM Conclusions GRAPH3 From the complete reversibility of adsorption the Adsorbent palladium on charcoal low isosteric heat of adsorption, and the continual X / m (CC./P.) 50 80 100 120 140 170 decrease of adsorption with increasing temperature Q (kca1.)-net energy it is concluded that physical adsorption is taking of adsorption 0 . 6 3 0 . 4 7 0.41 0 . 3 7 0 . 3 1 0.27 place. The expected decrease of the isosteric heat The diborane on boron nitride isoster showed a of adsorption with increasing values of X / m was obgood fit to a straight line over a wide range of pres- served for the palladium on charcoal adsorbent. sure. In this case &is,, was independent of the No evidence of decomposition was found. value of X l m , which varied from 2 to 10, and was (11) This value BI was ealoulated with the Trouton approximation.
