Continuous Flow Measurement of Desorption Isotherms S. Karp, S. Lowell,’ and A. Mustacciuolo C. W . Post College, Long Island Unisersily, Greenvale, N . Y. 11548 NON-STATICGAS adsorption methods have been used for several years (1-4). Perhaps the most popular of these is the continuous flow method of Nelson and Eggertson (3). This method uses a continuous flow of a n inert, non-adsorbable, carrier gas mixed with an adsorbate gas. When a cell containing the adsorbent is immersed in a coolant, some of the adsorbate, in the flowing mixture, is adsorbed until the surface comes into equilibrium with the constant composition gas stream. When the coolant is removed and the sample warms, desorption occurs into the flowing stream. Thermal conductivity detectors are used to measure the quantity of adsorbate taken up by the sample, and subsequently desorbed when the sample is allowed to warm. The desorption signal is calibrated by introducing a known volume of pure adsorbate. This procedure is repeated with different adsorbate relative pressures until all the data points on the adsorption isotherm are acquired. As pointed out by several authors (5, 6 ) , the Nelson and Eggertson method does not permit points on the desorption isotherm to be obtained. This is true because each data point reflects the amount adsorbed by a surface initially free of adsorbate. The desorption isotherm, however, must consist of data points indicating the amount desorbed from a surface which was previously saturated with adsorbate, and subsequently equilibrated with adsorbate of a specific relative pressure, but not exposed to relative pressures below that specific pressure. We have found that a n ambient pressure continuous flow method can be used to determine desorption isotherms, as well as scans of the hysteresis loop.
To scan the hysteresis loop from the adsorption to the desorption isotherm, the sample, immersed in the coolant, is equilibrated with a relative pressure gas mixture corresponding to the start of the scan on the adsorption isotherm. When adsorption is complete as indicated by a constant detector signal, the adsorbate concentration is reduced to a value corresponding to a relative pressure between the adsorption and desorption isotherms. When equilibrium with the flow is reestablished, as indicated by a constant detector reading, the coolant is removed and the desorption signal is monitored. Calibration, using pure adsorbate, yields a data point between the adsorption and desorption isotherms. Repetition of this procedure, each time using a slightly different relative pressure between the adsorption and desorption isotherms, yields a hysteresis loop scan from the adsorption to the desorption side. To scan the hysteresis loop from the desorption to the adsorption branch, pure adsorbate is first adsorbed, and then the adsorbate concentration is reduced to a value giving a relative pressure corresponding to the start of the scan o n the desorption isotherm. When equilibrium is reestablished, the adsorbate concentration is increased to give a relative pressure between the desorption and adsorption isotherms. After the detector gives a constant signal indicating that equilibrium is reestablished, the coolant is removed and the desorption signal is monitored. This signal, when calibrated, gives a data point between the desorption and adsorption isotherms. Repeating this procedure, each time using a different relative pressure between the two isotherms, will yield a hysteresis loop scan from the desorption to the adsorption branch.
GENERAL PROCEDURE
EXPERIMENTAL
The desorption isotherm is determined by first exposing the sample, while immersed in the coolant, to a flow of pure adsorbate until saturated. The flow is then changed to a n adsorbate concentration corresponding to a relative pressure below saturation. Some desorption occurs until the surface comes t o equilibrium as indicated by a constant thermal conductivity detector signal. The coolant is then removed and the desorption signal is monitored. This signal is calibrated by introducing a known volume of pure adsorbate and represents a data point on the desorption isotherm. The above procedure is repeated, each time using a different relative pressure, but always starting with a surface saturated first with pure adsorbate.
The apparatus used was a Quantasorb continuous flow adsorption system manufactured by Quantachrome Corporation, 337 Glen Cove Road, Greenvale, N.Y. 11548. An alumina sample was used, which was found to be amorphous by observing the absence of any X-ray diffraction peaks. A zinc oxide sample was supplied by the New Jersey Zinc Company. Nitrogen and helium gas mixtures were J. T. Baker Company zero grade. The concentration accuracy of the lesser component was guaranteed by the supplier to be within 1 % relative. A liquid nitrogen cold trap was employed to remove any hydrocarbons and water. Saturated vapor pressure measurements of the nitrogen adsorbate were made using the gauge provided for that purpose on the Quantasorb instrument. This gauge indicates the equilibrium vapor pressure of liquid nitrogen formed in the sample cell while immersed in liquid nitrogen coolant. The saturated nitrogen vapor pressure, Po,was measured after each data point was obtained. The liquid nitrogen Dewar flask contained 0.4 liter and was replaced after acquiring each data point. Calibration volumes of pure nitrogen were introduced into the flow stream through a septum using Hamilton precision gas sampling syringes with Chaney adapters. Signal areas were measured with the digital integrator built into the Quantasorb. Powder samples were outgassed, prior to analysis, for 1 h r at 150 “C under a helium purge. This was adequate to bring the samples to constant weight and to give reproducible data.
For reprint requests. ~
( 1 ) R. G. Davirs, Clrem. I i d . (Loirdoii), 160 (1952). (2) R. Stock, Ph.D. Thesis, London University, England, 1955. (3) F. M. Nelson and F. T. Eggertson, ANAL.CHEM.,30, 1387 (1958). (4) W. V. Loebenstein and V. R. Deitz, J . Res. Ncrt. Bur. Sraitd., 46, 51 (1951). ( 5 ) D. L. Kantro, S. Brunauer, and L. E. Copeland, “The Gas Solid Interface,” Vol. I, E. A. Flood. Ed., Marcel Dekker, New York, N.Y. 1967, Chap. 12. (6) L. S. Ettre, “Sorptometer Instruction Manual,” Perkin-Elmer Corporation, Norwalk, Conn., p 21.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2395
Figure 1. Adsorption and desorption isotherms of Nz for 0.106-g sample of alumina Adsorption, 0 ; desorption,
f.
V,(, in cm3
0
V
I 0.2
I
I
I
0.4
0.6
0.8
I
Figure 2. Hysteresis loop scan for same sample as Figure 1 Desorption to adsorption, desorption, 0
+;
0
6
Figure 3. Adsorption and desorption isotherms of N2for 0.358-g sample of zinc oxide Adsorption, 0 ; desorption,
+.
o/@
0
2396
/e
4-
Vstpin cm3
I
I
0.2
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
I
0.4
I 0.6
1
0.8
adsorption to
300
Figure 4. Cumulative pore volume for alumina sample calculated by method of Pierce ( 8 )using desorption isotherm of Figure 1
vPxlo3
I
2 00
P
V, ds pore volume in cm3, and 7, is pore radius
I
in A
I
100
I
l”
rp
6
d 0
I 50
-
I
I
100
I50
P ‘
Data were obtained in triplicate and were reproducible to within 1 %. RESULTS AND DISCUSSION Figure 1 shows, for a n amorphous alumina sample, the adsorption and desorption isotherms obtained by the method described above using nitrogen as the adsorbate and helium as the carrier gas. Figure 2 shows the scan of the hysteresis loop in both directions. The zinc oxide sample exhibited no adsorption hysteresis under the same experimental conditions used t o construct the alumina isotherms, as seen in Figure 3. The specific surface area of the amorphous alumina and the zinc oxide measured by the continuous flow method, and calculated by the B.E.T. (7) equation, was 206 mZ/g and 8.0 m2/g, respectively. The zinc oxide specific area was measured volumetrically by the manufacturer as 8.0 mz/g. The high specific area of the alumina sample is consistent with extensive pore structure and, therefore, adsorption hysteresis, while the lower area zinc oxide sample indicates none or very little porosity and does not show adsorption hysteresis. Figure 4 is a plot of the alumina cumulative pore volume cs. pore radius calculated from the desorption isotherm (Figure
1) by the method of Pierce (8). The specific area of the alumina sample calculated from the pore size distribution was 213 m2/gingood agreement with the B.E.T. area. It should be pointed out that quite small sample weights are required compared to static adsorption methods. Thus, the time required to establish equilibrium after changing gas mixture was less than 10 minutes in all cases and usually about 3 to 5 minutes. Small sample sizes also hasten the time for outgassing prior to analysis. Another distinct advantage of the method described here is the fact that no void volume measurements and corrections are necessary. Because most calculations of pore volumes use the desorption isotherm, the authors believe that the experimental methods described here can be useful for rapid and efficient pore volume measurements. ACKNOWLEDGMENT The authors wish to thank Spero Manolatos of the C . W. Post College Department of Physics for the X-ray analysis of the alumina sample. RECEIVED for review May 4,1972.
(7) S. Brunauer, P. H. Emmett, and E. Teller, J . Amer. Cliern. SOC., 60, 309 (1938).
Accepted August 9,1972.
(8) C . Pierce, J. Phys. Chem., 57, 149, (1953).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2397
