R. A. BEEBE,R. L. GALE,AND T. C. W. KLEINSTEUBER
4010
Calorimetric Heats of Adsorption of Nitrogen, Carbon Monoxide, and Argon on Graphon at -70"
by R. A. Beebe, R. L. Gale, and T. C. W. Kleinsteuber Department of Chemistry, Amherst College, Amherst, Massachusetts
01008
(Received July 86, 1966)
The heats of adsorption of N2, CO, and Ar on Graphon have been measured calorimetrically in the low coverage region and over the temperature range from -62 to -81". The calorimeter described by Gale, Haber, and Stone has been adapted and somewhat modified to meet the conditions of t,he present research. The observed heats are compared with recently reported isosteric heats which were chromatographically determined a t similar coverages and temperatures. The heat of adsorption was found to be independent of temperature in the range from -196 to -60", and no change in the heats with coverage was observed for these three gases on Graphon.
Introduction A number of recent papers have been addressed to the possibility of obtaining data on heats of adsorption by means of gas-solid chromatography.' I n this laboratory, studies have been made on gas-solid systems consisting of the elementary gases Nz, 0 2 , and Ar, as well as certain compound gases such as CO and C2Fs as adsorbates, on graphitized carbon blacks, and on bone mineral as adsorbent column materials. In this work both the pulse method2 and the continuous flow (frontal analysis) method3 of chromatography were employed. I n both it was possible to obtain values for the heats of adsorption at low coverages in the temperature range from -80' to room temperature by application of the ClapeyronClausius equation. The results of the two methods agree when the isotherms at low coverage follow Henry's law. When the isotherms were nonlinear, however, it was found that the heat values obtained by the pulse method deviated from the presumably correct values obtained by the continuous flow method in the direction predicted previously. Having obtained these differential heat values by means of chromatography, it is of interest to compare them with the calorimetrically measured values. There were no prior calorimetric data in the literature for the range of temperature and of coverage studied in the chromatographic work. Data were available from our earlier calorimetric work at a much lower The Journal of Physical Chemistry
temperature (-196') and at considerably higher c ~ v e r a g e . ~In ' ~ order to compare these data with the chromatographically determined values, it is necessary to extrapolate them to higher temperatures and lower coverages. These twice extrapolated values are open to the criticism that they involve the assumption that the heats vary only very little with either temperature or coverage. To fill in the logical gap, we have now undertaken to measure the calorimetric heats under conditions of temperature and coverage which duplicate as nearly as possible those of the chromatographic work. It was decided to develop a calorimeter to use in the temperature range from -80 to -50' (where most of our gas-solid chromatography had been done) with a sensitivity high enough to permit us to use it down to the low coverages involved in the chromatographic measurements. For this purpose we have modified the calorimeter described by Gale, Haber, and Stone,6and we have made a thorough study of the (1) An extensive bibliography is given by H. W. Habgood in "The Solid-Gas Interface," E. A. Flood, Ed., Marcel Dekker, New York, N. Y., to be published. See also references given in ref 2 and 3 of this paper. (2) R. L. Gale and R. A. Beebe, J . Phys. Chem., 68, 555 (1964). (3) R. A. Beebe, P. L. Evans, T. C. W. Kleinsteuber, and L. W. Richards, ibid., 70, 1009 (1966). (4) R. A. Beebe, J. Biscoe, W. R. Smith, and C. B. Wendell, J . A m .
Chem. Soc., 69, 95 (1947). (5) R. A. Beebe, B. Millard, and J. Cynarski, ibid., 75, 839 (1953). (6) R. L. Gale, J. Haber, and F. S. Stone, J . Catalvsis, 1 , 32 (1962).
CALORIMETRIC HEATSOF ADSORPTION OF Nz, CO, AND Ar
A
B
C.
D E
F
G H
.e' 4
I
i/ Figure 1. Cross section of the calorimeter (drawn to scale): A, connecting tube for evacuating the vacuum jacket; B, glass filler to reduce the dead space; C, Kovar glass seal; D, lead wires to thermometer and heater coil; E, glass-platinum seal; F, adsorbate; G, platinum walls; H, heater coil; I, thermometer coil.
behavior of this calorimeter, paying particular attention to the determination of the heat capacity. The calorimeter was then applied to the measurement of the heats of adsorption of Ar, CO, and N2 on Graphon.
Experimental Section Apparatus and Procedure. The modified calorimeter is shown in Figure 1. In order to improve the heat distribution, the two concentric tubes G were made out of platinum 0.15 mm thick instead of glass, and they are joined at their common base by vacuum-tight silver soldering. The adsorbent was placed in the annular space between the platinum tubes. Two identical coils of 0.0508-mm diameter nickel wire of high temperature coefficient were interwound noninductively upon the outer platinum tube. In order to secure the wires and at the same time insulate them electrically from the platinum, a thin layer of Dow Corning 935 silicone varnish was painted on the plati-
4011
num tube before winding the wires onto it. After curing for approximately 30 min at 250°, the wires were found to be well secured and insulated and the varnish film withstood repeated heating and cooling between -80 and 250'. The coils had a resistance of 168 ohms at Oo. One coil was connected to a Muller bridge and served as a resistance thermometer which was Calibrated by comparison with a platinum resistance thermometer standard. The other coil was used as a heater in the determination of the heat capacity of the calorimeter. The whole assembly was immersed in a 15-1. dewar flask filled with methanol which served as a cryostat. Its temperature was controlled by the device described by Smith? and by Graham.? The temperature change of the calorimeter was observed by means of a sensitive Leeds & Northrup galvanometer which recorded the out-of-balance current of the Muller bridge. The response of the calorimeter was extremely rapid and hence a slight drift of the bath temperature could be tolerated. The maximum temperature reading was generally reached in less than 20 sec, and it was therefore found sufficient to record the time-temperature curve for 400 sec after the generation of heat. Figure 2 shows a typical response curve from an adsorption measurement. We determined the temperature rise AT, due to the heat of adsorption, graphically from the time-temperature curve. The product of AT times the heat capacity is then the adiabatic heat of adsorption. We measured heatsvarying from 0.03 to 0.3 cal. An excessively high rate of admission of the gas to the calorimeter was avoided by sending the initial major part of any gas increment through a by-pass tube of very small (0.1 mm) diameter. Materials. Because of its high specific surface area (89 m2/g) in comparison with the other graphitized carbon blacks,2 we have chosen to use Graphon in the present calorimetric work. This material has been used as one of the adsorbents in our gas chromatographic columns and is described in an earlier publicat i ~ n . A~ 6.8-g sample of the Graphon was used in the calorimeter. All gases used were of prepurified grade supplied by the Matheson Co. Determination of the Heat Capacity. The calorimeter used in the present research was designed to be calibrated by the equilibrium method as described by Wahba and Kemball,s Klemperer and Stone,g and Gale, Haber, and Stone.B Unfortunately, this method ~
~~
(7) F. W. Smith, Ph.D. Thesis, St. Andrews University, Great Britain, 1952; D. Graham, J . Phys. Chem., 66, 1815 (1962). (8) M.Wahba and C. Kemball, Trans. Faraday Soc., 49,1351 (1953). (9) D. F. Klemperer and F. S. Stone, Proc. Roy. SOC.(London), A243, 375 (1957).
Volume 70, Number 13 December 1966
R. A. BEEBE,R. L. GALE,AND T. C. W. KLEINSTEUBER
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r - $4
I
L
I
100
1
200
I
300
400
500
600
I
700
I
800
Time, eec. Figure 2. Typical response curve of an adsorption experiment. NI on Graphon; heat observed, 0.089 cal.
24
0
especially at -67O, we showed that the observed value for the heat capacity of the calorimeter was independent of the size of the energy pulse inserted over the range from 0.008 to 8 cal. The observed heat capacity was likewise independent of the rate of energy input over the range 0.0057 to 0.086 cal/sec. Figure 3 gives a typical response curve of a calibration measurement. The excessively high readings between 550 and 600 sec are due to an emf induced in the thermometer coil in switching off the heater current. We extrapolated the cooling curve to the time at which the two shaded areas are equal, as is common practice in calorimetry.l0Vl1 A rough estimate was made of the amount of heat radiated directly from the heater coil to the vacuum jacket during calibration. The estimated heat loss was less than 1% of the total heat input and could therefore be neglected. The data for the heat capacity measurements are given in Table I. Table I: Heat Capacities of Calorimeter Filled with Graphon Temp, O C
-55 -67 -75 -80 100
200
300
400 500 Time, sec.
600
700
16
50 14 18
Mean value in oal/deg
3.51 i 1 . 2 % 3.52 f 0.9% 3.49 f 1.2oj, 3.50 f 1 . 2 %
800
Figure 3. Typical response curve of a calibration experiment. Temperature, -67”; heat input, 0.95 cal.
requires a steadier base line than we have been able to achieve in our cryostat in the low-temperature range from -80 to -50’. It therefore seemed to us preferable to inject measured quantities of energy in the form of electrical pulses of short duration, thus simulating more closely the actual adsorption experiments (see Figures 2 and 3). We measured the potential drop across the heater coil with a precision voltmeter, the time of heat input with an accurate stopwatch, and the electrical resistance value by means of the Muller bridge. The electrical energy input was determined to =k0.6%. It is of course desirable to distribute the heat quickly and uniformly throughout the whole mass of the calorimeter. I n order to assist the heat distribution during the calibration, a low pressure of helium (1-5 mm) was included in the annular space containing the adsorbent. By an extensive series of calibration experiments, The Journal of Physical Chemistry
No. of measurements
Correction for the Heat of Compression. It is well known that the heat of adsorption observed in an adiabatic calorimeter contains a term which is due to the compression of the gas phase in the dead space of the calorimeter. This effect was first observed and considered by Ward.12 The detailed discussion of the thermodynamics of gas adsorption by HiIll3accounts for this term implicitly; it has since been discussed in greater detail by Kington and Aston14 and by Young and Cr0wel1.l~ The heat of compression is of im(10) M.Braun and R. Kohlhaas, Z. Angew. Phys., 14, 91 (1962). (11) I n order t o check our method of determining the heat capacity, we plotted log (Tc - Tb) 8s. time, where Tc is the temperature of the calorimeter and Tb is the temperature of the bath. If the heat dis-
tribution through the calorimeter is sufficiently rapid, this plot should give a straight line. A linear plot was indeed obtained and this justifies our practice of limiting the observation of the cooling curve to 400 6ec (see Figures 2 and 3). (12) A. F. H.Ward, Proc. Roy. Soc. (London), A133, 506 (1931). (13)T.L. Hill, J . Chem. Phys., 17, 520 (1949). (14) G. L. Kington and J. G. Aston, J . Am. Chem. SOC.,73, 1929 (1951). (15) D. M. Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworth and Co. Ltd., London, 1962,p 74.
CALORIMETRIC HEATSOF ADSORPTION OF Nz, CO, AND Ar
4013
Table 11: Heats of Adsorption, qst, of Nz, CO, and Ar on Graphon (2)
(3)
System
Coverage in Yo of monolayer, this paper
No. of measuremente, this paper
NrGraphon CO-Graphon Ar-Graphon
0.4-1.0 0.5-2.6 1.0-1.2
16 8 4
(1)
(5)
(6)
Temp range, this paper
q a t by calorimetry, this paper
by calorimetry, - 196'
-81 to -65" -81 to -52" -81 to -65"
2 . 9 f0 . 1 2 . 9 f0 . 1 2 . 7 f0 . 2
(4)
gat
3.0'
(7) by chromatography, pulse
(8)
qat
q e t by chromatography, frontal
3 . 0 f 0.28
2.72
...
. ..
...
2.66
2.P
2.9
0.23
portance if calorimetric and isosteric heats of adsorption are to be compared with each other. The following relationship is given by Young and Crowell
where qcal is the differential heat calorimetrically measured under adiabatic conditions, qst is the heat derived from isosteres by the Clapeyron-Clausius relationship, V , is the volume of the gas phase in the calorimeter vessel, and (bp,/bns)ad is the change of pressure due to the adsorption of bn, moles if the total entropy is kept constant. We are dealing with a reversible adsorption process and measuring under adiabatic conditions. Furthermore, the ideal gas law is valid in our systems. Therefore, the correction for the heat of compression for a small increment is made as follows Ansqcai
-
V,(pt - pi) =
Ansqst
(2)
where AnBis the amount adsorbed, and pi and p f are, respectively, the pressures before and after the addition of an increment. This is the same correction term as Ward12 gave. Frequently, the heat of compression, V,(pr - pi), is very small compared to the heat of adsorption, Ansqcal. In our measurements, however, this term amounted to a significant correction, approximately 10 to 20%. Therefore, we tested this relation by admitting helium into the calorimeter. Since helium is presumably not adsorbed a t the temperatures used, we are measuring the heat of compression only. The results are shown in Figure 4. As predicted, the observed heat values are directly proportional to the pressure changes. I n a recent publication, Smith and Ford16have reported an experimentally observed linear relationship between pressure change and the heats of compression as well as the heats of expansion. Using the dimensions of the platinum vessel of our calorimeter and the weight and density of the Graphon in the vessel, we have estimated the volume of the gas phase V,. The calculated values for
2
4
6
Pressure change in calorimeter ( p t
8
- pi), cm.
Figure 4. Observed heat of compression us. pressure change in calorimeter a t -67".
V,(pt - pi) and the experimentally observed heats of compression are in satisfactory agreement. All the heats of adsorption reported in this paper have been corrected for the heat of compression and can therefore be compared to the isosteric heats of adsorption as obtained by gas chromatography.
Results and Discussion The results of our present calorimetric work with N2, CO, and Ar on Graphon are presented in column 5 of Table 11. As indicated above, these data are corrected for heat of compression. The conditions of the experiments in the present research are given in columns 1-4. All but five measurements referred to in column 3 were made on the admission of measured small quantities to the previously evacuated calorimeter. Four of the measurements with Nz-Graphon and one with CO-Graphon were taken for second increments added without pumping out the gas of the initial increment. Although there was some scattering of the data, this effect was to a great extent offset by conducting a substantial number of determinations. As a result, the average deviation of the mean value was no more than 3% for the Nz-Graphon and COGraphon systems, and 6% for Ar-Graphon. (16) W. R.Smith and D. G. Ford, J. Phys. Chem., 69, 3587 (1965).
Volume 70. Number 18 December 1966
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For comparison with the data of the present work, we list in column 8 earlier calorimetric measurements at -198°.435 It was estimated that at -196' the correction for the heat of compression was negligible. Thus all the values of columns 5-8 are comparable in that they represent isosteric heats. Joyner and Emmett" have measured the isotherms for the system Nz-Graphon over the temperature range from -183 to -205'. The isosteric heats they derived are in excellent agreement with the calorimetrically determined values4 at -196'. I n columns 7 and 8 the data are given for the heats of adsorption obtained by chromatography by the pulse method2 and by the frontal analysis m e t h ~ d respectively. ,~ All the data of Table I1 are drawn from work done in this laboratory. It is difficult to put an exact error value on the data of columns 6 and 7; however, we estimate the error here to be roughly *0.2 kcal/mole. From the data of Table 11, within the limits of the inherent experimental errors, we may draw the following conclusions for the adsorption of NZ, CO, and Ar on the Graphon adsorbent. (1) There is no evidence for any temperature dependence of the heats of adsorption in any of the three systems studied in the (2) There is no evidence region from -50 to -198'.
The Journal of Physical Chemistry
R. A. BEEBE,R. L. GALE,AND T. C. W. KLEINSTEUBER
for any change in the heats of adsorption with coverage in the range up to 1.0 to 2.0% of a monolayer. The above conclusions are perhaps not unduly surprising when we remember that Graphon presents an essentially nonpolarizing ~ u r f a c e . As ~ ~ ~a result, even with molecules of a polarizable gas like CO, which also have a small permanent dipole, the forces involved in adsorption are essentially van der Waals in nature and there is probably small change in their nature over the ranges of temperature and coverage in question. For polarizing surfaces such as dry bone minera12p3 and TiOz,16there is definite evidence for a decrease in the heats of adsorption of N2 and CO with increasing coverage in the region of low coverage. At the present time we know of no definitive data to test the temperature dependence of the heat of adsorption on these strongly polarizing surfaces. I t is felt, that it will be very worthwhile to obtain such data.
Acknowledgments. Our grat,itude is due to the National Institutes of Health and the i'r'ational Science Foundation for financial support of this work. (17) L. G. Joyner and P. H. Emmett, J. Am. Chem. Soc., 7 0 , 2356 (1948).
