MEASUREMENT OF T H E RATE O F ADSORPTION AT CONSTANT PRESSURE PAUL V. McKINNEY Department of Chemistry, Rutgers Universitq, New Brunswick, New Jersey Received October 84, 1938
In the simplified formula for the process of adsorption as suggested by Langmuir, the rate of the process is dependent upon the pressure of the gas being adsorbed, For the simple case of a single gas the rate of condensation may be given by (1)
and the rate of evaporation by dx -dt
=
k2B
Because of the importance of the measurement of the rate of adsorption in the determination of the activation energy (2) of the adsorption process, it was of interest to determine actually the significance of this pressure factor in the measurement. Most recent workers have used apparatus similar to that described by Pease (3). A quantity of gas is admitted and the velocity of adsorption is determined by the rate of pressure change. In one case reported the pressure actually decreased from 624 mm. to 235 mm. in 71 minutes. In the measurement of adsorption isotherms for which the process was devised, this change in pressure is of no consequence, since the process continues to equilibrium. APPARATUS
The apparatus was constructed of Pyrex glass throughout and is illustrated in figure 1. Tube A containing the adsorbent was connected through a special stopcock with the gas burette E and manometer G. The traps B were cooled with solid carbon dioxide and ether. Capillary tubing was used except in the pump system. The manometer was arranged to act as a constant volume instrument and tungsten wire contacts were connected with a small electric light to serve as the indicator. The leveling bulb for the burette was supported by a wire over a pulley to a set of reducing gears. Thus the level of the mercury in the burette could be 381
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PAUL V. MCKINNEY
changedrapidly and with precision. In order to measure the rate of adsorption at constant pressure the manometer was adjusted to the reading for the pressure desired and the gas burette opened to the catalyst. The
E
n
J
t
V
FIG. 1. APPARATUSFOR THE MEASUREMENT O F THF. RATEO F ADSORPTION A, Adsorbent tube with boiler; B, traps cooled with carbon dioxide and ether; C? nlcLeod gage; D, mercury-vapor, Sprengle and oil pumps; E, gas burette; F, gas reservoirs and purification; G, constant volume manometer with electric contact; H, leveling bulbs.
volume change of the process, at constant pressure, could be read directly from the burette each minute, By closing the burette after admission of the gas to be adsorbed this apparatus was also used to determine the adsorption a t constant volume.
383
RATE O F ADSORPTION AT CONSTANT PRESSURE VARIATION O F RATE WITH PRESSURE
A mixed oxide catalyst, MnO-Crz03, was selected t o illustrate the effect of pressure on the rate of adsorption. For comparison the material was TABLE 1 Adsorption of hydrogen on a mixed oxide catalyst 24 grams of MnO-Cr20s at 132°C. ADBORPTION NO. 3 VOLU>lE CONSTANT TIME
Pressure mm.
2
3 4 5 6 7 8 9 10 11 12 13 14 15 17 20 25 30 40 50 60 75 90 105 120
480 467 451 439 430 420 413 406 400 394 388 383 379 374 366 355.5 339.5 326 305 287.5 273 255 240 227.5 216.5
Volume adsorbed cc.
6.5 7.2 8.1 8.8 9.3 9.9 10.25 10.7 11.o 11.35 11.7 12.0 12.2 12.5 12.9 13.8 14.4 15.2 16.4 17.4 18.2 19.4 20.05 20.89 21.4
ADSORPTION
N0.4. PRE5 SURE 480 M M .
I
ADSORPTION N O , 5 VOLUME CONBTANT
TINE
Volume adsorbed
Pressure
cc.
mm.
6.5 7.6 8.5 9.2 9.8 10.4 10.9 11.4 11.8 12.2 12.5 12.8 13.2 13.5 14.0 14.8 15.9 16.9 18.5 19.9 21.0 22.5 23.4 24.5 25.6
76 77 78 79 80 82 85 90 95 100
110 120
480 460 447.5 435 425 416 408 402 395 389 384 378.5 373 369 361 350 334 321 299.5 282 267.5 250.5 480
1
Volume adsorbed CC.
6.4 7.7 8.5 9.1 9.7 10.2 10.6 10.9 11.3 11.7 12.0 12.3 12.7 12.9 13.3 13.9 14.8 15.5 16.8 17.8 18.6 19.5 20.2 20.35 20.5 20.7 20.9 21.1 21.5 22.0 22.5 23.0 23.8 24.6
prepared as described by Taylor and Williamson (4). The data presented in table 1and figure 2 was obtained on 24 grams of the mixed oxides. Be-
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PAUL V. MCKINNEY
fore the measurement of adsorption the sample was reduced a t 350-370OC. for three days and evacuated twenty-four hours at 450470OC. by means of mercury-vapor and oil pumps. After the determination of the inert space of the system with purified nitrogen, the material was again evacuated twenty-four hours a t 450470OC. A similar evacuation of twenty hours followed each adsorption measurement. Adsorption No. 3 was carried out a t constant volume by the method previously described to obtain the rate of adsorption. With all other conditions identical except that the pressure was maintained constant, curve 4 was obtained. It is quite evident that as the formula predicts, the ad25
't
5
3
20
.E:
d. 15
f
9
f
3
IO
6 Time in minstes
FIQ.2. RATEOF ADSORPTION Curve 3 at constant volume; curve 4 at constant pressure
sorption a t constant pressure continues at a higher rate than in those cases in which the pressure decreases. At the end of two hours in this case the difference is an increased adsorption of 4.2 cc. by the method advocated, amounting to 19 per cent of the total amount adsorbed by the former method in this length of time. A fifth adsorption was made in which the process was a t constant volume during the first 75 minutes after which the pressure was increased immediately to 480 mm. and the adsorption measurements continued a t that pressure. In this adsorption, curve 5, it was surprising to find the high rate a t which after 75 minutes the adsorption increased toward the value previously obtained. After two additional adsorptions were made on the sample a t 132°C. a t constant pressure, which
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RATE O F ADSORPTION AT CONSTANT PRESSURE
in themselves agreed with number 4, similar results were obtained in adsorptions at 100°C. by the two methods. STABILITP O F THE ADSORBENT SURFACE
Adsorbent surfaces are known to be highly complex. However the author has contended that by rigorously following a definite method of preparation and cleaning of the surface, reproducible results in a given sample should be obtainable. Two possible methods of determining whether a surface has been completely degassed are the use of the McLeod gage or the repetition of the evacuation and adsorption. The complete cleansing may of course result not in a surface of maximum activity but merely in one of constant activity. The McLeod gage was used previously TABLE 2 ADBORPTION
TIME
minutes
10 30 60
ADSORPTION
TAYLOR A N D WILLIAMSON
cc. per gram
0.39 0.45 0.53
1ST ADSORPTION
2
cubic centimetera
3.9 11.9 14.6 17.8
2 30 60 120
SAMPLE
1
cubic centimeters
minutes
5.9 14.5 18.3 22.6
3 R D ADSORPTION
5TH ADSORPTION
.--____cc. per gram
0.34 0.50 0.61
cc. per gram
0.46 0.63 0.76
cc. per gram
0.47 0.65 0.77
( 5 ) in the study of palladium to check the completeness of the degassing process. In the present study the amount of hydrogen adsorbed was less in the first two trials, table 2, than in the subsequent experiments. The method and time of each step of the process was identical throughout. Among the possible factors are the following. The extra evacuation after each adsorption may have finally removed all interfering materials, or the added period of heating during evacuation may have produced activating changes in the surface. The hydrogen adsorbed may have an activating effect upon the surface, cleansing it of other gases, or actually completing t,he reduction of the oxide. Burrage (6) has recently discussed the necessity of flushing the surface. Whatever the process, it is evident that in the present case the surface reaches a maximum activity and remains quite constant. This final surface has a larger capacity than that of Taylor and Williamson (see table 3), although the samples agree with each other more
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PAUL V. McKINNEY
closely than the first and last adsorption on the present sample. It is perhaps superfluous to remark that this increased adsorption is produced by the same method as is used in producing the most active catalytic surface, i.e., by repeated reduction. ACTIVATION ENERGY
Although the purpose of this determination did not involve the calculation of activation energies, they may be obtained from the data at the two temperatures. Since the rates at each temperature are different depending upon the method used, it is not surprising to find some variation in the value of E resulting. This activation energy of adsorption varies with the fraction of the surface covered, the first gas adsorbed requiring less activation than the final amount. The data as presented in table 4 have been calculated so that they may be compared with the data of Taylor and Williamson. The present sample weighed 0.51 as much as that of the TABLE 4 Activation energy in adsorption Hydrogen on Mn0-Cr20s at 100-132°C. V O L U l l ADSORBED
E
IN CALORIES PER MOLE ADSORBED
Pressure cc.
7.8-10,3 10.3-12.9 12.9-15.5 15.5-18.1
4 ,580 6,700 9,800 12,600
I I
Volume
-
10,300 14,000 15,400
former authors and therefore the volume interval 15-20 cc. corresponds to that between 7.8-10.3 cc. for an equal fraction of the surface covered in the present sample, etc. CONCLUSION
The Langmuir theory of the adsorption process requires that the velocity of condensation be dependent upon the pressure. This has been proven to be experimentally true in the case of hydrogen adsorbed upon the mixed oxides of manganese and chromium. As a result, in the determination of the activation energy of the adsorption process from rate measurements it is necessary that the method used shall consider the effect of pressure. A method is here suggested for this measurement at constant pressure, previous work having been done at constant volume. Repeated adsorptions upon the same surface were found to increase in rates to a maximum on the third adsorption. After this amount of use the surface remained constant during five additional adsorptions. The conclusion is that the
RATE O F ADSORPTION AT CONSTANT PRESSURE
387
surface should first be shown to be stable before it may be used to obtain data a t various temperatures for calculation and comparison. This apparatus and method is also ideally suited for the direct determination of the adsorption isobar. REFERENCES (1) See TAYLOR, H. S.: Treatise on Physical Chemistry, Volume 11, p. 1072. D. Van Nostrand Go., New York (1931). (2) TAYLOR: J. Am. Chem. SOC. 63, 578 (1931). (3) PEASE:J. Am. Chem. SOC.46, 1196 (1923). (4) TAYLOR AND WILLIAMSON: J. Am. Chem. SOC.63,2168 (1931). (5) TAYLOR AND MCKINNEY: J. Am. Chem. SOC.63, note 30, p. 3613 (1931). Trans. Faraday SOC.28,192 (1932). The Adsorption of Gases. (6) BURRAQE:
