J. P. HOBSON AND R. A. ARMSTRONG
2000
Vol. 67
A STUDY OF PHYSICAL ADSORPTION AT VERY LOW PRESSURES USING ULTRAHIGH VACUUM TECHXIQUES BY J. P. HOBSON AXD R. A. ARMSTRONG Radio and Electrical Engineering Division, National Research Cou.zcil, Ottawa, Canada Received March Q. 1965 Ultrahigh vacuum techniques have been used in the measurement of the adsorption isotherms of nitrogen and argon on Pyrex glass in the range of relative pressures between lO-’3 and 10-8 and in the range of relative coverage between and 0.3. Temperatures between 63.3 and 902°K. were used. The Dubinin-Radushkevich isotherm equation was found empirically to describe all the data. This equation was also found t o describe the isotherm data of others a t low relative coverage on a variety of heterogeneous adsorbents. The energy distribution function giving rise to the Dubinin-Radushkevich isotherm is discussed. A simple analytical hypothesis is suggested for the extrapolation of isotherms on heterogeneous surfaces t o the region of Henry’s law.
I. Introduction Ultrahigh vacuum techniques have extended the range of pressures which can now be achieved and directly measured to about 10-14 mm.’ This development has made possible measurements of certain classical phenomena a t pressures many orders of magnitude below those previously investigated. This paper describes (sections I1 and 111) the application of ultrahigh vacuum techniques to the measurement of the physical adsorption isotherms of nitrogen and argon on Pyrex (Corning 7740) over ranges of temperature, pressure, and surface coverage of 63.3 T 90.2’K., 5 X P mix, and 0 6 0.3, respectively. conventional nieasurements on the adsorption of these vapors overlap only the upper ends of these ranges of P and 0. A characteristic of the extension of ultrahigh vacuum technology to quantitative measurements at very low pressures has been the demonstration of many complex reactions in the gages used.2 For this reason the experimental procedures described have been designed to check possible errorsan emphasis which would not be necessary with mellestablished techniques. Preliminary reports of this work have been published3 and the present paper summarizes the results to date. The lams governing physical adsorption a t very lorn pressures have become of practical importance, since they represent a means of achieving very lorn pressures. The conclusions of the ultrahigh vacuum results are examined in section IV in relation to conventional measurements. Since Henry’s lam was not found in the ultrahigh vacuum results it is concluded that the limiting law for the adsorption of vapors of nitrogen and argon has not been reached in the ranges of temperature, pressure, and coverage quoted above. The nature of the transition to Henry’s law is discussed not only because this transition is of theoretical interest but also because it is of practical importance in the design of cryogenic pumps. Hansen5 has recently published isotherm data for argon, krypton, and xenon on zirconium, comparable with the results given here.
<
€
67.I
1o1O-
O K
72.6-K 77.4'K
IO9CY
01
a
.
;; I O S
1
I
,
I
do
I
I
IO-^
IO+
id3
PRESSURE ABOVE ADSORBED LAYER (mrn Hg).
Fig. 3b.-Adsorption isotherms of Ar on Pyrex. Points are experimental. Lines obtained from Dubinin-Radushkevich isotherm equation (eq. 5 ) , with urn = 7 X 1Ol8, B = 7.2 X lo-'.
(2)
Here p o is the vapor pressure of the liquid adsorbate a t the temperature of the measurement. We have used for nitrogen p o for argon po
63.3 OK
6
(4)
are full of adsorbent in a liquid-like state and that areas (17) D. M. Young a n d A. D. Crowell, "Physical Adsorption of Gases," Butterworth and Co., London, 1962. (18) S. Brunauer, "Physical Adsorption of Gases and Vapors," Princeton University Press. Princeton. N. J., 1945.
$1
\
I-
"\
0 2 CL
W
0 10'
10"
IO'O
cr
10'~
d3
( MOLECULES ADSORBED
Fig. 4.-Isosteric
PER
10'~
10'~
C M 2 1.
heats of Na and Ar on Pyrex.
for which this relation is not satisfied make an unimportant contribution to the adsorption. The distribution of values of E on the surface will therefore determine the form of the characteristic curve and this form might be expected to vary with different adsorbate-adsorbent combinations, particularly for heterogeneous surfaces. However, we found empirically that the form of the
PHYSICAL ADSORPTION A T VERYLow PRESSURES
Oct., 1963
63.3 " K 67. I O K 72.6 'K T I 77.4 'K T
0
d
-loi4
81.0°K 84.9 " K T = 90.2"K
v T = 0 T = X
t
:
0 T = A T =
I -10'; ~
X
01
&?a
w
a n
-IOi2
E
0 (D
n
8"
I8
NZ
a :IO'
A A
X
I J
0.
3 V W
B Ao
s -I
-IOi0
0 0
-------IO8 4000
2003
lo-' and 7.20 X lo-' for nitrogen and argon, respectively. Equation 5 is an isotherm equation that was proposed by Dubinin and R a d u s h k e ~ i c hand ~ ~ has been applied extensively to adsorption on activated charcoal by DubininZ0and co-workers. KaganerZ1discusses this equation and writes
C = In urn (6) where (in the units of eq. 5 ) urnis the number of niolecules/cm.2 in a complete monolayer, and B is a constant characteristic of the adsorbent-adsorbate combination. Kaganer compares the values of urnwith surface areas obtained by the standard B.E.T. method and finds close agreement between the two methods. Kaganer, however, does not find eq. 5 valid for 8 = u/um 5 0.2, whereas we find it to give a good representation of the ultra high vacuum data down to 8 = lo+. All the adsorption data on Pyrex glass which we have taken over the past few years, which has included a measurement of the adsorption of helium at 4.2°K.,22have been consistent with eq. 5 and the values of B and urn obtained from these data are collected in Table I.
-I_-O
6000
lo-&. The picture of physigeneral physical basis for eq. 7. h possibility, which cal adsorption occurring as islands of condensation also has not been examined to our knowledge, is that eq. 7 agrees with the early consideratmioilsof Goldmaiin and arises from a distribution of cluster sizes on the surface. P ~ l a i i y i . ~However, ~ even if a certain distribution To be successful such a theory must describe the isofunction of energies is empirically useful it will finally be therm results and also explain how adsorption on necessary to justify it on a physical basis. heterogeneous and homogeneous adsorbents differ. (36) W. -4. Ptcele and G. D. IIalsey, J r . , J . Phys. Chem.. 59, 57 (1955). (37) S. Ross and J. 1'. Olivier, i b i d . , 65, 608 (1961). (c) Extrapolation of the Isotherms to the Henry's (38) J. H. de Boer, "The Dynamical Character of Adsorption," Clarendon Law Range.-As noted in the presentation of Fig. 3a Press, Oxford, 1953. 139) B. Goldmann and .\I. Polanyi, 2. physilc. Chem., A132, 321 (1928). (40) L. T. Radushkevioh, Zh. Fiz. Kham , 23, 1410 (1949). 1
>
>
PHYSICAL ADSORPTION AT VERYLow PRESSURES
Oct., 1963
and 3b, Henry's law was not found experimentally. On the other hand, it is difficult to fault Hill's4' derivation that it must be found a t sufficiently low pressures and coverages, particularly since this law is indeed found a t sufficiently high temperatures in Halsey's method. If, however, the Dubinin-Radushkevich isotherm (eq. 7) is extrapolated to indefinitely small values of p / p ~then the isotherm on a log-log plot first exceeds 45" and then approaches 90". The latter appears a most unlikely physical limit, and in the absence of any experimental data or theoretical considerations in the appropriate range of the variables, we arbitrarily suggest a simple method of extrapolation to Henry's law. We suggest that the Dubinin-Radushkevich isotherm be continued until I t reaches 45" on a log-log plot and thereafter be continued as a straight line a t 4 5 O , namely, Henry's law. This procedure is illustrated in Fig. 9a and 9b which have been constructed for nitrogen and argon, respectively, using only eq. 7 and values of B = 3.6 X lo-' for nitrogen and B = 7.2 X lo-' for argon. The dashed curve for argon a t 90°K. is the simple extension of the Dubinin-Radushkevich isotherm. The transition to Henry's law occurs along the broken line which has the simple analytic form
In this hypothesis the transition to Henry's law is therefore a simple function of the variables (e, P , 7'). Plotted also in Fig. 9a and 9b are a sampling of Borne of the experimental results a t the appropriate temperatures, quoted in section IV(a). Included in addition, are two isotherm points obtained from Steele and Haldata for nitrogen and argon on porous glass using a value of PO based on eq. 3 disregarding once again the fact that 500°K. is far above the critical temperature for both these gases. In view of the range of the variables involved, the approximate agreemenb of these high temperature points with the simple model postulated is interesting. It is emphasized that the postulated transition to Henry's law has only mathematical simplicity to recommend it at present. The experimental measurement of this region of the adsorption isotherm of vapors presents a formidable challenge even to modern ultrahigh vacuum techniques.
V. Conclusion It is found experimentally that the adsorption of vapors of nitrogen, argon, and helium in the range of relative pressures to 10-6 are consistent with a simple theory of surface condensation. It is found that this conclusion is valid for several sets of other data, covering a variety of adsorbents. The detailed mechanisms giving rise to the theory remain obscure. Acknowledgments.-During this work we have received the assistance of many. J. Earnshaw, v . Chuiig, and R. Verreault took much of the data. A. W. Pye and J. A. Marier were primarily responsible for construction of the apparatus. We have had valuable T. L. Hill, Advan. Catalysis, 4, 211 (1952). (42) W. A . Stsele and G. D. Halsey, Jr., J . Chem. Phys., 22, 979 (1954).
(41)
2007
discussions with E. A. Flood and J. A. Morrison, as we11 as with our coIIeagues P. A. Redhead, E. V. Kornelsen, and A. Szabo. DISCUSSION D. GRAHAM (E. I. du Pont de Semours and Company).Patchwise adsorption can be expected in systems involving a strongly interacting adsorbate with a surface containing widely spaced sites of such strength that the strength of the adjacent sites (as augmented by induction and lateral interaction) materially exceeds that of the majority sites. An example is found in the adsorption of water on Graphon. The "strong" sites in this system may involve chemically bound oxygen atoms.
B. BERQSNOV-HAKSEN (Stanford Research Institute).This paper is of the greatest interest for thoee of us who are interested in the mechanisms of gas-surface interaction under ultrahigh vacuum. However, I would like to point out that the results of Hobson and Armstrong do not necessarily apply to the sorption of nitrogen by other surfaces. I n the department of surface physics and chemistry a t Stanford Research Institute, which is headed by Dr. Pasternak, we have done similar expriments. Our results have some bearing on the paper presented here. A molybdenum film, prepared by electron beam evaporation a t a pressure 5 x 10-10 torr, was exposed to nitrogen a t room temperature. The rate of chemisorption and total amount of sorbed gas was measured. On cooling to liquid nitrogen temperature, additional sorption took place. No such sorption was observed on cooling to - 130". On warming the film, the sorbed gas was released. Our measurements took place over a pressure range from 5 x 10-9 t o 1 x 10-3 torr. The amount of reversible sorbed gas was found to be independent of pressure. The stickprobability was 0.4 and found independent of coverage almost to saturation. We think that a well diffused (probably molecular) sorption layer is formed on the molybdenum film which was already covered by a monolayer of chemisorbed nitrogen. The presence of such a layer has been found by Ehrlich for tungsten filament and by Roberts for molybdenum films. R. A. ARivsmoixG.-In your question you do not state the absolute amount of gas contained in the reversibly adsorbed layer, but from private conversation we understand this amount was close to a monolayer. Thus the central properties of the < P < 10-3 reversible layer are: (1)T = 77.4"K,e 1, 5 X mm.; (2) T = 143,2'K, 0 small, 5 X
