1630
J. L. SHERESHEFSKY AND BIBHUTI R. MAZUMDER
DISCUSSION F. M. F o w ~ ~(Shell , s Development Company).-The
dir&, heats of interaction of these metals with the amine and alcohol are very much greater than expected for electron donation to ions of these 15 kcal.,mole). Doesn't indicate that bonds in the organic molecules are broken and new species are formed? A. c. ZETTLEMOYER.-h the case of the alcohol, we do in-
Vol. 63
deed believe that the alcohol molecule breaks up, and to a large extent the hydrogen from the hydroxy1 group and the ProPoxY radica1 are adsorbed* The amine, On the Other hand, is very adsorbed through a coordinate covalent or dative bond created by the originally unshared nitrogen electrons. I n solution, water molecules must be separat.ed from the metal ions as such bonds are formed; the cost of this water displacement apparently accounts for the lower exothermic heats in such cases.
THE ADSORPTION OF SOME GASES ON EVAPORATED METAL FILMS AND ON OXIDIZED FILMS OF NICKEL1 BYJ. L. SHERESHEFSKY AND BIBHUTI R. MAZUMDER Contribution from the Chemistry Department, Howard University. Washington, D.c. Received May 1 1 , 1969
Low pressure adsorption isotherms a t 76.8 and 90.2"K. are reported for krypton, methane and ethane on copper and oriented nickel evaporated films, and on an oriented oxidized film of nickel. The krypton and methane isotherms on nickel oxide and the krypton isotherms on nickel follow the Langmuir equation. The methane isotherm at 76.8'K. on nickel is s-shaped, the low pressure region following the Langmuir equation. The ethane isotherms on copper, nickel and nickel oxide are all s-shaped. The ethane isotherm on nickel oxide shows a firsborder phase change. The nature of the adsorbed phase, the variation of the calculated heat of adsorption is discussed from the point of view of the van der Waals twodimensional equation, the entropy of adsorption and the effect of the polarizing field of the adsorbent on the van der Waals attraction. Proof is given for the orientation of ethane in the adsorbed phase, and a method for the preparation of oriented films of nickel oxide is described.
Introduction Two-dimensional condensations in monolayers spread on liquid substrates has been known and understoodtfor a long time. The similar phenomenon in gases adsorbed on solid surfaces has only recently attracted attention and is the subject of a number of investigations. It was first observed and recognized in this Laboratory in a study on the adsorption of nitrogen2 and oxygenSon glass spheres; it was confirmed and some of its properties studed by Ross and cow o r k e r ~ . ~These studies showed that the critical temperature and pressure of the transition, and the shape of the transition curve depend not only on the nature of the adsorbate but also on the adsorbent and the purity of the surface., Condensation was observed to take place on surfaces of sublimed sodium and potassium chloride, graphitized carbon and mixed surfaces of glass and minera,ls. To the present, two-dimensional condensation has not been observed on metal surfaces. The technique developed in recent years of producing evaporated metal films, the high purity of such surfaces and the homogeneous surface of the oriented (1) Presented at the Oolloid Symposium, University of Minnesota, June 18-20, 1959. This paper is based on a thesis submitted by Bibhuti R. Mazumder, in partial fulfillment of the requirements for the degree of Doator of Philosophy, to the Graduate Sahool of Howard University, June, 1958. The experimental observations are reported in full in the original Thesis, copies of which may be obtained from University Microfilms, Ann Arbor, Michigan. (2) J. L. Shereshefsky and C. E. Weir, J . Am. Cfiem. SOC.,58,2022 (1936). 60, 1162 (3) J. L. Shereshefsky and C. E. Weir, THIBJOURNAL, (1956). (4) 8. Ross, J . Am. Cfiem. SOC., 70,3830 (1948); 9.Rose and C. H. Seaoy, THIS JOURNAL, 68, 308 (1949); 8. Rosa, ibid., 68, 383 (1949); 8. Ross and W. Winkler, J . Am. Chem. SOC.,76, 2637 (1954); S. Ross and H. Clark, ibid., 76, 4291. 4297 (1854); 8. Ross and C. Sanford, 58,288 (1954); 8. Ross and W. Winkler, J . Colloid Sci., TRIEJOURNAL, 10,319, 330 (1955).
metal film induced us t o undertake the present study. In the course of the investigation it was found that also oriented films of nickel oxide can be produced, and the investigation was extended to include adsorption on this type of surface. Experimental The apparatus in which the determinations of the adsorption isotherms were made was of the conventional volumetric type, except that mercury cut-offs were used instead of stopcocks, and that it included a multi-range McLeod gauge. A double gas trap in the shape of an inverted u, between the adsorption vessel and the rest of the system, cooled by liquid air, kept mercury vapor out of the adsprption vessel. The latter, after being cleaned with chromic acid and distilled water, was baked at 400" in high vacuum for five hours, so that a vacuum of about 0.001 p could be maintained for several hours. The vessel consisted of a Pyrex glass tube 6 cm. long and 2 em. in diameter. It was connected to the apparatus through a narrower tube carrying two tungsten electrodes, one mm. in diameter, to which the metal filaments were attached. The films were deposited on the inner surface of the tube by subliming part of the filament. The temperature of the films was maintained with liquid nitr-gen or oxygen, kept in a liter Dewar flask constant to 0.1 The temperature was measured with a Leeds and Northrup latinum resistance thermometer. b. The nictel films were repared from nickel filaments 0.5 mm. in diameter. The f&ment was heated to a temperature of llOOo, by passing a current of 6.8 amp. The sublimation was carried out in an argon atmos here of 0.85 mm. of pressure, and the temperature of the fiyament was measured with a micro-optical pyrometer made by the Pyrometer Instrument Company. The temperature of the adsorption vessel during sublimation was kept at 23'. The weight of the film was determined by the difference in the weight of the filament before and after deposition. The films were on the average 55 mg. in weight and 1.6p in thickness. The films resulting from this manner of deposition were found to be oriented with the 110 plane in the surface, as determined a t the National Bureau of Standards by X-ray and electron diffraction methods. C The nickel oxide film was prepared from a nickel film deposited in the above manner, by oxidation in air a t 275' for one hour. The air was purified by passingAit over sodium
.
Oct., 1959
ADSORPTION OF
GASESIN EVAPORATED METALFILMS
hydroxide and through a trap cooled in liquid air. Electron diffraction studies showed the top layers of the film to be oriented with the 111 plane parallel to the substrate, and the deeper lying layers the original unoxidiaed nickel. The copper films were prepared from electrolytic copper which was electro-deposited on a molybdenum filament. The filament was heated in a high vacuum to 830°, by ass ing a current of 4.6 amp. During the deposition o f the metal, the walls of the vessel were kept at the ice-water temperature. The films thus prepared were unoriented, weighed about 10 mg. and were 0.29 in thickness, calculated from bulk density. The materials used in the determination of the isotherms of krypton, methane and ethane on films of copper, nickel and nickel oxide were of the following purity. Argon and krypton obtained from Matheson Co., Inc., were 99.9% pure; the methane and ethane gases obtained from the same source were, respectively, 99 and 95% pure. The latter were further purified by passing through traps cooled in liquid air, or in a Dry Ice and ether mixture. The nickel which was supplied by the courtesy of the Washington, D. C., office of the International Nickel Co. was nickel A, 99.4570 p q e . The filament was further purified by degassing at 880' in a high vacuum for five hours. The copper was obtained from J. T. Baker Chemical Co., as electrolytic copper foil. It was free of iron, antimony, tin or lead.
Experimental Results The results obtained on the adsorption isotherms are reported on the basis of 100 mg. of film. Krypton Isotherms.-The adsorption isotherms of krypton a t 76.8 and 90.2" K. on an evaporated nickel film, oriented with the 110 plane parallel to the substrate, extend from about 0.005 to 100 p a t 763°K. and from 0.05 to 500 p a t 90.2"K. The lower pressure regions up to 0.06 p for the former and 0.6 p for the latter are shown in Fig. 1, curves A and B ; the higher pressure regions up to 10 or 100 p are shown in Fig. 2. The application of the Langmuir equation to these isotherms is shown in Fig. 3. Adsorption a t pressures higher than those shown in Fig. 2 follows the straight lines of the Langmuir plots. Each isotherm forms two straight lines, intersecting a t about 40 and 45% coverage, as based on the monolayer constant, V,, determined by the B E T method. The constants determined by the Langmuir and B E T methods are given in Table 11. The isotherms of krypton on the 111 plane of nickel oxide films extend from 0.002 to 200 p a t 76.8"K., and from 0.05 to 100 p a t 90.2"K. The low pressure regions up to 0.65 p are shown in Fig. 1, curves C and D; the higher pressure regions up to 90 p are shown in Fig. 4. The Langmuir and B E T monolayer constants are given in columns 2 and 3 of Table 11. Methane Isotherms.-The methane isotherms on the 110 plane of evaporated nickel films extend from 0.002 to 1800 p a t 76.8"K. and from 0.012 t o 750 p a t 90.1"K. The isotherms up to a monolayer coverage are shown in Fig. 5, and those a t higher pressures in Fig. 6. The application of the B E T equation, including the parameter N , to the 76.8"K. isotherm, in accordance with the procedure of Joyner, Weinberger and Montgomery: is shown in Fig. 7. It is seen that one and one-half layers were built up within a relative pressure of two-tenth. The Langmuir and B E T monolayer constants are given in Table 11. The low pres( 5 ) L. G.Joyner, E. E. Weinberger and C. W. Montgomery, J . Am. Cham. Soo., 67, 2182 (1945).
1631
0
P X lo6, cm. 2 4
6
7
0
2
6
7
4 P X 106, cm.
Fig. 1.-Adsorption of krypton on the 110 plane of nickel and the 111 plane of nickel oxide: A, nickel at 76.8"K: B, nickel at 90.2"K.; C, nickel oxide a t 76.8"K.; D, nickei oxide a t 90.2"K.
P X los, cm. 0
2.5
5.0
7.5
9.0
0.4
2.5 5.0 7.5 9.0 P X lo4, cm. Fig. 2.-Adsorption of kryptm on the 110 plane of nickel: A, 76.8"K.; I),90.2"K.; 0 , adsorption; A, desorption. 0
sure regions of these isotherms are shown in Fig. 8, curves A and B. The methane isotherms on the 111 plane of nickel oxide extend from 0.05 to 100 p a t 763°K. and from 1.2 to 680 p at 90.2"K. The low pressure regions of these isotherms are shown in Fig. 8, curves C and D, where the maximum pressures are 0.6 and 5.7 p , respectively. I n Fig. 9 are shown the isotherms up to a pressure of 120 p . Ethane Isotherms.-The adsorption of ethane was measured only at one temperature, at 90.2"K., on films of copper, nickel and nickel oxide. The copper film was unoriented, and the adsorption on it extends from 0.1 t o 8.7 p . The adsorption on oriented nickel film extends from 0.016 to 6.2 1.1.
J. L.SHERESHEFSKY AND BIBHUTI R. MAZUMDER
1632
Vol. 63
TABLE I ENTROPY OF ADSORPTIONOF KRYPTON, METHANE A N D ETHANE O N ORIENTED FILMS OF NICKELAND NICKELOXIDE A T THE STANDARD STATE OF 8 a 0.5 OK.
Krypton on nickel
P
76.8 90.2 76.8 90.2 76.8 90.1 76.8 90.2 90.2 90.2 90.2
Krypton on nickel oxide Methane on nickel
- AF,
x
-AH,
kcal. mole-]
lo-', om.
3.24 18.4 2.06 5.01 0.543 12.16 1.70 3.80 0.30 0.0767 0.977
1.89 1.91 1.96 2.14 2.17 2.39 1.99 2.19 2.64 2.89 2.43
aSt ,
AS,
kcal. mole -1
aSL,
0.u.
e.u.
3.57
-20.0
32.9
3.53
-17.7
1.64
+ 7.7
Methane on nickel - 2.2 2.27 oxide Ethane on copper 4.45. -20.0 Ethane on nickel 4.60" -19.0 Ethane on nickel oxide 4.455 -22.4 Calculated from BET equation, taking the heat of vaporization as 3860 cal./mole.
0.U.
17.0 17.0
27.9
13.8 13.8
30.2
15.8 14.3 15.8
TABLE I1 SOMEPHYSICAL PROPERTIES OF ADSORBED KRYPTON, METHANE AND ETHANE ON EVAPORATED FILMS OF COPPER,NICKEL AND NICKELOXIDE 4
3 OK.
Krypton on nickel
76.8 90.2 76.8 90.2 76.8
Krypton on nickel oxide Methane on nickel
V m (BET),
VdW,
2
1
eo.
CC.
0.35;0.52 0.18;0.34 0.080
0.034 0.50;0.60 0.342 0.0795 0.0495
0.52 .34 .078 ,034 .497 .348 .0795 .050 .050 . .56 .0765
90.1 Methane on nickel 76.8 oxide 90.2 Ethane on copper 90.2 Ethane on nickel 90.0 Ethane on nickel 90.2 oxide At e = 0.25. b Oriented with long diameter normal to surface.
P X 103,cin.
-w,
e = 0.1
1.24 1.25 1.32 1.41 1.39 1.40 1.02" 0.72 1.13 1.27 1.33 1.57b
5
Debye--
G
e = 0.5
.--pl/2u, (e = 0.1)
0.73 .70 .69 .71 .71 .85 .84 .72 1.13 1.27 0.74 1.2gb
4.43 4.46 4.93 5.7 5.36 5.45 2.81Q 1.43 2.04 2.58 2.83 4.47b
kcsl. mole-
(e
= 0.5)
1.61 1.40 1.32 1.43 1.40 2.03 1.98 1.43 2.04 2.58 0.845 3.02b
7 -T&= 0.5)
(e
32 38 39 36 38 7 9 35 42 16 105 104b
Both isotherms are s-shaped and are shown in Fig. 10. The low pressure regions are shown in Fig. 8, curves E and F. The BET V , values are given in Table 11. On nickel, the monolayer is complete at a relative pressure of 0.1, and on copper at a relative pressure of 0.17. Figure 11 shows the adsorption of ethane on the 111 plane of nickel oxide. The isotherm extends from 0.08 to 6.7 p. In the first run, a t low pressures, adsorption increases almost linearly with pressure; the isotherm then turns first convex and then concave to the pressure axis, and on further adsorption the pressure falls to a lower value, as shown by the broken line and the numbered sequence of open circle points. After this pressure drop, adsorption continues to increase linearly with pressure. It is of interest to note that the point preceding the pressure drop, the opeii circle point 2 remained constant for about 12 hours, and t,he drop in pressure to point 3 occurred only after the addition of more gas. I n the second run, the points designated with triangles follow the first run in the linear, low pressure region of the isotherm. At about 1.1 p, on further addition of gas made in small increments, 0 2.5 5.0 7.5 9.0 the adsorption continues to rise from 0.05 cc. to P X lo4,cm. about 0.078 cc. a t a constant pressure of 1.15 p. Fig. 3.-The application of Langmuir's equation to the krypton isotherms on the 110 plane of nickel: A, 76.8"K.; Point 5, which follows the constant pressure interval, seems to fall on the higher pressure linear part B, 90.2'K. 0
2.5
5.0
7.5
9.0
ADSORPTION OF GASESIN EVAPORATED METALFILMS
Oct., 1959
1633
(PIP,) x 103. 59
0
118
177
210
0.9
8 7.5 0
s
0.8
2 PI i3
5 5.0
$
6
0
4
N-
4
2
0
s
2
0.7
&
2
=:
x 2.5
s
P
6
2 0.6
50
+-
-4 X
0
P
2.5 5 7.5 9.0 P x 103, cm. Fig. 4.-Adsorption of krypton on the 111 plane of nickel oxide: A, 76.8"K.; B, 90.2"K.; 0 , adsorption; A,desorption. 0
25
0.5
0 10 15 18 P X lo2, cm. Fig. 6.-Adsorption of methane on the 110 plane of nickel: A, 7624°K.: B, 90.2"1
