1626
YUNG-FANG Yu, J. J. CHESSICKAND A. C. ZETTLEMOYER
Vol. 63
ADSORPTION STUDIES ON METALS. VIII. MONOFUNCTIONAL ORGANIC MOLECULES ON REDUCED AND OXIDE-COATED NICKEL AND COPPER BY YUNG-FANG Yu, J. J. CHESSICK A N D A. C. ZETTLEMOYER Surface Chemistry Laboratory, Lehigh University, Bethlehem, Penna. Received March 9#1969
Monolayer coverage reversibilities and heats of adsorption of monofunctional organics were studied at 0 to 35” on reduced nickel and copper powders. More extensive and intensive chemisorption of 1-propanol, n- ropylamine and benzene occurred on the electron-deficient d-metal, nickel, than on the electron-rich s,p metal copper. Evidence for considerable surface reaction developed in the case of n-butyraldehyde on nickel. Exponentially decreasing heats of adsorption with increasing coverage were usually found for chemisorption. Only for the system propanol on nickel did the heats of adsorption remain constant up to essentially monolayer coverage; a dissociative adsorption mechanism is used to explain this phenomenon. The presence of biographical heterogeneities need not be considered to explain the heat curves. The relatively low heats of adsqrpt.ion for benzene on nickel are explained on the basis of energy absor tion needed to destroy the resonance of the aromatic ring. Comparisons were made with adsorption on thinly coated ox& films on the metals where the interactions were much less energetic and the packing of the adsorbate molecules less dense.
Tntroduction The nature of the adsorption of organic vapors by metal surfaces is of vital interest in catalysis, in lubrication and in corrosion inhibition. While some by-product information has been developed mostly from catalytic studies, very few direct studies of interactions of organic molecules possessing polar functional groups with bare metal surfaces are available. Chief among these was the development of an activity series for metal films by Trapnelll based on a semi-quantitative rating of their tendency t o chemisorb a number of gases including ethane and ethylene. In a later more detailed study, Trapnel12 ascribed the low activity of Fe, Co and Ni films for the chemisorption of methane and ethane to the ferromagnetic alignment of some of their unpaired electrons. Trapnella also reported that one ethylene molecule takes up four tungsten sites; this ratio can be explained if hydrogens split off from each of the carbons to occupy two additional tungsten atoms or if each of the unsaturated carbons interacts with two tungsten atoms. Whether ethylene adsorption is associative or dissociative in character on supported nickel has been resolved by Eischens and co-workers4 by means of infrared spectroscopy; on freshly reduced nickel films there is clear evidence for dissociative chemisorption which is eliminated when the surface is satisfied by the addition of small amounts of hydrogen. Contrary to the low activity of nickel films found by Trapnell, Selwood6 showed by changes in magnetic susceptibility that nickel supported on kieselguhr readily chemisorbs ethane. Furthermore, the greater effectiveness of ethane over ethylene in lowering magnetic susceptibility can be ascribed to the greater dissociative adsorption of the saturated molecule. In addition, Selwood showed that the initial rate of decrease of magnetic susceptibility with coverage for hydrogen, benzene and cyclohexane on supported nickel is in the ratio of 2:6:8. This relationship is to be expected if the hydrogen B. M. W. Trapnell, Proc. Rou. Soc. (London), A218, 566 (1953). B. M. W. Trapnell, Trans. Faraday Xoc., 62, 1619 (1956). B. M. W. Trapnell, ibid., 48, 160 (1952). R. P. Eischens and W. A. Pliskin, “Advances In Catalysia,” Vol. IX. Academic Press, Inc., New York, N. Y., 1957, p. 662. (5) P. W. Selwood, J . Am. Chum. Soo., 79, 4637 (1957). (1) (2) (3) (4)
splits t o become adsorbed as atoms, if no hydrogens split off the planar benzene ring and if one hydrogen splits off each of the four carbons in cyclohexane which contacts the surface in the “boat” form. When aromatic compounds chemisorb on metals, the ?r-electrons of the ring often add to the electrondeficient energy levels of the so-called d-metals to reduce the electrical resistance. Suhrmann? the chief proponent of this technique, showed the effectiveness of benzene and naphthalene adsorbed on nickel films. One of the few previous measurements of heats of adsorption of organic vapors onto metals is of interest here. Maxted and Josephs’ showed that the initial heat of adsorption of thiophene on platinum black is lower than its aliphatic counterpart ethyl sulfide; the difference is just the resonance energy of the thiophene which the system absorbs as the 7r-electrons add to the metal. Because of the paucity of data on the adsorption of organic molecules onto metals, it has not been possible to relate the electron donor tendency of various functional groups to the electronic configuration of metals. The present work was initiated to attempt to fill in some of this needed information by adsorption and calorimetric techniques. There is evidence* that the state of the metal, whether deposited film, .single crystal, supported crystallites or polycrystalline powder, considerably affects the surface states and hence the adsorption properties. The early work in this series was begun with metal powdersg and these substrates were used here with the realization that comparisons should be made with other forms. Nickel powder was chosen as a typical d-metal and copper as a typical s,p-metal. Experimental Materials.-The nickel and copper powders were prepared by thermal decomposition of C.P. nickel carbonate and basic copper carbonate, respectively. These powders were decomposed t o oxides at 400’ under reduced pressure and the (6) R. Suhrmann, “Advanaes in Catalysis,” Vol. VII, Academic p. 320; R. Suhrmann and K. Schule, J . CoEEoid Sci., Sup. 1, 55 (1954). (7) E. B. Maxted and M. Josephs, J . Chsm.Soc., 2635 (1956). (8) See for example J. H. deBoer, “Advances in Catalysis,” Vol. VIII, Academic Press, Inc., New York, N. Y., 1956, p. 109. (9) Y . F. Y u , J. J. Chessick and A. C. Zettlemoyer, “Advances in Catalysis,” IX,415 (1957) (no. V I of this series); A. C. Zettlemoyer, J. J. Chessick, Y . F. Y u and F. H. Healey, THIS JOURNAL, 61, 1319 (1957).
Press. Inc., New York, N. Y., 1955,
Oct., 1959
ADSORPTION STUDIESON METALS
outgassing was continued 12 hows beyond the point of no apparent gas evolution. The oxides were reduced with dry hydrogen at 400 and 300",res ectively; these temperatures used previously are t,he lowest feasible so that sintering could be minimized. After reduction, the metal powders were degassed at 10-5 mm. a t 450 and 350', respectively, just prior to the adsorption studies. The organic liquids used to provide the adsorption vapors were reagent grade. They were dried with anhydrous magnesium sulfate and then frozen and pumped through several cycles so that only the middle portion was taken for the adsorption measurements. Adsorption Apparatus.-The basic feature of the adsorption apparatus is the Teflon stopcocks. Besides the simplification these stopcocks made possible, they eliminated possible contamination of the metal samples with mercury from the cut-offs usually employed. These stopcocks would not hold a ood vacuum at the outset, but they steadily improved w%en operated frequently during the f i s t ten days of use. A liquid nitrogen trap was employed between the adsorption system and the pumps. The adsorption system includes a sample tube, glass break-seal, organic vapor reservoir and a calibrated doser, and it is terminated with a glass Bourdon s oon gage. The gage is balanced with helium pressure whicf is measured by an Apiezon B oil manometer; small deflections of the gage from zero are determined by a microscope eyepiece. The equilibrium pressures can be determined to f0.02 mm. and the range of the oil manometer is from 0-10 cm. A side arm is attached to the gage and immersed in silicone oil to damp the vibrations. The reduced samples were sealed off and transferred to the adsorption apparatus. The seal was broken by a magnetically operated plunger just prior to each run. For studies on oxide-coated surfaces, the reduced powders were first exposed to dry oxygen at 25" and 1 cm. for 10 minutes so that the oxide films resulting were from 10 to 15 A. thick. Calorimeter.-The basic calorimeter design was that used previously for heats of chemisorption of oxygen on these same metals.10 Modification includes a glass break-seal so the sample could be transferred from the apparatus on which it was reduced and .a twenty-five junction cop er constantan thermocouple with the reference junctions Reld within 0.1" of the adsorption temperature to minimize heat loss through the thermocouple wires. The e.m.f. is amplified then recorded on a Brown Recorder. The sensitivity of the calorimeter is 0.001' or about 0.01 cal.; the heat ca acity of the filled calorimeter ranged from 10 to 15 cal. per i g r e e . It was estimated for the calorimetric heats recorded here that over 90% of the heat evolution occurred within the first two minutes after the introduction of each portion of organic vapor.
Results A series of monofunctional compounds, both
1627
1
so
Fig. 1.-Heats of adsorption of n-propylamine on nickel: 0, reduced surface, run 1; A, reduced surface, run 2, degassed at 25", e = 0 at A; X, reduced surface, run 3, degassed at 25' and looo, 0 = 0 a t C; 0 , oxidized surface, 0 = 0 a t B.
20 '
i
5: 5
j I 4
IT;;
0
I
1
2
I
3
,i, , 4
5
6
AMOUNT ADSORDED IMOLEX1O~'l.
Fig. 2.-Heats of adsorption of n-propylamine on copper: 0, reduced surface, run 1; X, reduced surface, run 2, 0 = 0 at A ; 0 , oxidized surface, e = 0 at B.
donor and acceptor types, were chosen for adsorption studies on nickel. Table I shows that l-propanol, n-propyl bromide, n-propylamine, n-nitropropane, n-butyraldehyde and benzene were included. Four of the more interesting of these, namely, n-propyl alcohol, n-propylamine, n-nitropropane and benzene, were chosen for adsorption studies on copper. The isotherms as indicated in Table I were measured at temperatures ranging from 0 to 35" depending upon the vapor pressures of the adsorbates. The BET plots were linear except for butyraldehyde-on-nickel. This system gave indication of polymerization occurring on the surface; the apparent isotherm was almost linear I I I 1 I and a Vm could not be estimated from the BET plot. AMOUNT ADSORSEO I M M XlO*'), Table I lists the BET s2'1, for the other adsorb3.-Heats of adsorption of benzene on nickel: 0, ates on the freshly reduced powders as vm%. These Fig. reduced surface; 0 , oxidized surface, e = 0 at B. values are compared to the corresponding argon vm rather than assigning doubtful area values to each adsorbate molecule. The doubt arises particularly because the packing may be different in the portion (IO) J. J. Cbessiok, Y. F. Yu and A. C. Zettlemoyer, "Prooeedings of the Seoond International Congress on Surface Activity," Vol. 11, of the f i s t layer where chemisorption occurs and Aoademic Press, New York, N. Y.(no. VI1 of this aeries), p. 269. those portions physically adsorbed. I
,
I
I
I
1
1628
YUNG-FANG Yu, J. J. CHESSICK AND A. C. ZETTLEMOYER
Vol. 63
TABLE I ADSORPTION OF MONOFUNCTIONAL ORQANICS O N METALS Adsorbate
Temp.,
"C.
vmy urn
v#/ vmA
Rr, %
vmIII/ u&
v,IV/ UrnA
Nickel (1.2-1 . 3 m.Z/g.) 25 0.57 0.44 . . 0.45 0.29
y"! ?
0
I 1
I
2
I 3
I
I 1 I 4 1 6 AMOUNT ADSORBED IMOLEXIO" I
I 7
1
8
I
) ),
Fig. 4.-Heats of adsorption of benzene on copper: reduced surface; 0, oxidized surface.
I
0
i
Fig. 5.-Heats of adsorption of n-propyl alcohol on nickel: 0, reduced surface, run 1; X, reduced surface, run 2, e = 0 at A; 0 , oxidized surface.
P I 1
I
2
I! I t I 3 4 5 e AMOUNT ADSORBED W O L E X 10'1.
Fig. 6.-Heats of adsorption of n-propyl alcohol on copper: 0,reduced surface, run 1; X, ieduced surface, run 2; 0 , oxidized surface.
In each case, the residual vapor after completion of the isotherm was condensed in a liquid nitrogen trap. Only in the case of the propyl alcohol was
1-Propanol n-Propyl bromide 0 n-Propylamine 0 n-Nitropropane 35 *Butyraldehyde 25 Benzene 0
Ro, %
64
.75 .89
.58 .37
79 41
.42
.37 65 .44 105
.59
.30
50
.28
.25
89
..
..
..
.51
76
.86 .38
.68 .36
79
.68
.57
94
Copper (0.4-0.5 m.*/g.) 1-Propanol 25 0.31 0.29 94 0.31 0.25 80 n-Propylamine 0 .64 .51 81 .57 .41 72 n-Nitropropane 35 .37 .18 60 .23 .22 96 Benzene 0 -34 .34 100 .38 .38 100
any permanent gas detected, and this amount was quite small. Hence, dissociated fragments in the gas phase did not invalidate the pressure measurements. After each initial isotherm on the reduced surface was completed, the systems were degassed a t 25", employing a liquid nitrogen trap and high vacuum system, for about 16 hours to a vacuum of better than 10" mm. A good vacuum was usually attained after 4 hours. The isotherms were then repeated to give new BET v J h . The ratio 100 vmII/VmI is designated reversibility on the reduced surface, Rr; differences of a few per cent. between these values are no doubt meaningless. The reversible fraction of a monolayer is regarded to a first approximation as the physically adsorbed portion. Further detailed evidence emerges from the measurements of heats of adsorption. Similar repeat adsorption isotherms were measured on the oxidized metal surfaces. The respective Om values are listed in Table I as VmIII and vmIv. The ratio 100 Vmlv/vmIII is designated reversibility on the oxidized surface, Ro. On the oxidized surfaces it was expected that a complete monolayer would form mostly by physical adsorption; a comparison of these amounts adsorbed with the reversible portions (at 25") adsorbed on the reduced metals would then help to reveal differences in orientation and packing. The direct determinations of calorimetric heats of adsorption of the 1-propanol, n-propylamine and benzene on both reduced and oxidized nickel and copper powders are plotted against amount adsorbed in Figs. 1 to 6 . These measurements were made at 22" at which temperature the calorimeter functions best. Each set of runs is plotted so that the vm values are superimposed. For example, in Fig. 1, the zero point represents the start of the first run on the reduced surface; the BET Vm value from this first run is indicated at the appropriate amount adsorbed by a vertical dashed line. After degassing for 16 hours a t 25O, a second run gave a much smaller vm and the heat measurements made on this partially covered surface were plotted as though the
Oct., 1959
ADSORPTION STUDIES ON METALS
zero point for the new coverage was at (Urn' - Urn''), point A. Using this scheme, the heat values were found to coincide with the original values. If the amine-nickel system was degassed after this second run, first at 25" for 16 hours followed by one hour a t looo, a greater amount of adsorbate was removed. The isotherm on this surface revealed a much larger v,; subtracting this value from VmI gave point C for the zero point of the heat determinations on this more stringently outgassed surface. This heat of adsorption curve fell on a somewhat different curve t.han the first ones. Since the system was irreversibly altered by outgassing a t 100°, this procedure was not followed for the other systems. On the oxidized nickel surface, the measured vmI1I is represented by the difference between VJ and point B. These heats of adsorption fell somewhat lower than those on the reduced or outgassed surfaces. Discussion It is valuable to consider the heats of adsorption first. The initial values on the reduced metal surfaces are much higher for nickel than for copper; in accord with this finding, the reversibilities are less for nickel than for copper. The high initial heats for the alcohol and amine on nickel suggest electron transfer to the unfilled d-band. The electron-rich copper cannot participate in such transfer. The low initial heat for benzene on nickel a t first appears to be an anomaly since portions of the amine adsorbed with such low energies could be desorbed a t 25". I n accord with an earlier suggestion,' however, it is reasonable to conclude that transfer of the n-electrons of benzene only occurs a t the cost of the resonance energy. Thus, about 40 kcal./mole would be added to the heats of adsorption if this energy had not been absorbed. The possibility that some dissociative adsorption of benzene may also occur,ll cannot be discounted on the basis of the measurements made here. The complete reversibility and lower heats for benzene on copper, on the other hand, suggest no strong interactions of any kind. The initial constant high heat values for the alcohol on nickel versus the immediate gradual decrease for the amine on nickel deserves further examination. The unshared pair of nitrogen electrons in the amine no doubt add in coordinate covalent bond fashion to the d-band. Hence, dipoles with the positive end directed outward are developed. The addition of electrons to the metal gradually decreases the work function. Coupled with the dipole-dipole repulsion, these two factors cause the decrease in the heats of adsorption. If it were not for the discrete location of the dipoles and their mutual depolarization on near approach,12 the decrease with coverage would be even more rapid. On the other hand, the initial constant high heats for the alcohol on nickel up to coverages approaching one layer can only be explained if dipole-dipole repulsion and more significantly, the decrease in (11) J. R. Anderson and C . Kemball, "Advances in Catalyais,"lV, Academic Press, New York, N . Y.,1957, p. 51. (12) J. H. de Boer, in "Advances in Catalysis," VIII, Academia Press, New York, N. Y.,1956, p. 107 ff.
1629
the work functions are not large. Dissociative adsorption appears to be the likely factor; this interpretation is supported, as mentioned previously, by the evidence of uncondensed product on desorption. For this reason, Table I does not list the reversibility for the n-propyl alcohol although it was nominally 78%. I n addition, the heats from the second run after degassing a t 25" did not coincide in this case with those from the first run a t superimposed v, values. It appears likely that the propoxy radical adds its electron pair to the nickel as the proton from the hydroxyl group adsorbs ~eparately.'~ The tendency for decrease in the work function due to the electron donating capacity of the propoxy group is thus countered by the electron attracting capacity of the proton. The latter probably resides within the boundary of the top layer of metal atoms. Then mutual depolarization of the dipoles oreated would lessen repulsion as coverage increases. An approximate calculation of the heat of dissociative adsorption of the alcohol on nickel according to the method of Eley14 gave a value about 20 kcal./mole less than that measured. The heat curves for the amine and for benzene on the reduced metals for the second run after degassing a t 25" coincided well with those from the first run a t superimposed urn values. For these systems the adsorption and desorption steps do not alter the reversible portions of the surface. The low vapor pressure of nitropropane prevented valid determination of heats of adsorption, This adsorbate differs from the others in that it irreversibly adsorbs to the same extent on both nickel and copper. The electron donating capacity of the tautomeric form together with the electron accepting capacity of the nitro group may explain its dual activity on both the electron-poor nickel and the electron-rich copper. In contrast to the initial high heats obtained on the base metal surfaces, those for the oxidized surfaces are all much lower. There seems to be no doubt that here physical adsorption predominates. The generally lower reversibilities on oxide-coated nickel than on oxide-coated copper, and the higher heats of adsorption of the alcohol, may be ascribed to the larger concentration of 0- field-creating ions on the former.'O At first glance a t the ratios against argon, the good agreement between the values for the amounts physically adsorbed on the reduced surfaces, Vm'I/VmA, and those for the adsorption on the oxidized surfaces, VmIII/vmA, is striking. On further reflection, this agreement seems to be more fortuitous than meaningful. Apparent molecular areas of organic molecules on polar surfaces have often been reported to be larger than on graphitic or metal surfaces.16 The oriented chemisorption on the reduced surfaces aids in the development of the larger values for the VmI/vmA ratios. Acknowledgment.-The authors wish to express their appreciation for the financial support provided by the Office of Ordnance Research. (13) M. Inghram and R. Gomer, 2. Nuturforachun~,loa, 876 (1955). (14) D. D. Eley, Disc. Faraday Soc., 8, 105 (1950). (15) J. J. Van Voorhis, R. G. Craig and F. E. Bartell, T ~ r aJOURNAL, 61, 1513 (1957); see also ref. 11, p. 82.
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, TRIEJOURNAL, 58,288 (1954); 8. Ross and W. Winkler, J . Colloid Sci., 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
