Oct., 1963
REACTION BETWEEN OXYGENAND EVAPORATED FII.MSOF SODIUM
of 2.86 8.is probably not significantly shorter than this value but the two equivalent out-of-plane separations of 3.58 8.are significantly larger. I n the case of the cubic form of BaC12, the barium ion coordination sphere is eightfold with each Ba-C1 distance equivalent and equal to 3.17 8. in length. Although the average Ba-C1 distance in the orthorhombic form is somewhat larger than that in the cubic modification, the coordination number is higher and the structure is more closely packed. This fact is evident by comparing the molecular volume of 98.22 A.3/molecule in the cubic form to 87.64 8.3/molecule in the orthorhombic form. The environment around the chloride ions would require an average of four and one-half nearest barium ions to satisfy the stoichiometry of the compound. There are two crystallographically different chloride ions. That which is labeled in Table I11 as XI is surrounded by four barium ions in a distorted tetrahedral configuration a,t distances comparable to the combined ionic radii. The chloride ion labeled Xz is surrounded by five near ba,rium ions, two of which are a t distances significantly aonger than the sum of the ionic radii, namely 3.58 A. Both types of chloride ions have seven nearest chloride ion neighbors a t distances of less than 4.00 8. The distances observed range from 3.53 to 3.87 8.as presented in Table IV. These values are comparable to the accepted value of the sum of the ionic radii, namely 3.62 A. The packing in both BaBrz and Barz is quite similar
2135
to that of BaCL The Ba-X and X-X distances observed in these salts are also presented in Table IV. TABLE IV IXTERATOMIC DISTANCES IN BaClz, BaBrz, AXD Barl No. of equiv. Interaction distances
BaC12,
8.
BaBra.
BaIz, 8.
8.
B a-X 1 Insameplane 1 1 2 Outofplane 2 2
2.86f0.08 3 . 1 5 f .08 3 . 1 8 f .08 3 . 1 7 f .08 3 , 2 5 2 ~ .08 3 . 5 8 & .OS
3.21f0.04 3 . 3 2 f .04 3 . 2 6 f .04 3 . 1 9 f .04 3 . 3 8 & .04 3 . 7 3 f .04
3.38f0.04 3 . 6 3 f .04 3 . 5 5 % .04 3 . 5 5 f .04 3 . 5 8 5 .04 4 . 1 0 f .04
Xl-XI utofplane
2
3 . 7 3 & .10
3.71f
.04
3.96f
.04
2
3.87f
4.05f
.04
4.38f
.04
2 2 1
3 . 6 4 5 .10 3.53-1: .10 3 . 7 8 & .10
3 . 8 9 f .04 3 . 7 8 f .04 3 . 9 4 & .04
4.16f 4.08f 4.29f
.04 .04 .04
x2-x2
Outofplane
.10
xrx2 Outofplane In plane
Acknowledgments.-This work was supported by a grant from the National Aeronautics and Space Administration. The Rice Computer is supported by grant No. AT-(40-1)-1825 from the Atomic Energy Commission.
THE REACTION BETWEEN OXYGEN AND EVAPORATED FILNIS OF SODIUM BY J. R. ANDERSON AND N. J. CLARK Chemistry Department, University of Melbourne, Parkville N.2, Victoria, Australia Received January 7 , 1968 The kinetics of the reaction between oxygen and evaporated films of sodium have been studied a t 90, 195, and 273°K. in the pressure range 10-3 t o 10-1mm. Provided an adequate standard of purity was achieved for the reactants and provided the oxygen pressure above the film wm sufficiently high, a protective oxide layer was produced and a t 90 and 195°K. the results obeyed a logarithmic inverse rate law. At 273'K. no single rate law gave a satisfactory fit over the entire uptake range but a cubic rate law was found t o hold after an oxide layer of sufficient thickness had been formed. Data, including those from an electron diffraction examination, indicated that under these conditions the oxide layer corresponded to sodium superoxide, NaOt. The surface potential, measured in the initial stages of oxidation by the diode method, was positive. The slow addition of oxygen to the system which inaintained an oxygen pressure above the film not greater than about 10-3 mm. resulted in an augmented total oxygen uptake. Impure reactants resulted in extensive and irreproducible oxidation with no protective behavior and the oxide formed corresponded approximately to sodium monoxide, XasO. The mechanism of oxidation is discussed.
Introduction Although mainy kine tic studies have been made on the rate of oxidation of a wide variety of metals, little attention has been paid to the reaction between oxygen and sodium. Esome quantitative data for this system have been presented by Cathcart, Hall, and Smith,l but very high oxygen pressures were used, so that little information was obtained about the initial stages of the reaction and, fnrthermore, the structure of the oxide product was not determined. For the present study, this system was chosen because, by the absence of any cation other than Nai- in the reaction product, a less ambiguous interpretation of the mechanism was expected than may be possible for those metals in which the oxidation product contains variable valency cations. (1) J. V. Cathcart, L. L. Hall, and G . P. Smith, Acta Met., 5, 245 (1957).
Experimental A simple constant volume apparatus, Fig. I , was used for the determination of the kinetics. Normal high-vacuum techniques with Apiezon-greased taps were used. With the reaction vessel and protecting trap baked to 620°K. and then cooled, the pressure prior to the deposition of a film was -lo-? nim. The reaction vessel (RV) was made from a 250-ml. round-bottom flask. The filament ( F ) and the Pyrex glass thimble (C) a t the base of the vessel formed an electrolytic cell for the preparation of sodium using a sodium nitrite melt maintained a t 750-780°K. A detailed description of this technique is given by Strong2 and the modification3 for the use of an all-Pyrex apparatus was adopted. The electrolysis current was determined almost entirely by the filament temperature, and, for 0.1-mm. diameter tungsten wire operated at 2750'K. (1.8 amp.), an electrolysis current of about 50 ma. was achieved. Faraday's laws are obeyed4 so that a (2) J. Strong, "Modern Physical Laboratory Practice," Blackie and Sons Ltd., London, 1948. (3) E. W. Pike, Rev. Sei. Instr., 4, 687 (1933).
J. R. ANDERSONAND N. J. CLARK
2136
Fig. I.-Apparat'us
for oxidation of sodium films.
,,
t o vacuum etc.
U Fig. 2.---Apparatus for surface potential measurement. 7
70
40
5: z '+
m
30 I4
1
20
pressure was measured with an ion gage during film preparation in a separate grease-free apparatus. The resulting pressures are: mm.; (a) vessel a t 293°K. after baking, filament on, 1 X (b) sodium nitrite raised to 770"K., 2 X mm.; ( c ) elecmm.; ( d ) 30 sec. after (c), 5 X trolysis commenced, 5 X lO-'mm.; (e) 5 min. after (c), 2 x 10-8mm. The sharp but brief pressure rise at the commencement of electrolysis was probably due to gas evolved from the glass under the influence of the incident electrons. It is clear, however, that under these conditions most of the film was deposited a t a pressure not greater than about 10-8 mm. The electrolyzed sodium was condensed as a film in the spherical region of the reaction vessel which was carefully protected from the heat of the melt by asbestos shields so that during deposition of the sodium film, the temperature of the glass bulb of the reaction vessel did not rise above 310°K. The film was then maintained for 10 min. a t 293°K. to allow any rapid spontaneous recrystallization to occur and was then thermostated a t the reaction temperature. The geometric film area was 185 f 10 cm.2 and for a roughness factor of unity this is taken as the true film area. The sodium films thus were of average thickness about 1.2 X IO*A. Oxygen was metered to the film from the calibrated volume T.' (4.42 f 0.05 ml.) where the pressure was measured to better than 5~0.2%. Oxygen was prepared by thermal decomposition of A.R. grade potassium permanganate and considerable care was exercised to remove condensable impurities by extensive passage through liquid-air traps and to remove residual nitrogen by thorough pumping and by purging of the system during the permanganate decomposition. Nitrogen impurity was shown by Moyer? to accelerate the oxidation of sodium. The thermistor, T (S.T.C. Type R53), formed the sensitive element of a Pirani-type gage for pressure measurement in the mm. and during operation the range 5 X 10-1 to 1 X thermistor tube was maintained a t 273°K. in an ice bath. The decade box, D, was adjusted to keep the bridge in balance when the value of D was a direct measure of the oxygen pressure. The thermistor was calibrated directly against a McLeod gage using oxygen. As the resistance-pressure relation was nearly linear the precision of the readings-fO.l~o-did not vary with pressure. Surface potential measurements were made by the diode method using the modified reaction vessel shown in Fig. 2 and the technique of measurement was generally similar to that described by Anderson.8 Electron diffraction studies of oxidized sodium surfaces were made in a Metropolitan-Vickers electron diffraction camera using 50-kv. electrons at grazing incidence. After oxidation, the reaction vessel was sealed off and transferred to a drybox which had been flushed exhaustively with PnO,-dried argon. The reaction vessel was smashed within the drybox and the film sample mounted in the specimen holder and covered by a small glass bell jar which sealed against the flat plate forming the base of the holder. The sample, with cover in place, was mounted in the specimen chamber of the diffraction camera and after pumping down, the bell jar was removed by a hook operated from outside the camera through a single Wilson seal. Attempts t o use parts of the sodium film deposited on flattened areas of glass in the reaction vessel failed due to excessive electrostatic charging of the specimen and therefore the examination was made using a platinum foil substrate, 10 x 5 mm., which was recovered from the reaction vessel.
IFt 1" O°K+
273'K.
40
Fig. 3.-Dependence
120 Time, min.
200
of oxygen uptake on time.
current of 50 ma. corresponds to the introduction of sodium into the reaction vessel a t a rate of 45 mg./hr. Film weights were standardized a t 22 mg. Spectroscopic analysis of electrolytic sodium5 has s h o m the only metallic impurity to be 0.08% of potassium. The gas content of electrolytic sodium was shown by Andrews and Bacon6 to be much less than for redistilled sodium. As a check on possible gaseous contamination, the
(1920).
( 4 ) I. H. Hurter. Helv. Chim. Acta, 9 1009 ( 6 ) .I. R. Nielsen, I'bys. Rev., 31, 304 (1928). ( 6 , i\l. Andrews and J. Bacon, J . Am. Chem. Soc., 63, 1674 (1931).
Vol. 67
Results Kinetics of Oxygen Uptake.-The oxidation of (i) sodium was studied a t 273, 195, and 90°K. The uptake of oxygen consisted of an initial very fast process, complete within 1 min., followed by a slower uptake, the rate of which was measured. Although reproducible results were obtained a t each temperature, in occasional experiments the oxygen uptake showed marked deviation from the reproducible behavior in that the fast uptake was three or four times that observed for reproducible kinetics and in extreme cases, the complete film oxidized as quickly as the oxygen was dosed into the reaction vessel. Such irreproducible behavior was (7) J. W. hloyer. P h ~ sRev., . 83, 877 (1951). (8) J. R. Anderson, J . Phys. Chem. S o l d * , 16, 201 (19601.
REACTION BETWEEN OXYGEN AND EVAPORATED FILMS OF SODIUM
Oct., 1963
always found on those occasions when the film had been prepared under unsatisfactory vacuum conditions. Figures 3 and 4 show uptake vs. time plots for typical experiments at, the three temperatures. The curve for 90°K. in Fig. 3 shows the effect of subsequently warming the system first to 273 OK. and finally to 321 OK. For films which showed reproducible kinetics, there was little change in the appearance of the sodium surface a t the sodium-gas interface. However, all films which showed irrepraducible behavior with large and rapid uptakes produced oxide layers colored initially black to deep purple, changing through brown to yellow as the oxidation continued. (ii) Pressure Dependence of Uptake Rate.-To evaluate the pressure dependence of the oxidation rate, the pressure over an oxidizing film was raised or lowered suddenly and the change in reaction rate was noted. Figure 4 illustrates results obtained by this method a t 273 and 195°K. At 195°K. the rate was pressure independent and this is assumed to be true also a t 90"IC Table I lists for a number of experiments at 273°K. the ratio R of observlad rates, for changes of pressure by a factor P, together with the pressure dependence exponent x,defined by R = P".
2137
6.6
6.4 Lo
'
2
6.2
x
.
1.5
I fm I*3x10-' m H g 195'K.
1.3
I
# -
10
Fig. 4.-The
50
30 Time, min.
effect of changing oxygen pressure on rate of oxygen uptake.
TABLE I DEPENDENCE OF OXIDATION R
P
0.74 1.43 0.85 2.57 2.18 1.64
0.68 2.15 0. (50 10.0 5.0
RATEAT 273'K.
ON PRESSURE 1:
0.77 .47 .23 .44 .55 8.8 .23 0.45 =I=0.17mean" The mean of 2 is quoted for 90% confidence limits.
(iii) The EfYect of Method of Oxygen Admission and Surface F'otential Measurement.-To determine whether the method by which oxygen was admitted to the sodium affected the reaction kinetics, a leak which by-passed the closer was used. The leak consisted of a glass capillary, further constricted a t a point very close to the end which terminated a t the reaction vessel, and adjusted so that the oxygen flow rate through the leak admitted, over a period of 120 min., a quantity of oxygen into the reaction vessel equal to about twice the total uptake normally found in a similar time for reproducible films at 273 OK. I n a typical experiment, oxygen was leaked a t the rate of 7.5 X 1106' molecules min.-l into the reaction vessel with the sodium film a t 273°K. After 170 min. the film had taken up 12.8 X 1OI8 molecules, and the mm. oxygen pressure above the film was less than The oxidation product at this stage was colored purple. At this point the addition of a large quantity of oxygen through the doser (V) resulted in an initial fast uptake followed by a slow reaction. Out of a total of 14.4 X 10'8 molecules the number 12.3 X molecules were taken up, but by then the uptake had effectively ceased leaving an oxygen pressure over the film of 2 X 10-1 mm. Similar behavior was found in all of a series of three experiments, althLough the reproducibility of the fast uptake frorn the Jarge dose and of the rate of the subsequent slow process was poor. I n all of these experiments it was established that the vacuum conditions
0.6
1.2
1.8
2.4
Oxygen adsorbed, molecules X 10-18.
Fig. 5.-Dependence of surface potential on oxygen uptake: 0 and 0 , separate experiments.
were a t least as good as those normally used to give reproducible kinetic behavior, and there is no reason to believe that the films would not have shown reproducible kinetics had the oxygen been dosed into the reaction vebsel in the standard manner. Surface potentials were measured during fast oxygen uptake at 273°K. Gas doses each equivalent to onetenth of the total fast uptake a t 273°K. (total 4.5 X 10'8 molecules) were added from V; each was completely taken up and the resulting surface potentials are given in Fig. 5 on the convention that a positive surface potential corresponds to a work function reduction. surface potentials could not be measured by the diode method in the slow uptake region because of extreme nonparallelism of the diode curves. (iv) Characterization of the Reaction Product.Three oxides of sodium are known: the monoxide, Na20; the peroxide, Na20z; and the superoxide, NaOz. (a) Chemical Tests.-The aqueous solution obtained by washing the oxidized film from the reaction vessel would have chemical properties dependent on the
2138
J. R. ANDERSON AND N. J. CLARK
nature of the oxide, since both NaOz and P;azOzyield hydrogen peroxide on solution in water while Ka20 does not. The washings of all films which showed reproducible oxidation kinetics decolorized an acidified potassium permanganate test solution. Similar tests on the washings of films which showed abnormal, rapid, and irreproducible oxidation kinetics gave no reaction, from which it is concluded that the reaction product in such cases was NazO while the product from reproducible oxidation contains either or both NaOz and Xa20z. (b) Electron Diffraction Examination.-Results are collected in Table I1 for four films which showed reproducible oxidation kinetics. The reflection patterns obtained were not arcs of circles but single spots and the d-spacings t o which these spots correspond are tabulated. The three oxides of sodium have different crystal structures: SazO, antifluorite, a = 5.55 b. (LandoltBornstein Tabellen) ; Na202, tetragonal, a = 6.65 b.,c = 9.91 ISaOz has been reported by Zhdanov and ZvonkovalO to have a sodium chloride structure with a = 5.44 A. while Carter and Templetonll have deduced from single crystal X-ray diffraction a pyrite the difference in structure structure with a = 5.49 8., depending mainly on the degree of free rotation in the 02- ion. For the present purpose, the degree of uncertainty introduced into the expected electron diffraction pattern is small; with one exception the extra reflections expected from the pyrite structure are too weak to appear under the present conditions. The lattice constant of Carter and Templetonll will be used as a basis of comparison with the present results. The diffraction patterns expected for ?;azo and S a 0 2 are very similar; if the sodium chloride structure is assumed for Ka02 the expected patterns are of identical type, since then both structures wouId have the same space group (Oh5). Since the lattice constants differ by only about 1%, a direct distinction between the monoxide and superoxide is difficult, as the calibration obtained using magnesium oxide is accurate only to about 2%. However, the observed d-values cannot be fitted to KazOz and since the washings of all four films readily decolorized acidified potassium permanganate test solution, it is concluded that a substantial proportion of YaOz was present. A few weak reflections -those parenthesized in Table 11-do not correspond to NaO2, but these probably arise from a small amount of Na20 present. The extra reflection observed a t d = 1.83 is indexed as arising from the pyrite structure. Since the diffraction patterns consisted of spots rather than arcs of circles, the crystal size must have been quite large, and it is not surprising that in the pattern from each film not all reflections were present. (c) Oxygen Uptake for Complete Oxidation.Experiments were carried out with very thin films (about 1 nip.), which were left to oxidize to completion. For films showing reproducible kinetics, the rate of oxidation a t 293°K. became so slow after several days that a t this temperature several months would have elapsed before oxidation was complete. Therefore, (9) A. I. Kltaigorodskn, “X-Ray Structure Analysis of Mioroorystalline and Amorphous Materials,” (Roentgenostrukturnyl Analiz Melkokristallicheskikh I Amorfnykh Tel), G.I.T.T.L., Moscow, 1952. (10) G. S. Zhdanov and Z. V. Zvonkova, Dokl. Akad. Nauk S S S R , 83,743 (1952). (11) B, F, Carter and B. H. Templeton, J , Am. Chant. Soc , 78, 6247 (1953)
1701. 67
TABLE I1 ELECTROK DIFFRACTION RESULTSFROM OXIDIZED SODIUM FILMS
-
A.
d-syacings,
--Bulk
NazO
3.20
oxides-
NaOz
...
--------0 A
...
3.17
(3.27) 3.14
2.78
...
...
...
2.74
...
1.96
. ..
1.94
(2.08)
... ...
1.66 1.58 1.37 1.26 1.23 1.12 1.06 0.97
1.68
...
...
1.27 .
.
I
1.11 1.04
...
7
xidized ’ ’ films---C
B
... ...
.*.
... (2.90) 2.76 (2.06)
.
.
.
I
...
...
(2.08)
(2.08) 1.91
I ,68
1.68
... ...
... ...
...
1.24 1.22
... ...
... 1.83 1.65 1.61 I
7
n ... ... ...
.
1.27 1.24
1.19
... ... ...
...
...
...
... ...
0.95
0.98
after 7 days a t 293OK., the temperature was raised to 373°K. and after 48 hr. a t this temperature oxygen uptake ceased and the reaction mas apparently complete, the reaction product appearing as a fine white coating on the glass surface. The results are presented in Table 111, together for comparison with typical data for a sodium film which showed irreproducible behavior and oxidized rapidly to completion a t 273 OK. TABLE I11 UPTAKEFOR COMPLETE OXIDATION OF SODICX FILMS Numberofsodiumatomsinfilm ( a ) Initial fast oxygen uptake a t 293OK. (molecules) (b) Slow oxygen uptake a t 293°K. (molecules) (c) Oxygen uptake a t 373°K (molecules) Total oxygen uptake from a, b, and c for complete oxidation (molecules) Total oxygen uptake at 273°K. for complete oxidation (molecules) Ratio Na/O in oxide
“Reproducible” film
“Irreproducible” film
x
6.0 X lozo
2.9
lola
....
0.9 X
o
25
x
1019
o
55
x
1019
1.7
x
1019
.... 0.85
.... .... .... 1 6 X lozo 1 88
For the “reproducible” film, the total oxygen uptake exceeded by 17y0 the uptake expected if all the sodium reacted to form SazOz, clearly showing that at least some KaO2 must have been formed. On the other hand, the composition of the oxide from the “irreproducible” film approached that for iSazO. (d) Thermal Analysis of the Oxidation Product.The oxide layer produced from the “reproducible” film in Table I11 was subjected to thermal analysis in which the specimen, after pumping away residual oxygen a t 373”K., was isolated in a closed system and the quantity of oxygen evolved was measured manometrically as the temperature was increased in the range 383 to -730OK. The results are shown in Table I’V. These thermal data should be compared with the results obtained by Rode and Golder12 which may be summarized by the sequence (12) T. V. Rode and G. A. Colder, Bull. A c a d Sct. USSR, Dzv, Chemi Sci., 929 (19.56).
REACTION BETWEEN OXYGEN AND EVAPORATED FILNSOF SODIUM
Oct., 1963 1.
2. Ozf
OPT
3.
7t
OzT
NaOz -+Naz03.6-+ NazOz-+ NazO 393*K.
523'K.
623'K.i-
RESULTSFROM
3
2
5
20
10
t
- to, min.
t
- to, min.
30
50
100
200
4.5
4.0
TABLE IT' HEATING COMPLETELY OXIDIZED FILM
OK.
A
slow
I n the present case, the oxygen evolved in the range 383-423 OK. is attributed to reaction 1 in this sequence, while the subsequent reabsorption of this oxygen suggests the presence of a small amount of unreacted sodium, which agrees with the over-all Na/O ratio of 0.85 (cf. Table 111). The 6.0 X 1018 oxygen molecules evolved in the range 463-593°K. corresponds to reaction 2, so that there must have been a t least 12 X 1Ols molecules of sodium superoxide in the specimen. By comparison with the known over-all composition (cf. Table 111), there remain 5 X 10l8molecules of oxygen which must have combined with the remaining 17 X IO1* atoms of sodium to form sodium monoxide and sodium peroxide. These data then may be used for a mass balance in oxygen and sodium to evaluate the amounts of >\'azo and NazOz and it follows that the amount of NazOz in the product could not have been more than about 2 >< lo1*molecules.
Temperature range,
2139
Molecules oxygen evolved
383-423 2 x 1018 423-463 Some oxygen reabsorbed 463-593 6 0 X lo1* 593-680 Very slow oxygen evolution" The material still gave a positive tests with potassium permanganate after heating for 1 hr. at 680°K.
Discussion (i) Detailed Kinetics.--il preliminary examination showed that the oxidation data for 90 and 195°K. could only be fitted satisfactorily to a law in which the rate was some exp'onential function of 1/X, where X is the thickness of the oxide layer. From the theory of Cabrera and i\/Iott,13the integrated growth law in the very thin film region ( X
