GUSTAV F. HUTTIG,JOSEF FUNKE AND HERBERT KITTEL
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ANALYTICAL CHEMISTRY TECHNOLOGY ]
[CONTRIBUTION FROM THE INSTITUTE FOR INORGANIC AND
OF THE
VOl. 57 GERMANINSTITUTE
OF
The Changes of the Magnetic and Catalytic Properties during the Transformation of a Mixture of Calcium Oxide and Ferric Oxide into Calcium Ferrite’ BY GUSTAVF. HUTTIG, JOSEP FUNKE AND HERBERT HITTEL Purpose of the Investigation If a stoichiometrical mixture of the oxides of a bivalent metal and of a trivalent metal is brought to higher temperatures, in many cases a crystalline compound will be formed according to the scheme Me0 R208-+ MeRaOr. In several former communications it was shown2 by us that before the formation of the crystallized addition compound very active intermediate substances are formed, e. g., some with a very high catalytic effect, as high magnetic susceptibility and similar enhanced properties. From the magnetic, optic, x-ray spectroscopic, catalytic, and other properties, it could be concluded without doubt that these active intermediates are by no means mixtures of the starting materials (Me0 R203) with the crystallized end-products (MeR204) but that the reaction mixture ‘or parts of it are present in specifically characterized compositions. In agreement with the theory of W. JanderI3the experimental results pointed to the conclusion that these active intermediates consist of comparatively thin films which envelop the starting material, thus leaving the kernel entirely unaffected. It was possible to obtain about 30% in the system CaO/FezOs, about 15% in the system SrO/FezOa, and about 10% in the system BaO/Fe&a of the total mixture in the form of this film. If the heating is continued further, crystals of the compound MeR204 begin to form in increasing quantities while the amount of starting material (CaO -t- Fez03) diminishes. With the onset of this crystallization, the activity of the substance decreases and the film has but little more to do with the building up of the system. In view of the generality of this phenomenon and of the great significance of oxidic mix-catalyzers in industry, it seemed necessary to find an explanation of the nature of these carriers of the active properties and their changes with time by systematic investigations. For this purpose parts of a stoichiometrical mixture of calcium oxide 1.
+
+
(1) Active oxides, 81st communication. (2) A summary of 18 treatises has been given by G. F. Hiittig, Z. Elektrochcm., 41, 527 (1935). 131 \V. Jander and W. Scheele, Z. anorg. allgcm. Chcm.. 114, 56 (103’3);W . Jaoder. 2. ANeoYr C h s m . . 47, 233 (1034).
(as calcium carbonate) and ferric oxide were brought to various temperatures ( t l ) ; each substance thus prepared was tested for its magnetic properties, its catalytic effect a t different ternperatures (4)and other properties which might help to explain its nature. The results of the investigations and the conclusions reached are described in the following sections. 2.
Experimental Arrangement and Nomen-
clature The substances, after being prepared as described in the following section, were tested for their catalytic activity in the decomposition of nitrous oxide (2N20 -+ 2N2 0 2 ) a t different temperatures ( k ) . Further their magnetic susceptibility, their “powder-density” (weight of 1 cc. of the finely ground powder after being shaken to a fairly compact mass), their color, and (in the experimental series 11) their carbon dioxide content was determined. The catalytic activity in the formation of carbon dioxide (2CO 0 2 +2C02) as well as x-ray data, which had been obtained by us in former similar experiments, were sometimes used for comparison. The catalytic activity in the decomposition of nitrous oxide was measured with the same arrangement as has been described beforeS4 In all the experiments mentioned here 577 cc. (c, measured at 20’) of nitrous oxide, always purified and dried in the same way, were led past the catalyzer per hour. The cross section of the catalysis tube was 1.54 sq. cm. In experiments with pure, unmixed calcium oxide, the sample weighed 0.208 g. (0.00371 mol calcium oxide) in those with pure, unmixed ferric oxide 0.59 g. (0.00371 mol ferric oxide), and in the mixed catalyzer, consisting of calcium oxide and ferric oxide 0.800 g. (0.00371 mol CaO-FezOa). In substances containing the calcium oxide partially or wholly bound to carbon dioxide, 0.800 g. was also weighed out each time; in these cases the number of moles is somewhat smaller (depending on the carbon
+
+
(4) (a) Hiittig, Zinker and Kittel, Z. Elekfrochcm., 40, 307 (1934); (b) Hiittig. Novak-Schreiber and Kittel, 2. physie. Chcm., A l l l . 86 (1934); (c) Hiittig. Sieber and Kittel. Acta physicachimica. U ,R. S.S..4. 129 (1935),
TRANSFORMATION OF CALCIUM AND FERRIC OXIDES INTO CALCIUM FERRITE
Dec., 1935
dioxide content) than when carbon dioxide-free samples are used, and the measured catalytic activity is therefore not a t once comparable with that of the carbon dioxide-free substances. The percentage of nitrous oxide which was decomposed by the catalyzer is represented by cy, while tz is the temperature at which the catalysis was observed. The magnetic susceptibility was measured before the substarice had been used as catalyzer, employing two different field strengths (480 and 950 gauss) and an arrangement which has been applied and described in former investigations.6 x represents the “magnetic mass-susceptibility” (susceptibility referred to 1 g. of the substance) and p the “powder-density” (defined above). The color of the samples (Ow) is designated according to the color chart (with 24 divisions) of Wilhelm Ostwald (Grosser Farbatlas; editor Unesma G. m. 1s. H., Leipzig). In our case the following were important : 4
5
6
7
8
reddish
reddish with brown tint
reddishbrown
reddishpurple
purple
The two added letters indicate the content of gray. In samples where calcium carbonate was used as starting material, the carbon dioxide content of the catalyzer is given by the number of moles of carbon dioxide (n) which exist per 1 mole of calcium oxide; e. g., n = 0.33 signifies that onethird of the total calcium is present as calcium carbonate. 3.
Preparation of the Samples
Series I.-Starting materials: calcium oxide (Kahlbauin “pro analysi”) was heated for two hours to 1050’; it then showed a p = 0.957 and x = -0.58.10-6. The ferric oxide (FezOs) was prepared from an aqueous solution of ferric chloride by precipitation with ammoniae and ignition for two hours over the Bunsen burner of the ferric hydroxide formed. Preparation of the parts of the system CaO/Fe&: the atlcium oxide and the ferric oxide were mixed in molar proportions (lCaO:lFe*Os), pulverized in an agate mortar for one-half hour and pressed through a 10,000-mesh sieve; then they were mixed for two hours in a machine and afterward kept for six hours in a crucible at a temperature tl (see Table I). Every sample has been subjected to the clomplete temperature treatment of the substances prepared a t lower temperatures. The samples were cooled from temperature tt to room temperature in a vacuum desiccator. After this, they were once more ( 5 ) Hiittig and Kittel, 2.aiorg. allgcm. Chem.. 199. 136 (1931) (6) Hiittig and Ganridr, ibid., 119, 49 (1929)
2471
pressed through the same sieve and mixed for one hour; then they were ready for the investigation. Gi-eat care was taken to keep moisture and carbon dioxide away from the sample. Series 11.-Calcium carbonate (Schering and Kahlbaum, Berlin, D. A. B. 5 ; analysis 99.57% &COS, 0.43% HzO) was mixed with the same ferric oxide as in series I in molar proportions (lCaCOs:lFezO~),then pulverized for one-half hour in an agate mortar and finally mixed for twelve hours with a machine. The further treatment was as in series I, only that instead of pressing it through a sieve, the mixture was pulverized in an agate mortar. Series 111.-These samples were prepared exactly as those of series I1 with the only difference that after preparing the stoichiometrical mixture (lCaC03:lFegOs) a further 3% of crystalline calcium ferrite (CaFezOd) was admixed. The latter was identical with the product of series I after the sample had been heated to $1 = 1000”. Series IV.-In preparing the samples of this series, special care was taken that the heating, and therefore also the changes caused thereby, were the same in all layers of the sample. The preparation was the same as in series 11; however, the heating was not done in a crucible but the mixture was made into a paste with purest methanol (puriss. free from acetone) and with that the middle portion of a porcelain tube, which fitted exactly in the center of an Heraeus furnace, was covered evenly with a layer 0.25 mm. thick. Within two hours the furnace was brought to a temperature of 120’ while a stream of dry air was passed through it. After leaving it a t that temperature for two hours, the passage of air was stopped, the oven heat-insulated and the temperature gradually brought to t l in five hours where it was held for r = 6 hours. Here the various samples do not have the same temperature treatment as the substances prepared a t lower temperatures. A number of preliminary experiments have shown that the treatment with methanol is without influence upon the products formed. A crucible was divided into 2 symmetrical parts by an exactly fitting plate of eternite. One-half was filled with the mixture without methanol, while the other half contained the paste prepared with methanol. The whole was then heated as has been described above. By repeating the experiments a t different temperatures, tl, a series of samples were prepared. In all cases the contents of both halves of the crucible showed identical magnetic properties, in particular also the temperature tl, at which ferromagnetism a t first becomes apparent, is the same. The samples of series V were prepared exactly as those of series IV, with the only difference that the time of heating ( 7 ) to temperature tl was not the same in the various samples, as is stated in Table I , column 1.
4.
The Results Obtained
The observed catalytic activity in the decom position of nitrous oxide is given for all samples in Fig. 1. In the lower left corner of each graph the number of the experimental series corresponding to one of the above described modes of preparation is given. The temperature ( t l ) of prepara-
GUSTAV F. H~TTIG, JOSEF FUNKE AND HERBERTKITTEL
2472
Vol. 57 1.8
18
1.4 1.0
0.6
0.6
0.2
0.2 12.4
13.2
14.0 12.4
I / T ~x 104.
13.2
14.0
1/Ta X lo4.
12.4 i
13.2 14.0 12.4 13.2 14.0 / x~104. I / T ~x 104. Fig. 1.
tion is given with each curve and identifies the various substances within a given series. The 273) = 1/Tz is plotted as abscissa, value l/(h while the ordinates represent the corresponding value for loglo a. In the interval a = 2 to the experimental results form a straight line on the graph. It is therefore possible to represent these results by the equation loglo a = - (p’/Tz)
+
12.4
13.2
14.0 14.8
I / T ~x 104.
+ log n’,where p‘ and log n’ are individual constants for each sample. If p (cal.) is set equal to 1.986 X 2.303 p‘, then p represents in the usual notation the “heat of activation,” i. e., a number which for a given sample must be the smaller, the more catalytically active any single active place, while n’ represents a number which is proportional to the number of catalytically active places.
TABLE I 1 tl. o c .
20 200 300 400 450 500 550 600 700 800 850 900 1000
2
a, cal.
3
Log n’
31640 46280
10.233 13.746
38830 39198 39830
12.197 12.427 12.590
41060 36080 34600 19210
12.628 11.596 10.230 6.172
-x
4
480
6
6
7
8
950
Mean value
P
Color
X 10‘gauss-
Series I : CaO, Fe*08, CaO/Fe208 -0.6 -0.6 -0.6 37.4 37.5 37.4 30.1 30.0 30.1 26.6 26.8 26.7 29.7 29.9 29.8 25.6 25.7 25.6 28.7 28.7 28.7 25.2 25.0 25.1 25.3 25.3 25.3 29.5 29.6 29.5 156.2 131.0 395.5 300.2 ferromagnetic 443.3 766.4
0.96 2.56 1.08 1.08 1.36 1.16 1.51 1.54 1.32 1.14 1.31 1.71
7 pi 7 le 7 le 7 ic 7 ic 6 ic 7ng 6-7ng 6ng 5 ng 4ng
2.11 2.18
5-6 pi 8pl
9 7,
%
Series I1 : CaCOs, FezOa, CaCO,/FenOs A t the 50-degree intervals from 450 to 750°, inclusive, the following fractions of carbon dioxide were evolved: 0.003, 0.019, 0.027, 0.062, 0.279, 0.961, and 1.00. 20 200 300 400 450 500 550 600 650 700 750 800
4 1480 26390 22090 22500 21450 18750
12.15 8.19 7.44 7.52 7.06 6.16
21.5 21.6 21.5 21.6 22.0 22.2 22.5 23.3 29.9 87.2 133.9 117.7
21.5 21.5 21.4 21.6 21.9 22.2 22.5 23.5 29.9 80.4 116.6 103.2
21.5 21.6 21.5 21.6 21.9 22.2 22.5 23.4 29.9 ferromagnetic
1.58 1.58 1.58 1.56 1.58 1.59 1.60 1.61 1.46 1.65 1.65 1.93
7 le 7 le 7 le 7 le 6-7 le 6 le 6 le 5 ng 5 ng 4 ng 4ng 4-5 ng
+ + + +1.0
1.97 3.0 6.0 27.68
IrRANSFORMATION OF CALCIUM AND FERRIC OXIDES INTO CALCIUM FERRITE
Dec., 1935
2473
TABLE I (Concluded) Series 111: CaCOs/FetOa At 700' the carbon dioxide fraction evolved was 1.00. 1 11,
2 oc.
q. cal.
3 Log n'
20 600
24100
7.61
650
21950
7.43
700
20350
6.83
--x
4 480
950
54.0
46.0 (21.5) 49.1 (24.7) 94.5 (71.5) 102.5 (79.8)
(21.5) 56.1 (23.6) 99.6 (68.5) 110.7 (80.0)
20 400 630 680 690 700 710 720 800 6h at 717 12h at 714 70h a t 714 336h a t 714
22230
7.48
23460 25800 22870 17290
7.92 8.84 7.71 5.74
26340
9.14
5
6
X 10'gauss-
Mean value
(7) Compare E. W. I
