317
REDUCTION OF FERRIC OXIDE BY HYDROGEN
(c) the temperature a t which each secondary reaction begins; (d) the relative speeds of partial reactions; (e) the temperature of a modification in the kinetics of the reactions; and (f) the temperature of a modification in the kinetics of the reactions. From the inspection of curves obtained by the method, it is possible to study the various chemical reactions which occur in an industrial process. The reduction of ore in a blast furnace, the carburization of steel, the chemical reactions between the constituents of glass during the fusion, and the decomposition of an organic compound are all possible practical applications. One such example (the catalytic decomposition of carbon monoxide by ferromagnetic metals (3)) has been studied in detail by the author. A subsequent paper will set forth a practical and interesting realization of the method. REFERENCES (1) GUICHARD, M.: Bull. soc. chim. 39, 1113 (1926). (2) OLMER,F. J.: Thesis, “Reduction des oxydes de fer”, Rey, Lyon, France, 1941. (3) OLMER,F. J.: J. Phys. Chern. 46,405, 1942. (4) ST.JOHN,J. L.: J. Phys. Chem. 38, 1438 (1929). (5) VALLET,P.: Thesis, “Recherches sur la methode d’Btude des systemes chimiques en temperature variable”, Jouve, Paris, 1936.
REDUCTION OF FERRIC OXIDE BY HYDROGEN FRANCOIS OLMERI
Laboratory of Chemistry, &ole Nationale Supbrieure des Mines de Paris, France Received July 84, 194.8
Although numerous authors have studied the reduction of the oxides of iron in hydrogen, the kinetics of these reactions have not yet been clearly explained. Most of the hypotheses presented in the case of ferric oxide may be classified as follows: (1) Fes03-+ Fe (8, 12) (2) Fen08 3 FesOa 3 Fe (2,19) (3) Fez03-+ FeO + F e (5) (4) FezOs-+ FesOc 3 FeO -+Fe (13,17) (5) Fez03 -+ simultaneous mixture of all oxides 3 Fe (6) (Fez03 FesOc -+ Fe a t low temperature (16) \F@Os -+ Fe at high temperature
(4,11)
---f
A new method of studying the mechanism of chemical reactions by means of linearly increasing temperatures (14) has been applied to the question by the author of this paper. 1
Present address: Diamond Alkali Co., Painesville, Ohib.
318
FRANCOIS OLMER EXPERIMEKTAL
In the general exposition of the method mentioned, it was supposed that the progress of the reaction was followed by measuring the variations of a physical property such as the mass of one reagent.
FIG.1. Diagram of the apparatus
FIG.2. Reduction of ferric oxide by hydrogen
In the present case of the reduction of iron oxides, if the amount of hydrogen greatly exceeds the amount necessary to the reduction, the pressurc changes A P are small (2-3 cm.) compared to the mean pressure P of the experiment (75 cm.) and are, therefore, proportional to the changes AT' in volume V (VAP = PAV). These changes in pressure are also proportional to the amount of oxide reduced. The pressure was chosen as the suitable physical property for observing the progress of the reaction.
REDUCTION OF FERRIC OXIDE BY HYDROGEN
319
The diagram of the apparatus used is shown in figure 1. The oxide is introduced between two asbestos wads in the fused-quartz tube T placed in the center of the electric oven F. The volume of hydrogen is kept constant and the water produced is absorbed in B and D by phosphoric anhydride. In order to avoid an accumulation of water vapor around the oxide, the hydrogen is circulated in the apparatus through the mercury pump C. The speed of circulation is sufficiently great so that the water is rapidly carried away. I n the atmosphere around the oxide, the partial pressure of the water vapor is very small compared to the pressure of the hydrogen, and one may consider the reduction as being carried on in an atmosphere of pure hydrogen. The temperature of the oven F is measured by a thermocouple in contact with the oxide, and is increased regularly a t the rate of 100°C. per hour by the use of a Vallet device (18) or by the use of a Wheelco Program Controller. The
FIQ.3. Two characteristic curves for the action of hydrogen on ferric oxide
varying pressure in the barometer B is transformed by Jolibois' device (7) in a variable electric current. A Le Chatelier double galvanometer (figure 2) photographically registers the variations in pressure against the temperature. The electrical circuits are shown in figure 2. The ferric oxide was prepared by calcination of C. P. ferric nitrate a t 300" or by precipitation of a solution of C. P. ferric nitrate by ammonia. In both cases every precaution was taken to obtain products of absolute purity. RESULTS
When pure ferric oxide is reduced, it is observed that the temperature a t the beginning of the reaction depends on the temperature a t which the oxide was. prepared or calcined (sintered). Several authors ( 6 , 15, 19) have given different, explanations of this phenomenon. Figure 3 represents two characteristic curves of the action of hydrogen on!
320
FRANCOIS OLMER
ferric oxide. Both curves were obtained under the same conditions and from the same weight of the same sample of ferric oxide prepared a t low temperature (300°C.), but sample 2 was calcined a t 1200°C. prior to the experiment. The effect of sintering may be immediately noticed. The reduction of sample 1 begins at 23OOC.; that of sample 2 a t 400°C. What is more important, the curves differ in shape, indicating a different mechanism in the reduction of each sample. ( I ) Interpretation of c z m c 1: This curve is easily interpreted. The plateau B shows that ferric oxide i s first reduced to magnetic oxide, since: (a) The ordinates of the points '1, B, and C are in the ratio
corresponding to the chemical equations:
+ H20
3Fe2O85 HZ
-+
2Fe804
2Fe304
-+
6Fe
+ 8H,
+ 8H20
(1)
(2)
( b ) The portion BC of the curve is identical nith the curve obtained in the reduction of magnetic oxide (curve not represented here). (c) The chemical analysis of thc product obtained by stopping the experiment a t 300%. gives the formula Fes04. Magnetic oxide of iron begins to be reduced at 325°C. Yo plateau or break appears on the portion BC of the curve. The question as to whether ferrous oxide is formed during this reduction is not easily answered. Chaudron's equilibrium diagram (3) (figure 4) shows that ferrous oxide is reduced as follows
FeO
+ H)
-+
Fe
+ H20
(31
in pure hydrogen. Furthermore, it shows that the oxide is decomposed below 570°C. in any atmosphere, according to the reaction: 3Fe0 -+FezOa
+ Fe
(4)
Therefore, if ferrous oxide is formed during the reduction of magnetic oxide Fe804
+ H,
-+
3Fe0
+ H?O
(5)
it is immediatdy decomposed or reduced, or both, and its existence is only ephemeral. Hoxever, thermodynamical calculations (1, 9) indicate that the speed of reaction 4 is extremely great in the interval of temperature 350-600°C. illthough no data are available for the speed of reduction (equation 3)-if this reaction really occurs below 570"C.--it may be supposed that it begins slowly like all the other reactions observed in this study. On the other hand, the ferric oxide produced in reaction 4,as will be shown below, is reduced above 325OC. according to the mechanism: FezOs'f 3IIz -+ 2Fe
+ 3&0
(6)
321
REDUCTION OF FERRIC OXIDE BY HYDROGEN
and this reduction begins slowly a t 325°C. Therefore, a certain amount of ferric oxide should be obtained when the reduction of magnetic oxide is stopped , a few moments after its beginning. Experiments showed that ferric oxide was never observed, neither when the products of the reduction of magnetic oxide were cooled rapidly nor when the hydrogen was pumped off to prevent reductions 3 and 5. Magnetic oxide is reduced directly to iron in pure hydrogen, and ferrous oxide does not appear in the reduction.
%. 90-
80 . 70 60
\-r
\-%
"
Fe
. *
\
50 -
'Fs n.
40.
\'..
30. 20
-
IO
-
H20' zdoe
3000
4000 500. b b o o . 700' eo00 FIQ.4 . Chaudron's equilibrium diagram
900.
(2) Interpretation of curve 8: Two hypotheses may be given to explain the fact that curve 2 (figure 3) is perfectly regular: (a) The ferric oxide is reduced as before to magnetic oxide. The temperature being above 325"C., the latter is also reduced and the two reactions (1 and 2) are simultaneous. ( b ) The ferric oxide is reduced directly to metallic iron according to reaction 6. Intermediary curves of the reduction of ferric oxide calcined a t different temperatures were obtained to determine the gradual modification of curve 1 into curve 2. The enlarged beginning of these curves is shown on figure 5. If magnetic oxide were formed, the reduction would progress in three phases: From the start of the reduction to 325"C., the ferric oxide is reduced.
Fez03-+ Fe304
(1)
322
FRANCOIS OLMER
Above this temperature, both what is left of the ferric oxide and whatever magnetic oxide has been formed are reduced together.
When all of the ferric oxide has been reduced, only magnetic oxide is left and only reduction 2 occurs. It has been shown in a discussion of the theoretical background of this method of study (14) that the end of all the reactions is so abrupt that it is indicated on the curves by a break. Therefore, if the above mechanism really occurred, a break would appear, and only one, separating the second and third phases
FIG.5
of the reduction. The ordinate of this break would correspond to a greater amount of hydrogen than for cquation 1 alonc, sirice part of the magnetic oxide formed would have been reduced. The temperature of this break would vary for each curve, but would a l m p s be greater than 325°C. The curves should take the form of those shown in figure 6, which have been traced by analogy with other experimental curves obtained in this study. On the other hand, if the mechanism of the reduction of ferric oside changes abruptly a t 325°C. from FenOa-+ FesOa (1) to
REDUCTION OF FERRIC OXIDE BY HYDROGEN
FIG.6
I
I I
I
200°
I
I
I
306 FIG.7
NO0
323
324
FRAKCOIS OLXER
the difference in speed of these two reactions should be indicated by a break on the curves. The break should always appear a t 325°C. As reaction 1 has not been entirely completed, the ordinate of this break should always be less than that of the plateau of curve 1. The experimental curves of figure 5 show all these particularities. The break P observed in curve 3 corresponds to the end of the reduction of the magnetic oxide formed during the first phase of the reduction. The question may arise as to whet,her the phenomena which have been observed aboi-e may not he caused by the calcination of the ferric oxide; the heating might transform part of the oxide, this part reacting with the hydrogen in a different m y . Thc double mechanism of the reduction, however, has been observed on non-calcined samples of ferric oxide. It has been noted (14) that the slopc of each curve is inversely proportional to the rate of increase in the temperature of the experiment. Therefore, if the rate of increase is made greater, t’he end of any reaction will occur at a higher temperature. Figure 7 represents the beginning of the curves obt,ained by reducing the same weight of ferric oxide, from the same sample prepared a t low temperature, under identical conditions. Only the rate of increase in temperature was different in each experiment. All the curves begin a t the same temperature, since the oxide is the same in each case. The breaks N occur always a t 32S°C., and the ordinate shows that the ferric oxide has not entirely reacted. Reasoning similar to that followed in the case of calcined oxide s h o w that the double mechanism of the reduction is observed in noncalcined as well as in calcined ferric oxide. The “sintering” of ferric oxide, therefore, has no other influence than to raise the temperatwe a t which the oxide is reduced. CoNCLL-sIoKs
The reduction of ferric oxide and that of magnetic oxide in pure hydrogen lvere studied a t temperatures which were increased linearly with the time. The curves obtained show that ferric oxide is reduced to magnetic oxide at temperatures below 325°C. Above this temperature, ferric oxide is reduced directly to metallic iron. No magnetic or ferrous oxide is formed in this case. Magnetic oxide is reduced directly to metallic iron. The formation of ferrous oxide is not observed. REFERESCES BAIKOFF,A . A , : Metall. U.S.R.R. 3 (1926). REHGER,E.: Compt. rend. 174, 1343 (1922). CHAUDRON, G.: Ann. chim. 16, 253 (1921). HILPERT,S . : Ber. 42, 4575 (1909). (5) HOFIIIANK, K . : Z. angew. Chem. 38, 715 (1925). E . : Stahl u . Eiven 6, 1562 (1910). 46) JOISTEN, (7) JOLIBOIS, P.: Compt. rend. 172, 809, (1921). (8) KAMURA, H . : J. Iron Steel Inst. (London), September, 1925. ‘(9) K A T ~ U J I REO. ,: Science Repts. TBhoku Imp. Univ. 26, 562 (1938). (10) KAWAKITA, K , : Rev. Phys. Chem. Japan 14, 79 (1940). (1) (2) (3) (4)
MECHANISM OF FORMATION OF KOHLSCH~TTER’S SILVER SOL
325
MATHESIUP, F.: Stahl u. Eisen 34, 872 (1914). MATSUBARA, A . : Trans. Am. Inst. Mining Met. Eng. 3, 1051 (1921). MOISSAN,H . : Compt. rend. 84, 1296 (1877). OLMER,F.: J. Phys. Chem. 47, 313 (1943). J . : 2.Elektrochem. 29, 79 (1923); 30, 175 (1924). SAUERWALD, TAYLOR, G . B., AND STARKWEATHER, H . W. J . : J. Am. Chem. SOC.52,2314 (1930). TCHOUFFAROFF, G. I . : J. Phys. Chem. Russe 6, 1103 (1934); Acta Physicochim. U.R.S.S. 4, 617 (1936). (18) VALLET,P. : “Methode d’btude des systemes chimiques B temperature variable”, Thesis, Jouve, Paris, 1936. (19) WUST, F., AND RUTTEN,P. : Mitt. Kaiser-Wilhelm Inst. Eisenforsch. Diisseldorf 5, 6 (1924).
(11) (12) (13) (14) (15) (16) (17)
T H E MECHANISM OF THE FORMATION OF KOHLSCHUTTER’S SILVER SOL. II HARRY B. WEISER
AND
M A X F. ROY
Department of Chemistry, The Rice Institute, Houston, Texas Received December 4 , 1948
“Kohlschutter’s silver sol” (1) is the term commonly applied to the hydrosol formed by conducting hydrogen into a solution of silver oxide containing an excess of the solid oxide, maintained at a temperature of 50430°C. Carbon monoxide may be substituted for hydrogen as a reducing agent, but according to Kohlschutter the resulting sol is instable. The first paper (2) on this subject reported the results of tt study of the mechanism of the sol formation process with hydrogen as the reducing agent; this paper is concerned with the formation of Bilver sol using carbon monoxide as reducing agent. In the earlier paper it was demonstrated that sol formation results by the reduction of silver oxide a t 55-58°C. only when some of the solid oxide is suspended in the solution. In fused-silica or silver vessels, ultrafiltered silver oxide solutions are not reduced by hydrogen a t 55-58’C.; in glass and platinum vessels, the reduction is confined to the surface of the vessel, giving a thin silver mirror on glass and relatively large platelets of silver on platinum. The reduction of ultrafiltered silver oxide solution in a platinum vessel results from catalytic activation of the hydrogen at the surface of the metal. No catalytic activation and no reduction take place at the surface of silver or fused silica. The reduction a t the surface of glass does not result from catalytic activation of hydrogen, followed by reduction of dissolved silver oxide, but is due to reduction of a s l m of solid silver oxide precipitated on the surface of the glass by alkali dissolved from the glass; adsorption of the oxide from solution plays a minor r81e in forming the oxide film. The ease with which solid silver oxide in suspension or on the walls of the vessel is reduced indicates that hydrogen is activated a t the surface of silver oxide or a t the interface silver-silver oxide.
.
