DECOMPOSITIOX
OF CARBOX MOSOXIDE
B Y FERROYAGSETIC
METALS
405
2. Heats of mixing may be measured consistently to an accuracy of within 5 joules per mole using ordinary Pyres test tubes and thermometers graduated in tenths of a degree Centigrade. Care must be taken to have the liquids substantially at the steady temperature of the room at the instant before mixing, and the heat capacity of the apparatus must be considered. 3. The heat capacity of the apparatus is substantially the same for any apparatus of the same kind when used in the same way, to within these limits of accuracy. 4. The results of these measurements are adequate for survey and industrial purposes and for rough checking of solution theories. 5. Heats of mixing in the system ethanol-acetic acid-ethyl acetate cannot be represented adequately by the simpler theories of solutions. REFERENCES ( 1 ) HILDEBRAND: Solubility of .lion-electrolytes, 2nd edition. Reinhold Publishing Cor-
poration, New York (1936). (2) HIROBE:J. Fac. Sci. Imp. Cniv. Tokyo 1, 155 (1926). (3) International Critical Tables, Vol. V, p. 114. McGraw-Hill Book Company, Inc., S e w York (1929). (4) KELLEY: J. Am. Chem. SOC.61, 779 (1929). (5) LANGMUIR: Colloid Symposium Monograph 1946, 48. (6) MADGIN AND BRISCOE: 3. SOC.Chem. Ind. 46T, 107 (1927). (7) RSDEYANAND LUCAS:Ind. Eng. Chem., Anal Ed. 9, 621 (1937). (8) SCATCHARD: Trans. Faraday Sot. 93, 160 (1937). (9) TIYOFEEV: Compt. rend. 112, 1261 (1891).
DECOMPOSITION OF CARBOS MOSOXIDE BY FERROMAGSETIC METALS FRAKCOIS OLMER' Laboratory of Mineral Chemistry, $cole Nationale SupCrieure dhs Mines de Paris, France Received June 7 , lO4l INTRODUCTION
At temperatures below 1000°C. carbon monoxide is decomposed according to the following equation: 2co 4c
+ co,
Under ordinary circumstances this reaction cannot be observed, but when iron, nickel, or cobalt is present, a strong catalytic action is observed. The fact that the metals which best catalyze the reaction are the three ferromagnetic metals led the author to wonder whether their catalytic action is dependent on their 1
Present address: Department of Chemistry, University of Missouri, Columbia, Mis-
80Uri.
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FRANCOIS OLMER
magnetic properties. We attempted to discover in the following order: ( 1 ) whether other metals than the three mentioned above catalyze the decomposition of carbon monoxide; ( 2 ) whether chemical compounds of the known catalysts also manifest this property; and (3) how the mechanism of the catalysis is realized. The progress of the decomposition of carbon monoxide was studied by measuring the drop in pressure resulting from the absorption of the carbon dioxide produced. It was seen immediately that the catalytic phenomena involved were not reproducible. I t seemed useless, therefore, to study, throughout a series of distinct experiments, the variations in catalytic activity when a condition such as temperature, for instance, was varied. I t is for this reason, perhaps, that earlier works pertaining to this subject show certain discrepancies.
FIG. 1. Diagram of the apparatus
On the other hand, it is consistent to study the variations in catalytic activity
aa a function of the temperature during a single eqeriment, utilizing the same catalyst. This was done, the temperature of the reaction being made to increase proportionally to the time. Since the same catalyst was used throughout each experiment, its activity may be compared a t the various temperatures and certain interesting conclusions drawn. EXPERIMENTdL
The diagram of the apparatus is shown in figure 1. The volume of carbon monoxide is kept constant, and the carbon dioxide produced is absorbed in D by caustic potash. In order to avoid an accumulation of carbon dioxide in the atmosphere surrounding the catalyst, and to carry the former past the potash, it is necessary to circulate the gas in the apparatus through the mercury pump C. The reaction chamber T is a fused-quartz tube placed in the center of the elec-
DECOMPOSITION
OF CARBOY MOSOXIDE
BY FERROMAGNETIC
METALS
407
tric oven F. The catalyst is introduced between two asbestos wads in the center of the tube. A thermocouple, in contact with the catalyst, measures the temperature. The air is pumped out of the apparatus and the carbon monoxide introduced by means of stopcocks R1and R2. Ground joints facilitate the taking apart of the tube T for cleaning and recharging after each experiment. The total volume is about 3000 cc. The speed of circulation of carbon monoxide is relatively great around the catalyst and the carbon dioxide being formed is rapidly carried away; the atmosphere in the chamber T, if not composed of pure carbon monoxide, contains a t most a very small amount of carbon dioxide. The pressure is measured in B by Jolibois' device (11). Variations of both pressure and temperature are recorded on i,he photographic plate of a Le Chatelier registering galvanometer. This system gives a curve each point of which has as coordinate a practically linear function of pressure and temperature. The temperature of oven F is increased regularly a t the rate of 100°C. per hour by the use of Vallet's device (19). The author has already discussed (12) the problems encountered in carrying out the method explained here. RESULTS
The characteristic metals as possible catalysts
In addition to the ferromagnetic metals, chromium (17, 18), manganese, and aluminum (3) have been reported to catalyze the decomposition of carbon monoxide. The following metals, characteristic of the different chemical groups, were studied according to the method described above: magnesium, aluminum, chromium, manganese, copper, zinc, tin, antimony, iron, cobalt, nickel, lead, silver, platinum, molybdenum, and tungsten. These metals were obtained from pure laboratory products, according to well-known methods (12). For each experiment, 0.05 gram-atom of metal was used. The degree of sensitivity of the galvanometers was regulated so that a deposit of uj,a atom of carbon per atom of metal was registered. The experiments (12) show that only thc three ferromagnetic metals catalyze the decomposition of carbon monoxide. Of the other metals, copper, lead, zinc, tin, antimony, chromium, silver, and platinum have no action whatsoever a t a temperature below 1000°C. The passivity of chromium is in direct contradiction to the results obtained by the authors quoted above (17, 18). It is possible that the chromium they used contained slight traces of iron which served as a catalyst. The metals magnesium, manganese, and aluminum are oxidized by carbon monoxide toward 900°C. Molybdenum and tungsten, on the other hand, are carburized toward 1000°C. Both these reactions can be observed above 1000°C., a temperature a t which carbon monoxide is stable. The ferromagnetic metals and their compounds as possible catalysts Lack of space prevents the citation of all the work that was done by the author on the various compounds of iron, nickel, and cobalt. Perhaps the most interesting result obtained pertains to iron oxides, iron carbides, and iron itself,
408
FRhNCOIS OLMER
since their action as catalysts in the decomposition of carbon monoxide hm given rise to many contradictory claims. Some authors believe that iron and all iron oxides act m catalysts (1, 14); others hold that only iron possesses this property (13, 16). The most recent publications ( 2 , 4, 7, 8 ) attribute it only to the iron carbides. In this study, the author developed the principle on which Schenk and Zimmermann (16) based their experiment. Considering the equilibrium diagram of iron and iron oxides in carbon monoxide and carbon dioxide (figure 2 ) , we notice that there are regions which correspond a t once to the stability of some iron oxide and to the instability of carbon monoxide. Take, for instance, ferric oxide. Suppose the temperature and the ratio COt/CO to have values corresponding to a point P on the diagram. The ferric oxide is reduced to the ferrous state, since the point P is in the region of stability of the ferrous oxide. The
coz
I
XKT
+ob
'
I
I
500.
6w'
m* em.
I
I
w
1040'
FIQ.2. Equilibrium diagram of iron and iron oxides in carbon monoxide and carbon dioxide
reduction takes place without change in pressure. On the other hand, the presm,if ferrous oxide does catalyze the decomposition of carbon monoxide. should fall in proportion to the drop in volume represented by the equation: 2co 2 vol. ---f --f
c + eo2 1 vol.
since the point P is in the region of instability of carbon monoxide. Several experiments were carried out with mixtures of carbon monoxide and carbon dioxide of daerent compositions. No potash absorber was used. The temperature increased linearly with the time, and the progress of the reaction waa represented by a straight line such as ABCD. In none of the experiments in which the fraction of carbon monoxide wm less than 50 per cent was there observed a drop in pressure,-that is to say, any decomposition of carbon monoxide. The residue was completely soluble in
DECOMPOSITION OF CARBON MONOXIDE BY FERROMAQNETIC METALS
409
hydrochloric acid and presented no trace of carbon. Each time, on the contrary, that the gases contained more than 55 per cent of carbon monoxide, a strong drop in pressure took place a t the temperature a t which the reduced iron appeared and ceased a t the temperature a t which the region of stability of carbon monoxide was reached. In other words, decomposition of carbon monoxide occurred only in the shaded portion of the diagram, which corresponds to the region of reduced iron. The curves of figure 3 show that the decomposition of carbon monoxide in the presence of iron begins a t about 300°C. At this temperature no one has ever observed the formation of FerC through the action of carbon monoxide or carbon itself on pure iron. FeoC, therefore, cannot cause the catalysis of the reaction. The passivity of this iron carbide was c o n k e d by carrying out several experiments on this compound prepared in the reaction chamber, a t high temperatures.
2W
%8 WO'
506
600.
700'
100'
900'
lOOO*
FIG.3. Decomposition of carbon monoxide in the presence of iron, nickel, and cobalt The resulting curves are all horizontal straight lines, showing that no decomposition of carbon monoxide takes place.
Mechanism of the catalysis ( 1 ) The first question which must be solved is whether there is a relationship
between the catalytic activity and the magnetic properties of iron, nickel, and cobalt, the only elements to possess both these characteristics. As has been seen, the author's method of study, using a linearly increasing temperature, allows one to observe variations in the catalytic activity of a substance continuously as a function of the temperature. The catalyst itself remains the same throughout the experiment. By means of a preliminary experiment, it was verified that even when a fairly large quantity of carbpn was deposited on the catalyst, the activity of the latter was not modified. If, therefore, the quantity of carbon deposited during the
410
FRANCOIS OLMER
entire duration of such an experiment is not extremely large, the curve will show the comparative speed of catalysis a t any temperature. Figure 3 represents one of these curves for each of the three ferromagnetic metals. The curves of figure 4,obtained by plotting the slope of the preceding curves as a function of the temperature, give a better picture of the phenomenon. The ordinate of any point of a curve of figure 4 is proportional to the speed of the catalysis, since the variation in temperature (linearly increasing) is constant. Xumerous curves similar to those shown in figures 3 and 4 were obtained. The maxima and minima vary in size but the curves all have the same aspect, and, which is even more important, the temperatures at which maxima and minima occur are very nearly the same in each case. Below are given these
n
NO"
500'
4M)"
600'
800'
700.
900'
-
1WO'
FIG.4 . Plot of the slope of the preceding curves as a function of the temperature
temperatures as determined from all the curves and, in particular, from those reproduced here: METAL
1 I
Iron Nickel Cobalt
FIPSTYUXlYUM
"C
580 495 475
~
PlNIYM
1 I
"c 710
I
600
575
1
SECONDWXIYUM
1, I
"C
77C-780 740-770 680-710
It is known, on the other hand, that the temperatures of the Curie point and of the change in structure of the three ferromagnetic metals are as follows :
1
Iron.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .! Cobalt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
"C . 770 3-365 1150
I
1 I
"C.
sw-900 425
None
It is easily seen that there is no satisfactory relationship to be established between the two series of temperatures.
DECOMPOSITION OF CARBOX MONOXIDE BY FERROMAGKETIC METALS
411
The improbability that ferromagnetism might play some r6le in the catalytic phenomena was checked even more directly. There exist certain compounds of manganese which are also ferromagnetic (6,9, lo), although their constituents do not possess this property. The following, especially, were worked on: CPvln,, BMn, and SbMn. Their preparation has been explained in an earlier study (12). The curves which were obtained by the use of the new method are all straight horizontal lines for the temperatures at which carbon monoxide is unstable. Toward 1OOO'C. a slight drop in pressure is observed. This comes from the decomposition of the substance tested with oxidation or carburization of its components. So it is seen that none of these compounds catalyzes the decomposition of carbon monoxide. Thus it seems certain that ferromagnetism and the catalytic property studied are independent phenomena. It has been shown in the study already mentioned that a chemical mechanism is highly improbable as an explanation for the catalytic action. We are, therefore, led to consider the hypothesis of a physical mechanism. (2) It was noticed, during the course of previous experiments, that the catalytic properties of iron wew weakened when it had been heated to a high temperature beforehand. This phenomenon was now studied in greater detail. The catalyst, used in the same quantity each time, was prepared in the following manner: The asbestos wad of the reaction chamber was soaked in a given solution of ferric nitrate and then heated to 1000°C. in order to obtain the oxide, which was reduced by hydrogen at 400°C. The apparatus was cleared of hydrogen and the iron calcined in the vacuum, the temperature varying for each trial. After cooling, the carbon monoxide was introduced and the usual experiment carried out with a linearly increasing temperature. Thus, each of the catalysts had been prepared in the same way, except that each calcination had taken place a t a different temperature. The resulting curves are represented in figure 5. As in all these catalysis experiments, the curves are not rigorously reproducible; nevertheless, the discontinuity is always quite apparent, and it always is noticed between the same temperatures. Whenever the calcination temperature lies below 580"C., the curves resemble one another and also those shotvn in figure 3. (Only the beginning of the curves appears in figure 5 . ) On the contrary, those curves corresponding to calcinations a t 590%. or above show a considerable weakening of the catalytic power of the metal. On the other hand, if the slope of these latter curves is plotted as a function of the temperature, as was done for figure 4, a bell-shaped curve (figure 4, dotted curve) with a single m a x i m u m is obtained. Similar experiments were repeated for nickel. The discontinuity is less clear than for iron, extending over the temperature interval 490-515'C. It may be noticed that these temperatures are in concordance with those of the maxima of the figure 4 curve. (3) It is interesting, subsequently, to see whether this transformation of the catalytic properties is reversible. The experiments were repeated, after allowing an interval of 48 hr. to elapse after the calcination, during which time the catalyst was maintained in a vacuum to avoid any possible chemical reaction with carbon
4 12
FRANCOIS OLYER
monoxide. Several experiments were also tried on Armco iron (obtained at a high temperature), which had been delivered to the laboratory several years previously. In no case waa a slope curve with two maxima found. These last experiments would be interesting to carry out in systematic fashion in order to verify the results conclusively.
I
.
300'
L
~~
440'
500'
6GQ'
700.
8OQ'
900'
1000'
ho. 5. Effect of calcination on the catalytic activity of iron
FIG.6. Effect of magnesia upon the catalytic action of iron in the decomposition of carbon monoxide
( 4 ) It also seemed interesting to try the same experiments with reduced iron mixed with a neutral substance. Among other substances magnesia waa used, which has no action on iron, carbon, or carbon monoxide at the temperatures of the experiments. Figure 6 shows the slope curves resulting from the experimental curves obtained from mixtures of 100,500, 1O00,and 10,OOOmolecules of magnesia to one atom of iron. The figure is self-explanatory.
DECOMPOSITION OF CARBON MONOXIDE BY FERROMAQNETIC METALS
413
CONCLUSION
Thus it appears that iron undergoes a modification between 580° and 590°C. which weakens its catalytic properties, and that this modification becomes less noticeable as the iron is mixed with a larger and larger amount of a neutral substance such as magnesia. Two studies mentioning such a discontinuity in the properties of iron at about the temperature a t which the author noticed the change in the catalytic power have been found. Goerens (5) points out a discontinuity in the degree of hardness and of resilience of iron obtained a t a low temperature when it is heated above 580°C. Sauerwald (15) states that, at a certain temperature, powdered substances undergo an important increase in hardness which comes from a “superficial agglomeration” (Verschweissung) of the granules. He states that this phenomenon takes place for iron at about 600°C. All this leads us to the adoption of the hypothesis of a physical mechanism to explain the catalysis of the decomposition of carbon monoxide. This may very well be dependent on a superficial adsorption of carbon monoxide on the surface of the granules of the catalyst. The extent of adsorption depends on the radius of curvature of these granules, which is very small when the iron has been obtained at a low temperature. The speed of catalysis increases from zero as the temperature rises. When the temperature reaches 580”C., the iron undergoes a physical modification such as the one observed by Sauerwald; the particles of iron agglomerate suddenly, lesseningthe curvature ratio and causing the capacity of adsorption to decrease rapidly. The catalytic activity also drops, as the author has observed experimentally. As the temperature continues to go up, the speed of the reaction begins to rise again, reaches a second maximum, and again drops toward a zero limit as the experiment reaches temperatures at which carbon monoxide is stable. On the contrary, if the catalyst is mixed with a neutral substance, the granules of iron are separated, the calcination no longer has any effect, and the catalyst retains its activity even after having been calcined beyond 590°C. The several experiments carried out for nickel and cobalt tend to confirm the hypothesis that the catalysis of the decomposition of carbon monoxide by the th;ee ferromagnetic metals is in accord with the physical mechanism jut proposed by the author in the case of iron. REFERENCES
(1) ALMQUIST, J. A., AND BLACK,C. A , : J . Am. Chem. SOC.48, 2815 (1926). (2) BAHR,T.,AND JESSEN,V . : Ber. 66, 1238 (1933). (3) BANKLOA, W.,AND HIEBER,G.: 2.anorg. Chem. 226, 321 (1936). (4) GLUUD,W.,OTTO,K . V . , AND RITIER,H.: Ber. 62, 8483 (1929). P.:Ferrum 10, 112 (1912). (5) GOERENS, (6) HILPERT, S.,AND DIECKMANN, T.: Ber. 44, 2831 (1911). (7) HILPERT,S.,AND DIECKMANN, T.:Ber. 48, 1281 (1915). (8) HOFMANN, M.:2. anorg. Chem. 181, 414 (1930). (9) HONDAAND ISIWARA:Rev. m6t. 16, 127 (1918). (10)JASSONNBIX, BINNETDU: Bull. 8 0 C . chim. 88, 102 (1906).
414
SYDSEY ROSS
(11) (12) (13) (14) (15)
JOLIBOIS, P.: Compt. rend. 172, 809 (1021). OLMER,F.: “Reduction des oxydes de fer”, Thesis, Rey, Lyon, 1941, p. 15. SABATIER, P.: La catalyse, p. 29. Paris. SAITO,H.: Science Repts. TGhoku Imp. Univ. 16, 186 (1927). SAUERWALD, F.: Z. anorg. Chem. 122, 277 (1922); Z. Elektrochem. 29, 79 (1023); 30,
(16) (17) (18) (19)
176 (1924). SCHENK, R . , .4ND ZIMYERMAKN, F.: Ber. 36, 1232 (1903). SLADE,R. E., AKD HIGSOK, G. J.: J . Chern. SOC.116, 205 (1919). TROPSCH, H., AND PHILIPPOYICH, A . v . : Abhandl. Kenntniskohle 7, 44 (1929). VALLET,P.: “Methode d’6tude des systhmes chirniques d temperature va,riable”. Thesis, Jouve, Paris, 1936.
ORIENTATIOS O F AIICELLES IN S0.4P FIBERS SYDXEY ROSS Department of Chemistry, Stanford Unzuerszty, Californza Receivod LVovemberIS, 1941
The ease with which molecules of soaps and fatty acids orient themselves is a well-known and characteristic property. Investigation by x-rays has indicated fibrillation when the material is either pressed into rods (l), or smeared on metallic surfaces (lo), or picked up from aqueous surfaces by the Langmuir and Blodgett technique (2), or spun into threads from concentrated gels (7). The fiber structure thus revealed by x-rays may well be expected to exhibit features in common with other fibers composed of organic molecules linked to one another along a polymeric chain. The exhaustive x-ray researches already applied with such valuable results to the study of cellulose could be used as a guide for further studies of the nature of soap fibers. However, soap fibers differ in one respect from the polymeric fibers in that the molecules lie transverse to the fiber axis (8) and are not linked together by anything more than “crystal” forces, whereas in all other natural and synthetic fibers the macromolecules lie along the fiber axis. Fiber photographs can be obtained by pressing the powdered material into capillary tubes, so the presence of micelles in the soap fiber need not be assumed merely by analogy with the case of the cellulose fibers. I t would be impossible to suppose that the original particles lose their separate identity after orientation. That the micelles are oriented is immediately evident from the x-ray picture. The positions of the intensity maxima on the fiber diagram indicate the direction of the orientation, and the sharpness of the localized maxima can be used to ascertain how nearly the orientation is completed. The arrangement of the soap molecules inside the micelle has already been derived from x-ray data by Thiessen and his collaborators (7, 8, 9). The next step in the elucidation of the fiber structure,-the orientation of the micelles in the fiber,-can also be revealed by x-ray diffraction photographs. The purpose
