THERMAL S T A B I L I T Y O F COBALT CARBIDE
BG1
(6) RRUSAVER,E M M E T TA,S D T E L L E R J. : Am. Chem. Soc. 60, 309 (10383. ( 7 ) E I S C H E KASD S SELT\-OOD: J. Am. Chem. SOC. 69, 1500 (1947). (8) E~IIIETT: I n E. 0.Kraemer's Atlt'ances i n Colloid Science, pp. 1-36. Interscience Publishers, Inc., S e w York (1942). (9) EMMETT AND B R T K A U E R J.: .4m. Chem. soc. 59, 310 (1937). (10) EMMETT A S D CITES: J. Phys. Colloid Chem. 51, 1248 (1947). (11) E l I l I E T T BSD D E K I T T :Ind. Eng. Chem., Anal. E d . 13, 28 (1941). (12) EIIIIET,Y AXD DEITITT: J . Am. Chem. Soc. 66, 1253 (1943) (13) EWISG:J. Am. Chem. SOC.61, 1317 (1939). (14) GLEYSTEES A X U D I E T Z :J. Research Katl. Bur. Standards 35,288 (1945). (15) H m s i s s ASD Gass: J. Phys. Chem. 36, 86 (1932). (16) HARKISS A K D J C R A :J. Am. Chem. Soc. 66, 1362 (1944). (17) HARKIXS ASC JYRA: J . Am, Chem. SOC.66, 1366 (1944). (18) HOIAIES. ~ K DE\r\fCTT: J . Phys. Colloid Chem. 51, 1262 (1917). (19) JOYKER, WEIKBERGER? A X U ~IOSTGOMERI-: J. Am. Chem. Soc. 67, 2182 (1945). (20) PIERCE : Tiventy-first Kational Colloid Symposium, Pnlo Alt,o, California, June, 1947. (21) RIES, JOHTSOS, ASD MELIK:J , Chem. Phys. 1 4 , 465 (1946). .JOHSSOS, ASD BAUERXEISTER: J . Am. Chem. SOC.67, 1242 (22) RIES, VAX SORDSTR-ISD, (1945). (23) IIIES,I-AXXORDSTRIKD, ASL, K R E G E RJ. : Am. Chem. Soc. 69, 35 (1947). (24) KIES! \-AX SORDSTR.IND, A S D T E T E R :I n d . Eng. Chem. 37, 310 (1945). ( 2 5 ) RITTERASD D R A K E Ind. : Eng. Chem. 17, 782 (1945). (26) RI-SSEI.LAK;L, STOKES:Iiid. Eng. Chem. 38,1071 (1946). (27) SELWOOD: Chem. Revs. 38, 41 (1946). (28) S m n i A S D FI-ZEK: J. Am. Cheni. soc. 68, 229 (1946). (29) STORCH, H. H . : Private communication. (30) T A Y L O R I I,. S . : Private communicntion. (31) T L T E R .U.S . patent 2,381,473 (.'iugust 7 , 1945); Chem. Abstracts 39, 5253 (1945). (32) \-asSORDSTRASD, K R E G E RASD , RIES: Paper presented at the ll2t)h llecting of the American Chemical Society, Se;v T o r k , September, 1047. (33) I - I S S E RChem. : Weekblad 42, 127 (1945). (34) ~I:TTLF;\IOTF:R ASI) \TALKER:Ind. Eng. Cheni. 39, 69 (1947).
ISOTHERRIAL DECOillPOSITIOS OF T H E CARBIDE I N A CARBURr ZED COBALT FISCHER-TROPSCH CATALYST A X D R.C . PEEBLES of Synthetic Laquid Flcels, Bureau of J f z n e s ,
L. .I. E HOE'ER, E 11. C O R S , Rpaenlch ntid 1)Pielopiiieiii Bi.nnch, O$ce
F:iiiceion, PennsylLnnia
Rrceived .-lugtist 27, 1948 INTRODCCTIOK
I n their study of cobalt catalpts which are used for the synthesis of gasoline, Hofer and Peebles prepared and identified a neIT cobalt carbide, CozC ( 5 ) , and observed the rates of formation and of hydrogenation of this carbide in the standard cobalt-thoria-kieselguhr Fischer-Tropsch catalyst (6). The following is a study of the thermal stability of this cobalt carbide as it occurs in a -arhiirized catalyst. The reaction takes place according to the equation
Co2C
+ 2cu-Co
+ (free) C
662
L. J. E . H O F E R , E. M. C O H S AND ‘W. C . PEEBLES
nhere reactant and products are solids. The course of the reaction could be followed magnetically, and the data thus obtained were substantiated by a chemical methcd. MAGNETIC METHOD
It is possible under certain circumstances to follow quantitatively the course of an isothermal reaction in the solid phase by observing the change of the force exerted by a magnetic field on a sample that undergoes a chemical and simultaneous ferromagnetic change. This continuous physical analysis has been found to be especially useful in the present case, where a weakly magnetic substance is converted to a strongly ferromagnetic one (under the experimental conditions). Since the method appears to have been used very little as yet (9), it is desirable to discuss some of the factors involved. The force F , exerted by a magnetic field with field gradient dH/dx on a single, magnetically saturated phase of mass m and having a specific magnetization u is
where mn is the magnetic moment, -11, of the phase. If more than one ferromagnetic phase is present, each contributes its share to the total magnetic moment, ?.Il,
JI, =
o, m,
(i
=
numbar of p!ias?s)
1
and the force exerted on the sample is
F
= (coiT H , ) i
tlH dx
The specific niagnetizatiori is a characteristic property of a ferromngrietic phase which decrease, ~vithincreasing temperature and disappears a t a temperature characteristic of the phase, the transition temperature or Curie point. (For practical purposes, the Curie point is usually defined as the point of inflection of a specific magnetization-temperature plot.) The terms “ferromagnetic” and “specific magnetization” as used above refer to the temperature a t which a particular rcaction is observed. Thus, a phase n-hich is ferromagnetic at room temperature is non-ferromagnetic and has a negligible specific magnetization if thc reaction temperature is higher than its Curie point. It is sometimes possible to choose the ieaction temperature SO that only one of tn-o or more ferromagnetic phases is present below its Curie point. Similarly, if only one ferromagnetic phase is present, the reaction temperature must not only be below its Curie point but also sufficiently far below it so that the specific magnetization is large enough for convenient measurements. Although the specific magnetization may m r y considerably over the temperature range investigated, only the accuracy of measurements at a given temperature
THERMAL STABILITY O F COB.ILT CARBIDE
6G3
is thereby affected. KO correction need be made in correlating measurements made over a temperature range, since the amount of ferromagnetic phase present a t any temperature is proportional to its magnetic moment and its specific magnetization remains invariant a t that temperature. Thus. it is pcssible to express the measured force directly as the fraction (or per cent) of ferromagnetic phase present. I n the simplest case, a ferrcmagnetic substance is converted into a nonferromagnetic one, or cice t w s a . -1s a first approximation, the measured force is proportional to the amount of magnetically saturated ferromagnetic phase present in a constant magnetic field of constant field gradient. In the case in which one ferromagnetic phase is transfcrmed into ancther having a different specific magnetization, it is necessary to calculate the magnetic contribution of each phase. The method is less accurate the more nearly equal the specific magnetizations of the tn-o phases are. -1s the number of ferromagnetic phases present during a reaction increases, interpretation of the data becomes much more difficult. The measured force is not strictly proportional to the amount of ferromagnetic phace, because the ferromagnetic effect is a rcoperative or “domain” effect; that is, it is due to the interaction of a group of atcms or molecules. The minimum size of a ferromagncetic particle has been determined to be 10-12 8. for iron (8) and about 30-40 A. for Y-Fe20: (3). As the ferromagnetic particles grow larger than the critical size, the specific magnetization increases rapidly and reaches a constant value IT hen the sample becomes magnetically saturated. Thus, the actual amount of ferromagnetic phase present at any time during the reaction may be somewhat greater than that which is calculated ircm the measured magnetic moment and from the specific magnetization. This discrepancy cannot be corrected, because no means are available so far for estimating the amount of ferromagnetic substance present in aggregates of subdomain size, and for estimating the amount and size distribution of particles in n-hich the magnetization is not yet fully developed. If solid solutions are involved in a reaction, the Curie point and the specific magnetization of the ferromagnetic phase may change continuously, and a quantitative intcrpretation of the data may become impossible without auxiliary data. APPARATUS
The magnetic balance has been described previously (10) ; the suspension assembly and heating unit used in this study xi11 be described in another paper. AIATERIALS
X 3-g. sample of a r a r , pelleted cobalt-thoria-kieselguhr (1OO:lS :I 00) catalyst (Bureau of Mines KO. 108B (10)) was rediiced with flon-ing (2 l./hr./ sample), dry, oxygen-free hydrogen (passed at 350°C. over palladized asbestos, soda lime, calcium chloride, and magnesium perchlorate) at 398°C. i 2” for 40 hr., after which it was exposed to hydrogen under the same conditions for 44 hr.
with virtually no further \\-eight loss. It was then carburized with floiving (1 l./hr./sample) dry, oxygen-free carbon monoxide (passed over red-hot copper, soda lime, calcium chloride, and magnesium perchlorate) at 208°C. f 2" for 72 hr. and further exposed to carbon monoxide under the same conditions for 48.5 hr. \\ ith virtually no \\-eight increase. The fully cnrl)urized sample>\\ as cooled jn pure nitrogen (passed over red-hot copper, soda lime, calciiim (~hloridP,and magnesium perchlorate), removed in an atmosphere of carbon dioxide a t dry-ice temperature, and stored iinder petroleum ether in a glash h t t l e filled with carbon dioxide. Carbonyl formution and oxidation 1)y contact \\-ith air \\ t w thus prevented. The completely rediiccd sample (before c.arbiii.ization) aho\i ecl an x-ray diffraction pattern of B-cobalt, as described in a previous paper (6), while the carbided sample shoved cobalt carbide lines. Iiieselguhr and thoria were essentially amorphous. Because the quantity of carbon taken up by the sample (10.03 g. per 100 g. of cobalt) corresponded closely to that required for CozC (10.17 g. of carbon for 100 g. of cobalt), and because the carburized sample showed no @-cobaltlines one might expect that virtually all of the cobalt had been converted into cobalt carbide. h h r m d Jessen (11) stated that only carbidic carbon is formed by the reaction of carbon monoxide with cobalt under these conditions. PKOLLDURI'.
'l'he experiments \\-ere carried out \\-ith eight pellets each of the carbided cutalyst, equivalent to 0.06-0.08 g. of cobalt as found by electrolysis of acid-dissolved samples. The sample holder was filled \\-ith petroleum ether, and the pellets were transferred quickly from the storage bottle to the holder. h glass rod was then pushed down into the holder to crush the pellets and subsequently used to insert a plug of Pyrex glass wool. Excess petroleum ether was then decanted, the holder was connected to a vacuum pump by means of a groundglass seal, and the samples were evacuated a t room temperature for at least 1 hr. After this time, the sample holder \vas sealed off a t a previously prepared constriction and inserted into the magnetic balance. The force of the magnetic field on the sample was measured a t room temperature. The sample was then heated as rapidly as possible to the reaction temperature a t which it was kept during the remainder of the experiment, while the force on the sample was measured periodically. HE su LTb
Preliminary experiments, sonie carried out with another similar preparation, had shown that the carbide was weakly magnetic, that the force of the magnetic field on the samples remained constant for extended periods up to about 270°C., and that it increased sloivly with time a t this temperature as the reaction proceeded. At 272"C., for example, the (weak) magnetic moment ruse to twice the original value after heating for about 4.5 hr. An x-ray diffraction pattern of the same sample showed only lines due to kieselguhr and cobalt carbide after
about 6 hr. of heating a t 272°C Another bample \\ab heated to about 360°C. Its magnetic moment increased rapidly at first, then approached a constant value more slou1y, and finally remained unchanged after the sample had been kept a t the reaction temperature for about 1 hr. (The transition temperature of cohalt is 400°C. i 20" (2, 4). Therefore only a-cobalt should form in the temperature range investigated, L'ZZ., 300-360°C ) Subsequent x-ray analysis of the apparently unsintered sample revealed the presence of a-cobalt and kieselguhr only, there v a s no cobalt carbidr left. These results suggest that the increase in magnetization n as clue t o decomposition of cobalt carbide into ferromagnetic a-cobalt and free carbon and that thiq phenomenon co~ildhr 1lsed to follon the reac6on
FIG 1. ThPnnaI dec~oiiil)ositi o i i of carhide i n cnrhided cobalt catalyst in magnetic bnlince at 315°C
Decomposition rates
\\
ere nicasured at six temperatures : 300", 32S", 335''
145". 35.i", and 359°C. Figure 1 is R typical experimental curve, obtained a t 1 4 5 O C ' It phon-s the magnetic moment, in arliitrary units, as a function of time. EY;\LUATIOS
O F DATA
The rate of decomposition of cobalt carbide n a s measured by the change of he magnetic force experienced by a decomposing sample in a constant magnetic ield of constant field gradient. Above about 240°C., a field strength of 2160 ;auss [vas sufficient for magnetic saturation of the ferromagnetic a-cobalt which \-as formed, as shown by the fact that at and above this temperature reversing If the polarity of the field did not change the magnetic force. This is also to be xpected from Honda and Masumoto's (7) study. Thus, the force was directly iroportional to the mass of a-cobalt. Local variations of dH/dr due to movenent of the sample were eliminated by continually moving the sample holder lack t o its original position. To accomplish this. the torsion wire from which he .ample holder was suspended was twisted. The angle of twist was directly roportionnl t o the magnetic moment and thus t o the mass of ferromagnetic laterial.
666
L. J. E . H O F E R , E. R1. COHN AiYD 11‘. C. PEEBLES
The measured values were converted t o per cent cobalt carbide with the following two assumptions: (1) T h e initial magnetization of the sample w a s due to free cobalt. Although the x-ray patterns of the carbided catalysts showed apparently only cobalt carbide lines, the presence of a-cobalt cannot be excluded by means of x-ray diffraction analysis ( 5 ) . The initial magnetic moment can be accounted for by 6-7 per cent of the cobalt remaining as free metal, the corresponding quantity of carbon being present as free carbon. (The rate of deposition of free carbon n-ould be extremely slow a t low carburization temperatures, and the resulting weight increase of the sample could hardly be detected in reasonable time intevvals.) Although pure cobalt carbide (Co,C) appears to be ferromagnetic ( l l ) , it behaved like a paramagnetic substance in the catalyst. This must be ascribed to its state of high subdivision and the low magnetic permeability of the carbide phase. (The permeability can decrease with particle size t o such an extent that ferromagnetism is no longer detectable.) ( 2 ) T h e force was directly proportional to the amount of cobalt present. While this cannot be strictly true (see magnetic method, above), the actual quantity of cobalt present in small aggregates (smaller than those in which full magnetization is developed) x a s small and nithin the limits of error of the measuremeCts, as borne out by the chemical experiments described below. With these two assumptions, the magnetic force was converted into per cent cobalt carbide by letting zero force correspond to 100 per cent carbide and letting the final constant force, f , correspond to 100 per cent cobalt. Any intermediate force, f t , then corresponded to a percentage of carbide. Per cent carbide
=
(
103 1 - -) w ;
It may be noted that the specific magnetization of cobalt is virtually constant over the temperature range of this investigation (‘i), so that the magnetic force on a given quantity of cobalt does not change detectably between 300” and 360°C. The results of all runs are summarized graphically in figure 2, where the percentage of cobalt carbide is plotted as a function of time, which ivas recorded from the moment the heating started. To present a clearer picture, some of the experimental points are not shonn. A theoretical interpretation of the data was not attempted, because the mechanism of the decomposition is obscure. Hoxever, an empirical evaluation was made as follows: From the apparent zero order of the reaction from about 80-25 per cent cobalt carbide content, one can calculate apparent specific reaction rate constants, li, covering this range of the decomposition. They are tabulated in table 1, and their common logarithms are plotted as a function of reciprocal temperature in figure 3. The apparent activation energy is 54.3 kcal. per mole. The “frequency factor”, A , of the Arrhenius equation = Ae-E/RT
is 3.89
x
10‘5 per second. The dimensions of k and -1 are (set.)-', because
THERhfAL S T A B I L I T Y O F COBALT CARBIDE
667
FIG.2. Isothermal decomposition of carbide in carbided cobalt Fischcr-Tropsch :atslyst. TABLE 1 4pparent zero-order rate constants f o r isothermal decomposition of a carbided cobalt catalyst; ranoe atout 80-26 ver cent cobalt carbide
300 325
7.96
x
5 . 4 0 X lo-&
FIG. 3. Apparent specific reaction rate constants as a function of temperature
668
L. J. E. HOFER, E . R I . C O H S A S D TV. C . PEEBLES
the calculations were carried out using "fraction of the original carbide rtmaiiiing a t any time". The shape of the curves in figure 2 as well as the above numerical yalueq can be considered to be characteristic only of the particular catalyst preparation. Preliminary results indicate that the same catalyst, but partly carbided, and :L Hall-type (10) cobalt catalyst shoir- somewhat different behavior. CHEMICAL NETHOD
T o check the validity of the magnetic data, a series of experiments n-ab madc in which the quantity of cobalt carbide (Co&) was determined by the change in weight of partly decomposed samples upon hydrogenation. Four 4-g. qamples of the same catalyst were reduced and carburized up to 94.3-99.3 per cent carbide under the same Conditions as the sample which was used for the magnetic study One of these four was hydrogenated at 21OOC. for Stis hi.., after
ORIGINAL PER CENT CARBIDED
(35,s 96.0 99.3 91.3
-
I
Temperature
Time
335 325 315
7s 140 171
Per cent found
Per cent calculated from magnetic data
(0.6 1 49.1 17.1 '1.3
_I.)- 7 47 I 26 ,5
which 99.4 per cent of the av:iilablc rurbon had ijeen r e n m ed, pri)ving that virtually all of the carbon prcqent 11 as carbidic (1 1). The other three samples were heated in nitrogen a t different temperatures and for various periodh, as indicated in table 2. They \yere then hydrogenated at 210°C. to remove the unchanged carbidic carbon. The amount of free carbon formed during heating was calculated from the weight differences. l'his free carhon correspondccl to the amount of cobalt carbide decomposed during heating. l'htx conditions were chosen so that decomposition should be hetneen 25 and 80 per cent of carbide. As table 2 shows, the percentages found by chemical anal>-& compare favorably with those calculated from the magnetic data. The apparent zero-order specific rate constants which were calculated from these data are shown in fig1u.t. 3. It is thus evident that the magnetic method gave a true indication of the amount of cobalt present, that the quantity of cobalt present in subdomain aggregates was small, and that nitrogen had no effect upon the thermal decomposition of the carbide. SCMMARl
1. The rate of isothermal decomposition of cobalt, carbide in a carburized Fischer-Tropsch catalyst \I as studied magnetically and chemirnlly between 300" and 360°C.
2. The cobalt carbide in the preparation used \vas either paramagnetic or very weakly ferromagnetic. 3. In the range from about 80 to 25 per cent cobalt carbide, the decomposition-time curves \\-c.re linear and hcnce the reaction \vas of apparent zero order in that range. 4. An empirical activation energy cf 54.3 lrcwl. per mole n.as found for the range of apparent zero order. 5. The details of a magnetic method are dewibed, by means of which the complete course of a reaction in thc solid state can he follon-ed under certain condit inns. The authors wish to thank W. E. Dieter for the ainalyses for cobalt. REFEIIESCES (1) BAHR,H. A . , A X D JESSEN, V.: Ber. 63, 2226 (1030). (2) EXAIETT, P.H., A N D SHULTZ, J . F . : J . Am. Chcm. Soc. 51, 3249 (1929). (3) H A U L R , . , ASD SCHOOS, T.: Z. Elektrochcm. 45, 663 (1939); Chem. Abstracts 34, 298. (4) MENDRICKS,B., JEFFERSON, 11.E., ASD SHULTZ, J. F.: Z.Krist. 73, 376 (1930). ( 5 ) H O F E RL. , J. E., A N D P E E R L E ER. , C . : J. Am. Chem. Soc. 69,893 (1947). , C.: J. Am. Chem. SOC.69, 2497 (1947). (6) H O F E RL. , J. E . , ASD P E E B L E SW. ( 7 ) H O N D AI
