A. K. KURIAKOSE AND J. L. MARGRAVE
2772
Kinetics of the Reactions of Elemental Fluorine. IV.
Fluorination of Graphite
by A. K. Kuriakose and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas (Received March $6,1966)
The reaction of graphite with fluorine has been investigated between 315 and 900" a t various fluorine partial pressures ranging from 6.5 to 75 torr in helium. Graphite gains weight in fluorine a t temperatures between 315 and 530" linearly because of the formation the solid intercalation compound poly (carbon monofluoride) (C,F,) , while above 600" it loses weight with the formation of only gaseous fluorides. Between 530 and 600", there is an overlapping of the two types of reactions. At all the temperatures, the reaction rate is proportional to the fluorine partial pressure. An activation energy of 42.5 kcal./mole is calculated for the reaction forming poly (carbon monofluoride) between 315 and 400", and the dissociation of fluorine on the graphite surface by chemisorption is believed to be the rate-determining step in this process. From 400 to 900" the Arrhenius plots are discontinuous, and two other regions with activation energies 2 and 2.5 kcal./mole are observed in the temperature ranges 400-493 and 650-goo",respectively. Diffusion mechanisms seem to operate in these temperature ranges.
Amorphous forms of carbon burn in fluorine at room temperature, but graphite and diamond are not attacked by fluorine at ordinary temperatures.l Ruff and Bretschneider2seem to be the first to have found that around 420' graphite reacts with fluorine to form a gray solid which they called carbon monofluoride (C,F,). From 460to 700°the reaction is sometimesexplosive,and above 700' graphite burns in fluorine with the production of fluorocarbons, mainly CF,. Later work by Riidorff and Riidoffa-5 showed that, depending on the temperature (between 410 to 550') and the particle size of graphite, various compositions from CFo.68 to CFo.asscould be obtained and that hydrogen fluoride was a catalyst in the reaction. The same authors6e6 also synthesized another solid product of composition approximately C4F in the presence of fluorine and Very hydrogen fluoride at temperatures below 75'. recently, Watanabe, et al.,? have also reported the formation of carbon monofluoride from graphite and fluorine in the temperature range 300-500°. As a continuation of studies on the kinetics of the reactions of elemental fluorine, the fluorination rate of graphite was measured under various conditions of tempere ture and fluorine pressure. The term poly(carbon monofluoride) will be used for the solid carbon monofluoride in order to distinguish it from the gaseous CF. The Journal of Physical Chemistry
Experimental The apparatus and general experimental procedures have been described earlier.8 The only modification was that, for runs below 600°, a small aluminum or nickel cup was attached beneath the graphite samples in order to collect any falling particles from the specimen, and the sample, together with the cup, was suspended from the quartz spring balance. The reaction rates of the aluminum or nickel cups were negligibly small compared to the fluorination rate of graphite a t the low temperatures. The samples were cut from a single spectroscopically pure graphite electrode, about 9 mm, in diameter, manufactured by the National Carbon Co. The circular pieces, about 2 mm. thick, were polished on abrasive (1) H.Moissan, Compt. rend., 110,276 (1890);Ann. chim. phys., [6] 24, 242 (1891). (2) 0.Ruff and 0. Bretschneider, 2. anorg. allgem. Chem., 217, 1 (1934). (3) W.RudoriT and G. Rtldorff, ibid., 253, 281 (1947). (4) W.Rudorff and G. Rtldorff, Chem. Ber., 80,413 (1947). (5) W.RtldorfF, Advan. Inorg. Chem. Radiochem., 1, 230 (1959). (6) W.RtldoriT and G. RtldorfY, Chem. Ber., 80, 417 (1947). (7) N.Watanabe, Y. Koyama, and S. Yoshjzawa, Denki Kagaku, 31, 756 (1963); J. Electrochem. SOC.Japan, 31 (4),187 (1963). (8) A. K. Kuriakose and J. L. Margrave, J. Phys. Chem., 68, 290 (1964).
KINETICS OF THE REACTIONS OF ELEMENTAL FLUOR~NE
2773
Table I : Kinetic Data on the Graphite-Fluorine Reaction
Temp., 'C.
P F ~torr ,
ki, mgJcm.2 min. of graphite
315 315 315 333 333 333 360 360 360 360 360 360 360 396 396 396 396 396 411 411 411 411 422 422 446 446 446 446 464 464 464 464 486
13.2 52.5 74.7 13.2 52.5 74.7 13.2 13.2 21.2 30.6 52.5 52.5 74.7 2.8 13.2 30.6 52.5 74.7 13.2 30.6 52.5 74.7 13.2 30.6 13.2 30.6 52.5 74.7 13.2 30.6 52.5 74.7 13.2
0.008 0.020 0.028 0.017 0.045 0.087 0.123 0.136 0 * 210 0.328 0.476 0.439 0.820 0.263 0.451 0.861 1.105 1.658 0.557 0.710 1.579 2.672 0.573 0.932 0.585 1.149 1.666 2.267 0.609 1.081 1.710 2.239 0.567
Remarks
Temp., OC.
PF,. torr
Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt . gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain
486 486 486 493 493 493 493 530 530 530 530 570 570 570
30.6 52.5 74.7 13.2 30.6 52.5 74.7 13.2 30.6 52.5 74.7 13.2 30.6 35.5
570 570 600 600 600 600 600 650 650 650 650 700 700 750 750 800 800 900 900
52.5 74.7 6.5 6.5 13.2 30.6 52.5 6.5 13.2 30.6 52.5 6.5 13.2 6.5 13.2 6.5 13.2 6.5 13.2
cloth and finally on ordinary cloth. The surfaces were then cleaned with a blast of air to remove any adhering loose particles. The areas of the specimens were calculated from average micrometer readings of the diameter and thickness.
Results and Discussion No measurable reaction occurred between graphite and fluorine up to about 300" and 1 atm. fluorine pressure. Detectable reaction started at about 315" a t a low fluorine pressure of 13.2 torr, and the kinetic studies were carried out between 315 and 900". The reaction rate was linear under all experimental conditions except for an initial fast spurt which might be the reaction of residual graphite dust still within the pores of the graphite. The linear rate constants were calculated only after the fast reaction had subsided. After taking the necessary kinetic readings a t
ki, mg./cm.P min. of graphite
Remarks
Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Wt. gain Nonlinear Nonlinear No weight change
1.066 1.722 2.456 0.656 1.151 1.829 2.249 0.533 0.810 1.447 I.877
...
*..
... ... ...
Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear Wt. loss, nonlinear
0.327 0.286 0.655 1.027 1.917 0.525 1.040 2.400 3.647 0.557 1.093 0.625 1,192 0.652 1.324 0.673 1.500
the required fluorine partial pressure, the fluorination was completed by increasing the fluorine pressure to 1 atm. From the weight gain data, up to about 530", carbon monofluoride with compositions ranging from CFo.asto CFo.*was the product with the higher fluorine content being obtained a t the higher temperature. The product was collected in the attached cup and was gray in color. A small quantity of the product was obtained also a t 600" which was white in color. An infrared spectrum of the product showed a single absorption band a t 1220 cm.-l characteristic of the poly(carbon monofluoride) The kinetic data on the fluorination of graphite from 315 to 900" are presented in Table I. The rate constants are given in terms of the graphite reacted in mg./cm.2 min. Figure 1 is an Arrhenius plot of the .51
(9) W. Rtldorff and K. Brodersen, 2.Naturforsch., 12b, 595 (1957).
Volume 69, Number 8 August 1966
A. K. KURIAKOSE AND J. L. MARGRAVE
2774
t0.4
-
0.0
-0.4
-
-0.8
r m
-1.2
-3
-I .6
-
-2.0
t
b\
I O ~ / OTK I
0.6
I .o
I
1.4
I
I
1.8
Figure 1. Arrhenius plot for the flyorination of graphite a t various fluorine partial pressures with He: A, @, 6 . 5 ; B, 0 , 13.2; C,e, 30.6; D, A, 52.5; E, a, 74.7 torr.
results a t various fluorine partial pressures in helium and shows a family of nearly parallel curves with breaks a t particular temperatures. Up to 530", the points represent weight gain, and above 600°, weight loss data. From 315 to about 400°, there is a rapid increase in the reaction rate while the rate between 410 and 493 is rather insensitive to temperature. There is a slight decrease in the rate a t about 530" using the weight gain data which indicates that poly(carbon monofluoride) and volatile products are being formed simultaneously a t this temperature. Above 650" no solid products are observed, and the reaction rate remains rather insensitive to temperature up to 900", the highest temperature investigated. The breaks in the curves may be attributed to changes in the mechanism of the fluorination of graphite. From 315 to 400" the reaction seems to be chemisorption controlled because of the relatively high activation energy, 42.5 kcal./mole, obtained in this temperature range. This also suggests that the activated complex involves dissociation of fluorine on the graphite surface since the dissociation energy of fluorine, 37.7 kcal./ mole, is very close to the observed activation energy, as was the case in the fluorination of boron.10 A mechanism of this type readily explains the formation The Journal of Physical Chemistry
of poly(carbon monofluoride). Between 410 and 493" the activation energy is only 2 kcal./mole although the product is still mainly poly(carbon monofluoride). Why the activation energy drops in this temperature range is not quite clear. The diffusion of fluorine through the graphite pores may be the rate-determining step under these conditions, and, also, gas phase diffusion is becoming significant since mass spectrometric studies have shown that the monofluoride decomposes partly in this temperature region. The low activation energy of 2.5 kcal./mole between 650 and 900" definitely means that gas phase diffusion of fluorine to the graphite surface through the CF, product gas is the rate-limiting process in the high-temperature range. Variation of the fluorine partial pressure between 2.8 and 74.7 torr in helium showed that throughout the temperature range studied, the reaction rate was directly proportional to the fluorine pressure. However, the behavior of the reaction a t 570" is interesting. Up to a fluorine partial pressure of 30.6 torr the graphite gains weight, although nonlinearly, and above 40.5 torr it loses weight, while in the region of about 35 torr it neither loses nor gains. The reason for this is that under the latter conditions the rate of formation of poly(carbon monofluoride) is equal to the rate of formation of volatile products. The temperatures corresponding to the breaks in the Arrhenius plots seem to be independent of the fluorine partial pressure. This shows that surface temperature changes during the formation of poly(carbon monofluoride) are not detectable under low fluorine partial pressure conditions. There are no data on the heat of reaction of fluorine with graphite to form C,F,, but for l/n(C,F,) it should be more positive than one-fourth of the heat of formation of CF,, ie., the exothermicity of the reaction would be less than 55 kcal./mole. Table I1 shows the graphite surface temperature rises for various fluorine partial pressures a t a furnace temperature of 790", as observed with an optical pyrometer, with samples of nearly identical dimensions (surface area = 2 cm.2; weight = 0.25 g.). The reaction product is mainly CF,(g). If the fluorination rates a t 400 to 500" are approximately one-third to one-half of the rate a t 800", then the graphite surface temperature increase in the 400 to 500" range should be about one-tenth of the surface temperature increase at 800" for any particular fluorine partial pressure, assuming that the temperature rise is directly proportional to the rate constant. At a partial pressure of 13.2 torr, thus, the surface temperature (10) A. K. Kuriakose and J. L. Margrave, J . Phys. Chem., 6 8 , 2671 (1964).
2775
REACTION OF TRIPHENYLPHOSPHINE WITH ~ K A L METALS I
effects would be comparable with the experimental error of temperature measurement ( Alo). Therefore, no corrections are necessary for the temperatures with PFz = 13.2 torr, and the calculated activation Table 11: Surface Temperatures of Graphite in Various Fluorine Partial Pressures a t a Furnace Temperature of 790' PF?, torr
Surface temp.
,OC.
Temp. rise, OC.
0 2.8
790 795
0 5
6.5
10
13.2
800 810
52.5 74.7
890 940
20 100 150
energies are valid for the fluorination of graphite a t this partial pressure. At higher fluorine pressures, there will be correspondingly greater surface effects, and temperature corrections may be significant. However, the lack of thermochemical data on C,F, precludes quantitative corrections, and one expects them to be small since the monofluoride detaches itself from the graphite surface as soon as it is formed a t the higher temperatures.
Acknowledgment. This work was supported by the Aeronautical Research Laboratory, Wright Air Development Division, United States Air Force, in part, under a subcontract with A. D. Little, Inc., administered by Dr. Leslie A. McClaine.
The Reaction of Triphenylphosphine with Alkali Metals in Tetrahydrofuran'
by A. D. Britt and E. T. Kaiser2 Department of Chemktry, Univmsity of Chicago, Chicago 67,Illinois (Received March 89, 1966)
The first reaction of triphenylphosphine with alkali metals which has been detected is a phenyl cleavage rather than formation of the triphenylphosphine negative ion. Radical formation occurs in a second reaction when the metal diphenylphosphine produced by the f i s t reaction is reduced by additional metal. The resultant radical has the formula (CeHs)2PM-,as identified by electron spin resonance and chemical tests.
Introduction The reaction of triphenylphosphine ((CsHs)&?) and alkali metals in tetrahydrofuran (THF) has been reported to produce the mononegative ion ((C&3&€"), identsed by its electron spin resonance (e.s.r.) spect r w n 8 Since the phenyl cleavage of (C&&)J? by alkali metals in T H F is a well-known r e a ~ t i o n , ~the -~ nature of this reported radical has been reinvestigated. The reaction was studied as a function of the concentration of (CaH&P for each of the following alkali metals: K, Na, and Li. The course of the reaction was determined by chemical tests and e.s.r.
Experimental
Materials. The solvents and metals were purified and the solutions prepared in the usual Com(1) Grateful acknowledgment is made to the donors of the Pe troleum Research Fund for support of this research. (2) To whom inquiries regarding this article should be made. (3) M. W. Hanna, J. Chem. Phua., 37, 685 (1962). (4) D.Wittenberg and H. Gilman, J. Org. C h m . , 2 3 , 1063 (1958). (5) I(.Isdeib and H. 0. Frohlich, 2.Naturfwsch., 14b, 349 (1959). (6) A. M.Aguair, J. Beialer, and A. Mills, J. 0 ~ g Chem., . 27, 1001 (1962). (7) D.E.Paul, D. Lipkin, and S. I. Weissman, J. Am. Chem. SOC., 78, 116 (1956).
Volume 69,Number 8
August 1966
