S', S" aut1 their sum S, by using the constants CI, P2 and C3calculated by M a r g e n a ~ . ~ In Table I it is quite clear that while the contri1)ution of the dipole-quadripole p:u't of the interaciion potential is considerable, tlie contribution of the quadripole-quadripole term (and similarly the higher terms) is negligible. From these calculations one observes that a t higher temperatures when the dipole-quadripole contribution becomes apprecialile, the value of Sutherland coilstant increases. This has been observed experimentally by Rilaud and \'asilesco' and others. Bearing in mind the fact that the above calculations are very sensitive to the values of 6 ,as they appear in high powers, the agreement between the calculated and observed values is reasonable with the exception of fen. gases. Some of these gases such as hydrogen and helium are known not to obey the Sntherland law; while the o t h m such as CO?, C1, and HC1 which are polar gases require in their expression for the interaction potential a term depending on their dipole moments and varies inversely as r3. SURFACE AREA MEASUREMENTS ON C.-lRBON BLaCK PRODUCED BY THE CATALYTIC DECOMPOSITION OF CA4RBON MONOXIDE OVER IRON BYDONALD S. AIACIVER AND PAUL, H. EMMETT Contribution f r o m the Mialtiple Fellozcslrip of G u l f Research and Dewlopme11t Coni p a n y , ilfellon Instit ule, PiflsburQh, Pen nsv1i:aiiia Receivod S p r i l 2.5 1956
Various ~ v o r k e r s ~have - ~ observed that when carbon monoxide is decomposed over iron a t 450-600", the carbon is deposited as minute, thread-like growths having thicknesses of the order of several hundred Bngstrom units. The threads often appear to be twisted into rope-like formations and always seem to contain iron, usually in the form of carbides and usually to the extent of one such particle per thread. It is difficult to tell from electron micrographs whether the threads are tubular or flat ribbons. I n view of the unusual nature of the carbon deposit, it seemed of interest to investigate the nature of the carbon threads by the standard gas adsorption technique using nitrogen and carboil monoxide as adsorbates a t - 1%".
measurements was t h e prepurified tnnk grndc; t)he helium, for cnlibrat,ion purposes, wns a conimerci:il gi~ndeand was purified by passage over degnssetl charcoal a t -395". Tnnlc carbon monoside from the Air Reduct.ion Company \vas condensed nt -195' :tnd distilled, tlie iniddle portion being taken for the adsorption mensurcinents. The reduction and adsorptions \yere carried out,in a sample tube equipped with an inlet and outlet, as well as a sidrarm containing a manifold of thin-walled capillary tubes. By means of the Intter, it was possible to remove small port,ions of the sample in vacuo for X-ray diffraction studies. The carbon black 'rvns placed in the snmple tube and degassed a t 150' for 2 hours. A nitrogen adsorption isotherm was then run on the sample a t -195". From this, the surface area was calculated from the familiar B E T plot.8 The sample was then reduced for 48 hours at 425" in n stream of hydrogen at 1 atmosphere. A small portion of the sample was removed for X-ray annlysis and t'he surface area of the remainder measured by nitrogen adsorption. Following this, the adsorption of carbon monoside at - 195" was ineasured. The sample WRS then evacuated for one hour at -78" and another c:irbon monoxide isotherm run a t - 195'. A portion of the original, unreduced carbon black \vns +ijected to an X-ray analysis and to n wet annlysis for total iron.
Results According to electron microscope pictures hy Dr. E. Sterling,4 the carbon black is thread-like in form, the threads having an apparent thickness of 500 to 1000 A. An iroii analysis indicated that the material contained (5.94% Fe. X-Ray analysis showed that the original sample contained l0-15% Hagg Fe2C with a trace of Fe304. The average crystallite size of the Fe2C as estimated from the diffraction patJternsappeared to be i n the range of 150-250 A. The nitrogen adsorption-desorption isotherm obtained on the unreduced carbon black is shown in Fig. I. The lack of a hysteresis effect suggests6 the probable absence of pores in the range 20 to loo@& The BET surface area as calculated from the nitrogen isotherm was 145 nx2//g. This correspoiids to a calculated diameter of 132 8. for the carbon threads, and indicates that they ha1.e :L roughness
2zQl
200
Experimental The carbon black used wns prepared by Dr. E. Sterling4 l y decompoising pure cnrlion monoside over a finely divided iron cat:ilyst at 400". The portion of the carbon black porntier passing through a 40-mesh sieve was taken for study after removing the massive catalyst particles initially present. The volumetric adsorption apparatus employed in the present worlr was similar to the one desci,ibecl in the liternture several The nitrogen used for adsorption (1) L. V. Rstdusiikevich and V. M. Luk'yanovicli, Z h u r . F i t . K h i m . , 26, 88 (1952); known tlirongh C.A., 4 7 , G210 (1953). ( 2 ) JV. R. Dnvies, R . J. Slawson and G. R. Ri(:by, A'nture, 171,751; ( 1953). ( 3 ) V. J. ICehrer, J r . , and H. Leidheiser, tTr., T H I S.JOURNAL, 56, 550 (1954). (4) E. Sterling, L. J. llofer and J. T. hIcCartney, to he p u l ~ l k h e d . ( 5 ) P. H. Eininett, A m . A'oc. Tevting Aloterials, 41, 05 (1941). (11) P. H. Emiiiett. .4dunuccs in Colloidal Sci., 1 , 1 (1912). (7) P. H. P : ~ t l ~ ~ lnnd e t t 8. Brunnuer, .I. A m . C h ~ m Sac., . 56, RR (1934).
o
at
I
I
a2
0.3
I I I 04 0.5 0.6 Relollre Preaiure
.
I
I
I
0.7
08
09
I
Fig. 1 .-Kit,roge;i adsorption isotherm on carbon black at - I95 : 0, adsorption; 0 , desorption. -
( 8 ) S. Biunatmr, .'k
H. Eininett nnd E. Teller, ibid., 6 0 , 309 (1038).
1110
TTol.
NOTES
factor of about five in the form of cracks and crevices too small t o cause hysteresis. The reduced carbon black had a BET surface area of 144 me2/g. A comparison of the two carbon monoxide isotherms a t - 195" before and after an intervening evacuation a t - 78" in the manner suggested by Emmett and B r u n a ~ e r indicates ,~ a chemisorption of 2.4 cc. (STP) of carbon monoxide per gram of carbon black. Assuming a chemisorbed molecule of carbon monoxide occupies the same area as a molecule of physically adsorbed nitrogen, one can calculate that about 7% of the surface of the carbon black sample consists of iron. This corresponds t o an average diameter of about 50 A. for each particle of iron. An X-ray analysis of the redticed sample showed particles of a-iron about 300 A. in size. This j s probably in satisfactory agreement with the 50 A. size estimated from the adsorption measurements in view of the fact that the iron particles may have a roughness factor that makes the surface area several-fold greater (and the calculated particle size several-fold smaller) than would be deduced from X-ray analysis. (9) P. H. E m m e t t and S. Brunauer, J . A m . Chrm. Soc., 59, 310 (1937).
RADIATION-INDUCED EXCHANGE OF HYDROGEN ISOTOPES: CHAIN INHIBITION BYLEONRI. D O R F M A AND N ~F. J. SHIPKO Knolls Atomic Pouer Laboratoiy.2 General Electric Company, Schenectady, N . Y . Received M a y 8, 1956
5s
was present a t pressures below 0.09 mm. A single run on the hydrogen-tritium exchange, a t very much higher radiation intensity, was completed. Experimental The reaction cell was a spherical Pyrex bulb, diameter 5.7 cm. The reaction bulb waR closed off by a mercury cut-off to avoid any possible effect of stopcock grease, and analytical samples were taken by letting one or two bubbles of gas out through the cut-off. The analyses were performed on a General Electric analytical mass spectrometer. I n tjhe hydrogen-deuterium run8 the pressure of tritium was so low that its presence did not siguificantly affect the :tiialyses for the other two isotopes. The reaction bulb was flamed 2nd sparked t o assist in degassing prior to the runs. The isotopic gases were purified on separate uranium beds. In the deuterium runs the tritium used (approsimately 937, tritium, 770 hydrogen) was analyzed beforehand and introduced to the reaction cell a t a low pressure. This pressure WRS then read on a sensitive McLeod gage. The first two hydrogen-deuterium runs were carried out in separate bulbs which were degassed simultaneously. The same batches of reactants and of tritium were used in these two runs. The third run and the hydrogen-tritium run were done a t separate times. I n the hydrogen-deuterium runs the progress of the reaction was followed for two or three days, during which time five analytical samples were t,alcen. The hydrogen-tritium run, at a much higher rndiation intensity, was followed for three hours.
Results and Discussion The data for each run gave good linear plots of the exponential equation HDm - HDt
=
HDme-kL
(1)
and the initial rates of formation of the mixed isotopic molecule were obtained from
(dq) =kHDm
t i 0
Studies of the eschange reaction of hydrogen and deuterium initiated by a-particles3s4and measurements of the hydrogen-tritium eschange5have demonstrated the chain-character of these reactions. Yields on the order of 2 X l o 3molecules/100 e.v. a t one atmosphere pressure and lower yields a t lower pressures have been obt'ained. The effect of minute quantities of oxygen in drastically lowering these yields has been sh0n.11.~ This note describes a brief continuation of these experiments in which yields as high as 2 x l o 4 molecules/100 e.v., a t only 270 mm. total pressure, have been obtained for the hydrogen-deuterinm exchange. These experiments, carried out in the absence of stopcock grease and witjh special care in heating and degassing the react,ion cell prior to the runs, emphasize the high chain-length of the reaction, and indicate that earlier rate measurements represent the rates for inhibited chains. Three i'uiis on the hydrogen-deuterium exchange, initiated by P-part,icles a t extremely low radiation intensities, were carried out at, room temperature. In these runs the source of radiat,ion, tritium gas, (1) General Electric Research Lalioratory, Sohenectady, N. Y .
(2) T h e Knolls Atomic Power Laboratory is operated b y the General Electric Company for the .4toniic Enerxy Commission. T h e work reported liere w a y carried o u t under Contract No. W-31-100Eng-52. (3) W. M u n d , T. dehlenten de Homes and 11. Van RIeemchc, BdZ. S O C . chim. Belg., 56, 386 (1947). (4) W. M u n d and RI. Vnn Meersclie, i b i d . , 57, 88 (1948). ( 5 ) L. R I . Dorfinan and H. C. RIntti,aiv, T H I SJ O U R N A L , 57, 723 (1953).
where (HD)b and (HD), are the concentrations a t time t and a t equilibrium, and k is a constant. These equations have been discussed p r e v i ~ u s l y . ~ The absorption of tritium P-particles in hydrogen has been measured6 and the fraction absorbed is known within a few per cent. a t the higher pressures and within about & l o % nt the lower pressures. The absorbed intensity is thus readily determinable from the measured concentration and the known nuclear constants for tritium: half-life 12.4 years,' average energy 5.G9 kev.* The results of the runs are given in Table I. The yields for the exchange reaction listed in Table I are substantially higher than those which have been obtained in any of the earlier ~ v o r k . ~ ~ ~ The yields for the hydrogeu-deuterium runs are approximately 20 times as high as those observed in the a-particle studies.4 The yield obtained in the hydrogen-tritium run is about 15 times as high as yields reported earlier.; .4 yield of approximately 4 X l o 4 for the hydrogen-deuterium eschange initiated by polonium a-pa,rticles has recently been ~ b t a i n e d . ~ The yields for the hydrogen-deuterium runs i n Table I increase with increasing pressure, consist(0) L. M. Dorfman, Phus. Reu., 95, 393 (1954). (7) G. H. Jenks, F. N. Sweeton a n d J. A. Gliormley, ibid., 8 0 , 990 (1950). ( 8 ) G. H . Jenks, J. A. Ghorinley and F. N. Sweeton, ibid., 7 6 , 701 (1949). (9) S. 0. Thompson a n d 0. A . Rchaeffer, private conimunicntion; J . Phus. Cliern., 23, 759 (1955).
