Sept.. 1060
HYDROGEN SORPTION
OX
GRAPHITE A T E L E V A T E D TEMPERATURES
1093
HYDROGEN SORPTIOK ON GRAPHITE A T ELEVATED BY J. P. REDMOND AND 1'. L. W..ZLHER, JR. Lk paitmcnt of Fuel Technology, T h e Pennsylvania State Uniz~ersity,University Park, Pennsylvania Received March 7 . 1980
The sorption of hydrogen on selected types of nuclear and spectroscopic graphite has been studied. On nuclear graphite, the desorption of hydrogen over the temperature range 1035 to 1375' follows the Elovich equation. The activation energy of desorption over the coverage range (e), 0.32 to 0.68, can be approximated as E' = 137 - 42(8) kcal./mole. On the basis of limited results, it is found that the adsorption of hydrogen a t 1835" on nuclear graphite can also be expressed by the Elovich equation. Isotherms for sorption of hydrogen on both nuclear and spectroscopic graphites over the temperature range 920 to 1495' are of the Temkin-type. However, a t temperatures of 1085' and above, the isotherms consist of two straight-line regions of markedly different slopes. From the isotherms a t 1335 and 1495' for the nuclear graphite, the heat of adsorption over the coverage range, 0.50 to 0.80, can be approximated as Q = 58 - 56(0) kcal./mole. It is concluded that the adsorption of hydrogen occurs on carbon atoms a t the edge of crystallites and that significant intracrystalline sorption does n > t occur.
Introduction drtificial graphite of use in graphite-moderated nuclear reactors, as anodes for mercury vapor rectifiers, and crucibles for vacuum fusion work should contain little gas. Otherwise, when the graphite is heated to operating temperatures, the gas desorbs from the graphite result.iiig in adverse effects. For example, in the graphite-moderated nuclear reactor, hydrogen and carbon monoxide released from the graphite can interact with the fuel cladding material (usually zirconium or stainless steel) resulting in hardening and embritt'lemeiit of the metal. Released carbon monoxide can disproportionate on the cooler metal heat exchanger surfaces depositing carbon,3 resulting in an adverse effect on the heat transfer and :low properties of the system. A number of ~ o r k e r s ~have - ~ determined the amount and analysis of gases which are desorbed from graphite and amorphous carbons upon heating in vacuo. 111 general, they report that hydrogen and carbon monoxide are the major constituents of the released gas (especially a t the higher temperatures), wit'h Significant amounts of carbon dioxide and water vapor released at the lower temperatures. In some case!;, hydrocarbons also are released. Little definitive work is available on the rates of sorption (adsorption and/or desorption) of gases from graphit 3. Eltzin and Jewlew9 conclude from rat,e measurements t,hnt the evolution of gas from graphite consists of tlw first-order processes-desorption from the external surface and diffusion from the interior of t,hc graphite structure to the surface. Barrer,10 d i o s e work will he discussed (1) Based on a Ph.D. thesis submitted by J. P. Redmond t o thR Graduate SCh3ol ,f T!ie I'rnnaylvania State University, June, 1959. ( 2 ) ThiJ paper presents t h e results of o n e phase of research carried oiit under C o n t r t c t S o . 4T:XO-1)-1710, sponsored by t h e Atomic Energy Commissim. (8) P. L. \Talher. J r . , .I. 1'. Rakszawski and Q. R. Imperial, THIS JOUHYAL 63, , 133 (lQ.59). (4) P. Lebeau imd 21. Picon, Compl. rend.. 179, 264 (1924). ( 5 ) E. J. Norton and A . L. Marshall, Trans. Am. Inst. Mining Met. Engrs., 166, 351 (1914). (ti) R. L. C n r t t r a n d R. R. Eggleston. Proceedings of t h e First a n d Second Carbon Ginference, U. of Buffalo. S. 'IT.. pp. 149-153, 19.56. (7) R. 13. Andemon and P. H. E m m e t t , Tim J O U R N A L61, , 1308 (lQ47). (8) R. B. A n d e w m and P. H. E m m e t t , ibid., 66, 753 (1952). (9) I. A. E l t a i r . a n d A. P. Jewlew, P h y s i k . 2. Sowjetunion, 6, 687 (1934).
(IO) R. h l . Barrer, J . Chem. Soc., 1266 (1936).
later, studied the rate of hydrogen adsorption on graphite and diamond. In recent years, many workers have found the equation
to be applicable to their adsorption data for a wide variety of systems, where dq/dt is the rate of adsorption and a and d are constants. Equation 1, which is now commonly called either the Elovich or Roginsky-Zeldovich equation can be derived theoretically on the basis of a linear incresse in activation energy of adsorption with increase in surface coverage.l' As will be seen, equation 1 will be applicable in the present studies. In the present studies, the rate of hydrogen adsorption and desorption from graphite has been investigated. Also adsorption isotherms for hydrogen on graphite have been obtained. Hydrogen has been chosen for the initial study since it is the major constituent released from nuclear and spectroscopic graphites a t elevated temperatures. Additional work is in progress studying the sorption of carbon monoxide and mixtures of hydrogen and carbon monoxide on graphite. Experimental Apparatus.-The graphite samples used in this study were cylindrically shaped, 1 in. long by 1/2 in. in diameter. A 0.035 in. diameter hole, 1/8 in. deep, was drilled into one end of each sample. The sample was supported by a tungsten wire (0.028 in. in diameter and 2.5 in. long) which was inserted into the hole in the graphite sample and in turn cemented to a 7/8 in. diameter mullite base. The base supporting the sample sat in the bottom of a 1 in. diameter quartz tube which served a~ the sorption chamber. A quartz window waa sealed to the bottom of the quartz tube and a standard taper connected to the top. A Pyrex section containing a mating standard taper and an optical window connected the quartz tube to the pumping and measuring system. The pumping system consisted of a liquid nitrogen cold trap and a two stage Van Hespen mercury diffusion pump. The evolved gases were removed continually from the desorption chamber during a run, with the pump capable of operating against a back pressure of ca. 10 mm. The evolved gases not condensed in the liquid nitrogen trap (including all of the hydrogen) were pumped into a reservoir consisting of two 2 liter bulbs and 1.22 liters of connecting tubing. At intervals, a small fraction of the accumulated gas was bled through a Knudsen leak into a mass spectrom(11) B. M. W. Trapnell, "Chemisorption," Butterworth Scientific Publications, London, 1955, pp. 103-106.
J. P. REDMOXI) .GI) P.I,. WALKER, JR.
1094
TIME m m u l l l
Fig. 1.-Rate of hydrogen desorption from virgin nuclear giaphite (TSP) a t different temperatures. r
-
63- 4 8 C
1-
66-
-
-
1
-77
~
7
r
1-01. 61;
made. With a reservoir volume of 5.2 liters, there RBS not a large pressure change during a typical run. At the conclusion of a run, the sample was cooled quickly t o room temperature (by turning off the induction power supply) and the hydrogen remaining in the gas phase was evacuated. The sample was then reheated to 2000°, the quantity of desorbed hydrogen measured and compared with the smount calculated to have been adsorbed. The agreement, for the run reported, was within 5%. Procedure for Measuring Adsorption Isotherms The graphite was cleaned by degassing at 2000°, prior to cooling to room temperature. A known pressure of hydrogen was admitted, the sample heated quickly to a predetermined temperature, and the sample held a t this temperature for a t least one hour. The sample was quickly cooled to room temperature and the hydrogen remaining in the gas phase removed. The sample was again heated to 2000” and the amount of hydrogen desorbed determined. Isotherms were obtained by repeating this procedure for various pressures of hydrogen a t a constant temperature. Description of Graphites Used.-One grade of nuclear graphite (TSP) and one grade of spectroscopic graphite (AGKSP) manufactured by the National Carbon Company were used in this study. The raw materials used for the production of the graphites were petroleum coke and coal tar pitch. A detailed description of the manufacture and general properties of these graphites can be found elsewhere.12-14 Table I lists some selected properties for thp graphites. TABLE
I
SELECTED PROPERTIES O F THE GRAPHITES yGraphite-Property TSP AGKSP
FIGURE
2
Fig. 2.-Elovich plots for hydrogen desorption from virgin nuclear graphite (TSPj a t different temperatures. eter to determine the hydrogen pressure. With the leak used, the maximum pressure which could be measured by the mass spectrometer was ca. 200 p . Power to heat the graphite sample was supplied by a 5 Kw. high-frequency induction generator. A “radiamatic” pyrometer located above the optical window in the sorption chamber sighted on the graphite sample, with its signal being fed into a recorder-controller. The output from the controller was fed in turn to a saturable reactor which gave a two-position control action for the induction generator. This regulation system controlled the temperature to fIO”. The “rzidiamatic” pyrometer did not indicate the “true” temperature of the graphite sample because the target was too small to be focused properly on the thermopile. Therefore, this temperature was read by a disappearing filament optical pyrometer. In turn, this temperature was corrected for the emissivity of the graphite and absorptivity of the optical window t o yield the true temperature. Procedure for Measuring the Rate of Desorption of Hydrogen from Graphite.-After placing the graphite sample in the sorption chamber, the apparatus was outgassed a t room temperature for a t least 12 hours. A run was initiated when the induction generator was turned on and the graphite heated to a predetermined temperature. A4t regular intervals, $1, sample of the accumulated gas was bled into the mass spectrometer for hydrogen analysis. The lag time between the release of the hydrogen from the graphite sample and the recording of desorbed hydrogen by the mass spectrometer was ca. 10 seconds. Procedure for Measuring the Rate of Adsorption of Hydrogen on Graphite.-The graphite was cleaned by degassing i t at 2000” for about 30 minutes, prior to cooling the sample to room temperature under vacuum. A known pressure of prepurified hydrogen (less than 200 p ) was admitted to the system a t room temperature and the graphite sample rapidly heated to the adsorption temperature. The decrease in pressure of hydrogen with time was followed by introducing some gas into the mass spectrometer a t known time intervals. A correction for the decrease in pressure caused by loss of hydrogen to the mass spectrometer was
BET surface area, m.Z/g. Apparent density, g./cc. True density, g./cc. Porosity, % Total ash content, 70
0 30 1 ’io 2 26 25 2 0 004
0 40 1 56 2 26 31 2
