.

6.

Yu. V. Belokopytov, Yu. I. Pyatnitskii, and Yu. N. Grebennikov, "Programmed thermal desorption applied to the interaction of benzene with vanadium-molybdenum oxide cata- lysts," Teor. Eksp. Khim., 21, No. 2, 252-255 (1985). Yu. I. Pyatnitskii, "Gas-phase heterogeneous-catalytic oxidation of aromatic hydrocar- bons," Usp. Khim., 45, No. 8, 1505-1532 (1976).

THERMAL DESORPTION OF BROMINE FROM A GRAPHITE--BROMINE INTERCALATION COMPOUND

L. S. Lysyuk, K. N. Khomenko, and A. A. Chuiko

UDC 541.128+543.544

Gas chromatography and static adsorption have been used to examine the bromine-- graphite aceeptor system, which has intercalant structures comparable with the ad- jacentl graphite:layegs. The;bromine adsorption-~desorption isobars and isotherms have been determined at 278-393 K, whose shapes indicate reversible phase transi- tions between intercalation stages 2 and 4. Measurements at 390-550 K give the de- sorption activation energies for the two forms of intercalated bromine; the heat in- volved in the conversion of the ionized intercalant form to the molecular one is 10.4 kJ/mole.

The structural and energy characteristics for intercalated layers are important in elucidating the physicochemical parameters for graphite intercalation compounds, and particular interest attaches to the graphite-~romine system, where two-dimensional planar structures occur, whose parameters are comparable with those of the adjacent graphite layers [1-3]. The bromine-~raphite interaction is [4-6] dependent on the initial material and the reaction conditions. X-ray spectroscopy, electron microscopy, and diffractometry have been applied to the intercalation kinetics and the two-dimensional phase transitions in the layer [7-10]. Bromine desorption measurements have usually been made at room temperature over long periods (up to 20 days [9]) or on heatin~ ~elow 400 K [3, I0]. The desorption causes the intercalated- compound composition to vary from CsBr to C3sBr via the stages Cz~Br, C=IBr, C28Br [3, 9, i0], where the intercalant structure alters but the planar unit cell persists.

We have constructed isobars and isotherms for bromine adsorption and desorption in the graphite-4Dromine system at 278-393 K and have also examined the later stage of high-tempera, ture thermal desorption from the residual graphite-bromine compound. The measurements above 400 K were made in flowing helium by gas chromatography.

The initial GAK-2 graphite consisted of single-crystal flakes about 0.2 mm in size. The oxygen-bearing groups on the surface were removed along with volatiles by heating the specimens under vacuum at 1300 K. The bromination was from the vapor in a two-zone gas-transport system [ii]. The mass increment was measured with a McBenn--Bacra spring quartz balance. The reac- tion was performed under static conditions with relative pressure P/Ps = i.

The composition of the compound is determined by the temperature difference between the graphite and the intercalant AT = Tg - TBr (Fig. la). The sorption-desorption isobar is a hysteresis loop, with two steps on the curve, which correspond to reversible phase transitions between stages 2, 3, and 4 in the intercalation. The lower step corresponds to a compound containing about 5% bromine. This result agrees well with the [ii] data for highly oriented pyrolytic graphite.

The bromine vapor adsorption isotherm (Fig. ib) has a threshold pressure (P/Ps = 0.i), below which there is no intercalation. The resulting compound is close in composition to the saturation compounds CsBr but is unstable, so the adsorption isotherm showsa hysteresis loop. The absorbed bromine is not removed completely in a low-temperature adsorption--desorp- tion cycle; the residual compound contains about 30% absa~bed bromine and is reasonably stable and corresponds to the C~sBr in the intercalation stage 4.

Surface Chemistry Institute, Ukrainian Academy of Sciences, Kiev. Translated from Teoreticheskaya i ~ksperimental'naya Khimiya, Vol. 25, No. i, pp. 120a123, January-February, 1989~ original~article submitted January 5, 1987.

108 0040-5760/89/2501-0108512.50 �9 1989 Plenum Publishing Corporation

6:~ 8:" 4o "~-K --- C2::"

20 20

,, f I ~ . , , { r 1 , I i I I i

0 20 40 60 80 lO0 , d ~ K a

50

~0 ! -~ Br . . . . . . . . .

C2/Br

............. CZ8 Br

I ~ i , , I

0,2 o: o'8 ' 0,8 :/:, b

Fig. i~ Isobar (a) and isotherm (b) for sorption (open cir- cle) and desorption (filled circle) for bromine on GAK-2 graphite; AT = i0 K.

Measurements were made on the thermal desorption from the residual compound formed under vacuum at 298 K with a Tracor 570 chromatograph and thermal-conductivity detector. A weighed specimen was placed in a glass column and used with linear programmed heating from 313 to 550 K. Preliminary tests with the initial graphite (unbrominated) showed that there were no volatile products in that range. The heating rate was varied over the range 3-15 K/min and the peak temperatures Tm for desorption were determined, from which we calculated the de- sorption energies Ed by the [12] method from

21nTm--ln ~ = Ed/RT+In(Ed/AR ),

in which ~ is the heating rate, A the Arrhenius constant, and R the universal gas constant.

The peak numbers and shapes (Fig. 2) were dependent on the heating rate. Usually, the peaks become narrower and more symmetrical as ~ increases, but the graphite--bromine system shows the converse:~s peaks broaden at h~ghrates, evidently because certain products are released at similar temperture s as $ increases and are recorded as diffuse unresolved peaks (Fig. 2b). Table igivesbasic characteristics for the products from the residual ~raphite- bromine compound. Desorption activation energies are given for the two forms of intercalated bromine seen on the chromatogram (Fig. 2) as peaks i and 3. They agree well with the [13] data for graphitized carbon black, which implied a bromine desorption activation energy of 62-83 kJ/mole.

Electrical conductivity measurements in intercalated compounds [I, 6] indicate that the bromine in the present compound may be present either as neutral molecules or as ions: Br~'2Br2, with the intercalant anions oriented in accordance with the carbon macrocations [i]. At low temperatures, equilibrium is established in the electron transfer between the carbon macromolecules and the interchelated bromine. The equilibrium presumably shifts as the temperature rises, with some of the bromine ions going over to molecular form and being released. Some of the bromine ions are irreversibly bound to the graphite (possible at de- fect sites), and further heating partially destroys the surface compound.

Table i confirms this scheme indirectly. The peak area corresponds to the amount of material released, so the peak dimensions enable one to judge the size of the intercalant domain between the graphite planes. As a rule, the domain dimensions decrease as the in- dices in the intercalation stages increase. For the first five peaks in Table I, the ratios between the sizes, which are represented by ~, fall approximately in the sequence 4:2:4:2:1, and we assume that three types of intercalant domains are disrupted on desorption, which cor- respond to three successive intercalation:stages, with the physically bound molecular form of bromine being desorbed first as three successively decreasing peaks. Then the same domains release molecular bromine formed by high-temperature disruption of the ionized part. The first peak in this series appears to coincide in time and temperature with the last and weakest peak in the previous triad and conceals it. Then the actual peak size ratios may be close to 4:2:1:4:2:1. The last and strongest peak we assign to release of the compound of bromine with carbon (probably CBr4, boiling point 473 K). However, it was impossible to identify this compound under the conditions used.

The above desorption scheme suggests that peaks i and 3 (Fig. 2) characterize bromine release fmom one domain type. The difference between the forms lies in that peak 3 is mainly

109

l. mA f

/UL' ~-

]7---

f~

,/ j ! l ~ ' a

[ , \

�9 "'~, min ::' ":" "~"b ! t mxn

Fig. 2. Bromine thermal desorption chromatogram for the re- sidual graphite--bromine compound; heating rates in K/min: a) 5; b) 15.

TABLE i. Bromine Thermal Desorption Product Char- acteristics for the Graph- ite-Bromine Compound

Peak K/mxn) 1 peak area mole

1 383 25,4 60,3 2 429 13,4 -- 3 488 22.0 70,7 4 507 10.6 -- 5 516 2.0 -- 6 550 26,5 --

due to the high-temperature transition

BrF -+ Br 2 + e - + AQ.

Then the difference between the Ed for peaks i and 3 gives the heat of that reaction, i.e., el~ tron transfer from the ionized form of bromine to the graphite plane, which is 10.4 kJ/mole at 383-488 K.

LITERATURE CITED

I. A. R. Ubbelhode and F. A. Lewis, Graphite and its Crystalline Compounds [Russian transla- tion], Mir, Moscow (1965).

2. D. Chosh and D. D. L. Chung, "Two-dimensional structure of bromine intercalated graphite," Mater. Res. Bull., 18, No. 9, 1179-1187 (i983).

3. Ao Erbil, G. Dresselhaus, and M. S. Dresselhaus, "Raman scattering as a probe of structura phase transitions in the intercalated graphite--bromine sytem," Phys. Rev. B, 25, Noo 8, 5451-5460 (1982).

4. A. Herold, "Recherches sur les composes d'insertion du graphite," Bull. Soc. Chim. France, No. 7/8, 999-1013 (1955).

5. F. Bloc and Ao Herold, "Recherches sur les variations dimensionnelles des graphites arti- ficiels polycristallins lots de l'insertiondubrome," Carbon, 6, No. 6, 771-780 (1968).

6. M. S~ Dresselhaus and G. Dresselhaus, "Intercalation compounds:of graphite," Adv. Phys., 30, No. 2, 139-326 (1981).

7. A. Erbilo G. Timp. A. R. Kortan, et al., "Structure and phase transitions in bromine and potassium-mercury intercalated graphite," Synth. Met., [, ~73-281 (1983).

8. A. R. Kortan, A. Erbil, R. Jo Birgeneau, and M. S. Dresselhaus, "Commensurate-incommen- surate transition in bromine-intercalated graphite: a model strip domain system," Phys. Rev. Lett., 49, No. 19, 1427-1430 (1982)o

9. D. Chosh and D. Do L. Chung, "Effect of intercalate desorption on the two-dimensional structure of graphite-bromine," Synth. Met., ~, 283-288 (1983).

i0. K. K. Bardhan, "Commensurate-incommensurate transition and disordering in dilute graphite- bromine," Solid State Commun., 44, No. 5, 583-586 (1982).

ii. G. Furdin, "Etude thermogravimetrique et dilatometrique du systeme pyrographite HOPG-Brome Synth. Met., 2, 101-166 (1983).

12. R. J. Cvetanovic and Y. Amenomiya, "Application of a temperature-programmed desorption technique to catalyst studies," Adv. Catal., 17, 103-149 (1967).

13. A. Marchand, J. C. Ronillon, and F. C. ' D Arcollieress, "Etude des cinetiques successions d'insertion et de d6sorption du brome dans un pyrocarbone," Carbon, l_!l, NO. 2, 113-126 ( 1 9 7 3 ) .

Ii0