Kinetics and thermodynamics of intercalation of bromine in graphite-I. Experimental

The intercalation of bromine in highly oriented pyrolytic graphite was performed at various combinations of temperature T (20-160°C) and bromine vapor pressure P (0.01-20 atm). The phase diagram (logP vs 1 T) for the formation of various stages showed that the enthalpies of reaction were -10.0, -10.8 and -11.0 kcal mol^-1 Br₂ for stages 2, 3 and 4, respectively, and that the entropies of reaction were -29.9, -30.8 and -30.6 cal mol^-1 Br₂ K^-1 for stages 2, 3 and 4, respectively. The progress of intercalation was monitored in situ by optical measurements and ex situ by gravimetry, X-ray absorption and X-ray diffraction. The kinetic data yielded the first time-temperature-transformation diagram for intercalation. In particular, the rate of formation of stage 2 or 3 was found to first increase and then decrease as the reaction temperature increased and the thermal phase boundary (70°C for stage 2, 120°C for stage 3) was approached. In addition, the intercalation rate was determined as a function of the bromine potential, thereby yielding a time-concentration-transformation diagram.