A study of the reaction of carbon with sulphur vapor was undertaken in order to bring further information about the mechanism of carbon gasification at high temperatures (lOI%2000°C) and very low pressures (lo-'-lo-' Torr).
The first part of this paper is concerned with the temperature dependence of the reaction kinetics.
Experiments were done with graphite, pyrocarbon, vitreous and amorphous carbon samples. Since the molecular composition of sulphur vapor cannot be determined, there is some uncertainty about pressure values and consequently about rates expressed as collision yield F.
Carbon disulfide CS2 is the main primary reaction product. However, some monosulfide CS is formed, perhaps partly from a secondary reaction.
Amorphous carbons are much more reactive than graphitized carbons, and show gasification rates as high as those obtained for oxygen. Kinetic investigations show that similar surface phenomena take place which have been previously observed in other carbon gasification reactions: intrinsic reactivity of a carbon surface is dependent upon temperature. Changes in intrinsic reactivity do not appear experimentally instantaneously: after a temperature change the new characteristic rate of reaction-the stationary rate-is attained only after a transient period (Fig. 5).
These so-called hysteresis effects show that temperature increase contributes to carbon deactivation. The changes in intrinsic reactivity (concentration of reactive centers) explain the unusual profiles of the F(T) curves where stationary collision yields are plotted as a function of temperature (Figs. 2 and3): they all exhibit a maximum at about 1600°C.
These results generalise the well-known kinetic features of the reactions of carbon with oxygenated compounds. Following Duval's explanation such a behaviour must be related to the lability of the reactive centers present on the carbon surface which disappear at very high temperatures via a superficial annealing process.
A further complication is that another surface phenomenon occurs for the non-graphitized samples: namely a very slow but continuous decrease in reaction rates (Fig. IO) and a progressive flattening of F(T) curves (Figs. 9 and 10). Elimination of the superficial layer of such a "degenerate" carbon sample, by means of carbon evaporation or carbon gasification with oxygen, restores the characteristics kinetic features as observed initially for any new sample (Fig. 9). This behaviour seems to be due to an irreversible change in the carbon reactivity itself rather than to a poisoning by sulphurised surface compounds.