Carbon oxidation reactions at high temperatures

• Main Author: Egerton, Julian Robert
• Published: Imperial College London 1968
• Subjects: 542
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.623010

A study of the reaction of oxygen with pyrolytic carbon filaments at 50)1 pressure showed a well defined maximum in the rate and the hysteresis effect, in the temperature range 900-2000°C. The hysteresis changed sense at high temperatures, about 1500°C, giving hysteresis above in the same sense as found earlier at 0.76u. This change over in the hysteresis was shown to occur at lower temperatures for lower pressures. It was also shown that differences in the quantities of carbon reacted and in filaments used were not responsible for different hysteresis effects observed previously. The Nagle Strickland-Constable theory was found to give a considerably better prediction of the maximum and minimum if account was taken of the variation of active sites as found by the hysteresis results. A simple mechanism is proposed based on annealing and creation of active sites which gives a good fit to the hysteresis data at 50 and 0.76u. Reaction in N20 was not found to affect subsequent reaction in 02 and vice versa, suggesting different active sites are involved. Various additives were added to the filament. Boron and Silicon were found to be without effect, while Fe, W, Zr, U, Mo and Pt all catalysed the reaction. The basic mechanism of the reaction did not appear to be changed fundamentally by catalysis, maxima in the rates still being evident. There was also a strong suggestion of hysteresis. The oxygen balances suggested in most cases that lower oxides still existed on the surface, but at the carbon-metal interface it was thought likely free metal or carbides were the active agents. The experimental evidence strongly suggests an electronic mode of catalysis.