THE DUAL NATURE OF CARBON - CATALYST AND INHIBITOR

The nature of carbon in the form of thin films as an inhibitor and as a catalyst was studied using the conversion of methane as a test react ion. The carbon films were formed by pyrolysis of either propylene (C( p)) or methane (C(m)) in a quartz reactor. The conversion reaction was performed using methane pressures ranging from 20 to 641 Torr (3 to 8 5 kPa) and in the temperature range of 600 to 750-degrees-C. The resul ts showed that small amounts of carbon, in the neighborhood of 1/10 of a monolayer, caused inhibition of the rate of conversion of methane; but much larger amounts, corresponding to films of 50-100 nm thickness , caused a significant acceleration of the rate. On such films formed from propylene, the rate was 40 times greater than the rate on a quart z surface, and on the films formed from methane, the rate was about ei ght times that on quartz. When short, consecutive reactions were perfo rmed on both carbon films without removal of deposited carbon, the rat e decreased with each successive experiment. This decrease was attribu ted to the inhibiting effect of the very small amounts of carbon forme d in each reaction. The activation energy of the rate of decomposition of methane over the carbon films, 303 kJmol-1, was lower than the val ue observed for the homogeneous reaction, although the product distrib ution was similar. It was concluded that surface processes play an imp ortant role in the reaction of methane in the presence of carbon.