The dual nature of carbon: Catalyst and inhibitor

Abstract

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 reaction. 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 85 kPa) and in the temperature range of 600 to 750°C. The results 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 quartz surface, and on the films formed from methane, the rate was about eight times that on quartz. When short, consecutive reactions were performed on both carbon films without removal of deposited carbon, the rate decreased with each successive experiment. This decrease was attributed to the inhibiting effect of the very small amounts of carbon formed in each reaction. The activation energy of the rate of decomposition of methane over the carbon films, 303 kJmol −1, was lower than the value observed for the homogeneous reaction, although the product distribution was similar. It was concluded that surface processes play an important role in the reaction of methane in the presence of carbon.