Kinetics and mechanism of carbon film deposition by acetylene pyrolysis

The pyrolysis of a mixture of 2.5% acetylene in helium in a flow system with and without added hydrogen has been studied between 1100 and 1250 K, under conditions favoring carbon film deposition. The rate of carbon deposition was measured continuously during pyrolysis as a function of the increasing conductivity of the growing film, while the effluent gas was analyzed for ethane, ethylene, and acetylene. An activation energy of 278 kJ/mol was obtained for carbon deposition. While an increase in the partial pressure of hydrogen in the input gas is accompanied by an increase in the concentration of ethylene in the effluent, no corresponding decrease in acetylene concentration is observed; the ratio of output acetylene to input remaining unchanged regardless of the amount of hydrogen added. Acetylene consumption is dependent solely upon residence time in the reaction zone, indicating a zero-order process. The rate of carbon deposition is inversely related to the hydrogen partial pressure. The isolation of carbon and ethylene formation from acetylene consumption is consistent with a process involving surface saturation. A mechanism is proposed involving a slow reaction at active sites, by which acetylene is chemically adsorbed as surface species which then serve as the common source of carbon, ethylene, and other pyrolysis products. Active sites are regenerated in the process, so that the total concentration of active sites remains constant.