CARBON DEPOSITION OVER FE, NI, AND CO FOILS FROM CO-H2-CH4-CO2-H2O, CO-CO2, CH4-H2, AND CO-H2-H2O GAS-MIXTURES .2. KINETICS

Nonequilibrium C-H-O phase diagrams were used to investigate the effect of gas composition on the kinetics of carbon deposition over Fe, Ni, and Co foils at 900 K and 1 atm. The gas-phase carbon activity, which is proportional to the driving force for carbon deposition, was fixed at 10.0 for all experiments. The reduced metal foils were initially exposed to five-component gas mixtures of CO, H-2, CH4, CO2, and H2O, after which the gas composition was switched to a binary CO-CO2 or CH4-H-2 gas mixture, or a ternary CO-H-2-H2O gas mixture. On exposure to either of the binary gas pairs, the rate of carbon deposition over all three metals was found to be approximately the same (almost-equal-to 10* 10(-6) min-1), and the rate was not observed to be a function of the partial pressure of the carbon-bearing gas alone, either CO or CH4. In the case of Fe and Co, the rate of carbon deposition from the binary gas pairs dropped two orders of magnitude (2500* 10(-6) to almost-equal-to 10* 10(-6) min-1) and one order of magnitude (500* 10(-6) to almost-equal-to 10* 10(-6) min-1), respectively, compared to the rate from the five-component gas mixture. In the case of Ni, the rate of carbon deposition from the binary gas pairs was not observed to change from the rate of the five-component gas mixture (almost-equal-to 10* 10(-6) min-1). On exposure of Fe and Co foils to ternary gas mixtures, the rate changed proportionally to the product of the P(CO)P(H-2). Exposure of Ni foils to the ternary gas mixture had no effect on the rate of carbon deposition.