Carbon deposition over Fe, Ni, and Co foils from CO-H 2-CH 4-CO 2-H 2O, CO-CO 2, CH 4-H 2, and CO-H 2-H 2O gas mixtures: II. Kinetics

Abstract

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 gasphase 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, CH 4, CO 2, and H 2O, after which the gas composition was switched to a binary CO-CO 2 or CH 4-H 2 gas mixture, or a ternary CO-H 2-H 2O 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 ( ≈10 ∗ 10 −6 min −1 ), and the rate was not observed to be a function of the partial pressure of the carbonbearing gas alone, either CO or CH 4. 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 ≈10 ∗ 10 −6 min −1 ) and one order of magnitude ( 500 ∗ 10 −6 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 ( ≈10 ∗ 10 −6 min −6 ). On exposure of Fe and Co foils to ternary gas mixtures, the rate changed proportionally to the product of the P COP H2. Exposure of Ni foils to the ternary gas mixture had no effect on the rate of carbon deposition.