Detailed Kinetic Modeling of Iron Nanoparticle Synthesis from the Decomposition of Fe(CO)5

A detailed chemical kinetic model for gas-phase synthesis of iron nanoparticles is presented in this work. The thermochemical data for Fen clusters (n ≥ 2), iron carbonyls, and iron-cluster complexes with CO were computed using density functional theory at the B3PW91/6-311+G(d) level of theory. Chemically activated and fall-off reaction rates were estimated by the QRRK method and three-body reaction theory. Kinetic models were developed for two pressures (0.3 and 1.2 atm) and validated against literature shock-tube measurements of Fe concentrations and averaged nanoparticle diameters. The new model indicates that the nanoparticle formation chemistry is much more complex than that assumed in earlier studies. For the important temperature range near 800 K in a CO atmosphere, the Fe atom formation and consumption are largely controlled by the chemistry of Fe(CO)2, especially the reactions Fe(CO)2 ⇌ FeCO + CO, Fe + Fe(CO)2 ⇌ Fe2CO + CO, and Fe(CO)2 + Fe(CO)2 ⇌ Fe2(CO)3 + CO. The decomposition of Fe(CO)5 is restricted by the rate of the spin-forbidden reaction, Fe(CO)5 ⇌ Fe(CO)4 + CO. This model facilitates the understanding of how the reaction conditions affect the yield and size distribution of iron nanoparticles, which will be a crucial aspect in the gas-phase synthesis of carbon nanotubes.