CO dissociation on iron nanoparticles: size and geometry effects.

The reactivity of 0.5-1.4 nm iron nanoparticles and corresponding bulk surfaces has been systematically studied using density functional theory. The study includes both ideally symmetric and more realistic rugged nanoparticles. The activation energies for CO dissociation vary between 1.1 and 2.1 eV. An increasing particle size and roughness result in lower activation energies. For a single particle, variations as large as 0.9 eV occur indicating the importance of local particle morphology. Depending on the nanoparticle size, geometry, and CO dissociation pathway the reaction rates span five orders of magnitude at conditions relevant for gas-phase chemical vapor deposition synthesis of carbon nanotubes. Studies on the smallest particles and bulk surfaces show that these systems cannot be used as reliable models for catalysis on larger iron nanoparticles. It has also been demonstrated that predictive d-band and linear-energy relationships cannot be used to explain the reactivity of iron for CO dissociation as reaction mechanisms vary from one particle to another. The changes in reaction mechanisms can be rationalized by the varying Fe-Fe bond lengths in different particles leading to changes in back-bonding between the iron surface and CO. CO dissociation on nano-sized iron seems to be more complex than that seen on more conventional non-magnetic noble metal particles.

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