Spin-polarized density functional theory calculations were performed to investigate CO dissociation on clean and hydrogen precovered Fe(111) at 1/3 monolayer coverage. On clean Fe(111), the adsorbed CO first diffuses from the shallow-hollow site to the bridge-like site by elevating 0.20 eV in energy, and then dissociates into C and 0 atoms by overcoming a barrier of 1.53 eV. Interestingly, the CO dissociation process is accelerated in the presence of H-2 via intermediate CHOads (COads + 2H(ads) -> CHOads + H-ads -> CHads + Oads + H-ads). This stepwise path is kinetically more favored with the lowest barrier of 1.17 eV. In contrast, the previously suggested paths, COads + 2H(ads) -> C-ads + O-ads + 2H(ads) and COads + 2H(ads) -> C-ads + OHads + H-ads, are not competitive due to higher barriers (1.76 and 1.79 eV, respectively). The activity of different low-index Fe surfaces toward CO is also compared. (c) 2007 Elsevier Inc. All rights reserved.