Facile dissociation of CO on Fe{2Â 1Â 1}: Evidence from microcalorimetry and first-principles theory

Chemisorption of CO on the Fe{2 1 1} surface is studied within first-principles density functional theory (DFT) and single-crystal adsorption calorimetry (SCAC). The most stable molecular adsorption state corresponds to CO bound in a three-fold site involving one metal atom from the top layer and two metal atoms in the second layer. In this configuration, CO is tilted and elongated with a considerably red-shifted stretching frequency (calculated to be 1634 cm−1 as opposed to 2143 cm−1 for gas-phase CO). This state is very similar to that of CO on Fe{1 0 0} and Fe{1 1 1}, which is believed to be a precursor state to dissociation at relatively modest temperatures. However, dissociation of CO is found by DFT to be particularly facile on Fe{2 1 1}, with a dissociation barrier noticeably lower than that for CO on Fe{1 0 0} or Fe{1 1 1}. The 300 K coverage-dependent calorimetric data is consistent with either molecular or dissociative adsorption, with an initial adsorption heat of 160 kJ/mol. At higher coverages, the heat of adsorption and sticking probability data exhibit a forced oscillatory behaviour, which can be explained by assuming CO dissociation and subsequent diffusion of atomic carbon and/or oxygen into the substrate. It is argued that conditions for CO dissociation on Fe{2 1 1} are nearly optimal for Fischer-Tropsch catalysis.