Article,
Facile dissociation of CO on Fe2�1�1: Evidence from microcalorimetry and first-principles theory
Surface Science, 602 (13): 2325 - 2332 (2008)
DOI: DOI: 10.1016/j.susc.2008.05.014
Abstract
Chemisorption of CO on the Fe2�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 Fe1�0�0
and Fe1�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 Fe2�1�1, with a dissociation
barrier noticeably lower than that for CO on Fe1�0�0 or Fe1�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 Fe2�1�1 are nearly
optimal for Fischer-Tropsch catalysis.
