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Carl R. F. Lund, University at
Buffalo, SUNY
Fe3O4-based
High and Low Temperature Shift Catalysts
Abstract
Density functional theory and kinetic modeling were combined to probe
the mechanism of water-gas shift (wgs) over catalysts based upon iron
oxide. The heat of formation of an adsorbed oxygen atom from kinetic
modeling was compared to the values computed for {100}, {110} and {111}
surfaces using DFT. The results for the {111} surface displayed the
best agreement. DFT was then used to compute the structure and heat of
formation of several possible wgs intermediates on the {111} surface of
magnetite. Two commonly-cited pathways for shift were studied: a redox
pathway and a formate pathway. Mechanistic kinetic modeling showed that
for each of the pathways there are at least two non-equivalent sets of
kinetic parameters that lead to equally good fits to experimental data.
Overall it was found that the redox pathway was most consistent with
available data. When the catalyst was promoted with copper, the
severity of inhibition by carbon dioxide decreased. Mechanistic
modeling showed that the kinetic parameters changed only slightly upon
promoting the catalyst with copper. Combining this observation with DFT
results for promoted catalysts suggests that substitutional copper, and
not surface copper atoms or clusters, is responsible for the
promotional effects. When gold is added to the catalyst it is found to
be active at very low temperature. Kinetic modeling reveals that in
this case the mechanistic pathway is not the same as for the higher
temperature catalysts.
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