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Mechanistic studies on the Water Gas Shift reaction

The Water Gas Shift (WGS) reaction is important for the production of hydrogen that is used in hydrogen fuel cells. Carbon monoxide, even at very low concentrations, poisons the fuel cell electrodes so it must be removed. The WGS reaction is one way of doing this:

CO + H2O  = CO2 + H2

High activity catalysts based on gold have been developed using reducible oxide supports such as ceria-zirconia. Typical kinetic resutls are shown below:

 

The remarkable activity of gold when supported on a reducible oxide (CeZrO4) compared to platinum has led to a lot of research on gold catalysts to try to understand the high activity.

 

A combination of experimental work (structural, kinetic and spectroscopic studies) and density functional theory calculations has led us to propose a model for the active site wich is shown below:

 

In this model a gold "ion" is strongly attached to the support by being located at a cerium cation vacancy. This weakens the oxygen ion beside the cerium vacancy and creates a site for the activation of water to produce a surface OH group. This OH reacts with adsorbed CO to give CO2 and release hydrogen by the coupling of two hydrogen atoms on the metallic gold atoms in the gold nano-particle.  Other "ionic" models for the water gas shift reaction have been proposed but the complexity of the reaction would seem to require a more complex active site such as proposed in the model above.