Density functional theory -DFT- calculations were used to study the mechanism of water gas shift -WGS- reaction on Ni -111-surfaces. Three sets of elementary reactions based on the formate intermediate and oxidation-reduction mechanisms are consid-ered in this study. Formate intermediate is produced via dissociation and non-dissociation adsorption of H2O in the proposedmechanisms. The adsorption energy for all surface species and the activation barriers for the rate-determining steps in these WGSmechanisms were calculated. The overall reaction rates were developed based on the considered mechanisms. Based on theSabatier principle, the effects of CO and H2O adsorption energies on the activation energy of the rate-determining reactions in theproposed mechanisms are considered. According to the calculated overall activation energies, the formate intermediates pro-duced from the reaction between adsorbed H2O and CO species provide the best condition for the overall WGS reaction.

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