Water-Gas-Shift Reaction on Molybdenum Carbide Surfaces:  Essential Role of the Oxycarbide

Density functional theory (DFT) was employed to investigate the behavior of Mo carbides in the water-gas-shift reaction (WGS, CO + H2O → H2 +CO2). The kinetics of the WGS reaction was studied on the surfaces of Mo-terminated Mo2C(001) (Mo−Mo2C), C-terminated Mo2C(001) (C−Mo2C), and Cu(111) as a know...

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Published in:The journal of physical chemistry. B 2006-10, Vol.110 (39), p.19418-19425
Main Authors: Liu, Ping, Rodriguez, José A
Format: Article
Language:English
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Summary:Density functional theory (DFT) was employed to investigate the behavior of Mo carbides in the water-gas-shift reaction (WGS, CO + H2O → H2 +CO2). The kinetics of the WGS reaction was studied on the surfaces of Mo-terminated Mo2C(001) (Mo−Mo2C), C-terminated Mo2C(001) (C−Mo2C), and Cu(111) as a known active catalyst. Our results show that the WGS activity decreases in a sequence:  Cu > C−Mo2C > Mo−Mo2C. The slow kinetics on C−Mo2C and Mo−Mo2C is due to the fact that the C or Mo sites bond oxygen too strongly to allow the facile removal of this species. In fact, due to the strong O−Mo and O−C interactions, the carbide surfaces are likely to be covered by O produced from the H2O dissociation. It is shown that the O-covered Mo-terminated Mo2C(001) (O_Mo−Mo2C) surface displays the lowest WGS activity of all. With the Mo oxide in the surface, O_Mo−Mo2C is too inert to adsorb CO or to dissociate H2O. In contrast, the same amount of O on the C−Mo2C surface (O_C−Mo2C) does not lead to deactivation, but enhances the rate of the WGS reaction and makes this system even more active than Cu. The good behavior of O_C−Mo2C is attributed to the formation of a Mo oxycarbide in the surface. The C atoms destabilize O-poisoning by forming CO species, which shift away from the Mo hollow sites when the surface reacts with other adsorbates. In this way, the Mo sites are able to provide a moderate bond to the reaction intermediates. In addition, both C and O atoms are not spectators and directly participate in the WGS reaction.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp0621629