Density functional theory study into H2O dissociative adsorption on the Fe5C2(0 1 0) surface

Authors:  Rui Gao, Dong-Bo Cao, Shaoli Liu, Yong Yang, Yong-Wang Li, Jianguo Wang, Haijun Jiao

Abstract: 
Spin-polarized density functional theory calculations (GGA–PBE) have been carried out to study H₂O adsorption and dissociation on the Fe₅C₂(0 1 0) surface. It is found that the iron region on the Fe₅C₂(0 1 0) surface is active for H₂O adsorption and dissociation, while the carbon region is inactive. For H₂O adsorption in the iron region, H₂O prefers the top site ofthe surface iron atoms, and significant hydrogen bonding interaction has been found at high H₂O coverage on the basis ofthe computed adsorption energies and the intermolecular O–H distances. In the iron region H2O dissociation (H₂O→H + OH; OH→H + O) is favored both kinetically and thermodynamically. On one O pre-covered surface, O-assisted H₂O dissociation becomes favorable kinetically (O + H₂O→OH + OH) and further OH dissociation (OH→H + O) becomes difficultthermodynamically. Upon the increase of surface O coverage, H₂O dissociation becomes difficult, while H2 formation from the surface adsorbed H atoms becomes easy. On the potential energy surface,the dissociation of four H2O molecules into four surface O and four H₂ molecules (4H₂O(g)→4O(s) + 4H₂(g)) is still thermodynamically favorable by 0.63 eV, and the iron region is fully covered by surface oxygen atoms. Thermodynamic analysis reveals clearly that the catalyst surface has always adsorbed oxygen atoms under water environment and their number in the iron region depends on temperature and water content; and high temperature and low H₂O partial pressure can maintain the catalyst stability and excess H₂O partial pressure will result in full oxidation. For the oxidation of one surface carbon atom, it is necessary to migrate one of the four adsorbed oxygen atoms from the iron region to the carbon region, and the H₂O assisted CO₂ formation is more favorable than the direct CO₂ formation. The overall surface carbon oxidation, FexCy + 4H₂O(g)→O₂FexCy–1 + CO₂(g) + 4H₂(g), is thermodynamically accessible. Detailed comparisons show that the Fe(1 0 0) and Fe₅C₂(0 1 0) surfaces are very similar in H₂O adsorption and dissociation at low coverage.

Keywords:
DFT
Iron carbide
H₂O dissociation
Surface oxidation

Published in: Applied catalysis A: General   (Volume 468, Pages 1-474, 5 November 2013)

Publisher: Elsevier  

ISSN Information: 0926-860X

Density functional theory study into H2O dissociative adsorption on the Fe5C2(0 1 0) surface

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