Adsorption and energetics of H2O molecules and O atoms on the χ-Fe5C2 (1 1 1), (−4 1 1) and (0 0 1) surfaces from DFT

• Most, medium and least stable Fe5C2(1 1 1), (−4 1 1) and (0 0 1) surfaces are used.
• Adsorption of H2O molecules and O atoms was calculated at high coverage.
• Surface stability correlates with the adsorption energies of H2O and O.
• High coverage H2O prefers hydrogen bonding instead of adsorption on iron.
• The number of surface O atoms depends on temperature and H2O/H2 ratio.

The adsorption of H2O molecules and O atoms on the most (1 1 1), medium (−4 1 1) and least exposed (0 0 1) surfaces of χ-Fe5C2 has been investigated with spin-polarized density functional theory (GGA-PBE) calculations. The computed adsorption energies of H2O molecules and O atoms correlate with the surface stability, strongest on the least stable (0 0 1) surface and weakest on the most stable (1 1 1) surface. At high coverage, the H2O molecules are floating over the surface via hydrogen bonding, and the hydrogen bonding interaction is stronger than H2O adsorption on the surface iron atoms, as revealed by the computed average and stepwise adsorption energies. On the basis of the computed adsorption energies of O atoms, atomistic thermodynamic analysis reveals that these surfaces can have adsorbed oxygen atoms under water environment and the number depends on temperature and water content, high temperature and low H2O partial pressure can maintain the catalyst stability and excess H2O partial pressure will result in full oxidation of catalyst surfaces. The number of surface O atoms is the largest on the least stable (0 0 1) surface, and smallest on the most stable (1 1 1) surface under the same condition.

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