Competitive adsorption of CO and H<sub>2</sub> on strained Fe(110) surface

Abstract

In this work, the competitive adsorption behavior of H<sub>2</sub> and CO on strained Fe(110) are investigated by the first-principles method based on the spin-polarized density functional theory to study the hydrogen embrittlement of steels. The results show that the most stable adsorption site for CO is top site, and the orbital of CO molecule hybridizing with Fe 3p and 4s states illustrates a strong electronic interaction between them. The adsorption energy values of CO at the four calculated adsorption sites are more negative than those of H<sub>2</sub>, which favors the binding with Fe(110) surface. The potential energy variations for CO and H<sub>2</sub> molecules close to the surface are calculated. The attractive force of the Fe(110) surface acting on CO in 1.5–3 Å is greater than that acting on H<sub>2</sub>. The pre-adsorbed CO increases the dissociation energy barrier of H<sub>2</sub> from 0.08 eV to 0.13 eV but reduces the force between H<sub>2</sub> and surface. The surface tensile strain enhances the interaction between hydrogen and Fe(110), which, however, is reduced by the compressive strain. The opposite tendency is found in the adsorption of CO. The binding strength of CO is stronger than that of H<sub>2</sub> on the strained Fe(110) surface. The difference in adsorption energy between CO and H<sub>2</sub> decreases with tensile strain increasing. The effect of surface strain and partial pressure of CO gas phase on the surface coverage ratio of H atom are also calculated quantitatively based on thermodynamics at 298 K, with the partial pressure of H<sub>2</sub> set to be 10 MPa. The surface ratio of the H atom decreases with partial pressure of CO increasing. The hydrogen coverage drops nearly to zero when the partial pressure of CO reaches a certain value. This result reveals that CO can inhibit hydrogen adsorption on Fe surface. In the case where the surface ratio of hydrogen decreases to 1%, the corresponding CO partial pressures are 105 Pa, 1.1 × 10<sup>3</sup> Pa, 2.4 × 10<sup>5</sup> Pa on –2%, 0, 2% strained Fe(110) surface, respectively. High CO partial pressure is needed to suppress the hydrogen adsorption since the binding strength of CO is close to that of H<sub>2</sub> on the expanded surface.