Coadsorption of CO and H₂ on an Iron Surface and Its Implication on the Hydrogen Embrittlement of Iron

We investigated the competitive coadsorption of carbon monoxide and hydrogen gas on an iron surface with a 110 facet using density functional theory. Our study discusses the hydrogen dissociation reaction on a fresh iron surface and a surface with varying carbon monoxide coverage. Additionally, we investigated the carbon monoxide surface adsorption as a function of the carbon monoxide surface coverage. Our results show different trends for the carbon monoxide adsorption and hydrogen dissociation on surfaces with low and high CO coverage. Those opposite trends were related to the charge of the surface iron atoms and the available surface electron density which is necessary to facilitate the carbon monoxide adsorption and catalyze the hydrogen dissociation reaction. The subsurface diffusion of predissociated surface hydrogen atoms has been included in the model. It was found that the atomistic hydrogen diffusion into the material is also related to the carbon monoxide surface coverage. Our theoretical results confirmed that a small amount of carbon monoxide as an impurity in the hydrogen gas can mitigate the effect of hydrogen embrittlement by significantly reducing the rate of hydrogen dissociation on the iron surface and thus reduce the hydrogen uptake into the bulk of the material. To verify the theoretical results, we carried out a fracture toughness test of pure iron in a high-purity H₂, CO and H₂ mixture, and N₂ gases. This material suffered from hydrogen embrittlement, in other words, reduction in the fracture toughness due to hydrogen. We could derive the complex dependence on the hydrogen embrittlement manifestation as a function of the H₂/CO gas mixture ratio and gas exposure time.