Non-additive effects of CO and H2O impurities on H2 dissociation and subsurface diffusion on Fe(110): A first-principles study

Impurities such as CO and H 2 O are inevitably present in hydrogen transport pipelines and can strongly influence hydrogen uptake and embrittlement of structural steels. However, the fundamental mechanisms by which mixed trace gases jointly regulate hydrogen dissociation and subsurface diffusion on iron surfaces remain unclear. In this study, first-principles calculations were performed on the low-energy Fe(110) surface to elucidate the individual and combined effects of CO and H 2 O pre-adsorption on H 2 activation and permeation. Results demonstrate that CO exhibits strong chemisorption and substantially increases the H₂ dissociation barrier, whereas H 2 O interacts weakly with Fe(110) and does not hinder H₂ activation. In the CO–H₂O co-adsorption system, H 2 O partially mitigates CO’s inhibitory effect, reducing the dissociation barrier from 0.10 to 0.07 eV. Hydrogen diffusion displays pronounced spatial heterogeneity: barriers decrease near H 2 O but increase near CO. This heterogeneity becomes more extreme under co-adsorption, with barriers dropping to 0.68 eV near H 2 O, accelerating hydrogen dissolution by over three orders of magnitude. Charge-density mapping reveals that CO and H 2 O introduce non-additive electronic restructuring, generating alternating low- and high-electron-density regions that selectively promote or suppress hydrogen mobility. These findings demonstrate that mixed CO–H 2 O impurities produce a heterogeneous energy landscape for hydrogen on Fe(110).