Adsorption behaviors of NH₃ and HCl molecules on Fe-based crystal planes: A DFT study

By adopting the First-principles method based on the density functional theory (DFT), the adsorption properties of NH₃ and HCl molecules intermingled in crude oil on the Fe-based crystal planes exposed on the inner surface of a duct in a crude distillation column have been conducted. Specifically, the calculated adsorption energy of NH₃ and HCl reveal the most stable adsorption sites on the three planes, and the free energy at different temperatures was corrected. Furthermore, the charge density difference and the Bader charge obviously illustrate the direction of charge transfer and the valence of the atoms after adsorption. The results of projected electron density of states (PDOS), electron localization function (ELF) and the crystal orbital Hamilton population (COHP) reveal the nature of chemical bonds and the influence of specific atomic orbitals. Accordingly, the adsorption type of the two molecules on the Fe-based crystal planes is chemical adsorption. Considering the nature of the chemical bond and the multi-component environment of the distillation column, we infer that the possible causes of corrosion are coordination ions, hydrogen embrittlement and Cl⁻ concentration.