Density functional theory study of aromatic adsorption on iron surfaces

This thesis studies the adsorption behaviour of aromatic molecules on iron surfaces using density functional theory (DFT) calculations. The adsorbates studied are benzene (C₆H₆), naphthalene (C₈H₁₀) and quinolinium (C₇H₁₀N⁺) as well as a molecule composed of these fragments known to inhibit acid corrosion on steels used in the oil and gas industry (naphthylmethylquinolinium, C₂₀H₁₆N⁺ or NMQ+). This work represents an effort towards a mechanistic understanding of acid corrosion inhibition of steel as well the development of a general understanding of aromatic adsorption on iron, which has rarely been studied computationally or experimentally. First, the results of a DFT study of benzene adsorption on the three most stable surface facets of bcc iron, including flat {110}, kinked {100} and stepped {211} surfaces, are presented. All stable adsorption sites are identified and the most energetically favourable adsorption sites are compared across the three surfaces. In general, sites which appear centered over hollow-like surface sites are preferred. The effect of van der Waals (vdW) corrected DFT on binding site energetics and geometries has also been studied by way of the Tkatchenko-Scheffler (TS) correction. It shows a strong influence on the adsorption energies, some effect on the relative energetic ordering of sites and little to no effect on adsorption geometries. Second, the adsorption of naphthalene and quinolinium is studied on the most stable surface of bcc iron, Fe{110}, using DFT. Quinolinium and naphthalene differ only by one atom but their electronic structure differences result in significant changes in preferred adsorption site energetics and geometries. Quinolinium tends to adsorb preferentially in geometries which allow its nitrogen atom to bind in an atop position on the Fe{110} surface, in agreement with prior DFT work on $NH_{x}$ adsorbed on Fe{211} showing tetravalent arrangements of adsorbed nitrogen are preferred. The preferred naphthalene adsorption configuration presents the highest overall symmetry of all investigated naphthalene sites. The adsorption energy for the top quinolinium geometry is 1.12~eV stronger than that found for the best naphthalene adsorption geometry. Finally, the adsorption behaviour of NMQ+ on the Fe{110} surface is studied using DFT. Prior to the DFT adsorption study, a semi-empirical level conformational search on the NMQ+ ion is conducted to identify preferred gas phase conformations of the ion. Energetically favoured structures are optimised using DFT, which further refines the search and identifies two different favourable gas phase NMQ+ conformations. Six different starting NMQ+ geometries on the Fe{110} surface are tested based on the two favourable gas phase conformations, and reveal a strongly favoured site (1.4 eV stronger adsorption than the next best site) which presents double dehydrogenation of the quinolinium moiety. The next best site also presents double dehydrogenation, but on the methyl linker and quinolinium moiety. Beyond providing insight into the mode of action of molecules intended for corrosion inhibition of steels, this work provides a fundamental understanding of the behaviour of adsorbed aromatic molecules on iron surfaces, which can play a role in a number of industrially relevant applications, including organic solar cells, transistors and LEDs, heterogeneous catalysts and medical implants.