DFT Study of Azole Corrosion Inhibitors on Cu2O Model of Oxidized Copper Surfaces: I. Molecule–Surface and Cl–Surface Bonding

Abstract

Abstract:The adsorption of three simple azole molecules-imidazole, triazole, and tetrazole-and Cl on various sites of several Cu 2 O(111)-and Cu 2 O(110)-type surfaces, including Cu and O vacancies, was characterized using density functional theory (DFT) calculations; the three molecules can be seen as models of azole corrosion inhibitors and Cl as a corrosion activator. Both non-dissociative and dissociative adsorption modes were considered for azole molecules; the latter involves the N-H bond cleavage, hence we also addressed the adsorption of H, which is a co-product of the dissociative adsorption. We find that molecules and Cl bind much stronger to unsaturated Cu sites compared to saturated ones. Dissociated molecules bind considerably stronger to the surface compared to the intact molecules, although even the latter can bind rather strongly to specific unsaturated Cu sites. Bader analysis reveals that binding energies of dissociated molecules at various Cu sites correlate with Bader charges of Cu ions before molecular adsorption, i.e., the smaller the Cu charge, the stronger the molecular bonding. All three azole molecules display similar non-dissociative adsorption energies, but significant difference between them appears for dissociative adsorption mode, i.e., dissociated triazole and tetrazole bind much stronger than dissociated imidazole because the former two can form two strong N-Cu bonds, but imidazole cannot due to its incompatible molecular geometry. Dissociative adsorption is consequently favorable only for triazole and tetrazole, but only at oxygen vacancy sites, where it proceeds barrierlessly (or almost so). This observation may suggest that, for imidazole, only the neutral form, but, for triazole and tetrazole, also their deprotonated forms are the active species for inhibiting corrosion under near neutral pH conditions, where copper surfaces are expected to be oxidized. As for the comparison with the Cl-surface bonding, the calculations indicate that only dissociated triazole and tetrazole bind strong enough to rival the Cl-surface bonds.

DFT Study of Azole Corrosion Inhibitors on Cu2O Model of Oxidized Copper Surfaces: II. Lateral Interactions and Thermodynamic Stability

“…The top row of Figure 3 summarizes the results presented in Part I [1], that is, adsorption bond-strengths for MolH * and Mol * adsorption modes of imidazole, triazole, and tetrazole at various considered Cu sites on Cu 2 O surfaces and the respective reaction energies for MolH * + * → Mol * + H * dissociation. These results were obtained for a coverage, which is low enough that it allows for focussing predominantly on the molecule-surface interactions (see Figure 3 in Part I); currently this coverage can be seen as intermediate, because herein we consider also lower and higher coverages.…”

“…It is worth noting that the comparison between GGA and GGA+U calculations (see Appendix A in Part I [1]) seems to indicate that GGA does not overestimate molecular bonding to Cu CUS sites, because GGA+U predicts slightly stronger bonding. In both aforementioned cases, it is clear that molecular adsorption stabilizes the thermodynamic deficiency of coordinatively unsaturated Cu CUS ions.…”

“…The reason for this choice is motivated by the observation that the GGA+U method, which is often used for the description of transition-metal oxides, does not substantially improve the band-gap of Cu 2 O [4,5]. Nevertheless, the effect of the +U correction on the adsorption properties of current molecules on Cu 2 O surfaces was evaluated to some extent and the comparison between the PBE and PBE+U methods is provided in Appendix A of Part I [1].…”

“…Cu 2 O slabs without a metal support underneath were used as models of oxidized copper surfaces; the justification of this choice is provided in Part I [1]. We use the following surface models (cf.…”

“…In Part I [1] we focused on the azole-surface bonding described at lower coverage without reference to lateral intermolecular interactions-these are addressed in the current Part II-and compared it to the Cl-surface bonding. While the three azole molecules (their molecular structures are shown in Scheme 1) can be seen as models of azole corrosion inhibitors, chloride is a prototypical corrosion activator.…”