A DFT study of adsorption of imidazole, triazole, and tetrazole on oxidized copper surfaces: Cu₂O(111) and Cu₂O(111)-w/o-Cu^CUS

Abstract

Azoles and their derivatives are known for their corrosion inhibition ability for copper. For this reason the bonding of imidazole, triazole, and tetrazole-used as archetypal models of azole corrosion inhibitors-to Cu2O(111) and Cu2O(111)-w/o-Cu(CUS) was characterized using density functional theory (DFT) calculations. The former surface contains coordinatively-saturated (CSA) and coordinatively-unsaturated (CUS) Cu sites, whereas the latter lacks the CUS sites. We find that the molecules preferentially bond with a single unsaturated N atom to a surface Cu ion and concomitantly form a hydrogen bond with the surface O ion. They adsorb rather strongly at CUS sites with an adsorption energy of about -1.6 eV (as calculated with the PBE functional), whereas the bonding at CSA sites is about three times weaker thus being similar as on metallic Cu(111). The impact of van der Waals dispersion interactions on molecular adsorption bonding is also addressed. Depending on specifics of the adsorption structure, they strengthen the adsorption bonding by about 0.2-0.5 eV. Due to this specific bonding enhancement, dispersion interactions alter the relative stability of adsorption modes for tetrazole. An atomistic thermodynamics approach was used to construct two-dimensional phase diagrams for all the three molecules. In the viable range of oxygen chemical potential only three phases appear in the phase-diagrams, two of which are the high coverage (1 × 1) molecular phases (one on Cu2O(111) and the other on Cu2O(111)-w/o-Cu(CUS)) and the third is clean Cu2O(111)-w/o-Cu(CUS). The current results indicate that molecular adsorption at CUS sites is strong enough to compensate the thermodynamic deficiency of stoichiometric Cu2O(111) thus making it more stable than Cu2O(111)-w/o-Cu(CUS), unless the conditions are too oxygen rich and/or for azole lean. This finding may tentatively suggest that the corrosion inhibition capability of azoles stems from their ability to passivate reactive surface sites.

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

“…The choice of the Cu 2 O(111) w/o +1Cu CUS model can be further justified by the fact that the relaxed stoichiometric Cu 2 O(111)r, though being a local minimum structure, is not thermodynamically stable [27]. However, in our previous study [17], we showed that molecular adsorption is able to compensate the thermodynamic deficiency of Cu CUS ions. Hence, it seems appropriate to maintain only those Cu CUS ions that bond with adsorbed molecules.…”

“…By lower coverage, we do not mean an extra low coverage, but instead some intermediate coverage, which is low enough that it allows for focusing predominantly on the molecule-surface interactions, even though at such coverage the long-ranged dipole-dipole interactions [15,17,48] may not yet be insignificant. Intact molecules (MolH) typically adsorb with the molecular plane perpendicular to the surface (or nearly so) and form one N-Cu bond and one X-H· · · O hydrogen bond (X = C2 for ImiH or N1 for TriH and TetH), except when bonding to Cu Ovac sites (Figure 6), where ImiH binds with the N3 atom to two adjacent Cu Ovac ions and forms the C2-H· · · O sub hydrogen bond.…”

“…In compliance with our previous publication [17], we used Cu 2 O slabs without a metal support underneath as a model of oxidized copper surfaces. Such models are appropriate for cases where the oxide layer on top of metal is not ultrathin (note that the reactivity of few Å thick oxide films supported on metals can be very different from the reactivity of bulk oxides [37]).…”

“…Molecular structures of imidazole, triazole, and tetrazole are shown in Scheme 1. In previous publications, the adsorption of these molecules was characterized on metallic Cu(111) [15,16] as well as on Cu 2 O(111) [17]. It was shown that neutral molecules bind weakly to Cu(111) via unsaturated N heteroatoms through σ-type bonding and the magnitude of adsorption energy decreases from imidazole to tetrazole.…”

“…However, oxide-free copper surfaces are more relevant at acidic pH, but, under other conditions, copper surfaces are often oxidized [19]. To this end, we have chosen Cu 2 O as a model of the oxidized copper surface and, in our previous publication [17], we characterized the non-dissociative adsorption of these molecules on Cu 2 O(111) and Cu 2 O(111)-w/o-Cu CUS surfaces. In the current two-part article series, we extend that study in two ways: first, we consider additional Cu 2 O surfaces and, secondly, we also consider the feasibility of dissociative adsorption that results from the cleavage of the N1-H bond upon adsorption; this mode of adsorption was not considered in our previous publication.…”