Model systems are very important to identify the working principles of real catalysts, and to develop concepts that can be used in the design of new catalytic materials. In this review we report examples of the use of model systems to better understand and control the occurrence of charge transfer at the interface between supported metal nanoparticles and oxide surfaces. In the first part of this article we concentrate on the nature of the support, and on the basic difference in metal/oxide bonding going from a wide-gap non-reducible oxide material to reducible oxide semiconductors. The roles of oxide nanostructuring, bulk and surface defectiveness, and doping with hetero-atoms are also addressed, as they are all aspects that severely affect the metal/oxide interaction. Particular attention is given to the experimental measures of the occurrence of charge transfer at the metal/oxide interface. In this respect, systems based on oxide ultrathin films are particularly important as they allow the use of scanning probe spectroscopies which, often in combination with other measurements and with first principles theoretical simulations, allow full characterization of small supported nanoparticles and their charge state. In a few selected cases, a precise count of the electrons transferred between the oxide and the supported nanoparticle has been possible. Charge transfer can occur through thin, two-dimensional oxide layers also thanks to their structural flexibility. The flow of charge through the oxide film and the formation of charged adsorbates are accompanied in fact by a substantial polaronic relaxation of the film surface which can be rationalized based on electrostatic arguments. In the final part of this review the relationships between model systems and real catalysts are addressed by discussing some examples of how lessons learned from model systems have helped in rationalizing the behavior of real catalysts under working conditions.

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Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

Molecular modeling of organic corrosion inhibitors: Calculations, pitfalls, and conceptualization of molecule–surface bonding

Oxide passive films are a key for the durability of metals and alloys components as well as a central issue in corrosion science and engineering. Herein, we discuss current developments of the nanometer and sub-nanometer scale knowledge of the barrier properties and adsorption properties of passive oxide films brought by recent model experimental and theoretical investigations. The discussed aspects include (i) the chromium enrichment and its homogeneity at the nanoscale in passive films formed on Cr-bearing alloys such as stainless steel, (ii) the corrosion properties of grain boundaries in early intergranular corrosion before penetration and propagation in the grain boundary network, and (iii) the interaction of organic inhibitor molecules with incompletely passivated metallic surfaces. In all three cases, key issues are highlighted and future developments that we consider as most relevant are identified.

Current developments of nanoscale insight into corrosion protection by passive oxide films

In order to model the adsorption of organic corrosion inhibitors on ultrathin oxide layers on aluminum, the formation of a self-assembled layer of gallic acid, the smallest tannin molecule, at the surface of an Al2O3 ultrathin layer supported by Al(111) was studied using periodic density functional theory including dispersion forces (DFT-D). A dense self-assembled layer (SAM) was formed with a density of 3.6 molecules/nm2, adsorbed in a monodentate way by ligand exchange, with an adsorption energy of 2.27 eV/molecule. The electronic analysis shows that the oxide levels are significantly stabilized by the covalent bonding with the COOH moiety of the molecule. The permittivity of the surface layers falls from 109 for the ultrathin Al2O3 film to 4 for the joint oxide and organic film. The permittivity of the surface constrained gallic acid SAM is 1.4, a value slightly lower than that of the free hexagonal compact, SAM (2.4). The organic layer forms an efficient barrier against electron transfer to dioxygen, ...

DFT Modeling of Corrosion Inhibition by Organic Molecules: Carboxylates as Inhibitors of Aluminum Corrosion

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-CuCUS 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-CuCUS) and the third is clean Cu2O(111)-w/o-CuCUS. 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-CuCUS, 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.

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

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O2. Three components of the complex film formed on Al(111) can be identified: an ultrathin under-stoichiometric AlxOy interface layer, an intermediate Al2O3 phase with γ-alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O2 molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al2O3 phase. The electron transfer takes place both from the metal-oxide interface and the oxide surface to the adsorbed O2 molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O2 − species to capture protons and transform into hydrogen peroxide in a non-activated process. H2O2 is further reduced to two water molecules, in a series of two-electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al2O3 inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

Francesco Mercuri

Papers

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film