Bare Metal Surface Research Articles

Effects of Catalyst Model and High Adsorbate Coverages in ab Initio Studies of Alkane Hydrogenolysis

Bare, low-index periodic surface models are typically used to examine metal-catalyzed reactions in density functional theory (DFT) studies, and these most closely resemble low-pressure surface science reactions and catalyzed reactions that occur on large terraces that prevail on large (>5 nm) supported nanoparticles. Many catalytic reactions, however, occur near conditions at which catalytic surfaces are saturated by one or more adsorbed intermediates, leading to strong coadsorbate interactions and surface reconstruction leading to increased curvature. Alkane hydrogenolysis is such a reaction and has been extensively studied using DFT—often on bare metal surfaces—with the assumption that omitted coadsorbed hydrogen atoms (H*) do not significantly alter the relative activation barriers and with ad hoc assumptions about the site requirements for relevant reactions. Here, we use ethane hydrogenolysis on H*-covered Ir catalysts (using a periodic surface model and a nanoparticle model) as a probe reaction to e...

Understanding Corrosion of Ni-Cr Alloys in Molten Chloride Salts

Molten chloride salts possess many attractive properties for use as a coolant and storage medium in advanced nuclear reactors and in concentrating solar power systems. Among the many challenges is the corrosion of salt-facing structural components. In molten halide salts, the passive films that typically form on the surface of corrosion resistant alloys either do not form, or are unstable due to the very low oxygen activities. Therefore, corrosion of materials is dependent largely on interfacial dissolution reactions between the molten salt and the bare metal surface in which alloy constituents with the most stable chlorides tend to be the most likely to dissolve. For most common structural alloys, chromium has the most stable chloride, and thus selective dissolution of chromium from the alloy into the salt is the dominant form of material degradation in molten halides. While this fact is well known and backed-up by experimental data, the mechanism of chromium depletion from alloys is not yet known. To truly understand corrosion in molten salts, the mechanism of chromium depletion from alloys must be understood and quantified. This talk will discuss results of a combined experimental and computational strategy aimed at fundamentally understanding corrosion of alloys in molten chloride salts. Controlled capsule tests are conducted in which model Ni-Cr alloys are exposed to molten KCl-MgCl2 at temperatures in excess of 600°C to measure degradation in molten salt. In an effort to maintain greater control of the molten salt environment, new processes to sufficiently purify salts are discussed. Quantification of salt impurities is matched to corrosion results in an effort to quantify the effect of salt impurity concentrations on corrosion rates. Traditional experimentation is coupled with x-ray spectroscopy and thermodynamic modeling to identify relevant reactions and to develop a thermodynamic description of the alloy-salt system. This work is funded by the Laboratory Directed Research and Development Fund at Oak Ridge National Laboratory.

Hole patterns in ultrathin vanadium oxide layers on a Rh(111) surface during catalytic oxidation reactions with NO.

Various oxidation reactions with NO as oxidant have been investigated on a partially VOx covered Rh(111) surface (θV = 0.3 MLE) in the 10-4 mbar range, using photoelectron emission microscopy (PEEM) as spatially resolving method. The PEEM studies are complemented by rate measurements and by low-energy electron diffraction. In catalytic methanol oxidation with NO and in the NH3 + NO reaction, we observe that starting from a homogeneous surface with increasing temperature first a stripe pattern develops, followed by a pattern in which macroscopic holes of nearly bare metal surface are surrounded by a VOx film. These hole patterns represent just the inverse of the VOx distribution patterns seen if O2 instead of NO is used as oxidant.

Morphology-Driven Emissivity of Microscale Tree-like Structures for Radiative Thermal Management

ABSTRACTSpectral emissivity of surface materials has a strong impact on thermal properties of systems that are exposed in the ambient environment. While the solar spectrum heating up the surface ranges from 200 to 2,500 nm, the atmospheric transmission spectrum allowed for infrared cooling ranges from 8 to 14 µm. However, conventional surface materials have emissivity values that are either high or low throughout the spectrum. For example, ceramic materials are typically emissive and metallic materials are typically reflective and not emissive. Here, we show that surface materials with artificial periodicities can have a selectively controlled emissivity and that the surface morphology can transform ceramic materials to be reflective or metallic materials to be emissive. As a model system, we use microscale tree-like structures, or briefly micro-trees, to demonstrate wide variations of morphology-driven emissivity spectra. Our computation based on the rigorous coupled-wave analysis shows that optimal designs of micro-trees can act as a nearly perfect reflector or a black body depending on the spectral range and offer radiative cooling or heating capabilities beyond the limits of conventional materials. For cooling, metallic micro-trees provide a surface temperature 10 K lower than that of bare metallic surfaces in a normal ambient condition, and for heating, ceramic micro-trees provide a surface temperature 8 K higher than that of bare ceramic materials. The morphology-driven emissivity of micro-trees can offer a net cooling power of 136 W/m2 or a net heating power of 12 W/m2 depending on the application without requiring any active devices, and these results guide optimal designs of artificial materials for thermal management.

Oxidation versus salt-film formation: Competitive adsorption on a series of metals from first-principles

Chloride ion is known to accelerate corrosion in numerous metallic systems, although it is not well understood why. Using first-principles methods, such as density functional theory, it is possible to directly compute the energy of adsorption for chloride, and, using a thermodynamic framework based on statistical mechanics, to predict the environmental conditions under which it may be anticipated that chloride could displace other ions (such as hydroxide, incipient oxide, or other potentially corrosion inhibiting species) from the bare metal and/or oxide surface. By reviewing the literature, as well as performing some original density functional theory calculations, the authors present in this work the dependence of the surface coverage for H2O, OH, O and Cl on the electrochemical potential, pH and chloride concentration for a number of metallic systems, including nickel, iron, magnesium and aluminum, to predict the fundamental surface processes that may related to the role of chloride in potentially interfering with processes such as repassivation and metal dissolution. It is found that under some conditions certain metals can possess zones of mutual stability where both chloride and hydroxide may be coadsorbed, whereas other metals appear to have intrinsic resistance to chloride adsorption. Variations among the available first-principles adsorption energies obtained from different sources indicate that uncertainty analysis is necessary moving forwards. An analogous approach could be taken to consider the interaction of chloride with clean and defective oxide and/or hydroxylated surfaces.

Interaction of Model Inhibitor Compounds with Minimalist Cluster Representations of Hydroxyl Terminated Metal Oxide Surfaces

The computational modeling of corrosion inhibitors at the level of molecular interactions has been pursued for decades, and recent developments are allowing increasingly realistic models to be developed for inhibitor–inhibitor, inhibitor–solvent and inhibitor–metal interactions. At the same time, there remains a need for simplistic models to be used for the purpose of screening molecules for proposed inhibitor performance. Herein, we apply a reductionist model for metal surfaces consisting of a metal cation with hydroxide ligands and use quantum chemical modeling to approximate the free energy of adsorption for several imidazoline class candidate corrosion inhibitors. The approximation is made using the binding energy and the partition coefficient. As in some previous work, we consider different methods for incorporating solvent and reference systems for the partition coefficient. We compare the findings from this short study with some previous theoretical work on similar systems. The binding energies for the inhibitors to the metal hydroxide clusters are found to be intermediate to the binding energies calculated in other work for bare metal vs. metal oxide surfaces. The method is applied to copper, iron, aluminum and nickel metal systems.

Crossover of 2D graphene and 3D carbon island growth on Cu–In alloy surface

Nucleation and initial stage of graphene growth in chemical vapor deposition on the Cu–In alloy surface were studied by in situ scanning electron microscopy together with ex situ Auger electron microscopy, atomic force microscopy, and Raman spectroscopy. Because of the lower melting point of Cu–In alloy, around 800°C depending on the In content, growth on solid and liquid Cu–In surfaces could be compared. On both the solid and liquid surfaces, 2D graphene and 3D carbon island growth coexisted in the initial stage. On a wide terrace, 3D carbon island nucleation dominated over 2D graphene nucleation and 3D carbon islands showed dendritic growth. When the bare area on the Cu–In surface became narrow with an increase in the island coverage, 2D graphene nucleation and growth dominated. The growth mode change from 3D to 2D was attributed to the reduction of carbon adatom flux to the existing islands due to depletion of supersaturation and reduction of the size of bare metal surface.

Redefining the Electrochemical Kinetics of Redox Reactions on Passive Metals

The kinetics of redox reactions are described by specifying a standard exchange current density and a transfer coefficient that locates the position along the reaction coordinate at which the activated complex (transition state) occurs. However, this formulation was originally developed to describe charge transfer reactions on bare metal surfaces. In the case of a passive metal, however, the barrier layer of the passive film represents a barrier to the quantum mechanical tunneling of charge carriers (electrons and electron holes) between the metal and the redox reaction center and theory shows that this barrier has a profound effect upon the tunneling probability and hence upon the values of the exchange current density and the Tafel constant. In this presentation, the role of quantum-mechanical tunneling of charge carriers through the barrier oxide layer on a passive metal, in determining the electrochemical kinetic parameters is explored theoretically. The exchange current density for a redox reaction occurring on the passive metal surface is assumed to be the product of the exchange current density on the bare metal surface and the quantum mechanical tunneling probability (QMTP) of charge carriers through the barrier layer, which is inversely proportional to the exponential of the barrier thickness and the tunneling constant that in turn is proportional to the square root of the product of barrier height and the effective electron mass. The QMTP is used to redefine the exchange current density for the cathodic reaction (e.g., reduction of oxygen), as measured on the passive surface, to that on the (hypothetical) bare metal alone. Thus, knowing the voltage, the barrier layer thickness can be calculated from the Point Defect Model (PDM), which in turn can be used to estimate the QMTP for that particular set of conditions. Then, the exchange current density for the redox reaction on the passive metal surface is calculated and used, if the barrier height and the effective electron mass are constants, in the generalized Butler-Volmer equation to describe the current/voltage characteristics of the redox reaction. In doing so, it becomes evident that the kinetics of a redox reaction are fully determined by specifying a (hypothetical) exchange current density and transfer coefficient of the reaction on the bare metal (“hypothetical” because the bare metal condition may not be realizable experimentally), the QMTP, the electric field strength within the barrier layer, the polarizability of the barrier layer/solution interface, and the potential of zero thickness of the barrier layer. The application of this theory is illustrated with respect to the hydrogen electrode reaction (HER) on passive carbon steel and on aluminum and in predicting the corrosion potentials of stainless steels and nickel alloys in the primary coolant circuits of water-cooled nuclear reactors.

Effect of Combination of Anionic Polymer with Calcium Phosphate Coating on Corrosion Behavior of Magnesium Alloy in Physiological Solution

Magnesium alloys attract attention as a biodegradable/bioabsorbable bone plates & screws and stents owing to their biodegradability and good mechanical integrity. Various surface modifications have been examined to control corrosion rate and to improve biocompatibility of the magnesium alloys. Bone plates are deformed to adjust the shape of bone and screws are rubbed with bone during screwing, which sometimes cause cracking and delamination of modified surfaces. We developed hydroxyapatite (HAp) coating, and revealed that the HAp coating improved the corrosion resistance of Mg-3mass% Al-1mass% Zn (AZ31) in cell culture medium (in vitro) and in subcutaneous tissue of mouse (in vivo) [1]. Effect of scratching HAp coated surface on the corrosion behavior of HAp-coated AZ31 was examined in cell culture medium [2]. Corrosion of the scratched surface was not accelerated in comparison with the surface without scratch. A calcium phosphate deposit layer covered the bare metal surface inside the scratch. These results suggest that it is effective to enhance calcium phosphate deposition at the defect of HAp coating to enhance the self-healing ability of the coating. Combination of HAp coating with polymer molecules which act as a nucleus for calcium phosphate deposition is a potential way to enhance the self-healing ability. It was reported that an anionic polymer, sodium polyacrylate (SPA), acted as a nucleus for calcium phosphate deposition [3]. In this study, SPA molecule was combined with HAp coating and the corrosion behavior was examined by a static immersion test and a slow strain rate tensile (SSRT) test in a simulated body fluid [4]. HAp was coated on AZ31 at 60ºC for 1 hour in a previously reported solution [1]. SPA molecule was dissolved in a phosphate buffer at pH 8 and the HAp-coated AZ31 was dipped at room temperature for 10-30 sec. Scratches were formed on the surface of HAp- and SPA-HAp-coated AZ31 specimens with a knife. The scratched specimens were statically immersed in Hanks’ solution for 7 days. Volume of H2 gas generated was measured. The surface after the immersion was characterized using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). A SSRT test of HAp- and SPA-HAp-coated AZ31 tensile test specimens was performed in Hanks. Scratched HAp- and SPA-HAp-coated AZ31 showed no significant difference in H2 gas volume generated under the static immersion. Inside the scratches were covered with a calcium phosphate deposit layer regardless of the presence of SPA. The deposit layer with SPA was uniform and prevented the corrosion initiation; whereas, local corrosion initiated from inside the scratches without SPA. Moreover, SPA enhanced the deposition of calcium phosphate granules inside the scratches. In the SSRT test in Hanks, HAp-coated AZ31 specimen fractured just over the yield strength with small elongation of ca. 1.5%. This result indicates that HAp-AZ31 is susceptible to stress corrosion cracking (SCC). SPA-HAp-coated AZ31 specimen showed larger elongation of ca. 2.4%. Time to failure of HAp-coated AZ31 was ca. 8.4 hours and it was elongated by 45% with SPA to ca. 12.2 hours. Some cracks of SPA-HAp coating were closed at crack mouth, suggesting that SPA enhanced the healing of cracks of HAp coating. It was revealed that the combination of SPA molecules with HAp coating enhanced the self-healing ability of HAp coating and reduced the SCC susceptibility of HAp-coated AZ31. [1] S. Hiromoto et al., Acta Biomater., 11 (2015) 520. [2] S. Hiromoto, Corros. Sci., 100 (2015) 284. [3] A. Bigi et al., Biomacromolecules, 4 (2000) 752. [4] S. Hiromoto et al., CORROSION (submitted).

Adsorption and electronic properties of pentacene on thin dielectric decoupling layers.

With the increasing use of thin dielectric decoupling layers to study the electronic properties of organic molecules on metal surfaces, comparative studies are needed in order to generalize findings and formulate practical rules. In this paper we study the adsorption and electronic properties of pentacene deposited onto h-BN/Rh(111) and compare them with those of pentacene deposited onto KCl on various metal surfaces. When deposited onto KCl, the HOMO and LUMO energies of the pentacene molecules scale with the work functions of the combined KCl/metal surface. The magnitude of the variation between the respective KCl/metal systems indicates the degree of interaction of the frontier orbitals with the underlying metal. The results confirm that the so-called IDIS model developed by Willenbockel et al. applies not only to molecular layers on bare metal surfaces, but also to individual molecules on thin electronically decoupling layers. Depositing pentacene onto h-BN/Rh(111) results in significantly different adsorption characteristics, due to the topographic corrugation of the surface as well as the lateral electric fields it presents. These properties are reflected in the divergence from the aforementioned trend for the orbital energies of pentacene deposited onto h-BN/Rh(111), as well as in the different adsorption geometry. Thus, the highly desirable capacity of h-BN to trap molecules comes at the price of enhanced metal–molecule interaction, which decreases the HOMO–LUMO gap of the molecules. In spite of the enhanced interaction, the molecular orbitals are evident in scanning tunnelling spectroscopy (STS) and their shapes can be resolved by spectroscopic mapping.

Critical Role of Oxygen in Silver-Catalyzed Glaser–Hay Coupling on Ag(100) under Vacuum and in Solution on Ag Particles

The essential role of oxygen in enabling heterogeneously catalyzed Glaser-Hay coupling of phenylacetylene on the Ag(100) was elucidated by STM, laboratory and synchrotron photoemission and DFT calculations. In the absence of co-adsorbed oxygen, phenylacetylene formed well-ordered dense overlayers which, with increasing temperature, desorbed without reaction. In striking contrast, even at 120 K, the presence of oxygen led to immediate and complete disruption of the organic layer due to abstraction of acetylenic hydrogen with formation of a disordered mixed layer containing immobile adsorbed phenylacetylide. At higher temperatures phenylacetylide underwent Glaser-Hay coupling to form highly ordered domains of diphenyldiacetylene that eventually desorbed without decomposition leaving the bare metal surface. DFT calculations showed that while acetylenic H abstraction was otherwise an endothermic process, oxygen adatoms triggered a reaction-initiating exothermic pathway leading to OH(a) + phenylacetylide, consistent with the experimental observations. Moreover, it was found that with a solution of phenylacetylene in nonane and in the presence of O2, Ag particles catalyzed Glaser-Hay coupling with high selectivity. Rigorous exclusion of oxygen from the reactor strongly suppressed the catalytic reaction. Interestingly, too much oxygen lowers the selectivity towards diphenyldiacetylene. Thus vacuum studies and theoretical calculations revealed the key role of oxygen in the reaction mechanism, subsequently borne out by catalytic studies with Ag particles that confirmed the presence of oxygen as a necessary and sufficient condition for the coupling reaction to occur. The direct relevance of model studies to mechanistic understanding of coupling reactions under conditions of practical catalysis was reaffirmed.

Application of wire beam electrode technique to investigate the migrating behavior of corrosion inhibitors in mortar

The migration rate and inhibitive effect on rebar corrosion of migrating corrosion inhibitors (MCIs) in mortar were studied using wire beam electrode technique with high temporal and spatial resolution. The MCIs readily diffused through the mortar but exhibited preferential adsorption on the bare metal surfaces rather than on the rust layer on rebars. The ingress of MCIs in mortar increased the resistance on general corrosion much more than localized corrosion. This increase was mainly attributed to the sealing effect on mortar pores (pore-blocking effect) rather than on chemical absorption.

Obtaining the solid-liquid interfacial free energy via multi-scheme thermodynamic integration: Ag-ethylene glycol interfaces.

The solid-liquid interfacial free energy γsl is an important quantity in wetting, nucleation, and crystal growth. Although various methods have been developed to calculate γsl with atomic-scale simulations, such calculations still remain challenging for multi-component interfaces between molecular fluids and solids. We present a multi-scheme thermodynamic integration method that is inspired by the "cleaving-wall" method and aimed at obtaining γsl for such systems using open-source simulation packages. This method advances two aspects of its predecessor methods. First, we incorporate separate schemes to resolve difficulties when manipulating periodic boundary conditions of the supercell using open-source simulation packages. Second, we introduce a numerical approximation to obtain thermodynamic integrands for complex force fields when an analytical differentiation is not readily available. To demonstrate this method, we obtain γsl for interfaces between Ag(100) and Ag(111) and ethylene glycol (EG). These interfacial free energies mirror interfacial potential energies for each facet. We also estimate entropies of interface formation and these are consistent with theoretical predictions in signs and trends. For the Ag-EG systems, we find that the largest contribution to γsl is the free energy to create the bare metal surfaces. The second-largest contribution to γsl is from the liquid-solid interaction. This user-friendly method will accelerate investigation in a broad range of research topics, such as the thermodynamic effect of structure-directing agents in solution-phase shape-controlled nanocrystal syntheses.

Pd Supported on Carbon Nitride Boosts the Direct Hydrogen Peroxide Synthesis

Herein, the development of an improved Pd on carbon nitride catalyst for direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used to characterize the chemical speciation of the Pd-selective and -unselective sites. Selectivity trends among the samples suggest that a bare metal surface with a differential heat of CO chemisorption ranging between 140 and 120 kJ mol–1 is responsible for the total O2 hydrogenation, while a maximum threshold value of differential heat of CO chemisorption of approximately 70 kJ mol–1 is necessary for the partial hydrogenation of O2 to H2O2. Such a low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron withdrawal from the surrounding ligands: i.e., the N functional groups on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide the following: a higher concentration of N sites, a flexible network of π-conjugated polymeric subunits with sp3 linking subuni...

Dithiocarbamate Self-Assembled Monolayers as Efficient Surface Modifiers for Low Work Function Noble Metals.

Tuning the work function of the electrode is one of the crucial steps to improve charge extraction in organic electronic devices. Here, we show that N,N-dialkyl dithiocarbamates (DTC) can be effectively employed to produce low work function noble metal electrodes. Work functions between 3.1 and 3.5 eV are observed for all metals investigated (Cu, Ag, and Au). Ultraviolet photoemission spectroscopy (UPS) reveals a maximum decrease in work function by 2.1 eV as compared to the bare metal surface. Electronic structure calculations elucidate how the complex interplay between intrinsic dipoles and dipoles induced by bond formation generates such large work function shifts. Subsequently, we quantify the improvement in contact resistance of organic thin film transistor devices with DTC coated source and drain electrodes. These findings demonstrate that DTC molecules can be employed as universal surface modifiers to produce stable electrodes for electron injection in high performance hybrid organic optoelectronics.

Laboratory‐scale testing of the anti‐corrosion effectiveness of a primer coating on a bare AA2024 surface

In this study, the behaviour of a cut‐edge electrode machined from a coated AA2024‐T3 plate, designed for an aircraft structure, has been used to mimic the electrochemical response occurring after a mechanical scratch on a coated structure.In situ pH imaging with an ion‐selective microprobe was used to confirm the competition between the effect of the inhibitor release and local corrosion activity around intermetallic (IM) particles. In this work, the validation of the described method is based on conventional chromate/dichromate inhibitors, assumed to be cathodic inhibitors.An experimental method, supported by mass transport modelling, was defined to characterize the competition between the triggering of localized microstructural corrosion and the passivation of a bare metal surface. Copyright © 2016 John Wiley & Sons, Ltd.

Self-Assembly of Graphene Nanoblisters Sealed to a Bare Metal Surface.

The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C-Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks.

Electric-field noise above a thin dielectric layer on metal electrodes

The electric-field noise above a layered structure composed of a planar metal electrode covered by a thin dielectric is evaluated and it is found that the dielectric film considerably increases the noise level, in proportion to its thickness. Importantly, even a thin (mono) layer of a low-loss dielectric can enhance the noise level by several orders of magnitude compared to the noise above a bare metal. Close to this layered surface, the power spectral density of the electric field varies with the inverse fourth power of the distance to the surface, rather than with the inverse square, as it would above a bare metal surface. Furthermore, compared to a clean metal, where the noise spectrum does not vary with frequency (in the radio-wave and microwave bands), the dielectric layer can generate electric-field noise which scales in inverse proportion to the frequency. For various realistic scenarios, the noise levels predicted from this model are comparable to those observed in trapped-ion experiments. Thus, these findings are of particular importance for the understanding and mitigation of unwanted heating and decoherence in miniaturized ion traps.

Promotional effects of chemisorbed oxygen and hydroxide in the activation of C–H and O–H bonds over transition metal surfaces

Electronegative coadsorbates such as atomic oxygen (O⁎) and hydroxide (OH⁎) can act as Brønsted bases when bound to Group 11 as well as particular Group 8–10 metal surfaces and aid in the activation of X–H bonds. First-principle density functional theory calculations were carried out to systematically explore the reactivity of the C–H bonds of methane and surface methyl intermediates as well as the O–H bond of methanol directly and with the assistance of coadsorbed O⁎ and OH⁎ intermediates over Group 11 (Cu, Ag, and Au) and Group 8–10 transition metal (Ru, Rh, Pd, Os, Ir, and Pt) surfaces. C–H as well as O–H bond activation over the metal proceeds via a classic oxidative addition type mechanism involving the insertion of the metal center into the C–H or O–H bond. O⁎ and OH⁎ assist C–H and O–H activation over particular Group 11 and Group 8–10 metal surfaces via a σ-bond metathesis type mechanism involving the oxidative addition of the C–H or O–H bond to the metal along with a reductive deprotonation of the acidic C–H and O–H bond over the M–O⁎ or M–OH⁎ site pair. The O⁎- and OH⁎-assisted C–H activation paths are energetically preferred over the direct metal catalyzed C–H scission for all Group 11 metals (Cu, Ag, and Au) with barriers that are 0.4–1.5eV lower than those for the unassisted routes. The barriers for O⁎- and OH⁎-assisted C–H activation of CH4 on the Group 8–10 transition metals, however, are higher than those over the bare transition metal surfaces by as much as 1.4eV. The C–H activation of adsorbed methyl species show very similar trends to those for CH4 despite the differences in structure between the weakly bound methane and the covalently adsorbed methyl intermediates. The activation of the O–H bond of methanol is significantly promoted by O⁎ as well as OH⁎ intermediates over both the Group 11 metals (Cu, Ag, and Au) as well as on all Group 8–10 metals studied (Ru, Rh, Pd, Os, Ir, and Pt). The O⁎- and OH⁎-assisted CH3O–H barriers are 0.6 to 2.0eV lower than unassisted barriers, with the largest differences occurring on Group 11 metals. The higher degree of O⁎- and OH⁎-promotion in activating methanol over that in methane and methyl is due to the stronger interaction between the basic O⁎ and OH⁎ sites and the acidic proton in the O–H bond of methanol versus the non-acidic H in the C–H bond of methane. A detailed analysis of the binding energies and the charges for O⁎ and OH⁎ on different metal surfaces indicates that the marked differences in the properties and reactivity of O⁎ and OH⁎ between the Group 11 and Group 8–10 metals is due to the increased negative charge on the O-atoms (in O⁎ as well as OH⁎) bound to Group 11 metals. The promotional effects of O⁎ and OH⁎ are consistent with a proton-coupled electron transfer and the cooperative role of the metal-O⁎ or metal-OH⁎ pair in carrying out the oxidative addition and reductive deprotonation of the acidic C–H and O–H bonds. Ultimately, the ability of O⁎ or OH⁎ to act as a Brønsted base depends upon its charge, its binding energy on the metal surface (due to shifts in its position during X–H activation), and the acidity of the H-atom being abstracted.

Influence of mass transport on the competition between corrosion and passivation by inhibitor release after coating breakdown

Abstract Interactive combination of the properties of storage, release of pigments and transport of inhibitors are controlling the efficacy of inhibitive coatings deposited as primer on 2024 alloys subject to environments with a risk of atmospheric corrosion. A test method is proposed to characterize the competition between the triggering of localized microstructural corrosion and the passivation of a bare metal surface. The behaviour of a cut-edge electrode machined from a coated AA2024-T3 plate has been used to mimic the electrochemical response after a breakdown in the surface coating. In this work, the calibration of the described method is based on conventional chromate/dichromate inhibitors, assumed to be cathodic inhibitor. In situ pH imaging with an ion-selective microprobe was used to confirm the effect of the inhibiting pigment depletion inside the primer coating on the competition between corrosion and passivation. This test method is intended as useful for selection of chromate replacement.