One-step synthesis and growth mechanism of nitrate intercalated ZnAl LDH conversion coatings on zinc

An Inhibitor-Loaded LDH- and MOF-Based Bilayer Hybrid System for Active Corrosion Protection of Aluminum Alloys

A novel inhibitor-loaded bilayer hybrid system based on the LDH inner layer and MOF outer layer is designed on an aluminum alloy 2A12 surface to improve corrosion performance. The hybrid film system covers the inherent cavities and intercrystalline defects of the LDH film using the affinity between the LDH and the MOF compounds. The results demonstrate that the LDH-inhI precursor film is entirely covered by new Zn-based MOF microrods. The LDH-inhI precursor film is partially dissolved and recrystallized in favor of MOF crystal growth to strengthen the binding adhesion between LDH and MOF films. The LDH-inhI/MOF-inhII bilayer film shows significantly enhanced corrosion resistance through the synergistic action of LDH and MOF nanocontainers doped with different corrosion inhibitors (vanadates, 2,5-furandicarboxylic acid, and benzotriazoles). Due to the multiple loadings of the MOF film and the sustained-release of the LDH film, this method provides an effective approach to developing new anticorrosion systems and enhancing both the barrier ability and active corrosion protection performance of LDH-based conversion treatments.

Surface Hydroxyl and Oxygen Vacancies Engineering in ZnSnAl LDH: Synergistic Promotion of Photocatalytic Oxidation of Aromatic VOCs

Photocatalytic oxidation has gained great interest in environmental remediation, but it is still limited by its low efficiency and catalytic deactivation in the degradation of aromatic VOCs. In this study, we concurrently regulated the surface hydroxyl and oxygen vacancies by introducing Al into ZnSn layered double hydroxide (LDH). The presence of distorted Al species induced local charge redistribution, leading to the remarkable formation of oxygen vacancies. These oxygen vacancies subsequently increased the amount of surface hydroxyl and elongated its bond length. The synergistic effects of surface hydroxyl and oxygen vacancies greatly enhanced reactant adsorption-activation and facilitated charge transfer to generate •OH, •O2-, and 1O2, resulting in highly efficient oxidation and ring-opening of various aromatic VOCs. Compared with commercial TiO2, the optimized ZnSnAl-50 catalyst exhibited about 2-fold activity for the toluene and styrene degradation and 10-fold activity for the chlorobenzene degradation. Moreover, ZnSnAl-50 demonstrated exceptional stability in the photocatalytic oxidation of toluene under a wide humidity range of 0-75%. This work marvelously improves the photocatalytic efficiency, stability, and adaptability through a novel strategy of surface hydroxyl and oxygen vacancies engineering.

Zinc-aluminum layered double hydroxide and double oxide for room-temperature oxidation of sulfur dioxide gas

Sulfur dioxide (SO2) gas at trace levels challenges the consumption of fuel gases and cleaning of flue gases originating from diverse anthropogenic sources. We have demonstrated Zn-Al layered double hydroxide (LDH) and layered double oxide (LDO) as low-cost and effective adsorbents in removing lowly concentrated SO2 gas at room temperature. Water in the adsorbent bed significantly improved the performance, where the maximum adsorption capacity of 38.0 mg g-1 was achieved for LDO. Based on the spectroscopic findings, the adsorbed gas molecules were oxidized to surface-bound sulfate/bisulfate species, showing complete mineralization of SO2 molecules. By employing an inexpensive NaOH-H2O2 solution-based regeneration strategy, we successfully regenerated the spent LDO, significantly restoring its gas uptake capacity. The regenerated oxide exhibited an increased gas uptake capacity ranging from 38.0 to 98.5 mg g-1, highlighting the practicality and economic feasibility of our approach. LDH/LDO materials are promising regenerable adsorbents for removing low concentrations of SO2 gas in ambient conditions.

Corrosion Inhibitors Intercalated into Layered Double Hydroxides Prepared In Situ on AZ91 Magnesium Alloys: Structure and Protection Ability

This work first describes the intercalation of corrosion inhibitors into layered double hydroxides LDH-OH/CO₃ nanocontainers (parental LDH) obtained in situ on the surface of magnesium alloy AZ91 in the presence of a chelating agent. Vanadate, as a typical broad inhibitor for active metals, and oxalate, as an inhibitor suitable for magnesium, were selected as a first approach. The optimization of exchange conditions was performed, and the optimal parameters (ambient pressure and 95 °C) were selected. The corrosion protective properties of obtained LDH-based layers were studied using immersion and salt spray tests in NaCl solution, supported by electrochemical impedance spectroscopy and atomic emission spectroelectrochemistry. It is demonstrated that vanadate intercalated into LDH is more effective for the active protection of AZ91 in comparison to the performance of oxalate. A possible mechanism of corrosion inhibition based on the application of LDH nanocontainers is suggested and discussed.

Functional Regulation of ZnAl-LDHs and Mechanism of Photocatalytic Reduction of CO2: A DFT Study

Defect engineering and heteroatom doping can significantly enhance the activity of zinc-aluminum layered double hydroxides (ZnAl-LDHs) in photocatalytic CO₂ reduction to fuel. However, the in-depth understanding of the associated intrinsic mechanisms is limited. Herein, we systematically investigated Zn vacancies (V_Zn), oxygen vacancies (V_O), and Cu doping on the geometry and electronic structure of ZnAl-LDH using density functional theory (DFT). We also revealed the related reaction mechanism. The results reveal the concerted roles of V_O, V_Zn, and doped-Cu facilitate the formation of the unsaturated metal complexes (Zn^δ+-V_O and Cu^δ+-V_O). They can localize the charge density distribution, function as new active centers, and form the intermediate band. Simultaneously, the intermediate band of functionalized ZnAl-LDHs narrows the band gap and lowers the band edge location. Therefore, it can broaden the absorption range of light and improve the selectivity of CO. Additionally, the unsaturated metal complex lowers the Gibbs free energy barrier for effective CO₂ activation by bringing the d-band center level closer to the Fermi level. The work provided guidance for developing LDH photocatalysts with high activity and selectivity.

Layered double hydroxides for corrosion-related applications—Main developments from 20 years of research at CICECO

This work describes the main advances carried out in the field of corrosion protection using layered double hydroxides (LDH), both as additive/pigment-based systems in organic coatings and as conversion films/pre-treatments. In the context of the research topic “Celebrating 20 years of CICECO”, the main works reported herein are based on SECOP’s group (CICECO) main advances over the years. More specifically, this review describes structure and properties of LDH, delving into the corrosion field with description of pioneering works, use of LDH as additives to organic coatings, conversion layers, application in reinforced concrete and corrosion detection, and environmental impact of these materials. Moreover, the use of computational tools for the design of LDH materials and understanding of ion-exchange reactions is also presented. The review ends with a critical analysis of the field and future perspectives on the use of LDH for corrosion protection. From the work carried out LDH seem very tenable, versatile, and advantageous for corrosion protection applications, although several obstacles will have to be overcome before their use become commonplace.

Environmentally Friendly Layered Double Hydroxide Conversion Layers: Formation Kinetics on Zn-Al-Mg-Coated Steel

Phosphate- or chromate-based industrially produced conversion layers, while effectively increasing adhesion for organic coatings and corrosion resistance, come at the cost of environmentally problematic and harmful treatment solutions and waste. In this respect, layered double hydroxide (LDH)-based conversion layers offer an environmentally benign alternative without toxicologically concerning compounds in the treatment solution. Here, we study an LDH conversion layer on Zn-Al-Mg-coated steel (ZM-coated steel), which was produced by immersion into a carbonate- and magnesium-containing alkaline solution. The mechanism and kinetics of the conversion layer formation were investigated with in situ open circuit potential measurements, cyclic voltammetry (CV), and scanning electron microscopy (SEM). Acceleration of the LDH layer formation through high convection in the treatment solution was found. This was attributed to a higher oxygen availability at the metal/solution interface because no diffusion-limited state during the layer formation is reached due to high convection. The importance of oxygen within the kinetics indicates a corrosion-like mechanism, with cathodic and anodic sites on the steel sample. The LDH formation happens by co-precipitation of ions present in the treatment solution and dissolved ions from the ZM-coated steel. With CV, SEM, and X-ray diffraction, the growth of the LDH conversion layer was investigated with respect to the immersion time. It was found that after 30 s, the sample surface was almost fully covered with an LDH layer, and with the increasing immersion time, the layer grows in thickness. Increased understanding on the kinetics and mechanism of the LDH conversion layer formation on ZM-coated steel gives rise to a targeted optimization of the treatment solution and process parameters.

In situ kinetics studies of Zn-Al LDH intercalation with corrosion related species

Kinetic parameters for three anion exchange reactions - Zn-LDH-NO₃→ Zn-LDH-Cl, Zn-LDH-NO₃→ Zn-LDH-SO₄ and Zn-LDH-NO₃→ Zn-LDH-VO_x- were obtained by in situ synchrotron study. The first and the second ones are two-stage reactions; the first stage is characterized by the two-dimensional diffusion-controlled reaction following deceleratory nucleation and the second stage is a one-dimensional diffusion-controlled reaction also with a decelerator nucleation effect. In the case of exchange NO₃^-→ Cl^- host anions are completely released, while in the case of NO₃^-→ SO₄^2- the reaction ends without complete release of nitrate anions. The exchange of Zn-LDH-NO₃→ Zn-LDH-VO_x is a one-stage reaction and goes much slower than the previous two cases. The latter is characterized by a one stage two-dimensional reaction with an instantaneous nucleation. As a result, at the end of this process there are two crystalline phases with different polyvanadate species, presumably V₄O₁₂^4- and V₂O₇^4-, nitrate anions were not completely released. The rate of replacing NO₃^- anions by guest ones can be represented as Cl^- > SO₄^2- > VO_x^y-.

Zn-Al Layered Double Hydroxide Thin Film Fabricated by the Sputtering Method and Aqueous Solution Treatment

Zn-Al layered double hydroxides (LDHs) were synthesized herein via a simple process. First, Al-doped ZnO film was deposited onto a glass substrate using the facing target sputtering system. Successful synthesis of the Zn–Al LDH was achieved via a treatment process using an aqueous solution which contains NO3− anions. X-ray diffraction analysis confirmed that it was consistent with the previous Zn–Al LDH synthesis experiment data, and the calculated d-value was 9.1 Å. Scanning electron microscopy observations revealed that the as-synthesized sample had a plate-like structure.

Layered Double Hydroxide Protective Films Developed on Aluminum and Aluminum Alloys: Synthetic Methods and Anti-Corrosion Mechanisms

This work reviews the characteristics of layered double hydroxides (LDHs) in the context of protective thin films to enhance the corrosion resistance properties of aluminum alloys. A discussion is made in detail about the LDH protection mechanism and the effect of synthesis approaches on LDH structural variations and the corresponding anti-corrosion behavior. LDHs anion-exchange behavior to host inorganic/organic anions makes them a potential material to investigate for anti-corrosion film. This unique advantage and the availability of a wide range of metal oxide-based layers, interlayer anions, and self-healing properties make LDH family an attractive choice for the development of compact LDHs based smart coating systems.

Chlorides Entrapment Capability of Various In-Situ Grown NiAl-LDHs: Structural and Corrosion Resistance Properties

In this work, various NiAl-LDH thin films, exhibiting specific surface morphologies, were developed directly on aluminum AA 6082 substrate to understand the two main characteristics of layered double hydroxide (LDH), i.e., ion-exchange behavior and barrier properties, which are found to have a significant influence on the LDH corrosion resistance properties. The as-prepared NiAl-LDH films were analyzed through the scanning electronic microscope (SEM), X-ray diffraction (XRD), while the corrosion behavior of the synthesized films was investigated by the electrochemical impedance spectroscopy (EIS) and potentiodynamic curves. The results indicated that NiAl-LDH microcrystals grow in various fashions, from porous relatively flat domains to well-developed platelet structure, with the variation of nickel nitrate to ammonium nitrate salts molar ratios. The LDH structure is observed in all cases and is found to cover the aluminum surface uniformly in the lamellar order. All the developed NiAl-LDHs are found to enhance the corrosion resistance of the aluminum substrate, specifically, a well-developed platelet structure is found to be more effective in chloride adsorptive and entrapment capabilities, which caused higher corrosion resistance compared to other developed NiAl-LDHs. The comparison of the synthesized NiAl-LDH morphologies on their ion-exchange capabilities, barrier effect and their combined effect on corrosion resistance properties is reported.

The catalytic degradation of nitrobenzene by the Cu-Co-Fe-LDH through activated oxygen under ambient conditions

Efficient and low-cost catalysts for catalytic wet air oxidation (CWAO) under ambient conditions are of great significance for the degradation of hydrophobic organic contaminants. In this study, four LDH catalysts were prepared and their catalytic performance was studied by the degradation of nitrobenzene. The CuCoFe-LDH shows the best catalytic activity with an NB removal efficiency of 41.2%. The CuCoFe-LDH exhibited a typical layer structure, with a specific surface area of 167.32 m² g^-1, and Cu²⁺, Co²⁺ and Fe³⁺ were evenly dispersed on the crystal. The NB removal efficiency was increased by 12.5% through adding formic acid. After five recycling processes, the NB removal efficiency was 18.9% because 3.8 mg g^-1 of Co was leached out of the LDH. In the CWAO process, H₂O₂, ˙OH, ˙O₂^- and ¹O₂ were successfully formed through activated oxygen by the CuCoFe-LDH catalyst under ambient conditions. This work further broadens the application scope of layered double hydroxides (LDHs) in the degradation of organic pollutants by CWAO under ambient conditions.