Metal Hydroxide Layers Research Articles

Europium insertion into MgAl hydrotalcite-like compound to promote the photocatalytic oxidation of nitrogen oxides

Easy synthesis of efficient, non-toxic photocatalysts is a target to expand their potential applications. In this research, the role of Eu3+ doping in the non-toxic, affordable, and easily prepared MgAl hydrotalcite-like compounds (HTlcs) was explored in order to prepare visible light semiconductors. Eu doped MgAl-HTlcs (MA-xEu) samples were prepared using a simple coprecipitation method (water, room temperature and atmospheric pressure) and europium was successfully incorporated into MgAl HTlc frameworks at various concentrations, with x (Eu3+/M3+ percentage) ranging from 2 to 15. Due to the higher ionic radius and lower polarizability of Eu3+ cation, its presence in the metal hydroxide layer induces slight structural distortions, which eventually affect the growth of the particles. The specific surface area also increases with the Eu content. Moreover, the presence of Eu3+ 4f energy levels in the electronic structure enables the absorption of visible light in the doped MA-xEu samples and contributes to efficient electron-hole separation. The microstructural and electronic changes induced by the insertion of Eu enable the preparation of visible light MgAl-based HTlcs photocatalysts for air purification purposes. Specifically, the optimal HTlc photocatalyst showed improved NOx removal efficiency, ∼ 51% (UV–Vis) and 39% (visible light irradiation, 420 nm), with excellent selectivity (> 96 %), stability (> 7 h), and enhanced release of •O2− radicals. Such results demonstrate a simple way to design photocatalytic HTlcs suitable for air purification technologies.

Efficient phosphate removal by Mg-La binary layered double hydroxides: synthesis optimization, adsorption performance, and inner mechanism.

Layered double hydroxides (LDH) hold great promise as phosphate adsorbents; however, the conventional binary LDH exhibits low adsorption rate and adsorption capacity. In this study, Mg and La were chosen as binary metals in the synthesis of Mg-La LDH to enhance phosphate efficient adsorption. Different molar ratios of Mg to La (2:1, 3:1, and 4:1) were investigated to further enhance P adsorption. The best performing Mg-La LDH, with Mg to La ratio is 4:1 (LDH-4), presented a larger adsorption capacity and faster adsorption rate than other Mg-La LDH. The maximum adsorption capacity (87.23 mg/g) and the rapid adsorption rate in the initial 25 min of LDH-4 (70 mg/(g·h)) were at least 1.6 times and 1.8 times higher than the others. The kinetics, isotherms, the effect of initial pH and co-existing anions, and the adsorption-desorption cycle experiment were studied. The batch experiment results proved that the chemisorption progress occurred on the single-layered LDH surface and the optimized LDH exhibited strong anti-interference capability. Furthermore, the structural characteristics and adsorption mechanism were further investigated by SEM, BET, FTIR, XRD, and XPS. The characterization results showed that the different metal ratios could lead to changes in the metal hydroxide layer and the main ions inside. At lower Mg/La ratios, distortion occurred in the hydroxide layer, resulting in lower crystallinity and lower performance. The characterization results also proved that the main mechanisms of phosphate adsorption are electrostatic adsorption, ion exchange, and inner-sphere complexation. The results emphasized that the Mg-La LDH was efficient in phosphate removal and could be successfully used for this purpose.

The crystal structure of charmarite – the first case of a 11 × 11 Å superstructure mesh in layered double hydroxides

Abstract Charmarite, Mn4Al2(OH)12CO3⋅3H2O, is a hydrotalcite supergroup member (or layered double hydroxide, LDH) with a previously unknown crystal structure and a Mn2+-analogue of quintinite (commonly erroneously reported as ‘2:1 hydrotalcite’). The single-crystal X-ray diffraction (XRD) data were obtained from the specimen from Mont Saint-Hilaire, Québec, Canada and are best processed in the space group P $\bar{3}$ , a = 10.9630(4), c = 15.0732(5) Å and V = 1568.89(12) Å3. The crystal structure has been solved by direct methods and refined to R1 = 0.0750 for 3801 unique reflections with Fo > 2σ(Fo). The charmarite structure has long-range periodicity in the xy plane due to $2\sqrt 3$ a’ × $2\sqrt 3$ a’ scheme (or 11 × 11 Å) determined for LDHs for the first time (where a’ is a subcell parameter ≈ 3.2 Å). This periodicity is produced by the combination of two superstructures formed by: (1) Mn2+ and Al3+ ordering in the metal-hydroxide layers [Mn4Al2(OH)12]2+ according to the $\sqrt 3$ a’ × $\sqrt 3$ a’ pattern and (2) the (CO3)2– ordering according to the 2a’ × 2a’ pattern in the [CO3(H2O)3]2– interlayer sheet in order to avoid close contacts between adjacent carbonate groups. The $2\sqrt 3$ a’ × $2\sqrt 3$ a’ superstructure is an example of the adaptability of the components of the interlayer space to the charge distribution of the metal-hydroxyl layers. The Mn2+ and Al3+ cations have a large difference in size, which apparently leads to the considerable degree of their order as di- and trivalent cations resulting in a higher degree of statistical order of the interlayer components. Both powder and single-crystal XRD data show that the samples studied belong to the hexagonal branch of two-layer polytypes (2T or 2H) with d00n ≈ 7.57 Å. The chemical composition of the samples studied is close to the ideal formula. The Raman spectrum of charmarite is reported and the band assignment is provided.

Comprehensive Analysis of the Chemical and Structural Transformations of Mg-Al-CO3 Layered Double Hydroxides with Different Mg/Al Ratios at Elevated Temperatures.

Mg-Al layered double hydroxides (LDHs) with CO32- interlayer anions are promising CO2 adsorbents. Here, we analyzed the quantitative gas evolution behaviors of Mg-Al LDH particles with different Mg/Al ratios during the multistep chemical/structural transformations at elevated temperatures. The Mg/Al molar ratio strongly affects the behavior: the transformation changes from two apparent steps to three steps depending on the Mg/Al ratio. The transformation occurs in essentially the same way as that observed for large Mg-Al LDH crystals: (1) release of the interlayer water, (2) partial dehydroxylation of the hydroxyl layers followed by coordination of carbonate ions to the metals, and (3) collapse of the layered structure. We provide a molecular/atomic level picture of the structure in each step of the transformation by first-principles density functional theory (DFT) calculation. The structurally optimized model and reexamination of experimental data showed that step (1) results in a large decrease in the interlayer distance of the LDH from ∼7.6 to ∼6.7 Å (a decrease of ∼0.9 Å) and the possible explanation is the waving of the metal hydroxide layers. This study provides a comprehensive understanding of the structural changes of LDHs with various Mg/Al ratios to resolve the various interpretations in the literature.

Nanocomposite Anion Exchange Membranes for Aems Electrolyzers: Ion Transport and Carbonation Studies

Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different electrochemical energy conversion and storage (EECS) devices. [1] AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes. However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]In this work, nanocomposite AEMs based on Polysulfone ionomer (PSU) and two different nanoadditives, Layered Double Hydroxide (LDH) and Nanoscale Ionic Materials (NIM), have been developed and studied. [3] LDHs are mineral anionic clays with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism. NIMs have a nanostructure consisting of a spherical silica core functionalized with an ionic oligomeric corona. Due to the extremely high number of functional groups present, these materials are able to significantly increase the ionic conduction capacity of the resulting electrolyte. [4]PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree between 60 and 80%, preserving its stability.Nanocomposites AEMs were investigated in both OH- and HCO3 - forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate. Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance.

Synthesis, structure and canted antiferromagnetism of layered cobalt hydroxide sorbate

Layered hybrid organic–inorganic compounds of the transition metal hydroxides, in which part of the hydroxide ions are exchanged for organic ligands that act as spacers of the 2-dimensional inorganic layers, elicit new or enhanced properties. The interlayer ligands enable fine-tuning of the physicochemical properties of the metal hydroxide layers, including their magnetism. Specifically, within the Co(OH)2 family of transition ion hydroxides, this approach has been leveraged to stabilize antiferromagnetic and ferrimagnetic ground states for pillared and layered complexes, respectively. Here, we further investigate the effects of changing dimensionality, local Co coordination, and interlayer spacing on the magnetic ground state of brucite-like β-Co(OH) layered complexes. We report the hydrothermal synthesis of a new crystalline material, Co(OH)(sorb), where the inorganic layers are spaced by monotopic, doubly unsaturated sorbate ligands (sorb = [C6H7O2]−). The material was structurally characterized using powder X-ray and neutron diffraction measurements and the crystal structure was solved ab initio from powder diffraction data and refined using the Rietveld method. The magnetic susceptibility and magnetization measurements reveal canted antiferromagnetic ordering with TN = 41.7 K. This report augments our understanding of tuning magnetism through dimensionality in layered complexes and represents a step forward in the design of new 2D layered hybrid compounds.

Molecular-Level Pictures of Chemical and Structural Transformations of Mg–Al Layered Double Hydroxide Crystals (Mg/Al = 2) at Elevated Temperatures

Mg–Al layered double hydroxides (Mg–Al LDHs) are crystalline compounds with layered structures composed of hydroxide layers and interlayer anions such as CO32–. A detailed understanding of the thermal decomposition behaviors is indispensable for materials design toward promising applications such as CO2 adsorbents. Here, we report that the thermal decomposition behavior of a well-crystallized Mg–Al LDH having an Mg/Al atomic ratio of 2, where all hydroxyl groups experience an identical chemical environment, provides quantitative evidence for and clear molecular-/atomic-level pictures of the multi-step transformation of the crystals at elevated temperature. Thermal decomposition is found to occur in multi-steps of (1) release of the interlayer water, (2) dehydroxylation of just one-third of hydroxyl groups accompanied by the formation of coordinatively unsaturated sites followed by coordination of carbonate to metals, and (3) collapse of the layered structure at a higher temperature. The stepwise structural transformations are not ascribable to different coordination environments of hydroxyl groups. The reason is possibly that the structure after the partial dehydroxylation of the metal hydroxide layers is rather stable. Structural optimization by first-principles DFT calculations and its powder X-ray diffraction simulation supported the interpretations for and the molecular-level pictures of the structural transformations. These results resolve the various interpretations on the structural change of the crystals.

Role of Anions in Stabilizing the [Zn–Al] Layered Double Hydroxides: A Thermodynamic Study

Room-temperature acid solution calorimetry, high-temperature oxide melt solution calorimetry, and low-temperature heat capacity measurements were employed to calculate the thermodynamic stabilities of the [Zn–Al–X] layered double hydroxides (LDH) containing different anions (X = Cl–, CO32–, and SO42–). Cryogenic heat capacity measurements demonstrated a Schottky-type anomaly in the heat capacity of all three LDHs below 11 K. This anomaly is attributed to the tunneling of protons between adjacent oxygen atoms in the LDH interlayer as this creates an energy system similar to a two-level system modeled with a Schottky term. These heat capacity measurements were also used to determine vibrational entropies which, when combined with configurational entropies, provide standard entropies of these LDHs. Enthalpies of formation of LDHs from binary components were determined and combined with the entropies of formation to calculate Gibbs free energies. Based on these values, the order of stability is [Zn–Al–SO4] > [Zn–Al–CO3] > [Zn–Al–Cl]. This trend results from a combination of the interlayer spacing, amount of water in the interlayer, interactions among the interlayer species, and interactions between the metal hydroxide layer and the interlayer.

Synthesis and Structure Refinement of [Co-Al4-X] LDHs (X = NO3 - and SO4 2-) from Nordstrandite.

[M-Al4-X] LDHs (M = divalent metal and X = anion) are a class of aluminum-rich layered double hydroxides synthesized from both the gibbsite and bayerite polymorphs of Al(OH)3. Henceforth, "g" and "b" are used to indicate gibbsite and bayerite. Despite the differences in the stacking arrangement of the hydroxyl layers in the precursor polymorphs, [M-Al4-X] LDHs whether synthesized from gibbsite or bayerite were seen to be structurally the same. In this work, we report the first ever synthesis of [M-Al4-X] LDHs (M = Zn, Ni, and Co and X = NO3 - and SO4 2-) from nordstrandite, which is yet another polymorph of Al(OH)3. Hereafter, "n" represents the nordstrandite precursor. We report that n-[M-Al4-X] LDHs do not differ structurally from those prepared from gibbsite and bayerite. We also report the structural refinement of n-[Co-Al4-X] LDHs, where X = NO3 - and SO4 2-. This work is also significant as it gives for the very first time the refined structure of a [Co-Al4-NO3] LDH, though there are earlier reports on the synthesis of this LDH from both gibbsite and bayerite. The NO3 - ion in the interlayer makes an angle of ∼48° with the plane of the metal hydroxide layer, and its symmetry reduces from D 3h to C 2v . Similarly, the change in the symmetry of the SO4 2- ion in the interlayer is from T d to C 3v .

(Invited) Local and Macroscopic Probes of Semiconductor/Electrocatalyst Photochemical Interfaces

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. These measurements are compared and contrasted to a new approach to “wirelessly” measure interfacial for different contact materials by analyzing shifts in element-specific shifts in x-ray photoelectron emission energies. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

(Invited) Ions Transport Properties and Carbonation Process Investigation of Nanocomposite Anion Exchange Membranes Containing Layered Double Hydroxide

Electrochemical energy conversion and storage (EECS) technologies such as fuel cells and electrolyzers are expected to play a pivotal role owing to their compatibility with the environment and high energy conversion efficiency. Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different EECS devices.[1] They have numerous advantages respect to the proton-exchange membranes, such as operation at high pH conditions, significantly lower cost (based on low-cost raw materials), less corrosive environment, and significantly lower fuel crossover (one order of magnitude lower than the acidic counterparts). AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes.However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]We report on an extensive study on nanocomposite AEMs based on tetramethylammonium Polysulfone ionomer (PSU) and Layered Double Hydroxide (LDH) as nanofiller. [3] PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree of about 80%, preserving its stability. Concerning the LDHs, these mineral anionic clays are excellent inorganic anionic conductors with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism.PSU/LDH AEMs were investigated in both OH- and forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate.In addition, the 13C-NMR technique was used to investigate carbonation processes in the presence of CO2, showing that the presence of the LDH platelets into the AEM remarkably reduces the conversion rate of hydroxyl groups, as much as slow down the diffusion of carbonate ions.Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance. Acknowledgments This work has been supported by the Italian Ministry for University and Research (MUR) for funding through the FISR 2019 project AMPERE (FISR2019_01294). References Vincent, I. and Bessarabov, D. (2018) Renewable and Sustainable Energy Reviews 81, 1690–704G. Arges, L. Zhang, ACS Appl. Energy Mater. 1 (2018) 2991–3012,Simari, C.,...and Nicotera, I. (2022) Electrochimica Acta, 403, p. 139713.

(Invited) Nanoscale Electrocatalyst/Semiconductor Interfaces As Charge-Carrier-Selective Contacts in Photocatalytic and Photoelectrochemical Systems

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

Enhanced catalytic activity of layered double hydroxides via in-situ reconstruction for conversion of glucose/food waste to methyl lactate in biorefinery

Conversion of food waste into valuable chemicals under mild conditions has attracted increasing attention. Herein, a series of nano-sized MgAl layered double hydroxides (LDHs) were firstly developed as solid base catalyst for the methyl lactate (MLA) production directly from glucose/food waste. Glucose, which could be easily obtained from cellulose or starch-rich food waste via hydrolysis, was thus selected as the model compound. It is inspiring to find that the metal hydroxide layer in prepared LDHs was highly stable and suitable enlarged interlayer distance was reconstructed owing to in-situ intercalation of formed aromatics during the reaction, which was demonstrated by 27Al magic angle spinning nuclear magnetic resonance and time-of-flight secondary ion mass spectrometry analysis. As a result, in-situ activation of the catalysts along with gradually enhanced catalytic activity was obtained in the recycling runs and the highest MLA yield of 47.6% from glucose was achieved over LDHs (5:1) after 5 runs at 150 °C. Most importantly, the scope was further extended to other typical substrates (e.g. Chinese cabbage and rice) and the results demonstrated the effectiveness of present conversion system for real food waste.

Oriented Films of Metal-Organic Frameworks on Metal Hydroxides via Heteroepitaxial Growth

Abstract Framework compounds such as metal-organic frameworks (MOFs) are characterized by their micropores, which provide a large accessible surface for applications such as gas storage and catalysis. Additional functionalities for electronics, photonics, and magnetic applications can be attained by accommodating functional guests (molecules, ions, or nano-objects) into the pores. Device-scale MOF thin films with controlled crystallographic orientations must be achieved to enhance these sophisticated functionalities for practical applications. Recently, we reported the heteroepitaxial growth of MOFs via solution processing of precursor metal hydroxide-oriented films. The resultant MOF film exhibited crystallographic orientation along all three axes, which can be used as quasi-single crystals because the pores are aligned in specific directions at the macroscopic scale. The orientation primarily depends on the crystallographic plane of the seed metal hydroxide layer. In this article, recent advances in the fabrication and application of oriented MOF films are reviewed.

Valleriite, a Natural Two-Dimensional Composite: X-ray Absorption, Photoelectron, and Mössbauer Spectroscopy, and Magnetic Characterization.

Valleriite is of interest as a mineral source of basic and precious metals and as an unusual material composed of two-dimensional (2D) Fe–Cu sulfide and magnesium hydroxide layers, whose characteristics are still very poorly understood. Here, the mineral samples of two types with about 50% of valleriites from Noril’sk ore provenance, Russia, were examined using Cu K- and Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), 57Fe Mössbauer spectroscopy, and magnetic measurements. The Cu K X-ray absorption near-edge structures (XANES) spectra resemble those of chalcopyrite, however, with a higher electron density at Cu+ centers and essentially differ from those of bornite Cu5FeS4; the Fe K-edge was less informative because of accompanying oxidized Fe-containing phases. The post-edge XANES and extended XAFS (EXAFS) analysis reveal differences in the bond lengths, e.g., additional metal–metal distances in valleriites as compared with chalcopyrite. The XPS spectra confirmed the Cu+ and Fe3+ state in the sulfide sheets and suggest that they are in electron equilibrium with (Mg, Al) hydroxide layers. Mössbauer spectra measured at room temperature comprise central doublets of paramagnetic Fe3+, which decreased at 78 K and almost disappeared at 4.2 K, producing a series of hyperfine Zeeman sextets due to internal magnetic fields arising in valleriites. Magnetic measurements do not reveal antiferromagnetic transitions known for bornite. The specific structure and properties of valleriite are discussed in particular as a platform for composites of the 2D transition metal sulfide and hydroxide (mono)layers stacked by the electrical charges, promising for a variety of applications.

A high-performance oxygen evolution electrode of nanoporous Ni-based solid solution by simulating natural meteorites

Transforming the intermittent solar and wind energies into green hydrogen energy by water electrolysis is deemed to be a promising approach to substitute traditional fossil energies. However, the scarcity of precious IrO2/RuO2-based metal oxides for oxygen evolution reaction hinders the large-scale utilization. Here, based on the nanoscale Kirkendall effect, we fabricate nanoporous γ-Ni based solid solution with the simulated FeCoNi surface compositions of Gibeon meteorites by a facile microwave sintering method. A metal (oxy)hydroxide layer with a thickness of approximately 9 nm was formed on the surface of as-prepared electrode after aging, achieving an overpotential of only 254 mV at 10 mA cm−2 and a low Tafel slope of 37.0 mV dec−1. The electrode exhibits outstanding long-term stability in a wide range of current densities without compromising the electrocatalytic activity, which is attributed to the 3D-interconnected nanoporous structure, synergistic effect of different species, high conductivity and excellent wettability. The fabrication of nanoporous non-precious solid solution architectures which mimic naturally-formed materials provides a feasible strategy for the design of high-performance electrocatalysts towards clean energy applications.

Thermal Evolution of Natural Layered Double Hydroxides: Insight from Quintinite, Hydrotalcite, Stichtite, and Iowaite as Reference Samples for CO3- and Cl-Members of the Hydrotalcite Supergroup

In Situ high-temperature powder X-ray diffraction experiments were undertaken for the coarse crystalline natural layered double hydroxides (LDHs) quintinite, hydrotalcite, stichtite, and iowaite in the temperature range 25–1000 °C, with thermal analyses of these minerals and their annealed forms carried out in parallel. In the temperature range from 25 °C to 170–210 °C quintinite, hydrotalcite, and stichtite (carbonate members of the LDH family) demonstrated contraction of the basal d00n-value of 0.1–0.3 Å, followed by a sharp contraction of 1.0–1.1 Å at T > 170–210 °C. The high-temperature modified states were stable up to 380–420 °C, before decomposing to an amorphous phase. Iowaite (chloride member of the family) was stable up to 320 °C and transformed to an amorphous phase at higher temperature. Iowaite experiences continuous contraction of the d00n-value of up to 0.5 Å in the temperature range 25–200 °C, reaching a plateau at a temperature range of 200–320 °C. Assessing the reversibility of thermal transformation shows complete reconstruction of the crystal structure of the hydrotalcite and iowaite heated to 300 °C. Solid-state nuclear magnetic resonance analysis shows that some Al changes coordination from 6- to 4-fold, synchronously with quintinite transformation to the amorphous phase. All phases transform to periclase and a spinel-type compound upon further heating. Thermal analysis of samples annealed at 125 °C shows that carbonate members do not have a tendency to form dehydrated phases, whereas for iowaite, a dehydrated phase having 0.9 apfu lesser water content as in the initial sample has been obtained. Thermal evolution of LDHs is found to depend on the nature of the interaction of interlayer species and water molecules to H atoms of the metal-hydroxide layer.

Unveiling the local structure of 2-mercaptobenzothiazole intercalated in (Zn2Al) layered double hydroxides

The structure and composition of a zinc‑aluminum layered double hydroxide (Zn2Al LDH) with the intercalated 2-mercaptobenzothiazole corrosion inhibitor (a.k.a. benzo[d]thiazole-2-thiol) are interpreted by means of atomistic molecular dynamics (MD) simulations. The results concerning the proportion of intercalated 2-mercaptobenzothiazole and water species in the Zn2Al LDH interlayer were correlated with experimental X-ray diffraction (XRD) and thermogravimetric analysis (TGA) data of samples obtained at pH 8.5, 10 and 11.5. While the sample synthesized at the lowest pH is almost free of contaminants, the sample obtained at the highest pH is contaminated by a small fraction of a material with intercalated OH−. The comparison of the calculated and XRD interlayer distances suggests that the most stable structure has a ratio of ~4.5 water molecules per intercalated organic species, which is higher than the ratio of ~2 typically reported in the literature. The distribution of molecules in the LDH interlayer consists of a layer of water near the hydroxides, a second layer grown over the first layer, with the 2-mercaptobenzothiazole species adopting conformations with the sulfur of the thioamide group facing the hydroxide/water layers and the 6-member ring oriented towards the middle of the interlayer. Different structural analyses were done to explain the equilibria between the different species in the interlayer space, and their molecular interactions with the LDH metal hydroxide layers.

Reversible hydration of the perchlorate-intercalated layered double hydroxides of Li and Al: structure models for the dehydrated phases

Imbibition of lithium sulphate into aluminium hydroxide is known to result in a sulphate-intercalated layered double hydroxide (LDH) of Li and Al. The perchlorate ion has the same size and molecular symmetry as the sulphate ion, but only half its charge. Consequently, twice the number of \(\hbox {ClO}_{4}^{-}\) ions is needed to balance LDHs the charge on the metal hydroxide layer, compared to the \(\hbox {SO}_{4}^{2-}\) ions. In this work, the \(\hbox {ClO}_{4}^{-}\)-intercalated LDHs were obtained from both the bayerite and gibbsite precursors. Inclusion of the hydration sphere along with the \(\hbox {ClO}_{4}^{-}\) anion, induced turbostratic disorder in the stacking of the metal hydroxide layers. Temperature-induced dehydration (\(T \sim 100\)–\(140{^{\circ }}\hbox {C}\)) brought about a partial ordering in the interlayer region and the \(\hbox {ClO}_{{4}}^{{-}}\) ion oriented itself with one of its \(C_{{2}}\)-axes parallel to the metal hydroxide layer. The close packing of \(\hbox {ClO}_{4}^{-}\) ions could be realized by the complete dehydration of LDH and the distribution of the \(\hbox {ClO}_{4}^{-}\) ions in all the available interlayer sites. In contrast, within the crystal of the sulphate analogue, the sulphate ions occupy only half the number of interlayer sites. The other half is occupied by the residual water molecules, as the sulphate analogue does not fully dehydrate even at elevated temperatures. This difference in the behaviour of the two LDHs has its origin in the large difference in the hydration enthalpies of the two anions.

CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

Green single‐crystals of the hydroxochromate(III) CaNa[Cr(OH)6] were grown under highly alkaline hydrothermal conditions at about 200 °C. The starting materials Ca(NO3)2·6H2O and Cr(NO3)3·9H2O were reacted in a mixture of water and sodium hydroxide with the molar ratio of 2.8:1. CaNa[Cr(OH)6] crystallizes in the non‐centrosymmetric trigonal space group R3 with the lattice parameters a = 583.86(2) pm and c = 1428.73(6) pm [T = 100(1) K]. Characteristically, the crystals are reverse‐obverse as well as inversion twins. The crystal structure is a stack of uncharged metal hydroxide layers, which can be regarded as a cation‐ordered rhombohedral variant of the Mg(OH)2 (brucite) structure type. The oxidation state of chromium(III) and its coordination by hydroxide groups was confirmed by UV/Vis and IR spectroscopy, respectively. Temperature‐dependent magnetic measurements revealed paramagnetic behavior with an effective moment of 3.82 μB per chromium atom. The thermal decomposition of CaNa[Cr(OH)6] takes place at about 225 °C, where the fast elimination of 1.5 equivalents of water is followed by the oxidation of chromium(III) to chromium(VI). Upon further heating to 1000 °C and 1200 °C, the intermediate decomposition products CaCrO4 and Na2CrO4 transform into the oxochromates(V) Ca5(CrO4)3O0.5 and Ca3(CrO4)2, respectively.