Double Hydroxide Structure Research Articles

Photocatalytic Degradation of 2,4,6-Trichlorophenol by MgO–MgFe2O4 Derived from Layered Double Hydroxide Structures

In recent years, the search for solutions for the treatment of water pollution by toxic compounds such as phenols and chlorophenols has been increasing. Phenols and their derivatives are widely used in the manufacture of pesticides, insecticides, paper, and wood preservers, among other things. Chlorophenols are partially biodegradable but not directly photodegradable by sunlight and are extremely toxic—especially 2,4,6-trichlorophenol, which is considered to be potentially carcinogenic. As a viable proposal to be applied in the treatment of water contaminated with 2,4,6-trichlorophenol, this paper presents an application study of the thermally activated Mg/Fe layered double hydroxides as photocatalysts for the mineralization of this contaminant. Activated Mg/Fe layered double hydroxides were characterized by X-ray diffraction, thermal analysis, N2 physisorption, and scanning electron microscopy with X-ray dispersive energy. The results of the photocatalytic degradation of 2,4,6-trichlorophenol in aqueous solution showed good photocatalytic activity, with an efficiency of degradation of up to 93% and mineralization of 82%; degradation values which are higher than that of TiO2-P25, which only reached 18% degradation. The degradation capacity is attributed to the structure of the MgO–MgFe2O4 oxides derived from double laminate hydroxide Mg/Fe. A path of degradation based on a mechanism of superoxide and hollow radicals is proposed.

Novel Ag-ZnO-La2O2CO3 photocatalysts derived from the Layered Double Hydroxide structure with excellent photocatalytic performance for the degradation of pharmaceutical compounds

Abstract In this work, we have prepared the Ag-ZnO-La2O2CO3 nanomaterials as promising photocatalysts for the photocatalytic degradation of pharmaceutical pollutants. Firstly, a series of ZnAl1-xLax(CO3) (0 ≤ x ≤ 0.5) layered double hydroxides (LDHs) were synthesized by the co-precipitation method at the component molar ratio of Zn/(Al + La = 3, where La/Al = 0, 0.25 and 0.5). Photocatalysts were prepared by the calcination of the LDH precursors at different temperatures of 300, 400, 500, 600, 800 and 1000 °C. The effects of the La/Al molar ratio and the calcination temperature on the photocatalytic activity of the catalysts were evaluated by the degradation of caffeine as a model pharmaceutical pollutant in aqueous solutions under the UV irradiation. Thereafter, in order to increase the photocatalytic activity, the catalysts obtained at the optimal La/Al molar ratio and calcination temperature were doped with the Ag noble metal at various concentrations (i.e. 1, 3 and 5 wt%) using the ceramic preparation process to obtain the desired Ag-ZnO-La2O2CO3 catalysts. The synthesized photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS). Detailed photocatalytic experiments based on the effects of the irradiation time, the dopant amount, the catalyst dose, the initial solution pH and reuseability were performed and discussed in this study. The Ag doped material showed significantly a higher photocatalytic activity compared to the undoped, pure ZnO and P-25 catalysts. The experimental results show that the highest photocatalytic activity was obtained from the Ag (5%) doped Zn-0.75Al-0.25La-CO3 photocatalysts calcined at 500 °C with a degradation efficiency of 99,4% after 40 min of irradiation only. This study could provide a new route for the fabrication of high performance photocatalysts and facilitate their application in the environmental remediation issues.

Structural evolution of NiAl-based layered nanostructures grown by a low-temperature hydrothermal method

We report on the growth mechanism of NiAl-based layered nanostructures on rigid substrates using a low-temperature hydrothermal method. NiAl-layered double hydroxide (LDH) structures were grown with various source concentrations and growth times. Structural and optical investigations showed that uniform NiAl-LDHs were grown on the substrates because Al(OH)3 was sufficiently supplied from the aluminum thin film seed layer when the Ni source concentration was low and in the initial stage of the growth. However, a bilayer of NiAl-LDH and α-Ni(OH)2 was formed when the Ni sources were sufficiently supplied and the growth time was long enough because of the deficient Al(OH)3 sources at the growth front.

Synthesis of α- and β-CoNi binary hydroxides nanostructures and luminol chemiluminescence study for H2O2 detection

Herein, we firstly reported utilization of α- and β-CoNi hydroxides nanostructures as efficient catalyst for the luminol-H2O2 CL reaction. The catalysts were synthesized by hydrothermal method and characterized by XRD, FT-IR, FE-SEM/EDX, and UV–vis. The α-form had layered double hydroxide (LDH) structure witout trivalent cations in the metal hydroxide layer, while the structure of β-phase was similar to brucite. UV–vis spectra demonstrated that the partial oxidation of Co2+ and Ni2+ did not occur in the both catalysts. It was found that α- and β-CoNi could significantly enhance CL reaction of luminol-H2O2 at pH = 11. During study of experimental parameters the higher relative intensity of CL signal in the presence of α-Co-Ni hydroxide was observed. This may be due to the greater Co2+/Ni2+ ratio in this catalyst than β-phase. Therefore, a facile and rapid method was developed to assay H2O2 using α-CoNi binary hydroxide as catalyst. Under optimum conditions, in the range 0.1–12 μM of H2O2 concentrations the CL intensity was linear and a detection limit (S/N = 3) of 0.05 μM for H2O2 was obtained. The relative standard deviation for 6 repetitive determinations was less than 2.9%. The proposed method was successfully applied to detect H2O2 in river water samples with recoveries of 97.3% and 106%.

Oxygen Evolution Catalysis with Mössbauerite-A Trivalent Iron-Only Layered Double Hydroxide.

Mössbauerite is investigated for the first time as an "iron-only" mineral for the electrocatalytic oxygen evolution reaction in alkaline media. The synthesis proceeds via intermediate mixed-valence green rust that is rapidly oxidized in situ while conserving the layered double hydroxide structure. The material catalyzes the oxygen evolution reaction on a glassy carbon electrode with a current density of 10 mA cm-2 at 1.63 V versus the reversible hydrogen electrode. Stability measurements, as well as post-electrolysis characterization are presented. This work demonstrates the applicability of iron-only layered double hydroxides as earth-abundant oxygen evolution electrocatalysts. Mössbauerite is of fundamental importance since as an all Fe3+ material its performance has no contributions from unknown synergistic effects as encountered for mixed valence Co/Ni/Fe LDH.

Novel NiFe/NiFe-LDH composites as competitive catalysts for clean energy purposes

The electrodeposition of metals generally employs several additives to avoid the formation of undesirable byproducts such as oxides and hydroxides. Although the deposition of metals is still the main goal in the most metals electroplating, the applicability of these byproducts might be an interesting field which is not explored in detail so far. In this work, the significance of water splitting reaction in clean energy production, employing NiFe hydroxides formed during the metals electrodeposition, is demonstrated for oxygen evolution reaction. The synthetized materials are composites of three components easily prepared in one-step by means of electrodeposition. Specifically, a granular NiFe alloy is obtained over which local pH variation and chloride presence induce the formation of a layered double hydroxide structure. The study of the influence of solution composition, deposition time, and deposition potential on the catalytic properties of the composites with respect to the oxygen evolution reaction are analyzed. Deposition times of few seconds, deposition potentials in the range −1.4 to −1.6 V vs. Ag/AgCl/KCl3M, and solutions containing Fe(II), Ni(II) and high chloride concentrations, lead to the best catalysts, showing an η10 mA cm−2 about 0.280 V.

Controlled Release of Phosphate from Layered Double Hydroxide Structures: Dynamics in Soil and Application as Smart Fertilizer

A route is proposed to produce a hydrotalcite-like layered double hydroxide structure ([Mg-Al]-LDH) for phosphate fertilization. The mechanism of controlled phosphate release from the structure was investigated. The preparation strategy resulted in a phosphorus content of around 40 mg·g–1 LDH, which was higher than previously reported for related fertilizers. The release of phosphate into water from [Mg-Al-PO4]-LDH continued over a 10-fold longer period, compared to release from KH2PO4. Analysis using 31P NMR elucidated the nature of the interactions of phosphate with the LDH matrix. In soil experiments, the main interaction of P was with Fe3+, while the Al3+ content of LDH had no effect on immobilization of the nutrient. Assays of wheat (Triticum aestivum) growth showed that [Mg-Al-PO4]-LDH was able to provide the same level of phosphate nutrition as other typical sources during short periods, while maintaining higher availability of phosphate over longer periods. These characteristics confirmed the potential of this preparation route for producing controlled release fertilizers, and also revealed fundamental aspects concerning the interactions of phosphate within these structures.

A review on the synthesis and applications of green rust for environmental pollutant remediation

This paper discusses the various approaches to the synthesis of green rust. Green rusts are synthesized by changing the average oxidation state of iron compounds so that the ratio of ferric/ferrous ions is between 0.25 and 0.33. This can be achieved by mixing ferrous and ferric hydroxides, oxidation of ferrous compounds or reduction of ferric compounds. Characterization of the samples with analytical methods such as Mossbauer spectroscopy, X-ray diffraction spectroscopy, scanning electron microscopy, transmission electron microscopy and atomic-force microscopy is discussed. Green rusts have a layered double hydroxide structure, with an interchangeable anion layer and a mixed ferrous–ferric cation layer. This structure can adsorb ions by intercalation or exchange anions with its environment. The cation layer can oxidize, which gives a reducing nature to green rusts. These properties make them excellent candidates for the reduction and immobilization of several inorganic pollutants. Green rusts also have applications in dechlorination of organic compounds and in the synthesis of nanohybrid materials. These environmental and nanochemistry applications are also covered in this paper.

Incorporation of palladium nanoparticles and 10-molybdovanado phosphoric acid in FeCo layered double hydroxide structure: electrochemical and catalytic investigation for Mizoroki–Heck coupling reactions

Palladium (Pd) has been immobilized on FeCo layered double hydroxide (LDH) intercalated 10-molybdovanado phosphate (FeCo/Mo10V2-Pd) for the first time. The structure was characterized using various characterization techniques. The electrocatalytic behavior of FeCo/Mo10V2-Pd was investigated using cyclic voltammetry and linear sweep voltammetry techniques. The presence of Mo10V2 enhanced the catalytic activity of FeCo-Pd in the Heck coupling reaction. Also, the collaboration of Pd and Mo10V2 with LDH as a mesoporous support was studied. These reusable solid catalysts exhibited excellent activity, and the methodology is applicable to diverse substrates providing good to excellent yields of the desired products. This method has advantages of high yields, low reaction times, elimination of ligand and base, heterogeneous catalysts and simple methodology. It is notable that FeCo/Mo10V2-Pd can easily separate from the reaction mixture using an external magnet and reused for at least four successive runs without any considerable decrease in its catalytic activity.

Mechanochemical versus co-precipitated synthesized lanthanum-doped layered materials for olefin oxidation

Layered catalytic materials with a double hydroxide structure and Mg2+/(Al3++La3+) =3 (Al3+/La3+ molar ratio of 1) were prepared via co-precipitation and mechanochemical routes. In the second one, stoichiometric amounts of nitrates in mixture with Na2CO3 and NaOH were directly milled in a ceramic mortar under ambient conditions. XRD characterization confirmed the formation of hydrotalcite structure for the samples containing Mg and Al, and of mixtures of La2(CO3)2(OH)2 and poor crystallized Mg,Al-layered double hydroxides for the samples modified by La. Catalytic tests carried out in the oxidation of cyclohexene by hydrogen peroxide in acetonitrile shown that the presence of lanthanum enhances the activity of the MgAlLa catalyst compared to unmodified MgAl. For MgAl the reconstruction of the LDH structure is a fast process occuring even under the reaction conditions that is favorized by the replacement of CO32− by OH−. This in situ anion replacement allows an increase of the catalytic activity. The addition of La to MgAl eliminates the advantages induced by the reconstruction but preserves the system epoxidation activity. On the other hand, the presence of La generates catalysts with improved stability and recyclability. This behavior has been assigned to the cooperative participation of the different crystallographic phases.

Antimicrobial activity of 2-mercaptobenzothiazole released from environmentally friendly nanostructured layered double hydroxides.

The objective of this work was to assess the antibacterial effect of 2-mercaptobenzothiazole (MBT), used as model-biocide, immobilized in a layered double hydroxide (LDH) structure, under different conditions of pH and salinity, envisaging possible applications of the system in active antifouling and anticorrosion coatings. Biological effects of MBT immobilized in LDH were assessed by monitoring bacterial bioluminescence of cell suspensions of either Allivibrio fischeri or a recombinant strain of Escherichia coli, as a proxy for bacterial activity. Salinity (1, 2 and 3% NaCl) and pH (4, 5, 6 and 7) of the suspension media were experimentally manipulated and biocide release tests were performed in parallel. The release profiles obtained by UV-visible spectrophotometry indicated a fast release of biocide from MBT@LDH, slightly enhanced in 3% NaCl and under alkaline conditions. However, biological effects were more pronounced at 1% NaCl and at neutral pH. The release and toxic effect of MBT immobilized in LDH is dependent on the concentration of solutes in the suspension medium. The results confirm LDH as a biologically compatible material with potential to be used for biocide delivery.

3D hierarchical and porous layered double hydroxide structures: an overview of synthesis methods and applications

Nanostructured layered double hydroxide (LDH) materials with unique diffusion properties, large surface area along with desired functionalities have recently been produced for a number of well-established and advanced fields of applications. In this review, we describe and discuss the main synthetic methods that have been reported for the fabrication of porous LDH with tailored chemical composition and porosity. The efficiency of soft and hard templating approaches is particularly reviewed. A special emphasis is put on the microstructure and porosity of the materials according to the synthetic method involved. Finally, the performance enhancement of the materials due to the presence of porosity, especially macroporosity, in applications such as pollutant removal, catalysis and energy storage and conversion is overviewed.

Structural Analysis of Magnesium-Aluminium Hydrotalcites Modified with Iron III Obtained by Hydroxide Precipitation Method

Hydrotalcite-type anionic clays are a group of important materials used in adsorption processes, mainly for organic pollutants removal due the layered double hydroxide structure. The layer-interlayer interactions provide a structural memory even after dehydration and dehydroxylation process, since a very stable interlayer anions are part of material composition, like the carbonate one. A limited numbers of trivalent modifier cations can replace the aluminium cation due the ionic radii mismatch or oxidation state restrictions. Transition metal cations can replace the aluminium one in octahedral site of hydroxide lamellas in order to improve the adsorptive behaviors. In this work, we have investigate three compositions of carbonated magnesium-aluminium hydrotalcite with dif-ferent iron (III) contents through the co-precipitation method at pH 11 and aging step at 60°C for 6 hours. Thermal analysis was performed aiming the determination of the hydration water and hydroxyl amounts in dried precipitate samples, taking in account the results obtained for X-ray diffractometry, infrared spectroscopy, and nitrogen adsorption-desorption characterization for several thermally treated samples. All of synthesized samples showed high surface areas, even for high temperature of thermal treatment. The co-substitution with iron (III) reduced the temperature of dehydration and dehydroxylation process, but the co-substitution at 5 mol% provides other desirables characteristics, like a more amount of rhombohedral HDL phase and higher porosity, even after the thermal treatment at 500°C for 4 hours. This result makes that composition very applicable as a reusable adsorbent material in order to removal several types of micro-pollutant compounds in aqueous media.

Simple Aqueous Preparation of High Activity and Stability NiFe Hydrous Oxide Catalysts for Water Oxidation

A series of Ni1–xFex hydrous oxide nanoparticles (HO-nps, x = 0–0.5, 1) were prepared by stirring 0.1 M KOH solutions of NiCl2 and FeCl2 under air at room temperature. The resulting HO-nps can be used directly, or washed and isolated without loss of activity. The Ni1–xFex HO-nps are outstandingly active and stable toward the water oxidation reaction (WOR) in base. The Ni0.75Fe0.25 HO-np was the most active mass-normalized WOR catalyst in the series, with onset overpotential = 190 mV, Tafel slope = 24 mV dec–1, overpotential at 10 mA cm–2 = 234 mV, and loadings Ni + Fe = 0.135 mg cm–2. The catalysts were robust under long-term galvanostatic and duty-cycle testing. The Ni0.75Fe0.25 HO-np catalyst has a layered double hydroxide structure, and it appears that Fe enhances the activity of Ni toward the WOR by decreasing its electron density.

Enhancement of Mercury Sorbent Using Metal Aluminate Carbonates with Chloride under Hydrothermal Conditions

Mercury sorbents M–Al–CO3 (M=Mg2+, Mn2+, Fe2+, Cu2+, or Zn2+) were prepared by the coprecipitation of M2 + and Al3+ in an alkaline NaOH/Na2CO3 solution. The formation of a layered double hydroxide structure significantly enhanced Hg removal, uniquely with a suitable binder as a support. Both Hg(NO3 )2 and HgCl2 were used to perform the capture test. The calcined sorbent exhibited superior efficiency, particularly for materials having bivalent states of mixed oxide (MO), such as calcined Mn and Fe. Chloride ions effectively raised the mercury scrubber efficiency, regardless of the type of sorbent. The results show that synthetic sorbents can provide 96% removal of Hg at 200°C. In addition, a suitable binder such as TiO2 can be used as a support for dispersing metal oxides, which significantly decreases the MO content while maintaining a high equivalent capture capacity.

Design and development of an in situ synthesized layered double hydroxide structure of Fe-induced hydroxyapatite for drug carriers

Possible mechanisms: (i) formation of LDH structure of Fe-induced hydroxyapatite (HA), (ii) intercalation of Aceclofenac (AF) drug in LDH structure of FH carriers and (iii) releasing of drug via hydrolysis and/or reaction with other biomolecules.

Determination and reduction of Fe(III) incorporated into Mg–Fe layered double hydroxide structures

The syntheses of Mg–Fe(III) layered double hydroxides with different compositions at varied temperature through a co-precipitation method were attempted. The well-crystallized and white colored layered double hydroxides with chloride anions were obtained from the samples adjusted to the composition of Mg/Fe=4 and aged at higher temperature. The Fe species in the white colored Mg–Fe layered double hydroxides were evaluated as all Fe(III) species by a Mössbauer measurement. The synthesized Mg–Fe(III) layered double hydroxide was treated with a sugar alcohol at temperature above its melting point in order to reduce Fe(III) incorporated into the hydroxide layers. The Mg–Fe layered double hydroxide resulting from a sugar alcohol treatment showed the reduction property in an aqueous solution.

Dysprosium-containing layered double hydroxides nanoparticles intercalated with biologically active species as an approach for theranostic systems

Abstract A layered double hydroxide structure including dysprosium cations was prepared by co-precipitation. The nanoparticles showed a linear relationship with the reciprocal relaxation spin-lattice (T1) time of water protons which is reflected as contrast in aqueous suspensions analyzed by magnetic resonance imaging. The interlayer space of dysprosium containing LDH was successfully intercalated with folate, ibuprofen and gallate ions, which are key molecules for recognition of some cancer cells and treatment of diseases. The paramagnetic property of the dysprosium-containing LDH detected in this work beside the ability to transport drugs open up the opportunity to design theranostic materials in a single crystal phase with nanometric dimensions.

Dual nutraceutical nanohybrids of folic acid and calcium containing layered double hydroxides

Dual nutraceutical nanohybrids consisting of organic nutrient, folic acid (FA), and mineral nutrient, calcium, were prepared based on layered double hydroxide (LDH) structure. Among various hybridization methods such as coprecipitation, ion exchange, solid phase reaction and exfoliation-reassembly, it was found that exfoliation-reassembly was the most effective in terms of intercalation of FA moiety between Ca-containing LDH layers. X-ray diffraction patterns and infrared spectra indicated that FA molecules were well stabilized in the interlayer space of LDHs through electrostatic interaction. From the atomic force and scanning electron microscopic studies, particle thickness of LDH was determined to be varied with tens, a few and again tens of nanometers in pristine, exfoliated and reassembled state, respectively, while preserving particle diameter. The result confirmed layer-by-layer hybrid structure of FA and LDHs was obtained by exfoliation-reassembly. Solid UV–vis spectra showed 2-dimensional molecular arrangement of FA moiety in hybrid, exhibiting slight red shift in n→π* and π→π* transition. The chemical formulae of FA intercalated Ca-containing LDH were determined to Ca1.30Al(OH)4.6FA0.74·3.33H2O and Ca1.53Fe(OH)5.06FA2.24·9.94H2O by inductively coupled plasma-atomic emission spectroscopy, high performance liquid chromatography and thermogravimetry, showing high nutraceutical content of FA and Ca.

Vibrational spectroscopic study of sol–gel layered double hydroxides containing different tri- and tetravalent cations

Five different layered double hydroxides were synthesized from magnesium ethoxide in the presence of aluminium, gallium, indium, tin and zirconium acetylacetonates by using the sol–gel technique. The colloid suspensions initially obtained were gelled and separated by centrifugation. XRD patterns confirmed that the five solids thus obtained possessed a layered double hydroxide structure. Also, IR and Raman spectra revealed differences between the solids.