Studying Hydrotalcite-Like Compounds Isomorphically Substituted with Iron and Cobalt via Inverse Temperature-Programmed Reduction

Synthesis and evaluation of layered double hydroxide based sorbent for hot gas cleanup of hydrogen chloride

The aim of this work is to explore the possibility of use of layered double hydroxides (LDH) as a photocatalyst for the selective oxidation of toxic nitrogen oxides, NO and NO2 (NOx ), to nitrate. Recent studies suggest that layered double hydroxides could be suitable for this aim. LDH consists of brucite-like layers, with a partial MII/MIII substitution, which causes a positive charge in the layer. The excess of charge generated is replaced by the intercalation of anions in the interlayer space. LDHs are layered materials with general formula [MII1-xMIIIx(OH)2]Xn-x/n·nH2O where MII = Mg2+, Zn2+, etc; MIII = Al3+, Fe3+, etc; and Xn- an interlayer anion which could be a great variety of inorganic and organic species depending on the preparation method. Calcination at 500 ºC converts the LDHs into mixed oxides, rehydration of which restores the original LDH structure by interaction anions from aqueous solution (reconstruction). Thus, we synthesized MgAl-CrO4 LDH by reconstruction method, from the precursor MgAl-CO3 with Mg/Al ratio of 3, obtained by coprecipitation method, and compared these two LDH as photocatalysts for decomposition of NOx gases (deNOx). Techniques such as XRD, FT-IR and UV-Vis spectroscopy have been used to characterize the samples. Compared to the pristine LDH system, the textural characteristics, optical and electronic properties of chromium containing LDH samples exhibited a better harvesting of sunlight and the highest specific surface area (> 50 m2. g-1). In addition, the photocatalytic results showed high efficiency of LDH (60%) towards the complete oxidation of NOx.

Selective adsorption and mechanism of Se(Ⅳ) via electric field-enhanced interfacial charge characteristics of Mg-Al layered double hydroxides.

Mg-Al layered double hydroxides (LDHs) containing chloride (LDH-Cl) or carbonate (LDH-CO(3)) in the interlayer were obtained at room temperature and after calcitation at 450 degrees C (LDH-Cl-450 and LDH-CO(3)-450) and were characterized by X-ray diffraction and Fourier transform infrared analyses. Sorption isotherms of a humic acidlike fraction naturally occurring in olive oil mill waste waters, named polymerin, on these LDH minerals were carried out. Because LDH-CO(3) showed the highest capacity to sorb polymerin among the four LDH minerals synthesized, two organo-mineral complexes named LDH-CO(3)-LP (low polymerin) and LDH-CO(3)-HP (high polymerin) were prepared by coprecipitation (LDH-CO(3)-LP) and sorption onto a preformed LDH-CO(3) mineral (LDH-CO(3)-HP). These complexes were characterized chemically and physicochemically, and their stability to pH and after thermal treatment at 80 degrees C were evaluated. The diffuse reflectance infrared Fourier transform and X-ray analysis of the complexes indicated that polymerin was sorbed only on the external surfaces of LDH-CO(3) and no intercalation occurred. The LDH-polymerin complexes appeared to be more stable than LDH-CO(3) at pH 4.0 and showed that they were able to sorb both As(V) and Zn. Because the waste waters are usually contaminated with mixtures of pollutants in cationic and anionic forms, the LDH-polymerin complexes appear more suitable than the sorbents in a potential water remediation process.

The role of dispersion of LDH in fire retardancy: The effect of different divalent metals in benzoic acid modified LDH on dispersion and fire retardant properties of polystyrene– and poly(methyl-methacrylate)–LDH–B nanocomposites

Study on the sorption–desorption–regeneration performance of Ca-, Mg- and CaMg-based layered double hydroxides for removing phosphate from water

Layered double hydroxides (LDHs) are a class of anionic clays with a layered structure, that can be exfoliated giving place in 2D materials. Recently, these materials are attracting increasing attention from the point of view of their electrochemical performance towards energy storage and conversion applications. [1] In the field of electrocatalysts for the oxygen evolution reaction, Fe-based LDHs stand out as one of the best oxygen evolution reactions (OER) electrocatalysts in alkaline conditions. In the attempt to improve the electrochemical performance of Fe-LDHs-based electrodes, the effect of the microstructure, exfoliation, metal composition, crystallinity, doping, defect engineering and so on has been reported to increase the knowledge of electrochemical behavior and obtained better performances.[2] Beyond these well-studied parameters, interlayer space, Fe-clustering and covalent functionalization have been not explored, being the last two synthetic challenges within the field of LDHs.On the one hand, to explore the influence of interlayer space in the behaviour of Fe–LDHs as an oxygen evolution reaction (OER) electrocatalyst, we develop a family of NiFe-LDHs with the same metal composition, morphology, and dimensions and different alkyl sulfate to reach extreme ranges of basal spacing (from 8 to 32 Å). Increasing the basal space of the LDH results in a higher electrochemical surface area and a reduction of the resistance related to the chemisorption of oxygen leading to better kinetic behaviour. Indeed, the Tafel slope for the NiFe–LDHs with the highest basal space studied is similar to that obtained for benchmark exfoliated NiFe nanosheets and shows better stability as a consequence of the tridimensional robustness of the hybrid material. However, an excessive increment of the interlayer space compromises the onset potential.[3]On the other hand, to shed light on the influence of Fe-clustering on this topic, firstly, we have selected MgFe-based LDHs phases as model systems to decipher whether the Fe-clustering exert an effect on the OER performance. By optimization of a hydrothermal method based on triethanolamine different Fe-clustering degree has been achieved, and thanks to the diamagnetic behaviour of Mg, a simple magnetic characterisation allows us to identify the Fe-clustering degree. Nevertheless, both samples behave identically in terms of OER performance when glassy carbon electrodes are used. Surprisingly, when the samples are tested on the most employed electrode, nickel foam, striking differences arise. Specifically, the sample exhibiting the lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA/cm2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based ones during the electrochemical tests.[4]Last but not least, the covalent functionalization of LDHs has been explored to enhance their processability and stability as an electrocatalyst, for this, we develop for the first time the covalent functionalization of Fe-LDHs beyond silane-based molecules due to the reversible character of this bond in alkaline media. By a modified hydrothermal method from Kuroda et al.,[5] the covalent functionalization of NiFe-LDHs with tris(hydroxymethyl) aminomethane (Tris) was performed. The NiFe@Tris LDH were fully characterized using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis coupled with gas chromatography-mass spectrometry, magnetic measurement, XAS in order to identify the structural differences with a bare-LDH. At the same time, the stability of the covalent functionalization in basic media was proved. Interestingly, the Tris molecule anchored to LDHs increases the electrocatalytic stability in absence of binder material. Thus, after 40 hours of the OER, the overpotential increase is 37% lower in the NiFe@Tris than in the NiFe-Cl@bare. To check the origin of this overpotential decrease, ICP analysis of electrolytes pre-treated with acid after the stability test was performed. A reduction of 27% and 17% of Ni and Fe respectively is observed in the electrolyte for functionalized samples. Consequently, it is demonstrated that, regardless of the decomposition process (detachment or dissolution), the NiFe-LDH functionalized with the tripodal ligand Tris presents greater stability.[6]Overall, we demonstrated as molecular and cation engineering of LDHs can be used to enhance the electrochemical activity and increase the stability for their use as electrocatalysts for the oxygen evolution reaction.

BACKGROUND AND OBJECTIVES: This study aims down to evaluate the ability of chloride magnesium- aluminium- layered double hydroxides (4:1) for nitrate adsorption from the soil solution in successive cropping periods. METHODS: The study was conductedunder long-term cropping periods, including first crop): bell pepper; second crop: mentheae; third crop: cherry tomato; and fort h crop: wheat), absorption of soil mineral nitrate in fallow periods and nitrate absorption from plants by layered double hydroxides. The effect of layered double hydroxides on qualitative and quantitative characteristics of plants was also studied. FINDING: Results indicated that layered double hydroxides were able to induce long-term nitrate exchange in crop and fallow sequences. Layered double hydroxides can adsorb soil excessive nitrates in cropping periods and reduce nitrate concentration in the soil solution. Compared to control, the treatment with 16 gram layered double hydroxide/kilogram soil could reduce nitrate concentration in the soil solution by 95%. During two-week fallow periods, the amount of nitrates mineralized in the soil solution was increased, but layered double hydroxides treatments could adsorb them well and maintained the N-nitrate concentration in the soil solution at a low level. Additionally, Results indicated that application of 2, 4, 8 and 16 gram layered double hydroxides/kilogram soil led to 34%, 44%, 58% and 69% reduction in N-nitrate concentration of soil leachates, respectively, compared to control. By increasing nitrogen availability, layered double hydroxides improved the quantitative and qualitative properties of plants. Application of 2, 4, 8 and 16 gram layered double hydroxides/ kilogram soil increased the plant height (cherry tomato) by 14%, 26%, 50% and 80%, respectively. CONCLUSION: It is concluded that the layered double hydroxides has a potential to be used as a long-term nitrate exchanger to control the movement of nitrate in soil, and thereby reduce risks of nitrate leaching in crop production in sensible areas.

: In this study the preparation of layered double hydroxide (LDH)/sugar nanocomposites using an exfoliated MgAl-lactate LDH (Mg: Al = 3:1) and a sugar (sucrose and maltose) as precursors is described. The products have been characterised by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The properties of the composite materials have been further studied using differential scanning calorimetry (DSC). The expanded interlayer spacing of the lactate LDH suggests the intercalation of lactate ions within the gallery region. Stirring of the MgAl-lactate LDH in water for 24 hours at room temperature resulted in a clear colloidal dispersion. Upon solvent evaporation, the resulting material consisted of a disorganized restacked LDH phase. The most significant evidence for the formation of nanocomposites from the exfoliated clay materials is suggested by the absence of reflections associated with the LDH in the PXRD patterns. DSC results suggest that the glass transition temperatures of the composite materials with 5%, 10% and 20% LDH loading are superior to the parent maltose. Therefore, the LDH nanolayers give the LDH filled sugar nanocomposites better thermal stability than the pristine sugar, thus these composites could be used as drug excipients in the pharmaceutical industry. Keywords : Amorphous sugar, exfoliation, layered double hydroxides, nanocomposites, thermal stability. Doi: 10.4038/jnsfsr.v39i2.3171 J.Natn.Sci.Foundation Sri Lanka 2011 39 (2): 113 - 119

This research paper provides a comprehensive overview of the use of layered double hydroxides (LDH) in the removal of selenium species from contaminated water sources. Key studies on sorption mechanisms and the impact of competing ions on selenium removal are presented, and the effectiveness of LDH is compared across different structures and compositions. Scholarly sources extensively document the application of conventional LDH for effective selenium removal, with notable advancements achieved through innovative synthesis approaches. Comparative studies between LDH synthesized through various methods reveal the potential of tailored LDH for enhanced selenium adsorption. The paper further explores the influence of competing anions on LDH efficacy, emphasizing the impact of sulfate on selenium removal. Additionally, investigations into calcined LDH and commercially available variants underscore the potential for industrial applications. Beyond conventional LDH, the paper delves into iron-based LDH, LDH with intercalated thiomolybdate anions, and layered rare earth hydroxides, exploring their effectiveness in separating different selenium species. The role of pH in the removal of selenium species and the impact of three-metal cation LDH are also discussed. The study extends to nanocomposites, combining LDH with zero-valent iron, carbon-based materials, and organic compounds, illustrating their potential for selenium species immobilization. The presented findings offer valuable insights for researchers and practitioners in environmental science, addressing the growing demand for efficient selenium remediation strategies.

Synthesis of Cu-Al layered double hydroxides from aluminum saline slags

Layered Double Hydroxide (LDH) of Mg–Al–Acetate (Mg/Al = 3) was prepared by an anion exchange method starting from a nitrate precursor. The high hydration energy of acetate was exploited to exfoliate LDH into nanosheets having a thickness of 2–3 nm. A colloidal suspension of LDH layers was freeze dried to retain the layers in the exfoliated state. Thermal decomposition of exfoliated LDH at 400 °C resulted in the mixed metal oxide having a surface area of 275 m2 g−1 which is higher than pristine LDH oxide (100 m2 g−1). This enhanced surface area of exfoliated LDH is responsible for a large amount of CO2 sorption (1.4 mmol g−1) at room temperature as compared to pristine LDH (0.63 mmol g−1). This enhanced CO2 uptake is equal to some of the porous solids at similar conditions. The combination of aqueous exfoliation and the freeze drying technique has resulted in the successful synthesis of a novel CO2 adsorption sorbent based on LDHs.

Investigation of the effect of Mg-Al-LDH on pavement performance and aging resistance of styrene-butadiene-styrene modified asphalt

Advances in layered double hydroxide (LDH)-based elastomer composites

FTIR characterization of layered double hydroxides and modified layered double hydroxides