Enhanced CO2 capture from methane-stream using MII -Al LDH prepared by microwave-assisted urea hydrolysis

In this study, layered double hydroxide [M^II-Al LDH (M^II = Mg/Zn)] were used as selective CO₂ adsorbents from methane stream. It prepared by microwave (MW) assisted homogenous precipitations via urea hydrolysis. The LDH structures and the type of intercalated anions were confirmed by the X-ray diffraction (XRD), Fourier transform infrared (FTIR), thermal analysis (DSC and TGA), and elemental analysis, and scanning and transmission electron microscopes (SEM and TEM) imaging. The results indicated that; the LDH morphology and the intercalated anions (carbonate and urea species) were dependent on MW power, time, and M^II type. The predominant urea species were carbamate and isocyanate for Mg-Al LDH and Zn-Al LDH, respectively. The efficiencies of the LDH were tested towards CO₂ capture from methane stream using dynamic flow system under atmospheric pressure. Effect of temperature and adsorption/desorption cycles were studied. The best CO₂ adsorption capacities obtained at (30 °C) were 3.25, and 2.47 mmol g⁻¹ for Mg-Al-LDH and Zn-Al LDH, respectively. Whereas, those for the calcined LDH (at 550 °C) were 2.96 and 2.29 mmol g⁻¹ for Mg-Al-LDO and Zn-Al LDO, respectively. The results revealed the role of the intercalated urea derived anions type in enhancing the LDH adsorption properties to selectively capture CO₂ from methane stream.     

A critical assessment of the methods for intercalating anionic surfactants in layered double hydroxides

Anionic surfactant intercalated layered double hydroxides (LDH) of high purity are easily prepared via direct coprecipitation and also by the ion exchange method provided that the precursor contains a monovalent anion, e.g., LDH–Cl or LDH–NO₃. However, LDH–CO₃ is an attractive starting material as it is commercially available in bulk form owing to large-scale applications as a PVC stabilizer and acid scavenger in polyolefins. Thus, intercalation of dodecyl sulfate and dodecylbenzenesulfonate into a commercial (LDH) with approximate composition [Mg_0.654Al_0.346(OH)₂](CO₃)_0.173 · 0.5H₂O] was explored. Direct ion exchange is difficult as the carbonate is held tenaciously. In the regeneration method it is removed by thermal treatment and the surfactant form obtained by reaction with the layered double hydroxide that forms in aqueous medium. Unfortunately the resulting products are impure, poorly crystallized and only partial intercalation is achieved. Better results were obtained using water-soluble organic acids, e.g., acetic, butyric, or hexanoic acid, to aid decarbonation of LDH–CO₃. Intercalation proceeded at ambient temperatures with the precursor powder suspended in an aqueous dispersion of the anionic surfactant. The carboxylic acids are believed to assist intercalation by facilitating the elimination of the carbonate ions present in the anionic clay galleries.     

Stearate intercalated layered double hydroxides: effect on the physical properties of dextrin-alginate films

Glycerol-plasticized dextrin-alginate films were prepared by solution casting. They contained a fixed amount (16.6% mass/dry film mass) of functional filler based on the reaction products of the LDH, Mg₄Al₂(OH)₁₂CO₃·3H₂O, and stearic acid (SA). The films were characterized using infrared (IR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effect of filler composition on water vapour permeability and film stiffness was determined. The ratio of stearic acid (SA) to the LDH (Mg₄Al₂(OH)₁₂CO₃·3H₂O) was varied over the full composition range. Infrared spectroscopy and X-ray diffraction studies confirmed that the SA intercalated into the LDH. The Young’s modulus of films attained a maximum value (more than double the value for the neat film) at a filler composition of 60% SA. The water vapour permeability showed a broad minimum at filler compositions of 50–80% SA. Scanning electron microscopy revealed that in this composition range the filler assumes a high-aspect-ratio platelet morphology. This contrasts with the sand rose morphology of the LDH starting material and the globular dispersion of 100% SA in the film.     

Oxidizing synthesis of Ni–Mn layered double hydroxide with good crystallinity

Ni²⁺–Mn³⁺ layered double hydroxide (LDHs) with good crystallinity and uniform morphology has been hydrothermally synthesized at 180 °C for 2 days using urea as hydrolysis agent and ammonium peroxodisulfate as oxidant. The obtained Ni²⁺–Mn³⁺ LDHs material has been characterized by XRD, SEM, XPS, FT-IR, and TG–DTA. Ammonium peroxodisulfate as oxidant plays an important role for the formation of Ni²⁺–Mn³⁺ CO₃²⁻ LDHs material, and Mn²⁺ ions are oxidized into Mn³⁺ ones during the precipitation of Mn²⁺ ions, giving rise to layered hydroxide with the hydrotalcite structure. Ni²⁺–Mn³⁺ LDHs material with Ni/Mn molar ratio of 4 has a layered structure with a basal spacing of 0.739 nm. The morphology, size, and uniformity of the as-prepared materials connect with the hydrothermal treatment temperatures, and uniform and regular flowerlike spheres with a mean lateral size of 3.5 μm are observed for Ni²⁺–Mn³⁺ LDHs material with good crystallinity and uniform morphology.     

Mg–Al and Zn–Fe layered double hydroxides used for organic species storage and controlled release

Layered double hydroxides (LDH) containing (Mg and Al) or (Zn and Fe) were prepared by coprecipitation at constant pH, using NaOH and urea as precipitation agents. The most pure LDH phase in the Zn/Fe system was obtained with urea and in Mg/Al system when using NaOH. The incorporation of phenyl-alanine (Phe) anions in the interlayer of the LDH was performed by direct coprecipitation, ionic exchange and structure reconstruction of the mixed oxide obtained by the calcination of the coprecipitated product at 400 °C. The reconstruction method and the direct coprecipitation in a medium containing Phe in the initial mixture were less successful in terms of high yields of organic–mineral composite than the ionic exchange method. A spectacular change in sample morphology and yield in exchanged solid was noticed for the Zn₃Fe sample obtained by ionic exchange for 6 h with Phe solution. A delivery test in PBS of pH = 7.4 showed the release of the Phe in several steps up to 25 h indicating different host–guest interactions between the Phe and the LDH matrix. This behavior makes the preparation useful to obtain late delivery drugs, by the incorporation of the anion inside the LDH layer.     

Influence of synthesis conditions on NO oxidation and NOx storage performances of La0.7Sr0.3MnO3 perovskite-type catalyst in lean-burn atmospheres

Synthesis conditions of catalysts can significantly affect catalytic activities for a certain reaction. Here, a series of the La_0.7Sr_0.3MnO₃ perovskite-type catalysts was prepared by the sol–gel method under the different synthesis conditions. The faster calefactive velocities during calcination of the xerogel precursors would produce a lot of the impurities and cause the dropped amount of the excessive oxygen in perovskite, as well as the aggregated particles and the decreased surface areas; the higher calcination temperature would sinter the perovskite phases seriously; and the initial pH value of the precursor solution would greatly affect the morphology of the catalysts including the shape and the size, which directly linked to their NO_x storage capacity. Moreover, our findings revealed that the NO oxidation ability was determined by the amount of the excessive oxygen species in the perovskite. Here, the optimum synthesis conditions were achieved with the calcination temperature of 700 °C, the calefactive velocity of 2 °C min⁻¹, and the precursor solution of pH = 8. This catalyst presented the best performances for the NO oxidization and NO_x storage, i.e. the NO-to-NO₂ conversion of 70.2% and the NO_x storage capacity of 170.4 μmol g⁻¹.     

The multi-staged formation process of titanium oxide nanotubes and its thermal stability

The multi-staged formation process of titanium oxide nanotubes was investigated in detail under a hydrothermal treatment. During the synthesis procedure, an intermediate stage (tree-like structures) was observed before the formation of the titanium oxide layered structures. The layered structure of titanium oxide generally was considered to exfoliate directly from raw TiO₂ materials through the alkaline hydrothermal treatment. The rolling process of the layered structures of titanium oxide was confirmed by TEM observation after the alkaline hydrothermal treatment for the raw TiO₂ materials, followed by washing with 4 M HNO₃ aqueous solution. The thermal stability of the tube products was investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The result showed that both the morphology and crystal phase of titanium oxide nanotubes could be retained even after calcination at 650 °C.     

Preparation and photocatalytic study of fibrous K0.3Ti4O7.3(OH)1.7–anatase TiO2 nanocomposite photocatalyst

Nanocomposite of K_0.3Ti₄O_7.3(OH)_1.7 fiber and anatase TiO₂ nanoparticle was prepared by hydrothermal treatment of the K_0.3Ti₄O_7.3(OH)_1.7 which was synthesized by calcination of K₂CO₃ and TiO₂ at 1250 °C followed by refluxing in nitric acid. Effects of hydrothermal treatment conditions such as temperature and time on morphology, phase composition and crystal structure of the nanocomposites were extensively studied. Photocatalytic activities of the catalysts prepared at various hydrothermal conditions were evaluated by means of methylene blue decomposition under blacklight irradiation.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

Atomically Dispersed Silver Atoms Embedded in NiCo Layer Double Hydroxide Boost Oxygen Evolution Reaction

Bimetallic layered double hydroxides (LDHs) are promising catalysts for anodic oxygen evolution reaction (OER) in alkaline media. Despite good stability, NiCo LDH displays an unsatisfactory OER activity relative to the most robust NiFe LDH and CoFe LDH. Herein, a novel NiCo LDH electrocatalyst modified with single-atom silver grown on carbon cloth (Ag_SA-NiCo LDH/CC) that exhibits exceptional OER activity and stability in 1.0 m KOH is reported. The Ag_SA-NiCo LDH/CC catalyst only requires a low overpotential of 192 mV to reach a current density of 10 mA cm⁻², obviously boosting the OER activity of NiCo LDH/CC (410 mV@10 mA cm⁻²). Inspiringly, Ag_SA-NiCo LDH/CC can maintain its high activity for up to 500 h at a large current density of 100 mA cm⁻², exceeding most single-atom OER catalysts. In situ Raman spectroscopy studies uncover that the in situ formed NiCoOOH during OER is the real active species. Hard X-ray absorption spectrum (XAS) and density functional theory (DFT) calculations validate that single-atom Ag occupying Ni site increases the chemical valence of Ni elements, and then weakens the adsorption of oxygen-contained intermediates on Ni sites, fundamentally accounting for the enhanced OER performance.     

Metal-organic framework derived hexagonal layered cobalt oxides with {1 1 2} facets and rich oxygen vacancies: High efficiency catalysts for total oxidation of propane

A cobalt-based metal–organic framework was used as a precursor to synthesize Co₃O₄ catalysts exhibiting a hexagonal layered morphology by calcination at varying temperatures. Various characterization techniques, such as XRD, SEM, Raman, H₂-TPR, O₂-TPD and N₂ adsorption–desorption, were used to study the effects of calcination temperature on the grain size, surface area, and pore volume of the catalysts. The Co₃O₄ catalyst obtained by calcination at 350 °C (Co₃O₄-350) exhibited the highest catalytic activity for the total oxidation of propane. Furthermore, the small grain size and layered structure of Co₃O₄-350 allowed it to possess a high specific surface area, a highly exposed {1 1 2} facets, and abundant oxygen defects that facilitated a favorable low-temperature reducibility and oxygen mobility, thereby improving catalytic activity. This research offers a simple strategy for synthesis of Co₃O₄ with layered structure, highly exposed {1 1 2} facets and rich oxygen defects.     

A low temperature route to prepare LaFeO3 and LaCoO3

A combination of digestion and further low temperature calcination to crystallize the product was employed to prepare LaFeO₃ (LF) and LaCoO₃ (LC) powders. Freshly co-precipitated lanthanum and ferric (or cobalt) hydroxide gels by sodium hydroxide were allowed to react at 100 °C under refluxing and stirring conditions for 4-6 h. These oven dried powders were heated at 450 °C to form crystalline LF (or LC) powders. The phase contents and lattice parameters were investigated by X-ray diffraction (XRD). Transmission electron microscope (TEM) investigations were carried out to examine the morphology and average particle size of these powders.     

MgO-templated porous carbons-based CO2 adsorbents produced by KOH activation

Nanoporous (styrene–divinylbenzene)-based ion exchange resin-based carbons (MPCs) were prepared by MgO-templating synthesis and activated by KOH. MPCs were prepared from a (styrene–divinylbenzene)-based ion exchange resin by the carbonization of a mixture with Mg gluconate at 900 °C. And then, the prepared MPCs were treated with KOH at KOH/MPCs ratios ranging from 0.5 to 4 at 800 °C. Low KOH/MPCs ratios (KOH/MPCs ratio = 1) tended to favor the formation of micropores, whereas higher KOH/MPCs (KOH/MPCs ratio = 4) led to the formation of mesopores. The treated MPCs with a KOH/MPCs ratio = 1 exhibited the best CO₂ adsorption value of 266 mg g⁻¹ at 1 bar. However, the treated MPCs with a KOH/MPCs ratio = 3 exhibited the best CO₂ adsorption value of 1385 mg g⁻¹ at 30 bar. This result indicated that the CO₂ adsorption capacity of nanoporous carbons attributed to the mesopore volume fraction at higher pressure.     

Layered‐Double‐Hydroxide Nanosheets as Efficient Visible‐Light‐Driven Photocatalysts for Dinitrogen Fixation

Semiconductor photocatalysis attracts widespread interest in water splitting, CO₂ reduction, and N₂ fixation. N₂ reduction to NH₃ is essential to the chemical industry and to the Earth's nitrogen cycle. Industrially, NH₃ is synthesized by the Haber–Bosch process under extreme conditions (400–500 °C, 200–250 bar), stimulating research into the development of sustainable technologies for NH₃ production. Herein, this study demonstrates that ultrathin layered‐double‐hydroxide (LDH) photocatalysts, in particular CuCr‐LDH nanosheets, possess remarkable photocatalytic activity for the photoreduction of N₂ to NH₃ in water at 25 °C under visible‐light irradiation. The excellent activity can be attributed to the severely distorted structure and compressive strain in the LDH nanosheets, which significantly enhances N₂ chemisorption and thereby promotes NH₃ formation.     

One-pot synthetic method to prepare highly N-doped nanoporous carbons for CO2 adsorption

A one-pot synthetic method was used for the preparation of nanoporous carbon containing nitrogen from polypyrrole (PPY) using NaOH as the activated agent. The activation process was carried out under set conditions (NaOH/PPY = 2 and NaOH/PPY = 4) at different temperatures in 600–900 °C for 2 h. The effect of the activation conditions on the pore structure, surface functional groups and CO₂ adsorption capacities of the prepared N-doped activated carbons was examined. The carbon was analyzed by X-ray photoelectron spectroscopy (XPS), N2/77 K full isotherms, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The CO₂ adsorption capacity of the N-doped activated carbon was measured at 298 K and 1 bar. By dissolving the activation agents, the N-doped activated carbon exhibited high specific surface areas (755–2169 m² g⁻¹) and high pore volumes (0.394–1.591 cm³ g⁻¹). In addition, the N-doped activated carbons contained a high N content at lower activation temperatures (7.05 wt.%). The N-doped activated carbons showed a very high CO₂ adsorption capacity of 177 mg g⁻¹ at 298 K and 1 bar. The CO₂ adsorption capacity was found to be dependent on the microporosity and N contents.     

Urea biosensor based on Zn3Al–Urease layered double hydroxides nanohybrid coated on insulated silicon structures

Urea biosensors for medical diagnostic monitoring were developed based on the immobilization of urease within layered double hydroxides (LDH). The urease–LDH material was obtained by a stepwise exchange reaction by urease of a Zn₃Al–dodecyl sulphate (ZnAl–DS) colloidal suspension. XR diffraction and FTIR analysis show that this method gives rise to a Zn₃Al–Urease LDH nanohybrid material with urease dispersion and textural properties. An aqueous suspension of this urease–LDH nanohybrid material was deposited on an insulated semiconductor (IS) structure. Biosensor responses to urea additions were obtained using capacitance (C vs. V) and impedance (Z vs. ω) measurements. An enhanced maximum limit of the dynamic range was observed in the case of the impedance measurements (110 mM) compared to (5.6 mM) the capacitive urea biosensor. The Michaelis–Menten constant was also calculated according to the Lineweaver–Burk plot. It was found that the K_m value with immobilized enzymes was lower (K_m = 0.67 mM) in comparison with free enzymes. This K_m value obtained from the capacitance measurements indicates that the urea degradation is performed within any inhibition action on the IS/Zn₃Al–Urease LDH electrode. A comparative study was carried out between these results and those obtained previously, using urease/ZnAl–Cl layered double hydroxides mixture coated on the pH-ISFET transducer.     

Effect of carbon content and calcination temperature on the electrochemical performance of lithium iron phosphate/carbon composites as cathode materials for lithium-ion batteries

Lithium iron phosphate/carbon (LiFePO₄/C) composites were prepared by a convenient method with water-soluble phenol-formaldehyde resin as the carbon precursor. The morphology, crystalline structure, thermal stability, and composition of as-prepared LiFePO₄/C composites were investigated by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and Raman spectrometry. Their electrochemical performance was examined based on cyclic voltammogram with a LAND battery testing system while the effect of carbon content and calcination temperature was highlighted. Results show that carbon content and calcination temperature dramatically influence the discharge capacities and rate performance of LiFePO₄/C composites. The optimal calcination temperature is 700 °C, and the optimal carbon content (mass fraction) is 8.7%. The LiFePO₄/C composite prepared under the optimal conditions exhibits an initial room temperature discharge capacity of 150.2 mA h g^?1 at a 0.2 C rate and a constant discharge capacity of about 105.7 mA h g^?1 at a 20.0 C rate after 50 cycles, showing promising potential as a novel cathode material for lithium ion batteries.     

Characterization and optical properties of m-Li2ZrO3 prepared by optimized solid-state reaction

m-Li₂ZrO₃ powders were successfully prepared by solid-state reaction method using Li₂CO₃ and ZrO₂ as raw materials. The synthesis was optimized by varying the ball-milling time (0–96 h); Li₂CO₃ excess (0 or 5 wt%), reaction temperature (700, 800, 900 or 1000 °C), and reaction time (3, 6, 9 or 12 h). The structural, morphological and optical properties of m-Li₂ZrO₃ powders were examined by X-Ray Diffraction, Thermogravimetric and Differential-Thermal analysis, Scanning Electron Microscopy, High-Resolution Transmission Electron Microscopy, Laser Diffraction, Dynamic Light Scattering and UV–Vis Diffuse Reflectance Spectroscopy. The results show that precursors suitable for the synthesis of fine powders require ball-milling times longer than or equal to 6 h. Highly crystalline m-Li₂ZrO₃ was synthesized under two distinctive calcination conditions as follows: 900 °C/6h without Li₂CO₃ excess or 1000 °C/12 h using 5 wt% of Li₂CO₃ excess. Particle size of as-synthesized powders was found to be in the range from 200 nm to 1 µm. m-Li₂ZrO₃ was found to be a wide band gap material with apparent optical band gap of 5.5 eV (direct) and 5.1 eV (indirect), which can be used in UV-C applications.     

Alumina‐Supported CoFe Alloy Catalysts Derived from Layered‐Double‐Hydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons

A series of novel CoFe‐based catalysts are successfully fabricated by hydrogen reduction of CoFeAl layered‐double‐hydroxide (LDH) nanosheets at 300–700 °C. The chemical composition and morphology of the reaction products (denoted herein as CoFe‐x) are highly dependent on the reduction temperature (x). CO₂ hydrogenation experiments are conducted on the CoFe‐x catalysts under UV–vis excitation. With increasing LDH‐nanosheet reduction temperature, the CoFe‐x catalysts show a progressive selectivity shift from CO to CH₄, and eventually to high‐value hydrocarbons (C₂₊). CoFe‐650 shows remarkable selectivity toward hydrocarbons (60% CH₄, 35% C₂₊). X‐ray absorption fine structure, high‐resolution transmission electron microscopy, Mössbauer spectroscopy, and density functional theory calculations demonstrate that alumina‐supported CoFe‐alloy nanoparticles are responsible for the high selectivity of CoFe‐650 for C₂₊ hydrocarbons, also allowing exploitation of photothermal effects. This study demonstrates a vibrant new catalyst platform for harnessing clean, abundant solar‐energy to produce valuable chemicals and fuels from CO₂.     

High-temperature chemical and microstructural transformations of an organic-inorganic nanohybrid captopril intercalated Mg-Al layered double hydroxide

The thermal evolution of a crystalline organic-inorganic nanohybrid captopril intercalated Mg-Al layered double hydroxide (LDH) [Mg_0.68Al_0.32(OH)₂] (C₉H₁₃NO₃S)_0.130(CO₃)_0.030·0.53H₂O obtained by coprecipitation method is studied based upon in situ high-temperature X-ray diffraction, in situ infrared and thermogravimetric analysis coupled with mass spectroscopy analysis. The results reveal that a metastable quasi-interstratified layered nanohybrid involving carbonate-LDH and reoriented less ordered captopril-LDH was firstly observed as captopril-LDH heat-treated between 140 and 230 °C. The major decomposition/combustion of interlayer organics occur between 270 and 550 °C. A schematic model on chemical and microstructural evolution of this particular drug-inorganic nanohybrid upon heating in air atmosphere is proposed.