Layered Double Hydroxides Layers Research Articles

Modification of Mg/Al-LDH by vanadate: effects on tribological properties and corrosion resistance.

In this study, Mg/Al layered double hydroxide (LDH) composite coatings were prepared on the surface of anodized 1060 aluminum alloy by an in situ growth method, and then the vanadate anions were embedded in the interlayer corridor of LDH by an ion exchange process. The morphology, structure and composition of the composite coatings were investigated using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry and Fourier transform infrared spectroscopy. Ball-and-disk friction wear experiments were carried out to measure the coefficient of friction, the amount of wear, and the morphology of the worn surface. The corrosion resistance of the coating is studied using dynamic potential polarisation (Tafel) and electrochemical impedance spectroscopy (EIS). The results showed that the LDH composite coating with unique layered nanostructure as a solid lubricating film can effectively improve the friction and wear reduction performance of the metal substrate. Chemical modification treatment by embedding vanadate anions in the LDH coating leads to the change of LDH layer spacing and the increase of interlayer channels, resulting in the best friction and wear reduction and corrosion resistance of the LDH coating. Finally, the mechanism of hydrotalcite coating as a solid lubricating film for friction and wear reduction is proposed.

Sequentially activating macrophages M1 and M2 phenotypes by lipopolysaccharide-containing Mg-Fe layered double hydroxides coating on the Ti substrate

As cells of innate immunity, macrophages are a class of innate immune cells existing in almost all tissues and play a crucial role in bone repair. However, it remains a challenge to modulate the sequential activation of the deferent phenotypes in macrophage when designing the titanium (Ti) implants. In this study, the Mg-Fe layered double hydroxides (LDHs) was coated on Ti substrate through hydrothermal treatment. Further on lipopolysaccharide (LPS) was introduced onto the LDHs through adsorption and ions exchange. The adsorption efficiency of the coating on LPS reached 72.8% in 24 h due to the anion exchange and electrostatic interactions between the LPS and the LDH layers in deionized water. The LDHs-LPS coating released a large amount of LPS in the early stage, which induced macrophages into M1 phenotype via activating TLR-4 → MyD88 and TLR-4 → Ticam-1/2 signal pathways. Subsequently, the M1 macrophages were transformed into M2 phenotype by regulating the integrin α5β1 of cells by the nanostructures, wetting angle and Mg2+ of the coating. The LDHs-LPS coating endows Ti with the ability of stage immunomodulation, indicating the positive osteoimmunomodulatory property.

Construction of layered double hydroxide-modified silica integrated multilayer shell phase change capsule with flame retardancy and highly efficient thermoregulation performance

Current energy problems have driven the development of thermal storage technology based on phase change materials (PCMs). However, the leakage and flammability of organic PCMs appeared as troublesome agendas that deserve attention. In this article, the strategy of constructing phase change capsules with paraffin wax (PW) core and multilayer shell was proposed aiming to address the above issues. For the integrated multi-layer shell, silica (SiO2) acts as initial layer to encapsulate the PW core, and then tannic acid (TA) was creatively taken to bridge the subsequent flame retardant layered double hydroxides (LDH) layer on the surface of the silica shell, and polyvinyl alcohol (PVA) layer was further introduced to improve the compatibility of phase change capsules with polymer matrix. Characterization of chemical structure and morphology confirmed that phase change capsules with LDH modified silica multilayer shell (M-EPCMs) was successfully prepared. The results indicated that M-EPCMs, particularly the representative M-EPCM-5, possessed outstanding thermal stability, excellent leak proofness, good thermal regulation performance as well as long-term cycling stability. Importantly, M-EPCM-5 with the thickest LDH layer can self-extinguish when exposed to fire, and especially can keep the integrity of its shell without breakage after being heated at high temperature. Furthermore, the silicone rubber foam (SRF) composite containing M-EPCM-5 can reach UL-94V-1 level, indicating the flame retardancy of SRF matrix was improved significantly. The possible flame retardant mechanism revels that LDH can not only accelerate the formation of dense ceramic protective layer in condensed phase, but also release non-combustible gases (H2O, CO2) in gas phase, thus improving the fire retardancy of M-EPCMs. Therefore, the construction strategy proposed in this article represents a powerful means to improve flame retardancy and prevent leakage of phase change capsules at the same time, which will greatly expand the application scope of PCMs.

Molecular orientation rules the efficiency of immobilized antioxidants

Tannic acid (TA) and glutathione (GSH) are important molecular antioxidants against reactive oxygen species. Their efficiency is limited by low solubility and high sensitivity, which may be solved by confinement in composite materials. Here, effect of immobilization of these antioxidants on their radical scavenging activity was investigated using layered double hydroxide (LDH) nanoparticles as hosts. Different preparation methods were applied to build composite systems leading to variations in the molecular orientation of both TA and GSH on the surface or among the layers of LDHs. Systematic combination of spectroscopy (FT-IR, Raman, UV–VIS-NIR-DRS), diffraction (XRD) and microscopy (SEM) methods revealed perpendicular or parallel orientation of TA on the surface of LDH depending on the preparation approach applied. Immobilization of GSH protected the antioxidant molecules from degradation. Radical scavenging tests evidenced that the activity of the antioxidants strongly depends on the molecular orientation. The LDH supported GSH and TA proved as durable and reusable antioxidant agents to be applied as radical scavengers in medical therapies or in industrial processes.

Construction and Evaluation of Hyaluronic Acid-Coated Flurbiprofen-Layered Double Hydroxide Ocular Drug Delivery System.

In this study, flurbiprofen (FB) was selected as the model drug, and hyaluronic acid-coated flurbiprofen-layered double hydroxide ophthalmic drug delivery system (HA-FB-LDH) was designed and prepared. In this system, the model drug flurbiprofen was intercalated in layered double hydroxide and coated with hyaluronic acid (HA), so as to prolong the cornealresidencetime and increase the corneal permeability of the drug. Layered double hydroxide (LDH) was prepared by alcohol-water coprecipitation method. Through single factor investigation, the optimum preparation conditions were obtained as follows: The Mg/Al ratio was 2:1, the reaction pH was 11.0, the hydrothermal reaction time was 24h, and the hydrothermal reaction temperature was 100°C. Under these conditions, the particle size of LDH was 116.4 ± 0.8nm, the potential was 42.2 ± 1.2mV, and a relatively regular crystal structure could be had. Then FB was intercalated into the LDH layer to prepare flurbiprofen-layered double hydroxide (FB-LDH). In the end, HA-FB-LDH was prepared by the stirring-ultrasonic method, in which through prescription screening, the molecular weight of HA was 200-400kDa and the concentration of HA solution was 1.25mg·mL -1, the final particle size of HA-FB-LDH was 185.8 ± 3.3nm, and potential of - 31.4 ± 0.7mV. The successful loading of FB and the coating of HA were verified by XRD, FTIR, TGA, TEM, and other characterization methods. The results of in vitro stability experiment indicated that the coating of HA could significantly enhance the stability of LDH in the presence of electrolytes. The in vitro release results suggested that the cumulative release amounts of FB-LDH and HA-FB-LDH within 12h were 92.99 ± 0.37% and 74.82 ± 0.29% respectively, showing a certain sustained release effect. At the same time, the release mechanism of FB-LDH was preliminarily explored by in vitro release experiment, which proved that the release mechanism of FB-LDH was mainly ion exchange. The results of in vivo ocular irritation experiments demonstrated that the ophthalmic preparation studied in this paper was safe and non-irritating. The results of tear pharmacokinetics in rabbits showed that the area under the curve(AUC), the average residence time (MRT), and the highest concentration (Cmax) in tears in the HA-FB-LDH group were 4.43, 4.48, and 2.27 times higher than those in eye drops group separately. Furthermore, the AUC of the HA-FB-LDH group was 1.48 times higher than that of the FB-LDH group. The above results suggested that HA-FB-LDH could improve the precorneal residence time. The results of aqueous humor pharmacokinetics in rabbits indicated that the AUC, MRT, and maximum concentration (Cmax) in aqueous humor in the HA-FB-LDH group were 6.88, 2.15, and 4.08 times of those in the eye drop group respectively. Additionally, the AUC and MRT of the HA-FB-LDH group were 1.55 and 1.63 times those of the FB-LDH group separately. These mentioned findings verified that HA-FB-LDH could enhance the corneal permeability of the drug. The fluorescent substance-fluoresce isothiocyanate (FITC) was substituted for FB intercalation in LDH for in vitro tissue imaging study of rabbits, whose results stated clearly that FITC-LDH and HA-FITC-LDH could both prolong the precorneal residence time of drugs, and HA-FITC-LDH could increase the corneal permeability of the drug to a certain extent. To sum up, HA-FB-LDH, which can overcome the shortcomings of low bioavailability of traditional eye drops to a certain degree, is a safe and effective ophthalmic drug delivery system.

Design and preparation magnetic bio-surfactant rhamnolipid-layered double hydroxide nanocomposite as an efficient and recyclable adsorbent for the removal of Rifampin from aqueous solution

In this study, a magnetic core–shell rhamnolipid (MCSR) intercalated cobalt/aluminum layered double hydroxide (LDH) has been used for water remediation and rifampin (RIF) removal. Biosurfactants, including rhamnolipid (RL), exhibit advantages such as low toxicity and environmental compatibility. Moreover, rhamnolipid can promote the removal efficiency of the adsorbent. The effect of major factors on the adsorption process, including pH of the solution, initial RIF concentration, contact time, and adsorbent dosage were examined and optimized in detail. Accordingly, in acidic pH the adsorption efficiency was the highest because in this condition, OH functional groups of RIF molecules may be protonated leading to interacting with positively charged LDH layers. The results showed that the RIF removal efficiency is 96 %. According to the Langmuir model, the maximum adsorption capacity obtained for 0.67 g/L of adsorbent (20 mg of adsorbent in 30 mL of RIF solution) was 221.30 mg/g. In accordance with the modeling results, the Temkin isotherm better expresses the adsorption mechanism at the surface of MCSR-LDH. Furthermore, the kinetic data was better suited to the pseudo-first-order model, indicating that the solution reaches equilibrium between 20 and 30 min. Evaluation of thermodynamic parameters indicated the exothermic nature of adsorption process (ΔH° = −68362.70 J/mol). The magnetic property of MCSR-LDH leads to the easy recovery of the adsorbent by an external magnet at the end of the adsorption process, so that it can be regenerated for repeated cycles of at least four cycles without a significant decrease in efficiency.

Synthesis of linear and cyclic organic sulfonic acid-modified Cu-Al layered double hydroxides and their adsorption properties

Over the past few years, many substances have been investigated for their potential application as adsorbent materials. The combination of a layered double hydroxide (LDH) and CH/π interactions enables selective adsorption through the differences in the electron density of the aromatic ring. Thus, in this study, two organic anions, namely the linear 1-octane sulfonate (OS-) and cyclic sulfonated-β-cyclodextrin (SCD13-), were intercalated into the interlayers of a Cu-Al LDH by coprecipitation (Cu/Al ratio, 3.0; chemical ratio, 2.0). The SCD13- anion was selected because β-cyclodextrin comprises hydrophobic cavities that allow the formation of inclusion compounds. X-ray diffraction (XRD), total organic carbon (TOC) analysis, and inductively coupled plasma-atomic emission spectrometry (ICP-AES) revealed that the as-synthesized OS-Cu-Al LDH and SCD-Cu-Al LDH were both oriented perpendicular to the LDH layer. Subsequent absorption experiments with m-aminophenol (AP) and m-nitrophenol (NP), which have similar structures but different aromatic ring electron densities, confirmed a Freundlich-type adsorption mechanism and revealed that adsorption occurred on a non-uniform surface for both LDHs. The results also revealed excellent NP and AP adsorptions for the linear OS•Cu-Al LDH and cyclic SCD•Cu-Al LDH, respectively. This was attributed to the strong hydrophobic interactions in the former and strong CH/π interactions in the latter. This study shows the potential of using linear and cyclic organic sulfonic acid-modified LDHs for the selective recovery of the desired aromatic organic compounds.

Self-templated transformation of Co-ZIF-L into hierarchical porous CoS2/Co-Ni LDHs with improved electrochemical activities

CoS2/Co-Ni layered double hydroxides (LDHs) with a rational tailored pore structure are developed by partial sulfurization of Co-ZIF-L and subsequent Ni2+ etching for the transformation of zeolitic imidazolate frameworks (ZIFs) to layered double hydroxides. The as-synthesized CoS2/Co-Ni LDHs maintain the leaf-like structure and have rich hierarchical pores. More importantly, CoS2 nanoparticles are uniformly embedded among Co-Ni LDH layers, facilitating mass diffusion and favorably exposing more active sites. Benefiting from these desirable compositional and structural features, a remarkable specific capacitance (capacity) of 1784 F/g (223 mAh/g) at 1 A/g is delivered by CoS2/Co-Ni LDHs. Furthermore, the as-assembled CoS2/Co-Ni LDH//activated carbon as the asymmetric supercapacitor device also exhibits a high energy density of 125 Wh/kg at a power density of 800 W/kg and a superb capacitance retention of 98 % after 5000 cycles, outperforming many Co-Ni LDH based electrode materials reported in the literature.

Enhanced corrosion resistance of AZ31 Mg alloy by one-step formation of PEO/Mg-Al LDH composite coating

In the present work, a novel one-step plasma electrolytic oxidation (PEO) method is developed for the formation of PEO/Mg-Al layered double hydroxide (LDH) composite coating on AZ31 Mg alloy. Through the control of electrolyte concentration, a Mg-Al LDH layer was formed on the surface of the resultant PEO coating. The corrosion properties of the composite coating were evaluated by electrochemical analysis, hydrogen evolution measurement and salt spraying testing, which exhibited improved corrosion resistance due to the presence of the Mg-Al LDH layer. This work provides a new strategy to take advantage of alloying elements in Mg alloys in PEO treatment to enhance corrosion resistance.

Corrosion protection of Zn–Al–Mg‐coated steel by a layered double hydroxide conversion layer

AbstractConversion layers produced on an industrial scale are typically based on phosphates or chromates and involve toxicological and environmental problematic treatment solutions and waste. Concerning this matter, layered double hydroxide (LDH)‐based conversion layers prepared by the process described in our study offer a promising alternative where the treatment solution comes with no environmental or toxicological concerns. The corrosion protection of Zn–Al–Mg (ZM)‐coated steel by an in situ grown LDH conversion layer is investigated by potentiodynamic polarization measurements, linear polarization resistance determination, neutral salt spray test and mass loss test. An LDH layer formation time as short as 30 s considerably improves the corrosion resistance of ZM‐coated steel in electrochemical testing as well as in industrially relevant corrosion testing under chloride‐containing atmospheres. With prolonged LDH formation times, further improvement in corrosion protection is obtained. The significantly increased corrosion resistance of ZM‐coated steel after the industrially feasible treatment time of 30 s makes LDH‐based conversion layers a promising candidate for novel industrially viable conversion layers, with enhanced corrosion performance and environmentally benign as well as sustainable chemistry.

Experimental and Theoretical Studies on the Intercalation of Naproxen into the Mg2Al and Zn2Al Layered Double Hydroxides by Ion Exchange Reaction

In this work, Layered Double Hydroxide (LDH) materials carrying the worldwide administered non-steroidal anti-inflammatory drug naproxen (NAP), and the sodium naproxenate salt (NaNAP) for comparison, were studied by computational approaches aiming to model the structure of hybrid LDH-drug and shed light on NAP intercalation process. Atomic modeling calculations were performed at the quantum mechanical level based on Density Functional Theory and classical force fields based on empirical interatomic potentials. LDHNAP materials were prepared by ion exchange reaction from Mg2Al(OH)6Cl and Zn2Al(OH)6Cl pristine phases. The characterization of the materials confirmed NAP intercalation and also the permanence of the pristine phases in the isolated materials after ion exchange. Crystallographic lattice parameters, elemental analysis, and TGA experimental results were then employed in the calculations, which revealed that NAP anions can completely neutralize the positive charge of the LDH layers: both Mg2Al and Zn2Al LDH structures could be optimized with all Cl− anions substituted by NAP. The drug assumed different dispositions in the NaNAP crystal or when intercalated into LDH. Additionally, infrared wavenumbers calculations agreed with the experimental results and showed useful to support LDHNAP bands assignment. The employed theoretical models to represent the structure of LDHNAP systems are expected to assist the interpretation of future experimental results and to be used as auxiliary tools to tune properties of LDH-drug pharmaceutical formulations.

Improvement of corrosion resistance of PEO coatings on Al alloy by formation of ZnAl layered double hydroxide

Thin plasma electrolytic oxidation (PEO) coating was produced on 2024 aluminum alloy as pre-treatment process, which was subsequently treated by hydrothermal method to grow layered double hydroxide (LDH) layer to seal the open pores and defects to enhance corrosion performance of the oxide layer. Vanadate and molybdate was exchanged and loaded into LDH layer to further improve the corrosion resistance of the composite coating. The porous PEO coating was sealed substantially after growth of LDH, although thickness of the composite layer was reduced due to dissolution during post-treatment process. Exchange of inhibitors can greatly enhance the corrosion performance of the coating. The impedance of vanadate containing coating was maintained in the level of 106 Ω·cm2 for 20 days during immersion in 3.5 wt% NaCl solution and showed superior corrosion protection for Al surface after performing neutral salt spray test for 60 days.

Enhanced sorption of perfluorooctanoic acid with organically functionalized layered double hydroxide

Sorption is recognized as an effective and economical approach for the practical treatment of water contaminated with per- and polyfluoroalkyl substances (PFAS). In this study, we reported the development, characterization, and evaluation of a family of new organically functionalized layered double hydroxide (LDH) adsorbents for PFAS removal with a primary focus on perfluorooctanoic acid (PFOA). The organic functional groups were introduced to the Zn-Al LDH by covalent bonding through a post-grafting process. The organically functionalized LDHs were very efficient for PFOA adsorption, likely due to the synergy of the positively charged structural layers of LDHs that provide strong electrostatic interactions and the modified organic functional groups that provide enhanced hydrophobic interactions to capture PFOA. Specifically, our adsorption kinetics and isotherm studies found that organically functionalized LDHs exhibited faster adsorption kinetics and greater adsorption capacities for PFOA than the unmodified LDH. Furthermore, organically functionalized LDHs showed substantially improved performance for PFOA removal under various water chemistry conditions in comparison to the unmodified LDH, and remained more effective following repeated adsorption-regeneration cycles. The best-performed organically functionalized LDH also exhibited high efficiency for the treatment of a mixture of PFAS with varied fluoroalkyl chain lengths and functionalities. Results of this work suggested that organically functionalized LDHs hold great promises for the treatment of PFAS-contaminated water.

Remediation of Cd-contaminated soil through different layered double hydroxides: The weakness of delamination and mechanism

The great variability of the reaction of layered double hydroxides (LDHs) with anions and cations, especially the ultra-thin thickness, large specific surface area and fully-exposed active sites of single LDHs, may be crucial factors in the remediation of Cd-contaminated soil. In this study, the environmental behavior of Cd was investigated through sequential extraction method and plant potting tests, so as to verify the immobilization efficiency of LDHs and single LDHs in Cd-polluted soil. The results revealed that S-LDHs differed significantly from the original LDHs, and Ca-LDH were the best material for immobilizing Cd. Through sequential extraction procedures, the exchangeable fraction of Cd decreased and its residual fraction increased by forming CdCO3(s) and Cd5(OH)8(NO3)2.2 H2O. After treating the Cd-contaminated soil, the LDH layers were restacked through the XRD-pattern-confirmed S-Mg-LDH, while S-Ca-LDH were more converted to CaCO3 and Cd5(OH)8(NO3)2.2 H2O. Through investigating the structural changes of stabilizers in soil remediation via a series of characterization, the difference between Mg-LDH and S-Mg-LDH was in the generation of CdCO3(s), while that between Ca-LDH and S-Ca-LDH was in the formation of CdAl-LDH-CO3. Single LDHs and LDHs showed the opposite effect when reacting with different pollutants (Cd and Cr), and single LDHs were not as effective for Cd in soil as was previously reported. These results helped with a better understanding of the use of LDHs as a solidified agent as well as the mechanisms involved.

Adsorption characteristics of assembled and unassembled Ni/Cr layered double hydroxides towards methyl orange

The lamella aggregation state of layered double hydroxides (LDHs) may affect their sorption capacity for organic compounds. The dried LDH samples (Ni/Cr LDH-FA-D and Ni/Cr LDH-H2O-D) and the undried samples (Ni/Cr LDH-FA-W and Ni/Cr LDH-H2O-W) were flexibly prepared by a co-precipitation method in formamide (FA) and water, respectively. The results of X-ray diffraction (XRD) and transmission electron microscope (TEM) showed that the undried LDHs were unassembled, which had no the stacking layers but had a pseudohexagonal nanosheet lamella structure. And the unassembled LDH layers can be assembled again during the dry process. Ni/Cr LDH-FA-W and Ni/Cr LDH-H2O-W showed much greater adsorption capacities towards methyl orange (MO) than Ni/Cr LDH-FA-D and Ni/Cr LDH-H2O-D, as well as shorter time to reach equilibrium. The maximum adsorption capacity of MO could be calculated to 806 mg/g and 740 mg/g for Ni/Cr LDH-FA-W and Ni/Cr LDH-H2O-W by Langmuir-type simulation. The greater adsorption capacities of unassembled LDH could be attributed to the loosen structure and much more exposed adsorption sites. It could be concluded that unassembled LDHs were an effective and conducive preparation pathway for the exploration of the adsorption sites of LDHs.

Preparation and characterization of layered-double hydroxides decorated on graphene oxide for dye removal from aqueous solution

In this study, coprecipitation method followed by sonochemical treatment was used to prepare layered-double hydroxides (LDH) and graphene oxide-layered-double hydroxides (GO-LDH). GO-LDH and LDH adsorption performance were evaluated using aqueous solution of Methyl Orange dye (MO). The LDH and GO-LDH were characterized using various advanced techniques. The analysis of data collected from the XRD pattern and size distribution showed that the presence of GO will increase the inter-space between the LDH layers and the polydispersity index than pure LDH. This will make the ion exchange be easier and improve the adsorption process. The adsorption kinetic studies presented that the adsorption of MO dye on LDH and GO-LDH follows a pseudo-second-order reaction. Moreover, according to intraparticle diffusion, GO-LDH has a faster diffusion constant for MO molecules than LDH. Furthermore, as predicted by Langmuir, the maximum adsorption ability of MO using GO-LDH (505.1 mgg−1) which was higher than LDH (400 mgg−1). Finally, LDH and GO-LDH were successfully regenerated using 1.0 M NaOH and Methanol, respectively. Moreover, the GO-LDH showed excellent reusability behavior compared to LDH. This suggests that GO-LDH could be a costless adsorbent compared to LDH.

Preferable phosphate sequestration using polymer-supported Mg/Al layered double hydroxide nanosheets

The efficient removal of phosphate from waters is critical to mitigating eutrophication. Recently, layered double hydroxides (LDHs) have been believed to be promising adsorbents for phosphate removal. Nevertheless, the scaled-up application of LDHs is limited by the difficulties of separation, excessive pressure drops, and potential metal leaching. In this study, a millimeter-sized nanocomposite, MgAl-201, was fabricated by impregnating Mg/Al LDH nanosheets into a polystyrene anion exchanger D201. The resulting MgAl-201 combines the inherent affinity of Mg/Al LDH toward phosphate and the excellent hydrodynamic performance of the support material. Benefiting from the shielding effect from the cross-linked polymeric host, MgAl-201 exhibits satisfactory chemical stability in the range of pH 3–11 with a negligible metal release. Adsorption experiments show that MgAl-201 has superb applicability to neutral phosphate-contaminated waters. It reaches adsorption equilibrium within 270 min, and the maximum adsorption capacity calculated by the double Langmuir model is 52.0 mg/g. Meanwhile, MgAl-201 exhibits more preferable adsorption toward phosphate than D201 when coexisting anions are at relatively high levels. FTIR and XPS surveys revealed that two distinct adsorption interactions were involved in phosphate removal, that is, electrostatic interactions from the quaternary ammonium groups bonded on the host and the interlayer exchangeable anions in the encapsulated Mg/Al LDH, and specific inner-sphere complexation from the −OH groups in the Mg/Al LDH layers. For wastewater application, a satisfactory treatable volume of 580 BV was achieved to reduce the effluents from 2.0 mg/L to 0.5 mg/L, which was up to 8 times that of the traditional anion exchanger D201. Furthermore, MgAl-201 could be easily regenerated using the Na2CO3-NaCl binary solution and maintained good reusability without significant capacity loss after 5 adsorption-desorption cycles. The results suggest that MgAl-201 is of great application capability for preferable phosphate sequestration in advanced wastewater treatment.

Discretization and Encapsulation of Palladium inside the Cavity of Crown Ether within the Interlayer of Layered Double Hydroxide for Enhanced Activity: A Case Study with Hydrogenation Reaction

AbstractThe activity of the noble metal‐based heterogeneous catalysts is limited by weak metal–support interactions, aggregation, and low surface to volume (S/V) ratio. The activity can be augmented in many ways. Among them, the discretization of the active sites and redistribution of electron density around the metal atom is an important one. In this work, these two phenomena are studied concerning a model reaction, hydrogenation of p‐nitrophenol (p‐NP). Herein, 1,4,7,10,13‐pentaoxacyclopentadecane ether is introduced in the basal space of layered double hydroxide (LDH) to encapsulate noble Pd0 atom inside the cavity of the crown molecule strategically. The modified LDH (Pda‐ECC‐L0.10@in situ CoAl LDH) augments the properties, like, high S/V ratio and nonaggregation of active sites by forming nonagglomerative discrete catalytic (DNSC) sites within the cavity of crown ether in the basal space. The developed catalyst exhibits higher turnover frequency demonstrating the improved activity due to the formed DNSC sites and redistributes electron density around the Pd atoms by LDH layers and crown molecules. Thus, the present material synthesis route can be considered as a stand‐alone method for preparation of the supported sub‐nanometer noble metal catalyst with higher activity and can be exploited for reactions where noble metal catalyst are used.

An integration system derived from LDHs for CO2 direct capture and photocatalytic coupling reaction

Aiming at integration of CO 2 capture and utilization, we put forward a nanocatalyst for coupling photocatalytic CO 2 reduction with 5-hydroxymethylfurfural (5-HMF) oxidation. Specifically, we synthesize a carbonate-type CuCoAl-layered double hydroxide (LDH) to pre-enrich the raw material of the half reduction reaction. In the inert atmosphere, CO 3 2− located in the interlayers realized in situ reduction catalyzed by the metal cations on the layers, and this was coupled with 5-HMF oxidation catalyzed by the hydroxyl sites, thus generating CO as a reduction product and 2.5-furandiformaldehyde (DFF), 5-formyl-2-furanacarboxylic acid (FFCA), and 2.5-furan dicarboxylic acid (FDCA) as oxidation products. A series of in situ Fourier transform infrared (FTIR) spectroscopy with assistance of 13 C labeling characterizations clarified the above in situ transformation process and further revealed the important role of tuning the electronic structure by selectively etching the LDHs layers in promoting catalytic performance. More importantly, the consumed CO 3 2− could be easily supplied by CO 2 in air, which would endowed the nanocatalyst with good recycle performance. • Constructing CO 3 2− -CuCoAl-LDHs for CO 2 reduction and 5-HMF oxidation coupling • Selective etching laminate cations is conducive to improving the performance • Directly supplying the consumed CO 3 2− from CO 2 in air endows good reusability • Designing in situ characterizations to reveal coupling mechanism under light Under the background of global efforts to be carbon neutral, comprehensive utilization of CO 2 and high-value conversion of biomass have become the mainstream trend in the field of energy research. Herein, we propose a new concept regarding the integration of CO 2 direct capture, utilization, and biomass high-value conversion, and thus we construct a carbonate-type layered double hydroxide (LDH) catalyst, which opens a green path not only for CO 2 capture, utilization, and storage (CCUS) technology but also for the synthesis of fine chemicals. Moreover, the smart employment of LDHs taps the application potential of 2D materials and simultaneously provides a reference for exploring a novel coupling process associated with traditional carbonate decomposition, which will be of great significance for reducing rigid CO 2 . A carbonate-type CuCoAl-layered double hydroxide (LDH) was synthesized to pre-enrich the raw material of the half reduction reaction. In the inert atmosphere, CO 3 2− located in the interlayers realized in situ reduction, which was coupled with 5-HMF oxidation, thus generating CO as reduction product and 2.5-furandiformaldehyde (DFF), 5-formyl-2-furanacarboxylic acid (FFCA), and 2.5-furan dicarboxylic acid (FDCA) as oxidation products. More importantly, the consumed CO 3 2− could be easily supplied by CO 2 in air, which would endow the nanocatalyst with good recycle performance.

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

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