Search Results (7)

Silica aerogels are highly attractive due to their outstanding properties, including their low density, ultralow thermal conductivity, large porosity, high optical transparency, and strong sorption activity. However, their inherent brittleness has limited widespread applications. Constructing a robust, highly porous three-dimensional network is critical to achieving the desired mechanical properties in aerogels. In this study, we introduce a novel synthesis route for fabricating lightweight and mechanically strong aerogels by incorporating poly(ethylene-co-vinyl alcohol) (EVOH) through thermally induced phase separation (TIPS). EVOH exhibits upper critical solution temperature (UCST) behavior in a mixture of isopropanol (IPA) and water, which can be utilized to reinforce the silica skeletal structure. Robust aerogels were prepared via the sol–gel process and TIPS method, followed by supercritical CO₂ drying, yielding samples with bulk densities ranging from 0.136 to 0.200 g/cm³. N₂ physisorption analysis revealed a mesoporous structure, with the specific surface area decreasing from 874 to 401 m²/g as EVOH content increased from 0 to 80 mg/mL. The introduced EVOH significantly enhanced mechanical performance, raising the flexural strength and compressive strength to 0.545 MPa and 18.37 MPa, respectively—far exceeding those of pure silica aerogel (0.098 MPa and 0.74 MPa). This work demonstrates the effectiveness of the TIPS strategy for developing high-strength, low-density silica aerogels with well-preserved porosity. Full article

A series of 5%Ni/Ce_1-xTi_xO₂ catalysts was prepared with nickel impregnation of mixed Ce–Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO₂ or mixed Ce–Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO₃ perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane. Full article

The chemical immobilization of cobalt(II) ions in a silica aerogel matrix enabled the synthesis of the first representative example of aerogel-based single-ion magnets. For the synthesis of the lyogels, methyl-trimethoxysilane and N-3-(trimethoxysilyl)propyl ethylenediamine were co-hydrolyzed, then the ethylenediamine groups that were immobilized on the silica matrix enabled the subsequent binding of cobalt(II) ions. Lyogels with various amounts of ethylenediamine moieties (0.1–15 mol %) were soaked in isopropanol solutions of cobalt(II) nitrate and further supercritically dried in carbon dioxide to obtain aerogels with a specific surface area of 210–596 m²·g⁻¹, an apparent density of 0.403–0.740 cm³·g⁻¹ and a porosity of 60–78%. The actual cobalt content in the aerogels was 0.01–1.50 mmol per 1 g of SiO₂, which could easily be tuned by the concentration of ethylenediamine moieties in the silica matrix. The introduction of cobalt(II) ions into the ethylenediamine-modified silica aerogel promoted the stability of the diamine moieties at the supercritical drying stage. The molecular prototype of the immobilized cobalt(II) complex, bearing one ethylenediamine ligand [Co(en)(MeCN)(NO₃)₂], was synthesized and structurally characterized. Using magnetometry in the DC mode, it was shown that cobalt(II)-modified silica aerogels exhibited slow magnetic relaxation in a nonzero field. A decrease in cobalt(II) concentration in aerogels from 1.5 mmol to 0.14 mmol per 1 g of SiO₂ resulted in a weakening of inter-ion interactions; the magnetization reversal energy barrier likewise increased from 4 to 18 K. Full article

The aim of the study is to investigate the influence of the solvents applied both in sol–gel process and for supercritical drying (SCD) on NiO aerogels’ properties. NiO aerogels were synthesized using methanol and 2-methoxy-ethanol (MeGl) as sol solvents. SCD was performed using iso-propanol, methanol and tert-butyl-methyl ether as supercritical fluids. The obtained samples were characterized using low-temperature nitrogen adsorption, X-ray diffraction analysis, mass-spectra analysis and STEM and TEM methods. It was found that specific surface area and the phase and chemical composition strongly depend on the synthesis conditions. We revealed that Ni²⁺ cations were reduced into Ni⁰ when 2-methoxy-ethanol was applied as a sol solvent. The mechanism of the Ni²⁺ cations reduction is proposed. We consider that at the stage of sol preparation, the Ni²⁺–MeGl chelate was formed. This chelate decomposes at the SCD stage with the release of MeGl, which, in turn, eliminates methanol and leads to the formation of aldehyde. The latter is responsible for the nickel reduction. The proposed mechanism was confirmed experimentally. Full article

Two series of Ni/Ce(Ti/Nb)ZrO₂ catalysts were prepared using citrate route and original solvothermal continuous flow synthesis in supercritical isopropanol and studied in dry reforming of methane (DRM). TEM, XPS and FTIRS of adsorbed CO confirm influence of support composition and preparation method on the catalysts’ morphology and surface features. The oxygen mobility was studied by isotope heteroexchange with C¹⁸O₂. After testing in DRM, carbon deposits after catalysts’ testing in DRM were investigated by temperature-programmed oxidation with thermo-gravimetric analysis. The lowest amounts of carbon deposits were obtained for unmodified Ni-CeZr and Ni-CeNbZr compositions. Ti addition lead to an increased amount of carbon, which was removed at higher temperatures. The use of supercritical supports also resulted in the formation of a higher amount of coke. Catalysts prepared by the supercritical synthesis were tested in DRM for 25 h. The highest activity drop was observed in the first three hours. For all compositions, close values of carbon deposits were revealed. Full article

Orthoniobates of rare-earth elements are a promising group of materials attractive for the design of nanocomposite hydrogen separation membranes owing to a perspective type of proton conductivity, good mechanical properties, and high stability in H₂O- and CO₂-containing atmospheres. In general, the promising method involves the synthesis of nanocomposites with transition metals (Cu, Ni, and Cu-Ni alloys) and their oxides with high electronic conductivity. For the first time, lanthanum orthoniobates and nanocomposites with NiCu and NiCo nanoparticles were synthesized using alcohol solutions of salts of the corresponding metals by the solvothermal method in a flow reactor in a supercritical isopropanol medium. This method made it possible to obtain single-phase La_0.99Ca_0.01NbO_4–δ oxide. The introduction of doping titanium cations did not allow obtaining a single-phase La_0.99Ca_0.01Nb_0.98Ti_0.02O_4–δ sample, as impurities in lanthanum methaniobate and La₂Ti₂O₇ were found. Calcined powders and gastight pellets of orthoniobates and nanocomposites were characterized by X-ray diffraction analysis as well as scanning and transmission electron microscopy. Transport characteristics were investigated by the Van der Pauw technique, varying measurement temperatures in a wet H₂ atmosphere. The oxygen mobility was estimated by the oxygen isotope heteroexchange with C¹⁸O₂. Full article

A series of 5%Ni-Ce_xZr_1−xO₂ (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of Ce_xZr_1−xO₂ obtained by the same route. The textural, structural, red-ox, and catalytic properties in methane dry reforming (MDR) of Ni-modified Ce-Zr oxides synthesized by two routes have been compared. It was shown by XRD, TEM, and Raman spectroscopy that the method of Ni introduction does not affect the phase composition of the catalysts, but determines the dispersion of NiO. Despite a high dispersion of NiO and near-uniform distribution of Ni within Ce-Zr particles observed for the one-pot catalysts, they have shown a lower activity and stability in MDR as compared with impregnated ones. This is a result of a low Ni concentration in the surface layer due to segregation of Ce and decoration of nickel nanoparticles with support species. Full article