High-temperature catalyst supports and ceramic membranes: Metastability and particle packing

Parameters and/or processes responsible for the stability of catalyst supports and ceramic membranes are discussed. Two major parameters/processes were identified which are responsible for the stability of sol-gel derived nanostructured oxides at elevated temperatures. They are metastable-to-stable phase transformation and structure and packing of primary particles within the aggregate. Based on these observations, strategies to develop thermostable nanostructured oxides for high-temperature membrane and catalyst applications are discussed by taking titania and titania-alumina nanocomposites as examples

Effect of sintering atmosphere on the pore-structure stability of cerium-doped nanostructured alumina

Pore-structure stability of pure and Ce-doped alumina in air and argon atmospheres was studied using DTA, TGA, N2 ads./des. and XRD with a view to understand the importance of the ionic size of the dopant cation on the pore-structure stability of alumina. The ionic size effect was studied by heat treating the Ce-alumina system in both oxidizing and reducing atmospheres to have Ce4+ (87 pm) and Ce3+ (106 pm) respectively. No compound formation between Ce and alumina was observed. In the case of pure alumina there is a drastic reduction in porosity during the transformation to α-alumina. Ce-doped alumina has a higher DSC transformation temperature corresponding to the α-alumina transformation compared to pure alumina. Ce-doped alumina showed higher pore-structure stability compared with pure alumina and the stability was relatively higher in reducing atmosphere (higher Ce3+/Ce4+ ratio, higher effective ionic size) compared with oxidizing conditions (lower Ce3+/Ce4+ ratio, lower effective ionic size)

Effect of peptization on densification and phase transformation behaviour of sol-gel-derived nanostructured titania

Porosity reduction, packing, pore-size distribution, and anatase to rutile phase transformation behavior of nanostructured titania ceramics prepared from both peptized and unpeptized sols were studied and compared using XRD, DSC, and nitrogen-gas physisorption techniques. Precursor gels prepared from the peptized sol had a green density of about 70% after drying at 40°C, whereas the samples prepared from the unpeptized sol had a green density of only 50%. Samples prepared from the peptized sol showed higher sintering and phase transformation rates compared to the unpeptized sols

Effect of Post-Precipitation Treatment on the Pore-Structure Stability of Sol-Gel-Derived Lanthanum Zirconate

The importance of post-precipitation treatments (pore-fluid exchange and its removal) on the evolution of the texture of coprecipitated lanthanum zirconate has been investigated. The nature of the pore fluid and the type of fluid-removal (drying) process have shown a profound effect on the aggregate structure (packing of primary particles within the aggregates of the gel). Exchanging the pore fluid with organic fluids such as ethanol or tetrahydrofuran resulted in higher specific surface area, larger pore volume, and, most frequently, larger pore radii. The effect has been attributed to the surface tension of the pore fluid rather than to the polycondensation effect. Freeze drying and microwave drying of ethanol-washed lanthanum zirconate gel also resulted in higher surface area, compared to the oven-dried gel. Freeze drying and microwave drying yielded a surface area of -30 m2/g, whereas oven drying yielded a surface area of 20 m2/g after heating at 800°C

Influence of Peptization and Ethanol Washing on the Pore-Structure Evolution of Sol-Gel-Derived Alumina Catalyst Supports

Alkoxide-derived boehmite precipitates were subjected to different post-precipitation treatments to obtain a peptized sol, an unpeptized slurry, and an ethanol-washed gel precipitate. These materials were dried further to obtain the corresponding gels. Changes in the surface area, porosity, and pore size with temperature and the phase-transformation behavior from gamma-alumina to alpha-alumina were studied using X-ray diffractometry, differential scanning calorimetry, and physical adsorption/desorption measurements. Gels that were obtained from the peptized sol retained the lowest porosity, in comparison to the ethanol-washed gel and the gel that was prepared from the aqueous slurry.\ud \u

Sintering of Lanthanum Zirconate

Lanthanum zirconate (La2Zr2O7) was prepared by coprecipitating lanthanum nitrate and zirconyl oxychloride at pH 10, followed by ethanol washing. The initial high surface area of 304 m2·g−1 decreased very rapidly with increased sintering temperature and decreased to an immeasurably small value after heating at 1200°C for 15 h. The major parameters studied were phase evolution, crystallite size, porosity, surface area reduction, and shrinkage during sintering. Three temperature regions were identified based on these studies: below the crystallization temperature, between the crystallization temperature and 1100°C, and above 1100°C. The main contribution of surface area reduction in the region 800°–1100°C was due to surface diffusion; the main contribution above 1100°C was due to grain-boundary diffusion coupled with surface diffusion

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

Pure Al2O3 and different compositions of La2O3–Al2O3 samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La2O3·11Al2O3 and La2O3·13Al2O3 had higher surface area compared to the pure Al2O3 and the La2O3·Al2O3 composition. Ethanol washing is an effective way for improving the textural stability of pure Al2O3 and La2O3–Al2O3 samples. The effect of steam on the thermal stability of La2O3·11Al2O3 has also been studied. La2O3·11Al2O3 sample is found to be stable in steam.\ud \u

Chrysin inhibits adipogenesis by modulating PPARγ: <i>in silico</i> and <i>in vitro</i> studies

Adipose tissue is the major storage site of lipids and plays a vital role in energy homeostasis. Adipogenesis is a well-regulated process wherein preadipocytes differentiate into adipocytes. It requires the sequential activation of numerous transcription factors, including peroxisome proliferator activated receptor-γ (PPAR-γ). Phytochemicals have been reported to regulate adipogenesis and flavonoids represent the most researched groups of phytochemicals with regard to their effect on adipogenesis. Chrysin is a naturally occurring flavone and is reported to have anti-inflammatory effects in obese conditions. The present study was aimed to examine the effect of chrysin on adipogenesis. In silico Molecular docking, dynamic simulation studies and in vitro cell-based assays showed that chrysin inhibited adipogenesis by modulating key adipogenic transcription factor PPARγ. Enhanced adipogenesis leads to obesity and targeting adipogenesis is potential in regulating adipose tissue development. So, these investigations may provide important information for designing therapeutic interventions to control adiposity. Communicated by Ramaswamy H. Sarma</p