Synthesis of manganese titanate and its precursors from xerogel

An overview of research on the synthesis of manganese titanates is presented. The xerogel of Mn–Ti–O–C–H composition was synthesized from manganese acetate and titanium tetrabutylate via liquid-phase method using organic solvents. The calcination of xerogel in air at 450 °C and 700 °C yielded manganese titanate precursors in the form of a nanostructured mixture of Mn₂O₃ and TiO₂. Annealing at 1000 °C, manganese metatitanate MnTiO₃ was obtained. Reference experiments with initial reagents included, separately, thermal decomposition of Mn(CH₃COO)₂×4H₂O and the product of Ti(OC₄H₉)₄ hydrolysis. The composition, structure, and properties of the products were studied using X-ray diffraction, scanning electron microscopy, elemental analysis, diffuse reflectance IR Fourier spectroscopy, thermogravimetry, and by measuring specific surface area. The data presented by these different techniques are basically consistent with each other (with an increase in the annealing temperature, an increase in globule size and decrease in specific surface area are observed; structuring occurs within the long- and short-range order; the size of the crystallites does not exceed that of the globules; elemental composition correlates with phase composition; the endothermic character of the reaction of MnTiO₃ formation at 900 °C is confirmed by a thermodynamic calculation). Nevertheless, some unexpected effects were revealed (based on the FTIR diffuse reflection spectra, mixed oxide Mn–Ti–O is formed in the surface layer of particles already at 450 °C and 700 °C; etc.). Application of the proposed technique for modifying Al₂O₃ powders, with the aim of implementing low-temperature sintering of corundum ceramics, is discussed.     

Development and investigation of sol–gel solutions for the formation of TiO2 coatings

We show a technological approach for the sol–gel processing of stabilized xerogel colloidal titanium oxide films. Glycerol was used as a drying control additive agent. Glycerol helped in stabilizing the solution. The thermal transformation of a xerogel film was studied by differential thermal analysis (DTA) and powder X-ray diffraction. The optical index of the annealed coatings was evaluated using UV–VIS–NIR spectrophotometry. The results showed that a nanocrystalline titania anatase film of high optical quality (n=2.34 at 600 nm) can be obtained by the sol–gel process.     

Electrode materials for ionic liquid-based supercapacitors

The use of ionic liquid (IL) electrolytes is a promising strategy to enhance the performance of supercapacitors above room temperature. In this paper we present the results of a study on optimization of electrode materials for IL-based supercapacitors featuring a hybrid configuration with carbon negative electrode and poly(3-methylthiophene) (pMeT) as positive operating at 60 °C with the ILs N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI). As it concerns the carbon electrode two routes have been pursued: (i) surface modification of commercial activated carbon and (ii) synthesis of mesoporous cryo- and xerogel carbons. Pore size distribution and electrochemical characterization data are related and suggest that the second route should be the most promising for carbons of high specific capacitance and low time constant in IL. For the polymer electrode the nature of the galvanostatic polymerization bath plays a crucial role to provide pMeT of high specific capacitance and the best results may be obtained when pMeT is electropolymerized in the same IL used for the capacitance tests. The strategy of using the acid additive trifluoromethanesulfonimide in IL-based polymerization baths is also described in some detail. This strategy that provides pMeT featuring more than 200 F g⁻¹ in IL is a clean procedure which prevents consumption of the ionic liquid with great advantage in terms of costs.     

Effects of drying conditions on physicochemical properties of epoxide sol−gel derived α-Fe2O3 and NiO: A comparison between xerogels and aerogels

A simple and easily operated supercritical CO₂ dryer was designed and manufactured with the aim of producing high-surface-area mesoporous α-Fe₂O₃ (hematite) and NiO aerogels. The gels were synthesized by a sol?gel method with the aid of propylene oxide (PO), as the gelation agent, and then dried and calcined at different conditions. The effects of drying and calcination conditions on the physicochemical properties of the final aerogels were investigated using X-ray diffraction (XRD), N₂ adsorption-desorption, Fourier-transformed infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM) analyses. It was demonstrated that α-Fe₂O₃ and NiO aerogels with high surface areas and mesoporosities could be successfully synthesized using the home-made supercritical CO₂ dryer. Supercritical drying of the gels resulted in α-Fe₂O₃ (186 m²/g) and NiO (178 m²/g) aerogels with 186% and 34% higher surface areas, respectively, than xerogels obtained via simple drying at 80°C using a laboratory oven. In addition, the results showed that supercritical CO₂ drying could enhance preservation of the porous network of the oxide nanostructures at high calcination temperatures via suppression of sintering phenomenon. Calcination of α-Fe₂O₃ and NiO aerogels at 600°C yielded 225% and 53% higher surface areas than the corresponding xerogel, confirming the significance of drying step in the sol?gel method. Asphaltene adsorption from a model oil with asphaltene concentration of 3000 ppm indicated that the aerogels possessed higher adsorption capacities for the bulky asphaltene molecules than xerogels calcined at the same temperature of 600°C, which was due to their enhanced textural properties.     

Preparation of organic and carbon xerogels using high-temperature–pressure sol–gel polymerization

To prepare organic gels at temperatures higher than normal boiling point of solvent, a method was developed using sol–gel polymerization in atmosphere saturated by vapor of solvent. To illustrate the advantages of proposed method, two series of gels were prepared using the conventional (T_curing = 70 °C) and the high temperature (T_curing = 140–170 °C) sol–gel polymerization. While no drying shrinkage was observed in our proposed method, 5–18% linear shrinkage occurred in conventional method depending on resin concentration in sol. Moreover, rising of curing temperature reduced the required time for preparation of organic gels from 5 days to lower than 5 h. The effects of processing parameters were investigated on physical and mechanical properties of organic xerogels. The results revealed that resin concentration significantly affects both density and compressive strength of final xerogels. While the curing temperature had no obvious effect on density, the raising of curing temperature significantly enhance the strength of organic xerogels. Carbon xerogels prepared by pyrolysis of novolac aerogels in inert atmosphere. The textures of the carbon xerogels were denser than corresponding organic xerogels, as evidenced by scanning electron microscopy (SEM) images. N₂ adsorption tests indicated that carbon aerogels were mainly meso or macroporous depending on resin concentration in initial sol.     

Cryo- and xerogel carbon supported PtRu for DMFC anodes

The specific catalytic activity of DMFC anodes based on PtRu may be improved using conducting carbon supports of high surface area and mesoporosity with pore size >20 nm for a high accessible surface area. To this purpose we pursued the strategy of developing PtRu catalysts deposited by chemical and electrochemical route on mesoporous cryo- and xerogel carbons. Here, we report the preparation and characterization data of different mesoporous cryo- and xerogel carbons as well as we present and discuss the results of the structural and morphological study and the catalytic activity data of PtRu catalysts chemically and electrochemically prepared, also by pulse techniques, on such carbons. The results are also compared to those obtained with PtRu supported on the generally used Vulcan carbon support.     

Graphene and carbon nanotube structures supported on mesoporous xerogel carbon as catalysts for oxygen reduction reaction in proton-exchange-membrane fuel cells

Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity.     

Selective catalytic oxidation of H<Subscript>2</Subscript>S using nonhydrolytic vanadia-titania xerogels

A series of vanadia-titania (V-Ti) xerogel catalysts were prepared by nonhydrolytic sol-gel method. These catalysts showed much higher surface area and total pore volumes than the conventional V₂O₅-TiO₂ xerogel. Two species of surface vanadium in the xerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Xerogel catalysts from the nonhydrolytic method showed very high conversion of H₂S without harmful emission of SO₂. The conversion of H₂S increased with increasing vanadia loading up to 10V-Ti; however, it decreased at higher vanadia loading (12V-Ti and 18V-Ti) probably due to the formation of crystalline V₂O₅.     

Influence of acetic acid and ammonia catalyst on microstructure and properties of fluorinated polyimide-organosilicate hybrids

This work investigates the preparation, microstructure and thermal/electrical properties of fluorinated polyimide-organosilicate hybrids utilizing acetic acid and ammonia as the catalysts for sol-gel process to grow organosilicate filler particles. Nano-scale organosilicate was observed in the base-catalyzed hybrids, which was ascribed to the relative inertness of base catalysts in promoting the hydrolysis reaction. This postponed the formation of particles and hence smaller particle size. Electrical measurements found that the base-catalyzed hybrids possess lower dielectric constants (κ = 2.40 at nominal Si content = 0.4 mol) due to the absence of polar groups and formation of silica xerogels with high porosity in the samples. In both hybrids, leakage current densities increase with the increase of Si content. However, leakage current density of base-catalyzed hybrid was higher than that of acid-catalyzed hybrid at the same Si content due to the smaller particle size and highly porous feature of organosilicate embedded in the base-catalyzed hybrids.