High-temperature Supercritical Drying Research Articles

Unlocking the electrochemical potential of carbon aerogels: Tailored performance via controlled carbonization and activation process

In this study, Carbon Aerogels (CAs) were prepared by high-temperature supercritical drying (HTSD) in ethanol for supercapacitors (SCs). HTSD eliminates the acetone-liquid CO2 exchange step and offers a time-efficient alternative to traditional liquid CO2 drying methods. This study investigated the effect of varying carbonization temperatures (600, 700, 800, and 900 °C) on resorcinol-formaldehyde aerogel and assessed their impact on structural, morphological, compositional, and surface area properties. Amongst, CA carbonized at 700 °C (CA-7) exhibited superior performance with a specific capacitance (Cs) of 78 F/g, energy density (Ed) of 39 Wh/kg at 2 A/g in 1 M Na2SO4. After KOH activation, CA-7 demonstrated two-fold enhancement in Cs (184 F/g at 2 A/g) and three-fold in Ed (93 Wh/kg) while maintaining 98 % capacitance retention over 7000 cycles. The symmetric aqueous device (ACA\\SSM||ACA\\SSM) exhibited a Cs of 32 F/g, Ed of 25 Wh/kg, Pd of 2308 W/kg, and 88.6 % retention over 2500 cycles at 2.4 V. This research highlights the advantages of HTSD-based CA synthesis and the potential of activated CA and Na2SO4 to achieve high Ed for SCs.

Evaluation the life cycle carbon emissions of the aerogel glazing under different weather conditions

Carbon emissions are widely recognized as the primary cause of global warming over the past few decades. The Aerogel Glazing System (AGS) stands out as one of the most advanced glazing systems currently available. In order to evaluate the carbon-saving potential of AGS, the life cycle carbon emission model of AGS was established. The carbon emission of production, transportation, construction, operation, maintenance, and waste disposal are considered. The results showed that AGS has higher carbon-saving potential. The life cycle carbon emissions of AGS in commercial, double glazing system (DGS), triple glazing system (TGS) are −144.8 kg CO2/m2, 593.6 kg CO2/m2, 250.8 kg CO2/m2 in Beijing horizontal direction. Under high temperature supercritical drying (in Commercial), low temperature supercritical drying, and freeze-drying, the life carbon emissions of AGS are lower compared to both DGS and TGS. Then the life cycle carbon emissions of AGS in different climates were analyzed. The results show that the life cycle carbon emissions of AGS under different climatic conditions are as follows (in ascending order): subtropical climate < polar climate < temperate climate < arid climate < tropical climate, with a maximum of 4929.9 kg CO2/m2 and a minimum of −634.5 kg CO2/m2. The use of AGS in tropical climate produced 5.9 times increase in life carbon emissions compared to its use in subtropical climate. In the heating-dominated climate, AGS has the greatest potential for carbon savings when orientated southwards. In cooling dominated climates, the carbon emissions are higher in the southward direction, with a weaker potential for carbon savings. Interestingly, in polar and tropical climates, increasing the aerogel thickness layer effectively reduces the life cycle carbon emissions of AGS. This study provides valuable insights for selecting sustainable curtain wall systems and window configurations, offering a pathway to minimize environmental impacts.

Engineering SiO2–TiO2 binary aerogels for sun protection and cosmetic applications

SiO2‒TiO2 binary aerogels obtained by the methods of low-temperature (carbon dioxide) and high-temperature (isopropanol, hexafluoroisopropanol and methyl tert-butyl ether) supercritical drying are considered as multifunctional cosmetic pigments with high anti-shine power and photoprotective properties. The composition and structure of SiO2–TiO2 aerogels obtained by supercritical drying in various fluids were studied by IR spectroscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and low-temperature nitrogen adsorption. The values of the sun protection factors SPF and UVAPF for the obtained materials are comparable to those of the commercial sunscreens Kronos 1171 and Kronos 2971, and the anti-shine power is approximately 1.5 times higher than that of kaolin-based materials.

Sol–gel synthesis of highly TiO2 aerogel photocatalyst via high temperature supercritical drying

Nanocrystalline powders of TiO2 xerogel and aerogel were prepared by using acid-modified sol–gel approach. For TiO2 aerogel material (TA), the solvent was high temperature supercritically extracted at 300°C and 100bars. However, the TiO2 xerogel material (TX) was dried at 200°C and ambient pressure. The effects of the drying processes on the crystalline structure, phase transformation and grain growth were determined by Raman spectroscopy, SAED and X-ray diffraction (XRD) analyses using Rietveld refinement method. The TiO2 aerogel was composed of anatase crystalline structure. The TiO2 xerogel material was composed of anatase, brookite and small amount of amorphous phase with anatase as dominant phase. The TX sample still contains a relatively high concentration of carbon than that of TA, indicating the amorphous character of TiO2 xerogel. These materials were applied as catalyst for the degradation of indigo carmine in aqueous medium. Photo-degradation ability of TA and TX was compared to the TiO2 commercial Degussa P25. The photo-catalytic results showed that the degradation efficiency was in the order TA>P25>TX. The photo-degradation of indigo carmine followed pseudo first order reaction kinetics.

Heat-resistant, strong titania aerogels achieved by supercritical deposition

Heat-resistant, strong titania aerogels are prepared via a novel supercritical deposition method. This deposition process is based on deposition by partially hydrolyzed titanium alkoxide and alkoxysilane during high-temperature supercritical fluid drying. The nanoparticles are significantly reinforced, and the crystal growth of titania upon heat treatment is effectively restricted by supercritical deposition, which leads to the enhanced mechanical strength and heat resistance of titania aerogels. The Young’s modulus of the resulting titania aerogel is enhanced up to 4.2MPa. The specific surface area of the optimized titania aerogel increases up to 147m2/g and the corresponding linear shrinkage decreases to as low as 11% after heat treatment at 1000°C. This may significantly contributed to the high-temperature applications of titania aerogels in thermal insulations, catalysts, catalyst supports, etc.

Modulating the textural properties and photocatalytic hydrogen production activity of TiO2 by high temperature supercritical drying

We report a facile method for the synthesis of TiO2 aerogels by a single step high temperature supercritical drying (HTSCD) of sol–gel derived TiO2. The morphological and structural features of the resultant materials were determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Diffuse Reflectance (DR) spectra, and Fourier Transform infra-Red spectroscopy (FT-IR) measurements. The materials exhibited enhanced solar hydrogen production from water using methanol as sacrificial reagent under Ultra-Violet (UV) light in the absence of Pt as a co-catalyst. Among the TiO2 aerogel samples synthesized, TiO2-M-6h evolved 390 μmol g−1 of H2 after 4 h of irradiation, whereas TiO2-M-2h produced 217 μmol g−1 of H2 after 4 h of irradiation under identical conditions, indicating the importance of aging the gels prior to HTSCD step. The enhancement was credited to increase in surface area, and decrease in particle size in TiO2-M-6h as evidenced from N2-sorption and DRS studies respectively. Upon comparison with a room temperature synthesized TiO2-xerogel, the aerogel materials exhibited enhanced hydrogen production. The results validate the superior performance of TiO2 aerogel materials over TiO2 xerogels and indicate the potential of HTSCD method for the preparation of titania aerogels for solar energy applications.

Fischer Tropsch synthesis over Co/SiO2 and Co-M (M: Ru, Re)/SiO2 catalysts prepared by a high-temperature super-critical drying method

Co/SiO2-based catalysts for Fischer Tropsch synthesis (FTS) were prepared by a high-temperature super-critical drying (HT-SCD) method, and the effects of the drying method on the catalytic properties were investigated. X-ray diffraction analysis (XRD) of the calcined catalysts revealed that the cobalt species formed in the HT-SCD catalysts is mainly a less-reducible cobalt silicate (Co2SiO4-like species), whereas that formed in catalysts prepared by a conventional thermal drying (TD) is mainly the easily reducible Co3O4 species. Moreover temperature programmed reduction (TPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and H2 chemisorption measurements revealed that the addition of rhenium or ruthenium to the HT-SCD catalysts improves the reducibility of the cobalt species and the cobalt dispersion, and partially suppresses the covering cobalt particles with SiO2 layer. The type of cobalt precursors used for catalyst preparation also had an effect on the cobalt species formed in the calcined catalysts. The HT-SCD catalysts with cobalt silicate exhibited higher olefin selectivities for FTS than the Co3O4-based catalysts prepared by TD, whereas the C11+ selectivities of the TD catalysts were higher than those of the HT-SCD catalysts. On the other hand, turn over frequency (TOF) estimated from the amount of H2 chemisorbed was constant without depending on the drying method and the type of cobalt precursors.

Synthesis and characterization of mesoporous VOx/MgO aerogels with high surface area

Homogeneously mixed mesoporous VOx/MgO aerogels with vanadium concentrations ranging up to 26wt.% with the surface area as high as 1200m2/g were synthesized by hydrolysis of a mixture of vanadium and magnesium alkoxides followed by gelation and high-temperature supercritical drying. Dehydration conditions were optimized to obtain nanocrystalline VOx/MgO oxides with the surface area exceeding 450m2/g after calcination at 550°C. The synthesized aerogels were extensively characterized by physicochemical methods (XRD, NMR, HRTEM, EDX). According to the EDX data, vanadium atoms were very uniformly distributed in the MgO structure of the mixed oxide aerogels. No magnesium vanadate phases were observed by XRD in the samples with the vanadium concentrations 26% or lower. As the vanadium concentration increased, vanadium ions were introduced into the MgO nanocrystals changing their symmetry from the initial cubic (Fm3m) to orthorhombic (Cmca). Simulation of the XRD patterns of as-prepared aerogels suggests that the Mg(OH)2 phase in these samples is substantially distorted due to the presence of methoxide groups between the layers.

Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying

When developing sustainable building fabric technologies, it is essential that the energy use and CO2 burden arising from manufacture does not outweigh the respective in-use savings. This study investigates this paradigm by carrying out a streamlined life cycle assessment (LCA) of silica aerogel. This unique, nanoporous translucent insulation material has the lowest thermal conductivity of any solid, retaining up to four times as much heat as conventional insulation, whilst being highly transparent to light and solar radiation. Monolithic silica aerogel has been cited as the ‘holy grail’ of future glazing technology. Alternatively, translucent granular aerogel is now being produced on a commercial scale. In each case, many solvents are used in production, often accompanied by intensive drying processes, which may consume large amounts of energy and CO2. To date, there has been no peer-reviewed LCA of this material conducted to the ISO 14000 standard.Primary data for this ‘cradle-to-factory gate’ LCA is collected for silica aerogel made by low and high temperature supercritical drying. In both cases, the mass of raw materials and electricity usage for each process is monitored to determine the total energy use and CO2 burden. Findings are compared against the predicted operational savings arising from retrofitting translucent silica aerogel to a single glazed window to upgrade its thermal performance. Results should be treated as a conservative estimate as the aerogel is produced in a laboratory, which has not been developed for mass manufacture or refined to reduce its environmental impact. Furthermore, the samples are small and assumptions to upscale the manufacturing volume occur without major changes to production steps or equipment used. Despite this, parity between the CO2 burden and CO2 savings is achieved in less than 2years, indicating that silica aerogel can provide a measurable environmental benefit.

Preliminary study on the selective oxidation of H2S over LaVO4 and Fe2(MoO4)3 oxides, produced by a solvothermal method

LaVO4 and β-FeMoO4 powders were synthesized by a solvothermal route followed by a high temperature supercritical drying step. The X-ray diffraction (XRD) patterns exhibit partly amorphous LaVO4 and crystalline Fe2(MoO4)3 phases. Subsequently heat treated samples were analyzed by the Fourier-transform infrared spectroscopy (FTIR) in order to determine the surface active species, which might be responsible for the selective oxidation of H2S. Catalytic activity tests were carried out in a fixed-bed reactor. The sulfur yield and selectivity for the LaVO4 was significantly higher than that of Fe2(MoO4)3 in the temperature range, i.e. 220–300 °C. It is proposed that the oxidation activity of these two catalysts investigated is based on a mechanism, which is similar to that in the case of alkane oxidation. Besides the effect of the formed surface species, the differences in the structure of the catalyst and its intrinsic reactivity should also be considered.

The influence of preparation method on the physicochemical properties of titania–silica aerogels

Titania–silica aerogels with different titania content were prepared. Four preparation methods differing mainly in approach to precursors hydrolysis were applied, while only three of them allowed total hydrolysis of silica precursor before titania precursor was added. The preparation of mixed products of titania and silica hydrolysis precursors containing gels was followed by high temperature supercritical drying (HTSCD) and thermal treatment at 500 °C. Obtained mixed oxides in form of aerogels were characterized by BET surface areas up to 1000 m2/g, mesopore volumes up to 1.6 cm3/g and bulk densities as low as 0.04 g/cm3. Even 18 h lasting aging did not allow to produce narrow diameter range mesoporous materials, their broad pore diameter distributions resulted in average pore sizes varying from 10 to nearly 30 nm. XRD measurements proved the presence of anatase crystalline form of titania, while silica was present in amorphous form. SEM studies indicated presence of isolated titania particles on titania–silica surface while joint hydrolysis method was applied. Titania–silica aerogels obtained by the simultaneous hydrolysis of precursors and the impregnation method showed high photocatalytic activity in degradation of salicylic acid in water. Activities of these mesoporous photocatalysts were higher than commercial P25 Degussa TiO2. Comparison of activity of pure TiO2 (P25 Degussa) and aerogels indicates higher utilization of titania present in mesoporous mixed oxides.

Surface characterization and H2S-sensing potential of iron molybdate particles produced by supercritical solvothermal method and subsequent oxidation

The mostly crystalline polymorph β-FeMoO4 was prepared by solvothermal synthesis from organic precursors, followed by high temperature supercritical drying in an autoclave. Crystallization of the synthesized particles occurred during subsequent heat treatment at 350 °C, confirmed by X-ray diffraction pattern analysis. The presence of Fe3+ ions in the powder, both well-crystallized and amorphous after heat treatment at 500 °C, was confirmed by room temperature Mossbauer spectrum. Thick-film gas sensors were prepared by conventional hand coating of a paste, the Fe2(MoO4)3 powder mixed with an α-terpineol-based solvent, over the Au electrodes. The response of the prepared sensors to H2S gas in the low concentration range 1–10 ppm in air was investigated. Moderately fast response and recovery times were observed. The iron molybdate, produced at low temperature, may be successfully used in the preparation of a H2S gas sensor.

Synthesis and Characterization of Iron Molybdate Aerogels Synthesized by the Sol-Gel Chemistry and High Temperature Supercritical Drying

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Structural, Textural, and Electronic Properties of a Nanosized Mesoporous ZnxTi1-xO2-x Solid Solution Prepared by a Supercritical Drying Route

Mesoporous nanosized TiO2 and Zn(x)Ti(1-x)O(2-x) solid solution having a Zn content below 10 mol % with a particles size between 13 and 17 nm are prepared by a template-free sol-gel method followed by high-temperature supercritical drying in 2-propanol. The structural, textural, and electronic properties of the obtained nanomaterials are methodically investigated by using XRD, SEM, TEM, ED, HREM, EDX, ICP-OES, N(2) adsorption-desorption, Raman spectroscopy, and diffuse reflectance UV-vis spectroscopy. It is shown that the proposed synthesis technique leads to the formation of a Zn(x)Ti(1-x)O(2-x) solid solution based on the anatase crystal structure rather than a two-phase sample. High-resolution electron microscopy and electron diffraction indicate that the distribution of zinc atoms over the anatase structure does not lead to a considerable deformation of the crystal structure.

Characterization of La2Mo2O9 Aerogels Synthesized by the Sol–Gel Chemistry and High-Temperature Supercritical Drying

A lanthanum molybdate aerogel, La2Mo2O9, with a mean particle size in the range from 100 to 150 nm, was synthesized by the sol-gel method and high-temperature supercritical drying. In this communication it is shown that control over the crystallinity of product aerogels can be exercised by changing the amount of water used for hydrolysis and the temperature for subsequent heat treatment. Methoxy species are formed on the surface during synthesis. The new aerogel may prove useful as a catalyst for the oxidation of hydrocarbons to oxygenated organic compounds.

Preparation and Characterization of FeCo-Al2O3and Al2O3Aerogels

Nanocrystalline γ-Al2O3 and FeCo-Al2O3 nanocomposites containing FeCo alloy nanoparticles well dispersed in the nanocrystalline γ-Al2O3 matrix were successfully obtained by the sol-gel method using aluminum tri-sec-butoxide and iron and cobalt nitrates as precursors. The gels were submitted to high temperature supercritical drying which allowed to obtain aerogels with high surface areas and pore volumes.

Evolution of the Structure and Magnetic Properties of FeCo Nanoparticles in an Alumina Aerogel Matrix

Nanocrystalline γ-Al2O3 and FeCo−Al2O3 nanocomposite aerogels with high surface areas and pore volumes were prepared by high-temperature supercritical drying of alcogels obtained by a fast sol−gel procedure. The formation of γ-Al2O3 occurs via a sequence of stages: in the parent aerogel an alkyl derivative of boehmite is observed whose calcination gives rise to a disordered phase and finally to γ-Al2O3 which is stable up to 1000 °C. In the presence of iron and cobalt, calcination of the aerogel gives rise to a spinel phase similar to γ-Al2O3 where metal ions partially fill the vacancies. Nanocomposites constituted of FeCo alloy nanoparticles dispersed into γ-Al2O3 matrix are obtained via reduction in hydrogen flow of the aerogels containing iron and cobalt. The amount and average size of the nanoparticles depends both on the temperature and time of the reduction treatment and affects the resulting magnetic properties. All the reduced aerogels show superparamagnetic behavior, but the collective magnetic p...

FeCo–SiO2 nanocomposite aerogels by high temperature supercritical drying

FeCo–SiO2 nanocomposite aerogels were prepared by high temperature supercritical drying of alcogels obtained using tetraethoxysilane, iron nitrate and cobalt nitrate as precursors. The structural evolution of the samples at the various stages of the preparation was studied by thermal analysis, transmission electron microscopy, nitrogen physisorption measurements and X-ray diffraction. Different experimental conditions of the supercritical drying process give rise to different porous structures in the silica matrix which have a strong influence on the formation of the FeCo alloy nanoparticles. Bcc FeCo alloy nanoparticles of the expected composition are obtained in the microporous samples while in the mesoporous samples a bcc alloy with a lower Co content is obtained which is accompanied by pure fcc Co.

Copper- and Silver–Zirconia Aerogels: Preparation, Structural Properties and Catalytic Behavior in Methanol Synthesis from Carbon Dioxide

Copper- and silver-zirconia aerogels containing 10 at% IB metal were prepared from tetra-n-butoxy zirconium(IV) and IB metal acetates using the solution sol-gel method and ensuing high-temperature (HT) and low-temperature (LT) supercritical drying, respectively. The influence of preparation parameters and calcination on the structural and catalytic properties of the aerogels for the synthesis of methanol from carbon dioxide and hydrogen was investigated. After calcination in air at 573 K, the catalysts had BET surface areas in the range of 100–143 m2·g−1(Cu/ZrO2) and 77–125 m2·g−1(Ag/ZrO2), respectively. Due to the reductive alcoholic atmosphere during high-temperature supercritical drying, metallic copper and silver existed in all raw HT-aerogels. The mean size of the copper crystallites was 30 nm. The silver crystallite size for the HT-aerogel prepared with nitric acid was 10 nm, whereas for samples prepared with acetic acid it was 5–7 nm. Calcination in air at 573 K led to the formation of highly dispersed amorphous copper oxide and silver. Comparing the catalytic behavior of the calcined copper–zirconia aerogels with corresponding xerogels prepared by coprecipitation revealed highest activity for the LT-aerogel, whereas the HT-aerogels were least active. In contrast, similar catalytic behavior was observed for the differently dried silver-zirconia samples. Generally, CO2-conversion of the copper–zirconia aerogels was markedly higher than that of the corresponding silver–zirconia aerogels, whereas methanol selectivity was similar.

Synthesis of Catalytically Active Sulfated Zirconia Aerogels

Sulfated zirconia aerogels were synthesized by a sol-gel method followed by high temperature supercritical drying. Aerogels with sulfur introduced during the gelation step had high surface areas and pore volumes. Their textural properties could be tuned by variation of the preparation conditions. Samples synthesized by this procedure and those obtained by incipient wetness impregnation of zirconia aerogels with (NH4)2SO4were shown to exhibit catalytic activity in the isomerization of n-butane comparable to conventional precipitated zirconia. Maximum catalytic activity of impregnated aerogels was observed at lower nominal sulfur loadings. They were shown to retain more surface sulfates after calcination compared to samples prepared by two other methods. The most active samples had close to monolayer surface coverage with sulfates with the decomposition peak at 820–830°C. Over-monolayer surface sulfates hindered the isomerization activity whereas bulk sulfates had little effect.