Facile preparation of ZrCO composite aerogel with high specific surface area and low thermal conductivity

A novel ZrCO composite aerogel is synthesized using zirconium oxychloride and resorcinol–formaldehyde (RF) as precursors through the sol–gel route and carbothermal reduction process. The effects of different Zr/R molar ratios and calcination temperatures on the physical chemistry properties of ZrCO aerogels are investigated. The ZrCO composite aerogel consists of the C/ZrO₂/ZrC ternary aerogel. The results show that with the increase of R/Zr molar ratios, the specific surface area and bulk density increase with calcination temperature up to 1300?°C, but decrease at even temperature (1500?°C). The specific surface area is as high as 637.4?m²/g for ZrCO composite aerogel (R:Zr?=?2:1), which was higher than ever reported. As the heat-treatment temperature increases to 1500?°C, the ZrC crystalline phase occurs and the t-ZrO₂ phase still appears within the composite. The thermal conductivity of the carbon fiber mat-reinforced composite aerogel is as low as 0.057?W/m/K at room temperature (25?°C).

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Activated carbon aerogels with high bimodal porosity for lithium/sulfur batteries

Activated carbon aerogels (ACAs) with high bimodal porosity were obtained for lithium/sulfur batteries by potassium hydroxide (KOH) activation. Then sulfur–activated carbon aerogels (S–ACAs) composites were synthesized by chemical deposition strategy. The S–ACAs composites were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy, and N₂ adsorption/desorption measurements. It is found that the activated carbon aerogels treated by KOH activation presents a porous structure, and sulfur is embedded into the pores of the ACAs network-like matrix after a chemical deposition process. The Li/S–ACAs (with 69.1 wt% active material) composite cathode exhibits discharge capacities of 1,493 mAh g^?1 in the first cycle and 528 mAh g^?1 after 100 cycles at a higher rate of C/5 (335 mA g^?1). The S–ACAs composite cathode exhibits better electrochemical reversibility, higher active material utilization, and less severe polysulfide shuttle than S–CAs composite cathode because of high bimodal porosity structure of the ACAs matrix.     

Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol–gel method

Ultra-fine zirconium carbide (ZrC) powders have been synthesized by carbothermal reduction reactions using inorganic precursors zirconium oxychloride (ZrOCl₂ · 8H₂O) as sources of zirconium and phenolic resin as the carbon source. The reactions were substantially completed at relatively lower temperatures (∼1400 °C/1 h) and the synthesized powders had a small average crystallite size (<200 nm) and a large specific area (54 m²/g). The oxygen content of the powder synthesized at 1400 °C/1 h was less than 1.0 wt%. The thermodynamic change process in the ZrO₂–C system and the synthesis mechanism were studied.     

Influence of supercritical drying fluids on structures and properties of low-density Cu-doped SiO2 composite aerogels

The Cu-doped SiO₂ composite aerogels were successfully prepared by sol–gel process and subsequently supercritical drying with ethanol and CO₂. The Cu-doped SiO₂ composite aerogels had porous texture, low density (<100 mg cm^?3) and high specific surface area (>800 m² g^?1), which were investigated by FESEM and nitrogen adsorption desorption porosimetry. The FTIR spectra of the aerogels showed that the ethanol-dried aerogels had been modified by ethyl while the corresponding CO₂-dried aerogels had more Si–OH groups. The phase structure and thermal stability were investigated by XRD and TGA, respectively. Due to the reducibility of ethanol, the copper was crystalline in ethanol-dried sample. The Cu-doped SiO₂ composite aerogels dried with supercritical ethanol had larger pore diameter and better thermal stability under 400 °C in comparison with CO₂-dried composite aerogels. The structures and properties of Cu-doped SiO₂ composite aerogels are obviously affected by supercritical drying conditions. The effect research could instruct the synthesis of different state of Cu in composite aerogels.     

Synthesis of TiO2–SiO2 aerogel via ambient pressure drying: effects of sol pre-modification on the microstructure and pore characteristics

TiO₂–SiO₂ composite aerogels were prepared via ambient pressure drying by sol–gel and surface modification for both the sol and gel samples. The organosilane reagents of decamethyltetrasiloxane (DMTSO)/trimethylchlorosilane (TMCS) and hexamethyldisiloxane (HMDSO)/TMCS were introduced into the TiO₂–SiO₂ composite sol for pre-modification respectively, and subsequently the TMCS/hexane solution was used for surface modification of the obtained TiO₂–SiO₂ composite gel. The effects of sol pre-modification on the microstructure and pore characteristics of TiO₂–SiO₂ composite aerogels were investigated. The results indicate that HMDSO/TMCS coupling reagents is more appropriate for the pre-modification of TiO₂–SiO₂ composite sol than the DMTSO/TMCS reagents. The best volume ratio of HMDSO/TMCS/composite sol for preparing mesoporous TiO₂–SiO₂ composite aerogels is in the range of 1:0.33:10–1:1.0:10, with which the specific surface area and pore volume of the obtained TiO₂–SiO₂ composite aerogels are 492–645 m²/g and 2.63–2.85 m³/g, respectively. The results of adsorption and photocatalytic degradation of rhodamine B show that the as-prepared TiO₂–SiO₂ composite aerogels have higher adsorption/photocatalysis. Particularly, the as-prepared TiO₂–SiO₂ composite aerogels with HMDSO/TMCS showed prominent adsorption capability with the adsorption rate attaining to 89.4 % within 60 min.     

Porous structure SnSb/amorphous carbon core-shell composite as high capacity anode materials for lithium ion batteries

SnSb/C core-shell powder has been successfully prepared by modified carbothermal reduction method. The shape, size, morphology, and electrochemical properties of the SnSb/C core-shell powder have been investigated. SnSb particles are completely encapsulated by amorphous carbon shell, and the surface of SnSb/C composite has been characterized with porous structure. The composite has a relatively high BET surface area of 253 m²g^?1. The composite exhibits relatively good capacity retention for 50 cycles at a constant current density of 100 mA g^?1 and show excellent rate performance when the current ranges from 50 to 200 mA g^?1. The improvement of reversible capacity and cyclic performance is attributed to loose and amorphous surface structure which could buffer volume variations through cycle process.     

Sol–gel preparation of ZrC–ZrO<Subscript>2</Subscript> composite microspheres using fructose as a carbon source

ZrC–ZrO₂ composite ceramic microspheres were prepared by internal gelation combined with carbothermic reduction using fructose as a chelating agent and carbon source. Fructose in the precursor solution formed complex with zirconium ions, which was conducive to the refining of the microstructure of the sintered composite. ZrC–ZrO₂ composite with ZrC content as high as 60?wt% could be prepared.

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In this paper, fructose was used as a chelating agent and an organic carbon source to prepare ZrCO microspheres by internal gelation and carbothermic reduction. The fructose in the precursor solution could form complex with zirconium ions, which was conducive to the refining of the microstructure of the sintered composite. ZrC–ZrO₂ composite with crystal size of ZrO₂ and ZrC in nanometer range and ZrC content as high as 60?wt% could be successfully prepared.

     

Preparation and characterization of a nitrogen-doped mesoporous carbon aerogel and its polymer precursor

Nitrogen-containing carbon aerogel was prepared from resorcinol–melamine–formaldehyde (R–M–F) polymer gel precursor. The polymer gel was supercritically dried with CO₂, and the carbonization of the resulting polymer aerogel under nitrogen atmosphere at 900 °C yielded the carbon aerogel. The polymer and carbon aerogels were characterized with TG/DTA–MS, low-temperature nitrogen adsorption/desorption (??196 °C), FTIR, Raman, powder XRD and SEM–EDX techniques. The thermal decomposition of the polymer aerogel had two major steps. The first step was at 150 °C, where the unreacted monomers and the residual solvent were released, and the second one at 300 °C, where the species belonging to the polymer network decomposition could be detected. The pyrolytic conversion of the polymer aerogel was successful, as 0.89 at.% nitrogen was retained in the carbon matrix. The nitrogen-doped carbon aerogel was amorphous and possessed a hierarchical porous structure. It had a significant specific surface area (890 m² g^?1) and pore volume (4.7 cm³ g^?1). TG/DTA–MS measurement revealed that during storage in ambient conditions surface functional groups formed, which were released upon annealing.     

Fabrication of titania and titania–silica aerogels using rapid supercritical extraction

Titania (TiO₂) and titania–silica (TiSi) aerogels are suitable for photocatalytic oxidation of volatile organic compounds for pollution mitigation; however, methods for fabricating these aerogels can be complex. In this work we describe the use of a rapid supercritical extraction (RSCE) technique to prepare TiO₂ and TiSi aerogels in as little as 8 h. The RSCE technique uses a metal mold and a four-step hydraulic hot press procedure to bring the solvents in the sol–gel pores to a supercritical state and control the supercritical fluid release process. Resulting TiO₂ aerogels were powdery with BET surface areas of 130–180 m²/g, pore volumes ~0.5 cm³/g and skeletal densities of 3.6 g/mL. Monolithic TiSi aerogels were made using two different methods. An impregnation process, in which titania precursor was added to a silica sol–gel, took 4–8 days to complete with a 7-h RSCE and resulted in translucent aerogels with high surface area (560–650 m²/g) and pore volume (2.0–2.6 cm³/g), bulk densities ranging from 0.1 to 0.4 g/mL and skeletal densities of 2.3 g/mL. A co-precursor method for preparing TiSi aerogels took 8 h to complete. The precursor chemical mixture was poured directly into the mold and processed in a 7-h RSCE process. The resulting aerogels were opaque, with high surface areas (510–580 m²/g), low bulk density (0.03 g/mL), skeletal densities of 2 g/mL and pore volumes of 2.6–3.5 cm³/g. Preliminary solar simulator studies show that TiO₂ and TiSi aerogels are capable of photocatalytic degradation of methylene blue in aqueous solution.     

Synthesis of zirconium diboride nano-powders by novel complex sol–gel technology at low temperature

Zirconium diboride nano-powders were synthesized by novel complex sol–gel technology at low temperature. Zirconium oxychloride as source of zirconium was complexed via adding propylene oxide that could form the stable zirconium sol by protonation and ring-opening reactions and boric acid as source of boron was dissolved effectively using glycerol that acted as complexing agent to create the boron network through forming stable coordination compound. Additionally, glycerol could be carbonized as source of carbon, which could replace the conventional carbon source additives. Then the mixed sols were gelated, dried, and exposed to 1,450 °C for 2 h with flowing argon in the furnace. High purity of ZrB₂ nano-powders with spherical shape of ca. 200 nm were synthesized via carbothermal reduction. The mechanism of two complexing agents were discussed combining with the infrared spectroscopy analysis results, and the powder characteristics were performed by using X-ray diffraction and transmission electron microscopy.     

LiFePO4/C composites from carbothermal reduction method

Using the cheap raw materials lithium carbonate, iron phosphate, and carbon, LiFePO₄/C composite can be obtained from the carbothermal reduction method. X-ray diffraction (XRD) and scanning electronic microscope (SEM) observations were used to investigate the structure and morphology of LiFePO₄/C. The LiFePO₄ particles were coated by smaller carbon particles. LiFePO₄/C obtained at 750 °C presents good electrochemical performance with an initial discharge capacity of 133 mAh/g, capacity retention of 128 mAh/g after 20 cycles, and a diffusion coefficient of lithium ions in the LiFePO₄/C of 8.80?×?10^?13 cm²/s, which is just a little lower than that of LiFePO₄/C obtained from the solid-state reaction (9.20?×?10^?13 cm²/s) by using FeC₂O₄ as a precursor.     

Synthesis of a hybrid MIL-101(Cr)/ZTC composite for hydrogen storage applications

Metal–organic frameworks (MOFs) hybrid composites have recently attracted considerable attention in hydrogen storage applications. In this study a hybrid composite of zeolite templated carbon (ZTC) and Cr-based MOF (MIL-101) was synthesised by adding the templated carbon in situ during the synthesis of MIL-101(Cr). The obtained sample was fully characterized and hydrogen adsorption measurements performed at 77 K up to 1 bar. The results showed that the surface areas and the hydrogen uptake capacities of individual MIL-101 (2552 m² g^?1, 1.91 wt%) and zeolite templated carbon (2577 m² g^?1, 2.39 wt%) could be enhanced when a hybrid MIL-101(Cr)/ZTC composite (2957 m² g^?1, 2.55 wt%) was synthesized. The procedure presents a simple way for enhancement of hydrogen uptake capacity of the individual Cr-MOF and templated carbon samples.     

Electrospun zirconium hydroxide nanoparticle fabrics as sorptive/reactive media

Good quality adsorbent fabrics were electrospun from slurries of zirconium hydroxide nanoparticles and polyvinyl butyral (PVB) in alcohols. The as-spun fabrics had very high porosity, about 80 vol%, and high zirconium hydroxide content, 50–95 wt%. The porosity and mechanical strength of the fabrics could be modified via compaction and/or exposure to alcohol vapor. SO₂ sorption capacity of the fabrics was high at 1.6–1.9 mol/kg and was relatively independent of the sample pretreatment, unlike the capacity of the pristine nanoparticles, which decreased when the powder was stored in a low relative humidity atmosphere. The as-purchased zirconium hydroxide nanoparticles contained about 45 wt% of water, equivalent to the formula ZrO₂·5.6H₂O. In contrast, the electrospun composite fabrics contained only 25 wt% of mostly coordinated water, leading to the formula ZrO₂·2.2H₂O. The presence of PVB binder inhibited the rehydration/dehydration processes and stabilized the SO₂ sorption capacity of the composite fibers. The electrospun fabrics could find applications as conformable, flexible filters in civilian and military applications.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

Carbon nanofibers–SiO<Subscript>2</Subscript> composites: Preparation and characterization

A method has been developed for preparing carbon fibers–SiO₂ composites using oligomethylhydridesiloxane (OMHS) as the precursor for SiO₂. The presence of active hydrogen in OMHS made it possible to attain chemical interaction between the surface of carbon nanofibers (CNFs) and the applied silicon oxide layer. The oxidation rate of CNF–SiO₂ composite is found to be lower by about one order of magnitude compared to that of the as-synthesized CNF. The thermal stability of CNF–SiO₂ composites has been studied. Under an inert atmosphere, CNF–SiO₂ composite has thermal stability up to 1300°C. At temperatures above 1350°C, silicon carbide (SiC) fibers are formed as a result of the carbothermal reduction of silicon oxide.     

Low temperature carbon monoxide catalytic oxidation at the Pd/Ce–Zr–Al–Ox catalyst

The homogeneous chemical composition ceria–zirconia–alumina (Ce–Zr–Al–O_x) nano-alloy were successfully synthesized by surfactant-assisted parallel flow co-precipitation method and applied as supports for low temperature CO oxidation. The experiment conditions were studied in detailed. At 0.92 wt% Pd loading, 30,000 ppm CO could be completely oxidized to CO₂ at 30 °C at a WHSV of 4,380 ml g^?1 h^?1 over the Pd/Ce–Zr–Al–O_x (n_Ce:n_Zr = 3:1) catalyst. Pd/Ce–Zr–Al–O_x catalysts were systematical studied by mean of BET, XRD and TEM analysis. XRD characterization showed that zirconium element entered into cubic structure of ceria and leaded to structure distortion. Addition of aluminum increased specific surface area of ceria–zirconia solid solution substantially. The average pore diameter of Ce–Zr–Al–O_x support palladium catalysts were the key impact factor for CO oxidation. When the Pd/Ce–Zr–Al–O_x catalysts had highly dispersed palladium nanoparticles, large average pore diameter, suitable surface area and pore volume, the activity of CO oxidation was the best.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels

In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm^?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm^?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m² g^?1 with a pore volume and average pore diameter of 1.24 cm³ g^?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m² g^?1, pore volume of 1.22 cm³ g^?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m² g^?1 with a total pore volume of 0.98 cm³ g^?1. The pyrolysed aerogels were also calcined to yield carbon free materials.     

VO<Subscript>x</Subscript>/Zr–SBA-15 catalysts for selective oxidation of ethanol to acetaldehyde

Effect of zirconium presence in the silica framework and content and speciation of vanadium surface oxo-complexes on the catalytic behavior of VO_x/Zr–SBA-15 catalysts in oxidative dehydrogenation of ethanol was investigated. Experimental results bring evidence of successful incorporation of zirconium into ordered mesoporous silica framework with the preservation of ordered mesoporosity by hydrothermal template base synthesis method. The presence of zirconium in the SBA-15 framework increases reducibility of vanadium species and acidity of the catalysts. It is reflected in higher activity of vanadium species expressed as turn-over frequency (e.g., TOF of 20 h^?1 for 5%VO_x/Zr–SBA-15 sample in comparison with TOF of 12 h^?1 for 5%VO_x/SBA-15 sample) and also in significant decrease of selectivity to acetaldehyde (65% in comparison with 90% for mentioned samples) followed by increase in selectivity to ethylene (25% in comparison with 5%). This change in distribution of reaction products is related to stronger acidity character of surface OH groups and inhibition effect of formed water vapours on the oxidative dehydrogenation products (acetaldehyde). Catalytic data also reveal that oligomeric/polymeric tetrahedrally coordinated vanadium species exhibit higher activity in ethanol oxidative dehydrogenation than monomeric complexes. In addition, comparison of the catalytic performance of VO_x/Zr–SBA-15 catalysts with VO_x/SBA-15 catalysts showed that catalytic properties of VO_x/Zr–SBA-15 catalysts can be tuned by incorporation of controlled amount of zirconium into silica framework.     

Synthesis and visible light photocatalytic activity of Ag spot-coated TiO2–60 wt% SrO composite powders

Nano-sized TiO₂–60 wt% SrO composite powders were synthesized from titanium isopropoxide and Sr(OH)₂·8H₂O by use of a sol–gel method. Ag spot-coated TiO₂–60 wt% SrO composite powders containing 3, 5, or 7 wt% Ag were synthesized by hydrothermal-assisted attachment, by use of Ag hydrosol in a high-pressure bomb at 250 °C and 450 psi. Nano-sized Ag particles approximately 5–25 nm in diameter adhered to the TiO₂–60 wt% SrO₂ composite powders. The photocatalytic activity of Ag spot-coated TiO₂–SrO powders in the degradation of phenol showed that all were highly active when irradiated with UV light. TiO₂–60 wt% SrO composite powder spot-coated with 5 wt% Ag was more photocatalytically active under visible light than TiO₂–SrO composite powder.