CO2‐driven synthesis of monodisperse barium titanate microspheres

Monodisperse BaTiO₃ microspheres were synthesized using Ba(OH)₂, a homemade amorphous TiO₂ precursor, and dissolved CO₂ as the raw materials. To dissolve sufficient CO₂ in the mixing solution, the reaction process was carried out in air. Then, a BaCO₃ layer, as a fixed barium source, was coated on the surface of the TiO₂ precursor template through a hydrothermal process at 90°C. Furthermore, the BaCO₃ reacted with the TiO₂ precursor to form pure BaTiO₃ through in situ conversion during subsequent heat treatment at 800°C. Compared to the commonly used acid soaking method for the removal of BaCO₃ after hydrothermal treatment, the method combining in‐air mixing with a low hydrothermal temperature and heat treatment yielded BaTiO₃ microspheres with a more monodisperse homogenous morphology, which have a greater potential in various applications.     

High-flux CO2-stable oxygen transport hollow fiber membranes through surface engineering

The influences of bulk diffusion and surface exchange on oxygen transport of (La_0.6Ca_0.4)(Co_0.8Fe_0.2)O_3-δ (LCCF) hollow fiber membranes were investigated. As an outcome, two strategies for increasing the oxygen permeation were pursued. First, porous LCCF hollow fibers as support were coated with a 22 μm dense LCCF separation layer through dip coating and co-sintering. The oxygen permeation of the porous fiber with dense layer reached up to 5.10 mL min^?1 cm^-2 at 1000 °C in a 50 % CO₂ atmosphere. Second, surface etching of dense LCCF hollow fibers with H₂SO₄ was applied. The surface etching of both inner and outer surfaces leads to a permeation improvement up to 86.0 %. This finding implies that the surface exchange reaction plays a key role in oxygen transport through LCCF hollow fibers. A good long-term (>250 h) stability of the asymmetric hollow fiber in a 50 % CO₂ atmosphere was found at 900 °C.     

On the formation of an oxycarbonate intermediate phase in the synthesis of BaTiO3 from (Ba,Ti)-polymeric organic precursors

The study of BaTiO₃ crystallization from X-ray amorphous (Ba,Ti) polymeric organic powders has been carried out on a (Ba,Ti)–citrate polymeric resin heat-treated in air at 250 °C. From thermal analysis, X-ray diffraction, infrared and Raman spectroscopies, and ¹³C NMR spectroscopy, it has been concluded that an intermediate oxycarbonate phase was formed prior to the formation of BaTiO₃ above 550 °C. Although not well crystallized, thermoanalytical measurements, the unique XRD pattern, and new IR, Raman, and ¹³C NMR structural features revealed that such a metastable intermediate oxycarbonate phase has a stoichiometry close to Ba₂Ti₂O₅CO₃, which is characterized by having CO₃^2- groups different to those of pure BaCO₃ located, probably, in an open interlayer BaTiO₃ metastable structure. A tentative structure for the oxycarbonate phase is proposed. From the XRD crystal size measurements and Raman spectra, it was suggested that the thermal decomposition of the (Ba, Ti) citrate polymeric organic precursor above 550 °C led to the formation of a mixture of tetragonal and hexagonal BaTiO₃ polymorphs rather than cubic. Despite the specific surface of the synthesized BaTiO₃ powders up to 700 °C, higher than 40 m²/g and an equivalent particle size smaller than 25 nm, Raman spectra indicated asymmetry inside the TiO₆ octahedra of the BaTiO₃ structure.     

Characterization of Solid‐State Reaction of Barium Carbonate and Titanium Dioxide by Spatially Resolved Electron Energy Loss Spectroscopy

A mixture of ultrafine submicrometer‐sized BaCO₃ powder and TiO₂ (rutile) powder was calcined in air at 700°C, 800°C, and 900°C, and then quenched to liquid nitrogen temperature in each case. The cross‐sectional quenched specimens were characterized by spatially resolved electron energy‐loss spectroscopy (SR‐EELS). The energy‐loss near‐edge structures (ELNES) were sequentially extracted at 1.3 to 5.3 nm in width from SR‐EELS image obtained from the rectangularly cut SR‐EELS slit aperture put on the synthesized BaTiO₃ layer and TiO₂ rutile powder. The ELNES of Ti‐L_2,3 edges and Ba‐M_4,5 edges clearly show fine structure changes from the surface of BaTiO₃ layer to the TiO₂ bulk region reflected from crystallinity of synthesized BaTiO₃, lattice distortion of TiO₂ caused by Ba diffusion, and lattice misfit between BaTiO₃ and TiO₂ without formation of Ba₂TiO₄ and other titanate phases.     

Direct dual layer spinning of aminosilica/Torlon® hollow fiber sorbents with a lumen layer for CO2 separation by rapid temperature swing adsorption

The direct dual layer spinning of Torlon^®/silica hollow fibers with a neat Torlon^® lumen layer is reported here for the first time. The dual layer fibers containing a porous Torlon^®/silica main structure and a dense, pure Torlon^® polymer bore‐side coating provide a simplified, scalable platform from which to construct hollow fiber amine sorbents for postcombustion CO₂ capture. After fiber spinning, an amine infusion process is applied to incorporate PEI into the silica pores. After combining dilute Neoprene treatment followed by poly(aramid)/PDMS treatment, a helium permeance of the fiber sorbents of 2 GPU with a He/N₂ selectivity of 7.4 is achieved. Ten of the optimized amine‐containing hollow fibers are incorporated into a 22‐inch long, 1/2 inch OD shell‐and‐tube module and the module is then exposed on the shell side to simulated flue gas with an inert tracer (14 mol % CO₂, 72 mol % N₂, 14 mol % He [at 100% R.H.]) at 1 atm and 35°C in a RTSA system for preliminary CO₂ sorption experiments. The fibers are found to have a breakthrough and equilibrium CO₂ capacity of 0.8 and 1.2 mmol/g‐ dry fiber sorbent, respectively. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41845.     

A CO2‐stable hollow‐fiber membrane with high hydrogen permeation flux

A Mo‐substituted lanthanum tungstate mixed proton‐electron conductor, La_5.5W_0.6Mo_0.4O_11.25?δ (LWM04), was synthesized using solid state reactions. Dense U‐shaped LWM04 hollow‐fiber membranes were successfully prepared using wet‐spinning phase‐inversion and sintering. The stability of LWM04 in a CO₂‐containing atmosphere and the permeation of hydrogen through the LWM04 hollow‐fiber membrane were investigated in detail. A high hydrogen permeation flux of 1.36 mL/min cm² was obtained for the U‐shaped LWM04 hollow‐fiber membranes at 975°C when a mixture of 80% H₂?20% He was used as the feed gas and the sweep side was humidified. Moreover, the hydrogen permeation flux did not significantly decrease over 70 h of operation when fed with a mixture containing 25% CO₂, 50% H₂, and 25% He, indicating that the LWM04 hollow‐fiber membrane has good stability under a CO₂‐containing atmosphere. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1997–2007, 2015     

Effects of the oxidation temperature on the structure and properties of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of the oxidation temperature of the PAN hollow fiber precursor on the microstructure, specific surface, pore size distribution, and adsorption properties of PAN‐based activated carbon hollow fiber (PAN‐ACHF) were studied. When PAN hollow fibers were oxidized at 270°C, because of drastic oxidation, chain scission occurred, and the number of pores within and on the surface of the resultant PAN‐ACHF increased, but the pores were just in the thinner region of the skin of PAN‐ACHF. The surface area of PAN‐ACHF reached a maximum when the oxidation temperature was 270°C. The adsorption ratios to creatinine were all higher than 90% at all oxidation temperatures, and the adsorption ratio to VB₁₂ reached a maximum (97%) at 230°C. The dominant pore sizes of the mesopores in PAN‐ACHF ranged from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 203–207, 2005     

Effects of BaCO3 addition on the microstructure and electrical properties of La-doped barium titanate ceramics prepared by reduction-reoxidation method

Effects of trace BaCO₃ addition were investigated on microstructure and electrical properties of La-doped BaTiO₃ ceramics prepared via reduction-reoxidation method. The addition of BaCO₃ could effectively facilitate the densification and grain growth of ceramics. SEM images suggested that numerous pores presented in ceramics due to the decomposition of BaCO₃ to expel generated CO₂ gas. The presence of pores promoted reoxidation of the reduced ceramics. As a result, lowering room-temperature (RT) resistivity and improving PTCR jump were simultaneously achieved as suitable amount of BaCO₃ was added. An optimal addition of 0.3 mol% BaCO₃ led to a low RT resistivity of about 28 Ω cm and a PTCR jump up to 10^3.7 after ceramics were fired at 1100 °C in high-purity N₂ and reoxidized at 800 °C in air.     

Oxygen separation through U‐shaped hollow fiber membrane using pure CO2 as sweep gas

A number of U‐shaped K₂NiF₄‐type oxide hollow fiber membranes based on (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ (PLNCG) were successfully prepared by a phase inversion spinning process. The PLNCG hollow fiber membranes were then used to investigate the effect of CO₂ concentration in both the sweep gas and the feed air on the oxygen permeation flux. With pure CO₂ as the sweep gas and even 10% CO₂ in the feed air, a steady oxygen permeation flux of 0.9 mL/min^·cm² (STP) is obtained at 975°C during 310 h, and no decline of the oxygen permeation flux is observed. XRD, SEM and EDS characterizations show the spent membrane still maintains the intact microstructure and perfect K₂NiF₄‐type phase structure without carbonate, which indicates that the U‐shaped PLNCG hollow fiber membrane is a very stable membrane under CO₂ atmosphere and has great potential for the practical application in oxyfuel techniques for CO₂ capture and storage.©2011 American Institute of Chemical Engineers AIChE J, 2012     

Application of Slurry Type Photocatalytic Oxidation‐Submerged Hollow Fiber Microfiltration Hybrid System for the Degradation of Bisphenol A (BPA)

Abstract

A pilot scale, slurry type photocatalytic reactor, followed by submerged hollow fiber microfiltration (MF) membrane hybrid system was evaluated for simultaneous and complete destruction of toxic organic chemical bisphenol A (BPA) and separation of photocatalyst TiO₂; in order to obtain a reusable quality water. With simple modification to the treatment operation, the effect of photocatalytic reaction at modest variations in temperature was examined. Adsorption pretreatment was carried out prior to photocatalysis (UV/TiO₂). BPA adsorption ability on TiO₂ was very less (about 15%) at 25°C. However, adsorption pretreatment followed by photocatalytic oxidation (UV/TiO₂) at an elevated nearly constant temperature (about 70°C) helped in increasing the BPA degradation efficiency. The effect of ozone introduction into the treatment stream was also analyzed. Applying ozone along with UV/TiO₂, brought about a synergistic effect on BPA degradation. Within 3 h, entire 10 ppm of BPA and the by‐product organic compounds were completely removed. TiO₂ particle separation performance using hollow fiber membrane was enhanced by adopting a two‐stage coagulation/sedimentation pretreatment. With initial turbidity of 4000 NTU, the turbidity of the final permeate water was well below 0.1 NTU. Almost complete removal of particles was achieved. Some of the main advantages of this hybrid treatment system include, large‐scale treatment, complete and efficient BPA and its organic intermediates degradation, TiO₂ easily separated after treatment and capable for reuse as it is free from chemical coagulant contaminants, reusable quality water is obtained, and the system has the potential for continuous operation with simple process modifications.     

The effects of carbonization temperature on the properties and structure of PAN‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate and then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of carbonization temperature of PAN hollow fiber precursor on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based carbon hollow fiber (PAN‐CHF) and PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the surface area of the PAN‐ACHF increased very remarkably, reaching 900 m² g^?1 when carbonization is 1000°C, and the adsorption ratios to creatinine and VB₁₂ of ACHF were much higher than those of CHF, especially to VB₁₂. The different adsorption ratios to two adsorbates including creatinine and VB₁₂ reflect the number of micropores and mesopores in PAN‐ACHF. The dominant pore sizes of mesopores in PAN‐ACHF are from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2155–2160, 2005     

Phase stability and equilibria in the La2O3–TiO2 system

XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La₂O₃–TiO₂ system. The diagram was redrawn to include the compound La₄Ti₃O₁₂ in addition to La₄Ti₉O₂₄, La₂Ti₂O₇ and La₂TiO₅. Above 1455°C a cation deficient perovskite La_2/3TiO₃ is stabilized by a small number of Ti³⁺ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.     

Decomposition pathways of glycolic acid on titanium dioxide

Fourier-transformed infrared spectroscopy has been employed to study the adsorption, thermal reactions and photodegradation of glycolic acid (HOCH₂COOH) on TiO₂ in a gas–solid system. The intriguing research focus is the reactivity and evolution of the two functional groups. Glycolic acid can exist on TiO₂ at 35 °C in two dissociative adsorption forms, OCH₂COOH and HOCH₂COO, which are derived from hydrogen loss of the COH and COOH groups, respectively. Heating the surface to a temperature higher than ～100 °C causes a largely enhanced carbonyl stretching band at ～1750 cm^?1, indicative of oxidation of the OCH₂ groups of the surface glycolic acid molecules. This chemical process is supported by the adsorption of glyoxylic acid (HCOCOOH) on TiO₂. As the surface temperature is further increased to 200 °C or higher, formate (HCOO) and methoxy (CH₃O) are produced. Their formation is proposed via dioxymethylene (OCH₂O) intermediate. CO and CO₂ are found to be the final thermal products. Photoirradiation of a TiO₂ surface covered with glycolic acid at ～325 nm leads to its decomposition, generating CO₂, CH₃O, HCOO and carbonate species. O₂ is found to promote the photochemical reactions of glycolic acid on TiO₂ to form CO₂, HCOO and carbonates. O₂ may play a role hampering recombination of photogenerated electron–hole pairs and participating in the formation of CO₂ and HCOO.     

Effects of activation temperature on the properties and structure of PAN‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of activation temperature of a precursor fiber on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the BET surface area of the PAN‐ACHF and surface area of mesopores in the PAN‐ACHF increased very remarkably and reached 1422 m² g^?1 and 1234 m² g^?1, respectively, when activation temperature is 1000°C. The adsorptions to creatinine and VB₁₂ of PAN‐ACHF were much high and reached 99 and 84% respectively. In PAN‐ACHF which went through the activation at 700°C and 800°C, the micropore filling mainly occurred at low relative pressures, multimolecular layer adsorption occurred with the increasing of relative pressure, and the filling and emptying of the mesopores by capillary condensation occurred at high relative pressures. But in PAN‐ACHF which went through the activation at 900°C, a mass of mesopores resulted in the large pore filling by capillary condensation. The dominant pore sizes of mesopores in PAN‐ACHF are from 2 nm to 5 nm. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3778–3783, 2006     

Effect of stabilization temperature on gas permeation properties of carbon hollow fiber membrane

Carbon hollow fiber membranes (CHFMs) derived from polymer blend of polyetherimide (PEI) and polyvinylpyrrolidone (PVP) were extensively prepared through stabilization under air atmosphere followed by carbonization under N₂ atmosphere. The effects of the stabilization temperature on the morphological structure, chemical structure, and gas permeation properties were investigated thoroughly by means of scanning electron microscopy, Fourier transform infrared spectroscopy, and single gas permeation system. The experiment results indicate that the transport mechanism of small gas molecules of N₂, CO₂, and CH₄ is dominated by the molecular sieving effect. Based on morphological structure and gas permeation properties, an optimum stabilization condition for the preparation of CHFM derived from PEI/PVP was found at 300°C under air atmosphere. The selectivity of ?55 and 41 for CO₂/CH₄ and CO₂/N₂, respectively, were obtained for CHFMs prepared at stabilization temperature of 300°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Template-assisted hydrothermal synthesis and photocatalytic activity of novel TiO2 hollow nanostructures

TiO₂ hollow nanostructures were successfully synthesized by a controlled hydrothermal precipitation reaction using Resorcinol–Formaldehyde resin spheres as templates in aqueous solution, and then removal of the RF resins spheres by calcination in air at 450 °C for 4 h. The obtained TiO₂ hollow spheres were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption–desorption analysis, and UV–visible diffuse reflectance spectroscopy. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic decolorization of rhodamine B aqueous solution at ambient temperature under UV illumination. The results indicated TiO₂ hollow nanostructures exhibit the excellent photocatalytic activity probably due to the unique hollow micro-architectures.     

Abnormal reoxidation effects in Ba‐excess La‐doped BaTiO3 ceramics prepared by the reduction‐reoxidation method

Ba‐excess La‐doped BaTiO₃ ceramics have been successfully applied to prepare laminated chip thermistor. Ceramics were firstly fired in a high‐purity N₂ flow and then reoxidized in air to obtain positive temperature coefficient of resistivity effect. However, an abnormal reduction trend of room‐temperature (RT) resistivity was found to be always beginning at ~800°C during reoxidation. This anomaly was found closely correlated with the insulating second phase (Ba₂TiO₄) at grain boundary, which destroyed conducting pathways between semiconducting BaTiO₃ grains. When reoxidation temperature was up to ~800°C, the insulating Ba₂TiO₄ could be gradually consumed and then conducting pathways reestablished leading to RT resistivity reduction. To further prove the proposed explanation, the reoxidation effects of TiO₂‐added ceramics were also studied, in which no Ba₂TiO₄ existed and certainly the abnormal RT resistivity reduction disappeared.     

Synthesis of barium titanate using supercritical CO2 drying of gels

BaTiO₃ powders from sol-gel derived gels were prepared using two different drying methods. In addition to the conventional drying of gels in air at 90°C the supercritical CO₂ drying method was also used. Results showed that the properties of BaTiO₃ powder produced by supercritical drying with CO₂ are better. The grain surface is less contaminated as a result of the supercritical drying and the microstructure development during sintering leads to a homogeneous fine-grained microstructure.     

Highly conductive mesoporous hierarchical carbon hollow fibers fabricated via confinement of TiO2 coating by ALD

A polysulfone (PSF) hollow fiber composed of interconnected nanofibers within its wall was employed as a template to deposit with a layer of TiO₂ by atomic layer deposition. Direct nitridation of the TiO₂-coated PSF hollow fiber at 800 and 1000°C was conducted, and a new hierarchical structure of TiO_xN_1−x and TiN@nitrogen-doped carbon hollow fibers, respectively, was formed. The PSF fiber served as the source of carbon and was directly transformed to a nitrogen-doped carbon fiber because the shape change was confined by the TiO₂ coating. In the meantime, TiO_xN_1−x or TiN was formed after the nitridation of TiO₂. X-ray photoelectron spectrometric analysis indicated that there was no chemical bonding between the nitridized coating and the carbon nanofibers. It implies that the nitridation of TiO₂ and carbonization of PSF proceed independently and simultaneously in the nitridation process. Raman spectroscopic analysis also confirmed the formation of graphitic lattice and Ti–N bonding. Electrical measurement indicated that both fibers were highly conductive, with the electrical resistivity in the order of 10⁻⁵ Ω m, which is lower than those of amorphous carbon and graphite along the direction perpendicular to the basal plane.     

A TEM and HREM study of particle formation during barium titanate synthesis in aqueous solution

The formation mechanisms of barium titanate particles from an amorphous TiO₂ gel during synthesis in aqueous solution at temperatures between 20 and 80°C have been investigated. It was found that barium ions diffuse into the gel almost immediately, with nanocrystalline BaTiO₃ particles being formed after heating to just 40°C. These particles grew to dimensions of about 100 nm as the temperature was increased to 80°C, consuming the remaining TiO₂ gel. Some remnants of gel were found on particle surfaces in a sample taken at this temperature but after holding the sol at 80°C for 2 or 4 h, the particle surfaces became “cleaner” and more rounded. It is proposed on the basis of these observations that the BaTiO₃ particles were formed by an in-situ transformation of the amorphous TiO₂ gel. The mechanism by which (i) the particles were then rounded off and (ii) the final gel fragments were incorporated into the main BaTiO₃ particles was, however, less clear.