Mechanism on reduction and nitridation of micrometer-sized titania with ammonia gas

Ammonia gas can be simultaneously used as a reductant and nitrogen source to prepare TiN from titania. In this work, the mechanisms on reduction and nitridation of micrometer-sized anatase with ammonia gas have been investigated, using both thermodynamic and experimental studies. The thermodynamic analysis indicated that reduction and nitridation of TiO₂ by NH₃ was feasible. Anatase will undergo different paths to form TiN, depending on the reaction temperature. Upon heating, NH₃ was seen to partially decompose into N₂ and H₂, although the actual NH₃ decomposition ratio was less than the theoretical value. The experimental results indicated that the obtained titanium nitride was non-stoichiometric (TiN_xO_1−x, x ≤ 1), as it contained a certain amount of oxygen. Based on the phase transformation and X-ray photoelectron spectroscopy analysis, the reduction and nitridation routes were deduced: TiO₂ reacted with NH₃ to form TiN_xO_1−x directly, at lower temperatures, and followed the path TiO₂ → Ti_nO_2n−1 → TiN_xO_1−x, at higher temperatures. Ti_nO_2n−1 was determined to be Ti₄O₇ and Ti₃O₅ at 1100°C and 1200°C, respectively. Reaction temperature and time significantly affected the oxygen and nitrogen contents in TiN_xO_1−x, with the lattice parameter of roasted products gradually increasing—approaching those of pure TiN—with an increase in reaction temperature and holding time. At the same time, the content of oxygen in TiN_xO_1−x decreased, and its nitrogen content correspondingly increased.     

Polyvinyl alcohol/polysulfone (PVA/PSF) hollow fiber composite membranes for pervaporation separation of ethanol/water solution

Polysulfone (PSF) hollow fiber membranes were spun by phase‐inversion method from 29 wt % solids of 29 : 65 : 6 PSF/NMP/glycerol and 29 : 64 : 7 PSF/DMAc/glycol using 93.5 : 6.5 NMP/water and 94.5 : 5.5 DMAc/water as bore fluids, respectively, while the external coagulant was water. Polyvinyl alcohol/polysulfone (PVA/PSF) hollow fiber composite membranes were prepared after PSF hollow fiber membranes were coated using different PVA aqueous solutions, which were composed of PVA, fatty alcohol polyoxyethylene ether (AEO₉), maleic acid (MAC), and water. Two coating methods (dip coating and vacuum coating) and different heat treatments were discussed. The effects of hollow fiber membrane treatment methods, membrane structures, ethanol solution temperatures, and MAC/PVA ratios on the pervaporation performance of 95 wt % ethanol/water solution were studied. Using the vacuum‐coating method, the suitable MAC/PVA ratio was 0.3 for the preparation of PVA/PSF hollow fiber composite membrane with the sponge‐like membrane structure. Its pervaporation performance was as follows: separation factor (α) was 185 while permeation flux (J) was 30g/m²·h at 50°C. Based on the experimental results, it was found that separation factor (α) of PVA/PSF composite membrane with single finger‐void membrane structure was higher than that with the sponge‐like membrane structure. Therefore, single finger‐void membrane structure as the supported membrane was more suitable than sponge‐like membrane structure for the preparation of PVA/PSF hollow fiber composite membrane. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 247–254, 2005     

Large-scale synthesis of centrifugally spun tantalum oxynitride fiber electrocatalysts for hydrogen evolution reaction

Tantalum oxynitride (TaO_xN_1−x) fibers were synthesized and evaluated for their electrocatalytic hydrogen activity using an in-house developed centrifugal spinning setup. By tailoring the composition of the spinning solution and optimizing collector distance and rotation speed of the spinneret, bead-free TaO_xN_1−x fibers with a diameter of 800 nm were obtained. The fibers were structurally characterized through phase and elemental analysis, confirming the formation of monoclinic TaO_xN_1−x with clear splitting of the X-ray photoelectron spectroscopy peaks indicating Ta was in +5 oxidation state. The resulting oxynitride fibers exhibited superior electrocatalytic performance with low overpotentials (250 mV) to generate 10 mA/cm² compared to Ta₂O₅ oxide fibers. Interestingly, the enhanced activity of oxynitride fibers was observed to be suppressed in basic medium due to the high oxophilicity of tantalum ions and a negative Gibbs adsorption-free energy, leading to poisoning of the active sites. This work demonstrates a facile pathway for the fabrication of high-performance electrocatalysts, based on TaO_xN_1−x fibers, from a cost-effective and energy-efficient centrifugal spinning technique.     

Hollow TiO2 containing multilayer nanofibers with enhanced photocatalytic activity

Hollow titania containing multilayer nanofibers was fabricated through the combination of electrospinning with layer-by-layer technique. Two different solvents, methylbenzene and THF were used to remove the template. The morphology of the obtained hollow multilayer nanofibers confirmed that THF is better than methylbenzene. The obtained hollow multilayer fiber has a diameter of about 700 nm and its shell thickness is about 140 nm. FTIR spectra show the fabrication of multilayer nanofibers. XPS measurement indicates that TiO₂ nanoparticles can be assembled successfully. The obtained multilayer hollow nanofibers have highest photocatalytic activity to degrade methylene blue solution comparing with the TiO₂ film due to their unique hollow structure.     

Dual-layer hollow carbon fiber membranes for gas separation consisting of carbon and mixed matrix layers

Hollow carbon fiber membranes for gas separation have been successfully fabricated for the first time by a special type of precursor. This precursor is dual-layer hollow fiber composed of a polysulfone-beta zeolite (PSF-beta) mixed matrix outer layer and a Matrimid inner layer. Pure gas permeation measurements show that the resultant hollow carbon fiber has O₂/N₂ and CO₂/CH₄ selectivities of 9.3 and 150, respectively; this performance is much better than that of the hollow carbon fiber derived from single-layer Matrimid hollow fiber. Mixed gas measurements show the CO₂/CH₄ selectivity of 128. After pyrolysis, the PSF-beta layer in the dual-layer precursor evolves into a continuous structure of closely packed zeolite particles embedded in the PSF carbon residue. TGA spectra suggest that the possible reason for the above observation is that the PSF-beta outer layer and Matrimid inner layer has significantly changed each other’s pyrolysis dynamics and thermal degradation process.     

Carbothermal synthesis of boron nitride coating on PAN carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fiber by dip coating method. Dip coating was carried out in saturated boric acid solution followed by nitridation at a temperature of 1200 °C in nitrogen at atmospheric pressure to produce BN coating. Chemical activation improved surface area of PAN fiber which favours in situ carbothermal reduction of boric acid. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) have shown the formation of boron nitride. The X-ray photoelectron spectroscopy reveals that the coating forms a composite layer of carbon, BN/BO_xN_y and some graphite like BCN with local structure of B–N–C and B(N–C)₃. The oxidation resistance of the coated fiber was significantly higher than uncoated carbon fiber. Tensile strength measurement reveals that the BN coated fiber maintained 90% of its original strength. As compared to chemical vapor deposition (CVD), this process is simple, non-hazardous and is expected to be cost effective.     

Facile synthesis of TiO2-supported Al2O3 ceramic hollow fiber substrates with extremely high photocatalytic activity and reusability

TiO₂ oxide was deposited on a microstructured α-Al₂O₃ ceramic hollow fiber substrate by a simple one-step solution-immersion process with tetraethyl orthosilicate (TEOS) as a binder. The effects of the starting coating solution's composition on the photocatalytic properties of TiO₂ powders deposited on a substrate was determined by using TiO₂-supported Al₂O₃ ceramic hollow fiber substrates fabricated from coating solutions with different compositions and examining the substrates' effect on the methylene blue (MetB) degradation reaction under ultraviolet light. A strong correlation was observed between the initial coating solution compositions and the final photocatalytic characteristics of the TiO₂-supported Al₂O₃ ceramic hollow fiber substrates. Under optimal conditions, the MetB removal efficiency reached about 91% in a few minutes. To the best of our knowledge, this is the highest and most rapidly attained MetB removal efficiency reported for TiO₂-supported Al₂O₃ ceramic hollow fiber substrates. Furthermore, apart from attaining an extremely high photocatalytic activity within minutes, the fabricated TiO₂-supported Al₂O₃ ceramic hollow fiber substrates exhibited high photocatalytic stability even after several cycles.     

Effect of nitriding atmosphere on the morphology of AlN nanofibers from solution blow spinning

High purity AlN fiber is a promising thermal conductive material. In this work, AlN fibers were prepared using solution blow spinning followed by nitridation under N₂ or NH₃ atmosphere. Soluble polymer, such as polyaluminoxane, and allyl-functional novolac resin were adopted as raw materials to form homogeneous distribution of Al₂O₃ and C nanoparticles within the fibers, which could inhibit the growth of alumina crystal and promote their nitridation process. The effect of nitriding atmosphere on the fiber morphology was investigated. XRD results showed that complete nitridation was achieved at 1300 °C in the NH₃ or at 1500 °C in the N₂ atmosphere. Hollowed fiber structure was observed when fiber was nitrided in N₂ at high temperature, which was caused by gaseous Al gas diffusion, and this phenomenon was eliminated in NH₃ atmosphere. The nitridation mechanisms in different atmosphere were analyzed in detail. It was demonstrated that the nitridation of Al₂O₃ fibers in the NH₃ atmosphere offered the favored AlN morphology and chemical quality. Flexible AlN fiber with O content of 0.7 wt% was achieved after nitriding in NH₃ at 1400 °C. The high quality AlN can be used in thermal conductive composite materials.     

Electrodeposition of fluorohydroxyapatite gradient coating on titanium alloy

In order to obtain the apatite implants which can meet the requirements for both bonding strength and bioactivity, the FHA(Ca₁₀(PO₄)₆OH_2−xF_x(x = 0 ~ 2)) gradient coating was prepared by electrophoretic deposition on the surface of titanium alloy (Ti–6Al–4V). The effects of fluorine substitution and gradient structure on the morphology, bonding strength, and bioactivity of the coating were investigated. The gradient coating has the dense inner layer and porous outer layer that were deposited by controlling the suspension concentration and the preparation process. Through controlling of the fluorine substitution gradient, the thermal expansion coefficient gradually changed to improve the bonding strength more than 30 Mpa. The inner layer with high fluorine content can improve the thermal stability of the coating and bonding strength between the coating and the substrate, while the proper content of fluorine in the outer layer improves the bioactivity of the gradient coating.     

Interface modification of carbon fibers with TiC/Ti2AlC coating and its effect on the tensile strength

A uniform TiC/Ti₂AlC gradient coating was obtained on carbon fibers via an in-situ reaction in molten salts. The results indicated that in-situ growth of TiC/Ti₂AlC coating caused strong interfacial bonding and surface defects. In this case, evident stress concentration was induced and cracks penetrated the fiber easily during tensile loading. Thus the tensile strength of carbon fibers was dramatically decreased to 78?±?13?MPa. In order to improve the performance of the as-prepared TiC/Ti₂AlC-coated carbon fibers, a pyrolytic carbon layer was pre-fabricated on carbon fibers. By introducing pyrolytic carbon layer, the interfacial bonding strength and surface defects were reduced accordingly. These improvements lead to a decrease of stress concentration and cracks propagation, and facilitate the interfacial debonding during tensile loading. As a result, the tensile strength of the fiber was significantly increased to 550?±?72?MPa. This fact indicates that pre-fabricating a pyrolytic carbon layer on carbon fibers is an effective method to improve the reliability of the TiC/Ti₂AlC-coated carbon fibers. The present work also provides a feasible way to fabricate TiC/Ti₂AlC interphase for high-performance Cf/SiC composites.     

Nanostructured Solids from Freeze‐Dried Precursors: Multigram Scale Synthesis of TiO2‐Based Powders

Nanocrystalline TiO₂ and Ti_1?xV_xO₂ (x = 0.01) powders have been prepared by thermal decomposition, in air, of amorphous precursors resulting from the freeze‐drying of appropriate solutions. In addition, TiO_2?xN_y (anatase and rutile) and TiO_xN_y (rock‐salt) have been prepared by thermal treatment in ammonia of a crystalline precursor (TiO₂ obtained at 673 K). TEM and SEM images, as well as the analysis of the X‐ray diffraction (XRD) patterns, show the nanoparticulated character of those solids obtained at low temperatures, with typical particle sizes in the 10–20 nm range when prepared at 673 K. The UV–Vis results indicate both the insertion of V in the anatase lattice and the feasibility of nitridation at low temperatures. The photocatalytic properties of these materials (as prepared and after their incorporation to mortar samples) in the degradation of nitrogen oxides have been preliminary evaluated. Although N‐doping enhances the photocatalytic activity of the TiO₂ matrix, V‐doping worsens it.     

Preparation and characterization of a novel activated carbon‐supported N‐doped visible light response photocatalyst (TiO2−xNy/AC)

A photocatalyst, TiO_2?xN_y/AC (activated carbon (AC) supported N‐doped TiO₂), highly active in both the Vis and UV range, was prepared by calcination of the TiO₂ precursor prepared by acid‐catalyzed hydrolysis in an ammonia atmosphere. The powders were characterized by diffuse reflectance spectroscopy, scanning electron microscopy, X‐ray diffraction, N₂ adsorption, Fourier transform infrared spectroscopy and phenol degradation. The doped N in the TiO₂ crystal lattice creates an electron‐occupied intra‐band gap allowing electron‐hole pair generation under Vis irradiation (500–560 nm). The TiO_2?xN_y/AC exhibited high levels of activity and the same activity trends for phenol degradation under both Vis and UV irradiation: TiO_2?xN_y/AC calcined at 500 °C for 4 h exhibited the highest activity. The band‐gap level newly formed by doped N can act as a center for the photo‐generated holes and is beneficial for the UV activity enhancement. The performance of the prepared TiO_2?xN_y/AC photocatalyst revealed its practical potential in the field of solar photocatalytic degradation of aqueous contaminants. Copyright © 2007 Society of Chemical Industry     

Photoelectrochemical Properties of NiO Coupled with TiO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Thin Film Prepared by Magnetron Sputtering Method

TiO_2−x N _x films were deposited on ITO glass by reactive dc magnetron sputtering method under different O₂/N₂ flow ratios. A NiO film was deposited by chemical bath deposition onto the as-prepared ITO/TiO_2−x N _x film to form an ITO/TiO_2−x N _x /NiO composite electrode. The morphology, crystal structure and composition of the TiO_2−x N _x films were characterized by SEM, XRD and XPS. The photoelectrochemical properties of the TiO_2−x N _x films were investigated by means of UV–Vis absorption spectra and photocurrent measurements. The results showed that the TiO_2−x N _x film sputtered under O₂/N₂ flow ratio of 1:2 exhibited a higher photocurrent response than the others. Correspondingly, the TiO_2−x N _x /NiO electrode consisting of the sputtered TiO_2−x N _x film under O₂/N₂ flow ratio of 1:2 also showed the best photoelectrochromic feature.     

Morphological and separation performance study of polysulfone/titanium dioxide (PSF/TiO2) ultrafiltration membranes for humic acid removal

In this study, polysulfone (PSF) hollow fiber membranes with enhanced performance for humic acid removal were prepared from a dope solution containing PSF/DMAc/PVP/TiO₂. The main reason for adding titanium oxide during dope solution preparation was to enhance the antifouling properties of membranes prepared. In the spinning process, air gap distance was varied in order to produce different properties of the hollow fiber membranes. Characterizations were conducted to determine membrane properties such as pure water flux, molecular weight cut off (MWCO), humic acid (HA) rejection and resistance to fouling tendency. The results indicated that the pure water flux and MWCO of membranes increased with an increase in air gap distance while HA retention decreased significantly with increasing air gap. Due to this, it is found that the PSF/TiO₂ membrane spun at zero air gap was the best amongst the membranes produced and demonstrated > 90% HA rejection. Analytical results from FESEM and AFM also provided supporting evidence to the experimental results obtained. Based on the anti-fouling performance investigation, it was found that membranes with the addition of TiO₂ were excellent in mitigating fouling particularly in reducing the fouling resistances due to concentration polarization, cake layer formation and absorption.     

Catalytic Membrane Reactors – Chemical Upgrading and Pollution Control

This article presents an overview on oxygen permeation, oxidative activation of light hydrocarbons and splitting of various molecules (H₂O, N₂O, NO_x) containing oxygen in a hollow fiber catalytic membrane reactor made of BaCo_xFe_yZr_zO_3–δ (BCFZ, x+y+z = 1) which is used to separate oxygen from oxygen containing gases. Innovative reactor concepts are introduced and performances for the various applications are discussed. A detailed view is also given on the thermodynamic and kinetic background with regard to the above mentioned reactions. The potentials in terms of product intensification and energy saving by usage of the BCFZ hollow fibers as reaction vessels are evaluated. However, there are still technological challenges that prevent the described processes from being practiced on commercial scale by now.     

Influence of remote plasma on PEDOT:PSS-coated carbon felt for improved activity of glucose oxidase

Increasing wettability of carbon felts is an important strategy to improve their efficiency in bio-electrochemical applications. Herein, influence of cold remote plasma (N₂ + O₂) treatment on surface properties of carbon felts with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coating was tested, aiming to improve immobilizing of glucose oxidase enzyme (GOx). Spectra of N 1s and O 1s confirmed the integration of carbonyl and ether as well as amide and amine groups on bare carbon fiber surface, while on coated fibers, carbonyl groups were pre-dominant. S 2p spectra confirmed oxidation of PEDOT:PSS coating with reduction of (S⁻) compared to (SO³⁻) group. GOx immobilized on different samples showed highest activity for PEDOT:PSS coating subjected to plasma with 2% O₂, maintaining up to 60% after immobilization, and 37% of its activity after six cycles for some samples. Enzymes immobilized on samples without plasma treatment lost their activity after four cycles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48521.     

UV photo‐stabilization of tetrabutyl titanate for aramid fibers via sol–gel surface modification

Tetrabutyl titanate was used as sol–gel precursor of a nanosized TiO₂ coating to improve the photo‐stability of aramid fibers. The nanosized TiO₂ coating was characterized by XRD and XPS. The influence of the TiO₂ coating on photo‐stability of aramid fibers was investigated by an accelerated photo‐ageing method. The photo‐stability of aramid fiber showed obvious improvement after coating. After 156 h of UV exposure, the coated fibers showed less deterioration in mechanical properties with the retained tensile strength and elongation at break greater than 36 and 50% of the original values, respectively, whereas the uncoated fibers degraded completely and became powdery. SEM analysis showed no significant surface morphological change on the coated fiber after the exposure, while some latitudinal crack fractures appeared on the uncoated aramid fiber. The effect of the nanosized TiO₂ coating was also well demonstrated by examining the difference of distributions of C1s in XPS deconvolution analysis on the surface of uncoated/coated fibers with increasing UV exposure time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3113–3119, 2007     

The oxygen reduction reaction on Pt/TiO x N y -based electrocatalyst for PEM fuel cell applications

Titanium oxy-nitride was developed for the first time as Pt electrocatalyst support for the ORR in PEM fuel applications. The conditions of the support preparation and the Pt/TiO _x N _y -based electrodes’ elaboration by chemical reduction method were determined. Comparison of the polarization curves of the carbon and the TiO _x N _y supported how clearly TiO _x N _y was more stable than the Vulcan XC-72R. It was found that the 40 wt% Pt/TiO _x N _y -based electrocatalyst is active for the ORR in acid medium, but the activity was less than that of Pt/C. The normalized electrochemical surface area degradation of Pt/TiO _x N _y was significantly less than that of Pt/C. The kinetics of the ORR on Pt/TiO _x N _y proceeded through a four-electron transfer process. The single-cell hydrogen/oxygen PEM fuel cell performances based on Pt/TiO _x N _y cathode electrocatalyst exhibited the same range of characteristics as those based on Pt/C.     

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.     

Preparation and photocatalysis of TiO2-fluoropolymer electrospun fiber nanocomposites

The preparation and photocatalytic properties of titanium dioxide (TiO₂)-fluoropolymer fiber nanocomposites were studied. The fluoropolymer nanofibers with carboxyl group were prepared by electrospinning. The complex was formed between carboxyl on fluoropolymer electrospun fiber surface and titanium ion, and then the TiO₂ nanoparticles were immobilized on the surface of fluoropolymer electrospun fibers through hydrothermal complex-precipitation. By controlling the reaction conditions, different sizes and numbers of TiO₂ nanocrystals can be obtained. The Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results reveal that an interaction exists between TiO₂ and fluoropolymer fibers. The degradation of methylene blue solution is performed by TiO₂-fluoropolymer fiber nanocomposites under UV irradiation. There may be an adsorption-migration-photodegradation process during the degradation of methylene blue by using TiO₂-fluoropolymer fiber nanocomposites as photocatalyst. The experimental results show that the TiO₂-fluoropolymer fiber nanocomposites have good photocatalytic ability, recycling and stability for the potential applicability in an environmental remediation.