Preparation and photocatalytic properties of macroporous honeycomb alumina ceramics used for water purification

In this work, porous alumina ceramics with highly ordered capillaries were successfully fabricated by ionotropic gelation process of alginate/alumina suspensions. By varying the initial solid loading (10–30 wt%) of slurries, the porosity of alumina ceramics ranged from 60.4% to 79.5% with controlled pore size (180–315 μm). Due to the well-crosslinked macroporous structure and large specific surface areas, the porous ceramics were utilized as the photocatalyst supports of TiO₂ catalysts whose photocatalytic activity was characterized by degrading methyl blue under UV irradiation. TiO₂ coatings prepared by sol–gel method demonstrated excellent adhesion to the substrates. When the solid loading of supports reached 15 wt%, the TiO₂ coatings showed the highest photocatalytic efficiency of 79.52%. Besides, TiO₂ films possessed nearly the same photocatalytic activity as titania/water suspension. Thus, the honeycomb ceramic prepared by self-organization process holds promise for use as photocatalyst supports in water purification without recycling process of powders.     

Photocatalytic degradation of methylene blue and acetaldehyde by TiO<Subscript>2</Subscript>/glaze coated porous red clay tile

Nanosized TiO₂ sol synthesized by sol-gel method was successfully coated on the porous red clay tile (PRC tile) with micrometer sized pores. PRC tile was first coated with a low-firing glaze (glaze-coated PRC tile) and then TiO₂ sol was coated on the glaze layer. A low-fired glaze was prepared at various blending ratios with frit and feldspar, and a blending ratio glazed at 700 °C was selected as an optimum condition. Then TiO₂ sol synthesized from TTIP was dip-coated on the glazed layer (TiO₂/glaze-coated PRC tile), and it was calcined again at 500 °C. Here, these optimum calcination temperatures were selected to derive a strong bonding by a partial sintering between TiO₂ sol particles and glaze layer. Photocatalytic activity on the TiO₂/glaze-coated PRC tile was evaluated by the extent of photocatalytic degradation of methylene blue and acetaldehyde. Methylene blue with the high concentration of 150 mg/l on the surface of TiO₂/glaze-coated PRC tile was almost photodegraded within 5 hours under the condition of average UV intensity of 0.275 mW/cm₂, while no photodegradation reaction of methylene blue occurred on the glaze-coated PRC tile without TiO₂. Another photocatalytic activity was also evaluated by measuring the extent of photocatalytic degradation of gaseous acetaldehyde. The photodegradation efficiency in TiO₂/glaze-coated PRC tile showed about 77% photocatalytic degradation of acetaldehyde from 45,480 mg/l to 10,536 mg/l after the UV irradiation of 14 hours, but only about 16% in the case of the glaze-coated PRC tile.     

Investigation of photocatalytic properties of TiO2 nanoparticle coating on fly ash and red mud based porous ceramic substrate

Titanium dioxide is one of the best semiconducting photocatalysts available for photocatalytic cleaning applications. Especially nano-sized TiO₂ particles deposited on porous substrates can be utilized as a filter for solid and liquid media. On the other hand, red mud and thermal plant fly ash are hazardous wastes that are produced in large quantities. Recycling/reuse of these waste material in a porous ceramic production would be beneficial both for environmental and economical issues. In the present study, a porous substrate was produced from red mud and fly ash with varying ratios and additives of H₃BO₃, CaCO₃, and MgCO₃ for lowering the melting temperature and porosity formation. Sintered ceramics were then coated with nano-sized TiO₂ particles by the sol-gel method. Ultrasonic dispersion of nano-sized TiO₂ nanoparticles was also utilized as an alternative method for impregnation of nanoparticles into the porous structure of the ceramic substrate. Finally, photocatalytic activities and degradation of methylene blue (MB) under UV radiation of substrates were investigated. According to the SEM investigations, the sol-gel method was observed to be a better way of nanoparticle deposition because deposited particles are homogenous throughout the ceramic body. Also, this method provides lower particle sizes than the ones that were deposited by the ultrasonic dispersion method. This results in higher surface area and better photocatalytic activities.     

Asymmetric TiO2 hybrid photocatalytic ceramic membrane with porosity gradient: Effect of structure directing agent on the resulting membranes architecture and performances

Asymmetric TiO₂ hybrid photocatalytic ceramic membranes with porosity gradient have been fabricated via acid-catalyzed sol–gel method. Different structure directing agents (SDAs) i.e. Pluronic P-123, Triton X-100, Tween 20 and Tween 80 were incorporated in the preparation of TiO₂ sol to obtain a porous multilayered TiO₂ coated on the alumina ceramic support. Six different SDA-modified membrane specimens were fabricated. Four of which were coated with the TiO₂ sols prepared using only one type of SDA. The remaining two specimens were fabricated via multilayer coating of different TiO₂ sols prepared using different types of SDAs. Physico-chemical and morphological properties of different TiO₂ layers were thoroughly investigated. The membrane M1 which had the most porous TiO₂ sub-layers showed a high pure water permeability of 155 L m⁻² h⁻¹ bar⁻¹. The membrane showed a relatively high Rhodamine B (RhB) removal of 2997 mg m⁻² over 8 h treatment duration in the batch photoreactor, second only to the Pluronic-based TiO₂ membrane (specific RhB removal of 3050 mg m⁻²). All membrane specimens exhibited good performances while operated in the flow-through photocatalytic membrane reactor. Over 91% of RhB removal capability was retained after 4 treatment cycles. All membranes also showed self-cleaning property by retaining >90% of initial flux after 4 treatment cycles. The flexibility of optimizing membrane performances by fine-tuning the porosity gradient configuration of the photocatalytic layer has also been demonstrated.     

Performance study of foam ceramics prepared by direct foaming method using red mud and K-feldspar washed waste

In this study, foam ceramics were prepared via a direct foaming method at high temperatures (1080–1120 °C), using red mud (RM) and K-feldspar washed waste (KFW) as the raw materials and SiC as the foaming agent, respectively. The chemical compositions and crystalline phases of the raw materials as well as the structural and mechanical properties of the foam ceramics were investigated. By adjusting the formulation and sintering process parameters, the porous structure of the foam ceramics could be effectively modulated. In addition to some residual crystalline phases in the raw materials, new phases, including rutile (TiO₂) and anorthite (CaAl₂Si₂O₈), were generated in foam ceramics. The compressive strength of the foam ceramics decreased with an increase in the KFW/RM ratio and sintering temperature, which was mainly related to the low density of the foam ceramics and the poor support of the pore walls to the structure. Among all the foam ceramics investigated, the foam ceramic with the KFW/RM ratio of 1:1, SiC content of 1 wt%, sintering temperature of 1100 °C and sintering time of 60 min showed the best overall performance with a bulk density, an apparent porosity, an average pore size and a compressive strength of 0.77 g/cm³, 61.89%, 0.52 mm, and 3.64 MPa, respectively. Its excellent porous structure and mechanical properties rendered it suitable for application as insulation materials or decorative materials for building partition walls.     

TiO<Subscript>2</Subscript> porous ceramic/Ag–AgCl composite for enhanced photocatalytic degradation of dyes under visible light irradiation

TiO₂ porous ceramic/Ag–AgCl composite was prepared by incorporating AgCl nanoparticles within the bulk of TiO₂ porous ceramic followed by reducing Ag⁺ in the AgCl particles to Ag⁰ species under visible light irradiation. The porous TiO₂ ceramic was physically robust and chemically durable, and the porous structure facilitated the implantation of AgCl NPs. Compared with the bare TiO₂ ceramic, TiO₂ porous ceramic/Ag–AgCl composite exhibited higher photocatalytic performance for the degradation of MO and RhB under visible light irradiation. The reaction rate constants k of MO and RhB degradation over TiO₂ porous ceramic/Ag–AgCl composite was respectively 6.25 times and 3.62 times higher than those recorded over the bare TiO₂ porous ceramic. The photocatalytic activity showed virtually no decline after four times cyclic experiments under visible light irradiation. Scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, photoluminescence spectra and X-ray photoelectron spectroscopy were used to characterize the TiO₂ porous ceramic/Ag–AgCl composite.     

Preparation of porous Al2TiO5-Mullite ceramic by starch consolidation casting and its corrosion resistance characterization

Stabilized Al₂TiO₅ (AT)-mullite (M) porous ceramics were fabricated by starch consolidation casting using corn starch as curing agent and their microstructure, mechanical properties, pore size distribution and corrosion resistance were examined. Results showed that AT-M porous ceramic with the flexural strength of 11.5 MPa, apparent porosity of about 54.7% and pore size distribution in the range of 1–15 µm could be obtained with 10 wt% corn starch addition. Corrosion resistance results showed mass losses in hot H₂SO₄ solution and NaOH solution for 10 h to decreased from 1.03% to 0.36% and 4.39–2% when the calcination temperature increased from 1400 °C to 1450 °C, which proved these AT-M porous ceramics to possess an excellent corrosion resistance in acidic condition when calcined at 1450 °C.     

Preparation of porous TiO2 by a novel freeze casting

Highly porous and open interconnected pore structural TiO₂ were prepared by a novel freeze casting method. In the experiment, the well-dispersed aqueous slurries were first frozen, and then dried at a reduced vacuum. Since the sublimation of ice crystals developed in the freezing process, the green bodies with highly porous were obtained. The phase composition and the microstructure of the sintered samples were characterized by XRD, SEM, porosity and the pore size distribution was measured by mercury porosimetry. The results demonstrated that the PVA concentration in the slurries remarkably affect the microstructure of TiO₂ ceramics. The pore morphology of TiO₂ ceramics with 3 wt.% polyvinyl alcohol (PVA) addition was dendritic, and however, the pore morphology of TiO₂ ceramics with 6 wt.% PVA addition changed into columnar. The reason for the variation of the pore morphology was ascribed to the effect of the PVA gelation on the growth behavior of the ice crystals.     

Photocatalytic properties of ZnO nanoparticle coating on porous ceramic substrates with varying porosities produced from fly ash and red mud

In order to improve filtering efficacy, nanoparticles are often deposited as photocatalytic degrading agents onto porous ceramics. This study aimed to deposit ZnO nanoparticles on ceramic substrates produced from fly ash and red mud with adjustable porosity and investigate their photocatalytic properties. To achieve this goal, at first porous ceramics were produced and sintered at various temperature/time intervals. It was observed that sintering at 800°C for 120 min provided a proper structure and porosity. In addition, MgO replacement with MgCO₃ lowered the water absorption of the samples from 25.63% to 11.45%. The samples were then coated with ZnO nanoparticles using the sol–gel method and the ZnO structures obtained were micron-sized plates. It was observed that increasing porosity increased the ZnO amount and accordingly the photocatalytic properties of the products. During the adsorption tests conducted in the dark, the coated ceramic samples were stained with MB with a maximum MB adsorption ratio of ∼14%. On the other hand, no visible MB stain was observed on the samples that were exposed to UV irradiation, and the MB removal after the UV irradiation was 93.6%; therefore, it was concluded that the dominant MB removal mechanism was photocatalytic.     

Homogeneous doping of ceramics by infiltration–gelation

Advanced ceramics require small amounts of cation dopants to improve the sintering process or achieve certain properties. Dopant precursors are often dissolved in initial processing liquids, which may adversely affect ceramic colloidal stability, and dopant transport during drying. This would lead to chemical and microstructural inhomogeniety in the eventual ceramics. Here we present a method that circumvents these problems using the example of Al³⁺-doped TiO₂ ceramics. Homogeneous TiO₂ compacts with 34% porosity are first prepared by colloidal casting and calcined at 700 °C. The obtained compacts are infiltrated with an aqueous solution of Al(NO₃)₃, citric acid and ethylene glycol. The solution composition is adjusted such that during drying, a gel forms that covers the internal pore surface. Subsequent decomposition of organics results in a homogeneous dopant distribution in the porous and then dense-sintered TiO₂ compacts. This is verified with SEM-EDX, and laser ablation coupled ICP-MS on dense compacts sintered at 1100 °C.     

Preparation of Crack-Free Nanocomposite Ceramic Membrane Intermediate Layers on α-alumina Tubular Supports

The objective of this work was to develop a crack-free membrane intermediate layer on tubular ceramic supports via dip-coating. TiO₂ submicron powder, Boehmite as a coupling agent, and SiO2 nanopowder as low melting-point sintering aid were used to deposit a thin and crack-free nanocomposite layer onto α-Al₂O₃ supports. Effects of key parameters such as solid content, number of coated layers, the presence of coupling agent, and a low melting point nanopowder on physicochemical properties like the thickness, microstructure, pore size, pure water flux, and gas permeance of the final modified ceramic supports were investigated. The obtained results showed that after twice coating of support in 3 wt.% bidispersed TiO₂-Boehmite suspension, the pore size of the alumina substrate, ～0.6 micron, was reduced to ～0.1 by the uniform membrane intermediate layers with low permeation resistance could be prepared. Moreover, by adding SiO₂ nanopowder, sintering temperature of intermediate layers decreases considerably (1000 to 700°C).     

Colloidal Assembly and Functionalization of Pore Channels in Polymer Foils

The deposition of ceramic colloidal particles into the pores of polymeric ion track membranes was studied. By means of a low-temperature sol-gel process, aqueous dispersions of ceramic nanoparticles can adhere to virtually any surface of a porous polymer foil (substrate) including the interior walls of the pores without adversely affecting the flexibility or ease of handling of the starting material. The coating method consists in depositing of a TiO₂-photocatalyst onto the substrate by providing either a SiO₂ or Ag specific layer or both between the photocatalyst and the substrate. Silica and metallic silver act as both binder and barrier against deterioration due to the photocatalytic action derived from TiO₂. On the other hand, the incorporation of such nanoparticles inside the pores provides active centres on which gas exchange coupled to photo-catalytic reactions can occur. Because of their small pore size, porous substrates can be very useful in a high purity filter application, for food package, gas-separation- and ion-exchange membranes. The structure and function of these ceramic-polymer composites were conceived and engineered taking ostrich eggshell as natural model. This bionic approach is based on the concept of immobilizing solid particles on a porous surface using polymer groups as tether.     

Porous aluminum titanate‒strontium feldspar‒mullite fiber composite ceramics with enhanced pore structures and mechanical properties

This study demonstrates the pore structures and mechanical properties of porous aluminum titanate‒strontium feldspar‒mullite fiber (ASM) composite ceramics. Samples were prepared using two different processes. A traditional reactive sintering method, with Al₂O₃ and TiO₂ as raw materials, was used to prepare one group of samples, and an improved method, using aluminum titanate (AT) clinkers, was used to prepare another group of samples. The effects of the processes and raw materials on the pore structure and mechanical properties of the composite ceramics were investigated. The properties of the sintered porous ceramics, including the microstructure, density, porosity, pore size, and mechanical properties, were analyzed. After sintered at 1400 °C, the ASM ceramics that were prepared using the improved method had a porosity level of 70% and a pore size of 24 µm, which were twice that of the traditional ASM ceramics, while both samples had identical flexural strength values of 2.27 MPa. The improved process endowed the porous ASM ceramics with excellent pore structures and mechanical properties, promoting their potential use in filter applications.     

Anatase TiO2 powder immobilized on reticulated Al2O3 ceramics as a photocatalyst for degradation of RO16 azo dye

The three-dimensional network of open-celled Al₂O₃ ceramic supports were first fabricated using a polymeric sponge replication technique. The polyurethane foams soaked with the highly loaded Al₂O₃ ceramic slurry were drained, slowly dried and sintered at 950 °C to complete the burn out process and to construct the ceramic preforms. They were then carefully soaked again with the moderately loaded Al₂O₃ slurry and sintered at 1500 °C to increase their strength. Finally, the photocatalyst ceramic powder covering the outer surfaces was placed layer by layer through dipping the rigid Al₂O₃ supports repeatedly in the slurry of TiO₂ powder. The dip-applied TiO₂ was sintered eventually at relatively low temperature of 700 °C to prevent phase transformation. The efficiencies of the TiO₂ coated Al₂O₃ samples were evaluated by the photocatalytic degradation of reactive orange 16 (RO16) azo dye molecules dissolved in water. Nearly complete color removals were achieved within 75 min under UVC irradiation.     

Effect of TiO2 addition on the microstructures,mechanical and dielectric properties of porous Si3N4-based ceramics

Porous Si₃N₄-based ceramics with different TiO₂ contents were prepared by gas pressure sintering method. The effects of TiO₂ addition ranging from 0 to 25?wt-% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. The addition of TiO₂ significantly promoted the density which increased from 1.64 to about 2.3?g?cm^?3. The mechanical properties of porous Si₃N₄-based ceramics with TiO₂ addition decreased first and then increased with the increase of TiO₂ content, and the flexural strength and elastic modulus are more than 167.4?MPa and 72.8?GPa, respectively, which were higher than that of the Si₃N₄ ceramic without TiO₂ addition. With the increase of TiO₂ content, both the dielectric constant and dielectric loss increased, and the dielectric constant enhanced obviously. These results suggested that the TiO₂ was beneficial for the improvement of mechanical properties and dielectric constant of porous Si₃N₄-based ceramics.     

Synthesis and Photocatalytic Activity of Anatase TiO2 Nanoparticles-coated Carbon Nanotubes

A simple and straightforward approach to prepare TiO₂-coated carbon nanotubes (CNTs) is presented. Anatase TiO₂ nanoparticles (NPs) with the average size ~8 nm were coated on CNTs from peroxo titanic acid (PTA) precursor even at low temperature of 100 °C. We demonstrate the effects of CNTs/TiO₂ molar ratio on the adsorption capability and photocatalytic efficiency under UV–visible irradiation. The samples showed not only good optical absorption in visible range, but also great adsorption capacity for methyl orange (MO) dye molecules. These properties facilitated the great enhancement of photocatalytic activity of TiO₂ NPs-coated CNTs photocatalysts. The TiO₂ NPs-coated CNTs exhibited 2.45 times higher photocatalytic activity for MO degradation than that of pure TiO₂.     

Role of Nano‐ and Micron‐Sized Particles of TiO2 Additive on Microwave Dielectric Properties of Li2ZnTi3O8 – 4 wt% TiO2 Ceramics

Microwave dielectric ceramics with the composition of Li₂ZnTi₃O₈ – 4 wt% TiO₂ were synthesized by the conventional solid‐state reaction. 4 wt% TiO₂ powders with different particles size were added to the Li₂ZnTi₃O₈ ceramic. Then the ceramic samples were sintered at temperatures 1075°C, 1050°C, 1000°C, and 950°C for 4 h. The effect of the particles size of TiO₂ additive on the microwave dielectric properties of the ceramics has been investigated. In the study two categories of particles size of TiO₂ additive have been used; (i) Nanoparticle (50 nm), (ii) Micron sized (40, 5, 1 μm) powder. X‐ray showed that the TiO₂ additive has not solved in the LZT structure and has not almost undergone chemical reaction with the LZT ceramic. The results showed that the addition of TiO₂ nanoparticles to the LZT ceramics significantly improved the density and a dense and uniform microstructure and also abnormal grain growth were observed by SEM. The use of TiO₂ nanoparticle reduces porosity and leads to an increase in green density. The maximum density was found to be 98.5% of the theoretical density and the best relative permittivity of 28, quality factor of 68000 GHz and τ_f value of ?2 ppm/°C were obtained for the samples added with 4 wt% of the TiO₂ nanoparticles, sintered at 1050°C for 4 h.     

Photocatalytic decolorization of rhodamine B by fluidized bed reactor with hollow ceramic ball photocatalyst

Photocatalytic degradation of organic contaminants in wastewater by TiO₂ has been introduced in both bench and pilot-scale applications in suspended state or immobilized state on supporting material. TiO₂ in suspended state gave less activity due to its coagency between particles. Recent advances in environmental photocatalysis have focused on enhancing the catalytic activity and improving the performance of photocatalytic reactors. This paper reports a preliminary design of a new immobilized TiO₂ photocatalyst and its photocatalytic fluidized bed reactor (PFBR) to apply photochemical degradation of a dye, Rhodamine B (RhB). But it was not easy to make a cost-effective and well activated immobilized TiO₂ particles. A kind of photocatalyst (named Photomedium), consisting of hollow ceramic balls coated with TiO₂-sol, which was capable of effective photodegradation of the dye, has been presented in this study. The photocatalytic oxidation of RhB was investigated by changing Photomedia concentrations, initial RhB concentrations, and UV intensity in PFBR This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Realizable recycling of coal fly ash from solid waste for the fabrication of porous Al2TiO5-Mullite composite ceramic

As the environment deteriorates, recycling of solid waste has become increasingly important. This study aimed to optimize the use of the Fe₂O₃, SiO₂, and CaO components in coal fly ash and to convert coal fly ash into stable porous Al₂TiO₅-mullite (AT–M) composite ceramic by sintering with AlOOH and TiO₂ additives at high temperatures. The phase composition, microstructure, apparent porosity, corrosion resistance, and mechanical properties of porous AT–M composite ceramics were systematically investigated. Results indicated that the sintered samples exhibited pore size distributions within the 0.16-2.9 μm, apparent porosities of approximately 52.8%, and flexural strength of 29.6 MPa. Corrosion resistance data revealed quality losses in the aqueous NaOH and H₂SO₄ solutions for 10 hours at 0.42% and 2.19%, respectively. After corrosion for 8 hours, the average flexural strength of the samples remained at 21.6 ± 0.53 and 20.84 ± 0.6 MPa, respectively. These findings show that these porous AT–M ceramics may provide enhanced corrosion resistance under alkaline conditions. The porous AT–M composite ceramics may fabricate high-performance composite membrane supports for the high temperature flue gas filtration.     

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO₂ filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO₂ fillers and SiO₂‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO₂ filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m²/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.