Effect of the microstructure of the supported catalysts CuO/TiO<Subscript>2</Subscript> and CuO/(CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>) on their catalytic properties in carbon monoxide oxidation

The effect of the microstructure of titanium dioxide on the structure, thermal stability, and catalytic properties of supported CuO/TiO₂ and CuO/(CeO₂-TiO₂) catalysts in CO oxidation was studied. The formation of a nanocrystalline structure was found in the CuO/TiO₂ catalysts calcined at 500°C. This nanocrystalline structure consisted of aggregated fine anatase particles about 10 nm in size and interblock boundaries between them, in which Cu²⁺ ions were stabilized. Heat treatment of this catalyst at 700°C led to a change in its microstructure with the formation of fine CuO particles 2.5–3 nm in size, which were strongly bound to the surface of TiO₂ (anatase) with a regular well-ordered crystal structure. In the CuO/(CeO₂-TiO₂) catalysts, the nanocrystalline structure of anatase was thermally more stable than in the CuO/TiO₂ catalyst, and it persisted up to 700°C. The study of the catalytic properties of the resulting catalysts showed that the CuO/(CeO₂-TiO₂) catalysts with the nanocrystalline structure of anatase were characterized by the high-est activity in CO oxidation to CO₂.     

Influence of NH<Subscript>3</Subscript>-treating temperature on visible light photocatalytic activity of N-doped P<Subscript>25</Subscript>-TiO<Subscript>2</Subscript>

The influence of NH₃-treating temperature on the visible light photocatalytic activity of N-doped P₂₅-TiO₂ as well as the relationship between the surface composition structure of TiO₂ and its visible light photocatalytic activity were investigated. The results showed that N-doped P₂₅-TiO₂ treated at 600°C had the highest activity. The structure of P₂₅-TiO₂ was converted from anatase to rutile at 700°C. Moreover, no N-doping was detected at the surface of P₂₅-TiO₂. There was no simply linear relationship between the visible light photocatalytic activity and the concentration of doped nitrogen, and visible light absorption. The visible light photocatalytic activity of N-doped P₂₅-TiO₂ was mainly influenced by the synergistic action of the following factors: (i) the formation of the single-electron-trapped oxygen vacancies (denoted as V_o^·); (ii) the doped nitrogen on the surface of TiO₂; (iii) the anatase TiO₂ structure.     

Synthesis,characterization and photocatalytic properties of tungsten-doped hydrothermal TiO<Subscript>2</Subscript>

Crystalline anatase phase TiO₂ with photocatalytic properties was obtained through a sol–gel low-temperature hydrothermal process. TiO₂ samples doped with tungsten oxide were also obtained by using this synthetic approach. The photocatalytic oxidation of methylene blue in water was monitored to study the influence of the tungsten doping degree on the photocatalytic degradation performance of TiO₂. The degradation rate constant was further increased by adjusting the tungsten doping degree of hydrothermal TiO₂. Also, a much faster photodegradation of methylene blue was achieved using tungsten doped samples baked at 450°C. The results were compared with those obtained with Degussa P25 used as photocatalyst. The structure and optical properties of tungsten-doped TiO₂ were studied by SEM, X-ray diffraction, UV–vis and DRIFT spectroscopy techniques.     

Synthesis of Mixed-Phase TiO<Subscript>2</Subscript> Nanopowders Using Atmospheric Pressure Plasma Jet Driven by Dual-Frequency Power Sources

Mixed-phase TiO₂ nanopowders with different ratios of anatase and rutile have been successfully synthesized using atmospheric pressure plasma jet driven by dual-frequency power sources. The crystal structures of the TiO₂ nanopowders were characterized by X-ray diffraction, SAED, HRTEM, and Raman shift spectroscopy. These results indicated that samples possessed anatase and rutile structure, in addition, the crystallinity of the TiO₂ nanopowders increased and the chlorine contamination decreased with discharge RF power increasing. The photocatalytic activity of the TiO₂ nanopowders was evaluated by decomposition methylene blue solution. The TiO₂ nanopowders which were produced at the discharge RF power of 110 W had the highest photocatalytic activity. Optical emission spectroscopy (OES) was used to detect various excited species in the plasma jet. The results indicate that the various RF power significantly changes the intensities of emission lines (Ar, Ar⁺, Ti, Ti⁺, Ti²⁺, Ti³⁺ and O), which results in the TiO₂ nanopowders a mixture of anatase and rutile phases. The nonequilibrium chemical composition could be formed in one step without anneal. It may have potential applications for synthesizing nanosized particles of high crystallinity by reactive nonthermal plasma processing.     

On the photocatalytic properties of nano-TiO<Subscript>2</Subscript> with dual-phase structure for aqueous methyl-red dye degradation

Heterogeneous photocatalytic removal of methyl-red (MR) dye from liquid phase was done using mixed-phase nanocrystalline TiO₂ for enhancement its photochemical decomposition capabilities. The mixed-phase nanocrystalline TiO₂ was characterized using various techniques to investigate its physicochemical properties. The photocatalytic efficiency of mixed-phase nanocrystalline titania was explored by monitoring the photochemical decay of aqueous MR dye. The results showed that the as-prepared mixed-phase nanocrystalline TiO₂ was excellent for degradation of MR molecule, and the impurity of rutile form increases the photochemical activity by a by a factor of 3. The reaction mechanism was proposed and the results demonstrate that the photocatalytic oxidation reaction followed a pseudo-first-order kinetics.     

The effect of anatase crystal morphology on the photocatalytic conversion of NO by TiO<Subscript>2</Subscript>-based nanomaterials

Hydrogen titanate nanotubes (H-TTNT) were synthesized by the alkali hydrothermal method followed by proton exchange and then submitted either to thermal treatment or to acid hydrothermal reaction to generate TiO₂-anatase nanocrystals of different morphologies. The samples were characterized by XRPD, TGA, sulfur analysis, N₂ physisorption, UV-Vis spectroscopy and TEM. Their photocatalytic activities were determined by measuring the NO conversion in inert gas stream passed through the powder catalyst bed under UV radiation. Incomplete transformation into anatase resulted in nanomaterials with low activity due to coexistence with H-TTNT or TiO₂-B precursors. Anatase specimens derived from H-TTNT aged in strong sulfuric acid media contained equidimensional nanoparticles, but retention of sulfate negatively affected their photocatalytic activity. Combining milder acidic pH with higher aging temperature, allowed synthesis of a sulfate free anatase with the same optical properties and specific surface area as the counterpart produced by calcination of H-TTNT at 550°C; however, the former exhibited truncated bi-pyramid nanocrystals and the other adopted the form of nanorods. This latter showed the highest photocatalytic activity for NO abatement, outperforming the benchmark photocatatyst TiO₂-P25; this improved activity was tentatively ascribed to the maximization of high energy {001} facets in anatase nanorods formed during calcination of H-TTNT.

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Preparation of nanocrystalline TiO<Subscript>2</Subscript> powders for photocatalytic oxidation of phenol

Nanocrystalline TiO₂ powders in the anatase, rutile, and mixed phases prepared by hydrolysis of TiCl₄ solution were of ultrafine size (<7.2 nm) with high specific surface areas in the range 167 to 388 m²/g. In the photocatalytic degradation of phenol as model reaction, the photocatalytic properties of TiO₂ nanoparticles were evaluated by use of UV–vis absorption spectroscopy and total organic carbon (TOC) content. The synthetic mixed-phase TiO₂ powder calcined at 400 °C had higher activity than pure anatase or rutile; it degraded more than 90% phenol to CO₂ (evaluated by TOC) after irradiation with near UV light for 90 min at a catalyst loading of 0.4 g/L. The TOC results indicated that rutile TiO₂ crystallites of particle size 7.2 nm resulted in much better photocatalytic performance than particles of larger size. This result suggested that some intermediates, not determined by UV–vis absorption spectroscopy, existed in the solution after the photocatalytic process over the rutile TiO₂ photocatalysts of larger crystallite size.     

Light expanded clay aggregate (LECA) as a support for TiO<Subscript>2</Subscript>, Fe/TiO<Subscript>2</Subscript>, and Cu/TiO<Subscript>2</Subscript> nanocrystalline photocatalysts: a comparative study on the structure,morphology, and activity

In this work, TiO₂ and doped TiO₂ photocatalysts (Fe/TiO₂ and Cu/TiO₂) were synthesized by the sol–gel method. The main objective of this study was to investigate the influence of dopants on the structure, morphology, and activity of the catalysts in powder and immobilized states. XRF, XRD, and SEM methods were used to characterize the catalysts. The structure and phase distribution of the nanocrystalline powders were identified by XRD. Nanoparticles crystallite size and the degree of crystallinity were affected by doping. The anatase contents of catalysts were achieved as follows: TiO₂ (5.89 %) < Fe/TiO₂ (42.17 %) < Cu/TiO₂ (70.28 %). It was indicated that the activity of the catalysts strongly depends on the anatase content. Under the same circumstances, copper-modified TiO₂ exhibited a twofold higher photocatalytic activity compared with TiO₂. The nanostructured catalysts were immobilized on light expanded clay aggregate (LECA) granules in order to investigate the effect of a novel support on the activity of the catalysts. Morphological changes are recognizable in the SEM images. Activity tests indicated that the best catalytic performance was assigned to Cu/TiO₂/LECA. After 120 min of irradiation, 61 % degradation of phenol in synthetic wastewater was achieved. The high photocatalytic activity of Cu/TiO₂/LECA confirms that LECA is as an excellent support.     

Vanadium-containing polyphosphomolybdate immobilized on TiO<Subscript>2</Subscript> nanoparticles: A recoverable and efficient catalyst for Photochemical,Sonochemical and photosonochemical degradation of dyes under irradiation of UV light

A new photocatalyst, nanoporous anatase TiO₂ crystalline particles coupled by Na₅PV₂Mo₁₀O₄₀ Keggin units, TiO₂-PVMo, was prepared by combination of the methods of sol-gel and hydrothermal treatment. The catalyst was characterized by X-ray diffraction (XRD), UV diffuse reflectance spectroscopy (UV-DRS), FT-IR spectroscopy, Scanning Electron Microscopy (SEM) and cyclovoltametery (CV). This photocatalyst exhibited a good photocatalytic (UV region) and sonocatalytic activity in the decomposition of different dyes in aqueous systems. The TiO₂-PVMo composite showed higher photocatalytic and sonocatalytic activity than pure polyoxometalate or pure TiO₂.     

Synthesis and photocatalytic activity of TiO<Subscript>2</Subscript> nanoparticles fluorine-modified with TiF<Subscript>4</Subscript>

Fluorine-modified TiO₂ nan oparticles were synthesized by introducing TiF₄ as a fluorine source either before or after the sufficient hydrolysis and condensation of Ti(OEt)₄. The photocatalytic activity of the fluorine-modified catalysts was found to be greatly affected by the fluorine position in TiO₂ nanoparticles. When TiF₄ and Ti(OEt)₄ hydrolyzed with synchronization, the fluorine tended to be doped in the lattice. The formation of Ti³⁺ defects could result in charge recombination in bulk and bring down the photocatalytic activity. In contrast, if TiF₄ was introduced after the sufficient hydrolysis and condensation of Ti(OEt)₄. Ti−F bonds could exist mainly on the TiO₂ particles surface, which not only prevented the growth of anatase crystals but also facilitated the transfer of organic compounds from solution to catalyst surface by reducing the hydrophilic properties.     

Preparation of TiO<Subscript>2</Subscript> nanofibers immobilized on quartz substrate by electrospinning for photocatalytic degradation of ranitidine

The photocatalytic activity of TiO₂ nanofibers immobilized on quartz substrates was investigated by evaluating the decomposition of organic pollutants. TiO₂ nanofibers were synthesized by electrospinning the Ti-precursor/polymer mixture solution, followed by hot-pressing for enhancing the adhesion of TiO₂-nanofiber films to the substrates. TiO₂ started to crystalize in the anatase form at 500 °C and reached the optimal photocatalytic anatase/rutile phase ratio of 70:30 at a calcination temperature of 600 °C. The TiO₂-nanofiber film was demonstrated to be an efficient photocatalyst by ranitidine decomposition under UV illumination and was proven to have a comparable photocatalytic activity with the well-known Degussa P25 nanoparticulate photocatalyst and excellent recyclability during 10 cycles of photocatalytic operation, indicating no loss of TiO₂ nanofibers during photocatalytic operations.     

Preparation of nanocrystalline Fe-doped TiO<Subscript>2</Subscript> powders as a visible-light-responsive photocatalyst

Nanocrystalline Fe-doped TiO₂ powders were prepared using TiOSO₄, urea, and Fe(NO₃)₃ · 9H₂O as precursors through a hydrothermal method. The as-synthesized yellowish-colored powders are composed of anatase TiO₂, identified by X-ray diffraction (XRD). The grain size ranged from 9.7 to 12.1 nm, calculated by Scherrer’s method. The specific surface area ranged from 141 to 170 m²/g, obtained by the Brunauer–Emmett–Teller (BET) method. The transmission electron microscopy (TEM) micrograph of the sample shows that the diameter of the grains is uniformly distributed at about 10 nm, which is consistent with that calculated by Scherrer’s method. Fe³⁺ and Fe²⁺ have been detected on the surface of TiO₂ powders by X-ray photoelectron spectroscopy (XPS). The UV–Vis diffuse reflection spectra indicate that the light absorption thresholds of the Fe-doped TiO₂ powders have been red-shifted into the visible light region. The photocatalytic activity of the Fe-doped TiO₂ was evaluated through the degradation of methylene blue (MB) under visible light irradiation. The Fe-doped TiO₂ powders have shown good visible-light photocatalytic activities and the maximum degradation ratio is achieved within 4.5 h.     

Graphite-like carbon deposited anatase TiO<Subscript>2</Subscript> single crystals as efficient visible-light photocatalysts

Graphite-like carbon deposited single-crystal anatase TiO₂ with exposed {001} facets was fabricated through a two-step solvothermal process by using glucose as carbon source. The physicochemical properties of the as-prepared samples were investigated by X-ray diffraction, Brunauer-Emmett-Teller, transmission electron microscopy, Raman, UV–vis diffuse reflectance spectra, electrochemical impedance spectroscopy and surface photovoltage spectroscopy. These results demonstrated that graphite-like carbon layers were deposited on the surface of TiO₂ single-crystal nanosheets with exposed highly reactive {001} facets via the dehydration of glucose during the process of hydrothermal treatment. The loading of the graphite-like carbon layers could effectively extend the light absorption edge of the single-crystal anatase TiO₂ nanosheets to visible light region and accelerate the separation of photo-generated electrons and holes, contributing an excellent visible-light driven photocatalytic performance to the graphite-like carbon deposited single-crystal anatase TiO₂ nanosheets for the degradation of methyl orange.     

Hydrothermal synthesis of mesostructured nanocrystalline TiO2 in an ionic liquid–water mixture and its photocatalytic performance

Anatase mesostructured TiO₂ nanocrystalline was prepared in a mixture of 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIM⁺BF₄⁻) ionic liquid and water by a low temperature hydrothermal method. The obtained materials were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption–desorption. The existence of BMIM⁺BF₄⁻ enhanced the polycondensation and crystallization rate, which encouraged the formation of anatase crystal. The TiO₂ particles were thermally very stable and thus resistant to anatase-rutile phase transformation during calcination at high temperatures. The anatase TiO₂ showed high photocatalytic activity in the degradation of p-chlorophenol than that of the commercially available TiO₂, Degussa P25. After 2 h reaction under the UV-irradiation of 250 W, the removing rate of p-chlorophenol was up to 96.3%.     

Degradation of organic compounds on TiO<Subscript>2</Subscript> photocatalysts prepared by a hydrothermal method in the presence of NH<Subscript>4</Subscript>F

TiO₂ photocatalysts were synthesized by a hydrothermal method from tetraisopropyl orthotitanate (TPOT) in the presence of NH₄F with different NH₄F/Ti molar ratios (0, 0.25, and 1). The formation of a well-crystallized anatase phase of TiO₂ and the suppression of phase transition to rutile were observed, even at high calcination temperature, owing to the effects of NH₄F. The TiO₂ synthesized hydrothermally with NH₄F exhibited absorption with a shift to the longer wavelengths of the visible-light region. The hydrothermally synthesized TiO₂ with a moderate amount of NH₄F exhibited high photocatalytic activity for the degradation of alcohol diluted in water under both UV-light and visible-light irradiations.     

Sol-gel preparation and characterization of Co/TiO<Subscript>2</Subscript> nanoparticles: Application to the degradation of methyl orange

Cobalt doped titania nanoparticles were synthesized by sol-gel method using titanium(IV) isopropoxide and cobalt nitrate as precursors. X-Ray diffraction (XRD) results showed that titania and Co/TiO₂ nanoparticles only include anatase phase. The framework substitution of Co in TiO₂ nanoparticles was established by XRD, scanning electron microscopy equipped with energy dispersive X-ray microanalysis (SEM-EDX) and Fourier transform infrared (FT-IR) techniques. Transmission electron microscopy (TEM) images confirmed the nanocrystalline nature of Co/TiO₂. The increase of cobalt doping enhanced “red-shift” in the UV-Vis absorption spectra. The dopant suppresses the growth of TiO₂ grains, agglomerates them and shifts the band absorption of TiO₂ from ultraviolet (UV) to visible region. The photocatalytic activity of samples was tested for degradation of methyl orange (MO) solutions. Although the photocatalytic activity of undoped TiO₂ was found to be higher than that of Co/TiO₂ under UV irradiation, the presence of 0.5% Co dopant in TiO₂ resulted in a catalyst with the highest activity under visible irradiation.     

Photocatalysis enhancement of CaAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript>, Nd<Superscript>3+</Superscript>@TiO<Subscript>2</Subscript> composite powders

CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders were synthesized by a sol–gel method under mild conditions (i.e. low temperature and ambient pressure). The as-prepared powders were characterized by transmission electron microscopy (TEM) and analyzed by X-ray diffraction (XRD). The photocatalytic behavior of the TiO₂-base surfaces was evaluated by the degradation of nitrogen monoxide gas. It suggested that CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders were composed of anatase titania and that CaAl₂O₄:Eu²⁺, Nd³⁺. TiO₂ particles were deposited on the surface of CaAl₂O₄:Eu²⁺, Nd³⁺ to form uniform film. CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders exhibited higher photocatalytic activity compared with pure TiO₂ under visible light. And the result also clearly indicated that the long afterglow phosphor absorbed and stored lights for the TiO₂ to remain photocatalytic activity in the dark.     

Influence of ThO<Subscript>2</Subscript> on the photocatalytic activity of TiO<Subscript>2</Subscript>

Microcomposites consisting of TiO₂ (or Ce-doped TiO₂) and ThO₂ (0.5–2% of the TiO₂ mass) are produced by sol-gel synthesis of TiO₂ in presence of ThO₂. X-ray diffraction study reveals the effects of ThO₂ (compared to the ThO₂-free TiO₂, obtained by the same method) on the anatase interplanar distances, crystallites size and phase composition. The photocatalytic tests in presence of the composites under UV irradiation reveal an increase of the Malachite Green degradation rate constant. The effect depends on the Th relative content, temperature of annealing of the catalyst and addition of other doping agent. The highest photocatalytic activity is observed for TiO₂ obtained at 550°C and containing 1% ThO₂. The composite exhibits activity in dark, also. The presence of Ce⁴⁺ ions is not an obligatory requirement for the realization of the ThO₂ effect. The reported results suggest that the radioactivity of the Th and/or its decay products is one of the main factors responsible for the increased photocatalytic activity of TiO₂.      

Microstructure and Photocatalytic Property of SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> Under Various Process Condition

Nanophase silica-titania particles were prepared by two different synthetic routes, namely, sol–gel and hydrothermal processing. The crystallinity and crystallographic phases, particle size and surface area of the materials were controlled by varying the calcination temperature, and/or the ratio of Si to Ti. It was determined by XRD that the crystallite sizes of SiO₂-TiO₂ prepared by sol–gel and hydrothermal processing decreased from 11 to 6 nm and 12 to 9 nm, respectively, as the mole fraction of silica was increased from 0.1 to 0.4. It is proposed that the presence of the amorphous silica suppresses the growth of anatase TiO₂ grains and their phase transformation to rutile. The photocatalytic decomposition rate of 1,4-dichlorobenzene (DCB) in aqueous solution with the sol–gel derived SiO₂-TiO₂ powder prepared at 750 °C was about 10 ± 5% higher than that observed with Degussa P25, whereas the SiO₂-TiO₂ samples prepared by hydrothermal processing at 250 °C showed a slightly lower decomposition rate than P25.     

钛酸纳米管前驱体水热制备TiO2：水热媒介对结晶度及光催化活性的影响

以钛酸纳米管为先驱体,180℃24 h同一水热条件下,利用H2O、HNO3、KBF4以及HNO3+KBF4的4种水热媒介溶液,制备了高结晶度高光催化活性的TiO2,并研究了水热媒介对TiO2结晶度、表面羟基和光催化活性的影响。通过扫描电镜(SEM)、X射线衍射(XRD)、红外测试(FTIR)、X射线光电子能谱(XPS)、紫外可见漫反射吸收光谱(UV-Vis)进行了结构表征。光催化活性通过降解甲基橙溶液进行了评价。结果表明,与其他3种媒介相比,HNO3+KBF4溶液为水热媒介制备的锐钛矿晶型TiO2,结晶度高,表面羟基含量高,因此,呈现高的光催化活性。