Preparation and Characterization of Multi-functional Titanium Dioxide Photocatalysts

Recent research trends of the preparation and characterization of highly efficient titanium oxide-based photocatalysts modified by different methods are reviewed on the basis of studies done in our laboratory. Special attention is focused on the preparation and characterization of TiO₂ photocatalysts prepared by the transitional metal doping and noble metal deposition method, especially combining above two methods. Fe³⁺ doped together with Au deposited TiO₂ (Au/Fe³⁺–TiO₂) was successfully prepared, which shows excellent photocatalytic activity for degradation of methyl orange (MO) under both UV and visible light (λ > 420 nm) illumination. Fe³⁺ has been confirmed by EPR to substitute for Ti⁴⁺ in the TiO₂ lattice, and Au exists as Au⁰ on the surface of the photocatalyst indicated by the results of XRD. Fe³⁺ and Au have synergistic effects on improving the photocatalytic activity of TiO₂. A proposed mechanism concerning the synergistic effects is discussed to explain the improvement of the photocatalytic activities.     

Preparation and characterization of TiO2 photocatalysts by Fe doping together with Au deposition for the degradation of organic pollutants

Fe³⁺ doped TiO₂ deposited with Au (Au/Fe–TiO₂) was successfully prepared with an attempt to extend light absorption of TiO₂ into the visible region and reduce the rapid recombination of electrons and holes. The samples were characterized by X-ray diffraction (XRD), N₂ physical adsorption, Raman spectroscopy, atomic absorption flame emission spectroscopy (AAS), UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectra. The photocatalytic activities of the samples were evaluated for the degradation of 2,4-chlorophenol in aqueous solutions under visible light (λ > 420 nm) and UV light irradiation. The results of XRD, XPS and high-resolution transmission electron microscopy (HRTEM) analysis indicated that Fe³⁺ substituted for Ti⁴⁺ in the lattice of TiO₂, Au existed as Au⁰ on the surface of the photocatalyst and the mean particle size of Au was 8 nm. Diffuse reflectance measurements showed an extension of light absorption into the visible region for Au/Fe–TiO₂, and PL analysis indicated that the electron–hole recombination rate has been effectively inhibited when Au deposited on the surface of Fe-doped TiO₂. Compared with Fe doped TiO₂ sample and Au deposited TiO₂ sample, the Au/Fe–TiO₂ photocatalyst exhibited excellent visible light and UV light activity and the synergistic effects of Fe³⁺ and Au was responsible for improving the photocatalytic activity.     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

Preparation, Characterization and Photocatalytic Activities of F-doped TiO2 Nanotubes

F-doped TiO₂ nanotubes were prepared by impregnation method. The prepared catalysts were characterized by XRD, TEM, and XPS. The photocatalytic activity of F-doped TiO₂ nanotubes was evaluated through the photodegradation of aqueous methyl orange. The experiments demonstrated that the F-doped TiO₂ nanotubes calcined at 300 °C possessed the best photocatalytic activity. Compared with pure TiO₂ nanotubes, the doping with F⁻ significantly enhanced the photocatalytic efficiency. The high photocatalytic activity was ascribed to several beneficial effects produced by F-doping: creation of oxygen vacancies, presence of Ti³⁺, and so on. An erratum to this article can be found at     

Study on the photocatalytic degradation of trichlorfon in suspension of titanium dioxide

In this paper, the photocatalytic degradation of trichlorfon, an organophosphorous pesticide, was studied by using TiO₂ as a photocatalyst. The effects of various parameters, such as the amount of the photocatalyst, illumination time, reaction temperature, electron acceptors, metal ions, anions, and initial pH value on the photocatalytic degradation of trichlorfon were investigated. The best conditions for the photocatalytic degradation of trichlorfon were obtained. The results show that the optimum amount of the photocatalyst used is 8.0 g L^− 1. The photodegradation efficiency of trichlorfon increases with the increase of the illumination time or reaction temperature. The photodegradation efficiency of trichlorfon is increased rapidly by adding a small amount of H₂O₂, K₂S₂O₈, KBrO₃, Fe³⁺ and Cu²⁺, however, with the addition of Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Co²⁺ and Ni²⁺, or with the addition of trace amount of SO₄²⁻, Cl⁻, Br⁻, there are no obvious effects on the photocatalytic degradation reactions. Alkaline mediums are favorable for the photocatalytic degradation of trichlorfon. The possible roles of the additives on the reactions and the possible mechanisms of effect were also discussed.     

Preparation and photocatalytic properties of a novel kind of loaded photocatalyst of TiO2/SiO2/γ‐Fe2O3

A novel kind of loaded photocatalyst of TiO₂/SiO₂/γ‐Fe₂O₃ (TSF) that can photodegrade effectively organic pollutants in the dispersion system and can be recycled easily by a magnetic field is reported in this paper. The γ‐Fe₂O₃ cores in TiO₂/γ‐Fe₂O₃ are found to reduce the activity of the TiO₂ photocatalyst in the photodegradation of dyes under either UV or visible light irradiation. Addition of a SiO₂ membrane between the γ‐Fe₂O₃ core and the TiO₂ shell weakens efficiently the influence of the γ‐Fe₂O₃ cores on the TiO₂ photocatalytic activity and leads to a highly active and magnetically separable photocatalyst on TSF. Comparison of the photodegradation processes of dyes under UV and visible irradiation is also carried out. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photocatalytic conversion of benzene to phenol using modified TiO2 and polyoxometalates

The application of photocatalytic reactions to organic synthesis has attracted interests in view of the development of environmentally benign synthetic processes. This study investigated the effects of various parameters (electron acceptor, surface modification, and the combination of photocatalysts) on the direct synthesis of phenol from benzene using photocatalytic oxidation processes. The OH radicals generated on UV-illuminated TiO₂ photocatalyst directly hydroxylate benzene to produce phenol, hydroquinone, and catechol. The addition of Fe³⁺, H₂O₂, or Fe³⁺ + H₂O₂ highly enhanced the phenol production yield and selectivity in TiO₂ suspension. Surface modifications of TiO₂ had significant influence on the phenol synthetic reaction. Depositing Pt nanoparticles on TiO₂ (Pt/TiO₂) markedly enhanced the yield and selectivity. Surface fluorination of TiO₂ (F-TiO₂) increased the phenol yield two-fold because of the enhanced production of mobile (free) OH radicals on F-TiO₂. Polyoxometalate (POM) in phenol synthesis played the dual role both as a homogeneous photocatalyst and as a reversible electron acceptor in TiO₂ suspension. POM alone was as efficient as TiO₂ alone in the phenol production. In particular, the addition of POM to the TiO₂ suspension increased the phenol yield from 2.6% to 11% (the highest yield obtained in this study). Reaction mechanisms for each photocatalytic system were discussed in relation to the phenol synthesis.     

Nano-gold supported on Fe2O3: A highly active catalyst for low temperature oxidative destruction of methane green house gas from exhaust/waste gases

A number of nano-gold catalysts were prepared by depositing gold on different metal oxides (viz. Fe₂O₃, Al₂O₃, Co₃O₄, MnO₂, CeO₂, MgO, Ga₂O₃ and TiO₂), using the homogeneous deposition precipitation (HDP) technique. The catalysts were evaluated for their performance in the combustion of methane (1 mol% in air) at different temperatures (300–600 °C) for a GHSV of 51,000 h⁻¹. The supported nano-gold catalysts have been characterized for their gold loading (by ICP) and gold particle size (by TEM/HRTEM or XRD peak broadening). Among these nano-gold catalysts, the Au/Fe₂O₃ (Au loading = 6.1% and Au particle size = 8.5 nm) showed excellent performance. For this catalyst, temperature required for half the methane combustion was 387 °C, which is lower than that required for Pd(1%)/Al₂O₃ (400 °C) and Pt(1%)/Al₂O₃ (500 °C) under identical conditions. A detailed investigation on the influence of space velocity (GHSV = 10,000–100,000 cm³ g⁻¹ h⁻¹) at different temperatures (200–600 °C) on the oxidative destruction of methane over the Au/Fe₂O₃ catalyst has also been carried out. The Au/Fe₂O₃ catalyst prepared by the HDP method showed much higher methane combustion activity than that prepared by the conventional deposition precipitation (DP) method. The XPS analysis showed the presence of Au in the different oxidation states (Au⁰, Au¹⁺ and Au³⁺) in the catalyst.     

Photocatalytic Properties of Silica-supported TiO2

TiO₂ supported on SiO₂ surface is effective on the recovery of photocatalyst, morphological control, and coating on the substrate. Furthermore, it shows much higher photocatalytic activity than pure TiO₂. The silica support is quite influential on the surface properties of TiO₂ supported on SiO₂. The enhanced photocatalytic activity of TiO₂–SiO₂ could be explained by the effects of surface area, adsorption, band-gap energy and local structure. However, it is difficult to say which one is the most important factor responsible for the photocatalytic property of TiO₂–SiO₂. For example, the reduction of particle size could effect on both of the surface area and band-gap energy. And, Ti–O–Si bonds could modify the band-gap energy and local structure. Therefore, the photocatalytic properties of TiO₂–SiO₂ should be expressed by sum of many factors such as surface area, adsorption, band-gap energy and local structure.     

Photo-catalytic degradation of Rhodamine B on C-, S-, N-, and Fe-doped TiO2 under visible-light irradiation

C-, S-, N-, and Fe-doped TiO₂ photocatalysts were synthesized by a facile sol–gel method. The structure and properties of catalysts were characterized by N₂ desorption–adsorption, X-ray diffraction (XRD), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results revealed that the surface area of the multi-doped TiO₂ was significantly increased and the crystallite size was smaller than the pure TiO₂ obtained by a similar route. Compared with TiO₂, the peak position in doped-TiO₂ XRD patterns was slightly shifted, which could be attributed to the distortion by the substitution of carbon, nitrogen, and sulfur dopants for some oxygen atoms and Fe³⁺ for Ti⁴⁺ in the lattice of TiO₂. These substitutions were confirmed by XPS. In addition, these dopants were responsible for narrowing the band gap of TiO₂ and shifting its optical response from ultraviolet (UV) to the visible-light region. The photocatalytic reactivities of these multi-doped TiO₂ catalysts were investigated by degrading Rhodamine B (RB) in aqueous solution under visible-light irradiation (λ > 420 nm). It was found out that the reactivity was significantly enhanced and the catalyst doped with nitrogen, carbon, sulfur, and 0.3 wt% iron had the highest photocatalytic activity.     

Homogenous photocatalytic decomposition of acetic acid using UV-Fe/Fe system in the absence of oxygen

Photocatalysis is a promising way to eliminate organic pollution. Here a novel homogenous non-oxygen photocatalytic system was studied. Acetic acid was used as a model substrate to be decomposed into hydrogen, methane, ethane and carbon dioxide continuously by photo-excited Fe²⁺ and Fe³⁺ ions. Fluorescence spectra and ultraviolet visible spectra showed that Fe²⁺ and Fe³⁺ were excited by ultraviolet light. The rate of acetic acid decomposition increased along with the increase of temperature, Fe²⁺ and acetic acid concentration. The photocatalytic mechanism and kinetics were proposed and the effects of pH value, reaction temperature, concentration of FeSO₄ and CH₃COOH were investigated.     

Enhanced Luminescence of Eu-Doped TiO2 Nanodots

Monodisperse and spherical Eu-doped TiO₂ nanodots were prepared on substrate by phase-separation-induced self-assembly. The average diameters of the nanodots can be 50 and 70 nm by changing the preparation condition. The calcined nanodots consist of an amorphous TiO₂ matrix with Eu³⁺ ions highly dispersed in it. The Eu-doped TiO₂ nanodots exhibit intense luminescence due to effective energy transfer from amorphous TiO₂ matrix to Eu³⁺ ions. The luminescence intensity is about 12.5 times of that of Eu-doped TiO₂ film and the luminescence lifetime can be as long as 960 μs.     

Ti addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate

Single-crystal α-Al₂O₃ hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 °C. Co-doping 1 mol% TiO₂ can increase the shape anisotropy of α-Al₂O₃ hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe₂O₃ and TiO₂ on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe³⁺ and Ti⁴⁺ can promote γ → α-Al₂O₃ phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of α-Al₂O₃ hexagonal flakes can be increased by adding TiO₂ due to the segregation of Ti⁴⁺ ions onto the surfaces of basal planes of α-Al₂O₃ single crystal particle.     

Novel Au/TiO2/Al2O3 · xH2O catalysts for CO oxidation

Au/Al₂O₃ · xH₂O and Au/TiO₂/Al₂O₃ · xH₂O (x = 0–3) catalysts were prepared by assembling gold nanoparticles on neat and TiO₂-modified Al₂O₃, AlOOH, and Al(OH)₃ supports, and their catalytic activity in CO oxidation was tested either as synthesized or after on-line pretreatment in O₂–He at 500 °C. A promotional effect of TiO₂ on the activity of gold catalysts was observed upon 500 °C-pretreatment. The catalyst stability as a function of time on stream was tested in the absence or presence of H₂, and physiochemical characterization applying BET, ICP-OES, XRD, TEM, and ²⁷Al MAS NMR was conducted.     

Effect of Mg-and Fe-ions on formation mechanism of aluminium titanate

The presence of Mg²⁺- and Fe³⁺-ions has an effect on the formation of Al₂TiO₅. Crystalline phases produced under the influence of the heat treatment have been identified in a heated X-ray diffraction chamber in the temperature range of 20–1500 °C. In the presence of Mg²⁺- and Fe³⁺-ions transitional phases are formed in the temperature range of 1000–1350 °C during Al₂TiO₅ formation. The XRD technique was used to identify the crystalline phases formed. On addition of MgO, chemical composition of the transitional phase formed is Mg_0.3Al_1.4Ti_1.3O₅, whereas on addition of Fe₂O₃ we have calculated a Powder Diffraction File card data for the transitional phase. Determination of the lattice parameters of the Al₂TiO₅ ceramics produced enabled verification of incorporation of Mg²⁺- or Fe³⁺-ions into the crystal lattice of Al₂TiO₅, i.e. the formation of Mg²⁺- and Fe³⁺-containing solid solutions.     

Efficient visible-light photocatalysts from Gd–La codoped TiO2 nanotubes

Robust visible-light Gd–La codoped TiO₂ nanotubes were successfully synthesized via an ultrasonic hydrothermal method and the photocatalytic activities were evaluated by photodegrading Rhodamine B (RB). The calcined Gd–La codoped TiO₂ nanotubes have significantly enhanced photocatalytic activities than the uncalcined ones. The La³⁺ and Gd³⁺ in the lattices of rare earth oxides may be substituted by Ti⁴⁺, creating abundant oxygen vacancies and surface defects for electron trapping and dye adsorption, accelerating the separation of photogenerated electron–hole pairs and RB photodegradation. The formation of an excitation energy level below the conduction band of TiO₂ from the binding of electrons and oxygen vacancies decreases the excitation energy of Gd–La codoped TiO₂ nanotubes, resulting in robust photocatalysts. The results suggest that Gd–La codoped TiO₂ nanotubes calcined at 500 °C are very promising for enhancing the photocatalytic activity of photocatalysts in visible-light region.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

Synthesis of a TiO2 ceramic membrane containing SrCo0.8Fe0.2O3 by the sol-gel method with a wet impregnation process for O2 and N2 permeation

A SrCo_0.8Fe_0.2O₃ impregnated TiO₂ membrane (TiO₂-SrCo_0.8Fe_0.2O₃ membrane) was successfully prepared using a sol-gel method in combination with a wet impregnation process. The membrane was subjected to a single gas permeance test using oxygen (O₂) and nitrogen (N₂). The TiO₂ membrane was immersed in the SrCo_0.8Fe_0.2O₃ solution, dried and then calcined to affix SrCo_0.8Fe_0.2O₃ into the membrane. The effect of the acid/alkoxide (H⁺/Ti⁴⁺) molar ratio of the TiO₂ sol on the TiO₂ phase transformation was investigated. The optimal molar ratio was found to be 0.5, which resulted in nanoparticles with a mean size of 5.30 nm after calcination at 400 °C. The effect of calcination temperature on the phase transformation of TiO₂ and SrCo_0.8Fe_0.2O₃ was investigated by varying the calcination temperature from 300 to 500 °C. X-ray diffraction spectroscopy (XRD) and Fourier transform infrared (FTIR) analysis confirmed that a calcination temperature of 400 °C was preferable for preparing a TiO₂-SrCo_0.8Fe_0.2O₃ membrane with fully crystallized anatase and SrCo_0.8Fe_0.2O₃ phases. The results also showed that polyvinyl alcohol (PVA) and hydroxypropyl cellulose (HPC) were completely removed. Field emission scanning electron microscopy (FESEM) analysis results showed that a crack-free and relatively dense TiO₂ membrane (∼0.75 μm thickness) was created with a multiple dip-coating process and calcination at 400 °C. The gas permeation results show that the TiO₂ and TiO₂-SrCo_0.8Fe_0.2O₃ membranes exhibited high permeances. The TiO₂-SrCo_0.8Fe_0.2O₃ membrane developed provided greater O₂/N₂ selectivity compared to the TiO₂ membrane alone.     

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.