Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO3/TiO2 photocatalysts under UV and visible light irradiation

Nanomaterials as Photocatalysts-Synthesis and Their Potential Applications

Increasing demand for energy and environmental degradation are the most serious problems facing the man. An interesting issue that can contribute to solving these problems is the use of photocatalysis. According to literature, solar energy in the presence of a photocatalyst can effectively (i) be converted into electricity/fuel, (ii) break down chemical and microbial pollutants, and (iii) help water purification. Therefore, the search for new, efficient, and stable photocatalysts with high application potential is a point of great interest. The photocatalysts must be characterized by the ability to absorb radiation from a wide spectral range of light, the appropriate position of the semiconductor energy bands in relation to the redox reaction potentials, and the long diffusion path of charge carriers, besides the thermodynamic, electrochemical, and photoelectrochemical stabilities. Meeting these requirements by semiconductors is very difficult. Therefore, efforts are being made to increase the efficiency of photo processes by changing the electron structure, surface morphology, and crystal structure of semiconductors. This paper reviews the recent literature covering the synthesis and application of nanomaterials in photocatalysis.

Improved photocatalytic oxidation of arsenic (III) with WO3/TiO2 nanomaterials synthesized by the sol-gel method

Photocatalytic oxidation of arsenite (As(III)) to arsenate (As(V)) was studied in aqueous solution using a series of WO₃/TiO₂ semiconductors readily synthesized through sol-gel method with WO₃ content in the range of 1-5 wt%. The resulting materials showed enhanced photocatalytic activity towards As(III) photo-oxidation compared to their individual counterparts under UV radiation. The amount of As(III) and As(V) species in the irradiated solution was determined using the molybdenum blue method. The efficiency of photoinduced carriers separation was further affirmed by electrical impedance spectroscopy (EIS) and photocurrent tests. The maximum catalytic efficiency was observed when the binary oxide 3%WO₃/TiO₂ (TW3) was used, reaching a 99% conversion of As(III) to As(V) within the first 25 min under UV irradiation. The enhanced photocatalytic performance of the heterostructures could be explained as consequent to an improved charge separation due to the migration of photoproduced holes in TW3 photocatalyst. Based on the electric band structure of WO₃ and TiO₂, a reasonable mechanism for the photo-oxidation of As(III) over TW3 novel catalyst has been proposed.

Green and facile sol-gel synthesis of the mesoporous SiO2-TiO2 catalyst by four different activation modes

The most environmentally friendly protocol for obtaining mesoporous SiO2-TiO2 catalysts has been sought. Water has been employed as a green solvent, the energy input has been minimized, and three further principles (1, 3, and 12) of Green Chemistry have been considered. Four different modes for promoting the reaction have been comparatively evaluated, namely near-infrared and microwave electromagnetic irradiations, ultrasound, and traditional mantle heating. Brunauer-Emmett-Teller (BET) analyses of the catalysts produced revealed that the non-conventional activation modes afforded both large surface areas (335-441 m2 g-1) and smaller crystal sizes (7.2-15.3 nm) than the mantle heating process. These modes also generated the catalysts in shorter reaction times than traditional mantle heating, 10-30 min versus 3 h, with anatase as the sole crystalline phase. The photocatalytic degradation of 4-chlorophenol has been carried out to assess the catalytic efficiencies of the hybrid materials. The catalyst synthesized with microwave assistance showed the best mineralization activity (97%), followed by those prepared with ultrasound, near-infrared, and mantle heating. The materials have been extensively characterized by FTIR, XRD, DRS-UV/Vis, SEM, 29Si MAS NMR, and BET analyses. To the best of our knowledge, this is the first such comparative assessment of green energetic alternatives in developing a sol-gel process.

Hybrid reactor based on hydrodynamic cavitation, ozonation, and persulfate oxidation for oxalic acid decomposition during rare-earth extraction processes

A cost-effective method for treating oxalic acid (OA) during rare-earth extraction was developed using hydrodynamic cavitation (HC), ozone (O₃), and persulfate (PS) (HC@PS@O₃ process). The results showed that the optimal inlet pressure during HC was 5.10 kg cm^-2 with an orifice plate diameter of 2 mm. Moreover, HC was shown to activate PS, providing an alternative activation method to base or heat as an ultrasound activation method for chemical oxidation. O₃ was also shown to activate PS. For OA oxidation using the HC@PS@O₃ process, the optimum pH was 3 and the reaction rate increased with increasing temperature. Further, the activation energy was 36.69 kJ mol^-1. The mechanisms unveiled in this study will allow optimization of the HC@PS@O₃ process as a chemical oxidation technology. The kinetic investigation and economic evaluation of the HC@PS@O₃ process can be used as the basis for real wastewater treatment processes in the future.

Resin-gel incorporation of high concentrations of W6+ and Zn2+ into TiO2-anatase crystal to form quaternary mixed-metal oxides: effect on the a lattice parameter and photodegradation efficiency

Incorporating up to 30% of W⁶⁺ and/or Zn²⁺ into TiO₂-anatase influences the lattice structure of TiO₂ thus its photocatalytic efficiency.

Easily Recoverable, Micrometer-Sized TiO2 Hierarchical Spheres Decorated with Cyclodextrin for Enhanced Photocatalytic Degradation of Organic Micropollutants

Micrometer-sized titanium dioxide hierarchical spheres (TiO₂-HS) were assembled from nanosheets to address two common limitations of photocatalytic water treatment: (1) inefficiency associated with scavenging of oxidation capacity by nontarget water constituents and (2) energy-intensive separation and recovery of the photocatalyst slurry. These micrometer-sized spheres are amenable to low-energy separation, and over 99% were recaptured from both batch and continuous flow reactors using microfiltration. Using nanosheets as building blocks resulted in a large specific surface area-3 times larger than that of commercially available TiO₂ powder (Evonik P25). Anchoring food-grade cyclodextrin onto TiO₂-HS (i.e., CD-TiO₂-HS) provided hydrophobic cavities to entrap organic contaminants for more effective utilization of photocatalytically generated reactive oxygen species. CD-TiO₂-HS removed over 99% of various contaminants with dissimilar hydrophobicity (i.e., bisphenol A, bisphenol S, 2-naphthol, and 2,4-dichlorophenol) within 2 h under a low-intensity UVA input (3.64 × 10^-6 einstein/L/s). As with other catalyst (including TiO₂ slurry), periodic replacement or replenishment would be needed to maintain high treatment efficiency (e.g., we demonstrate full reactivation through simple reanchoring of CD). Nevertheless, this task would be offset by significant savings in photocatalyst separation. Thus, CD-TiO₂-HS is an attractive candidate for photocatalytic water and wastewater treatment of recalcitrant organic pollutants.

Removal of chlorpyrifos, thiamethoxam, and tebuconazole from water using green synthesized metal hexacyanoferrate nanoparticles

The low-cost and highly efficient pesticides are largely used in residential, agricultural, and commercial applications. Their prevalent occurrence, bioaccumulation, and chronic toxicity to living beings have raised environmental concern and call for their whole eradication, especially from water. By virtue of semiconducting nature and high surface area, nanomaterials have become efficient adsorbent and photocatalyst in removal of toxins. To confirm this, the potential of highly crystalline metal hexacyanoferrates (MHCFs) of Zn, Cu, Co, and Ni was evaluated in deprivation of selected hazardous pesticides, viz., chlorpyrifos (CP), thiamethoxam (TH), and tebuconazole (TEB). Sharp nanocubes of ZnHCF (~ 100 nm), distorted nanocubes of CuHCF (~ 100 nm), and nanospheres of CoHCF and NiHCF (< 10 nm) were synthesized via green route using Sapindus mukorossi (raw ritha). At 50 mg L^-1 of pesticide, 15 mg of MHCF photocatalyst, neutral pH and sunlight irradiation, selected agrochemicals were degraded to maximum extent (91-98%) by ZnHCF followed by CuHCF (85-91%), NiHCF (73-85%), and CoHCF (70-83%). This might be because of highest zeta potential and BET surface area of ZnHCF. The highest adsorption of CP (83-98%) followed by TH (76-95%) and TEB (70-91%) on acidic surface of catalysts might be related to access of free electrons in their structures. On treatment with MHCF photocatalyst, targets underwent mineralization along with formation of some minor and non-toxic by-products such as (Z) but-2-enal, 3-aminopropanoic acid, and pyridin-3-ol, identified after mass spectrometric analysis of reaction mixture. Based on them, degradation pathways have been proposed to reveal the potential of MHCF for solar photocatalytic removal of organic pollutants in environment.

Degradation of toxic PAHs in water and soil using potassium zinc hexacyanoferrate nanocubes

Polycyclic aromatic hydrocarbons (PAHs) the ubiquitous, persistent and carcinogenic environmental contaminants have raised concern worldwide. Recently, their removal methodologies are advanced after exploring nanomaterials. Therefore, degradation of selected toxic PAHs (3-5 rings) using potassium zinc hexacyanoferrate (KZnHCF) nanocubes was studied. Highly crystalline and sharp KZnHCF nanocubes (∼100 nm) were obtained by green route using sapindus mukorossi. In both water and soil, anthracene and phenanthrene were degraded to maximum extent (80-93%), whereas, the degradation of fluorene, chrysene and benzo (a) pyrene were ∼70-80%.Because of small size (lower molecular weight), large number of anthracene and phenanthrene molecules were adsorbed on catalyst as compared to other PAHs. Higher degradation of PAHs in water than in the soil is attributed to the easy absorption of PAHs on catalyst in water and slow diffusion of PAHs on organic content of soil. PAHs were degraded at the concentration of 50 mg/L, 25 mg catalyst dose, neutral pH and solar irradiation. Higher proficiency of the catalyst was revealed by degradation of PAHs into small and non-toxic by-products such as malealdehyde, 4-oxobut-2-enoic acid and o-xylene. Overall, the potential KZnHCF nanostructures open future scope for eradication of other pollutants from the environment.

Self-Limiting Adsorption of WO3 Oligomers on Oxide Substrates in Solution

Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO₂(110) and magnetite Fe₃O₄(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO₃ to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO₃ spontaneously forms under all conditions of potential and pH at the tungsten-aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.

Photoelectrocatalytic degradation of oxalic acid using WO3 and stratified WO3/TiO2 photocatalysts under sunlight illumination

The WO₃ and stratified WO₃/TiO₂ thin films are successfully prepared by the spray pyrolysis method. The structural, morphological, compositional and photoelectrocatalytic properties of WO₃ and stratified WO₃/TiO₂ thin films are studied. XRD analysis confirms that films are polycrystalline with monoclinic and tetragonal crystal structures for WO₃ and TiO₂ respectively. The SEM images clearly show 3D sheeted porous structure of the as-prepared TiO₂ forms on WO₃ in stratified WO₃/TiO₂ samples. The synthesized photoelectrodes was used as catalyst for photoelectrocatalytic degradation of oxalic acid in aqueous medium. The rate constant (k) was evaluated as a function of the initial concentration of species. A significant decrease in concentrations of organic species was observed from COD analysis. The photoelectrocatalytic degradation effect is relatively higher in the case of the stratified WO₃/TiO₂ than WO₃ thin film photoelectrode in the degradation of oxalic acid and 83% removal efficiency of oxalic acid is obtained after 180min. Based on the obtained experimental data, the possible photoelectrocatalytic reaction mechanism was proposed. The photoelectrocatalytic experimental results indicate that stratified WO₃/TiO₂ photoelectrode is the promising material for removing of water pollutants.

Photocatalytic behaviour of WO3/TiO2-N for diclofenac degradation using simulated solar radiation as an activation source

In this study, the photocatalytic removal of an emerging contaminant, diclofenac (DCF) sodium, was performed using the nitrogen-doped WO₃/TiO₂-coupled oxide catalyst (WO₃/TiO₂-N). The catalyst synthesis was accomplished by a sol-gel method using tetrabutyl orthotitanate (C₁₆H₃₆O₄Ti), ammonium p-tungstate [(NH₄)₁₀H₂W₁₂O₄₂·4H₂O] and ammonium nitrate (NH₄NO₃) as the nitrogen source. For comparison, TiO₂ and WO₃/TiO₂ were also prepared under similar conditions. Analysis by X-ray diffraction (XRD), N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance UV-Vis spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the synthesized materials. The photocatalytic efficiency of the semiconductors was determined in a batch reactor irradiated with simulated solar light. Residual and mineralized DCF were quantified by high-performance liquid chromatography, total organic carbon analysis and ion exchange chromatography. The results indicated that the tungsten atoms were dispersed on the surface of TiO₂ as WO₃. The partial substitution of oxygen by nitrogen atoms into the lattice of TiO₂ was an important factor to improve the photocatalytic efficiency of WO₃/TiO₂. Therefore, the best photocatalytic activity was obtained with the WO₃/TiO₂-N_0.18 catalyst, reaching 100% DCF transformation at 250 kJ m^-2 and complete mineralization at 400 kJ m^-2 of solar-accumulated energy.

Effect of W doping level on TiO2 on the photocatalytic degradation of Diuron

In the present study, three compositions of W-doped titania nano-photocatalyst are synthesized via the sol-gel method. The powders obtained were characterized by X-ray diffraction, Raman spectroscopy and UV-visible diffuse reflectance spectroscopy. The photocatalytic performances of the different photocatalysts are tested with respect to the degradation of Diuron in water solutions under simulated solar light and visible light irradiation. The W_0.03Ti_0.97O₂ catalyst exhibits better photoactivity than the pure TiO₂ even under simulated solar light and visible light. This improvement in activity was attributed to photoelectron/hole separation efficiency.

Influence of titanium dioxide modification on the antibacterial properties

Antibacterial properties of 15 titania photocatalysts, mono- and dual-modified with nitrogen and carbon were examined. Amorphous TiO2, supplied by Azoty Group Chemical Factory Police S.A., was used as titania source (Ar-TiO2, C-TiO2, N-TiO2 and N,C-TiO2 calcined at 300°C, 400°C, 500°C, 600°C, 700°C). The disinfection ability was examined against Escherichia coli K12 under irradiation with UV and artificial sunlight and in dark conditions. It has been found the development of new photocatalysts with enhanced interaction ability with microorganisms might be a useful strategy to improve disinfection method conducted under artificial sunlight irradiation. The efficiency of disinfection process conducted under artificial sunlight irradiation with carbon (C-TiO2) and carbon/nitrogen (N,C-TiO2) photocatalysts was similar as obtained under UV irradiation. Furthermore, during dark incubation, any toxicity of the photocatalyst was noted.

Tungsten oxide--fly ash oxide composites in adsorption and photocatalysis

A novel composite based on tungsten oxide and fly ash was hydrothermally synthetized to be used as substrate in the advanced treatment of wastewaters with complex load resulted from the textile industry. The proposed treatment consists of one single step process combining photocatalysis and adsorption. The composite's crystalline structure was investigated by X-ray diffraction and FTIR, while atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the morphology. The adsorption capacity and photocatalytic properties of the material were tested on mono- and multi-pollutants systems containing two dyes (Bemacid Blau - BB and Bemacid Rot - BR) and one heavy metal ion-Cu(2+), and the optimized process conditions were identified. The results indicate better removal efficiencies using the novel composite material in the combined adsorption and photocatalysis, as compared to the separated processes. Dyes removal was significantly enhanced in the photocatalytic process by adding hydrogen peroxide and the mechanism was presented and discussed. The pseudo second order kinetics model best fitted the experimental data, both in the adsorption and in the combined processes. The kinetic parameters were calculated and correlated with the properties of the composite substrate.

One-dimensional densely aligned perovskite-decorated semiconductor heterojunctions with enhanced photocatalytic activity

By using one-dimensional rutile TiO(2) nanorod arrays as the structure-directing scaffold as well as the TiO(2) source to two consecutive hydrothermal reactions, densely aligned SrTiO(3) -modified rutile TiO(2) heterojunction photocatalysts are crafted for the first time. The first hydrothermal processing yielded nanostructured rutile TiO(2) with the hollow openings on the top of nanorods (i.e., partially etched rutile TiO(2) nanorod arrays; denoted PE-TNRAs). The subsequent second hydrothermal treatment in the presence of Sr(2+) transforms the surface of partially etched rutile TiO(2) nanorods into SrTiO(3) nanoparticles via the concurrent dissolution of TiO(2) and precipitation of SrTiO(3) while retaining the cylindrical shape (i.e., forming SrTiO(3) -decorated rutile TiO(2) composite nanorods; denoted STO-TNRAs). The structural and composition characterizations substantiate the success in achieving STO-TNRA nanostructures. In comparison to PE-TNRAs, STO-TNRA photocatalysts exhibit higher photocurrents and larger photocatalytic degradation rates of methylene blue (3.21 times over PE-TNRAs) under UV light illumination as a direct consequence of improved charge carrier mobility and enhanced electron/hole separation. Such 1D perovskite-decorated semiconductor nanoarrays are very attractive for optoelectronic applications in photovoltaics, photocatalytic hydrogen production, among other areas.

Degradation of methylene blue using porous WO3, SiO2-WO3, and their Au-loaded analogs: adsorption and photocatalytic studies

A facile sonochemical approach was used to deposit 3-5 nm monodisperse gold nanoparticles on porous SiO2-WO3 composite spheres, as confirmed by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). High-resolution TEM (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) further characterized the supported Au nanoparticles within the Au-SiO2-WO3 composite. These analyses showed isolated Au nanoparticles within both SiO2- and WO3-containing regions. Selective etching of the SiO2 matrix from Au-SiO2-WO3 yielded a pure Au-WO3 material with well-dispersed 10 nm Au nanoparticles and moderate porosity. This combined sonochemical-nanocasting technique has not been previously used to synthesize Au-WO3 photocatalysts. Methylene blue (MB) served as a probe for the adsorption capacity and visible light photocatalytic activity of these WO3-containing catalysts. Extensive MB demethylation (azures A, B, C, and thionine) and polymerization of these products occurred over WO3 under dark conditions, as confirmed by electrospray ionization mass spectrometry (ESI-MS). Photoirradiation of these suspensions led to further degradation primarily through demethylation and polymerization pathways, regardless of the presence of Au nanoparticles. Ring-opening sulfur oxidation to the sulfone was a secondary photocatalytic pathway. According to UV-vis spectroscopy, pure WO3 materials showed superior MB adsorption compared to SiO2-WO3 composites. Compared to their respective nonloaded catalysts, Au-SiO2-WO3 and Au-WO3 catalysts exhibited enhanced visible light photocatalytic activity toward the degradation of MB. Specifically, the rates of MB degradation over Au-WO3 and Au-SiO2-WO3 during 300 min of irradiation were faster than those over their nonloaded counterparts (WO3 and SiO2-WO3). These studies highlight the ability of Au-WO3 to serve as an excellent adsorbant and photodegradation catalyst toward MB.

A review on catalytic applications of Au/TiO2 nanoparticles in the removal of water pollutant

Nanomaterials are showing great potential for the improvement of water treatment technologies. In recent years, catalysis and photocatalysis processes using gold nanoparticles (Au-NPs) have received great attention due to their effectiveness in degrading and mineralizing organic compounds. This paper aims to review and summarize the recently published works and R & D progress in the field of photocatalytic oxidation of various water pollutants such as toxic organic compounds (i.e. azo dyes and phenols) by Au-NPs/TiO2 under solar, visible and UV irradiation. Extensive research which has focused on the enhancement of photocatalysis by modification of TiO2 employing Au-NPs is also reviewed. Moreover, the effects of various operating parameters on the photocatalytic activity of these catalysts, such as size and loading amount of Au-NPs, pH and calcination, are discussed. The support type, loading amount and particle size of deposited Au-NPs are the most important parameters for Au/TiO2 catalytic activity. Our study showed in particular that the modification of TiO2, including semiconductor coupling, can increase the photoactivity of Au/TiO2. In contrast, doping large gold NPs can mask or block the TiO2 active sites, reducing photocatalytic activity. The optimized loading amount of Au-NP varied for each experimental condition. Finally, research trends and prospects for the future are briefly discussed.

Synthesis, Surface Acidity and Photocatalytic Activity of WO3/TiO2 Nanocomposites – An Overview

Titania has been studied to be one of the best photocatalysts for the decomposition of many organic pollutants present in aqueous medium. Because of the wide band gap energy of TiO2 (3.2 eV) it can be activated only under UV light region which is present 4 % in the solar light spectrum. The photocatalytic activity of TiO2 was enhanced by means of several methods. TiO2 coupled with other semiconductor has gained additional importance, owing to improving the efficient charge separation by trapping the photogenerated electrons. WO3 is a better semiconductor having relatively lower band gap energy (2.8 eV) and absorb broad solar light spectrum. The formation of WOx monolayer on TiO2 notably increases the surface acidity of TiO2. This increasing the surface acidity of WO3/TiO2 photocatalyst facilitates the adsorption of the more hydroxyl group in addition to more organic reactants on its surface which obviously facilitate the enhancement of photocatalytic activity. In this review, the synthesis methods and photocatalytic activity of some selected and unique results related to WO3/TiO2 photocatalyst were discussed. The efficient charge separation, increased absorption of the reactants by increasing the surface acidity and high aspect ratio structures of WO3/TiO2 are also reviewed.

A facile synthesis of flower-shaped TiO2/Ag microspheres and their application in photocatalysts

Novel flower-shaped microspheres (FSMP) TiO₂/Ag nanoparticles composites were fabricated by the facile tyrosine-reduced method and used as a photocatalyst for the degradation of MB under light irradiation. The results indicate that as-prepared composite microspheres have a superior photocatalytic activity attributed to higher surface area, increased light absorption capability, the reduction of electron–hole pair recombination.

Solar photocatalytic activity of TiO2 modified with WO3 on the degradation of an organophosphorus pesticide

In this study, the solar photocatalytic activity (SPA) of WO3/TiO2 photocatalysts synthesized by the sol-gel method with two different percentages of WO3 (2 and 5%wt) was evaluated using malathion as a model contaminant. For comparative purpose bare TiO2 was also prepared by sol-gel process. The powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, diffuse reflectance UV-vis spectroscopy (DRUV-vis), specific surface area by the BET method (SSABET), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy with a high annular angle dark field detector (STEM-HAADF). The XRD, Raman, HRTEM and STEM-HAADF analyses indicated that WO3 was present as a monoclinic crystalline phase with nanometric cluster sizes (1.1 ± 0.1 nm for 2% WO3/TiO2 and 1.35 ± 0.3 nm for 5% WO3/TiO2) and uniformly dispersed on the surface of TiO2. The particle size of the materials was 19.4 ± 3.3 nm and 25.6 ± 3 nm for 2% and 5% WO3/TiO2, respectively. The SPA was evaluated on the degradation of commercial malathion pesticide using natural solar light. The 2% WO3/TiO2 photocatalyst exhibited the best photocatalytic activity achieving 76% of total organic carbon (TOC) abatement after 300 min compared to the 5% WO3/TiO2 and bare TiO2 photocatalysts, which achieved 28 and 47% mineralization, respectively. Finally, experiments were performed to assess 2% WO3/TiO2 catalyst activity on repeated uses; after several successive cycles its photocatalytic activity was retained showing long-term stability.