Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation

Nitrogen and sulfur co-doped and N-doped TiO₂ anatase TKP 102 (Tayca) were prepared by manual grinding with thiourea and urea, respectively, and annealing at 400 °C. Both materials showed visible-light absorption as measured by Diffuse Reflectance Spectroscopy (DRS). Interstitial N-doping, anionic and cationic S-doping was found when the TiO₂ was doped with thiourea while TiO₂ doped with urea showed only the presence of interstitial N-doping as measured by X-ray Photo-electron Spectroscopy (XPS). The N content on the surface of N-doped TKP 102 photocatalyst was 2.85 at.% and higher than the N content in the N, S co-doped TiO₂ photocatalyst (0.6 at.%).The photocatalytic activity of the doped catalysts was tested using phenol and Escherichia coli as chemical and biological targets, respectively, using N, S co-doped, N-doped TiO₂, undoped Degussa P-25 and undoped TKP 102 powders under simulated solar light. It was found that undoped Degussa P-25 was the photocatalyst with the highest photocatalytic activity towards phenol oxidation and E. coli inactivation. N, S co-doped powders showed almost the same photocatalytic activity as undoped TKP 102 while N-doped TKP 102 was the less active photocatalyst probably due the N impurities on the TiO₂ acting as recombination centers.     

Selective photocatalytic destruction of airborne VOCs

This article describes a photocatalytic method for selective oxidation of the airborne nitroglycerine (NG), in the presence of ethanol and acetone vapors at high concentrations (exceeding 1% by volume). Process selectivities toward NG photooxidation were examined for various photoreactor configurations and UV lamps and techniques used for the photocatalyst preparation. In addition, we have studied the effect of temperature, residence time, various additives (e.g. ozone, water vapor, nitrogen), and initial concentration of the solvents in air. Our data indicate that modifying TiO₂ with silico-tungstic acid (STA) catalyst results in selective NG oxidation without affecting ethanol and acetone significantly. Platinization of TiO₂ showed an adverse effect on NG destruction selectivity. In general, low residence times and initial concentration of oxidants and high temperatures and initial concentration of solvents favor selectivity toward NG destruction. In the case of temperature, our observation can be explained by the temperature dependent gas diffusion and surface processes. In most cases, the yield of ethanol oxidation was generally higher than that of acetone. Results from the bench-scale experiments using artificial UV light sources were used to build and test a solar photocatalytic oxidation reactor for selective NG treatment in the presence of ethanol and acetone.     

Molecular engineering control defects within carbon nitride for optimized co-catalyst Pt induced photocatalytic CO2 reduction and NO2 oxidation reaction

We confirmed a specific copolymerization (molecular doping) method for the covalent integration of 2,4-dihydroxyoxazole (DHO) monomer within the framework of carbon nitride (CN). The obtained composites xCN/DHO reveal a sophisticated dual-phase photocatalytic activity, which can effectively reduce and oxidize the CO₂ and NO₂ sources in an aqueous solution and simultaneously performed the oxidation of olefin (CC) in an organic state. This momentous dual state activity is concerned with the lipophilicity elevation from the convolution of oxazole (DHO) monomer within the shell of CN semiconductor. This modulation demonstrates the probability of hydrophobic olefin molecules, escorted in the bulk of CN and associated with the oxidation of hydroxyl radicals (1OH) caused by photogenerated electrons/holes. In this approach, the olefinic compound allusively consumes the photoinduced electrons/holes through elevated CN/DHO, thus stimulating the entire photocatalytic route. Recent research provides a novel strategy for the production of solar fuels upon organic synthesis via the oxidizing capacity of photoinduced holes within free semiconductors of amphiphilic metals. Likewise, the results of the NO₂ photocatalytic reaction demonstrated that molecular doping drastically reduces the oxidative capacity and improves its reducing propensity. More importantly, the CO₂ reduction process supervenes into an extreme aggregation of methane (CH₄) as well as carbon monoxide (CO) in the presence of co-catalyst Pt respectively. The photocatalytic results demonstrate that the copolymerized CN provide the greatest reduction/oxidation potential, which is due to its chemical oxidation phase that causes superior fluctuations in whole performance under solar irradiation.     

Effective degradation of aqueous bisphenol-A using novel Ag2C2O4/Ag@GNS photocatalyst under visible light

Photocatalytic oxidation of toxic pollutants is a proficient technique to solve the problems associated with the treatment of bisphenol-A which is classified as 1B reprotoxic substance. In this paper, Ag₂C₂O₄/Ag@GNS nanocomposite whereas Ag and graphene nanosheets (GNS) used as the charge carriers, which is combined through peroxymonosulfate (PMS) for the removal of bisphenol-A (BiP-A) for the first time. The XRD, UV-DRS, SEM, and TEM studies were performed to confirm the phase structure and the purity. Ag₂C₂O₄/Ag@GNS nanocomposite exhibited superior photocatalytic performance and removal rate when compared with pure Ag₂C₂O₄ and pure GNS. In Ag₂C₂O₄/Ag@GNS photocatalyst, the deposited Ag on the surface of Ag₂C₂O₄ rods effectively formed a metal and semiconductor heterostructure, thus photogenerated charge carriers were separated easily by the surface plasmon resonance effect (SPR) effect of noble Ag. Hence charge carriers lifetime has been extended to a great extent for the better photocatalytic performance. The experimental results confirmed that the ? O₂^?, ? OH, ? SO₄^? radicals were played major role in the photolysis process. Furthermore, the effect of the photocatalyst & PMS concentration, pH and co-existing ions towards the BiP-A degradation were studied in detail. According to the mass spectroscopy studies BiP-A pollutant was effectively deteriorated into smaller molecules and CO₂, H₂O. Furthermore, we have proposed the possible degradation pathway and photocatalytic mechanism for better understanding.     

Photocatalytic degradation of phenol: Comparison between pilot-plant-scale and laboratory results

The photocatalytic oxidation of phenol, using TiO₂ suspensions, has been studied at pilot-plant-scale with solar radiation, at the Plataforma Solar de Almería (PSA), Spain, and at the laboratory level with Xe lamps, at the University of Barcelona (UB). Two different reactor designs were tested at the PSA: a high concentrating radiation systems (Heliomans) and a low concentrating radiation systems (CPCs). Both were characterized from the point of view of the radiation field. The factors relating the solar radiation arriving at the radiation entering the photoreactors have been determined.Kinetic experiments were performed. In all the cases, the reaction rate shows linear dependence on the square root of the photonic flow entering the photoreactors. The kinetics is the first order with respect to the phenol concentration. The kinetic constants have been determined and compared for all the systems tested (laboratory and pilot-plant-scale). The efficiencies of Heliomans and CPCs modules have been compared.     

Influence of exposed facets,morphology and hetero-interfaces of BiVO4 on photocatalytic water oxidation: A review

The photocatalytic water splitting has a stupendous role to generate renewable hydrogen. However, the overall water splitting reaction is limited by the sluggish oxidation step. Bismuth vanadate (BiVO₄) has been identified as one of the most promising n-type semiconductors for photocatalytic water oxidation due to its small energy gap and favorable band alignment. Thus, it is necessary to summarize the recent progress done on BiVO₄ for the guidance of future research. In this review, we have discussed recent strategies that have been adopted to boost its photoconversion efficiency at three different levels: (1) facet control, (2) morphological control and, (3) interfacial control. The roles of high indexed facets, anisotropic morphologies, and mediator-based Z schemes are comprehensively discussed and the emphasis is given to find the suitable structural, morphological, and interfacial combination of BiVO₄ for efficient water oxidation.     

Integration of ReS2 on ZnIn2S4 for boosting the hydrogen evolution coupled with selective oxidation of biomass intermediate under visible light

Challenge remains to develop a high activity of photocatalyst for large-scale industrial application in photocatalytic selective conversion of biomass alcohols into the value-added chemicals accompany with H₂ evolution in aqueous solution. Herein, ReS₂ as high efficiency co-catalyst is utilized to modify the flower-like ZnIn₂S₄ (ZIS) microspheres to obtain heterojunction composite, result in dramatically enhancements in photocatalytic oxidation of furfural alcohols cooperative with H₂ evolution. Further studies show that the optimal catalyst containing 4.08 wt% ReS₂ (RZIS-3) realize remarkably generation rates of H₂ and furfural at 3092.9 and 2981.1 μmol g^?1 h^?1, respectively, nearly 12 times faster than that of blank ZnIn₂S₄. Mechanism studies verify that the migration of the photogenerated carriers from ZnIn₂S₄ to ReS₂ leading to the remarkably photoactivity of the composite. Moreover, the typical photocatalysis not rely on a single model substrate, and high performance of the composite has been identified for the oxidation of other alcohols biomass intermediate to value-added aldehydes/ketones, providing a new insight for design and fabrication of the novel photocatalytic hydrogen evolution systems.     

Photocatalytic oxidation of nitrite by sunlight using TiO2 supported on hollow glass microbeads

The feasibility of photocatalytic oxidation of nitrite using TiO₂ supported on hollow glass microbeads as a photocatalyst by sunlight was studied. The results showed that 1.1×10⁻⁴ mol/dm³ of nitrite can be completely photocatalytically oxidized after 120-min illumination by sunlight. The conversion of nitrite was increased rapidly by adding a small amount of H₂O₂, Fe³⁺ and Pd²⁺. The effects of parameters such as the amount of TiO₂/beads, air flow, initial pH, dichromate, SO₄²⁻, NO₃⁻, Cl⁻ and organic compounds on the photocatalytic oxidation of nitrite were also studied. The possible mechanisms of photocatalytic oxidation of nitrite were investigated. After 150-h illumination by sunlight, there was no significant loss of the photocatalytic activity of TiO₂/beads.     

Reactive species involved in photocatalytic transformations on zinc oxide

The main oxidizing species involved in photocatalytic transformations on ZnO are hydroxyl radicals and positive holes. Singlet oxygen, hydroperoxyl radicals and superoxide ions play a minor role. Several compounds such as furfuryl alcohol, sodium 1,3-cyclohexadiene-1,4-diethanoate (CHDDE), furoin, 1,4-phenylene diacetic acid and 2,5-furandimethanol were used as probe molecules. The formation of a stable endoperoxide from CHDDE is a better test to check the presence of singlet oxygen than the oxidation of furfuryl alcohol. The pathways non inhibited by ethanol or isopropanol are attributed to positive holes h⁺ formed on the surface of the photoconductive oxide. Both species (^.OH and h⁺) have different regiospecificities. With phenolic derivatives the oxidation by h⁺ is located at para position, whereas OH^.H reacts at both ortho and para positions, but mainly at ortho. Substituents in para position can be oxidized by both h⁺ and OH.Benzoquinone^.H.Benzoquinone and methylbenzoquinone are photoreduced on ZnO and on TiO₂ into the corresponding hydroquinones. They can be used as sacrificial electron scavengers to replace oxygen in photocatalytic oxidations.     

Anchoring single Pt atoms on hollow Ag3VO4 spheres for improved activity towards photocatalytic H2 evolution reaction

The high cost of noble metal catalysts has been a great bottleneck for the catalyst industry. Using the noble metal at a single-atom level for catalytic applications could dramatically decrease the cost. The impacts of single Pt atoms on the photocatalytic performance of Ag₃VO₄ have been investigated and reported. In this report, single Pt atoms were anchored on the surface of Ag₃VO₄ (AVO) as a cocatalyst, and the resultant composite photocatalyst has been studied for photocatalytic H₂ production from water driven by visible light. The as-prepared AVO particles are hollow nanospheres in the monoclinic phase with a bandgap of 2.20 eV. The light absorption edge of AVO/Pt is slightly red-shifted compared to that of the pristine AVO, indicating more visible light absorption of AVO/Pt. The XPS peaks of Ag, V, and Pt exhibit a significant shift after AVO and Pt get into contact, suggesting the strong interaction between the surface Ag and V atoms, and single Pt atoms. After 3-h illumination, the photocatalytic H₂ evolution amount from AVO/Pt is improved up to 1400 μmol, which is 2.8 times that on the bare AVO. Such efficient photocatalytic H₂ evolution on AVO/Pt is still maintained after five reaction cycles. The better photocatalytic performance of AVO/Pt has been attributed to the more efficient visible light utilization and the lower interfacial charge transfer resistance, as demonstrated in the DRS and EIS spectra. The presence of the surface Pt atoms also leads to a higher amount of reactive radicals, which could efficiently promote the surface redox reactions.     

Experimental study of homogeneous Hg oxidation in air and oxy-simulated flue gas

This study investigated the effects of Cl₂, SO₂, and NO on the mercury (Hg) speciation during oxy-combustion and compared it with the Hg speciation in air-simulated flue gas with Cl₂. Experiments were conducted in a bench-scale device at 200 °C. The results of Hg oxidation in an N₂ and CO₂ atmosphere with Cl₂ showed that CO₂ indirectly restrained Hg oxidation. Oxy-simulated flue gas promoted Hg oxidation more than air-simulated flue gas promoted that, because the oxygen in oxy-simulated flue gas indirectly promoted Hg oxidation using Cl₂. The percentage of Hg oxidized in both oxy-simulated flue gas and air-simulated flue gas with NO decreased as the concentration of Cl₂ increased because NO restrained Hg oxidation with Cl₂ through the elimination of the O and ClO radicals. SO₂ inhibited Hg oxidation with Cl₂ by consuming the O radicals. Moreover, when both NO and SO₂ were present in oxy-simulated flue gas with Cl₂, the effect of SO₂ on Hg oxidation was related to the NO concentration.     

Multiphase TiO2 surface coating g-C3N4 formed a sea urchin like structure with interface effects and improved visible-light photocatalytic performance for the degradation of ibuprofen

In this research, we did not use any template, but a prickly structure urchin like morphology made of composite TiO₂/g-C₃N₄ was synthesized by a one-step solvothermal method. It is generally reported that TiO₂ is a pure phase, besides, the photocatalytic effect of multiphase TiO₂ will be better. After the g-C₃N₄ was successfully loaded on the TiO₂ surface, the morphology did not change. The special sea urchin morphology provides more active sites for catalysis. The band gap becomes smaller, because two materials were combined to form an interface effect. Meanwhile, it could effectively separate electron hole pairs, and promote charge transfer efficiency. More active substances were produced under visible light, such as superoxide radicals and holes. The scavenging experiment further confirms that superoxide radicals and holes play an important role in the catalytic process. The photocatalytic degradation of ibuprofen under visible light was improved. A possible enhanced photocatalytic mechanism of g-C₃N₄/TiO₂ heterojunction photocatalysts was proposed.     

Photocatalytic conversion of ethane: status and perspective

Ethane (C₂H₆) is a main component of natural gas and a notable contributor to photochemical pollution and ozone production in the atmosphere. It is important to convert ethane to useful chemicals. The photocatalytic conversion of ethane is promising but challenging. As the first review article in this area, we summarize the recent important progresses in photocatalytic ethane conversion, with an emphasis on (1) homogeneously functionalization and (2) heterogeneously partial oxidation of ethane. Furthermore, the challenges and future directions are provided for the photocatalytic conversion of ethane.     

Photocatalytic degradation of organic compounds in aqueous solution by a TiO2-coated rotating-drum reactor using solar light

This paper deals with the degradation of aqueous phenol by a newly proposed rotating-drum reactor coated with a TiO₂ photocatalyst, in which TiO₂ powders loaded with Pt are immobilized on the outer surface of a glass-drum. The reactor can receive solar light and oxygen from the atmosphere effectively. It was shown experimentally that phenol can be decomposed rapidly by this reactor under solar light: with our experimental conditions the phenol with an initial concentration of 22.0 mg/dm³ was decomposed within 60 min and was completely mineralized through intermediate products within 100 min. The photonic efficiency under solar light was shown to take the value 0.00742 mol-C/Einstein. The photocatalytic decomposition processes of phenol by this reactor were also discussed on the basis of the Langmuir–Hinshelwood kinetic model.     

The enhancement of redox reactions with mixed oxide catalysts by the sol–gel process

The enhancement of the photocatalytic activity of TiO₂ was investigated as a function of added amount of other oxides to promote desired oxidation or reduction reactions. Mixed oxides of Nb₂O₅ or Li₂O with TiO₂ were prepared by the sol–gel process. The target material of dichloroacetic acid (DCA) was chosen for oxidation reactions and K₂Cr₂O₇ for reduction. While the Nb₂O₅/TiO₂ had a deleterious effect on the decomposition rate of DCA, the excess electrons due to the doping of Nb₂O₅ into TiO₂ promoted the reduction process for Cr⁶⁺. Li₂O (1 wt%) with TiO₂ was found the most efficient photocatalyst for DCA oxidation, resulting in photocatalytic activity of 50%.     

Solar photocatalytic treatment of simulated dyestuff effluents

The photocatalytic organic content reduction of two selected synthetic wastewater from the textile dyeing industry, by the use of heterogeneous and homogeneous photocatalytic methods under solar irradiation, has been studied at a pilot plant scale at the Plataforma Solar de Almeria. The effect of two different TiO₂ modifications with oxidants such as H₂O₂ and Na₂S₂O₈ on the decolourisation and the dissolved organic content reduction (DOC) of the wastewater was examined. The TiO₂/H₂O₂ system seems to be more efficient in comparison to the synergetic action that appears when using persulphate and TiO₂ in this specific wastewater. By an accumulated energy of 50 kJ l⁻¹ the synergetic effect of TiO₂ P-25 with H₂O₂ and Na₂S₂O₈ leads to a 70% and 57% DOC reduction, respectively, in the case of cotton synthetic wastewater, while the decolourisation was almost complete. The photocatalytic decolourisation, as well as the DOC reduction in the case of nylon simulated wastewater is a slower process and an accumulated energy of 50 kJ l⁻¹ leads to almost 54% mineralisation in both cases. The photo-Fenton process in both types of wastewater was more efficient in comparison to the TiO₂/oxidant system. An accumulated energy of 50 kJ l⁻¹ leads to 90% reduction of the organic content.     

In situ growing Cu2(OH)2CO3 on oxidized carbon nitride with enhanced photocatalytic hydrogen evolution and pollutant degradation

A novel composite has been successfully synthesized in situ via a coprecipitation method about the coupling of Cu₂(OH)₂CO₃ with oxidized carbon nitride (O-g-C₃N₄) forming Cu₂(OH)₂CO₃/O-g-C₃N₄ (CuCN) heterojunction structure. The as-prepared composites were characterized by diverse means. The CuCN composite with 3:5 mass ratio of Cu₂(OH)₂CO₃ to O-g-C₃N₄ (60CuCN) presented an extremely excellent photocatalytic activity. The photocatalytic H₂ evolution of 60CuCN was around 23.26 and 44.62 times higher than that of g-C₃N₄ and Cu₂(OH)₂CO₃, respectively. The photocatalytic degradation malachite green (MG) rate of 60CuCN was up to 91%, which was around 2.2 and 4.8 times as much as that of g-C₃N₄ and Cu₂(OH)₂CO₃, respectively. These results are mainly attributed to the structure property of O-g-C₃N₄ and the heterojunction structure of the composite, which could effectively accelerate the separation and transfer rate of photogenerated electrons and holes. The holes (h⁺) and superoxide radicals (·O₂⁻) played a dominant role in photocatalytic degradation MG reaction.     

Constructing direct Z-scheme photocatalysts with black N–TiO2-x/C and Cd0.5Zn0.5S for efficient H2 production

Black N doped TiO_2-x nanoparticles loaded on the carbon framework (N–TiO_2-x/C, NTC) were successfully fabricated by sol-gel method. In this work, the synthesized NTC material was combined with Cd_0.5Zn_0.5S (ZCS) short nanorods, forming a direct Z-scheme NTC/ZCS photocatalyst without using of electron mediator. The free radicals trapping experiment was also conducted through the degradation of RhB to determine photocatalytic mechanism. As a result, the 2% NTC/ZCS exhibited an optimal photocatalytic H₂ production rate which is up to 36.6 mmol/h/g and about 5.2 times higher than that of pristine ZCS nanorods. The apparent quantum efficiency (AQE) of the 2% NTC/ZCS hybrids at wavelength 420 nm was calculated to 51.2%. Moreover, long-term drying increases the close contact between NTC and ZCS, and this firm contact interface can promote charge transfer and improve photocatalytic stability of NTC/ZCS composite.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.     

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO₄ has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO₄ yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO₄ photoanode coupled with FeOOH (W–BiVO₄/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO₄. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO₄/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm², which is seven folds higher than that of the pristine BiVO₄ photoanode (0.31 mA/cm² 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO₄/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO₄/FeOOH photoanode.