Application of doped photocatalysts for organic pollutant degradation - A review

Ag loaded B-doped-g C3N4 nanosheet with efficient properties for photocatalysis

Three material engineering strategies in the form of doping (Boron-doping), nanostructuring (nanosheet (NS) formation) and decorating with plasmonic nanoparticles (loading with Ag metal), were integrated to improve the photocatalytic activity of graphitic carbon nitride (gC₃N₄). Concentrations of B-doping and Ag-loading were optimized to maximize the catalytic performance in the final nanocomposite of Ag-loaded B-doped gC₃N₄ NS. Combined effect of all three strategies successfully produced over 5 times higher rate towards degradation of organic dye pollutant, when compared to unmodified bulk gC₃N₄. Detailed characterization results revealed that incorporation of B in gC₃N₄ matrix reduces the band gap to increase the visible light absorption, while specific surface area is significantly enhanced upon formation of NS. Decoration of Ag nanoparticles (NPs) on B-doped gC₃N₄ NS assists in fast transfer of photogenerated electrons from gC₃N₄ to Ag NPs owing to the interfacial electric field across the junctions and thus reduces the recombination process. Investigations on individual strategies revealed that decoration of Ag NPs to induce better charge separation, is the most effective route for enhancing the photocatalytic activity.

Investigation of photocatalytic behavior of modified ZnS:Mn/MWCNTs nanocomposite for organic pollutants effective photodegradation

In this research, zinc sulfide (ZnS) doped with manganese (Mn) is synthesized on functionalized multiwall carbon nanotubes (MWCNTs) nanocomposite by a facile co-precipitation method as the photocatalysis. Due to the excellent electrical and optical characteristics of ZnS:Mn/MWCNTs nanocomposite, it is worth to investigate its photodegradation activity. To investigate the photocatalytic degradation properties of organic pollutants, the synthesis conditions were optimized in the presence of four substances: COOH, ethylene glycol, sodium dodecyl sulfate, and polyvinyl-pyrrolidone. Surface studies of the photocatalyst, i.e., structural, morphological, optical and physical properties, were characterized by FTIR, PL, XRD, SEM, and TEM analyses. The results showed that Mn ions decreased the band gap energy of the nanocomposites and there was excellent adhesion between ZnS and MWCNTs in the synthesized composite. According to the results, MWCNTs effectively increased the photocatalytic activity of the ZnS nanoparticles by the electron-hole pair recombination of ZnS and MWCNTs, and the composites with the carboxylic functional group showed greater photocatalytic activity. In addition, the kinetic studies showed that the photocatalytic process obeyed the pseudo-first-order kinetic model. To determine the exact mathematical formula of the photocatalysis, response surface methodology was modeled by the central composite design method. Various parameters, such as the time of the treatment process and initial concentration of the pollutants were studied for a quadratic model that fit all the cases well and their mathematical models were obtained.

Recent Advances in TiO2 Films Prepared by Sol-gel Methods for Photocatalytic Degradation of Organic Pollutants and Antibacterial Activities

Photocatalysis has recently emerged as an advanced, green, and eco-friendly process for the treatment of wastewater and air, and antimicrobial disinfection applications. In this context, TiO2 nanostructures have been shown to be the prominent photocatalyst candidates due to their low cost, non-toxicity, and ease of fabrication. This review highlights the investigation and development of TiO2 photocatalyst film by sol-gel method with special emphasis on the photodecolorization of synthetic dyes and antibacterial activities. Furthermore, various synthesis methods for the preparation of TiO2 films and their advantages, as well as limitations, are summarized. Finally, recent advances in TiO2 films by sol-gel method for dye degradation and antibacterial activities, challenges, and future perspective are discussed.

Zr-Modified ZnO for the Selective Oxidation of Cinnamaldehyde to Benzaldehyde

ZnO and Zr-modified ZnO were prepared using a precipitation method and used for the selective oxidation of cinnamaldehyde to benzaldehyde in the present study. The results showed that physicochemical properties of ZnO were significantly affected by the calcination temperature, and calcination of ZnO at 400 °C demonstrated the optimum catalytic activity for the selective oxidation of cinnamaldehyde to benzaldehyde. With 0.01 g ZnO calcined at 400 °C for 2 h as a catalyst, 8.0 g ethanol and 2.0 g cinnamaldehyde reacted at an oxygen pressure of 1.0 MPa and 70 °C for 60 min, resulting in benzaldehyde selectivity of 69.2% and cinnamaldehyde conversion of 16.1%. Zr was the optimal modifier for ZnO: when Zr-modified ZnO was used as the catalyst, benzaldehyde selectivity reached 86.2%, and cinnamaldehyde conversion was 17.6%. The X-ray diffractometer and N2 adsorption–desorption characterization indicated that doping with Zr could reduce the crystallite size of ZnO (101) and increase the specific surface area of the catalyst, which provided more active sites for the reaction. X-ray photoelectron spectrometer results showed that Zr-doping could exchange the electrons with ZnO and reduce the electron density in the outer layer of Zn, which would further affect benzaldehyde selectivity. The results of CO2 temperature-programmed desorption showed that Zr-modification enhanced the alkalinity of the catalyst surface, which caused the Zr–ZnO catalyst to exhibit higher catalytic activity.

Preparation of TiO2 and Fe-TiO2 with an Impinging Stream-Rotating Packed Bed by the Precipitation Method for the Photodegradation of Gaseous Toluene

Nano-TiO₂ has always been one of the most important topics in the research of photocatalysts due to its special activity and stability. However, it has always been difficult to obtain nano-TiO₂ with high dispersion, a small particle size and high photocatalytic activity. In this paper, nano-TiO₂ powder was prepared by combining the high-gravity technique and direct precipitation method in an impinging stream-rotating packed bed (IS-RPB) reactor followed by Fe³⁺ in-situ doping. TiOSO₄ and NH₃·H₂O solutions were cut into very small liquid microelements by high-speed rotating packing, and the mass transfer and microscopic mixing of the nucleation and growth processes of nano-TiO₂ were strengthened in IS-RPB, which was beneficial to the continuous production of high quality nano-TiO₂. Pure TiO₂ and iron-doped nano-TiO₂ (Fe-TiO₂) were obtained in IS-RPB and were investigated by means of X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and Brunauer-Emmett-Teller (BET) analysis, which found that pure TiO₂ had a particle size of about 12.5 nm, good dispersibility and a complete anatase crystal at the rotating speed of packing of 800 rpm and calcination temperature of 500 °C. The addition of Fe³⁺ did not change the crystalline structure of TiO₂. Iron was highly dispersed in TiO₂ without the detection of aggregates and was found to exist in a positive trivalent form by XPS. With the increase of iron doping, the photoresponse range of TiO₂ to visible light was broadened from 3.06 eV to 2.26 eV. The degradation efficiency of gaseous toluene by Fe-TiO₂ under ultraviolet light was higher than that of pure TiO₂ and commercial P25 due to Fe³⁺ effectively suppressing the recombination of TiO₂ electrons and holes; the highest efficiency produced by 1.0% Fe-TiO₂ was 95.7%.

Development of Smart Composites Based on Doped-TiO2 Nanoparticles with Visible Light Anticancer Properties

In this study, the synthesis of smart, polymerically embedded titanium dioxide (TiO2) nanoparticles aimed to exhibit photo-induced anticancer properties under visible light irradiation is investigated. The TiO2 nanoparticles were prepared by utilizing the sol gel method with different dopants, including nitrogen (N-doped), iron (Fe-doped), and nitrogen and iron (Fe,N-doped). The dopants were embedded in an interpenetrating (IP) network microgel synthesized by stimuli responsive poly (N-Isopropylacrylamide-co-polyacrylicacid)-pNipam-co-PAA forming composite particles. All the types of produced particles were characterized by X-ray powder diffraction, micro-Raman, Fourier-transform infrared, X-ray photoelectron, ultra-violet-visible spectroscopy, Field Emission Scanning Electron, Transmission Electron microscopy, and Dynamic Light Scattering techniques. The experimental findings indicate that the doped TiO2 nanoparticles were successfully embedded in the microgel. The N-doped TiO2 nano-powders and composite particles exhibit the best photocatalytic degradation of the pollutant methylene blue under visible light irradiation. Similarly, the highly malignant MDA-MB-231 breast cancer epithelial cells were susceptible to the inhibition of cell proliferation at visible light, especially in the presence of N-doped powders and composites, compared to the non-metastatic MCF-7 cells, which were not affected.

Synthesis and characterization of Ag2O/B2O3/TiO2 ternary nanocomposites for photocatalytic mineralization of local dyeing wastewater under artificial and natural sunlight irradiation

In this work, Ag₂O/B₂O₃/TiO₂ ternary nanocomposite was synthesized by a combination of green and precipitation method involving mixing of different concentrations of silver nitrate, boric acid, and titanium (IV) isopropoxide precursor with Plumeria acuminate leaf extract. The extract was obtained by boiling the mixture of distilled water and the powdered leaves in a beaker for few minutes followed by filtration. The microstructure, morphology, chemical composition, surface area, phase structure, and optical properties of the various prepared nanomaterials were determined by HRTEM, HRSEM, UV-Vis/DRS, BET, XRD, and XPS. The photocatalytic potential of TiO₂ nanoparticles and Ag₂O/B₂O₃/TiO₂ nanocomposites to degrade local dyeing wastewater under artificial and natural sunlight irradiation was investigated. The extent of degradation of the organic pollutants was measured using chemical oxygen demand (COD) and total organic carbon (TOC) as indicator parameters. The XRD pattern of Ag₂O/B₂O₃/TiO₂ nanocomposites revealed that the formation of pure anatase TiO₂ phase and the addition of both silver and boron precursors did not influenced the phase structure of the nanocomposites. The oxidation states of +1 and +3 for both Ag and B on the surface of Ag₂O/B₂O₃/TiO₂ nanocomposites were confirmed by XPS. Optical characterization of the sample revealed reduction of band gap energy from 2.6 to 2.0 eV for TiO₂ and Ag₂O/B₂O₃/TiO₂, respectively. The Ag₂O/B₂O_3/TiO₂ nanocomposites demonstrated excellent photocatalytic activity under natural sunlight and artificial light than mono and binary oxide systems with TOC and COD degradation efficiencies of 86.11% and 75.69%, respectively. The kinetics of degradation of organic dyes in the wastewater followed the order of Langmuir-Hinshelwood pseudo-first-order > Freundlich > Zero > Parabolic diffusion model. The coupling effect of Ag₂O and B₂O₃ onto TiO₂ framework was responsible for the enhanced photochemical stability of the nanocomposites even after five repeated cycles.

Water depollution using metal-organic frameworks-catalyzed advanced oxidation processes: A review

This paper presents a review on the environmental applications of metal-organic frameworks (MOFs), which are inorganic-organic hybrid highly porous crystalline materials, prepared from metal ion/clusters and multidentate organic ligands. The emphases are made on the enhancement of the performance of advanced oxidation processes (AOPs) (photocatalysis, Fenton reaction methods, and sulfate radical (SO₄^-)-mediated oxidations) using MOFs materials. MOFs act as adsorption and light absorbers, leading to superior performance of photocatalytic processes. More recent examples of photocatalytic degradation of dyes are presented. Additionally, it is commonly shown that Fe-based MOFs exhibited excellent catalytic performance on the Fenton-based and SO₄^•--mediated oxidations of organic pollutants (e.g., dyes, phenol and pharmaceuticals). The significantly enhanced generation of reactive species such as OH and/or SO₄^- by both homogeneous and heterogeneous catalysis was proposed as the possible mechanism for water depollution. Based on the existing literature, the challenge and future perspectives in MOF-based AOPs are addressed.

Decolorization of Orange-G Aqueous Solutions over C60/MCM-41 Photocatalysts

The majority of the photocatalysts studied for azo-dye degradation are based on semiconductor materials. Studies reported on non-semiconducting materials are very scarce. In the present work, we studied the fullerene (C60) ability to accelerate photodegradation of the dye’s azo bond in the presence of ascorbic acid. A series of C60 supported on ordered mesoporous silica (MCM-41) catalysts, containing 1, 3, 6, 9, and 12 wt % of fullerene C60, was studied using Orange G (OG) as representative azo-dye. This study showed that partial decolorization is achieved in the dark by simple adsorption of the dye on the bare surface of the carrier. The extent of decolorization increases with the irradiation of the suspension due to photocatalytic degradation of the azo-bond. This is maximized over the sample containing 3 wt % of C60 and it has been attributed to the best combination of the extent of the dye adsorption with the high intrinsic photocatalytic activity of small C60 clusters predominated in this sample. This catalyst proved to be quite stable upon five subsequent photocatalytic cycles, losing less than 5% of its initial activity. No degradation of OG takes place in the absence of ascorbic acid.

Enhanced photocatalytic performance of magnetic multi-walled carbon nanotubes/cerium dioxide nanocomposite

An easily separated photocatalyst, magnetic multi-walled carbon nanotubes/cerium dioxide (MMWCNTs-CeO₂) nanocomposite, has been successfully prepared by hydrothermal synthesis and subsequent loading of magnetic nanoparticles. The synthesized nanocomposite was characterized by scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, and vibrating sample magnetometer. The characterization results indicated the successful synthesis of MMWCNTs-CeO₂ nanocomposite. Photocatalytic degradation experiments were conducted to evaluate photocatalytic properties of MMWCNTs-CeO₂ by using methylene blue (MB) as a model pollutant. After a light irradiation of two hours, relatively high degradation efficiency (97.5%) of MB was achieved by using MMWCNTs-CeO₂ nanocomposite as photocatalyst in the presence of H₂O₂. The incorporation of magnetic nanoparticles could not only facilitate the separation of photocatalyst from the solution after treatment, but also enhance the photocatalytic degradation efficiency. The results of this study suggested that the synthesized MMWCNTs-CeO₂ nanocomposite showed an attractive prospect for application in the treatment of organic wastewater.

Metal/Semiconductor Nanocomposites for Photocatalysis: Fundamentals, Structures, Applications and Properties

Due to the capability of utilizing light energy to drive chemical reactions, photocatalysis has been widely accepted as a green technology to help us address the increasingly severe environment and energy issues facing human society. To date, a large amount of research has been devoted to enhancing the properties of photocatalysts. As reported, coupling semiconductors with metals is one of the most effective methods to achieve high-performance photocatalysts. The excellent properties of metal/semiconductor (M/S) nanocomposite photocatalysts originate in two aspects: (i) improved charge separation at the metal-semiconductor interface; and (ii) increased absorption of visible light due to the surface plasmon resonance of metals. So far, many M/S nanocomposite photocatalysts with different structures have been developed for the application in environmental remediation, selective organic transformation, hydrogen evolution, and disinfection. Herein, we will give a review on the M/S nanocomposite photocatalysts, regarding their fundamentals, structures (as well as their typical synthetic approaches), applications and properties. Finally, we will also present our perspective on the future development of M/S nanocomposite photocatalysts.

N–TiO2 Photocatalysts: A Review of Their Characteristics and Capacity for Emerging Contaminants Removal

Titanium dioxide is the most used photocatalyst in wastewater treatment; its semiconductor capacity allows the indirect production of reactive oxidative species. The main drawback of the application of TiO2 is related to its high band-gap energy. The nonmetal that is most often used as the doping element is nitrogen, which is due to its capacity to reduce the band-gap energy at low preparation costs. There are multiple and assorted methods of preparation. The main advantages and disadvantages of a wide range of preparation methods were discussed in this paper. Different sources of N were also analyzed, and their individual impact on the characteristics of N–TiO2 was assessed. The core of this paper was focused on the large spectrum of analytical techniques to detect modifications in the TiO2 structure from the incorporation of N. The effect of N–TiO2 co-doping was also analyzed, as well as the main characteristics that are relevant to the performance of the catalyst, such as its particle size, surface area, quantum size effect, crystalline phases, and the hydrophilicity of the catalyst surface. Powder is the most used form of N–TiO2, but the economic benefits and applications involving continuous reactors were also analyzed with supported N–TiO2. Moreover, the degradation of contaminants emerging from water and wastewater using N–TiO2 and co-doped TiO2 was also discussed.

Removal of acetylsalicylate and methyl-theobromine from aqueous environment using nano-photocatalyst WO3-TiO2 @g-C3N4 composite

Highly efficient, visible light-driven and a novel ternary hybrid photocatalyst WO₃-TiO₂-g-C₃N₄ with robust stabilities and versatile properties has been synthesized through facile hydrothermal method. This study considers the photo-degradation of aspirin (acetylsalicylate) and caffeine (methyl-theobromine) via photocatalysts (WO₃, WO₃/TiO_2, and WO₃/TiO₂/g-C₃N₄ (WTCN) composite) under visible-light irradiation. The SEM and TEM images show the formation of WO₃ nanoparticles with orthorhombic structure and average particle size of 65 nm. The photocatalyst WTCN composite possesses higher-catalytic activity when compared to that of WO₃ and WO₃/TiO₂ for degradation of aspirin and caffeine. The incorporation of g-C₃N₄ in WO₃/TiO₂ composite exhibited significant influence on the photocatalytic performance for both pollutants. Excellent photocatalytic performance of WTCN composite was observed owing to hydroxyl radical (OH) and superoxide radical (O₂^-) as main active species. The enhanced photocatalytic activity of WTCN composite can be attributed to following three reasons: (1) extended visible-light absorption; (2) extended surface area; (3) efficient charge-separation due to synergistic effects between g- and WO₃/TiO₂ composite. The removal efficiency of aspirin and caffeine (Methyl theobromine) could be achieved as much as 98% and 97% for acetylsalicylate and methyl-theobromine using WTCN composite material, respectively. This study could provide new insights into the synthesis of novel WO₃-based materials for environmental and energy applications.

Urban wastewater treatment by using Ag/ZnO and Pt/TiO2 photocatalysts

In this study, the treatment of wastewater coming from a river highly polluted with domestic and industrial effluents was evaluated. For this purpose, series of photocatalysts obtained by ZnO and TiO₂ modification were evaluated. The effect of metal addition and Ti precursor (in the case of the titania series) over the physicochemical and photocatalytic properties of the materials obtained was also analyzed. The evaluation of the photocatalytic activity showed that semiconductor modification and precursor used in the materials synthesis are important factors influencing the physicochemical and therefore the photocatalytic properties of the materials obtained. The water samples analyzed in the present work were taken from a highly polluted river, and it was found that the effectiveness of the photocatalytic treatment increases when the reaction time increases and for both, wastewater samples and isolated Escherichia coli strain follow the next order Pt/TiO₂ << ZnO. It was also observed that biochemical and chemical demand oxygen and turbidity significantly decrease after treatment, thus indicating that photocatalysis is a non-selective technology, which can lead to recover wastewater containing different pollutants.

Catalytic degradation of O-cresol using H2 O2 onto Algerian Clay-Na

Clay material is used as a catalyst to degrade an organic pollutant. This study focused on the O-cresol oxidative degradation in aqueous solution by adding H₂ O₂ and Mont-Na. The catalytic tests showed a high catalytic activity of Mont-Na, which made it possible to achieve more than 84.6% conversion after 90 min of reaction time at 55°C in 23.2 mM H₂ O₂ . The pH value was found to be negatively correlated with the degradation rate of O-cresol. UV-Vis spectrophotometry revealed that the increase of degradation rate at low pH is related to the formation of 2-methylbenzoquinone as intermediate product. In addition, the content of iron in Mont-Na decreased after the catalytic test, bringing further evidence about the O-cresol catalytic oxidation. The mineralization of O-cresol is also confirmed by the different methods of characterization of Mont-Na after the catalytic oxidation test. The effect of the O-cresol oxidation catalyzed by natural clay is significant. PRACTITIONER POINTS: Algerian Montmorillonite-Na is used as a catalyst to degrade an organic pollutant: O-cresol. It shows a great potential for catalyst properties in the presence of the oxidizing reagent H₂ O₂ . It proved to be an effective means for the degradation of O-cresol contained in wastewaters.

Bridging the gap between laboratory and application in photocatalytic water purification

Despite a large number of publications in the field, photocatalytic water treatment is still somewhat disconnected from real world application and we highlight recent developments to address this.

Ce-doped UiO-67 nanocrystals with improved adsorption property for removal of organic dyes

In this study, we report the synthesis of Ce-doped UiO-67 nanocrystals via one-step hydrothermal method and their potential use for waste water treatment to remove organic dyes. The as-prepared samples were characterized by using SEM, TEM, FT-IR, XRD, XPS and TG techniques and the results demonstrate the formation of the framework structure of the Ce-doped UiO-67. The adsorption study of the material shows that the Ce-doped UiO-67 nanoparticles preferentially adsorb the cationic dyes such as rhodamine B and malachite green rather than the anionic dyes such as methyl orange. Adsorption capacities are 754.4, 589.2 and 191.6 mg g⁻¹ for rhodamine B, malachite green and methyl orange respectively. Based on its zeta potential and adsorption isotherm, it is understood that Ce doping increases its electrostatic interactions, and as a consequence, improves the adsorption capacity for cationic dyes. In addition, it is found that a pseudo-second-order kinetics and the Langmuir isothermal model were suitable for describing the adsorption behavior of cationic dyes onto the Ce-doped UiO-67.

The Ce-doped UiO-67 nanocrystals were successfully synthesized via a one-step hydrothermal method. Ce doping increases the negative charge on the surface of the material, thus the adsorbent exhibits high adsorption capacity to cationic dyes.

Polyethylene glycol-doped BiZn2VO6 as a high-efficiency solar-light-activated photocatalyst with substantial durability toward photodegradation of organic contaminations

In this study, we focus on a simple, low-priced, and mild condition hydrothermal route to construct BiZn₂VO₆ nanocompounds (NCs) as a novel photocatalyst with strong solar light absorption ability for environmental purification using solar energy. NCs were further doped with polyethylene glycol (PEG) to improve their photocatalytic efficiency for photodegradation processes through inhibition of fast charge carrier recombination rates and higher charge separation efficiency. Surface morphology, phase structure, optical characteristics, and band structure of the as-prepared samples were analyzed using XRD, EDX, XPS, SEM, UV-vis spectroscopy, CL, and BET techniques. PEG-doped BiZn₂VO₆ NCs were applied as effective materials to degrade various kinds of organic pollutants including cationic and anionic types, and these NCs exhibited excellent photocatalytic efficiency as compared to traditional photocatalysts. In particular, the PEG-doped BiZn₂VO₆ (0.10% w/v) photocatalyst exhibited highly enhanced photocatalytic performance with improvements of about 46.4, 28.3, and 7.23 folds compared with PEG-doped ZnO nanorods (NRs), pristine BiVO₄, and BiZn₂VO₆ samples, respectively, for the decomposition of congo red (CR) dye. After 40 minutes of sunlight irradiation, 97.4% of CR was decomposed. In this study, scavenging experiments indicated that both hydroxyl radicals and holes play dominant roles in CR photodegradation under simulated solar light irradiation. Meanwhile, the optimal photocatalyst demonstrated good reproducibility and stability for successive cycles of photocatalysis.

Titanium Dioxide/Graphene and Titanium Dioxide/Graphene Oxide Nanocomposites: Synthesis, Characterization and Photocatalytic Applications for Water Decontamination

The use of titanium dioxide, TiO2 as a photocatalyst in water decontamination has witnessed continuous interest due to its efficiency, stability, low toxicity and cost-effectiveness. TiO2 use is limited by its large band gap energy leading to light absorbance in the UV region of the spectrum, and by the relatively fast rate of recombination of photogenerated electrons and positive holes. Both limitations can be mitigated by using carbon-TiO2 nanocomposites, such as those based on graphene (G) and graphene oxide (GO). Relative to bare TiO2, these nanocomposites have improved photocatalytic activity and stability under the UV–visible light, constituting a promising way forward for improved TiO2 photocatalytic performance. This review focuses on the recent developments in the chemistry of TiO2/G and TiO2/GO nanocomposites. It addresses the mechanistic fundamentals, briefly, of TiO2 and TiO2/G and TiO2/GO photocatalysts, the various synthesis strategies for preparing TiO2/G and TiO2/GO nanocomposites, and the different characterization techniques used to study TiO2/G and TiO2/GO nanocomposites. Some applications of the use of TiO2/G and TiO2/GO nanocomposites in water decontamination are included.

Photocatalytic-oxidation and photo-persulfate-oxidation of sulfadiazine in a laboratory-scale reactor: Analysis of catalyst support, oxidant dosage, removal-rate and degradation pathway

The extent of sulfadiazine (SDZ) removal via photo-degradation (UV-C), photocatalysis with TiO₂ (UV-C/TiO₂) and photo-persulfate-oxidation (UV-C/PS) was investigated in a batch reactor under different UV-C power levels (i.e. 14, 28, 42 and 56 W). Moreover, effects of suspended/immobilized catalyst, i.e. TiO₂ slurry/TiO₂ supported on granular activated carbon (GAC-TiO₂), on SDZ removal and corresponding SDZ degradation kinetics under different catalyst loading (1-6 g/L) were explored. Around 41.7% SDZ removal was observed after 120 min in UV-C system at the highest power level, i.e. 56 W. On the other hand, photocatalysis with TiO₂ and GAC-TiO₂ has shown better SDZ removal than photo-degradation. In UV-C/TiO₂ (4 g/L and 28 W) and UV-C/GAC-TiO₂ (5 g/L and 28 W) systems, SDZ removals were 91.8% after 120 min and 100% after 60 min, respectively; however, TOC analysis has revealed that 45.4% and 60.8% SDZ was mineralized in these systems, respectively. In UV-C/PS system, near complete degradation of SDZ (99.8%) was observed within 10 min under 50 mg/L of PS and 28 W UV illumination. On the other hand, complete SDZ removal was observed in PS alone system at a dosage of 1000 mg/L but the formation of SO₄^2- was found to be a drawback. In photolysis and photocatalysis systems, SDZ removal followed pseudo-first-order kinetics whereas the kinetics followed pseudo-second-order in UV-C/PS system. The comparison of electrical energy consumed (E_EO) in different systems revealed that UV-C/GAC-TiO₂ and UV-C/PS system were energy efficient compared with other systems. The LC-MS analysis has confirmed the cleavage of C-N bonds in the pyrimidine ring followed by S-N bonds in the sulfonyl group, which was found to be the major degradation pathway of SDZ.

Mesoporous TiO2 with WO3 functioning as dopant and light-sensitizer: A highly efficient photocatalyst for degradation of organic compound

The suitable doping or modification on TiO₂ holds promise for improving charge separation and extending light absorption range. Here, WO₃ modifying reduced band gap mesoporous TiO₂ (WO₃/RM-TiO₂) due to WO₃ doping was successfully fabricated by immersing mesoporous TiO₂ nanoparticles in the peroxotungstic acid sol with controllable reaction time (0-1 h). The W⁶⁺ ions were first incorporated into the TiO₂ lattice to form WOTi bonds, resulting in the formation of WO₃ doping TiO₂. Then, WO₃ nanoparticles gradually formed and attached on the TiO₂ surface, constructing a novel heterojunction catalyst with WO₃ serving as both dopant and light-sensitizer for TiO₂. Photocatalytic activity of the resulting WO₃/RM-TiO₂ depends on the immersing duration in the peroxotungstic acid. The BET analysis shows that 0.5 h-WO₃/RM-TiO₂ has the largest pore volume of 0.491 cm³ g^-1 and the highest surface area of 82.3 m² g^-1, whereas these values decline with prolonged immersing duration. As expected, the optimal efficiency in removing p-nitrophenol (PNP) is achieved over 0.5 h-WO₃/RM-TiO₂ under visible light irradiation, which is 2.33 times that of the unmodified M-TiO₂. This should be attributed to the suitable WO₃ doping and WO₃ modification.

A Facile Synthesis of Visible-Light Driven Rod-on-Rod like α-FeOOH/α-AgVO3 Nanocomposite as Greatly Enhanced Photocatalyst for Degradation of Rhodamine B

In this work, we have synthesized the rod-on-rod–like α-FeOOH/α-AgVO3 nanocomposite photocatalysts with the different amounts of solvothermally synthesized α-FeOOH nanorods via a simple co-precipitation method. The as-synthesized photocatalysts were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, UV−Visible diffuse reflectance spectroscopy, scanning electron microscopy (SEM), element mapping, high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS) analysis. The observed SEM images show that both α-AgVO3 and α-FeOOH exhibits the rod-shaped morphology with nano size. Furthermore, the photocatalytic activities of the obtained photocatalysts were evaluated towards the degradation of Rhodamine B (RhB) under visible-light irradiation. It is demonstrated that the 3 mg α-FeOOH added to the α-FeOOH/α-AgVO3 nanocomposite exhibited an enhanced photocatalytic performance as compared with the pure α-AgVO3 and α-FeOOH. This significant improvement can be attributed to the increased photo-excited electron-hole pair separation efficiency, large portion of visible-light absorption ability and the reduced recombination of the electron-hole pair. The recycling test revealed that the optimized nanocomposite exhibited good photostability and reusability properties. In addition, the believable photodegradation mechanism of RhB using α-FeOOH/α-AgVO3 nanocomposite is proposed. Hence, the developed α-FeOOH/α-AgVO3 nanocomposite is a promising material for the degradation of organic pollutants in an aqueous environment.

CO Preferential Photo-Oxidation in Excess of Hydrogen in Dark and Simulated Solar Light Irradiation over AuCu-Based Catalysts on SBA-15 Mesoporous Silica-Titania

In this work, SBA-15 silica and silica-titania have been used as supports for photocatalysts based on AuCu alloy (Au:Cu = 1) to be used in the preferential oxidation of CO (CO-PROX) in excess of hydrogen at room temperature and atmospheric pressure both in the dark and under simulated solar light irradiation. To study their textural, structural, chemical and optical properties, the samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), adsorption-desorption of N₂ at -196 °C, 13C and 29Si solid state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy. Titanium was present mainly in the form of titania aggregates, but also as small particles interacting with the SBA support. In both catalysts, the metal alloy nanoparticles displayed an average size of 4 nm as demonstrated by TEM measurements. AuCu/Ti-SBA turned out to be photoactive and selective in the photo-CO-PROX reaction showing the highest activity, with conversion and selectivity towards CO₂ of 80%, due both to the presence of titania incorporated in SBA-15 and to the synergistic effect of Cu when alloyed with Au.

Roles of Vanadium and Nitrogen in Photocatalytic Activity of VN-Codoped TiO2 Photocatalyst

In the present manuscript, we explore the VN-codoped TiO₂ system intended to understand the role played by each dopant in synergistic enhancement in performance of TiO₂ photocatalyst. The photocatalytic decomposition of organic pollutants in aqueous solution under visible light was used as a probe reaction to evaluate the performance of VN-codoped TiO₂ samples with different V and N concentrations. An optical measurement of VN-codoped TiO₂ shows considerable improved visible light absorption with increase in V-concentration as compared to increase in N concentration, which was due to the effective narrowing of the band gap by V-doping. The energy levels formed by N-dopants act as the trapping centers for photogenerated holes to suppress recombination process as indicated by PL and TRPL results. It is also observed that at high V-concentrations recombination centers are created in the form of oxygen vacancies as indicated by XPS and PL. In VN-codoped TiO₂ , addition of N partially fills these oxygen vacancies to reduce the number of recombination centers and prolong the lifetime of charge carriers. Thus, V improves the visible light absorption while N reduces the recombination of electron-hole pairs, thus creating the synergistic effect to produce three times better performance than pure TiO₂ .

Study on the degradation mechanism and pathway of benzene dye intermediate 4-methoxy-2-nitroaniline via multiple methods in Fenton oxidation process

Benzene dye intermediate (BDI) 4-methoxy-2-nitroaniline (4M2NA) wastewater has caused significant environmental concern due to its strong toxicity and potential carcinogenic effects. Reports concerning the degradation of 4M2NA by advanced oxidation process are limited. In this study, 4M2NA degradation by Fenton oxidation has been studied to obtain more insights into the reaction mechanism involved in the oxidation of 4M2NA. Results showed that when the 4M2NA (100 mg L-1) was completely decomposed, the TOC removal efficiency was only 30.70-31.54%, suggesting that some by-products highly recalcitrant to the Fenton oxidation were produced. UV-Vis spectra analysis based on Gauss peak fitting, HPLC analysis combined with two-dimensional correlation spectroscopy and GC-MS detection were carried out to clarify the degradation mechanism and pathway of 4M2NA. A total of nineteen reaction intermediates were identified and two possible degradation pathways were illustrated. Theoretical TOC calculated based on the concentration of oxalic acid, acetic acid, formic acid, and 4M2NA in the degradation process was nearly 94.41-97.11% of the measured TOC, indicating that the oxalic acid, acetic acid and formic acid were the main products. Finally, the predominant degradation pathway was proposed. These results could provide significant information to better understand the degradation mechanism of 4M2NA.

Reusable Photocatalytic Optical Fibers for Underground, Deep-Sea, and Turbid Water Remediation

An approach for underground, deep, and turbid water remediation is presented based on optical fibers with a photocatalytic coating. Thus, photocatalytic TiO2 P25 nanoparticles immobilized in a poly(vinylidene difluoride) (PVDF) matrix are coated on polymeric optical fibers (POFs) and the photocatalytic performance of the system is assessed under artificial sunlight. To the best of our knowledge, poly(methyl methacrylate)-POF coated with TiO2/PVDF and the reusability of any type of POF for photocatalytic applications are not previously reported. The photocatalytic efficiency of the hybrid material in the degradation of ciprofloxacin (CIP) and its reusability are evaluated here. It is shown that 50 w/w% of TiO2 P25 achieves a degradation of 95% after 72 h under artificial sunlight and a reusability of three times leads to a loss of activity inferior to 11%. The efficient removal of ciprofloxacin and the stability of the POF coated with TiO2 P25 successfully demonstrate its suitability in the degradation of pollutants with potential application in regions with low light illumination, as in underground and deep water.

A Semi-Pilot Photocatalytic Rotating Reactor (RFR) with Supported TiO₂/Ag Catalysts for Water Treatment

A four stage semi-pilot scale RFR reactor with ceramic disks as support for TiO₂ modified with silver particles was developed for the removal of organic pollutants. The design presented in this article is an adaptation of the rotating biological reactors (RBR) and its coupling with the modified catalyst provides additional advantages to designs where a catalyst in suspension is used. The optimal parameter of rotation was 54 rpm and the submerged surface of the disks offer a total contact area of 387 M². The modified solid showed a decrease in the value of its bandgap compared to commercial titanium. The system has a semi-automatic operation with a maximum reaction time of 50 h. Photo-activity tests show high conversion rates at low concentrations. The results conform to the Langmuir heterogeneous catalysis model.