Journal of Photocatalysis - Current Issue

Background: The necessity to have green and sustainable industrial processes has promoted new technologies for air and water purification together with the research of new energy sources. In this contest, the TiO2-based photocatalysis can be considered a promising route for both environmental applications and hydrogen production through water splitting.

Objective: In this work, we have investigated the photocatalytic performance of TiO2-CoOx composites on both photooxidation and photoreduction reactions. Specifically, we have compared the performance of the composites in the thermo-catalytic, photo-catalytic and photothermal-catalytic oxidation of ethanol chosen as model volatile organic compound (VOC) and in the photocatalytic hydrogen production by simulated solar light from aqueous solution of ethanol.

Methods: The samples were prepared with a simple impregnation method, and were characterized by Scanning Electron (SEM) and Transmission Electron (TEM) microscopies, X-ray powder diffraction (XRD), N2 adsorption-desorption measurements, Temperature Programmed Reduction in hydrogen (H2-TPR) and X-ray Photoelectron (XPS), Raman, UV-Vis Diffuse Reflectance (UV-Vis DRS) and Photoluminescence (PL) spectroscopies. The catalytic and photocatalytic activity were carried out on pyrex reactors irradiated with a solar lamp and analyzing the reactions products through gas chromatography.

Results: The presence and the amount of cobalt oxide were found crucial in determining the performance of the TiO2-based composites for both the catalytic and photocatalytic processes. In particular, the addition of 1 weight percent of CoOx led to the best performance in the photocatalytic processes, whereas a higher amount was beneficial in the thermo-catalytic tests. The multi-catalytic approach based on the synergistic effect of photocatalysis and thermocatalysis in the presence of the TiO2-1%CoOx sample allowed the temperature necessary to obtain 50% ethanol conversion and 50% yield in CO2 to be reduced by 40°C and 50°C, respectively. The same sample was also the best catalyst for photocatalytic solar H2 production.

Conclusion: The presence of small amounts of cobalt oxide leads to an efficient composite with TiO2 facilitating the space charge separation and increasing the lifetime of the generated photoholes and electrons. The wide versatility of TiO2-CoOx catalysts both for photooxidation and photoreduction reactions motivates to further exploit the use of these systems in real solar-driven photocatalysis.

Background: The influence of the potassium metal cation on the graphitic carbon nitride (g-C3N4) photocatalyst has been studied in the partial oxidation of 5-hydroxymethyl-2-furfural (HMF) to 2,5-furandicarboxaldehyde (FDC).

Objective: The aim of this study was to understand if the presence of K in C3N4 could increase the oxidative conversion of HMF to high added value species.

Methods: Two sets of photocatalysts were prepared by following two different methodologies. In both series of the materials, the precursor of C3N4 was melamine with different types of the K containing species including KCl and KOH in one case and KNO3 alone in the other case. However, for both series of photocatalysts, materials were prepared with different amounts of potassium.

Results: The results obtained by using materials prepared by the two different methodologies indicate that in both cases the presence of K was almost irrelevant at least for the lower amounts of potassium content. On the contrary, its presence was beneficial for the activity versus the photocatalytic partial oxidation reaction of the alcohol for the highest K content.

Conclusion: Some of the prepared K containing g-C3N4 materials showed increased photocatalytic activity for the partial oxidation reaction of HMF in water, particularly by using natural solar light as the irradiation source.

Background: Textile industries discharge harmful synthetic dyes to nearby water sources. These colour effluents should be treated before discharge to reduce the toxicity caused by synthetic colours.

Objective: To synthesize visible light active superstructures to reduce water pollution caused by textile industries.

Methods: We have successfully synthesized ZnO/Dy/NiO hybrid nanocomposites using waste curd as fuel by a simple combustion method. The obtained material was able to reduce recombination and enhanced the photocatalytic degradation of organic pollutants. The as-synthesized material was characterized by XRD, absorption spectroscopy, FESEM, EDAX, etc. The obtained hybrid nanostructure was used as a photocatalyst for the degradation of methylene blue under sunlight, UV light as well as in dark. Comparative experiments were carried out with a variation of catalytic load, pH, dye concentrations, etc. for a better understanding of the performance of the catalyst at various conditions.

Results and Conclusion: The ternary compound shows wide range of absorption by expanding absorption band both in UV and visible regions. ZnO/Dy/NiO hybrid nanocomposites performed well and showed uniqueness in the activity uder visible light.

Aims: Test the hypothesis that the catalytic activity of TiO2 nanoparticles towards a liquid-phase or mechanoactivated multicomponent reaction can be tuned by visible light and the shape of nanoparticles.

Background: Catalytic multicomponent reactions have been proven to be excellent synthetic approaches to a series of biologically relevant compounds including 2-amino-4H-benzo[b]pyrans. However, the potential photocatalytic activity and structural diversity of nanostructured catalysts remained underutilized in the design of new catalytic systems.

Objective: Harness the photocatalytic potential and diverse morphology of TiO2 particles as catalysts for the liquid phase and mechanoactivated multicomponent organic reactions.

Methods: The multicomponent reactions have been performed under catalytic, photocatalytic, liquid phase, and mechanoactivated conditions. The catalysts were characterized by XRD and TEM. The organic reactions products were isolated and characterized by NMR and mass spectroscopy.

Results: Catalytic activity of TiO2 nanoparticles towards multicomponent synthesis of 2-amino-4H-benzo[b]pyrans is increased by visible light. The nanorod-shaped TiO2 nanoparticles have shown substantially higher catalytic activity towards mechanoactivated multicomponent synthesis of 2-amino-4H-benzo[b]pyrans than their spherically-shaped counterparts.

Conclusion: An efficient methodology for the synthesis of 2-amino-4H-benzo[b]pyrans under ambient light condition has been developed using TiO2 nanorods (high aspect ratio anatase nanocrystals) as photocatalyst. This simple method furnished the corresponding terahydrobenzopyrans in high yields via three component reaction of aldehyde, malononitrile, and dimidone under solvent free reaction conditions at room temperature. The reaction takes 8-10 min at room temperature under ambient light condition and the catalyst can be reused multiple times. Utilization of light and the nanorod morphology of the catalyst through mechanoactivation has been applied for the -first time to the synthetic technique of multicomponent reactions. The synthetic procedures for 2-amino-4H-benzo[b]pyrans have been improved.

Background: Among Advanced oxidation processes, heterogeneous photocatalysis have a great interest, because it uses only light has a source of energy. One of the main limiting processes in photocatalysis is the high probability of electron-hole pair’s recombination in the volume or at the surface of the photocatalyst particles. TiO2 nanotubes grown by anodic synthesis are widely studied because of the large number of potential practical applications especially in photocatalytic or photoelectrochemical applications. However, the preparation of these electrodes at large scale is still challenging due to some technological obstacles such as the electrochemical cell design or the precise control of nanotubes morphology, especially regarding electrolyte ageing and overheating during the synthesis.

Objectives: This study examines the electrochemical synthesis of TiO2 nanotubes supported on large titanium electrodes.

Methods and Results: By understanding heat dissipation phenomenon during the synthesis, an optimized electrochemical cell was designed to prepare 6x4 cm 2 anodes. Then we aimed to control precisely the length of the nanotubes independently of electrolyte ageing. Indeed, It was previously observed that the electrolyte composition evolves (ageing) during the nanotubes synthesis and hence leads to non-reproducible nanotubes morphologies under time-controlled potentiostatic anodization conditions.

Conclusion and Perspectives: To overcome this issue, we developed a Coulometric approach that allows to synthesize, reusing the same electrolyte, several electrodes with a great precision and reproducibility on the length of the nanotubes (2,7 µm ± 160 nm) despite electrolyte ageing. Subsequently, these electrodes can be integrated in a photocatalytic or photoelectrocatalytic process in a real wastewater treatment sector would be very relevant.

Background: Photocatalytic oxidation is a promising tool for waste water treatment and decomposition of biologically non digestible substances. Immersed nanoscale catalyst particles from semiconductor materials such as TiO2 and ZnO can be excited by absorbed UV radiation, leading to hydroxyl-ion formation at the surface of the semiconductor and oxidative degradation of pollutants.

Objective: This contribution deals with reactors equipped with catalyst coated light guides to combine the advantages of immobilized catalysts with nearly homogeneous irradiation. With experimental and theoretical methods the coupling and decoupling of radiation were investigated and the performance of catalyst coated light guides was tested by means of methylene-blue degradation.

Methods: Radiation models, known from the recent literature, use single ray, parallel ray or multi ray models to approximate the light transmission. These models neglect Fresnel reflection and consider only coupling into the light guide. In this study, the LED was simulated as a Lambertian radiator using 10 4 rays with angle dependent intensities. This well-known model was extended with Fresnel-reflection, which predicted the measured coupling efficiencies accurately. The simulations predict the decoupling and catalyst activation at the lateral surface of the light guide for two boundary cases, ideal matt and ideal reflective surfaces. To generate matt surfaces, the light guides were either scratched or coated with TiO2 p25 nanopowder. Sol-gel coating methods were used, to create reflective surfaces.

Results: When using matt surfaces, the decoupling rate is very high: 80% of the radiant flux exits the light guide in less than 10 cm. If light guides with reflective surfaces are used, the radiant flux leaving the light guide is low: less than 10% of the radiation exited the light conductor in the first 10 cm. Methylene-blue degradation, seen as a model reaction, was used to determine the reactor performance by comparing the pseudo first order reaction coefficients. Due to the uniform light distribution along the length of the light guides and the resulting even formation of reactive radicals, the quantum yield was increased by a factor of 3, using sol-gel coated light guides, rather than powder coated light guides.

Conclusion: The effectiveness of LED driven optical fiber reactors was intensified, if reflective surfaces are used instead of matt surfaces. These surfaces are achieved by sol gel chemistry. However, to use the complete amount of photons, which entered the optical fiber, very long light guides are needed.

Background: Tetrabromobisphenol A (4,4’-isopropylidenebis(2,6-dibromophenol), TBBPA) is one of the most widely used brominated flame retardants. Due to its widespread use, high lipophilicity, and persistence, it has been detected in various environmental samples. Therefore, it is of great significance to develop methods to efficiently remove TBBPA from the contaminated environment.

Objective: The aim of our study was to examine photocatalytic dehalogenation of TBBPA on micro- and nano-sized Fe3O4 exposed to the visible light. The Fe3O4 catalyst was chosen due to its indisputable low impact on the environment.

Methods: A solution of TBBPA (1.84 × 10^-4 mol dm^-3) with a pH = 8 with suspended catalyst was illuminated (light intensity about 1.1x10¹⁷ photons per second, spectrum range 200-600 nm) for 1 hour. Analysis of the reaction progress was carried out by HPLC measurements of TBBPA decay and potentiometric measurements of an increase in bromide concentration.

Results: The degradation process seems to be the most effective for TBBPA in the reaction mixture containing the n-Fe3O4 (t0.5 ≈ 2 min). Slightly lower degradation efficacy is observed for TBBPA degradation in the presence of the µ-Fe3O4 (decay within the first 5 min). TBBPA decomposition of both n-Fe3O4 and µ-Fe3O4 is accompanied by different bromide concentrations time-profile.

Conclusion: The photogenerated electron-induced degradation by dissociative-attachment to the aromatic ring was followed by bromine ion expulsion. The micro-magnetite showed a strong tendency for adsorption of bromide anions during the process, which could be adventurous for the overall waste-decontamination process.

Aims: The study aimed at assessing the effectiveness of the PS/UV-C, PS/ZVA and PS/ZVA/UV-C processes in terms of ciprofloxacin, a fluoroquinolone type commercially important antibiotic, and toxicity abatements in raw surface water (RSW) and distilled water (DW).

Background: The occurrence of ciprofloxacin (CIP), the most widely prescribed second-generation fluoroquinolone antibiotic, even at trace level (ng/L) gives rise to antibiotic resistant bacteria and resistance genes, which can further impair the selection of genetic variants of microorganisms and impose adverse effect on human health.

Objective: The degradation and detoxification of ciprofloxacin with UV-C (PS/UV-C) and ZVA (PS/ZVA) activated PS oxidation systems were investigated in distilled water (DW) and raw (untreated) surface water (RSW) samples. Moreover, CIP degradation with the PS/ZVA/UV-C heterogeneous photochemical treatment combination was also studied.

Methods: The process performances of the investigated treatment systems were evaluated in terms of CIP abatement and PS consumption rates as well as dissolved organic carbon (DOC) removal efficiencies. The influence of common inorganic ions and natural organic matter (NOM) on CIP degradation was evaluated. Radical quenching experiments were conducted using probe compounds in order to elucidate the dominant reaction mechanism. In addition, acute toxicity of the original CIP and its degradation products were questioned by employing Vibrio fischeri (V. fischeri), the marine photobacterium, under optimized treatment conditions.

Results: CIP was completely degraded in distilled water (DW) and raw (untreated) surface water (RSW) samples after 15 min of treatment with the PS/UV-C process (PS=0.25mM; pH=3; UV-C=2.7W/m2). PS/UV-C experiments conducted with RSW at its natural pH (=8.5) resulted in 98% CIP and practically no DOC removal whereas 56% DOC was removed at pH 3 after 120 min. Radical quenching studies revealed that sulfate radicals prevailed over hydroxyl radicals. CIP degradation was significantly inhibited by the presence of humic acid due to the effect of UV absorption and free radical quenching. Acute toxicity tests with V. fischeri exhibited fluctuating trends throughout the investigated processes and did not change appreciably after 120 min of oxidation.

Conclusion: The results of this study demonstrated that PS/UV-C is superior to the PS/ZVA and PS/ZVA/UV-C treatment systems for both DW and RSW samples in terms of CIP removal rates. No additional positive effect was evident for simultaneous catalytic and photochemical PS activation (PS/ZVA/UV-C treatment system). It could be also demonstrated that the selected oxidation processes conducted in pure water might give an idea about the advanced treatment systems but realistic conditions with actual water/wastewater matrices still need to be further investigated to verify their feasibility and ecotoxicological safety.