Role of lamp type in conventional batch and micro-photoreactor for photocatalytic hydrogen production

The use of an irradiation source with a homogeneous distribution of irradiation in the volume of the reaction mixture belongs to the essential aspects of heterogeneous photocatalysis. First, the efficacy of six lamps with various radiation intensity and distribution characteristics is contrasted. The topic of discussion is the photocatalytic hydrogen production from a methanol-water solution in the presence of a NiO-TiO2 photocatalyst. The second section is focused on the potential of a micro-photoreactor system-the batch reactor with a micro-reactor with a circulating reaction mixture, in which the photocatalytic reaction takes place using TiO2 immobilized on borosilicate glass. Continuous photocatalytic hydrogen generation from a methanol-water solution is possible in a micro-photoreactor. This system produced 333.7 ± 21.1 µmol H2 (252.8 ± 16.0 mmol.m-2, the hydrogen formation per thin film area) in a reproducible manner during 168 h.

Self-cooling water disinfection reactor with ultraviolet-C light-emitting diodes

The use of ultraviolet-C (UV-C) light-emitting diodes (LEDs) as a water sterilization light source poses a serious challenge in heat dissipation. High junction temperatures reduce the radiant power and lifespan of UV-C LEDs. In this study, a novel self-cooling water disinfection reactor was developed to dissipate Joule heat from UV-C LEDs. The advantage of the self-cooling design is that cooling can be achieved without requiring additional power consumption and cooling liquid. The effects of the water flow rate and driving current of UV-C LEDs on the sterilization of Escherichia coli were investigated for a traditional flow-through reactor and a reactor with self-cooling. The experimental results indicated that an increase in driving current resulted in a considerable increase in the LED temperature of the flow-through reactor but only a marginal increase in the LED temperature of the self-cooling reactor. Under a driving current of 150 mA, the LED temperature of the self-cooling reactor was 55.5°C less than that of the flow-through reactor. The time required by the self-cooling reactor to reach the steady state decreased as the water flow rate increased. Under a flow rate of 100 mL/min, the self-cooling reactor reached the steady state within 62 and 70 s when the driving current was 100 and 150 mA, respectively. Moreover, the average irradiance and inactivation values of the self-cooling reactor were up to 16.5% and 26.0% higher than those of the flow-through reactor, respectively.

Recent developments in photocatalysis of industrial effluents ։ A review and example of phenolic compounds degradation

Industrial expansion and increased water consumption have created water scarcity concerns. Meanwhile, conventional wastewater purification methods have failed to degrade recalcitrant pollutants efficiently. The present review paper discusses the recent advances and challenges in photocatalytic processes applied for industrial effluents treatment, with respect to phenolic compounds degradation. Key operational parameters including the catalyst loading, light intensity, initial pollutants concentration, pH, and type and concentrations of oxidants are evaluated and discussed. Compared to the other examined controlling parameters, pH has the highest effect on the photo-oxidation of contaminants by means of the photocatalyst ionization degree and surface charge. Furthermore, major phenolic compounds derived from industrial sources are comprehensively presented and the applicability of photocatalytic processes and the barriers in practical applications, including high energy demand, technical challenges, photocatalyst stability, and recyclability have been explored. The importance of energy consumption and operational costs for realistic large-scale processes are also discussed. Finally, research gaps in this area and the suggested direction for improving degradation efficiencies in industrial applications are presented. In the light of these premises, selective degradation processes in real water matrices such as untreated sewage are proposed.

Impact of Reaction Parameters and Water Matrices on the Removal of Organic Pollutants by TiO2/LED and ZnO/LED Heterogeneous Photocatalysis Using 365 and 398 nm Radiation

In this work, the application of high-power LED_365nm and commercial, low-price LED_398nm for heterogeneous photocatalysis with TiO₂ and ZnO photocatalysts are studied and compared, focusing on the effect of light intensity, photon energy, quantum yield, electrical energy consumption, and effect of matrices and inorganic components on radical formation. Coumarin (COU) and its hydroxylated product (7-HC) were used to investigate operating parameters on the ^•OH formation rate. In addition to COU, two neonicotinoids, imidacloprid and thiacloprid, were also used to study the effect of various LEDs, matrices, and inorganic ions. The transformation of COU was slower for LED_398nm than for LED_365nm, but r₀^7-HC/r₀^COU ratio was significantly higher for LED_398nm. The COU mineralization rate was the same for both photocatalysts using LED_365nm, but a significant difference was observed using LED_398nm. The impact of matrices and their main inorganic components Cl^- and HCO₃^- were significantly different for ZnO and TiO₂. The negative effect of HCO₃^- was evident, however, in the case of high-power LED_365nm and TiO₂, and the formation of CO₃^•- almost doubled the r₀^7-HC and contributes to the conversion of neonicotinoids by altering the product distribution and mineralization rate.

The TiO2-ZnO Systems with Multifunctional Applications in Photoactive Processes-Efficient Photocatalyst under UV-LED Light and Electrode Materials in DSSCs

The main goal of the study was the hydrothermal-assisted synthesis of TiO₂-ZnO systems and their subsequent use in photoactive processes. Additionally, an important objective was to propose a method for synthesizing TiO₂-ZnO systems enabling the control of crystallinity and morphology through epitaxial growth of ZnO nanowires. Based on the results of X-ray diffraction analysis, in the case of materials containing a small addition of ZnO (≥5 wt.%), no crystalline phase of wurtzite was observed, proving that a high amount of modified titanium dioxide can inhibit the crystallization of ZnO. The transmission electron microscopy (TEM) results confirmed the formation of ZnO nanowires for systems containing ≥ 5% ZnO. Moreover, for the synthesized systems, there were no significant changes in the band gap energy. One of the primary purposes of this study was to test the TiO₂-ZnO system in the photodegradation process of 4-chlorophenol using low-power UV-LED lamps. The results of photo-oxidation studies showed that the obtained binary systems exhibit good photodegradation and mineralization efficiency. Additionally, it was also pointed out that the dye-sensitized solar cells can be a second application for the synthesized TiO₂-ZnO binary systems.

Investigation of the efficiency of BiOI/BiOCl composite photocatalysts using UV, cool and warm white LED light sources - Photon efficiency, toxicity, reusability, matrix effect, and energy consumption

BiOI, BiOCl, and their composites (BiOI:BiOCl) with molar ratios from 95:5 to 5:95 were synthesized and tested in the transformation of methyl orange (MO) and sulfamethoxypyridazine (SMP) antibiotic, using three various LED light sources: UV LEDs (398 nm), cool and warm white LEDs (400-700 nm). The 80:20 BiOI:BiOCl photocatalyst showed the best adsorption capacity for MO and enhanced activity compared to BiOI and BiOCl. The apparent quantum yield (Φ_app) of the MO and SMP transformation for cool and warm white light was slightly lower than for 398 nm UV radiation. The effect of methanol and 1,4-benzoquinone proved that the transformation is initiated mainly via direct charge transfer, resulting in the demethylation of MO and SO₂ extrusion from SMP. The change of photocatalytic efficiency was followed during three cycles. After the first one, the transformation rates decreased, but there was no significant difference between the second and third cycles. The decreased efficiency is most probably caused by the intermediates, whose continuous accumulation was observed during the cycles. Ecotoxicity measurements confirmed that no toxic substances were leached from the catalyst, but the transformation of both MO and SMP results in toxic intermediates. Using 80:20 BiOI:BiOCl and LED light source, the energy requirement of the removal is about half of the value determined using TiO₂ and a mercury vapor lamp. The effect of some components of wastewater (Cl^-, HCO₃^- and humic acids), pH, and two matrices on the composite photocatalysts' efficiency and stability were also investigated.

Photocatalytic ozonation of offshore produced water by TiO2 nanotube arrays coupled with UV-LED irradiation

Offshore produced water (OPW) containing hazardous substances such as polycyclic aromatic hydrocarbons (PAHs) needs to be treated prior to discharge. This study integrated a photocatalytic ozonation system with TiO₂ nanotube arrays (TNA) and UV-light-emitted diode (UV-LED) irradiation and applied to treat OPW. Experimental and modeling efforts were made to evaluate the degradation efficiencies of PAHs, examine the behaviors of the OPW composition (e.g., phenols, iodide, and bromide), and investigate the oxidation intermediates and the associated toxicity and biodegradability. The results indicated that ozone significantly enhanced the oxidation rates and removed the PAHs within 30 min, while the TNA showed strong photocatalytic capability. In the early stage, iodide was a strong ozone competitor, accelerating phenol degradation but inhibiting PAH oxidation, whereas UV-LED fortified the effect. The degradation of aromatics was altered by iodide and bromide at different stages. The contributions of four toxicants to the acute toxicity of OPW were quantified and ranked (PAHs > bromoform > phenols > dibromopentane). The EC₅₀ value increased from 3 % to 57 %, and the biodegradability was doubled with less footprint in 28-day biodegradation tests. Overall, it is recommended to sequentially oxidize the matrix of OPW by ozonation and PAHs by the UV-LED/TNA/ozone system.

The photocatalytic destruction of cinnamic acid and cinnamyl alcohol: Mechanism and the effect of aqueous ions

Cinnamic acid was chosen as an exemplar molecule to study the effect of potential contaminants on the kinetics and mechanism of the photocatalytic destruction of hydrocarbons in aqueous solutions. We identify the principal intermediates in the photocatalytic reaction of the acid and corresponding alcohol, and propose a mechanism that explains the presence of these species. The impact of two likely contaminants of aqueous systems, sulfate and chloride ions were also studied. Whereas sulfate ions inhibit the degradation reaction at all concentrations, chloride ions, up to a concentration of 0.5 M, accelerate the removal of cinnamic acid from solution by a factor of 1.6. However, although cinnamic acid is removed, the pathway to complete oxidation is blocked by the chloride, with the acid being converted (in the presence of oxygen) into new products including acetophenone, 2-chloroacetophenone, 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane. We speculate that the formation of these products involves chlorine radicals formed from the reaction of chloride ions with the photoinduced holes at the catalyst surface. Interestingly, we have shown that the 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane products form from 2-chloroacetophenone when irradiated with 365 nm light in the absence of the catalyst. The formation of potentially dangerous side products in this reaction suggest that the practical implementation of the photocatalytic purification of contaminated water needs to considered very carefully if chlorides are likely to be present.

Design and characterization of visible-light LED sources for microstructured photoreactors

Experimental and modeling toolbox to quantify the light uniformity, photon flux and energy efficiency of microstructured photoreactors.