Journal of Photocatalysis - Volume 3, Issue 1, 2022

Within the current production systems, the need for low environmental impact processes to produce chemicals, the treatment of aqueous effluents and the production of fuels is becoming an increasingly urgent challenge. In this context, photoelectrocatalysis (PEC), which couples the electrochemical method (EC) with photocatalysis (PC), can be considered a valid alternative to traditional catalytic processes. It increases the photocatalytic and the electrochemical efficiency by improving the separation of the photoproduced electrons and holes on the active electrode surface and addressing the partial oxidation products formation by selecting the applied potential values. This review is concerned with the possibility of forming PEC high-added-value chemicals, sometimes with the contemporary production of hydrogen, by using different materials for the fabrication of photoanodes and photocathodes in different solvents and system configuration, obtaining very interesting values of conversion, selectivity, and Faradaic efficiencies. In particular, the review presents results related to the production of valuable compounds by the valorisation of alcohols, biomass derivatives and some peculiar organic substrates, highlighting the influence of electrode composition, applied bias, electrolyte type, and solvent.

Background: Green and sustainable method for the oxidation of aldehydes into acids is now in demand as carboxylic acids are important and versatile intermediates. In this context, visible LED light-promoted aerial oxidation can be efficient and greener protocol.

Objective: Herein, we have demonstrated a visible LED light-promoted efficient and greener protocol for the aerial oxidation of aromatic aldehydes into corresponding acids in the absence of any additives, base, catalyst, and oxidant.

Methods: The oxidation of aromatic aldehydes was irradiated by an LED light at room temperature under an open atmosphere, monitored by TLC. The rate of oxidation was also monitored using different LED lights by UV-Vis studies. The products were purified by column chromatography using silica gel, and the mixture of ethyl acetate/petroleum ether as an eluting solvent and the pure products were confirmed by their melting point determination and NMR spectroscopic analysis.

Results: The aldehydes were successfully converted into corresponding acids with good isolated yields (60-90%) by this protocol, where blue LED light (l ~ 490 nm) was found to be the best choice.

Conclusion: The present protocol of aerial oxidation of aromatic aldehydes into corresponding acids under visible LED light has been carried out in the absence of catalyst, oxidant, base, and any other additives. The higher isolated yields, no byproduct formation, and neat reaction conditions are the major advantages of the protocol.

Background: Even though silver decorated reduced graphene oxide (Ag-rGO) shows maximum absorptivity in the UV region, most of the research on the degradation of dyes using Ag-rGO is in the visible region. Therefore the present work focused on the photocatalytic degradation of indigo carmine (IC) dye in the presence of Ag-rGO as a catalyst by UV light irradiation.

Methods: In this context, silver-decorated reduced graphene oxide hybrid material was fabricated and explored its potential for the photocatalytic degradation of aqueous IC solution in the UV region. The decoration of Ag nanoparticles on the surface of the rGO nanosheets is evidenced by TEM analysis. The extent of mineralization of the dye was measured by estimating chemical oxygen demand (COD) values before and after irradiation.

Results: The synthesized Ag-rGO binary composites displayed excellent photocatalytic activity in 2 Χ 10^-5 M IC concentration and 5mg catalyst loading. The optical absorption spectrum of Ag-rGO showed that the energy band-gap was found to be 2.27 eV, which is significantly smaller compared to the band-gap of GO. 5 mg of Ag-rGO was found to be an optimum quantity for the effective degradation of IC dye. The degradation rate increases with the decrease in the concentration of the dye at alkaline pH conditions. The photocatalytic efficiency was 92% for the second time.

Conclusion: The impact of the enhanced reactive species generation was consistent with higher photocatalytic dye degradation. The photocatalytic mechanism has been proposed and the hydroxyl radical was found to be the reactive species responsible for the degradation of dye. The feasibility of reusing the photocatalyst showed that the photocatalytic efficiency was very effective for the second time.

Background: Recent years have seen the increased use of antibiotics and hormones in domestic, agricultural and healthcare applications. As a result, waste streams contain more and more of these compounds, which eventually end up in the environment, where they might cause serious damage to flora and fauna, even in miniscule amounts. This issue is currently not resolved by conventional waste treatment plants, as their adequacy for handling these compounds, many of which are non-polar, is quite limited.

Objective: This work studies the effect of modifying the hydrophilic photocatalyst TiO2 with various rare earth oxides (REOs), of the lanthanide family (Er, La, Gd, Ce), on the photocatalytic activity toward degrading non-polar compounds. Ciprofloxacin, a widely used antibiotic, was chosen as a model hydrophobic compound. Its degradation rate was compared with that of caffeine, used as a model hydrophilic compound.

Methods: Fused silica plates were coated with REO-containing films comprising TiO2 and silica. The latter was used as a binder to assure high integrity and strong adherence of the films to their substrates. The plates were characterized by SEM, EDS, XPS, and scratch-resistance measurements. The photocatalytic kinetics were determined with UV-Vis spectroscopy (caffeine) or fluorescence spectroscopy (ciprofloxacin). Further information was obtained by measuring the kinetics in the presence of charge scavengers and by SEM-EDS mapping of the surface following photodeposition of platinum.

Results: Most REOs-modified TiO2 coatings showed increased activity and selectivity towards ciprofloxacin compared to coatings that did not contain REOs. A study on the silica binder's role suggests that the binder's hydrophobicity plays an important role in promoting ciprofloxacin degradation. With respect to REOs contribution, SEM-EDS mapping of REOs-containing films indicated that the REOs act as electron sinks, despite the position of their conduction bands. This charge accumulation is likely responsible for the contribution of the REOs to the enhanced degradation of ciprofloxacin. The hydrophobicity of lanthanide oxides, while affecting the adsorption of the non-polar ciprofloxacin, cannot explain the observed effects.

Conclusion: Oxides of erbium, gadolinium and lanthanum may be used to increase photocatalytic rates via electron accumulation, despite the location of their conduction bands. This is in parallel to their effect as adsorption promotors of hydrophobic compounds.