Advanced Oxidation Catalysts—Innovative Approaches and Emerging Trends

Novel oxidation catalysts in water treatment and purification have garnered major attention due to the growing demand for clean water resources. As the use of advanced oxidation technologies is recognized as one of the most sustainable methods for breaking down and removing organic pollutants, this Special Issue on advanced oxidation catalysts brings together 17 articles that explore the development, analysis, and use of advanced oxidation catalysts designed for eliminating organic pollutants and disinfecting water to improve its quality. This article emphasizes the most recent advancements and discoveries in this field and summarizes the key results and innovative findings presented in the selected papers.

Lahootifar and his team (Contribution 1) developed a novel photocatalyst composed of hetaerolite-embellished g-C₃N₄ dots. This novel material has increased photocatalytic effectiveness when exposed to visible light, allowing organic pollutants to be easily decomposed. The combination of these compounds causes a synergistic effect that increases the capacity of visible light absorption and extends the time of photo-generated charges, thus increasing the efficiency of pollutant degradation. This work confirms that creating heterojunctions can greatly boost the photocatalytic activity of chemicals, which is promising for their use in treating water pollutants.

An important study was carried out by Alharthi et al., building upon previous research. The authors provide a detailed explanation of the creation of a nanocrystalline composite material that includes metal tungstates, specifically NiWO₄ and CoWO₄ (Contribution 2). These components work together to form a heterojunction. This complex structure is particularly effective in breaking down paracetamol when exposed to ultraviolet (UV) radiation. When these materials are used in a heterojunction setup, they show an increased rate constant, demonstrating improved photocatalytic activity. This finding emphasizes the necessity of developing heterojunctions to accelerate the breakdown of pharmaceutical pollutants, which pose important environmental and health risks.

Cruz-Carrillo and other researchers focused on decomposing recalcitrant acidic pharmaceuticals, specifically clofibric acid, diclofenac, and indomethacin, through a combined biological and photocatalytic technique (Contribution 3). Their combination of biological processes and photocatalysis achieves almost complete degradation of such pharmaceuticals, where the latter treatment demonstrates excellent efficiency in breaking down any remaining substances. This method has confirmed potential for treating persistent pollutants that are difficult to eliminate using traditional methods.

Magomedova and colleagues focused on the construction of a magnetically divisible α/γ-Fe₂O₃ mixed-phase catalyst for the photo-Fenton-like oxidation process of Rhodamine B (Contribution 4). The magnetic properties of the catalyst make it very easy to separate and reuse, giving it great potential in practical applications. Their study also explored the effect of different factors on degradation efficiency and found that the catalyst performed well under optimal conditions, contributing to the field of sustainable wastewater treatment technology.

The research from Kamenicka and Weidlich investigates the use of different technologies to decompose the halogenated negatively charged textile dye Mordant Blue 9 (Contribution 5). Their study evaluates the efficiency of various oxidants and reducing agents and provides a comparative analysis of their economic feasibility and performance in dye decomposition. This study emphasizes the importance of exploring cost-effective methods to treat dye-contaminated water bodies.

Ma and his colleagues conducted research on the oxidation of carbon monoxide using a copper catalyst supported by alumina (Contribution 6). Their goal was to tackle the issue of toxic gases in industrial emissions. Their study investigated how various factors influence catalytic effectiveness, emphasizing that the characteristics of the substrates involved in mechanochemical activation are important in shaping the properties and reactivity of the resulting catalysts. This research contributes to the larger effort of using advanced catalytic technologies to purify air pollution.

Silerio-Vazquez and colleagues conducted a systematic review of sunlight-catalyzed heterogeneous water disinfection (Contribution 7). This study provides a comprehensive overview of the progress of photocatalytic water treatment technology, with a particular focus on solar energy utilization. The authors systematically analyzed 60 reports from 1043 records, extracting data on the reactor type, photocatalysts, microorganisms, and operational parameters. The results show that titanium dioxide is a widely used photocatalyst, while bismuth oxide and red phosphorus show promising prospects for visible-light-driven disinfection. The review clarifies the importance of further studies to optimize photocatalytic systems in practical water treatment applications.

Umar et al. conducted a thorough exploration of the capabilities of mycelium-based energy conversion and waste purification processes, providing a new view of bioremediation techniques (Contribution 8). Their research clarifies the capability of fungal fuel cells to simultaneously decompose organic pollutants and generate bioelectricity. The researchers investigated the degradation process of pollutants and emphasized the advantages of fungal biomass compared to conventional methods. Their study offers a novel approach to integrating biological processes with advanced oxidation technologies for sustainable wastewater treatment.

An in-depth study on the electrochemical coupling of the Elbaite/H₂O₂ system in the area of dye wastewater catalytic degradation introduces a new method for breaking down pollutants (Contribution 9). This study reveals the combined benefits of using pyrolytic carbon alongside hydrogen peroxide, resulting in the effective degradation of organic dyes. The results indicate that this innovative technique successfully purifies dye-containing wastewater while preventing the creation of external pollutants, positioning it as a potential solution for industrial use.

Falletta et al. carried out an extensive analysis focused on tungsten trioxide (WO₃) and its hybrid composite with titanium dioxide (TiO₂) in the photocatalytic degradation of NO_x and the inactivation of Escherichia coli (E. coli). This detailed study sought to tackle both air and water pollution simultaneously, demonstrating the important potential of these materials in environmental remediation (Contribution 10). Their research findings suggest that the combined composites exhibit a greater level of photocatalytic effectiveness when compared to WO₃ in its original form, indicating that adding TiO₂ enhances the degradation efficiency. This study emphasizes the promising applications of these composite materials in real-world scenarios.

The research conducted by Pavlova and her team investigated the impact of starting material characteristics on the formation, structural properties, and reactivity of Sr₂TiO₄ created through mechanical activation (Contribution 11). Their findings indicate that the mechanochemical activation process plays a critical role in influencing the catalytic activity of the produced materials. The experimental results demonstrate that the Sr₂TiO₄ catalyst, when activated through mechanical and chemical methods, exhibits enhanced catalytic performance, making it potentially useful for applications in various areas, including methane oxidation coupling.

Gote et al. present a new approach for the sonocatalytic breakdown process, using a NiFe₂O₄ catalyst created through ultrasonic irradiation (Contribution 12). The optimal conditions for degrading the azo dye crystal violet R are a hydrogen peroxide loading rate of 75 milligrams per minute, a pH level of 3, an ultrasonic working cycle of 70%, an output power of 120 watts, a reaction time of 160 min, and a catalyst loading rate of 5 g per liter. Under these conditions, the maximum efficiency for sound wave degradation can be achieved, reaching up to 92%. The process achieves a satisfying 83% success rate, demonstrating the effectiveness of ultrasound-enhanced catalytic breakdown for cleaning dye-contaminated water.

The research conducted by Wang and his colleagues examined how the photocatalytic activity of CeVO₄-V₂O₃ composites can be improved when exposed to visible light (Contribution 13). A detailed analysis of the effect of ethylene glycol on photocatalytic performance shows that the composite material exhibits increased photocatalytic reactivity under normal light conditions. This finding emphasizes the need to modify the structure and components of photocatalysts to enhance their ability to capture solar energy for water purification.

Alharthi and other researchers (Contribution 14) have conducted intensive research on the synthesis of Zn₃V₂O₈ and Ag-nanoparticle composite materials, and their results have attracted much attention. A large transition reducing the optical band gap from 2.33 eV to 2.19 eV was achieved by integrating Ag nanoparticles into a ZnV matrix. The widened band gap improves the light absorption efficiency and thus promotes the photocatalytic hydrogen generation process. The experimental results show that the Zn₃V₂O₈/Ag nanocomposite has outstanding performance in terms of its hydrogen production rate, reaching a high efficiency of 37.52 μmol g⁻¹ h⁻¹, showing its great potential as an economical and environmentally friendly photocatalyst in the field of hydrogen preparation.

The chosen papers often emphasize the significance of developing catalysts with enhanced properties through synthesis. One study describes the use of the hydrothermal method to create a nanocomposite that features bimetallic (Zn, Co) concurrent doping within a tungstate matrix, designed with a Z-scheme architecture (Contribution 15). This material demonstrates outstanding photocatalytic efficiency in breaking down Xylenol Orange (XO), greatly exceeding the performance of pure ZnWO₄ and CoWO₄, suggesting that co-doping can improve photocatalytic efficiency. The experimental findings confirm the essential role of superoxide anion radicals (O₂^•−) and light-generated electrons (e⁻) in the degradation process. This research emphasizes the nanocomposite’s potential as an eco-friendly and cost-effective material for use in industrial applications, especially for treating wastewater contaminated with XO pollutants.

A thorough evaluation of the research conducted by Weissenberger and colleagues has been completed. This study primarily aimed to explore the catalytic effects of active metals in the ammonia combustion process, with a particular emphasis on their role in NO_x formation (Contribution 16). Additionally, it analyzed the catalytic properties of noble and transition metals supported by alumina, specifically for the treatment of emissions from solid oxide fuel cells that arise from ammonia combustion. The tested catalysts can fully convert the hydrogen and ammonia present in the exhaust gases, and the efficiency of selectively generating NO_x improves as the reaction temperature rises. Under low-temperature conditions, the production of NO_x is effectively regulated, demonstrating the potential for optimizing catalysts to minimize NOx emissions in burner systems.

Overall, the articles in this Special Issue display the variety and depth of the catalytic processes applied for environmental remediation (Contributions 1–17). These papers emphasize the promise of innovative materials and integrated strategies in tackling water and air pollution. The guest editors express their sincere gratitude to the contributors for their valuable insights and to the evaluators for their comprehensive reviews. It is hoped that the findings presented in this article will inspire further research and innovations in advanced oxidation catalysts, eventually contributing to a cleaner and safer ecological environment.