Photocatalytic reactor applications are receiving increasing attention in environmental engineering, renewable energy, and chemical processing because they offer a clean and efficient way to drive reactions using light. A photocatalytic reactor is a device designed to expose a photocatalyst to light so that it can generate reactive species, such as hydroxyl radicals and electrons, which then promote the breakdown or transformation of target substances. By using sunlight or artificial light as the energy source, these reactors can support sustainable processes with reduced dependence on high temperatures, high pressures, or large amounts of chemical additives.One of the most important applications of photocatalytic reactors is water and wastewater treatment. They are used to degrade organic pollutants, including dyes, pharmaceutical residues, pesticides, and industrial chemicals that are difficult to remove by conventional treatment methods. When contaminated water flows through or around a photocatalyst under illumination, harmful molecules can be oxidized into less toxic compounds, and in some cases mineralized completely into carbon dioxide and water. This makes photocatalytic treatment attractive for municipal wastewater polishing, industrial effluent purification, and the remediation of groundwater or surface water contaminated by persistent pollutants.Air purification is another major application. Photocatalytic reactors can remove volatile organic compounds, odors, and airborne microorganisms from indoor or industrial environments. In these systems, air passes over a light-activated catalyst surface, where pollutants are decomposed into harmless products. This approach is useful in hospitals, laboratories, food-processing facilities, and public buildings where clean air is essential. It can also help reduce the concentration of gases such as formaldehyde, benzene, and nitrogen oxides.Photocatalytic reactors are also being explored for hydrogen production through water splitting. In this process, light energy is used to separate water into hydrogen and oxygen, providing a potential route to clean fuel generation. Although efficiency remains a challenge, advances in reactor design, light management, and catalyst development are improving performance. Similar systems are being studied for carbon dioxide reduction, where captured CO2 is converted into valuable fuels or chemicals, contributing to carbon utilization and climate mitigation.In addition, photocatalytic reactors have applications in self-cleaning surfaces, antimicrobial coatings, and chemical synthesis. Self-cleaning materials can degrade dirt and organic residues when exposed to light, reducing maintenance needs. Antimicrobial photocatalytic systems can inactivate bacteria and viruses on surfaces or in flowing fluids. In chemical manufacturing, photocatalytic reactors enable selective oxidation, reduction, and coupling reactions under mild conditions, offering a greener alternative to traditional methods.Overall, photocatalytic reactor applications span pollution control, energy conversion, and advanced manufacturing. As research continues to improve reactor efficiency, light utilization, and catalyst stability, these systems are expected to play an increasingly important role in sustainable technology development.