A photocatalytic reactor is a device that uses light energy to drive chemical reactions on the surface of a catalyst. Its working principle is based on the interaction among light, a semiconductor catalyst, water, oxygen, and pollutants or target reactants. When the reactor is exposed to light of sufficient energy, the catalyst absorbs photons and creates electron-hole pairs. These charge carriers then migrate to the catalyst surface, where they participate in redox reactions.The core of the process begins when light irradiates the photocatalyst, which is often a semiconductor material such as titanium dioxide, zinc oxide, or modified composites. If the photon energy is equal to or greater than the band gap of the catalyst, an electron in the valence band is excited to the conduction band. This leaves behind a positive hole in the valence band. The generated electrons and holes are highly reactive, but they can also recombine quickly, which reduces efficiency. Therefore, reactor design and catalyst structure are important for improving charge separation and utilization.At the surface of the catalyst, the photogenerated electrons react with oxygen molecules dissolved in water or present in the gas phase. This reaction produces superoxide radicals and other reactive oxygen species. Meanwhile, the holes can oxidize water molecules or hydroxide ions to form hydroxyl radicals. These radicals are extremely powerful oxidizing agents. They can break down organic compounds, destroy microorganisms, and convert harmful pollutants into less toxic substances such as carbon dioxide, water, and inorganic ions.A photocatalytic reactor is designed to maximize contact between light, catalyst, and reactants. Common reactor forms include slurry reactors, fixed-bed reactors, and immobilized-coating reactors. In a slurry reactor, catalyst particles are suspended in the liquid, providing a large surface area and good reaction efficiency, but requiring separation after treatment. In a fixed-bed reactor, the catalyst is supported on a solid substrate, which simplifies operation and recovery. The reactor may use ultraviolet light, visible light, or solar irradiation depending on the catalyst properties and application.Several factors influence reactor performance. These include light intensity, catalyst loading, flow rate, pollutant concentration, pH, temperature, and the presence of dissolved oxygen. Proper mixing helps ensure that reactants reach the catalyst surface and that the illuminated area is fully utilized. Reactor geometry also matters because it determines how well light penetrates and how evenly the catalyst is exposed.In practical applications, photocatalytic reactors are used for water purification, air treatment, odor removal, self-cleaning surfaces, and disinfection. They are valued because they can operate at ambient temperature and pressure, use light as the main energy source, and potentially degrade a wide range of contaminants. However, challenges remain, such as improving quantum efficiency, preventing catalyst deactivation, and scaling up from laboratory systems to industrial systems.Overall, the working principle of a photocatalytic reactor relies on light-induced charge generation in a catalyst and the subsequent formation of reactive species that oxidize or reduce target substances. Through careful design of the catalyst and reactor configuration, this technology can provide an effective and environmentally friendly method for pollution control and chemical conversion.