Photocatalysis harnesses light to speed up chemical reactions using special catalysts. It's a powerful tool that creates electron-hole pairs, driving redox reactions at the catalyst surface. Different types of photocatalysts, from semiconductors to metal-based ones, offer unique advantages.

Photocatalysis comes in two flavors: heterogeneous and homogeneous. It's widely used in environmental cleanup, from water purification to air cleaning. While it offers eco-friendly benefits, challenges like low efficiency and scaling issues remain. Future research aims to overcome these hurdles.

Fundamentals of Photocatalysis

Principles of photocatalysis

• Photocatalysis accelerates chemical reactions using light and a light-absorbing catalyst
• Light absorption by photocatalyst generates electron-hole pairs
• Charge carriers separate and migrate to catalyst surface
• Redox reactions occur at catalyst surface driving chemical transformations
• Bandgap energy determines light absorption range (UV, visible, infrared)
• Conduction and valence bands facilitate electron movement and hole formation
• Quantum efficiency measures photocatalytic performance comparing reacted molecules to absorbed photons

Types of photocatalysts

• Semiconductor photocatalysts utilize bandgap for charge separation (TiO2, ZnO, CdS)
• Metal-based photocatalysts exploit plasmonic effects (Au, Ag, Pt nanoparticles)
• Organic photocatalysts employ dye-sensitization or organic semiconductors
• Composite photocatalysts combine materials for enhanced performance (metal-semiconductor, semiconductor-semiconductor)
• Photocatalytic activity influenced by bandgap energy, surface area, particle size, crystal structure, and recombination rate

Heterogeneous vs homogeneous photocatalysis

• Heterogeneous photocatalysis uses solid catalyst separate from reactants
  1. Light absorption
  2. Charge separation
  3. Surface reactions
  4. Adsorption-desorption processes
• Surface defects and trap states affect heterogeneous reactions
• Homogeneous photocatalysis occurs with catalyst and reactants in same phase
  1. Photoexcitation of catalyst molecules
  2. Energy or electron transfer to substrates
  3. Regeneration of catalyst
• Heterogeneous and homogeneous differ in reaction environment, charge transfer, and catalyst recovery

Applications in environmental remediation

• Water purification degrades organic pollutants (pesticides, pharmaceuticals) and disinfects pathogens (bacteria, viruses)
• Air purification removes VOCs (benzene, formaldehyde) and reduces NOx emissions
• Soil decontamination treats contaminated sites (oil spills, heavy metals)
• Organic synthesis enables selective oxidation reactions and C-C bond formation
• Photocatalytic water splitting produces hydrogen for clean energy
• CO2 reduction creates value-added chemicals (methanol, formic acid)
• Energy applications include solar cells and artificial photosynthesis systems

Advantages and limitations of photocatalysis

• Advantages include environmentally friendly operation, room temperature reactions, complete pollutant mineralization, and solar light utilization
• Limitations involve low quantum efficiency, limited light penetration in turbid solutions, catalyst deactivation, and scaling challenges
• Future directions focus on improving visible light activity, enhancing charge separation, developing novel reactor designs, and scaling up for industrial use