Why in the news?

  • Delhi government has decided to conduct a study on the use of Smog Eating Photocatalytic Coatings on roads, pavements and public spaces to reduce pollutants such as nitrogen dioxide (NO₂) and harmful hydrocarbons.

Smog Eating Photocatalytic Coatings

  • What is it?:
    • These are surface coatings (often applied to building facades, roads, pavements, and tiles) that use photocatalysis to break down harmful air pollutants like NOx (nitrogen oxides), SOx, VOCs, and particulate matter.
    • Typically based on titanium dioxide (TiO₂) nanoparticles, which, when exposed to sunlight (UV), generate reactive oxygen species that oxidize pollutants into harmless substances (like nitrates, CO₂, and water).
  • How does it Work:
    • Sunlight Activation: UV rays excite TiO₂, releasing electrons and creating electron-hole pairs.
    • Reactive Oxygen Species (ROS): These react with oxygen and water vapor to produce hydroxyl radicals and superoxide ions.
    • Pollutant Breakdown: NOx, SOx, benzene, and other smog-forming compounds are oxidized into non-toxic byproducts.
    • Self-Cleaning Effect: The coatings also decompose organic dirt, keeping surfaces cleaner.
  • Applications:
    • Urban Infrastructure: Coated concrete, pavements, and tiles (e.g., “smog-eating” pavements in Mexico City, Milan, and Tokyo).
    • Building Facades: High-rise buildings in polluted cities (e.g., Torre de Especialidades Hospital, Mexico).
    • Transport Sector: Roadways and tunnels coated with TiO₂ to reduce vehicular NOx.
    • Consumer Products: Paints, tiles, glass, and textiles marketed as “air-purifying” or “self-cleaning.”
  • Significance for India:
    • Aligned with National Clean Air Programme (NCAP) goals.
    • Potential integration into Smart Cities Mission, Green Building Codes, and urban planning.
    • IITs and CSIR labs exploring TiO₂ and ZnO-based nanocoatings for smog mitigation.
    • Pilot projects could target Delhi, Kanpur, Lucknow, and other NCAP priority cities.
  • Advantages:
    • Air Quality Improvement: Reduces NOx concentrations locally by up to 30–80% in pilot projects.
    • Climate & Health Gains: Lowers smog formation, respiratory illnesses, and urban heat-island effects.
    • Durability: Long-lasting with minimal maintenance.
    • Co-benefits: Energy savings via cooler buildings (reflective surfaces).
  • Limitations:
    • Dependence on Sunlight: Less effective in shaded/indoor or low-UV areas.
    • By-products: Converts NOx to nitrates, which may accumulate and require surface washing.
    • Cost: Higher than conventional coatings.
    • Scale Challenge: City-wide coverage is expensive and logistically complex.
    • Efficacy Debate: Real-world reductions often smaller than lab claims.