Photocatalytic hydrophobic concrete coatings to combat air pollution

• SiO2 sol added to coating composition improved MB dye bleaching.
• SiO2–TiO2 coating presented enhanced photocatalytic degradation of NOx.
• Hydrophobic TiO2–siloxane coated concrete showed excellent NOx conversion.
• Adsorption vs. photo-oxidation compromise for increasing photoefficiency.
• TiO2 content threshold: 1% for hydrophilic vs. 5% for hydrophobic coatings.

Several low cost, user-friendly photocatalytic surface coatings were applied to cementitious surfaces and tested as a remediation technique for transportation NOx emissions and organic pollutants. All of the sols (aqueous suspensions of nanoparticles) coatings showed very high nitrogen oxide (NO) conversion (∼90%) and Methylene Blue dye bleaching when applied to concrete surfaces, even when the coatings were extremely thin or diluted, independently on the deposition method (spraying or dip coating). A novel hydrophobic coating based on photocatalyst nanoparticles suspended in a siloxane sealant showed similar conversion at low NO concentrations but required a greater loading of TiO2. At high NO concentrations the hydrophobic coating was less effective, even at greater TiO2 loadings, suggesting that the adsorption is the rate limiting step in this conversion process. A threshold of 1% TiO2 loading in hydrophilic coating and 5% in the case of hydrophobic was needed to obtain a quantitative NOx conversion. A home-made TiO2 sol showed lower NOx photodegradation but a more homogeneous and bonded layer.

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