Improvement of the photocatalytic activity of TiO2 induced by organic pollutant enrichment at the surface of the organografted catalyst
Résumé
The photocatalytic activity of TiO2 nanoparticles modified with hydrophobic hexadecyltrichlorosilane (HTS) is investigated. The strategy developed aims to concentrate the organic pollutants onto the surface. The impact of the grafted layer on the number and the nature of the surface photocatalytic sites involved in the production of radicals is analyzed. For this purpose, the surface of the TiO2 particles has been thoroughly characterized by total organic carbon analysis, Fourier transform infrared spectroscopy, wettability (capillary rise and flotation), nitrogen adsorption, UV/Vis spectroscopy and zeta potential. The adsorption capability and the photocatalytic activity of the materials have been tested in the degradation of salicylic acid and methyl orange at pH 3 and 10. The organic surface treatment reduces the overall production of radicals due to the diminution of the number of photocatalytic sites induced by the grafting of the HTS molecules onto the surface hydroxyl groups. Our analysis reveals that the TiOH2+ surface groups are mainly involved in the creation of radicals. The photodegradation performance and the adsorption capacity are significantly affected by the grafting density and the pH. At basic pH, the pollutant adsorbed amount remains low and roughly similar for all the systems. The hydrophobized particles exhibit lower degradation efficiency than that of the untreated material. At acidic pH, the superficial HTS monolayer promotes the adsorption of the pollutants onto the modified titania due to the hydrophobic interactions between the HIS molecules and the organic contaminants. The photocatalytic activity considerably depends on the pollutant enrichment at the surface of the catalyst. Despite the rather low number of photocatalytic sites, the greater degradation at larger pollutant adsorbed amount implies that the contaminant surface coverage on the catalyst plays a major role in the efficiency of the photocatalytic process.