Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1–C3 carbonyl pollutants

This overview of photocatalytic purification/deodorization of indoor air using TiO2 includes four parts. First, considering that TiO2 exposed to indoor air – which contains several thousands ppmv of water – is covered by layers of water molecules, it is suggested that: (i) the approach to the surface of pollutants at ppbv concentrations is hindered, (ii) the pollutants are not dissociated by mere adsorption, (iii) OH radicals are formed by mechanisms identical to those proposed for TiO2 exposed to liquid water, and (iv) OH radicals are mobile within the adsorbed water layers. Second, the formation and role of superoxide, singlet molecular oxygen and labile surface oxygen atoms of TiO2 are reviewed. Third, the competition between pollutants at ppbv levels for elimination under usual conditions of purifiers is discussed. Fourth, the question of the effect of indoor air photocatalytic treatment on the average concentration of low-molecular-mass carbonyls is addressed.