Photocatalytic products of the NO2 de-polluting process

MARÍA EUGENIA MONGE; CHRISTIAN GEORGE; BARBARA D'ANNA; JEAN-FRANÇOIS DOUSSIN; ADLA JAMMOUL; JUNNAN WANG; GRÉGORY EYGLUNENT; GÉRALDINE SOLIGNAC; VÉRONIQUE DAËLE; ABDELWAHID MELLOUKI

Orleans

Workshop; 2nd sino-french Joint Workshop on Atmospheric Environment; 2010

Titanium oxide, TiO2, has become the material of choice in a variety of remediation processes due to its photocatalytic properties as well as its favourable physical and chemical properties. Many studies have recently focused on the development of environmental friendly materials by adding TiO2 to ordinary building materials such as concrete or by preparing TiO2 film coatings in order to deplete air pollutants, such as NOx. Although various photocatalytic materials are already on the market, very little reliable information is available, except for limited technical data, regarding their impact on air quality. Recently, the photoenhanced NO2 uptake for real mineral dusts with HONO production was assigned to the chemistry occurring on the TiO2; and the photochemistry of nitrate doped dust samples was proved to be a potential renoxification process of the atmosphere. This work presents the study of the reactivity of irradiated TiO2 / SiO2 films with different TiO2 contents as proxies for de-polluting materials towards NO2 using a coated-wall flow tube reactor. The influence of the photocatalyst concentration, the role of molecular oxygen and the effect of nitrate on the reactivity of TiO2 films were investigated. NO, HNO2 and nitrate are produced as a consequence of the NO2 loss on UV-illuminated TiO2 films. A renoxification pathway that involves the photochemistry of the NO3 radical is involved in the release of NO, NO2, HNO2 and O3 from the TiO2 surface. Although O3 has been recently proven to decompose on illuminated TiO2 surfaces, its formation is observed when TiO2 treated surfaces are exposed to NOx under illumination. The presence of O2 in the carrier gas modifies the NO and HNO2 production yields in the heterogeneous reaction between NO2 and TiO2 as well as the products of the renoxification process. These processes need to be considered further as potentially important in polluted urban atmospheres; considering the role of buildings or roads and of mineral dust in promoting heterogeneous reactions.