Temperature Dependencies of the Degradation of NO, NO2 and HONO on a Photocatalytic Dispersion Paint

The photocatalytic decomposition of nitrogen oxides (NOx) has attracted significant interest as a potential measure of reducing NOx levels in the urban atmosphere. Since photocatalytic activity is highly variable depending on atmospheric conditions, the uptake of NO, NO2 and HONO was studied on a commercial photocatalytic dispersion paint in a flow photoreactor as a function of the relative humidity and temperature. Since the relative humidity is a function of the surface's temperature, here both dependencies were carefully decoupled for the first time. In addition, for the first time the temperature dependence of the whole NOx reaction system including the important intermediate HONO was investigated. While for NO and NO2 strong negative humidity dependencies were observed, the photocatalytic uptake of HONO increased with humidity. For constant relative humidity no temperature dependence of the photocatalytic oxidation of NO was observed, whereas the photocatalytic NO2 uptake decreased with increasing temperature, which is explained by a temperature dependent adsorption equilibrium of the surface active NO2. HONO uptake showed a positive temperature dependence confirming the proposed photocatalysis of nitrite in a layer of adsorbed water on the surface of the photocatalyst. The missing/negative temperature dependencies of the photocatalysis of NO/NO2 are overcompensated by their strong negative relative humidity dependencies, leading to increasing uptake for both pollutants when photocatalytic surfaces are heated by solar irradiation in the atmosphere.