O atoms losses compared to C2H2 losses on TiO2 surface after plasma exposure: an explanation for plasma/TiO2 synergy

Plasma photocatalyst coupling is an interesting combination for air treatment, but the mechanisms of plasma TiO₂ synergy is still not well understood. The time evolution of O atom density in the gas phase during the post-discharge of a pulsed N₂-O₂ DC plasma is studied using a plasma induced fluorescence technique (PIF) where a main long pulse creates the plasma and a shorter one re-excites atoms in the time post-discharge. The surface loss coefficient of O atoms on pyrex, porous silica, porous TiO₂ is measured, this latter being a photocatalytic material. When nano cluster of TiO₂ are deposited on porous silica, the loss coefficient is first high and comparable to the case of the porous silica, but decrease after 1 ms. In parallel the time evolution of the O atoms is monitored during the main plasma pulse as already measured (Guaitella et al. 2005). It is shown that, when nanoclusters of TiO₂ are deposited on silica fibers a large amount of O atoms are desorbed at the beginning of plasma pulses. These O atoms which stay available after duration longer than 1 s could explain the IR laser absorption measurement performed to monitor C₂H₂ decay on surface pre-treated by oxygen containing plasmas. Indeed, when TiO₂ surface is exposed during several minutes to air plasma, C₂H₂ introduced in the reactor after shut off the plasma is lost on the surface probably because of oxidation reactions at the surface with the adsorbed O atoms on TiO₂.