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.