NO oxidation on plasma pretreated Pyrex: the case for a distribution of reactivity of adsorbed O atoms

The formation of NO2 molecules on a Pyrex surface, as a result of NO oxidation by adsorbed O atoms on the wall, is experimentally demonstrated and quantified. The measurements reveal that the characteristic times of heterogeneous NO2 production and NO gas phase decay change from similar to 60 to similar to 1500 s as the gas phase concentration of NO introduced in a tube pretreated with an oxygen radiofrequency discharge increases from 10(13) to 10(15) cm(-3). Moreover, a clear variation of the characteristic loss frequency of NO molecules when small amounts of NO are successively injected in the tube is detected, between similar to 7 x 10(-2) and similar to 5 x 10(-3) s(-1). The complex surface kinetics observed is studied and interpreted with the help of a mesoscopic surface model accounting for Eley-Rideal NO oxidation and slow NO2 adsorption, confirming the existence of adsorption sites possessing a distribution of reactivity on the surface.