Effect of Modification with Vanadium or Carbon on Destructive Sorption of Halocarbons over Nanocrystalline MgO: The Role of Active Sites in Initiation of the Solid-State Reaction

Small amounts of vanadium or carbon added to nanocrystalline MgO aerogels were shown to promote their activity in destructive sorption of CF2Cl2 and CFCl3 halocarbons. This reaction is characterized by a prolonged induction period, which is considerably shortened after the addition of the studied promoters. It was demonstrated that the promoting effect of finely dispersed carbon did not depend on its location relative to the MgO nanoparticles. Approximately the same results were obtained when carbon was deposited as a thin layer on the surface of the MgO nanoparticles, when activated carbon was physically mixed with the nanocrystalline MgO or when it was located separately in the reactor. The modifying agents are believed to have a catalytic effect on the reaction of nanocrystalline MgO with halocarbons, accelerating the formation of active sites on the surface of the MgO nanoparticles. Electron-acceptor sites are shown to be possible candidates for the role of such active sites. The concentration of weak electron-acceptor sites tested by electron paramagnetic resonance (EPR) using anthracene as a spin probe increases during the induction period, reaching a well-defined maximum during the active stage of the reaction. Electron-acceptor sites of medium strength characterized by direct ionization of anthracene after adsorption were observed on the surface only during the active stage of the reaction. The studied reaction appears to be one of the first solid-state reactions where the role of surface active sites has been identified.