Title

Environmentally benign catalysis: I. Photocatalytic degradation of air contaminants. II. Thermocatalytic decomposition of nerve gas simulants

Date of Completion

January 2001

Keywords

Chemistry, Inorganic

Degree

Ph.D.

Abstract

Nanoscale SnO2 with a particle size of 5 nm was first synthesized for photocatalytic degradation of 1butene. Compared to standard P-25 TiO 2, nanoscale SnO2 exhibited a much high photoactivity, but this material could not tolerate high humidity due to the preferred adsorption of water on the active sites, surface hydroxyl groups. On the basis of initial work, nanoscale TiO2 photocatalysts prepared with a sol-gel method were developed to show excellent photoefficiency and water tolerance. Water was found to play a double role: maintaining the constant oxidation rates at low levels and inhibiting the adsorption of reactants at high levels. Using toluene as a substrate caused severe deactivation of catalysts due to the irreversible adsorption of partially oxidized intermediates, such as benzaldehyde and benzoic acid. Complete recovery of catalytic activity occurred only when the regeneration temperature was above 420°C. Loading platinum on TiO 2 lowered the regeneration temperature considerably at the expense of some photoactivity. The kinetic studies indicated a Langmuir-Hinshelwood reaction mechanism and linear dependence of initial oxidation rates on catalyst surface areas. ^ Studies of the decomposition of dimethyl methylphosphonate (DMMP) over metal oxide catalysts revealed that the vanadium catalyst with a loading of 10 wt % on SiO2 showed superior catalytic activity to that for a 1% Pt/Al2O3 catalyst, which has been widely reported in the literature. The occurrence of deactivation over Al2O 3 supported catalysts was due to the formation of noncrystalline AlPO 4 and the deposition of coke. In a search for an explanation of experimental observations, activated carbons with high surface areas, large mesopore and macropore volume, and good thermal stability in air were found to exhibit exceptional catalytic activity for the oxidative decomposition of DMMP. Although the catalysts lost their surface areas in the first several hours, the protection time was prolonged more than 100 h since P2O5 accumulated in the initial stage was able to catalyze the decomposition of DMMP. A two-stage reaction model and autocatalytic mechanism were proposed to explain existing experimental results obtained on various catalysts. ^