Adsorption and photocatalytic degradation of diisopropyl fluorophosphate and dimethyl methylphosphonate over dry and wet rutile TiO2

Nanosized, crystalline rutile TiO2 was synthesized at room temperature using a microemulsion-mediated system followed by hydrothermal treatment. The formed rutile had a specific surface area of about 40 m(2) g(-1) and the rutile crystals had dimensions of about 10 nm x 150 nm, which aggregated into 200-1000nm sized bundles. The adsorption and photocatalytic degradation of diisopropyl fluorophosphate (DFP) and dimethyl methylphosphonate (DMMP) over these rutile TiO2 nanoparticles in dry and wet synthetic air was investigated by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy during simulated solar light illumination. The methyl and isopropyl groups do not dissociate upon adsorption on either dry or humidified rutile nanoparticles. The F atom in DFP is, however, easily hydrolyzed and is readily dissociated upon interaction with hydroxyls on the TiO2 surfaces and leads to a destabilization of the DFP molecule. The initial solar light induced photodegradation rate for DFP and DMMP is 5.9 x 10(-4) and 1.0 x 10(-4) s(-1) in dry conditions and 8.1 x 10(-4) and 0.7 x 10(-4) s(-1) in wet conditions (corresponding to 2-3 monolayers (ML) water coverage), respectively. The main intermediate partial oxidation surface products are found to be surface bound formate-carboxylate-carbonate (R-COO-) and phosphate (R-POO-) species. Among them eta(1)-coordinated acetone and mu-formate, bicarbonate, and bidentate R-POO- moieties are detected. These surface species accumulate on the surface during the entire illumination period (60 min), and lead to a decreased total oxidation rate. Controlled humidification of the rutile surface leads to a reduction of the concentration of R-COO- intermediates, while at the same time maintaining approximately the same rate of DFP and DMMP photooxidation. The latter is due to blocking of Ti surface cation sites, which prevents the formation of strongly bonded surface compounds, in particular mu-coordinated R-COO- and R-POO- species. The findings show that, it is possible to optimize the sustained photocatalytic degradation of organic phosphorous compounds by controlled humidification of the reaction gas. (c) 2006 Elsevier B.V. All rights reserved.