Journal of Photochemistry and Photobiology A: Chemistry 184 (2006) 125–134 Adsorption and photocatalytic degradation of diisopropyl fluorophosphate and dimethyl methylphosphonate over dry and wet rutile TiO 2 A. Kiselev a,b , A. Mattson a , M. Andersson c , A.E.C. Palmqvist c , L. ¨ Osterlund a,∗ a Department of Environment and Protection, FOI NBC Defence, Cementv. 20, SE-901 82 Ume˚ a, Sweden b Department of Inorganic Chemistry, Ume˚ a University, SE-901 87 Ume˚ a, Sweden c Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 G¨ oteborg, Sweden Received 23 February 2006; received in revised form 30 March 2006; accepted 3 April 2006 Available online 18 April 2006 Abstract Nanosized, crystalline rutile TiO 2 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 × 150 nm, which aggregated into 200–1000 nm sized bundles. The adsorption and photocatalytic degradation of diisopropyl fluorophosphate (DFP) and dimethyl methylphosphonate (DMMP) over these rutile TiO 2 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 TiO 2 surfaces and leads to a destabilization of the DFP molecule. The initial solar light induced photodegradation rate for DFP and DMMP is 5.9 × 10 -4 and 1.0 × 10 -4 s -1 in dry conditions and 8.1 × 10 -4 and 0.7 × 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  1 -coordinated acetone and -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 -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. © 2006 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; TiO 2 ; Rutile; Nanoparticles; Organic phosphorous compounds; Dimethyl methylphosphonate; Diisopropyl fluorophosphate 1. Introduction Photocatalysis denotes the acceleration of a photoreaction by the action of a catalyst [1–4]. In the present context we use a more narrow description, i.e. the oxidative degradation of organic pol- lutants by an UV irradiated semiconductor, notably TiO 2 , which is by far the most commonly used photocatalyst. A wide array of organic substances containing O, N, S, P, and halogen atoms can be oxidized by this photochemical method. Hence, this route provides a convenient oxidative degradation method requiring ∗ Corresponding author. Tel.: +46 90 106900; fax: +46 90 106802. E-mail address: lars.osterlund@foi.se (L. ¨ Osterlund). no unstable and potentially dangerous chemical oxidants. Pho- tocatalysis is therefore, an attractive environmentally benign method for decontamination of organic phosphorous impurities, including warfare agents and pesticides. There is today a gen- eral understanding of the photocatalytic action of TiO 2 , albeit fundamental questions regarding, e.g. interfacial charge transfer mechanisms and deactivation still exist [1–3]. Briefly, light with energy larger than the optical band gap of TiO 2 (>3.0 eV for rutile) creates an electron hole pair, which is trapped via inter- facial electron transfer by appropriate redox-active adsorbates. On a humidified surface or in the aqueous phase, oxidation of adsorbed water or OH - by the hole produces hydroxyl radicals ( • OH), while reduction of O 2 produces O 2- or O - radicals, both of which are extremely powerful and indiscriminating oxidants 1010-6030/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2006.04.005