Thermal Decomposition of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Adsorbate Reactions with Lattice Oxygen as Studied by Infrared Spectroscopy

Abstract

The thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on high surface area TiO2 nanoparticles (Degussa P25) has been studied by transmission infrared spectroscopy. The dominant reaction channel in the low-temperature regime from 295 to 400 K is the nucleophilic attack of adsorbed DMMP by neighboring oxygen to produce Ti−OCH3 and a variety of P−Ox surface bound groups. Arrhenius studies reveal an activation energy of ∼48 kJ mol−1 for the conversion of surface bound phosphoryl (P═O) groups into P−Ox species. Above 400 K, thermally activated lattice oxygen begins to play a dominant role in driving the oxidation of surface Ti−OCH3 groups. The electrons left behind following the extraction of lattice oxygen are observed by a broad rise in the infrared absorbance as the electrons are excited from shallow traps into the conduction band. Tracking lattice oxygen removal as a function of temperature reveals an activation energy of ∼33 kJ mol−1, over the temperature range ∼400−600 K, for the high-temperature oxidation of strongly bound surface adsorbates. These measurements can be employed to provide a more complete understanding of the key role that lattice oxygen plays in the degradation of adsorbates on the surface of active nanoparticulate semiconductor oxides.