The decomposition of dimethyl methylphosphonate (DMMP, (CH3O)2P(O)(CH3)), a simulant to the toxic nerve agent Sarin, on the rutile TiO2(110) surface has been studied with temperature programmed desorption (TPD) and density...

Dimethyl methylphosphonate (DMMP) is used as a simulant for toxic nerve agents and pesticides, rendering the understanding of surface chemistry requisite to design effective materials for organophosphonate (catalytic) decomposition at room temperature. In this work, DMMP surface chemistry is studied on an iron oxide surface in a very well-defined environment using temperature-programmed reaction, isotopic labeling, scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory. DMMP, (CH 3 O) 2 P(O)(CH 3 ), dissociates to yield methoxy and methyl methylphosphonate, (CH 3 O)-P(O) 2 (CH 3 ), on the surface at room temperature. At higher temperatures, dimethyl ether is formed via intramolecular reaction, followed by the formation of formaldehyde and methanol from adsorbed methoxy decomposition during temperature-programmed reaction. Ultimately, stochiometric combustion at 870 K produces CO, H 2 CO, and CO 2 via reaction with lattice oxygen, with PO x remaining on the surface. Excess oxygen from the bulk is required to drive these higher temperature pathways. Neither hydrolysis nor a photoreaction is observed, when exposing the adsorbed DMMP to water or light above the band gap, respectively. No evolution of P-containing species is detected, indicating efficient trapping of this contaminant. The activity for DMMP decomposition at room temperature is reduced by the accumulation of PO x . However, a significant amount of reaction persists after multiple temperature-programmed reaction experiments.

To design effective personal protective equipment against chemical attacks, the understanding of chemical warfare agents (CWAs) decomposition chemistry is crucial. Metal oxides, particularly TiO 2 have been found to be promising materials to trap and decompose CWAs. This work explores the possible decomposition pathways of sarin on a model rutile TiO 2 (110) surface with and without the presence of surface oxygen vacancies. Sarin adsorbs on the surface mainly by its P�O unit via a dative P�O-Ti 5c bond, similar to its simulant dimethyl methylphosphonate (DMMP). Sarin decomposition on the pristine surface is possible at 455 K and proceeds via O−C bond cleavage, with a barrier of 1.17 eV, resulting in the production of surface-bonded monofluorophosphate and isopropoxy, while P−OR (R = C 3 H 7 isopropyl) or P−F cleavage is highly activated with barriers larger than 2 eV. However, the production of gas-phase propene after O−C cleavage has a high activation barrier (1.6 eV). In the presence of O vacancies, the barriers to cleave the P−F and P−OR bonds are greatly reduced and these cleavages become possible at a moderate temperature (425 K). In comparison to its simulant DMMP, the decomposition of sarin proceeds faster on the oxygen vacancy as the cleavage of the P−F bond is more facile and the binding of F on surface Ti creates a thermodynamically stable intermediate. The electronic effects of the F ligand also facilitate the P−OR bond cleavage at the O vacancy site. Frequency calculations validate the energy pathways: intact molecular adsorption of sarin can explain the experimental spectrum at room temperature, while further decomposition by C−O or P−F bond cleavage, presumably on the pristine surface and at O vacancies, respectively, is responsible for the spectral evolution seen at 500 K, in agreement with calculated barriers.