A comparative study of the adsorption of chloro- and non-chloro-containing organophosphorus compounds on WO3

The adsorption of dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and methyldichlorophosphate (MDCP) on monoclinic tungsten oxide (m-WO3) evacuated at various temperatures was investigated using infrared spectroscopy. DMMP is the most common molecule used for evaluating the performance of WO3 and other semiconducting metal oxide (SMO)-based sensors to phosphonate-based nerve agents. However, toxic nerve agents such as satin differ from DMMP in that they contain a functional group (P-F in sarin) that can be readily hydrolyzed. It is shown that the adsorption of organophosphates that contain P-Cl groups differs from nonhalogenated simulants such as DMMP and TMP on WO3 surfaces. Specifically, the nonchlorinated simulants DMMP and TMP adsorb on the surface solely through the P=O functionality with the surface water layer as well as the Lewis and Bronsted acid sites. The relative number of molecules bound on Lewis and Bronsted acid surface sites depends on the initial evacuation temperature of the WO3 surface. When MDCP adsorbs on WO3 through the P=O bond, it is accompanied by the hydrolysis of P-Cl groups by water vapor or the adsorbed water layer leading to additional phosphate-like species on the surface. The infrared data suggests that a halogenated phosphate like MDCP is a better simulant molecule for studies aimed at understanding the role of water and hydrolysis in the response of metal oxide-based sensors to nerve agents.