An Infrared Study of Adsorbed Organophosphonates on Silica:  A Prefiltering Strategy for the Detection of Nerve Agents on Metal Oxide Sensors

Abstract

The gas-phase adsorption of the nerve gas simulant dimethyl methylphosphonate (DMMP) along with trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and trichlorophosphate (TCP) on silica have been studied using infrared spectroscopy. Each phosphonate compound adsorbs through a different number of H-bonds of the methoxy and P O moieties with the surface hydroxyl groups on silica. The strength of the adsorption depends on the number and type of the H-bonds and follows the order TCP < MDCP < DMMP < TMP. TCP is completely removed from silica by evacuation at room temperature, adsorbed MDCP is removed by evacuation at 150 °C, DMMP requires an evacuation temperature of 300 °C, and TMP is eliminated at 400 °C. All phosphonate compounds molecularly desorb, and the silica returns to its original state. The differences in the reactivity of phosphonate compounds on silica from other oxides demonstrate the potential use of silica in prefiltering/preconcentrating strategies for semiconductive metal oxide based sensing devices. Specifically, it is shown that silica can be used to selectively adsorb DMMP from a gas stream containing methanol/DMMP mixtures.