An infrared study of adsorbed organophosphonates on silica: A prefilteringstrategy for the detection of nerve agents on metal oxide sensors

The gas-phase adsorption of the nerve gas simulant dimethyl methylphosphona te (DMMP) along with trimethyl phosphate (TMP), methyl dichlorophosphate (M DCP), and trichlorophosphate (TCP) on silica have been studied using infrar ed spectroscopy. Each phosphonate compound adsorbs through a different. num ber of H-bonds of the methoxy and P=O moieties with the surface hydroxyl gr oups on silica. The strength of the adsorption depends on the number and ty pe of the H-bonds and follows the order TCP < MDCP < DMMP < TMP. TCP is com pletely removed from silica by evacuation at room temperature, adsorbed MDC P is removed by evacuation at 150 <degrees>C, DMMP requires an evacuation t emperature of 300 degreesC, and TMP is eliminated at 400 degreesC. All phos phonate compounds molecularly desorb, and the silica returns to its origina l state. The differences in the reactivity of phosphonate compounds on sili ca from other oxides demonstrate the potential use of silica in prefilterin g/preconcentrating strategies for semiconductive metal oxide based sensing devices. Specifically, it is shown that silica can be used to selectively a dsorb DMMP from a gas stream containing methanol/DMMP mixtures.