Thermal Decomposition of Dimethyl Methylphosphonate on Size-Selected Clusters: A Comparative Study between Copper Metal and Cupric Oxide Clusters

Abstract

Room temperature decomposition and thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected copper clusters have been studied via combined X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Cu100 and (CuO)80, which have the same nominal masses, were chosen to present a direct comparison between the reactivity of metallic copper and that of cupric oxide with DMMP. Room temperature XPS results have shown that most of the DMMP molecules decompose completely and reductively into atomic phosphorus on Cu100, while almost all the DMMP molecules are only dissociatively adsorbed on (CuO)80 as methyl methylphosphonate (MMP). XPS and TPD have been carried out to analyze the thermal decomposition of adsorbed DMMP by identifying the surface species after annealing to certain temperatures and the gaseous products evolved during linear temperature ramps, respectively. Methanol, formaldehyde, and methane are the three most significant gaseous products for DMMP decomposition on both Cu100 and (CuO)80. Methanol and formaldehyde, which evolve in the low temperature region, are believed to originate from surface methoxy species. Methanol, formaldehyde, and methane evolved in the high temperature region are related to further decomposition of the phosphorus-containing surface species. A set of methanol-probed TPD experiments have also been carried out, which suggest that methane evolution originates from the methyl group within DMMP instead of the surface methoxy species.