Photoassisted Decomposition of Dimethyl Methylphosphonate over Amorphous Manganese Oxide Catalysts

Abstract

The gas-phase decomposition of dimethyl methylphosphonate (DMMP) has been studied over amorphous manganese oxide (AMO) catalysts in the presence of light (∼200−800 nm). The reaction was studied under oxidizing conditions using air at low temperatures (40−70 °C). DMMP and gas-phase products were studied using gas chromatography (GC). DMMP was found to adsorb strongly to the AMO surface and produce small amounts of methanol (MeOH) even in the absence of light. When AMO was irradiated with light of ∼200−800 nm, large amounts of MeOH and CO2 were initially formed. Following the initial period of high activity, strong deactivation was observed. After the reactions were performed, aqueous extracts from spent AMO were analyzed using ion chromatography (IC). The IC analyses indicated that several products accumulate on the AMO surface. These products include methyl methylphosphonate (MMP) and methylphosphonic acid (MPA). Greater amounts of MMP and MPA are produced after irradiation. Fourier transform infrared (FTIR) spectroscopy was used to examine adsorbed DMMP species on spent AMO. The IR results indicate that DMMP bonds to Mn Lewis acid sites on the AMO surface via the phosphoryl oxygen. On the basis of these results a mechanism is proposed for the adsorption and photoassisted decomposition of DMMP over AMO.