Adsorption and decomposition of dimethyl methylphosphonate on metal oxides

The adsorption and decomposition of dimethyl methylphosphonate (DMMP) have been examined on four different metal oxide surfaces: aluminum oxide, magnesium oxide, lanthanum oxide, and iron oxide. Aluminum, magnesium, and lanthanum oxides are observed to behave in much the same way, with initial binding of the P=O species to the surface at an acid site, followed by stepwise elimination of the methoxy groups, beginning at temperatures as low as 50 degrees C, which combine with surface hydrogens to yield methanol that evolves from the surface. The final product observed for these oxides is a surface-bound methylphosphonate, with the P-CH3 bond intact, which is resistant to further oxidation even in the presence of 70 Torr of oxygen at 300-400 degrees C. Adsorption on iron oxide yields a different sequence of events, with the initial adsorption occurring again with the P=O moiety binding to an acid site, although there is some indication of the formation of a second type of surface complex. The primary interaction on iron oxide appears to be much stronger than with the other oxides, and probably involves the unidentate coordination of the DMMP to a Lewis acid site on the surface. Nonselective elimination of both the methoxy and the phosphorus-bound methyl groups begins only after heating above 200 degrees C, but occurs with total elimination of the methyl and methoxy groups observed after heating above 300 degrees C in vacuum. The ease with which iron oxide cleaves the P-CH3 bond is attributed to the availability of multiple oxidation states to the iron atom. Participation of the Fe(III)/Fe(II) redox couple in the reaction provides a low-energy path for oxidative cleavage of the P-CH3 bond. The other oxide surfaces cannot provide a similar path, and on these surfaces the P-CH3 bond is resistant to cleavage. The use of infrared diffuse reflectance techniques, observing, in particular, the methyl stretch region of the infrared spectrum, has allowed the almost complete characterization of the decomposition processes which occur after DMMP adsorbs on aluminum oxide, magnesium oxide, lanthanum oxide, and iron oxide.