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 ox ide, magnesium oxide, lanthanum oxide, and iron oxide. Aluminum, magne sium, 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 si te, followed by stepwise elimination of the methoxy groups, beginning at temperatures as low as 50 degrees C, which combine with surface hyd rogens to yield methanol that evolves from the surface. The final prod uct observed for these oxides is a surface-bound methylphosphonate, wi th 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 sit e, 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 surfa ce. Nonselective elimination of both the methoxy and the phosphorus-bo und methyl groups begins only after heating above 200 degrees C, but o ccurs 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 mul tiple oxidation states to the iron atom. Participation of the Fe(III)/ Fe(II) redox couple in the reaction provides a low-energy path for oxi dative cleavage of the P-CH3 bond. The other oxide surfaces cannot pro vide a similar path, and on these surfaces the P-CH3 bond is resistant to cleavage. The use of infrared diffuse reflectance techniques, obse rving, in particular, the methyl stretch region of the infrared spectr um, has allowed the almost complete characterization of the decomposit ion processes which occur after DMMP adsorbs on aluminum oxide, magnes ium oxide, lanthanum oxide, and iron oxide.