Adsorption and Decomposition of Dimethyl Methylphosphonate on Metal Oxides

Abstract

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 °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 °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 °C, but occurs with total elimination of the methyl and methoxy groups observed after heating above 300 °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.

Thermal desorption of dimethyl methylphosphonate from MoO3

“…Since surface defects and hydroxylation promote DMMP decomposition, the small band at 971 cm −1 (Figure 1(b)) suggests the O 2 pre-treatment did not completely remove these sites. However, the similarity in the C-H stretch region of the IR spectra (Figure 1(a) [9]. Also, the OCH 3 symmetric stretch (1035 cm −1 ) of surface-adsorbed DMMP lies between the gas (1050 cm −1 ) and liquid (1032 cm −1 ) phase positions, indicating these groups interact with the MoO 3 surface, albeit weaker than the P=O group.…”

“…DMMP has been found to adsorb onto several metal oxides, including Al 2 O 3 [5,9,26,27], Fe 2 O 3 [9], MgO [5,9], ZnO [5], La 2 O 3 [9], SiO 2 [28,29], TiO 2 [5,21], CeO 2 [7,21], and WO 3 [5]. Of these, intact adsorption at room temperature occurs on TiO 2 [21], CeO 2 [21], and SiO 2 [28,29].…”

“…The current work continues this study by presenting more evidence for intact adsorption using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), a technique that has been used to characterize interactions between DMMP and various other metal oxides [7][8][9]. The DRIFTS experiments are supported by APXPS measurements in which we determine the effect of DMMP pressure and coverage on thermal desorption, focusing on three conditions: (i) after a 4 Langmuir (L, 1 × 10 −6 Torr for 1 s) dose of DMMP, (ii) under 10 mTorr of DMMP, and (iii) after extended exposure to 10 mTorr of DMMP.…”

“…To further investigate binding of DMMP on MoO 3 , a DRIFTS spectrum of MoO 3 nanoparticles exposed to a stream of DMMP vapor is compared to a spectrum of liquid DMMP (Figure 1). The peaks are assigned according to reference values reported in the literature [9]. Since surface defects and hydroxylation promote DMMP decomposition, the small band at 971 cm −1 (Figure 1(b)) suggests the O 2 pre-treatment did not completely remove these sites.…”

Decomposition of GD on CeO2/Alumina Adsorbents in a Gas Chromatograph On-Column Injector Tube Reactor

“…4 The mechanism is due to a change in the oxidation state of the metal, which creates a low-energy pathway for nucleophile oxygen (02) to react with the adsorbate (DMMP). 2 An example of the Mars-van Krevelen mechanism for the decomposition of DMMP on Fe 2 0 3 /alumina is shown in schemes 1 and 2.2…”