Adsorption of Dimethyl Methylphosphonate on MoO₃: The Role of Oxygen Vacancies

Abstract

Dimethyl methylphosphonate (DMMP) is a common chemical warfare agent simulant and is widely used in adsorption studies. To further increase the understanding of DMMP interactions with metal oxides, ambient pressure X-ray photoelectron spectroscopy was used to study the adsorption of DMMP on MoO3, including the effects of oxygen vacancies, surface hydroxyl groups, and adsorbed molecular water. Density functional theory calculations were used to aid in the interpretation of the APXPS results. An inherent lack of Lewis acid metal sites results in weak interactions of DMMP with MoO3. Adsorption is enhanced by the presence of oxygen vacancies, hydroxyl groups, and molecular water on the MoO3 surface, as measured by photoelectron spectroscopy. Computational results agree with these findings and suggest the

Thermal desorption of dimethyl methylphosphonate from MoO3

“…The interaction of the photon beam with the sample can cause changes to the spectra of MoO 3 [15] and DMMP (i.e. beam damage), as we have characterized previously [6]. Continually changing the irradiated sample area minimized photon beam exposure and the spectra reported here are free from photon-induced damage.…”

“…The most stable (0 1 0) surface of MoO 3 is oxygen-terminated with no inherent undercoordinated Mo atoms on the surface [4] and does not easily hydroxylate; thus, this surface contrasts with those previously studied [1,5]. Here, we continue our previous investigations of the adsorption of dimethyl methylphosphonate (DMMP), a common chemical warfare agent simulant, on polycrystalline MoO 3 [6]. Our past studies with ambient pressure X-ray photoelectron spectroscopy (APXPS) and density functional theory found evidence for weak, intact adsorption on pristine MoO 3 surfaces at room temperature, while decomposition to methanol upon adsorption was observed for hydroxylated surfaces.…”

“…At 373 K the O 1s spectrum becomes identical to that of pristine MoO 3 . Figure 3(b) shows the Mo 3d spectra, with a small amount of Mo 5+ indicated by the low binding energy shoulder at ~231.3 eV; we have assigned this peak to surface defects and possibly a lower oxidation state Magnéli phase resulting from our preparation method [6]. No changes are seen in the Mo 3d spectrum after DMMP dosing and heating.…”

“…Our previous studies suggested that DMMP adsorbs largely intact on polycrystalline MoO 3 through an interaction between the phosphoryl oxygen and the terminal oxygen atoms or hydroxyl groups of the molybdenum oxide surface [6]. 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).…”

“…Phosphorus-containing a feature corresponding to a small amount of hydroxyl groups on the surface (~10% of a monolayer, ML). A DMMP coverage of ~15% of a ML is calculated from the intensities of the molecular and oxide components, described previously [6,17]. At 373 K the O 1s spectrum becomes identical to that of pristine MoO 3 .…”