3652
Langmuir 2009, 25, 3652-3658
Uptake of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Reactive Adsorption and Active Site Poisoning Dimitar A. Panayotov and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 ReceiVed December 6, 2008. ReVised Manuscript ReceiVed December 28, 2008 Using Fourier transform infrared spectroscopy (FTIR) we studied the overall reaction pathways and ultimate fate of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on a commercial nanoparticulate (∼20 nm) titania material. Our data show that the initial uptake occurs through both molecular and reactive adsorption. Molecular adsorption is driven by hydrogen-bond formation to isolated hydroxyl groups. The reactive chemisorption appears to occur through interaction with both Lewis acid sites and active oxygen species present on the TiO2 surface. The reactive sites are found to be poisoned quickly by oxidation products that include a strongly bound, nonvolatile phosphorus compound. Thermal reactivation of the TiO2 in oxygen restores the physisorption capacity of the particles toward the DMMP, but the reactive adsorption pathway is nearly completely eliminated.
I. Introduction Understanding the interfacial chemistry of organophosphorous compounds on the surface of photoactive metal oxides and metal oxide nanoparticles is important for devising new methods for neutralization of chemical warfare agents (CWA’s).1 The surface chemistry of dimethyl methylphosphonate (DMMP), one of the most extensively studied CWA simulants,2 has been explored by a number of research groups. In particular, several investigations into the uptake and reaction pathways of DMMP on nanoparticulate titanium dioxide show the potential of this material as a reactive sorbant of CWAs. However, few researchers have studied the details of DMMP reactivity on clean, dehydroxylated TiO2, and very little is known about the role of surface adsorbates in the overall chemistry on this important material. In this paper, we report the high initial reactivity of dehydroxylated TiO2 in the absence of photoexcitation but show that the oxidation products of DMMP decomposition readily poison the active sites on the surface. Early research into the uptake of DMMP on surfaces focused on studying interfacial binding to the surface of metal oxides. Many of these studies employed infrared spectroscopy as a sensitive probe of the binding mechanism because the strength of the molecule-surface interaction is evidenced by spectral changes of the phosphoryl mode upon adsorption. Specifically, the ν(PdO) mode appears to red shift relative to the gas-phase frequency when bound to a number of materials. The gas-phase infrared mode for the PdO group appears at 1276 cm-1, while condensed DMMP on TiO2 (at
