(560fu) Chemical Warfare Agent Simulant Degradation over Metal Nanoparticles Supported on TiO2 Aerogels

Ashley M. Pennington,¹ Paul A. DeSario,² Catherine L. Pitman,¹ Debra R. Rolison,² Jeremy J. Pietron²

¹ National Research Council Postdoctoral Associate

² Chemistry Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA

The need to develop and implement materials that can degrade and decontaminate Chemical Warfare Agents (CWAs) has spurred research in the area of both sorbent and catalytic materials. We use titanium dioxide (TiO₂) aerogels, which offer a mesoporous network co-continuous with a covalently bonded solid network of TiO₂ nanoparticles, as catalytic supports for the degradation of chemical warfare agent simulants (CWSs). Mesoporous expressions of metal-oxide catalysts are advantageous as catalytic supports because their pore sizes allow uninhibited mass transport of the CWA to the catalytic active sites, unlike the many microporous structures currently under study. Adding metal (Cu or Au) nanoparticles (NPs) to the mesoporous oxide support adds thermocatalytic and photocatalytic functionality.

The aerogels are characterized via X-ray diffraction (XRD) for crystallinity, N₂ porosimetry for surface area and pore size distributions, and UV–visible spectroscopy for surface plasmon resonance (SPR) response of the aerogel-supported metal NPs. The degree and reversibility of agent binding to the catalyst is investigated via X-ray Photoelectron Spectroscopy (XPS) conducted before and after simulant exposure. Four classes of aerogels are evaluated for their ability to catalytically degrade CWA simulants: TiO₂ aerogels, Cu/TiO₂ with supported copper NPs, Au/TiO₂ with supported gold NPs, and 3D Au–TiO₂ with gold NPs entrained in the TiO₂ network. We measure catalytic and photocatalytic activity of these modified aerogels for degradation of two mimics for nerve agents such as (RS)-propan-2-yl methylphosphonofluoridate (Sarin, GB): dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP). Catalytic activity is monitored via in situ DRIFTS (diffuse reflectance infrared fourier-transform spectroscopy) in anaerobic (50 mL min^–1 He) and aerobic (40 mL min^–1 He,
10 mL min^–1 O₂) flow, under either white light irradiation or dark conditions for a reaction time of four hours at various relative humidities.

The pathways, rates, and product distributions of DMMP degradation differ tremendously based on the conditions to which the catalysts are exposed. Under atmospheric pressure, dry Cu/TiO₂ in the dark under both aerobic and anaerobic conditions exhibits minimal activity for DMMP degradation. Under white-light irradiation, Cu/TiO₂ is catalytically active for the degradation of DMMP under both aerobic and anaerobic conditions. DMMP fragments consistent with hydrolysis products are evident via DRIFTS on the catalyst surface under anaerobic conditions while oxidation products are evident under aerobic conditions. The activity and product distribution on Au–TiO₂ and Au/TiO₂ differ from those observed on Cu/TiO₂ aerogels. Preliminary results from in situ DRIFTS during DMMP degradation over 3D Au–TiO₂ indicate that water plays an important role in DMMP degradation in the dark, which is likely due to proton mobility along water wires that form on the surface of the networked oxide support. These results show the importance of conducting a detailed analysis of the effect of humidity ranges and illumination on the catalyst’s efficacy as a CWA decontaminant.

A fundamental understanding of the ability of metal/TiO₂ aerogels to bind and catalytically degrade CWSs under various atmospheric-relevant conditions will forge a path to test these materials using live CWAs, leading to the development and implementation of a highly active, inexpensive, catalytic alternative for field-decontamination.