Postdoctoral researcher, Lawrence Berkeley National Laboratory
Dimethyl methylphosphonate decomposition on CuOx studied with in situ photoelectron spectroscopy
Gas masks are the first line of defense for emergency responders and military personnel in the event of a toxic gas exposure. The filters used today in gas masks contain activated carbon decorated with metal oxide particles. Since the filters were first developed, newer nerve agents, such as sarin and VX, have been used in chemical warfare. The interaction of filter materials with these newer nerve agents is not well understood. For example, we don't know whether these nerve agents simply get stuck and absorbed by the filter, or if they react and break down to form other compounds. If they do break down, we don't know what new compounds they make, which may also be toxic for humans or could poison the filter and cause it to fail. We also don't know how different environments might change the interaction between the filter and the nerve agent. For example, the chemical reaction may be different in a humid forest environment compared to a dry desert. Also, common pollutants in the air that one might encounter in a densely populated city could affect the chemical reactions taking place inside the filter.
We want to make new filtration materials that can provide better protection for people who might be exposed to these toxic gases. Making new and better materials will be much easier if we can first understand how the current materials work and what their weak points are. To do this, we are studying how two of the metal oxides in the filters, copper(II) oxide and molybdenum(VI) oxide, interact with dimethyl methylphosphonate (DMMP). DMMP is a molecule that has similar reactivity to the nerve gas sarin, but is much less toxic so it is safe for us to work with.
We use high energy X-rays to watch how the DMMP molecules break down on the different metal oxide surfaces. The X-rays knock electrons out of the different atoms at our sample surface, and the energy we measure for each electron is like a fingerprint that tells us what type of atom it came from and what other atoms are nearby. We use this information to piece together the possible chemical reactions that might be happening, and we check which reaction pathways are the most likely using computer simulations. Our instrument also lets us add more than one gas at a time, so we can study how the reaction with DMMP changes if we make the environment more humid (by adding water vapor) or more polluted (by adding NOx and hydrocarbons to simulate fuel exhaust).
Abstract: Dimethyl methylphosphonate (DMMP) is one of the most widely used molecules to simulate chemical warfare agents in adsorption experiments. However, the details of the electronic structure of the isolated molecule have not yet been reported. We have directly probed the occupied valence and core levels using gas phase photoelectron spectroscopy and the unoccupied states using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Density functional theory (DFT) calculations were used to study the electronic structure, assign the spectral features, and visualize the molecular orbitals. Comparison with parent molecules shows that valence and core-level binding energies of DMMP follow trends of functional group substitution on the P center. The photoelectron and NEXAFS spectra of the isolated molecule will serve as a reference in studies of DMMP adsorbed on surfaces.
Pub.: 15 Mar '16, Pinned: 27 Jun '17
Abstract: Over the past several decades, ambient pressure x-ray photoelectron spectroscopy (APXPS) has emerged as a powerful technique for in situ and operando investigations of chemical reactions under relevant ambient atmospheres far from ultra-high vacuum conditions. This review focuses on exemplary cases of APXPS experiments, giving special consideration to experimental techniques, challenges, and limitations specific to distinct condensed matter interfaces. We discuss APXPS experiments on solid/vapor interfaces, including the special case of 2D films of graphene and hexagonal boron nitride on metal substrates with intercalated gas molecules, liquid/vapor interfaces, and liquid/solid interfaces, which are a relatively new class of interfaces being probed by APXPS. We also provide a critical evaluation of the persistent limitations and challenges of APXPS, as well as the current experimental frontiers.
Pub.: 03 Dec '16, Pinned: 27 Jun '17