Modeling the reactivity of Chemical Warfare Agents on metal oxides using computational chemistry methods
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Modeling the reactivity of Chemical Warfare Agents on metal oxides using computational chemistry methods

  • Author(s): Tesvara, Celine
  • Advisor(s): Sautet, Philippe PS
  • et al.
Abstract

To design efficient personal protective equipment against chemical warfare agents, there is a need to understand the fundamental pathway of decomposition of these chemicals on solid surfaces. In this thesis, the author investigates such pathways by employing computational chemistry methods, namely density functional theory (DFT) in link with experimental results obtained by temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and X-ray Photoelectron Spectroscopy (XPS).The thesis is divided into three parts, with the first part focused on the thermodynamic analysis of dimethyl methyl phosphonate (DMMP), a simulant of the nerve agent Sarin, interaction on Fe3O4(111) films grown on a Fe2O3(0001) crystal. DFT calculations reveal that dissociative adsorption of DMMP on Fe3O4(111) is very stable, which dissociates DMMP to surface methoxy and methyl methylphosphonate (MMP). Collaborative result of TPD and DFT shows three decomposition channels of DMMP: self-rearrangement of MMP to produce dimethyl ether (DME) at 600 K, surface methoxy disproportion reaction to produce CH3OH and CH2O at 700 K, and combustion of the remaining carbon-containing intermediates at 850 K. It was found that the dynamic interaction between Fe3O4 and subsurface Fe2O3 films results in dimethyl ether as an additional product, a behavior that is unique to this surface. The second part deals with elucidating the oxidative decomposition pathways of DMMP on pristine and defective rutile TiO2(110). Pathway searches were performed with the Nudged Elastic Band (NEB) method. Rate constants from Transition State Theory (TST) show that the decomposition of DMMP is slow. DMMP decomposes via O-CH3 bond cleavage on the pristine surface and P-OCH3 bond cleavage on the surface with oxygen vacancy, both cleavages happening at 600 K. Thermodynamic analyses show that P-CH3 bond cleavage of DMMP on pristine and defective surfaces are unlikely. We found that the presence of O vacancy facilitates P-OCH3 bond cleavage, adding another possible channel for active surface methoxy species creation. The last part investigates the decomposition of sarin on selected pathways over r-TiO2(110) to determine if DMMP is an adequate simulant of sarin. We conclude that the chemistry of DMMP does resemble Sarin’s well if only P-Oalko and O-C bond cleavages are considered, whereas P-F bond dissociation may show a different reactivity not seen on DMMP.

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