Computational Study of Environmental Effects in the Adsorption of DMMP, Sarin, and VX on γ-Al₂O₃: Photolysis and Surface Hydroxylation

Abstract

The adsorption of the chemical warfare agent simulant dimethyl methylphosphonate (DMMP) and the real agents Sarin and VX on the γ-Al2O3 surface has been studied using density functional theory. The focus is primarily on two different environmental effects, namely, surface hydroxylation and photoexcitation due to terrestrial solar radiation. Cluster models for the hydroxylated surface have been examined in detail, focusing on the chemical and electronic structure. The energy for formation via dissociative adsorption of H2O, the density of states of the occupied cluster orbitals and the OH deprotonation energies have been compared with results from two-dimensionally periodic slab calculations and, where available, with experimental data. For all three species, adsorption on an OH-free surface occurs via an Al(Td)—O═P dative bond to an unsaturated tetrahedral Al(Td) site. For the hydroxylated surface, OH sites which are 3-fold coordinated to Al are more reactive than one-fold coordinated sites, in agreement with experiment. In hydrogen-bond formation, the phosphonyl O atom is favored over other active centers (e.g., an alkoxy O atom); however, dative bonding remains the most stable mode of adsorption when OH-free Al(Td) sites are available. The tertiary amine group in VX is found to be sufficiently basic to extract an H+ from an acidic OH site. Even on a fully-OH-covered surface, only a single OH—O═P bond is formed, together with weaker CH—O bonds involving alkyl groups. These have been analyzed using the Atoms in Molecules theory. The threshold for electronic excitation of DMMP or Sarin, either in the gas phase or adsorbed on γ-Al2O3, is found to occur in the vacuum-ultraviolet, well beyond the upper limit of the terrestrial solar spectrum (∼4.5 eV). For VX, on the other hand, the threshold is at ∼4.2 eV in the gas phase and shifts slightly to the red when adsorbed. Adsorption-induced shifts in the threshold transition energies have been analyzed in terms of those in the initial and final states.