Decomposition of the Toxic Nerve Agent Sarin on Oxygen Vacancy Sites of Rutile TiO₂(110)

Abstract

To design effective personal protective equipment against chemical attacks, the understanding of chemical warfare agents (CWAs) decomposition chemistry is crucial. Metal oxides, particularly TiO 2 have been found to be promising materials to trap and decompose CWAs. This work explores the possible decomposition pathways of sarin on a model rutile TiO 2 (110) surface with and without the presence of surface oxygen vacancies. Sarin adsorbs on the surface mainly by its P�O unit via a dative P�O-Ti 5c bond, similar to its simulant dimethyl methylphosphonate (DMMP). Sarin decomposition on the pristine surface is possible at 455 K and proceeds via O−C bond cleavage, with a barrier of 1.17 eV, resulting in the production of surface-bonded monofluorophosphate and isopropoxy, while P−OR (R = C 3 H 7 isopropyl) or P−F cleavage is highly activated with barriers larger than 2 eV. However, the production of gas-phase propene after O−C cleavage has a high activation barrier (1.6 eV). In the presence of O vacancies, the barriers to cleave the P−F and P−OR bonds are greatly reduced and these cleavages become possible at a moderate temperature (425 K). In comparison to its simulant DMMP, the decomposition of sarin proceeds faster on the oxygen vacancy as the cleavage of the P−F bond is more facile and the binding of F on surface Ti creates a thermodynamically stable intermediate. The electronic effects of the F ligand also facilitate the P−OR bond cleavage at the O vacancy site. Frequency calculations validate the energy pathways: intact molecular adsorption of sarin can explain the experimental spectrum at room temperature, while further decomposition by C−O or P−F bond cleavage, presumably on the pristine surface and at O vacancies, respectively, is responsible for the spectral evolution seen at 500 K, in agreement with calculated barriers.

Diisopropyl Methylphosphonate and Sarin Decomposition on Pristine vs Hydroxylated Alumina Surfaces: Mechanistic Predictions from Ab Initio Molecular Dynamics

“…Figure f shows an adsorption energy of −17.62 kcal/mol, where sarin interacts with the surface through a P–F bond. Our findings corroborate previous studies that have identified the most favorable adsorption configurations arising from interactions between the O and F atoms of sarin with the surfaces of MgO, TiO, and ZnO …”

“…Metal oxide surfaces are commonly catalytic, which can adsorb and break down CWAs into harmless products. Prior experiments and theoretical calculations have shown that sarin simulants can adsorb onto metal oxides by forming a bond between a phosphoryl O atom and a metal atom. − When simulating the adsorption of DIMP on a pristine alumina (110) surface, we considered four possible stable arrangements, shown in Figure . The adsorption energies, E ads , were calculated using the following expression: E ads = E DIMP + surface − E surface − E DIMP where E DIMP+surface is the total energy of the adsorbed molecule and the surface, E DIMP is the energy of the isolated gas-phase DIMP molecule, and E surface is the energy of the bare alumina surface.…”

Understanding the Decomposition of Dimethyl Methyl Phosphonate on Metal-Modified TiO2(110) Surfaces Using Ensembles of Product Configurations

“…Although the lowest-energy adsorption structures for DMMP on Cu, K–Cu, and bare TiO 2 (110) surfaces are essentially the same, the DMMP adsorption energies show that addition of the Cu 4 cluster strengthens DMMP binding (−2.70 eV), while K addition destabilizes DMMP binding (−2.19 eV) relative to DMMP on the bare TiO 2 (110) surface (−2.35 eV). The latter value was recently reported by Tesvara et al using a similar level of DFT. , The lower DMMP adsorption energy on the K–Cu surface is attributed to the additional electron charge donated from the K atoms to the TiO 2 surface as indicated by the calculated Bader charges (Figure c,f). Lowering the binding energies of DMMP and its decomposition fragments is one of the expected outcomes of alkali addition, as electron transfer to the TiO 2 surface should effectively decrease the Lewis acidity of the Ti 5c cations and weaken bonds to electrophilic adsorbates like DMMP.…”

“…For the lowest-energy configurations (Figure a,e), DMMP binds in an η 2 -O–P–O­(CH 3 ) configuration to two Ti 5c 4+ cations aligned along the [001] direction of the TiO 2 (110) surface, while the second methoxy and the methyl groups are oriented along the perpendicular [11̅0] direction. This DMMP adsorption structure is essentially identical to that calculated for DMMP on the bare TiO 2 (110) surface. , On the Cu surface (Figure a), the Cu 4 cluster is located in the same row as DMMP and adopts a square geometry by forming four Cu–O bonds with nearby bridging O atoms. This is also the case on the K–Cu surface (Figure e) although the structure of the Cu 4 cluster is modified by direct interactions with the K atom.…”

Mechanistic Diversity in the Hydrolysis of Sarin by Single Transition-Metal Atoms on MOF-808

“…Instead, the alcohol binds to Ti­(IV), much as has been described at titania surfaces with open Ti­(IV) sites . This convergence of bonding patterns suggests that the voluminous work on the chemistry of nerve agents with extended metal-oxide surfaces − can inform studies on the discrete metal-oxide cores of MOFs. The P complex featuring an alcohol coordination to the Ti­(IV) Lewis acid is notably deeper (44 kJ/mol iPrOH Gibbs desorption energy in Ti­(IV)) than the hydrogen-bonded complex described for Zn­(II) in the same channel (10 kJ/mol, Figure ).…”