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Impact of Ambient Gases on the Mechanism of the [Cs8Nb6O19]-Promoted Nerve-Agent Decomposition

Abstract

The impact of the ambient gas molecules (X), NO2, CO2 and SO2, on the structure, stability and decontamination activity of the Cs8Nb6O19 polyoxometalate was studied computationally and experimentally. It was found that Cs8Nb6O19 absorbs these molecules stronger than it adsorbs water and Sarin (GB) and that they hinder nerve agent decontamination. Impact of the diamagnetic CO2 and SO2 molecules to polyoxoniobate Cs8Nb6O19 is fundamentally different than that of the NO2 radical. At ambient temperatures, a weak coordination of the first NO2 radical to Cs8Nb6O19 renders partial radical character to polyoxoniobate and promotes a stronger coordination of the second NO2 adsorbent to form stable diamagnetic Cs8Nb6O19/(NO2)2 species, while at low temperatures, NO2 radicals form a stable dinitrogen tetraoxide (N2O4) that weakly interacts with the Cs8Nb6O19. It is found that both in the absence and presence of ambient gas molecules, GB decontamination by the Cs8Nb6O19 species proceeds via general base hydrolysis involving: (a) the adsorption of water and the nerve agent on Cs8Nb6O19/(X), (b) concerted hydrolysis of the water molecule on a basic oxygen atom of the polyoxoniobate and nucleophilic addition of the nascent OH group to the phosphorus center of Sarin, and (c) rapid reorganization of the formed pentacoordinated-phosphorus intermediate followed by dissociation of either HF or isopropanol and formation of POM-bound isopropyl methyl phosphonic acid (i-MPA) or methyl phosphonofluoridic acid (MPFA), respectively. The presence of the ambient gas molecules raises the energy of the intermediate stationary points relative to the reactant asymptote, and slightly increases the hydrolysis barrier. These changes closely correlate with the Cs8Nb6O19-X complexation energy. The energetically most stable intermediates of the GB hydrolysis and decontamination reaction are found to be Cs8Nb6O19/X-MPFA-(i-POH) and Cs8Nb6O19/X-(i-MPA)-HF, both in the absence and presence of ambient gas molecules. The high stability of these intermediates is due in part to the strong hydrogen bonds between the adsorbates and the protonated [Cs8Nb6O19/X/H]+-core. Desorption of the HF or/and (i-POH) and regeneration of the catalyst requires deprotonation of the [Cs8Nb6O19/X/H]+-core and protonation of phosphonic acids (i-MPA) and MPFA. This catalyst regeneration is shown to be a highly endothermic process and the success-limiting step of the GB hydrolysis and decontamination reaction, both in the absence and presence of ambient gas molecules.

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Publication details

The article was received on 21 Nov 2017, accepted on 05 Jan 2018 and first published on 08 Jan 2018


Article type: Edge Article
DOI: 10.1039/C7SC04997H
Citation: Chem. Sci., 2018, Accepted Manuscript
  • Open access: Creative Commons BY license
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    Impact of Ambient Gases on the Mechanism of the [Cs8Nb6O19]-Promoted Nerve-Agent Decomposition

    A. L. Kaledin, D. M. Driscoll, D. Troya, D. Collins-Wildman, C. L. Hill, J. R. Morris and D. G. Musaev, Chem. Sci., 2018, Accepted Manuscript , DOI: 10.1039/C7SC04997H

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