Kinetics of Dimethyl Methylphosphonate Adsorption and Decomposition on Zirconium Hydroxide Using Variable Temperature In Situ Attenuated Total Reflection Infrared Spectroscopy

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Abstract

The decomposition mechanisms of dimethyl methylphosphonate (DMMP), a widely used simulant for organophosphorus chemical warfare agents (CWAs), are relatively well understood from previous studies. However, there still lacks a quantitative description of DMMP decomposition kinetics under ambient conditions that is relevant for sequestration applications. We investigated adsorption and decomposition kinetics of DMMP on amorphous zirconium hydroxide (ZH) using variable-temperature in situ attenuated total reflection (ATR) infrared spectroscopy. We demonstrate that quantifying DMMP decomposition kinetics using conventional methods, where the integrated absorbance of P-O vibrational modes is monitored, can be inaccurate because these spectra are also convoluted with C-O vibrational modes from transient surface methoxy species that are not proportional to DMMP decomposition due to methanol desorption. Here, we propose to use the ρ(PCH₃) modes as an alternative way to track DMMP adsorption and decomposition reactions. On the basis of density functional theory (DFT) simulations and comparisons to relatively unreactive monoclinic zirconia (m-ZrO₂), we assign the deconvoluted components of the ρ(PCH₃) region and use it to monitor decomposition products over time at various temperatures. Because the PCH₃ group is present in many toxic organophosphorus compounds, tracking the PCH₃ bands in time-dependent IR spectra is useful for measuring surface kinetics of CWAs and their simulants on various decontamination materials.

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