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

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.