The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP), a key chemical warfare agent (CWA) simulant, and Zr6-based metal organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP was found to adsorb molecularly to UiO-66 through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes on the surface of crystallites and not within the bulk MOF structure. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. In addition, analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations, used to guide the IR band assignments and to help interpret the XPS features, suggest that uptake is driven by nucleophilic addition of an OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). The rates of product formation indicate that there are likely two distinct uptake processes, requiring rate constants that differ by approximately an order of magnitude. However, the rates of molecular uptake were found to be nearly identical to the rates of reaction, which strongly suggests that the reaction rates are diffusion-limited. The final products were found to inhibit further reactions within the MOFs, and these products could not be thermally driven from the MOFs prior to decomposition of the MOFs themselves.

New materials for the rapid decomposition of chemical warfare agents (CWAs) are in high demand for protecting military and civilian populations from these weapons of mass destruction. The need for novel sorbents and decontamination catalysts has gained great urgency as terrorists groups demonstrate the ability to synthesize and deploy agents in chemical attacks. Although many new materials, such as metal-organic frameworks (MOFs), have been proposed to use as CWA filtration media, their eventual transition requires a detailed understanding of the atomic-scale reaction mechanisms. Zr-based MOFs were recently shown to be among the fastest catalysts of nerve-agent hydrolysis reaction in solution. We show the results of a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808 and NU-1000 MOFs (that have different pore sizes and connectivities) using synchrotron-based X-ray powder diffraction, X-ray absorption and infrared spectroscopies, which reveals key aspects of the reaction mechanism.[1] This study describes the implementation of a newly developed experimental setup for delivering vaporized DMMP to a reaction cell containing a MOF sample. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or in air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr 6 cores of all MOFs and its subsequent decomposition to phosphonate products. Further, we show that DMMP is actively adsorbed from air with good selectivity, even in the presence of humidity or other ambient gases, demonstrating that Zr 6-based MOFs may serve as effective sorbents for CWAs under ambient conditions. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities for rational design of new and superior decontamination materials.

Transmission FTIR spectroscopy is used to explore the electronic structure of excited TiO 2 nanoparticles. Broad infrared spectral features in UVphotoexcited, n-doped, and thermally reduced titania are found to be well-described by two theoretical models, which independently account for the creation of free conduction band electrons and trapped localized electrons that occupy states within the band gap. The infrared spectra indicate that the trapped electrons reside at shallow donor levels that exist 0.12−0.3 eV below the conduction band minimum. IR excitation of the trapped electrons is evidenced by a broad feature in the spectra, which exhibits a maximum that corresponds to the energy of the donor level. These features are well described by a hydrogenic-effective mass model. In addition, free conduction band electrons have a dramatic effect on the infrared spectra by exhibiting a broad featureless absorbance that increases exponentially across the entire mid-IR range. This absorbance is the result of intraconduction band transitions, for which free electron coupling to acoustic phonons is required to conserve momentum. Both localized (within the band gap) and delocalized (within the conduction band) electrons are found to exist in TiO 2 when excess electrons (are created by different means: UV photoexcitation in the presence of a hole scavenger (methanol), irradiation with atomic hydrogen, and thermal removal of lattice oxygen.

Using Fourier transform infrared spectroscopy (FTIR) we studied the overall reaction pathways and ultimate fate of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on a commercial nanoparticulate (approximately 20 nm) titania material. Our data show that the initial uptake occurs through both molecular and reactive adsorption. Molecular adsorption is driven by hydrogen-bond formation to isolated hydroxyl groups. The reactive chemisorption appears to occur through interaction with both Lewis acid sites and active oxygen species present on the TiO2 surface. The reactive sites are found to be poisoned quickly by oxidation products that include a strongly bound, nonvolatile phosphorus compound. Thermal reactivation of the TiO2 in oxygen restores the physisorption capacity of the particles toward the DMMP, but the reactive adsorption pathway is nearly completely eliminated.

Proton exchange, residence time, and gas uptake measurements are used to explore collisions and reactions of HCl, HBr, and HNO 3 with 70 wt % D 2 SO 4 at 213 K. These studies help to provide a detailed picture of HX (X ) Cl, Br, NO 3 ) energy transfer to sulfuric acid and the fate of the HX molecules immediately after thermalization at the D 2 O/D 2 SO 4 surface. We find that the three molecules readily dissipate their excess kinetic energy and become trapped momentarily in the interfacial region. However, only 11 ( 3% of the thermalized HCl and 22 ( 3% of the thermalized HBr molecules undergo H f D exchange; the HCl and HBr that do not react are found to desorb from the acid within 2 × 10 -6 s. In contrast, more than 95% of the initially trapped HNO 3 molecules are converted to DNO 3 . The HX molecules that undergo exchange dissolve within the deuterated acid for characteristic times of 5 × 10 -5 s (HCl), 3 × 10 -3 s (HBr), and 1 × 10 -1 s (HNO 3 ) before they desorb thermally as DX. The scattering experiments imply that the desorption of thermalized HCl and HBr molecules is, on average, faster than their solvation and reaction in the interfacial and bulk regions of 70 wt % D 2 SO 4 . Although HNO 3 is less acidic than HCl or HBr, it appears to hydrogen bond more strongly to surface D 2 O and D 2 SO 4 , enabling it to be captured by the acid in nearly every collision.