Interfacial Energy Transfer in Small Hydrocarbon Collisions with Organic Surfaces and the Decomposition of Chemical Warfare Agent Simulants within Metal-Organic Frameworks
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A molecular-level understanding of gas-surface energy exchange and reaction mechanisms will aid in the prediction of the environmental fate of pollutants and enable advances toward catalysts for the decomposition of toxic compounds. To this end, molecular beam scattering experiments performed in an ultra-high vacuum environment have provided key insights into the initial collision and outcome of critical interfacial processes on model systems. Results from these surface science experiments show that, upon gas-surface collisions, energy transfer depends, in subtle ways, on both the properties of the gas molecules and surfaces. Specifically, model organic surfaces, comprised of long-chain methyl- and hydroxyl-terminated self-assembled monolayers (SAMs) have been employed to test how an interfacial hydrogen bonding network may affect the ability of a gas-phase compound to thermally accommodate (typically, the first step in a reaction) with the surfaces. Results indeed show that small organic compounds transfer less energy to the interconnected hydroxyl-terminated SAM (OH-SAM) than to the organic surface with methyl groups at the interface. However, the dynamics also appear to depend on the polarizability of the impinging gas-phase molecule. The π electrons in the double bond of ethene (C2H4) and the triple bond in ethyne (C2H2) appear to act as hydrogen bond acceptors when the molecules collide with the OH-SAM. The molecular beam scattering studies have demonstrated that these weak attractive forces facilitate energy transfer. A positive correlation between energy transfer and solubilities for analogous solute-solvent combinations was observed for the CH3-SAM (TD fractions: C2H6 > C2H4 > C2H2), but not for the OH-SAM (TD fractions: C2H6 > C2H2 > C2H4). The extent of energy transfer between ethane, ethene, and ethyne and the CH3-SAM appears to be determined by the degrees of freedom or rigidity of the impinging compound, while gas-surface attractive forces play a more decisive role in controlling the scattering dynamics at the OH-SAM. Beyond fundamental studies of energy transfer, this thesis provides detailed surface-science-based studies of the mechanisms involved in the uptake and decomposition of chemical warfare agent (CWA) simulants on or within metal-organic frameworks (MOFs). The work presented here represents the first such study reported in with traditional surface-science based methods have been applied to the study of MOF chemistry. The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP) or dimethyl chlorophosphate (DMCP), key CWA simulants, 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 and DMCP were found to adsorb molecularly (physisorption) to the MOFs through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes or dangling -COH groups on the surface of crystallites. 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. Besides, the 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 (performed by Professor Troya at Virginia Tech), were used to guide the IR band assignments and to help interpret the XPS features, suggest that uptake is driven by nucleophilic addition of a surface OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). With similar IR features of MOF-808 upon DMCP exposure, the reaction pathway of DMCP in Zr6-MOFs may be similar to that for DMMP, but with the final product being methyl chlorophosphonic acid (elimination of the chlorine) or MMPA (elimination of a methoxy group). The rates of product formation upon DMMP exposure of the MOFs suggest that there are two distinct uptake processes. The rate constants for these processes were found to 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. Overall, and perhaps most importantly, this research has demonstrated that the final products inhibit further reactions within the MOFs. The strongly bound products could not be thermally driven from the MOFs prior to the decomposition of the MOFs themselves. Therefore, new materials are needed before the ultimate goal of creating a catalyst for the air-based destruction of traditional chemical nerve agents is realized.
- Doctoral Dissertations