Encapsulation of nanoparticles in metal-organic frameworks for air purification

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Tulig, Karen Renee
Walton, Krista S.
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Metal-organic frameworks (MOFs) and nanoparticles in MOFs (NP@MOF) are investigated for carbon monoxide adsorption and catalytic oxidation. In this work, gold nanoparticles (AuNPs) are encapsulated in UiO-66, a zirconium-based MOF. The use of zirconium propoxide (Zr(OnPr)) in place of zirconium chloride (ZrCl4) leads to an alternative synthesis route for producing high-quality crystals of UiO-66 without generating by-product HCl. This new method enables the inclusion of HCl-sensitive gold nanoparticles into the mother solution for encapsulation by UiO-66. Further investigations examine the effects of the solvent ratio, modulator concentration, AuNP capping agent, and UiO-66(Zr(OnPr)) modulator on the UiO-66(Zr(OnPr)) structure and porosity, AuNP diameter, UiO-66(Zr(OnPr)) particle geometry, and AuNP location. These studies show that the AuNP capping agent and UiO-66(Zr(OnPr)) modulator have the most significant effect on the Au@UiO-66(Zr(OnPr)) properties. Conclusive evidence showing that the AuNPs are completely confined within the UiO-66(Zr(OnPr)) particles is not attained, but the preliminary data will guide future endeavors. Additionally, this HCl-free synthesis is applied to the functional versions of UiO-66 resulting in the preparation of a series of UiO-66-X(Zr(OnPr)), where X = {–H, –NH2, –NO2, –Naph, –Anth, –Cl2, –Br, –(CH3)2, –COOH, –OH, and –(OH)}. In addition, the potential of UiO-66 as a catalyst support is probed using CO oxidation as a probe reaction throughout this work. First, preformed AuNPs are deposited onto the surfaces of UiO-66, titanium dioxide (TiO2), and zirconium dioxide (ZrO2). This colloidal deposition effectively decouples the AuNP factors such as size, shape, and oxidation state, from the support effect allowing a systematic study of the key support attributes. This study reveals a correlation between the oxygen storage capacity (OSC) and the catalytic activity of the materials with Au on UiO-66 exhibiting an enhanced OSC, due to the unusual chemistry introduced by the metal-linker interactions. Lastly, Au@UiO-66 prepared via encapsulation is compared to physical mixtures of Au on UiO-66 prepared with various AuNP diameters to probe the effects of the encapsulation procedure. Au@UiO-66 showed improved activity compared to the corresponding physical mixture. The enhanced catalytic activity suggests that synergism is introduced during the encapsulation procedure. This synergism potentially occurs due to partial confinement within UiO-66 particles and/or aggregates which increases the surface area of contact between the AuNPs and UiO-66. This increased contact area results in more interface sites which are typically believed to be responsible for the catalytic abilities of supported AuNPs. This dissertation concludes by summarizing the experimental results, determining trends between the chapters, and recommending topics for future research projects. In addition, limitations are acknowledged and possible solutions presented.
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