Date on Master's Thesis/Doctoral Dissertation

5-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Zamborini, Francis Patrick

Author's Keywords

Hydrogen sensing; Allyl alcohol; Metal nanoparticles; Hydrogenation and isomerization; Catalysis; Reactions

Subject

Chemical detectors; Nanoparticles

Abstract

This dissertation shows the hydrogen reactivity and stability of solid-state films and solutions of nanoparticles (NPs) for potential applications in hydrogen sensing and homogeneous catalysis of organic reactions. Mainly, this dissertation describes 1) the chemical synthesis of Pd and PdAg nanoparticles coated with various organic ligands and with different metal compositions, 2) the stability of solutions containing Pd and PdAg NPs in the presence of hydrogen, 3) the hydrogen sensing properties of films of NPs deposited onto Au microelectrodes, and 4) the evaluation of the catalytic activity of Pd and PdAg NPs for hydrogenation/isomerization of allyl alcohol. We chemically synthesized Pd and Pd-containing alloy (PdAg) NPs stabilized with alkanethiolates (CnS, n = 4, 6, 8, 12, 16, and 18), alkylamines (CnNH2, n = 8, 12, and 16), and mixed C8NH2/C6S ligands. All of the NPs were easily prepared and fully characterized by various analytical techniques to get their composition. By varying the initial mole ratio of Pd/CnNH2 and the chain lengths of CnNH2 and CnS, we were able to control the size, distribution, and stability of the NPs. When these NPs were exposed to 100% hydrogen, different stability was observed depending on the functionality and composition of Pd and PdAg NPs. While CnS Pd NPs (n = 6) had high stability against H2-induced aggregation because of the strong Pd-S interaction and moderate-sized alkane chain, the 6x and 12x C16NH2 Pd and Pd91Ag9 NPs exhibited the highest stability likely due to the formation of a bilayer of ligands on the Pd core that prevents the NP aggregation. For mixed ligand C8NH2/C6S Pd NPs, the stability strongly depended on the C8NH2/C6S ratio on the Pd NPs. The Pd NPs coated with ~3:1 or less C8NH2/C6S ligands showed excellent stability against H2-induced aggregation in solution. In the area of hydrogen sensing, neither C6S Pd nor C8NH2 Pd NPs displayed desirable sensing attributes. By synthesizing C8NH2/C6S mixed Pd NPs with controlled ratios, we were able to prepare films of these NPs that displayed stable, reversible sensing behavior to H2 gas down to 0.3%. In the area of catalysis, we synthesized a wide range of NPs with different composition and functionality. We discovered that the functionality plays a large role in the stability, selectivity, and reactivity of the catalysts. First, the NPs that are not stable showed very low turnover frequency (TOF) values since they were behaving as heterogeneous catalysts with lower surface area. For those that are stable, the strong thiolate ligands hindered the reaction rate, but provided a very interesting selectivity to form the aldehyde from allyl alcohol. In contrast, the very stable, but weak binding C16NH2 ligands led to Pd NPs with high reactivity, but little selectivity, forming both products. We learned that the long alkyl chain does not hinder the reaction very much as long as the ligand binds weakly. We also evaluated the catalytic properties of C6S and C16NH2 Pd and PdAg NPs for the hydrogenation/isomerization of various allyl alcohols that differ only slightly in chemical structure. We observed that the more branched substrates have low TOF, likely due to the strong Pd-S interaction and to restricted diffusion and access to active sites through the packed multilayer of C16NH2 ligands surrounding the Pd core. Interestingly, the C6S Pd NPs led to hydrogenation reaction when isomer formation is not possible. Importantly, by varying the H2 flow rate, we could favor the isomerization or hydrogenation. In the case of Pd NPs coated with CnS ligands, we determined that these catalysts are highly selective for isomerization reactions and that the carbon chain length does not impede the reactivity of the NPs. C8S Pd NPs may be the optimal catalysts in terms of reactivity and stability. Our research has led to new fundamental insights about the reactivity between hydrogen and various Pd-containing NPs that may allow for rational design of metal NPs for specific sensing and catalysis applications.

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