<p>    </p> <p>Surface functionalization of metallic nanoparticles is an attractive route to tailor the ensemble geometry and redox properties of active sites in heterogeneous catalysts. However, it is challenging to generate well-defined interfaces through conventional impregnation and one-pot colloidal synthesis methods. In this work, we utilize ligand exchange reactions for post synthetic surface modification of colloidal nanoparticles to generate unique core-shell and surface alloy structures. We use halometallate and metal chalcogenide complexes to create surface sites that are active for electrocatalytic hydrogen evolution reaction (HER). </p> <p>We synthesize a self-limiting monolayer of metal chalcogenides on colloidal Au nanoparticles through biphasic ligand exchange reaction between ammonium tetrathiomolybdate (NH₄)₂MoS₄ complex and Au nanoparticles. Through a combination of spectroscopy techniques and computational methods, we show that strong Au-S interactions introduce electronic and geometric distortion to the geometry and bond metrics of MoS₄^2-complex. Moreover, proximal MoS₄ units adsorbed on the Au surface interlink to form small MoSx oligomers with highly active bridging disulfide sites. Consequently, these core-shell AuMoS₄ nanoparticles exhibit significantly higher HER activity than MoS₄^2- supported on non-interacting carbon supports under highly acidic electrolyte conditions. Although post catalysis characterization reveals partial hydrolysis of surface adsorbed MoSx species, stable HER activity under bulk electrolysis condition indicates that active sites remain persistent. </p> <p>In an effort to extend these ligand exchange reactions to create metal/metal interfaces on other coinage metal nanoparticles such as Ag, we design metal-ligand coordination complexes to mitigate undesired galvanic replacement reactions. By varying the strength and number of coordinating ligands, we fine-tune the redox potential of oxidized noble metal precursors and confine the deposition of noble metals to a few surface layers of the Ag nanoparticles. We utilize organic amine and phosphine ligands to generate Ag@AgM core-shell nanoparticles, where M = Pd, Pt, and Au. Surface alloy or pure metal shells of Pd and Pt on Ag nanoparticles generated through this ligand-based strategy exhibited high precious metal atom utilization in electrocatalytic hydrogen evolution reaction. </p>

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Transition metal catalysis is a necessary branch of organic synthesis. Both monometallic and bimetallic catalysts can reduce reaction times, improve regio- and enantioselectivity, and minimize byproducts. Additionally, bimetallic catalysts can cooperatively activate substrates, which can enable new reactions and mechanistic pathways. The first half of this work will describe the synthesis and catalytic ability of our novel Pd(I) and Pd(II) dimers. Both dimers use a 2-phosphinoimidazole ligand scaffold to bring the metal centers in close proximity. The Pd(II) dimer can catalyze the synthesis of 1,3-disubstituted naphthalene rings from commercially available aryl iodides and methyl ketones with high regioselectivity and yields. Mechanistic and theoretical studies suggest the mechanism undergoes a Pd(III)–Pd(III) like intermediate. Additionally, we studied the impact of precatalyst oxidation state on C–N bonding reactions. We found that our Pd(I) dimer performed better in Buchwald-Hartwig aminations, while our Pd(II) dimer was shown to be extremely active in aminocarbonylation reactions. Both reactions gave C–N bonding products in good to excellent yield. The second portion of this work describes our novel Pd N–H NHC complex and its application in Suzuki-Miyaura cross couplings. In the presence of methanol, a Pd(II) salt will insert into the C–P bond of a 2-phosphinoimidazole ligand to give a protic NHC complex. The acidic hydrogen can be deprotonated under reaction conditions to give an anionic complex, which further increases the electron density on palladium as shown in Tolman Electronic Parameter studies. Application of the catalyst in Suzuki-Miyaura and Sonogashira coupling reactions gave product in high yield. Since our Pd N–H NHC complex with a diphenylphosphine ligand could not activate aryl chlorides, we then applied 2-dialkylphopshinoimidazole ligands. When the dialkyl ligands were stirred with Pd(II) salts in methanol, an equilibrium was observed between N–H NHC and P–N coordination complexes. When the catalytic mixture was applied to Suzuki-Miyaura cross-couplings, (hetero)aryl chlorides gave high yields with low catalyst loadings.

Recent advances in surface science technology have opened new opportunities for atomic scale studies in the field of nanoparticle (NP) catalysis. The 2007 Nobel Prize of Chemistry awarded to Prof. G. Ertl, a pioneer in introducing surface science techniques to the field of heterogeneous catalysis, shows the importance of the field and revealed some of the fundamental processes of how chemical reactions take place at extended surfaces. However, after several decades of intense research, fundamental understanding on the factors that dominate the activity, selectivity, and stability (life-time) of nanoscale catalysts are still not well understood. This dissertation aims to explore the basic processes taking place in NP catalyzed chemical reactions by systematically changing their size, shape, oxide support, and composition, one factor at a time. Low temperature oxidation of CO over gold NPs supported on different metal oxides and carbides (SiO2, TiO2, TiC, etc.) has been used as a model reaction. The fabrication of nanocatalysts with a narrow size and shape distribution is essential for the microscopic understanding of reaction kinetics on complex catalyst systems ("real-world" systems). Our NP synthesis tools are based on self-assembly techniques such as diblock-copolymer encapsulation and nanosphere lithography. The morphological, electronic and chemical properties of these nanocatalysts have been investigated by atomic force microscopy (AFM), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). Chapter 1 describes briefly the basic principles of the instrumentation used within this experimental dissertation. Since most of the state-of-art surface science characterization tools provide ensemble-averaged information, catalyst samples with well defined morphology and structure must be available to be able to extract meaningful information on how size and shape affect the physical and chemical properties of these structures. In chapter 2, the inverse-micelle encapsulation and nanosphere lithography methods used in this dissertation for synthesizing uniformly arranged and narrow size- and shape-selected spherical and triangular NPs are described. Chapter 3 describes morphological changes on individual Au NPs supported on SiO2 as function of the annealing temperature and gaseous environment. In addition, NP mobility is monitored. Chapter 4 explores size-effects on the electronic and catalytic properties of size-selected Au NPs supported on a transition metal carbide, TiC. The effect of interparticle interactions on the reactivity and stability (catalyst lifetime) of Au NPs deposited on TiC is discussed in chapter 5. Size and support effects on the formation and thermal stability of Au2O3, PtO and PtO2 on Au and Pt NPs supported on SiO2, TiO2 and ZrO2 is investigated in chapter 6. Emphasis is given to gaining insight into the role of the NP/support interface and that played by oxygen vacancies on the stability of the above metal oxides. Chapter 7 reports on the formation, thermal stability, and vibrational properties of mono- and bimetallic AuxFe1-x (x = 1, 0.8, 0.5, 0.2, 0) NPs supported on TiO2(110). At the end of the thesis, a brief summary describes the main highlights of this 5-year research program.

The main focus of the thesis is to have better understanding of the atomic and electronic structures, vibrational dynamics and thermodynamics of metallic surfaces and bi-metallic nanoparticles (NPs) via a multi-scale simulational approach. The research presented here involves the study of the physical and chemical properties of metallic surfaces and NPs that are useful to determine their functionality in building novel materials. The study follows the 'bottom-up' approach for which the knowledge gathered at the scale of atoms and NPs serves as a base to build, at the macroscopic scale, materials with desired physical and chemical properties. We use a variety of theoretical and computational tools with different degrees of accuracy to study problems in different time and length scales. Interactions between the atoms are derived using both Density Functional Theory (DFT) and Embedded Atom Method (EAM), depending on the scale of the problem at hand. For some cases, both methods are used for the purpose of comparison. For revealing the local contributions to the vibrational dynamics and thermodynamics for the systems possessing site-specific environments, a local approach in real-space is used, namely Real Space Green's Function method (RSGF). For simulating diffusion of atoms/clusters and growth on metal surfaces, Molecular Statics (MS) and Molecular Dynamics (MD) methods are employed.

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Laser powder bed fusion (LPBF) is a popular additive manufacturing (AM) technique that has demonstrated the capability to produce sophisticated engineering components. This work reports the crack-free fabrication of an SS316L/IN718 bimetallic structure via LPBF, along with compositional redistribution, phase transformations and microstructural development, and nanohardness variations. Constituent intermixing after LPBF was quantitatively estimated using thermo-kinetic coefficients of mass transport and compared with the diffusivity of Ni in the austenitic Fe-Ni system. The intermixing of primary solvents (Ni and Fe) in SS316L/IN718 bimetallic structures was observed for an intermixing zone of approximately 800 µm, and their intermixing coefficient was estimated to be in the order of 10−5 m2/s based on time of 10 ms. In addition, to understand the high temperature behavior, SS316L/IN718 bimetallic structures were annealed at 850, 950, and 1050 °C, for 120, 48, and 24h respectively, followed by water quenching (WQ). Furthermore, to better understand the intermixing of individual components (Ni and Fe) and to predict the varying (maximum) temperatures in LPBF of SS316L/IN718 bimetallic structures, solid-to-solid SS316L vs IN718 diffusion couples were examined at 850, 950, and 1050 °C, for 120, 48, and 24h respectively, followed by WQ. The investigation of SS316L vs IN718 diffusion couples yielded a maximum temperature of approximately 3400 K in the LPBF of SS316L/IN718 bimetallic structures. Finally, compositional redistribution, phase transformations and microstructural development, and nanohardness variations after LPBF of SS316L/IN625 bimetallic structure were also investigated to provide a better understanding of the LPBF process via bimetallic fabrication.

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