Core/Shell and Alloy Nanoparticles of Transition Metals for Heterogeneous Catalysis: Bridging the Gap between Experiment and Theory

This thesis describes the structural and catalytic properties of the architecturally-controlled bimetallic nanoparticles (NPs) of transition metals. In this study, bimetallic nanoparticles with well-defined architectures were synthesized, characterized and evaluated toward various heterogeneous reactions. Random alloy nanoparticles were compared to the core/shell nanoparticles (M@M' NPs where M is the core metal and M' is the shell metal), which is the synthetic counterpart of the theoretically well-studied Near Surface Alloys (NSAs). Thus, the long existing experimental gap with the theory can be bridged via the systematic evaluation of such architecturally-controlled bimetallic NPs. The M@Pt (M=Ru, Rh, Ir, Pd and Au) and Ru@M' (M'=Rh and Pd) core/shell NPs of tunable core sizes and shell thicknesses, and the PtRu alloy and PtRh alloy NPs of various compositions were prepared via poly-ol reduction reactions by using sequential deposition techniques. Seed NPs for the core/shell systems were synthesized via either poly-ol or NaBH4 reduction reactions. The wet-chemical co-deposition technique was employed to synthesize the alloy NPs. The core/shell and alloy NPs were characterized by using a combination of TEM, STEM-EDS, XRD, and FT-IR and Micro Raman -CO probe experiments. Full structural analysis employing techniques such as Extended X-Ray Absorption Fine Structure (EXAFS) and atomic Pair Distribution Function (PDF) was also performed for the 4.1 nm Ru@Pt NPs comprising of 3.0 nm cores and 1-2 MLs thick shells and the 4.4 nm Pt50Ru50 alloy NPs. Through collaborations, the nanoparticle structures were also modeled through EXAFS analyses, PDF fits, Rietveld Refinements and Debye Function simulations. The well-characterized core/shell and alloy NPs were evaluated for preferential oxidation of CO in H2 feeds (PROX). Catalytically, the core/shell NPs were superior to their alloy counterparts with similar particle sizes and identical compositions. The PROX reactivities of the M@Pt (M=Ru, Rh, Ir, Pd and Au) core/shell NPs increased in the order of Au@Pt 2-assisted O2 dissociation pathway on the electronically-altered Pt shells were suggested to bring on the room temperature CO oxidation and the subsequent H2 activation with enhanced PROX selectivity. The surface reactivities toward PROX and benzene hydrogenation reactions of the composition series of the PtRu alloy NPs exhibited the `Volcano' behavior, which invoked the Hammer-Norskov theory. The preliminary benzene hydrogenation results on the Ru@Pt NPs system presented in this study also showed a structure dependent correlation in surface activity

Room-temperature cycling of metal fluoride electrodes: Liquid electrolytes for high-energy fluoride ion cells

Fluoride ion batteries are potential “next-generation” electrochemical storage devices that offer high energy density. At present, such batteries are limited to operation at high temperatures because suitable fluoride ion–conducting electrolytes are known only in the solid state. We report a liquid fluoride ion–conducting electrolyte with high ionic conductivity, wide operating voltage, and robust chemical stability based on dry tetraalkylammonium fluoride salts in ether solvents. Pairing this liquid electrolyte with a copper–lanthanum trifluoride (Cu@LaF_3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in a fluoride ion electrochemical cell cycled at room temperature. Fluoride ion–mediated electrochemistry offers a pathway toward developing capacities beyond that of lithium ion technology

Monitoring Real-time Dynamics of Nanoparticle Formation via ‘Trading Space with Time’ Strategy and Synchrotron X-ray Absorption Spectroscopy Technique

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