Dendrimer-encapsulated nanoparticles as model electrocatalysts
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In this dissertation, dendrimer-encapsulated nanoparticles (DENs) are employed as model electrocatalysts. DENs are well-defined nanoparticles in the 1-2 nm size range. Nanoparticles consisting of 55-225 atoms are large enough to be synthesized and adequately characterized, but small enough to be modeled theoretically by density functional theory (DFT). In this size range, large deviations from bulk structural characteristics are observed. Characterization and electrocatalytic testing is performed for comparison to theoretical calculations. In this way we are able to better understand nanoparticle structure, as well as validate theoretical models. A solution based synthesis is used to produce Pd@Pt core@shell DENs, which has been theoretically predicted to have improved activity for the oxygen reduction reaction (ORR). However, through in situ extended X-ray absorption fine structure (EXAFS) spectroscopy characterization it is determined that structural inversion occurs in which the more noble Pt partitions to the core and the shell becomes enriched in Pd. Larger Pd@Pt nanoparticles (>3 nm) are known to retain their core@shell structure, but 1-2 nm nanoparticles tend to be structurally unstable. Therefore we infer that the observations reported here are driven by the energetics of the small number of atoms present in particles having sizes of <2 nm. A synthetic strategy for improved fully reduced Pt DENs is also outlined. DENs produced by galvanic exchange of Pt²⁺ salt for Cu DENs overcome the previous limitation of partial Pt reduction when a chemical reducing agent is used. Pt DENs by galvanic exchange are fully reduced and are an improved model electrocatalyst. These galvanic exchange Pt DENs are then studied by in situ electrochemical infrared spectroscopy to probe the stretching frequency of adsorbed CO. These studies show a redshift in the frequency of adsorbed CO, in relation to a Pt(111) single crystal surface, revealing stronger binding of CO to the Pt DENs surface. This shift is confirmed by theoretical calculations. This system is a good future platform to test predictions of the CO binding energy trends on DENs, and to test its use as a descriptor for more complex electrocatalytic reactions.