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

Abstract

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 < Pd@Pt < Ir@Pt < Rh@Pt < Ru@Pt, which is predicted by the NSA theory. Density Functional Theory (DFT) calculations performed by Prof. Mavrikakis at the University of Wisconsin helped elucidate the thermo-chemistry beyond the enhanced PROX activities and the observed surface reactivity trends for the core/shell architectures. The decreased equilibrium surface coverage of CO as well as the new H₂-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.