Browsing by Author "Fang, Yu-Lun"

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    Improving gold catalysis of nitroarene reduction with surface Pd
    (Elsevier, 2016) Pretzer, Lori A.; Heck, Kimberly N.; Kim, Sean S.; Fang, Yu-Lun; Zhao, Zhun; Guo, Neng; Wu, Tianpin; Miller, Jeffrey T.; Wong, Michael S.
    Nitroarene reduction reactions are commercialized catalytic processes that play a key role in the synthesis of many products including medicines, rubbers, dyes, and herbicides. Whereas bimetallic compositions have been studied, a better understanding of the bimetallic structure effects may lead to improved industrial catalysts. In this work, the influence of surface palladium atoms supported on 3-nm Au nanoparticles (Pd-on-Au NPs) on catalytic activity for 4-nitrophenol reduction is explored. Batch reactor studies indicate Pd-on-Au NPs exhibit maximum catalytic activity at a Pd surface coverage of 150 sc%, with an initial turnover frequency of ∼3.7 mol-nitrophenol/mol-metalsurface/s, which was ∼5.5× and ∼13× more active than pure Au NPs and Pd NPs, respectively. Pd NPs, Au NPs, and Pd-on-Au NPs below 175 sc% show compensation behavior. Three-dimensional Pd surface ensembles (with ∼4–5 atoms) previously identified through X-ray adsorption spectroscopy provide the active sites responsible for the catalytic maximum. These results demonstrate the ability to adjust systematically a structural feature (i.e., Pd surface coverage) to yield a more active material.
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    Thermal annealing effects on palladium-decorated gold nanoparticle catalysts
    (Elsevier, 2022) Fang, Yu-Lun; Zhao, Zhun; Heck, Kimberly N.; Pretzer, Lori A.; Guo, Neng; Wu, Tianpin; Zhang, Wenqing; Miller, Jeffrey T.; Wong, Michael S.
    Palladium metal supported on gold in the form of surface ensembles have enhanced catalytic properties compared to monometallic Pd, as exemplified by Pd-decorated Au nanoparticles (Pd-on-Au NPs) for various room-temperature reactions. Whereas the catalytic properties and nanostructure of Pd-on-Au NPs are not known at higher temperatures, this work focuses on thermal annealing effects on the Pd-on-Au NP nanostructure, bimetal distribution, and room-temperature water-phase trichloroethene hydrodechlorination (TCE HDC) as the model reaction. Analysis of the average coordination environment of Pd and Au atoms through x-ray absorption spectroscopy showed that as-synthesized Pd-on-Au NPs transitioned from a Au core/Pd shell structure to Au-rich core/PdAu surface alloy or PdAu mixed alloy structures depending on the Pd surface coverage (30–150 sc%) and annealing temperature (100–400 °C). The HDC activity strongly correlated with Pd ensemble size, where the as-formed Pd islands exhibited one order of magnitude enhanced activity compared to monometallic Pd. Higher annealing temperatures led to a surface/mixed alloy structure with smaller Pd ensemble size, resulting in lower activity but still ∼3 times more active than monometallic Pd. These results illustrate the importance of catalyst structure on activity and the usefulness of metal-decorated metal catalysts for higher-temperature reactions.
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    Understanding Structure-Property Relationships for Palladium-Gold Nanoparticles as Colloidal Catalysts
    (2011) Fang, Yu-Lun; Wong, Michael S.
    Bimetallic palladium-gold (PdAu) nanoparticle (NP) catalysts have been demonstrated for the better catalytic performance than monometallic Pd catalysts in various reactions; however, the enhancement mechanism is not completely clear for most reactions. This thesis addresses the investigation of PdAu NP catalysts with emphasis on the structureproperty relationships in water-phase reactions, using hydro dechlorination (HDC) of trichloroethene (TCE) as the model reaction. Catalyzed TCE HDC is a potential approach for water pollution control, in which colloidal Pd-decorated Au NPs (Pdf Au NPs) are known to be significantly better catalysts than monometallic Pd ones. X-ray absorption spectroscopy (XAS) of carbon-supported Pdf Au NPs with different surface Pd coverages verified their core-shell structure (Au-rich core and Pdrich shell). Structure evolution was observed upon heat treatment, in which Pd was in the form of surface Pd ensembles at room temperature. The metals formed a surface PdAu alloy or a bulk PdAu alloy above 200°C, as determined from the average coordination environment. Results suggested a new way to promote Pd catalysis, namely, by impregnating supported Pd catalysts with gold salt followed by thermal annealing; such post-impregnation with different heat treatments could lead to >15-fold increase in TCE HDC activity. Pd ensembles on the Au NP surface were demonstrated to be major active sites for TCE HDC as the reaction rates correlated strongly with the size of Pd ensembles determined from XAS. The geometric effect, in which atomic ensembles act as active sites, appeared to dominate over the mixed metal site effect and the electronic effect. Au NPs could stabilize surface Pd atoms in the metallic form, possibly leading to a set of highly active sites that is not present in monometallic Pd NPs. The TCE HDC reaction with Pdf Au NPs and Pd NPs was conducted as a closed batch system. Mass transfer effects in this three-phase reaction were assessed and quantified by analyzing observed reaction rates as functions of stirring rates and initial catalyst charges. The largest effect on observed reaction rates came from gas-liquid mass transfer. TCE HDC was modeled as a Langmuir-Hinshelwood mechanism involving competitive chemisorption of dihydrogen and TCE molecules.