Browsing by Author "Wu, Tianpin"
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Item 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.Item 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.Item Volcano-shape glycerol oxidation activity of palladium-decorated gold nanoparticles(Royal Society of Chemistry, 2014) Zhao, Zhun; Arentz, Joni; Pretzer, Lori A.; Limpornpipat, Pongsak; Clomburg, James M.; Gonzalez, Ramon; Schweitzer, Neil M.; Wu, Tianpin; Miller, Jeffrey T.; Wong, Michael S.Bimetallic PdAu catalysts are more active than monometallic ones for the selective oxidation of alcohols, but the reasons for improvement remain insufficiently detailed. A metal-on-metal material can probe the structure–catalysis relationship more clearly than conventionally prepared bimetallics. In this study, Pd-on-Au nanoparticles with variable Pd surface coverages (sc%) ranging from 10 to 300 sc% were synthesized and immobilized onto carbon (Pd-on-Au/C). Tested for glycerol oxidation at 60 °C, pH 13.5, and 1 atm under flowing oxygen, the series of Pd-on-Au/C materials showed volcano-shape catalytic activity dependence on Pd surface coverage. Increasing surface coverage led to higher catalytic activity, such that initial turnover frequency (TOF) reached a maximum of ̴6000 h−1 at 80 sc%. Activity decreased above 80 sc% mostly due to catalyst deactivation. Pd-on-Au/C at 80 sc% was >10 times more active than monometallic Au/C and Pd/C, with both exhibiting TOF values less than [similar]500 h−1. Glyceric acid was the dominant primary reaction product for all compositions, with its zero-conversion selectivity varying monotonically as a function of Pd surface coverage. Glyceric acid yield from Pd-on-Au/C (80 sc%) was 42%, almost double the yields from Au/C and Pd/C (16% and 22%, respectively). Ex situ X-ray absorption near edge structure analysis of two Pd-on-Au/C materials with comparable activities (60 sc% and 150 sc%) showed that the former had less oxidized Pd ensembles than the latter, and that both catalysts were less oxidized compared to Pd/C. That Au stabilizes the metallic state of surface Pd atoms may be responsible for activity enhancement observed in other PdAu-catalyzed oxidation reactions. Decorating a Au surface with Pd generates a catalyst that has the deactivation resistance of Au, the higher glyceric acid selectivity of Pd, and the synergistically higher activities that neither metal has.