Browsing by Author "Zhao, Zhun"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Catalytic Oxidation Properties of Palladium-decorated Gold Nanoparticles(2014-10-06) Zhao, Zhun; Wong, Michael S.; Gonzalez, Ramon; Zheng, JunrongBimetallic palladium gold (PdAu) catalysts have been shown to be superior to monometallic ones in many reactions, but the reasons for the enhancement are not thoroughly understood. In this work, palladium decorated gold nanoparticles (Pd-on-Au NPs) are used as structured model catalysts, allowing for the precise control of both size and metal distribution with Pd surface coverage (sc%). By testing reactions on a range of these catalysts, we hope to gain insight into the active site for a given reaction. In hydrodechlorination of perchloroethene (PCE), Pd surface coverage was found to be the key factor in catalyst activity, with the optimum at 80 sc%. A complete mechanistic model that coupled mass transfer processes with the surface reactions was further developed, consistent with the observed product profiles. Carbon supported Pd-on-Au NPs were tested for liquid phase glycerol oxidation for the first time. The best catalyst (80 sc%) had an initial TOF of ~6000 h-1, >10 times more active than Au/C and Pd/C. Catalytic activity, selectivity, activation energy and deactivation rate constant exhibited strong volcano-shaped dependences upon Pd sc%. Ex situ XANES results showed no to little change in surface Pd-O% for Au based catalysts, suggesting the possibility of Au suppressing Pd oxidation during reaction. Ex situ EXAFS results further confirmed the core-shell structures of 60 and 150 sc% Pd-on-Au/C catalysts via Punnett square analysis, and also ascertained no to little change in their oxidation states and coordination numbers post glycerol oxidation. EXAFS observations correlate with kinetics results, and lead to the conclusion that catalysts with a larger amount of 3-D Pd ensembles are more prone to oxidize during glycerol oxidation, making them less resistant to deactivation. Finally, Pd-on-Au/C catalysts were tested for room temperature formic acid decomposition. In situ XAS revealed that core-shell structures of 60, 150 and 300 sc% Pd-on-Au NPs maintained while oxidized Pd species was partially reduced during reaction. Catalyst with higher fraction of 3-D Pd ensembles showed much higher dehydrogenation activity than those with mostly 1-D or 2-D, correlating to the proposed mechanism that the dehydrogenation pathway is favored over metal terrace sites.Item Hydrogen-generating behavior of Pd-decorated gold nanoparticles via formic acid decomposition(Elsevier, 2019) Zhao, Zhun; Heck, Kimberly N.; Limpornpipat, Pongsak; Qian, Huifeng; Miller, Jeffrey T.; Wong, Michael S.Formic acid is a promising hydrogen storage material where hydrogen is generated via metal-catalyzed decomposition. Bimetallic catalysts are active for this reaction, but the mechanism has not been fully proven. Palladium metal supported on gold nanoparticles (Pd-on-Au NPs) has structural properties that are advantageous for studying aqueous-phase catalytic reactions. In this work, a series of Pd-on-Au NPs of varying Pd loadings (calculated in terms of Pd surface coverage, sc%) were synthesized, immobilized onto carbon, and studied for formic acid decomposition at room temperature. Pd-on-Au NPs were catalytically active, with a reaction rate constant as high as 137 m L-H2/gPd/min (corresponding to an initial turnover frequency TOF of 123 h−1) at a Pd loading of 300 sc%. In contrast, Au NPs were inactive, and Pd NPs were slightly active (5 mL-H2/gPd/min and TOF of 38 h−1). The Pd metal of Pd-on-Au catalysts are partially oxidized, and is readily reduced without changing the metal-on-metal structure during reaction, according to in situ x-ray adsorption spectroscopy measurements. CO formation was inhibited at a Pd loading of 300 sc%, suggesting that three-dimensional Pd ensembles favored the desired dehydrogenation pathway while single-atom and small two-dimensional Pd ensembles are active for the undesired dehydration pathway.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 Synergistic Gold-Bismuth Catalysis for Non-Mercury Hydrochlorination of Acetylene to Vinyl Chloride Monomer(American Chemical Society, 2014) Zhou, Kai; Wang, Wei; Zhao, Zhun; Luo, Guohua; Miller, Jeffrey T.; Wong, Michael S.; Wei, FeiGold has been proposed as an environmentally friendly catalyst for acetylene hydrochlorination for vinyl chloride monomer synthesis by replacing the commercially used mercury catalyst. However, long life with excellent activity is difficult to achieve because gold is readily reduced to metallic nanoparticles. The stability of gold limits its industrial application. In this paper, we promoted gold with bismuth for the hydrochlorination of acetylene. It was found that the Bi promotion leads to partial reduction to AuCl, rather than the complete reduction of Au to metallic nanoparticles in the absence of Bi. The optimized catalyst with a molar ratio of Bi/Au = 3:1 (0.3 wt % Au) showed comparable reactivity to 1.0 wt % Au catalyst and significantly improved stability. Furthermore, the gold-bismuth catalyst had higher activity and stability than the commercial mercury catalyst, is less toxic and more environmental-friendly, making it a potentially green, mercury-free industrial catalyst for acetylene hydrochlorination.Item Unknown 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 Unknown Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces(Springer Nature, 2016) Li, Jiebo; Qian, Huifeng; Chen, Hailong; Zhao, Zhun; Yuan, Kaijun; Chen, Guangxu; Miranda, Andrea; Guo, Xunmin; Chen, Yajing; Zheng, Nanfeng; Wong, Michael S.; Zheng, JunrongEnergy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.Item Unknown 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.