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Browsing Chemistry by Subject "Catalysis"
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Item Improving the Rates of Pd-Catalyzed Reactions by Exciting the Surface Plasmons of AuPd Bimetallic Nanotriangles(Royal Society of Chemistry, 2017) Gangishetty, Mahesh K; Fontes, Adriana, M.; Malta, Marcos; Kelly, Timothy L; Scott, Robert WJGold nanoparticles exhibit unique optical properties due to surface plasmon oscillations when they interact with light. By utilizing their optical properties, the rates of many chemical reactions have been improved in the presence of visible light. The properties of plasmonic nanoparticles are highly tunable based on the size and shape of the nanoparticle. Here, we have used anisotropic AuPd bimetallic nanotriangles to improve the rates of Pd-catalyzed reactions in the presence of visible light. We synthesized AuPd core–shell bimetallic nanotriangles and performed Suzuki cross-coupling and hydrogenation reactions in light and dark conditions. Upon illuminating AuPd nanotriangles with an array of green LEDs (power ∼ 500 mW), enhanced catalytic activity of palladium was observed. In order to understand the relative contributions of individual plasmonic effects, such as plasmonic hot electron transfer and plasmonic heating effects, the reaction temperatures were monitored, and careful control experiments were run at different temperatures. Our results indicated that the enhancement in the rate of these Pd-catalyzed reactions is primarily due to the plasmonic heating effect.Item in situ X-ray Absorption Spectroscopic Analysis of Gold-Palladium Bimetallic Nanoparticle Catalysts(American Chemical Society, 2013) MacLennan, Aimee; Banerjee, Abhinandan; Hu, Yongfeng; Miller, Jeffrey T; Scott, Robert WJGold–palladium core–shell nanoparticles have been previously shown to be extremely effective catalysts for a number of oxidation reactions including the aerobic oxidation of alcohols. However, the novel activity and durability of such catalysts are still poorly understood, and there are several putative mechanisms by which oxidation reactions can proceed. Previously we showed that Pd(II) salts in the presence of Au nanoparticles were also effective catalysts for the room temperature oxidation of crotyl alcohol. Herein we show an in situ X-ray absorption spectroscopy (XAS) study at both the Pd–K and Pd-LIII edges of Au nanoparticle/Pd(II) salt solutions in the presence of crotyl alcohol. Liquid cells with X-ray permeable windows were used to obtain quick-scan XAS data during the oxidation of crotyl alcohol, allowing for time-resolved Pd speciation information and information about the reaction mechanism and kinetics. XAS measurements definitively show that the first step of this reaction involves Pd reduction onto the Au nanoparticles; in addition, further studies of the stability of the resulting Au–Pd core–shell nanoparticles toward oxygen gas suggests that the role of Au in such catalysts is to prevent the reoxidation of the catalytically active surface Pd atoms. Catalytic crotyl alcohol oxidation measurements were done which validated that the in situ reduction of Pd(II) in the presence of Au nanoparticles did indeed result in catalytically active AuPd bimetallic catalysts.Item In situ X-ray absorption spectroscopic studies of magnetic Fe@FexOy/Pd nanoparticle catalysts for hydrogenation reactions(Elsevier, 2017) Yao, Yali; Rubino, Stefano; Gates, Byron D; Scott, Robert WJ; Hu, YongfengCore@shell Fe@FexOy nanoparticles (NPs) have attracted a great deal of interest as potential magnetic supports for catalytic metals via galvanic exchange reactions. In this study Fe@FexOy/Pd bimetallic NPs were synthesized through galvanic exchange reactions using 50:1, 20:1 and 5:1 molar ratios of Fe@FexOy NPs to Pd(NO3)2. The resulting Fe@FexOy/Pd NPs have Pd NPs on the Fe oxide surfaces, and still retain their response to external magnetic fields. The materials could be recovered after the reaction by an external magnetic field, and agitation of the solution via a magnetic field led to improvements of mass transfer of the substrates to the catalyst surface for hydrogenation reactions. The Fe@FexOy/Pd NPs derived from the 5:1 molar ratio of their respective salts (Fe:Pd) exhibited a higher catalytic activity than particles synthesized from 20:1 and 50:1 molar ratios for the hydrogenation of 2-methyl-3-buten-2-ol. The highest turnover frequency reached 3600 h−1 using ethanol as a solvent. In situ XANES spectra show that the Fe@FexOy NPs in the Fe@FexOy/Pd system are easily oxidized when dispersed in water, while they are very stable if ethanol is used as a solvent. This oxidative stability has important implications for the sustainable use of such particles in real world applications.Item Platinum Inhibits Low‐Temperature Dry Lean Methane Combustion through Palladium Reduction in Pd−Pt/Al2O3: An In Situ X‐ray Absorption Study(Wiley, 2017) Nassiri, Hanieh; Lee, Kee Eun; Hu, Yongfeng; Hayes, Robert E; Scott, Robert WJ; Semagina, NataliaPalladium–platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X‐ray absorption near‐edge structure and extended X‐ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd−Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium reduction, even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd−Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200–500 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low‐temperature dry lean methane combustion conditions, which is important for methane‐emission control devices.Item Rational Design and Characterization of Bimetallic Gold-Palladium Nanoparticle Catalysts(Wiley, 2015) Scott, Robert WJThis feature article covers a long‐term project in our laboratory at the University of Saskatchewan towards the rational synthesis of controlled‐architectures of gold‐palladium bimetallic nanoparticle catalysts for use as low‐temperature alcohol oxidation catalysts. Syntheses involve controlled growth of nanoparticle architectures via solution‐based nanoparticle synthetic strategies, followed by deposition of the designed particles onto solid supports. In addition, characterization methods used to elucidate structures of the synthesized particles before and after activation for catalysis will be discussed. This includes traditional characterization methods such as transmission electron microscopy and X‐ray absorption fine structure spectroscopy (EXAFS), but also includes, more recently, the use of in situ X‐ray absorption spectroscopy studies of gold‐palladium nanoparticle catalysts. Strategies and challenges towards the rational synthesis of heterogeneous supported‐nanoparticle catalysts based on bimetallic nanoparticle precursors will be detailed and contrasted with more traditional routes to synthesize such catalytic materials.Item Supported Bimetallic AuPd Clusters Using Activated Au25 Clusters(Elsevier, 2017) Lee, Kee Eun; Shivhare, Atal; Hu, Yongfeng; Scott, Robert WJBimetallic AuPd nanoparticles on alumina supports were prepared using Au25(SR)18 precursors activated by mild calcination or LiBH4 treatment, followed by selective deposition of Pd via ascorbic acid reduction. Comparison of their catalytic activity for the oxidation of crotyl alcohol showed that bimetallic structure had significantly improved catalysis compared to Pd/Al2O3. In particular, AuPd samples grown from LiBH4-activated Au25 clusters exhibit the highest catalytic activity as well as high selectivity towards crotonaldehyde formation, likely due to their smaller particle sizes as compared to AuPd samples grown from calcined Au25 clusters. X-ray absorption spectroscopy (XAS) at the Au L3-edge, Pd L3-edge and Pd K-edges showed that the resulting bimetallic AuPd nanoparticles had Au-Pd core-shell structures with a 4d-electron poor Pd surface.Item Synthesis of Sinter-Resistant Au@Silica Catalysts Derived from Au25 Clusters(2017) Sudheeshkumar, V; Shivhare, Atal; Scott, Robert WJGold clusters exhibit remarkable catalytic activity for many reactions such as carbon monoxide oxidation, alcohol, alkene, and hydrocarbon oxidations, and reduction reactions at low temperatures. However, several previous studies show that Au clusters undergo problematic sintering at temperatures above 250 °C, which makes them unsuitable catalysts for high-temperature oxidation reactions. Here we report the coating of Au25(11-MUA)18 clusters (where 11-MUA = mercaptoundecanoic acid) by silica to produce sinter-resistant Au@SiO2 catalysts. The structure of the resulting materials before and after calcination at temperatures up to 650 °C was followed by TEM and extended X-ray absorption fine structure spectroscopy (EXAFS) analyses, which showed that the Au@SiO2 catalysts created were much more stable to sintering compared to control materials; with average particles sizes of 2.2 nm after calcination at 250 °C and just over 3 nm after calcination at 650 °C. In addition, we explored the activity of the resulting materials for the 4-nitrophenol reduction and styrene epoxidation reactions; results clearly showed that the Au surfaces are accessible for reactants and that the kinetics of 4-nitrophenol reduction was directly related to the dispersion of the Au particles, as measured via the first shell Au–Au coordination numbers by EXAFS. Styrene epoxidation results show that the Au@SiO2 materials have excellent activity and recyclability.Item Water shifts PdO-catalyzed lean methane combustion to Pt-catalyzed rich combustion in Pd-Pt catalysts: In situ X-ray absorption spectroscopy(Elsevier, 2017) Nassiri, Hani; Lee, Kee Eun; Hu, Yongfeng; Hayes, Robert E; Scott, Robert WJ; Semagina, NataliaThe addition of platinum to palladium is known to provide bimetallic catalysts that are relatively active in lean methane combustion in the presence of water at temperatures below 773 K, which is of practical interest for exhaust treatment from natural gas vehicles. The study provides insight into the wet lean methane combustion mechanism on the Pd–Pt/Al2O3 catalyst via in situ X-ray absorption spectroscopy studies at temperatures 473–873 K. The presence of water leads to an increased fraction of metallic Pd due to the lack of surface oxygen. The fraction of metallic Pd drops as the temperature increases. Oxygen deficit results in Pt atoms available for methane dissociation, which does not occur in the dry methane-lean feed, in which oxygen poisons Pt.