Understanding Structure-Property Relationships forPalladium-Gold Nanoparticles as Colloidal Catalysts

[摘要] Bimetallic palladium-gold (PdAu) nanoparticle (NP) catalysts have been demonstratedfor the better catalytic performance than monometallic Pd catalysts in various reactions;however, the enhancement mechanism is not completely clear for most reactions. Thisthesis addresses the investigation of PdAu NP catalysts with emphasis on the structurepropertyrelationships in water-phase reactions, using hydro dechlorination (HDC) oftrichloroethene (TCE) as the model reaction. Catalyzed TCE HDC is a potential approachfor water pollution control, in which colloidal Pd-decorated Au NPs (Pdf Au NPs) areknown to be significantly better catalysts than monometallic Pd ones.X-ray absorption spectroscopy (XAS) of carbon-supported Pdf Au NPs withdifferent surface Pd coverages verified their core-shell structure (Au-rich core and Pdrichshell). Structure evolution was observed upon heat treatment, in which Pd was in theform of surface Pd ensembles at room temperature. The metals formed a surface PdAualloy or a bulk PdAu alloy above 200°C, as determined from the average coordinationenvironment. Results suggested a new way to promote Pd catalysis, namely, byimpregnating supported Pd catalysts with gold salt followed by thermal annealing; suchpost-impregnation with different heat treatments could lead to >15-fold increase in TCEHDC activity.Pd ensembles on the Au NP surface were demonstrated to be major active sitesfor TCE HDC as the reaction rates correlated strongly with the size of Pd ensemblesdetermined from XAS. The geometric effect, in which atomic ensembles act as activesites, appeared to dominate over the mixed metal site effect and the electronic effect. AuNPs could stabilize surface Pd atoms in the metallic form, possibly leading to a set ofhighly 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 closedbatch system. Mass transfer effects in this three-phase reaction were assessed andquantified by analyzing observed reaction rates as functions of stirring rates and initialcatalyst charges. The largest effect on observed reaction rates came from gas-liquid masstransfer. TCE HDC was modeled as a Langmuir-Hinshelwood mechanism involvingcompetitive chemisorption of dihydrogen and TCE molecules.

[发布机构] Rice University

[效力级别] engineering