Controlled Synthesis And Catalytic Study Of Palladium And Platinum Nanocrystals

Due to their unique catalytic properties and scarce reserves, the efficient use of Pdand Pt nano-catalysts has become an urgent scientific and social problem to be resolved.It is well recognized that the catalytic properties of nano-catalysts can be dictated bytheir size, shape, composition, elemental spatial distribution, and other factors.Therefore, to address the problem requires advanced synthetic approaches ofwell-defined nanostructures and better understanding of catalytic mechanisms at themolecular level. In this dissertation, we focused on the controlled synthesis of Pd-andPt-based nanostructures and their catalytic applications, in order to unveil thestructure-activity relationship and to develop superior catalysts.We started from the investigation of the shape control mechanism of Pdnanocrystals. With formaldehyde as the selective capping agent for Pd（111） facets,icosahedral, decahedral, octahedral, tetrahedral, and triangular plat-like Pd nanocrystalswere prepared by changing the concentration of the surfactant, i.e. oleylamine. Thegrowth process of nanocrystals was monitored by UV–vis study, based on which athree-step “intermediate formation–nucleation–growth” mechanism was proposed.Thermodynamic model analysis further suggested that the driving force of the shapeevolution from icosahedron to tetrahedron lay in the competition between the surfaceenergy and the strained energy of a single nanocrystal.We then focused our study on the exploration of the structure-activityrelationship of high-quality Pd-based nanocrystals. Firstly, the structuralchanges of two kinds of rationally designed Pd-based catalysts were carefullyexamined before and after the Suzuki-Miyaura coupling reaction, whichsuggested an oxidative-addition promoted Pd leaching mechanism. The resultsalso suggested that the activity nature of heterogeneous Pd-catalyzedSuzuki-Miyaura coupling reaction is homogeneous in origin. Secondly, in aneffort to observe the structural changes of nano-catalysts in operando, hollow Pdnanocrystals with multiple cavities were prepared. The surface plasmonresonance properties of the hollow Pd nanocrystals were subsequently studiedby experimental spectroscopy and theoretical simulations. Their SPR absorption were found to be highly structure-dependent, providing great potential to in-situinvestigation of structural changes of Pd nano-catalysts. Thirdly, to study theinfluence of elemental spatial distribution of bimetallic nanocrystals on theircatalytic properties, we systematically studied the performance of Pd, Pd/Ag₂physical mixture, Pd-on-Ag₂island structure, PdAg₂alloy and Pd2Ag alloy withcomparable active ensemble sizes in the selective hydrogenation of acetylene.Our results put the widely accepted notion of reduced ensemble effect inquestion.We further undertook the integrated design of Pt-based nano-catalystsunder the guidance of known structure-activity relationship to conclude thedissertation. Highly-branched Pt-Ni bimetallic nanocrytals enclosed by steppedsurface were prepared by a seed-based diffusion route. Combining theadvantages of the CO-resistant surface structure and composition as well as theaggregation-resistant overall morphology, the as-prepared Pt-Ni nano-catalystsexhibited superior activity and durability in the methanol electro-oxidationreaction than ordinary Pt nanoparticles and commercial Pt balck.