Controllable Synthesis Of Thermodynamically Metastable Pt-based Nanocrystals And Study On Their Catalytic Properties

Pt-based nanocrystals play key roles in fields of fuel cells and industrial catalysis.As noble metal Pt is particularly expensive and has rare reserves,it remains an important task for the researches to reduce the amount of Pt requirement and improve their catalytic activities.Tremendous efforts have been devoted to the systematic study of the relationship between their catalytic properties and their sizes,compositions and surface structures.It should be mentioned that the metastable nanocrystals(including metastable crystal phase,high energy structure and cyclic-penta-twinned structure)perform more excellent chemical physical properties than their stable counterparts.However,there are relatively few studies devoted to systematic study concerning their synthesis and properties of metastable nanocrystals.In this thesis,we endeavored to explore their formation mechanism,controllable synthesis and structure-dependent catalytic properties of metastable Pt-based nanocrystals(focusing on cyclic-penta-twined structure).The main research progress is summarized as follows:1.The theory of chemical potential has been proposed for the kinetically controllable synthesis of cyclic-penta-twined nanocrystals.The crystal nucleus of low chemical potential was favored in nucleation environment with low chemical potential.By regulating the nucleation rate and nucleation sites,which is related to chemical potential,Pt icosahedral nucleus with low chemical potential and Pt cube nucleus with high chemical potential could be successfully synthesized.From the theory of chemical potential,the kinetically controlled synthesis of cyclic-penta-twined nanocrystals can be understood in essence.Besides,the new theory throws a new light on the controllable synthesis of metastable nanocrystals(such as metastable crystal phase and high energy surface structure).2.The relationship between the Pt nanocrystals with shapes of icosahedra and cubes and their catalytic performance was studied.We show the features of icosahedra and cubes from the new view point of geometry.Isosahedra have smaller contact areas and surface areas than cubes,which could prevent Ostwald ripening and aggregation during catalytic process.Chosing Methanol oxidation as probe reaction,the CNTs supported icosahedra performed more excellent stabilities and activities in the methanol catalytic oxidation than Pt cubes.3.Concave octahedral Pt-Pd alloy nanocryrstals with {hhl} facets mediated by glycine and PVP were synthesized.With the increasement of glycine added,the structures of Pt-Pd alloy envolved from octahedron to concave octahedron.The concave octahedral Pt-Pd alloy nanocrystals performed more excellent activities and stabilities than commercial Pt black in electrooxidation of formic acid.4.We proposed that the symmetry breaking of Pd-Au nano-heterostructures during the seed-mediated growth is related to the difference of equilibrium electrochemical potentials(corresponding to their Fermi levels)of two metals in the growth solution from point of view of thermodynamics.By reversing the relative position of Fermi levels of the Pd nanocube seeds and the second metal Au with changing the concentration of reductant(L-ascorbic acid)in the growth solution,the structure of as-prepared products successfully evolved from centrosymmetric Pd@Au core-shell trisoctahedra to asymmetric Pd-Au hetero-dimers.The idea was further demonstrated by the growth of Ag on the Pd seeds.The theory has potentially significance to achieve the rational design of complex Pt-based bimetal nano-heterostructures with special properties and functions.