Mijong Kim

Porosity Control of Pd@SiO2 Yolk–Shell Nanocatalysts by the Formation of Nickel Phyllosilicate and Its Influence on Suzuki Coupling Reactions

The surface of Pd@SiO(2) core-shell nanoparticles (1) was simply modified by the formation of nickel phyllosilicate. The addition of nickel salts formed branched nickel phyllosilicates and generated pores in the silica shells, yielding Pd@SiO(2)-Niphy nanoparticles (Niphy = nickel phyllosilicate; 2, 3). By removal of the silica residue, Pd@Niphy yolk-shell nanoparticles (4) was uniformly obtained. The four distinct nanostructures (1-4) were employed as catalysts for Suzuki coupling reactions with aryl bromide and phenylboronic acid, and the conversion yields were in the order of 1 < 2 < 3 < 4 as the pore volume and surface area of the catalysts increased. The reaction rates were strongly correlated with shell porosity and surface exposure of the metal cores. The chemical inertness of nickel phyllosilicate under the basic conditions rendered the catalysts reusable for more than five times without loss of activity.

Gram-Scale Synthesis of Magnetically Separable and Recyclable Co@SiO2 Yolk-Shell Nanocatalysts for Phenoxycarbonylation Reactions

Co@SiO2 yolk‐shell nanostructures were fabricated on a gram scale by simple hydrogen reduction of CoO@SiO2 core‐shell nanoparticles without using an etching step. The cobalt core diameters were reduced by the lattice volume contraction of CoO to Co, forming vacancy between the cores and the silica shells. The Co@SiO2 yolk‐shell nanocatalysts were employed for the phenoxycarbonylation of iodobenzene, and exhibited high activity and reusability. The magnetic property of the cobalt cores enabled the facile separation of catalysts from the products.

Precise adjustment of structural anisotropy and crystallinity on metal–Fe3O4 hybrid nanoparticles and its influence on magnetic and catalytic properties

In the present study, we demonstrate the precise adjustment of the morphology and crystallinity of metal (Pd or Au)–Fe3O4 hybrid nanoparticles by reaction kinetics control. The nucleation and growth of the Fe component on the Pd surface are precisely controlled by using a mixture of capping agents, oleylamine and oleic acid. After the oxidation, the resulting Pd–Fe3O4 structures are produced as yolk–shell, irregular core–shell, and dumbbell-like NPs. Along with the morphology change, the average crystal domain size of Fe3O4 and the void gap between the metal cores and the shells are simultaneously adjusted. The crystal domain sizes of Fe3O4 directly influence the magnetic properties, and the structural arrangement of the Pd cores and the Fe3O4 shells leads to a large difference of conversion yields in the Suzuki coupling reactions. Our approach is successfully extended to other metal–Fe3O4 hybrid systems, such as those of Au and Fe3O4.

Directed C−H Activation and Tandem Cross-Coupling Reactions Using Palladium Nanocatalysts with Controlled Oxidation

Controlled oxidation of palladium nanoparticles provided high-valent PdIV oxo-clusters which efficiently promote directed C-H halogenation reactions. In addition, palladium nanoparticles can undergo changes in oxidation states to provide both high-valent PdIV and low-valent Pd0 species within one system, and thus a tandem reaction of C-H halogenation and cross-coupling (C-N, C-C, and C-S bond formation) was successfully established.

Synthesis of Co/SiO2 hybrid nanocatalyst via twisted Co3Si2O5(OH)4 nanosheets for high-temperature Fischer–Tropsch reaction

A cobalt-silica hybrid nanocatalyst bearing small cobalt particles of diameter ~5 nm was prepared through a hydrothermal reaction and hydrogen reduction. The resulting material showed very high CO conversion (>82%) and high hydrocarbon productivity (~1.0 gHC·g−1cat·h−1) with high activity (~8.5 × 10−5 molCO·gCo−1·s−1) in the Fischer–Tropsch synthesis reaction.

Selective Growth and Structural Analysis of Regular MnO Nanooctapods Bearing Multiple High‐Index Surface Facets

Although numerous morphologies of MnO nanostructures have been reported, an exact structural analysis and mechanistic study has been lacking. In the present study, the formation of regular MnO octapods was demonstrated in a simple procedure, comprising the thermal decomposition of manganese oleate. Because of their structural uniformity, an ideal three-dimensional model was successfully constructed. The eight arms protruded from the cubic center with tip angles of 38° and surface facets of {311} and {533} with rounded edges. The concentrations of oleate and chloride ions were the determining factors for the octapod formation. Selective coordination of the oleate ions to the {100} faces led to edge growth along the <111> direction, which was then limited by the chloride ions bound to the high-index surface facets. These structural and mechanistic analyses should be helpful for understanding the complex nanostructures and for tuning their structure-related properties.