Shape controlled synthesis of multimetallic nanocrystals and their application as organic catalysts

Abstract

Heterogeneous catalysts have been received attention from field of catalysis. They have recyclability, and advantages with ease of separation from reaction system unlike homogeneous counterpart. Metal nanoparticle is one of the largely studied research topics in heterogeneous catalysis because metals have outstanding catalytic activities either homogeneous or heterogeneous. From a few decades researches about metal nanoparticle catalysis have been carried out, and some factors which control the activity of nano-sized catalysts were verified from those studies. Size, shape, and metal composition are main components which influenced catalytic activities. That is, the surface modification of nanoparticle is important to determine reactivity. Firstly, size of nanoparticle has an effect on the surface area. When the volume of metal was fixed, surface area was getting larger with smaller size. This means that the numbers of surface metal atoms which can participate in the reaction are controlled by size effect. In other hand, by controlling shape of nanoparticle, arrangement structure among surface atoms is changed. Surface energy of nanoparticles, coordination number of surface atoms, and the number of edges and vertices, namely physical and chemical properties of nanoparticles were affected by shape control. Finally, one nanoparticle could be synthesized with two or more kinds of metals. These structures mainly composed with alloy, core-shell, and heterostructure. In the case of multimetallic structure, compositional change of metal has an effect on bifunctional effect or binding energy with reactant molecule. In conclusion, catalytic activities of nanoparticles are determined by surface control, and this means that the researches about nanoparticle surface with detailed control are needed. In chater 2, we have studied for surface control with Pd nanoparticles especially to identify catalytic active sites of nanoparticles. There are several researches that defect atoms like edge and vertex atoms have greater activities compared with face atoms with indirect evidences. However, to suggest direct evidence of catalytic active site of metal nanoparticle, nanoparticle was synthesized with selective deposited of Au on the surface of Pd nanoparticles. Reaction was progressed with Pd rhombic dodecahedron (RD). With Pd RD nanoparticles, edge-covered RD (E-CRD) Pd nanoparticles were synthesized by selective blocking of edge and vertex sites with catalytically inert Au due to iodide ion in synthesis. These two kinds of nanoparticles, RD and E-CRD, were utilized in Suzuki coupling as a model reaction. E-CRD nanoparticles were added in reaction mixture considering surface area, and then turn over frequencies (TOFs) of two types of catalysis was obtained. In the case of E-CRD, obtained TOF value is solely dependent on the terrace atoms because there are not exposed Pd atoms on edge and vertex sites. Compared with TOF values of Pd RD nanoparticles, TOF of defect atoms were deduced by considering atomic ratio between terrace and defect atoms. Calculated TOF values of defect atoms were higher compared with those of terrace atoms. Through this result, defect atoms are catalytically very important parts of metal nanoparticle catalysts. In chapter 3, we have studied change of reactivity of nanoparticles through composition control. Polyoxometalates (POMs) are metal-oxygen cluster compounds, and those are well known about stabilization effect with colloidal metal nanoparticles in solution. Furthermore, redox, catalytic, and photocatalytic properties of POMs accord unique reactivities to POM-stabilized metal nanoparticles. POM and POM derivatives with kinds of metals were synthesized, and utilized in $CO_2$ conversion reaction through application with ZnO. Because $CO_2$ is one of the main factors which cause greenhouse effect, to decrease the amounts of $CO_2$ in atmosphere has been a great task around the world. The transformation of $CO_2$ into other chemical is getting more important. Here, we studied $CO_2$ fixation with propylene oxide. Synthesized cyclic carbonate can be used as an organic solvent, or converted to other carbonates through transesterification. Through this mechanism, $CO_2$ in atmosphere could be decreased. This reaction was carried out with POM and POM derivatives, and it is shown that composition change of POM could influence to the reaction.In conclusion, several kinds of nanoparticle catalysts were synthesized, and chemical activities were changed by control with shape and composition. Therefore, further researches about more detail control with nanoparticle will promise the more efficient heterogeneous metal nanoparticle catalysts.