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.
불균일 촉매는 촉매 분야에서 많은 관심을 받아왔다. 균일 촉매에 비해 재사용성 및 분리의 용이성을 가지고 있기 때문이다. 금속 나노입자는 금속의 높은 반응성으로 인해 불균일 촉매에서 활발히 연구된 분야 중 하나이다. 금속 나노입자는 크기, 모양, 그리고 금속의 구성에 따라 촉매 활성에 영향을 받는다. 이는 나노입자의 표면 환경이 촉매 활성을 조절함을 의미한다. 우선 나노입자의 크기는 나노입자의 겉넓이에 영향을 미치며, 반응에 참여하는 표면 원자의 수를 조절함에 따라 반응성이 바뀐다. 그리고 나노입자의 모양을 바꾸면 표면입자의 배열이 바뀌게 되고, 표면 에너지나 표면원자의 배위수, 그리고 모서리에 위치한 원자의 수 등이 조절되며 물리적, 화학적 성질에 영향을 준다. 또한 다종금속으로 이루어진 나노입자의 금속 구성을 조절하여 결합 에너지를 조절하거나 금속간의 상호작용을 통해 반응성을 조절한다. 종합해보면 나노입자의 촉매 활성은 표면의 조절에 의해서 결정되며, 이는 나노입자 표면에 대한 더 세부적인 연구가 필요함을 나타낸다.
2장에서 팔라듐 나노입자의 표면을 조절하여 촉매 활성 부위를 알아내는 실험을 진행하였다. 모서리에 있는 원자들이 면에 있는 원자들보다 반응성이 좋다는 연구는 간접적인 연구를 통해 밝혀져 왔다. 하지만 직접적인 증거를 제시하기 위해 팔라듐 나노입자 표면에 모서리 부분에만 촉매 활성이 없는 금이 선택적으로 도포된 나노입자를 합성하였다. 두가지 나노입자는 스즈키 결합 반응에 촉매로 사용되었다. 모서리가 막힌 나노입자의 경우 반응이 면에 존재하는 원자에서 반응이 일어나게 되므로 두 반응의 반응성을 비교하면 면과 모서리에 위치한 원자의 반응성 차이를 확인할 수 있었으며, 모서리에 위치한 원자가 반응성이 약 100배 이상 높은 것을 확인하였다.
3장에서는 금속 구성에 따른 나노입자의 반응성 차이를 확인하였다. 폴리옥소메탈레이트는 금속 나노입자의 안정화를 도우며, 산화환원 및 화학 반응성을 가지므로 금속 나노입자의 반응성에 영향을 미친다. 폴리옥소메탈레이트와 금속입자와의 결합체를 합성하여 이산화탄소 고리화 첨가반응에 촉매로 사용하였다. 이산화탄소는 지구온난화에 영향을 미치므로 이산화탄소의 저감의 중요성이 높아짐에 따라 이산화탄소를 다른 화합물로 전환하는 반응이 주목을 받고있다. 우리는 다양한 종류의 폴리옥소메탈레이트를 사용하여 산화 프로필렌에 이산화탄소를 고정시켜 고리형 카보네이트 구조를 합성하였으며, 위 반응이 폴리옥소메탈레이트의 구성에 따라 반응성의 차이를 보임을 확인하였다. 결론적으로 나노입자 촉매를 성공적으로 합성하여 그의 모양 및 구성 조절을 통해 촉매 활성을 조절할 수 있었다. 추후 나노입자의 표면에 관한 더 세부적인 연구를 통해 보다 발전한 불균일 금속 나노촉매를 합성할 수 있을 것이라 기대된다.
