Colloidal metallic nanoparticles have been interesting materials due to their use in catalysis. Despite their excellent catalytic activities, the use of ensemble catalysts has been limited to probe a catalytic property of a single particle in that those are averaged outcomes of significant amount of catalytic particles. The ideal nanoreactor is composed of an active metal core and silica hollow shell through which reactants pass and exhibit catalytic reaction on the surface of the active metal. Because the single active metal is isolated by solid shell, catalytic reaction occurs under homogeneous reaction condition in the shell like metal particles are evenly dispersed on the supports. The outer shell hinders the aggregation of neighboring particles and stabilizes the particle even under harsh reaction conditions such as high temperature or high pressure. The active metal of nanoreactor can be controlled to have various sizes and shapes and the silica shell can be prepared to have several diffusion coefficients. Moreover, selective ligand anchored onto metal surface can enhance the reaction rate. In this work, the new heterogeneous bifunctional catalyst, metal@silica nanoreactor framework with these advantages is represented as well as its catalytic application is demonstrated to prove the benefits.
The fabricated gold nanoreactors which have different core size were applied to the catalytic reduction of p-nitrophenol. The turnover frequency of the gold nanoreactor showed a steady rise with decreasing the size of gold core, meaning that the reduction of p-nitrophenol is the size-dependent reaction.
The functional group on the shell of gold nanoreactor could be modified for further application. Hydroxyl groups on the silica surface could be modified to trimethylsiloxyl groups by trimethylsilylation and the gold nanoreactor is anticipated to be used in different media with different property.
It was also suggested that porous silica coating could realize to control catalytic efficiency by varying diffusion coefficient of reactants to catalytic active site. The catalytic efficiency was also controlled by using group specific interaction between specific anchored molecules on the surface of catalyst and approaching reactants.
현존하는 불균일촉매는 높은 촉매적 활성을 보이고 있으나, 사용되는 촉매입자 전체에서 나타나는 평균적인 특성만 예측할 뿐 촉매입자 한 개가 어떤 촉매적 특성을 갖는지는 예측할 수 없었다. 이를 해결하기 위해 래틀타입의 코어@쉘 구조를 응용하여 이상적인 금@실리카 나노반응기를 제조하고 촉매반응에 적용시켰다. 금@실리카 나노반응기는 코어 크기의 조절, 쉘의 작용기 변화, 확산 조절이라는 세가지 접근을 통해 그 촉매적 특성이 변화 및 향상되었다.
코어의 금입자 크기가 서로 다르게 제조된 나노반응기는 p-나이트로페놀의 환원반응에 적용되었는데, 단위면적당 한 개의 활성원자에서 결과물로 전환되는 반응물의 개수를 나타내는 TOF가 코어 크기가 작은 촉매일수록 높은 수치를 나타내는 경향성을 보였다.
실리카 쉘의 경우, 표면의 친수성 하이드록실기를 친유성 트리메틸실록시기로 변화시킬 수 있었는데 이것은 제조된 나노반응기를 간단한 처리만으로 성질을 변화시켜 이성 용매에서도 사용 가능할 것이라는 기대를 가져온다.
반응물의 확산 조절은 다공성 실리카 쉘의 합성조건을 변화하여 실현시킬 수 있었다. 또한 3-머캡토프로피오닉산을 금 입자 표면에 결합시켜 반응물 작용기와의 특정 상호인력을 통하여 촉매효율을 조절할 수 있었는데, 처리하는 3-머캡토프로피오닉산의 양에 따라 촉매의 반응상수를 증가시키거나 혹은 감소시킨다는 사실도 확인할 수 있었다.
