Approaches to Enhance the Catalytic Hydrogen Production by Dinuclear Rhodium Complexes

Abstract

화석 연료의 고갈로 인해 수소 대체 에너지 연료에 대한 관심이 높아지고 있다. 그러나 수소는 폭발 위험 가능성이 높고, 에너지 밀도 대비 부피가 커 저장과 운반이 어렵다는 단점을 가지고 있다. 이는 이산화탄소와 반응시켜 화학적으로 안정한 formic acid로 변화시킴으로써 충분히 극복할 수 있다. Formic acid를 다시 수소로 환원시키는 반응 역시 촉매가 필요한데, 이 연구에서는 dinuclear rhodium 촉매를 사용하였다. 이 연구는 dinuclear rhodium 촉매의 수소생성에 대한 반응성를 향상시킬 수 있는 방법들을 모색하는데 초점을 두었다. Chapter I에서는 dinuclear rhodium 촉매의 bridging ligand가 π전자가 풍부한aromatic ring들로 구성되어 있다는 점을 이용하여 rGO과 π- π결합을 하도록 하였다. 이를 통해 촉매의 수소생성에 대한 반응성의 향상 정도를 보았고, bridging ligand의 길이 별 interaction의 차이에 따른 활성도 변화를 관찰하였다. Chapter II에서는 2,2’-bipyridine과 유사한 구조의 bidentate ligand를 가지는 dinuclear rhodium 촉매들의 수소생성에 대한 반응성을 광화학 및 전기화학 조건에서 관찰하였다. 또한, 이산화탄소 환경 조건에서 electrolysis를 하여 proton 환원에 의한 수소 생성뿐만 아니라 이산화탄소의 환원에 의한 formic acid 생성 및 그로부터 향상된 수소 생성을 보고자 하였다. ;Abstract

As the world population and economy grow rapidly, the demand for energy will keep increasing in the near future. This demand could be met from fossil energy resources, particularly coal. However, prices of crude oil have risen significantly due to the depletion of fossil fuels and other political issues in producing countries. In addition, emission of carbon dioxide caused by the burning of fossil has caused various alarming environmental problems. Therefore, the development of alternative energy sources is crucial. Among many renewable energy resources, hydrogen is one of the most powerful candidates to replace the fossil energy resources. The reaction of hydrogen and oxygen produces sufficient amount of energy to support the world population, and its side product is just harmless water. However, hydrogen is yet far from replacing the fossil energy resources due to its disadvantages. Hydrogen is explosive and has a low volumetric energy density, so the storage and delivery of hydrogen are difficult. In Chapter I, hydrogen is first transformed into chemically more stable formic acid, or its conjugate base, formate. Formic acid is formed from a reaction of hydrogen and carbon dioxide in the presence of a suitable catalyst. After the storage and transportation processes, formic acid needs to be converted back into hydrogen. However, formic acid can be decomposed via dehydrogenation and dehydration methods. To selectively generate hydrogen from dehydrogenation pathway, we prepared a series of rhodium catalysts varying in the size of ligands. To enhance the catalytic activities of rhodium complexes, reduced graphene oxide (rGO) was added to the photochemical system for hydrogen production. A 2,2’-bipyridine ligand has been extensively used in complexation of metal ions due to its ability to store electrons in vacant π* levels. Thus, the ligand has outstanding redox stability. Some researches have shown that dinulear metal complexes exhibites several times higher catalytic activity compared to the mononuclear complexes. This may be explained by the synergistic electronic effect of the second metal center on the first one through the π-system of the bridging ligand. To investigate this synergistic electronic effect in dinuclear complexes, we have performed experiments for hydrogen production in both electrochemical and photochemical system.