Study on the mechanisms of photo- and electro-catalytic system for water splitting reaction = 물분해 반응을 위한 광-, 전기-촉매 시스템의 반응 메커니즘 연구

다국어 초록 (Multilingual Abstract)

Developing the suitable catalyst system for desired reaction is one of the most important goal of research community that has great significance both industrially and scientifically. In particular, catalytic systems that convert solar energy to hydrogen fuel by photo-/electro-chemical water splitting, have been attracting much attention recently as being an ideal technology for solving the environmental and energy problems of mankind and achieving a sustainable future energy system. As a result of many previous researches, the efficiency of catalyst system has been continuously improved stand on the descriptor based volcano approach that is supported by various experimental and calculation results. Although the catalyst system design guided by the descriptor based volcano approach has leaded to achieve an integrated understanding of the system and increase the efficiency continuously, a new system design approach is needed to break the volcano limitation of previous approach and to attain the practically economic efficiency of the catalyst system. In this regard, detailed understanding the reaction mechanism of the efficient catalyst system is essential to be the basis for the rational design of the improved catalyst system.
In general, the catalytic reaction cycle is not a simple single-step but a complex multi-step involving activation, substrate binding, product liberation, and regeneration process. Since the catalyst system itself undergoes a wide variety of intermediate states during the reaction, the analysis of each intermediate species of catalyst material is the first step in understanding the reaction mechanism. To understand the reaction mechanism of efficient catalyst systems for water splitting reaction, characterization of both the initial and intermediate states of the catalyst system is performed by various in situ/ex situ spectroscopy analysis.
Chapter 2 investigates sub-10 nm sized Mn3O4 nanoparticle (NPs) for electrocatalytic water oxidation reaction under the neutral pH condition. Through the in situ Raman spectroscopy analysis, we found out that the initial Mn3O4 NPs undergoes totally different reaction mechanism depending on the type of anion in the electrolyte. Further ex situ electron paramagnetic resonance (EPR), in situ X-ray absorption near edge structure (XANES), in situ UV-Vis spectroscopy analysis reveal that the anion with proton accepting ability is essential for efficient water oxidation reaction with Mn3O4 NPs catalyst by generating high-valent Mn-oxo species through the proton-coupled electron transfer manner.
Based on the above-mentioned mechanism, we have shown that the high-valent Mn-oxo species is a key intermediate species that determines the overall reaction rate, and in Chapter 3, we conduct studies to control this reaction intermediate species. By substitute Ni atoms to Mn3O4 NPs lattice, new intermediate signal is detected by ex situ EPR measurement. EPR simulation and DFT calculation show that substituted Ni can induce the compressed distortion in the active Mn site rather than directly participate in the reaction, forming new intermediate species with low spin electronic configuration (Mn(IV)=O, S=1/2).
Chapter 4 investigates single atom Cu/TiO2 for photocatalytic hydrogen generation reaction. Single Cu atoms are successfully substituted to the Ti sites in anatase crystal structure of TiO2 material by modified wrap-bake-peel process. Synthesized Cu/TiO2 system undergoes unique reversible and cooperative photoactivation process. From ex situ UV-Vis, photoluminescence, XANES, and EPR spectroscopic analysis and DFT calculation, we found out that the redox state of single Cu atom is reversible changed, which modulate the optoelectronic properties and photocatalytic activity of the overall Cu/TiO2 system.
In conclusion, we try to characterize and control the intermediate species of the catalyst systems for water splitting reaction. Our study provides a significant mechanistic insight for water splitting reaction and suggest the direction of the catalyst system design that could extend the boundaries of conventional heterogenous catalyst system.

국문 초록 (Abstract)

원하는 반응에 적합한 촉매 시스템을 개발하는 것은 산업적으로, 과학적으로 중요한 의미를 갖는 중요한 목표이다. 특히 태양 에너지를 수소 에너지원으로 바꿀 수 있는 광-/전기-화학 물분해 촉매 시스템은, 인류가 당면한 환경 및 에너지 문제를 해결하고 지속가능한 미래 에너지 시스템을 달성하기 위한 이상적인 기술로 매우 각광받고 있다. 많은 계산결과와 실험 결과들로 뒷받침된 descriptor 기반의 volcano 촉매 시스템 디자인 접근을 통해 촉매 시스템의 효율은 점진적으로 개선되어 왔다. Descriptor 기반의 volcano 디자인은 촉매 시스템 전반에 대한 이해도를 높이고 시스템 효율 증가에 기여해 왔지만, volcano limitation 을 넘어 가격 경쟁력을 갖춘 촉매 시스템 효율 달성을 위해서는 새로운 촉매 시스템 디자인 접근법이 필요한 상황이다. 이를 위해서는 촉매 시스템의 반응 메커니즘에 대한 구체적인 이해가 필수적이다.
보통 촉매 반응 사이클은 단순한 한 단계의 스텝이 아니라, 활성화 과정, 반응물 흡착 과정, 생성물 탈착 과정, 재생 과정 등의 여러가지 복잡한 스텝으로 이루어져 있다. 그리고 이 다양한 과정에서 촉매 물질 자체가 다양한 형태의 중간 상태를 거치게 되고, 이 각각의 중간 상태를 분석하는 것은 반응 메커니즘 이해의 첫 걸음이 될 것이다. 본 연구에서는 특히 효율 높은 물분해 반응 촉매 시스템에 다양한 실시간 분광학 분석법을 적용해 촉매 물질의 초기 상태와 다양한 중간 상태를 분석하여 반응 메커니즘을 이해하고자 하였다.
Chapter 2 에서는, 중성 pH 조건의 전기화학 물산화 반응용 10 nm 이하의 크기의 Mn3O4 나노입자 시스템에 대한 연구를 진행하였다. 실시간 Raman 분광학 분석을 통해, 전해질의 음이온 종류에 따라서 같은 Mn3O4 나노입자도 완전히 다른 반응 메커니즘을 갖는 것을 발견했다. 추가적으로, 실시간 EPR, XANES, UV-Vis 분광학 분석을 통해 Mn3O4 나노입자는 proton accepting 능력을 가진 음이온이 존재하는 경우 proton-coupled electron transfer 를 통해 high-valent Mn-oxo 종을 형성하며 물산화 반응을 진행하는 것을 밝힐 수 있었다.
위에서 밝힌 반응 메커니즘을 기반으로 Chapter 3 에서는, 전체 촉매 반응 속도를 더욱 향상시키기 위해 중요 중간체인 high-valnet Mn-oxo 종을 제어하는 연구를 진행하였다. Mn3O4 나노입자 격자구조에, Ni 원자를 치환시켜주는 경우, 물산화 반응 조건 하에서 새로운 형태의 EPR 신호를 관찰 할 수 있었다. SQUID 분석, EPR 시뮬레이션, DFT 계산을 통해 치환된 Ni 원자라 주변 Mn 활성점의 구조를 압축시켜 low spin electronic configuration 을 갖는 Mn(IV)=O (S=1/2) 중간체를 형성할 수 있다는 것을 확인하였다.
Chapter 4 에서는 광촉매 수소생산용 Cu/TiO2 촉매 시스템에 대한 연구를 진행하였다. Modified wrap-bake-peel 공정을 통해 성공적으로 Cu 단원자를 anatase TiO2 나노입자 격자내의 Ti site 에 치환시킬 수 있었다. 이렇게 합성된 Cu/TiO2 광촉매 시스템은 독특한 reversible and cooperative 광활성 과정을 거치며 매우 높은 광촉매 수소생산 활성을 보였다. UV-Vis, photoluminescence, XANES, EPR 분석을 통해, 단원자 Cu 의 산화가가 가역적으로 2+, 1+ 사이를 오가며 주변 TiO2 격자에 distortion을 유도하고 물질의 optoelectronic 성질 및 광촉매 활성을 제어한다는 것을 확인하였다.
종합적으로 본 연구에서는 물분해 반응용 촉매 시스템의 다양한 중간 상태를 분석하고 제어하고자 하였다. 이러한 연구 결과들은 물분해 반응 메커니즘 이해에 매우 중요한 기본 바탕이 되며 기존의 비균질 촉매 시스템의 한계를 뛰어 넘는 촉매 시스템 디자인을 위한 방향성을 제시할 수 있을 것으로 기대한다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 The nessecity for the devlopment of alternative energy source for future sustainable energy system 1
• 1.2 Hydrogen as an energy fuel 2
• 1.3 Solar hydrogen conversion 3
• 1.4 Water splitting reaction 8
• 1.4.1 General description for water splitting reaction 8
• 1.4.2 Photocatalytic hydrogen evolution reaction (HER) 11
• 1.4.3 Electrocatalytic oxygen evolution reaction (OER) 20
• 1.5 Previous approach for catalyst material design: descriptor based catalytic system design 38
• 1.5.1 Volcano plot suggested by Trassatti 38
• 1.5.2 Thermochemical calculation for activity prediction 41
• 1.6 Catalytic cycle mechanism understanding 48
• 1.6.1 Suggested mechanism for water oxidation reaction 48
• 1.6.2 In situ/ex situ spectroscopic analysis on heterogeneous catalyst system for mechanistic understanding 58
• 1.7 Scope of the Thesis 62
• 1.8 References 65
• Chapter 2 Mechanistic invesitigation of Mn3O4 nanoparticles (NPs) during electrocatalytic OER 80
• 2.1 Introduction 80
• 2.2 Experimental methods 103
• 2.2.1 Materials 103
• 2.2.2 Nanoparticle synthesis 104
• 2.2.3 Electrode preparation 105
• 2.2.4 Electrochemical analysis 106
• 2.2.5 In situ/ex situ spectroscopy analysis 107
• 2.3 Results and discussion 110
• 2.3.1 Electrode material characterization 110
• 2.3.2 Electrochemical kinetic analysis 114
• 2.3.3 In situ Raman spectroscopy analysis 125
• 2.3.4 Further spectroscopy analysis under phosphate electrolyte 130
• 2.4. Concluding remarks 140
• 2.5. References 141
• Chapter 3. Ni-Mn3O4 NPs with low spin Mn(IV)-oxo intermediate species during electrocatalytic OER 148
• 3.1 Introduction 148
• 3.2 Experimental methods 151
• 3.2.1 Mateirals 151
• 3.2.2 Electrode preparation 152
• 3.2.3 Mateiral characterization 154
• 3.2.4 Electrochemical analysis 155
• 3.2.5 Electroparamagnetic resonance (EPR) study 157
• 3.2.6 Computation study 158
• 3.3 Results and discussion 159
• 3.3.1 Electrode material characterization 159
• 3.3.2 Electrochemical kinetic analysis 165
• 3.3.3 In situ XANES and UV-Vis spectroscopy analysis 172
• 3.3.4 Ex situ EPR and SQUID spectroscopy analysis 176
• 3.3.5 DFT calculation of Ni-Mn3O4 NPs system 187
• 3.4 Concluding remarks 192
• 3.5 References 193
• Chapter 4. Reversible and cooperative photoactivation of Cu/TiO2 for photocatalytic HER 199
• 4.1 Introduction 199
• 4.2 Experimental methods 201
• 4.2.1 Materials 201
• 4.2.2 Synthesis of M/TiO2 202
• 4.2.3 Characterization of M/TiO2 205
• 4.2.4 Photocatalyst studies 208
• 4.2.5 Computational details of DFT analysis 211
• 4.3 Results and discussion 213
• 4.3.1 Synthetic strategy for site-specific SACs (M/TiO2) 213
• 4.3.2 Photocatalytic HER activity of M/TiO2 220
• 4.3.3 Initial state characterization of M/TiO2 227
• 4.3.4 Activated state characterization of Cu/TiO2 by various ex situ spectroscopic analysis 252
• 4.3.5 DFT calculation results for activation process of Cu/TiO2 267
• 4.3.6 Suggested photoactivation cycle of Cu/TiO2 271
• 4.4 Concluding remarks 274
• 4.5 References 275
• Chapter 5. Conclusion 282
• Abstract in Korean 284
• Research achievements 286