DGIST Scholar: Cocatalyst state engineering on TiO₂ photocatalysts for selective and stable C₁ production in gas-phase CO₂ reduction

Title

Cocatalyst state engineering on TiO₂ photocatalysts for selective and stable C₁ production in gas-phase CO₂ reduction

Alternative Title

TiO₂ 기반 광촉매의 조촉매 상태 제어를 통한 기상 CO₂ 환원에서의 선택적 C₁ 생성물 형성

DGIST Authors

Dongyun Kim ; Su-Il In ; Soonhyun Kim

Advisor

인수일

Co-Advisor(s)

Soonhyun Kim

Issued Date

2026

Awarded Date

2026-02-01

Type

Thesis

Description

CO₂ reduction: TiO₂-based photocatalyst: cocatalyst engineering: electron transfer: single-atom catalyst: rGO: MXene

Abstract

Photocatalytic CO₂ reduction on TiO₂ has been widely studied as a promising route to mitigate greenhouse gas emissions and enable solar fuel production. However, its efficiency and stability are still limited by fast charge recombination, insufficient control of electron distribution, and poorly regulated multi-electron reduction pathways. We establish cocatalyst state engineering as a unifying design framework to address these challenges. Stepwise strategies were introduced through a series of cocatalyst designs. These included phase-controlled MoS₂ for heterojunction formation, conductive rGO shells for charge transfer, single-atom catalysts for tuning the local environment at the atomic level to steer reaction pathways. In addition, ternary Cu/G/RT composites were designed for synergistic surface interactions, and ligand-modified MXene was employed for enhanced stability. Through these approaches, we revealed mechanistic principles linking electron transfer, stability, and product selectivity. MoS₂ heterostructures enabled stable CO generation; rGO shells enhanced charge separation and durability; Fe and Cu single-atom catalysts steered selectivity toward CO or multielectron hydrocarbons; and ligand-modified MXene achieved over 200-fold higher CH₄ activity with multicycle stability. Together, these results demonstrate how tailoring cocatalyst states systematically enhances the activity by ensuring durability. This work not only advances TiO₂-based systems for efficient CO₂ photoreduction but also provides a roadmap for rational catalyst design toward practical solar-to-fuel technologies.|본 연구는 TiO₂ 기반 광촉매에서 조촉매 상태 제어를 통해 이산화탄소의 선택적 C1 물질 생성을 달성하기 위한 체계적인 전략을 제시하였다. MoS₂의 상 제어와 환원된 TiO₂ (RT)와의 이종접합을 통해 귀금속 없이 안정적인 CO 생성을 확인하였으며, rGO를 적용하여 전자 재결합 억제와 안정성을 확보하였다. 또한 단일 원자 촉매를 도입하여 금속–지지체 상호작용이 CO와 탄화수소 생성 선택성을 제어할 수 있음을 규명하였다.
이러한 결과를 바탕으로, 환원된 TiO₂ 나노시트, rGO 전자 채널, Cu 단일 원자를 결합한 삼원계 복합체(Cu/rGO/2D-RT)를 설계하여 CH₄ 생산성을 60배 향상시켰으며, DRIFTS와 XAS를 통해 반응 조건에서의 조촉매 상태 변화를 규명하였다. 또한 PCA–PCR으로 코팅한 MXene 기반 조촉매를 개발하여 산화 안정성과 장기 내구성을 확보하고, 200배 이상의 활성을 달성하였다.
종합적으로, 본 논문은 MoS₂ 상 제어, rGO 전자 매개체, 단일 원자 촉매, 삼원계 복합체, MXene 표면처리 등 다양한 전략을 통해 전자 밀도, 전하 전달 경로, 계면 화학을 제어함으로써 선택적이고 안정적인 C1 물질 생성을 구현할 수 있음을 제시하였다. 따라서 본 연구는 TiO₂ 기반 광촉매의 조촉매 상태 제어 전략을 구체화함으로써 차세대 태양연료 전환 기술 개발에 중요한 기여를 한다. 향후 연구는 단일 원자 조촉매의 확장, 실제 태양광 및 스케일업 조건에서의 검증, 전기화학·열화학 시스템과의 융합으로 확장될 수 있으며, 이를 통해 실용적이고 지속 가능한 CO₂ 전환 기술로의 구현 가능성을 더욱 높일 수 있을 것이다.

Table Of Contents

Ⅰ. Introduction 1
1. Photocatalytic CO₂ conversion 1
1.1. Transition metal dichalcogenides (MoS₂) 3
1.2. Reduced graphene oxide (rGO) 4
1.3. Single atom catalyst (SAC) 5
1.4. MXene-based cocatalysts (Ti₃C₂TX) 7
1.5. Research gap and objectives 9

ⅠI. Characterization and synthesis equipment 10
2.1 Characterization 10
2.1.1 X-ray diffraction 10
2.1.2 X-ray photoelectron spectroscopy 11
2.1.3 X-ray Absorption Fine Structure 12
2.1.4 UV-Visible spectroscopy 12
2.1.5 Raman spectroscopy 14
2.1.6 Fourier-transform infrared spectroscopy 15
2.1.7 Photoluminescence spectroscopy 16
2.1.8 Electron paramagnetic resonance 17
2.1.9 Temperature programmed desorption 18
2.1.10 Transmission electron microscopy 19
2.1.11 Energy-dispersive X-ray spectroscopy 21
2.1.12 Gas chromatograph 22
2.2 Materials synthesis 23
2.2.1 Autoclave 23
2.3 Calculation 24
2.3.1 GC yield 24
2.3.2 AQY 25

III. Multi-functional hybrid 1T/2H-MoS₂ for photocatalytic CO₂ reduction 26
3.1 Introduction 26
3.2 Experiment 26
3.2.1 Materials 26
3.2.2 Synthesis of multiphase 1T/2H-MoS₂ 2-dimensional nanosheets 27
3.2.3 Synthesis of reduced TiO₂ (RT) nanoparticles 27
3.2.4 Fabrication of multiphase 1T/2H-MoS₂@R-TiO₂ (MRT) composites 27
3.2.5 Characterization 28
3.2.6 Photocatalytic CO₂ reduction tests 30
3.3 Results and discussion 31
3.3.1 Characterization 31
3.3.2 Photocatalytic CO₂ reduction with water vapor 45
3.3.3 Mechanism of photocatalytic CO₂ reduction 52
3.4 Conclusions 56

IV. Effect of r-GO shell on TiO₂ photocatalyst for CO₂ reduction 57
4.1 Introduction 57
4.2 Experiment 57
4.2.1 Materials 57
4.2.2 Instruments 58
4.2.3 Synthesis of reduced TiO₂ nanoparticles (blue TiO₂ (b-TiO₂)) 58
4.2.4 Preparation of graphene oxide (GO) 59
4.2.5 Preparation of reduced-graphene oxide (r-GO) 59
4.2.6 Synthesis of r-GO-coated TiO₂ (TiO₂@r-GO) and b-TiO₂ (b-TiO₂@r-GO) ·· 60
4.2.7 Photocatalytic CO₂ reduction 60
4.2.8 Setup of four-wave mixing (FWM) microspectroscopy 60
4.3 Results and discussion 61
4.3.1 Synthesis and characterization 61
4.3.2 Band gap analysis of TiO₂ r-GO shell 66
4.3.3 Photocatalytic performance of TiO₂ r-GO shell 68
4.3.4 Photocatalytic stability 72
4.3.5 Four-wave mixing microspectroscopy studies 76
4.4 Conclusions 82

V. Mechanism of single metal photocatalyst for CO₂ reduction 83
5.1 Introduction 83
5.2 Experiment 83
5.2.1 Materials 83
5.2.2 Synthesis of Fe/TiO₂ and Cu/TiO₂ 84
5.2.3 Material Characterization 86
5.2.4 Photochemical experiments 87
5.2.5 DRIFT analysis 88
5.2.6 Theoretical simulation 88
5.3 Results and discussion 89
5.3.1 Material synthesis and characterization 89
5.3.2 Photocatalytic CO₂ reduction experiment 98
5.3.3 CO₂ photoreduction mechanism 104
5.4 Conclusions 109

VI. Single atom Cu-decorated rGO/RT photocatalyst for CO₂ reduction 111
6.1 Introduction 111
6.2 Experiment 111
6.2.1 Materials 111
6.2.2 Synthesis of 2D reduced TiO₂ (2D-RT) 112
6.2.3 Synthesis of reduced GO/2D-RT composite (G/2D-RT) 112
6.2.4 Synthesis of Cu/G/2D-RT composite 112
6.2.5 Characterization 113
6.2.6 Solar-light-driven catalytic CO₂ reduction with H₂O vapors 114
6.3 Results and discussion 115
6.3.1 Characterization 115
6.3.2 Photocatalytic CO₂ reduction 127
6.4 Conclusions 134

VII. MXene, electron transfer assistant in CO₂ photoreduction 136
7.1 Introduction 136
7.2 Experiment 136
7.2.1 Materials 136
7.2.2 Synthesis of PCA-PCR (Poly(catechol/p-cresol) 137
7.2.3 Synthesis of PCA-PCR OMX (Organic dispersed MXene) 138
7.2.4 Synthesis of TiO₂ Nanosheet 138
7.2.5 Synthesis of reduced TiO₂ 138
7.2.6 Fabrication of f-MXene/R-TiO₂ composites 139
7.2.7 Synthesis of Cu decorated f-MXene/R-TiO₂ composites 139
7.2.8 Characterization 140
7.2.9 Photocatalytic CO₂ reduction 141
7.3 Results and discussion 142
7.3.1 Characterization 142
7.3.2 Photocatalytic CO₂ reduction with water vapor 147
7.4 Conclusions 152

VIII. Conclusions 154
8.1 Conclusions 154

References 156
요약문 188

URI

https://scholar.dgist.ac.kr/handle/20.500.11750/59622
http://dgist.dcollection.net/common/orgView/200000948314

DOI

10.22677/THESIS.200000948314

Degree

Doctor

Department

Department of Energy Science and Engineering

Publisher

DGIST

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