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Solar Light Activated Photocatalysts for Enhanced CO2 Conversion to Hydrocarbon Fuels

Solar Light Activated Photocatalysts for Enhanced CO2 Conversion to Hydrocarbon Fuels
Abdul Razzaq
DGIST Authors
Abdul Razzaq; In, Su Il; Lee, Soo Keun
In, Su Il
Lee, Soo Keun
Issue Date
Available Date
Degree Date
2017. 2
Solar light activated photocatalystsPhotocatalytic CO2 conversionTiO2 nanotubesCo-doping strategyReduced Graphene OxideReduced TiO2광촉매적 CO2 전환태양광 활성 광촉매TiO2 나노튜브동반도핑환원된 산화 그래핀환원된 TiO2
The enormous and continuous release of anthropogenic CO2 into earth’s atmosphere have excessively increased the atmospheric CO2 level, resulting in natural carbon cycle disturbance and stemming the critical issues of global warming, climate change and environmental pollution. Amongst greenhouse gasses (GHG), CO2 is a prominent gas responsible for proliferating the greenhouse effect. An effective approach to overcome such issue of elevated atmospheric CO2 level is to capture CO2 followed by its utilization in industrial processes and/or its conversion to useful chemicals/fuels. As revealed by thermodynamic studies, CO2 is a very stable molecule, demanding additional energy to overcome the uphill barrier for its conversion to useful chemicals. In this regard, several approaches have been developed to overcome the overpotential in its conversion to useful chemicals. Various techniques include chemical conversion, thermal conversion, biological conversion, electrocatalytic conversion, photoelectrochemical conversion and photocatalytic conversion. Amongst these approaches, photocatalytic CO2 conversion/CO2 photoreduction via solar light to useful hydrocarbons and chemicals seems to be an appealing and compelling strategy, well-fitting to the objectives of renewable energy utilization and, environment and energy infrastructure in a sustainable manner. Despite of extensive research and development, the respective field still remain in its infancy and demand enormous amount of efforts for improved photocatalytic performance and product selectivity. A plenty of photocatalytic materials have been developed for improved CO2 photoreduction, amongst which Titanium dioxide/Titania (TiO2) and/or TiO2 based photocatalysts are extensively studied within the scientific society. TiO2 offers several advantages such as corrosion stability, abundant availability and low cost, though its performance is largely limited due to inadequate light absorption, mainly attributed to its wide band gap (~3.2 eV) and low quantum yield in sunlight due to surface and bulk volume charge recombination. However, despite of such critical disadvantages, TiO2 still remains a champion material in the field of photocatalysis due to its stable and commendable properties. A number of approaches have been developed to overcome the issues of limited light absorption and efficient charge separation, including doping with non-metal or noble metal co-catalysts, coupling with low band gap semiconductors, and the synthesis of carbon-based TiO2 composites. Hence, with the aim of improving the photocatalytic performance of TiO2 and TiO2 based materials, the experimental works performed and investigated in this thesis consists of three key strategies leading to enhanced light absorption and improved charge separation for TiO2 and TiO2 based materials. Mainly three experimental works are done and investigated during Ph.D. research which include approaches such as (i) foreign element doped sodium titanate nanotubes (Na+-TNT), (ii) synthesis of reduced graphene oxide (rGO) coupled TiO2 nanotube arrays, a novel heterostructured photocatalyst and (iii) development of reduced TiO2 by a newly developed approach. During past few years, TiO2 nanotubes (TNT), a one dimensional (1-D) TiO2 nanostructures have attracted a great interest among the photocatalysis research community, offering more active sites and improved charge separation by its high surface area and directional charge transport. In the first experimental approach of the thesis, an attempt was made to enhance the photocatalytic performance of sodium titanate nanotubes (Na+-TNT) by a co-doping strategy of foreign elements. Carbon and nitrogen co-doped sodium titanate nanotubes (C,N-TNT) are synthesized by designing a simple two-step process, comprising of an alkaline hydrothermal technique followed by calcining the well mixture of Na+-TNT (obtained from alkaline hydrothermal method) with varied amounts of urea (as a nitrogen and carbon dopant source). The photocatalysts are characterized using numerous experimental techniques, and investigated under simulated solar light spectrum for the photocatalytic conversion of CO2 and water vapor to methane (CH4). The C,N-TNT sample with optimum dopant concentration yields the maximum methane yield of 230.80 ppm•g-1•h-1, 2.63 times more than pure Na+-TNT. The key factors contributing to enhanced photocatalytic performance include increased light absorption, surface area and Na+ ions concentration in TNT which acts as a CO2 adsorption site and photogenerated electrons recombination centers. It is observed that higher doping of the TNT, resulted in lower photocatalytic performance which might be due to decreased surface area or increased recombination centers. Our results suggest, co-doping of nanostructured photocatalysts is an excellent pathway for improving textural and photocatalytic properties for the respective application domain. Graphene based TiO2 nanostructures have also been found to offer improved photocatalytic/photoelectrochemical properties, with graphene contents enhancing light absorption as well as promoting rapid charge transfer. With the aim of improved photocatalytic performance via enhanced light absorption and efficient charge separation, an attempt is made in second experimental work of the thesis for the synthesis of novel heterostructure comprising of reduced graphene oxide (rGO) coupled with 1-D TiO2 nanotube (TNT) arrays. A facile synthesis approach is designed resulting in a noble metal-free novel nanostructured photocatalytic material, comprising of one-dimensional arrays of TNT covered with reduced graphene oxide and TiO2 nanoparticles termed as rGO-TNTNP. The probable mechanism which might be involved in the fabrication of such novel nanostructured photocatalyst is proposed on the basis of reported literature and experimental results specifically, Raman spectra, XPS data and SEM images. The novel nanostructure exhibits significantly improved photocurrent density and photochemical activity via photocatalytic conversion of CO2 into CH4 under simulated solar light irradiation. The rGO-TNTNP produces CH4 with an evolution rate of 5.67 ppm•cm2•h-1, 4.4 times more than pure TNT sample (1.28 ppm•cm2•h-1). The improved performance appears due to the combined effect of enhanced light absorption and effective charge separation promoted by the rGO content over photocatalyst surface. The discovery of black or reduced TiO2 materials with extended light absorption and suitable band structure has offered improved photocatalytic properties. Until now a variety of methods have been reported for the synthesis of reduced TiO2 (RT), suggesting different material properties which can be manipulated by a number of process parameters. In the third and last experimental work of the thesis, the performance of RT for CO2 photoreduction with water vapor to hydrocarbons mainly CH4, is investigated under simulated solar light irradiation. The RT employed in this work is synthesized by a newly developed reduction process using dual reducing agents i.e. Mg in 5% H2/Ar. Further, to improve the charge separation efficiency, platinum (Pt) nanoparticles as co-catalyst are loaded by a photodeposition method and Pt concentration is optimized on the RT surface. With optimally photodeposited Pt nanoparticles on RT, it exhibits a stable performance and a threefold increase in CH4 production rate (1640.58 ppm•g-1•h-1 or 1.13 µmol•g-1•h-1) as compared to Pt photodeposited pure commercial nano-TiO2 (546.98 ppm•g-1•h-1, 0.38 µmol•g-1•h-1). The improved photocatalytic performance is mainly attributed to the suitable band gap with enhanced light absorption, well-aligned position of band edges against CO2/CH4 redox potential and efficient photogenerated charge separation by well-dispersed Pt nanoparticles co-catalyst having optimum size, concentration and well dispersion. ⓒ 2017 DGIST
Table Of Contents
Chapter 1. Introduction 1-- 1.1 Impact of industrialization on environment 1-- 1.2 Carbon dioxide (CO2): A potential greenhouse gas 3-- 1.2.1 Climate change 4-- 1.2.2 Increase in earths average temperature 4-- 1.2.3 Rise in sea level 5-- 1.2.4 Spreading of diseases 5-- 1.2.5 Disturbing ecosystems 5-- 1.2.6 Crops cultivation 6-- 1.3 The “Carbon Cycle” disturbance 7-- 1.4 Normalizing excess atmospheric CO2 level 9-- 1.4.1 CO2 capture and separation 9-- 1.4.2 CO2 utilization and conversion 10-- 1.5 Photocatalytic CO2 conversion/reduction 14-- 1.5.1 Introduction 14-- 1.5.2 Thermodynamics 16-- 1.5.3 Mechanism and reactions pathways 19-- 1.6 Photocatalytic materials trend 23-- 1.6.1 Single semiconductor photocatalysts 24-- 1.6.2 Doped semiconductor photocatalysts 25-- 1.6.3 Nanostructured semiconductor photocatalysts 27-- 1.6.4 Graphene based photocatalysts 28-- 1.6.5 Metal loaded photocatalysts 30-- 1.6.6 Semiconductor-semiconductor junction photocatalysts 32-- 1.7 Research objectives and approaches 35-- 1.8 References 39-- Chapter 2. Characterization and analysis tools 48-- 2.1 Characterization tools 48-- 2.1.1 X-ray diffraction (XRD) 48-- 2.1.2 Raman spectroscopy 52-- 2.1.3 Scanning electron microscopy (SEM) 55-- 2.1.4 Transmission electron microscopy (TEM) 57-- 2.1.5 X-ray photoelectron spectroscopy (XPS) 59-- 2.1.6 UV-vis diffuse reflectance spectroscopy (UV-vis DRS) 62-- 2.1.7 Photoluminescence (PL) spectroscopy 64-- 2.1.8 Physisorption analysis 66-- 2.2 Analysis techniques 71-- 2.2.1 Gas Chromatographic (GC) analysis 71-- 2.2.2 Gas Chromatography-Mass spectroscopy (GC-MS) analysis 79-- 2.2.3 Photocurrent measurements 89-- 2.3 Experimental setup for photocatalytic CO2 conversion 92-- 2.3.1 Experimental assembly 92-- 2.3.2 Experiment preparation 93-- 2.3.3 Experiment operation 94-- 2.3.4 Control test and carbon source investigation 95-- 2.4 References 97-- Chapter 3. Photocatalytic Conversion of CO2 to Hydrocarbon fuel using Carbon and Nitrogen co-doped Sodium Titanate Nanotubes 99-- 3.1 Introduction 99-- 3.2 Experimental section 105-- 3.2.1 Materials and reagents 105-- 3.2.2 Preparation of carbon and nitrogen co-doped sodium titanate nanotubes (C,N-TNT) 105-- 3.2.3 Characterization 106-- 3.2.4 Photocatalytic CO2 conversion 107-- 3.3 Results and discussion 109-- 3.3.1 Crystallographic study 109-- 3.3.2 Morphological analysis 111-- 3.3.3 Light absorption and band gap estimation 113-- 3.3.4 N2-physisorption analysis 114-- 3.3.5 X-ray photoelectron spectroscopy (XPS) analysis 117-- 3.3.6 Photocatalytic CO2 conversion 120-- 3.4 Conclusions 124-- 3.5 References 125-- Chapter 4. TiO2 Nanotube Arrays Covered with Reduced Graphene: A Facile Fabrication approach towards a Noble Metal-free photocatalyst and its application in Photocatalytic CO2 conversion to methane 132-- 4.1 Introduction 132-- 4.2 Experimental section 135-- 4.2.1 Materials and reagents 135-- 4.2.2 Synthesis of graphene oxide (GO) 135-- 4.2.3 Synthesis of rGO-TNTNP 136-- 4.2.4 Characterization 137-- 4.2.5 Photocurrent measurements 138-- 4.2.6 Photocatalytic CO2 conversion 139-- 4.3 Results and discussion 140-- 4.3.1 Morphological analysis 140-- 4.3.2 Crystallographic study 142-- 4.3.3 X-ray photoelectron spectroscopy (XPS) analysis 144-- 4.3.4 rGO-TNTNP formation mechanism 148-- 4.3.5 Optical properties and photocurrent measurements 151-- 4.3.6 Photocatalytic CO2 conversion 153-- 4.4 Conclusions 156-- 4.5 References 157-- Chapter 5. Reduced TiO2 (TiO2-x) Photocatalysts prepared by a New Approach for Efficient Solar Light CO2 conversion to Hydrocarbon fuels 163-- 5.1 Introduction 163-- 5.2 Experimental section 169-- 5.2.1 Materials and reagents 169-- 5.2.2 Synthesis of reduced TiO2 (RT) 169-- 5.2.3 Synthesis of Pt deposited reduced TiO2 (RT) 170-- 5.2.4 Characterization 170-- 5.2.5 Band gap estimation 172-- 5.2.6 Photocatalytic CO2 conversion 172-- 5.2.7 13CO2 isotopic experiment 174-- 5.3 Results and discussion 175-- 5.3.1 Crystallographic study 175-- 5.3.2 Light absorbance and photoluminescence (PL) spectroscopy 178-- 5.3.3 X-ray photoelectron spectroscopy (XPS) analysis 180-- 5.3.4 Electron paramagnetic resonance (EPR) analysis 184-- 5.3.5 N2-physisorption analysis 185-- 5.3.6 Band gap estimation 186-- 5.3.7 Photocatalytic CO2 conversion 188-- 5.3.8 CO2 conversion mechanism 198-- 5.4 Conclusions 199-- 5.5 References 201-- Chapter 6. Concluding remarks 208-- Appendix 1. Abstract in Korean language 212-- Acknowledgements 217
Energy Systems Engineering
Energy Science and EngineeringThesesPh.D.

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