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Graphitic Carbon Nitride (G-C3N4), an Organic Semiconductor That Proved a Suitable Photocatalyst for Hydrogen Production from Water

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Graphitic Carbon Nitride (G-C3N4), an Organic Semiconductor That Proved a Suitable Photocatalyst for Hydrogen Production from Water METAL LOADED G-C₃N₄ FOR VISIBLE LIGHT-DRIVEN H₂ PRODUCTION Federica Fina A Thesis Submitted for the Degree of PhD at the University of St Andrews 2014 Full metadata for this item is available in St Andrews Research Repository at: http://research-repository.st-andrews.ac.uk/ Please use this identifier to cite or link to this item: http://hdl.handle.net/10023/6322 This item is protected by original copyright This item is licensed under a Creative Commons Licence Metal loaded g-C3N4 for visible light-driven H2 production by Federica Fina A thesis submitted in partial fulfilment for the degree of PhD at the University of St Andrews 2014 I, Federica Fina hereby certify that this thesis, which is approximately 50,000 words in length, has been written by me, and that it is the record of work carried out by me and that it has not been submitted in any previous application for a higher degree. I was admitted as a research student in January 2011 and as a candidate for the degree of Doctor of Philosophy in August 2014; the higher study for which this is a record was carried out in the University of St Andrews between 2011 and 2014. Date …………….………….. Signature of candidate …………….………….. I hereby certify that the candidate has fulfilled the conditions of the Resolution and Regulations appropriate for the degree of Doctor of Philosophy in the University of St Andrews and that the candidate is qualified to submit this thesis in application for that degree. Date …………….………….. Signature of supervisor …………….………….. In submitting this thesis to the University of St Andrews I understand that I am giving permission for it to be made available for use in accordance with the regulations of the University Library for the time being in force, subject to any copyright vested in the work not being affected thereby. I also understand that the title and the abstract will be published, and that a copy of the work may be made and supplied to any bona fide library or research worker, that my thesis will be electronically accessible for personal or research use unless exempt by award of an embargo as requested below, and that the library has the right to migrate my thesis into new electronic forms as required to ensure continued access to the thesis. I have requested the appropriate embargo below. The following is an agreed request by candidate and supervisor regarding the publication of this thesis: Embargo on all of electronic copy for a period of 2 years on the following ground: Publication would preclude future publication. Date …………….………….. Signature of candidate …………….………….. Date …………….………….. Signature of supervisor …………….………….. Acknowledgements I would like to thank my supervisor Professor John T. S. Irvine for giving me the opportunity to do my PhD and for his guidance and advice. I would like to thank Dr. Xiaoxiang Xu for helping me at the beginning of the project. My gratitude goes also to the JTSI group especially to Dr. Paul Connor for the discussions and the advice on the set-up, and to Dr. Dragos Neagu for the assistance on the XRD. A special thank you goes to George Anthony from the mechanical workshop for making the reactor, for being there when things needed fixing and for his advice. Thank you to Ross Blackley for being patient in teaching me to use the SEM and the TEM. I would also like to thank Sylvia Williamson for BET and TGA analysis, Derek Waddell for XRD measurements and Dr. Hervé Ménard (Sasol Ltd.) for XPS analysis. Finally, I would like to express my gratitude to my family and in particular my parents who supported me for all this time. "Yes, my friends, I believe that water will someday be employed as fuel, that hydrogen and oxygen, which constitute it, used singly or together, will furnish an inexhaustible source of heat and light….I believe, then, that when the deposits of coal are exhausted, we shall heat and warm ourselves with water. Water will be the coal of the future." The Mysterious Island, Jules Verne, 18741 i Abstract The need for green and renewable fuels has led to the investigation of ways to exploit renewable resources. Solar among all the renewables is the most powerful and its conversion into usable energy would help in solving the energy problem our society is facing. Photocatalytic water splitting for hydrogen production is an example of solar energy storage into chemical bonds. The hydrogen produced in this way can then be employed as carbon free fuel creating the “Hydrogen Cycle”. This work investigates the structure and the activity of graphitic carbon nitride (g-C3N4), an organic semiconductor that proved a suitable photocatalyst for hydrogen production from water. Synthesised by thermal polycondensation of melamine it is a graphitic like material with a band gap of 2.7 eV which makes it a visible light-active catalyst. In a first instance the effect of the synthesis conditions on its structure and morphology are investigated to find the optimum parameters. The temperature of condensation is varied from 450 °C up to 650 °C and the length from 2.5 h to 15 h. The structural changes are monitored via x-ray diffraction (XRD) and elemental analysis while the effect on the morphology and the band gap of g-C3N4 are investigated by mean of scanning electron microscopy and UV-Vis absorption. Subsequently, a study of the crystal structure of the catalyst is carried out. Using structures proposed in the literature, x-ray diffraction and neutron scattering simulations are used to narrow down the number of possible 3D structures. After structural characterisation, the activity of g-C3N4 for photocatalytic hydrogen evolution is evaluated. It is confirmed that loading 1 wt.% Pt on its surface significantly increases the hydrogen evolution rate. The attention then focuses on the loading ii procedures, the reduction pre-treatments of the co-catalyst and the reasons of the different performances when different procedures are employed. The catalytic system is characterised by mean of x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and XRD. By investigating the composition and the morphology of the platinum nanoparticles under different conditions, the main factors responsible for the changes in activity of g-C3N4 for hydrogen evolution are identified. Additionally, the role of the co-catalyst and its interaction with g-C3N4 is also elucidated. Finally, taking forward the knowledge acquired on the Pt-g-C3N4 system, the effect on the hydrogen evolution rate of alloying platinum with a second metal (Cu, Ag, Ni and Co) is studied. The nanoparticles are characterised by XRD and TEM. A screening of the loading procedures and bimetallic systems is performed to identify the most promising for photocatalytic hydrogen evolution with the aim of bringing them towards further investigation. Contents iii Contents Contents .......................................................................................................................... iii Table of figures .............................................................................................................. vi List of tables .................................................................................................................... x Glossary ......................................................................................................................... xii Chapter 1 Introduction ................................................................................................. 1 1.1. Energy scenario ......................................................................................................... 1 1.1.1. Current energy situation .................................................................................. 1 1.1.2. Renewable alternatives ................................................................................... 2 1.2. Hydrogen Economy ................................................................................................... 3 1.2.1. What is the Hydrogen Economy? ................................................................... 3 1.2.2. Why hydrogen? ............................................................................................... 4 1.2.3. Hydrogen production: today and tomorrow. ................................................... 5 1.3. Photocatalytic water splitting .................................................................................... 6 1.3.1. General principles and mechanism ................................................................. 7 1.3.2. Improving the activity of the catalyst ........................................................... 10 1.3.3. Overall water splitting ................................................................................... 13 1.3.4. Half reaction and sacrificial reagents ............................................................ 14 1.4. Graphitic carbon nitride ........................................................................................... 15 1.4.1. General description ....................................................................................... 15 1.4.2. State of the art: synthesis .............................................................................. 17 1.4.3. State of the art: photocatalytic applications .................................................. 18 1.5. Aim of the work ....................................................................................................... 21 Contents iv Chapter 2 Experimental .............................................................................................
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