Solar Light Assisted Reductive Hydrogen Peroxide Production from Dioxygen and Water

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Title
Solar Light Assisted Reductive Hydrogen Peroxide Production from Dioxygen and Water
Author
Hong, Yerin
Advisor
Jang, Ji-Wook
Keywords
Photocatalyst, hydrogen peroxide production
Issue Date
2019-08
Publisher
Graduate School of UNIST
Abstract
The finite nature of fossil fuel reserves and the increasing pace of climate change mean that we must find and harness clean and sustainable energy sources. H2O2 can be one such energy source as it generates more energy than any other fuel without generating any pollutant. Moreover, H2O2 has widespread applications in chemical industries, medicinal science, water treatment, etc. However, the commercial multistep anthraquinone (AQ) process for H2O2 production requires high energy input for hydrogenation/oxidation reaction, gases supply (H2 and O2), and noble metal catalysts. The usage of organic solvents and generation of by-products during extraction makes it non-eco-friendly process. However, photocatalytic H2O2 generation can be a potential replacement to AQ process, as it doesn’t require explosive hydrogen gas, expensive noble metals, etc. Considering, H2O2 as a potential future energy carrier, the research will be much focused on constructing an efficient and sustainable photochemical solar H2O2 production system capable of producing high concentrations of H2O2. In this dissertation, three different potential light absorbers (pristine as well as hybrid) have been synthesized directly/ indirectly to construct a sustainable powder based photocatalytic system capable of producing high concentrations of H2O2 under visible light irradiation at ambient conditions. In this reported research work, both organic as well as inorganic photocatalysts have been utilized to scale up the H2O2. In Chapter 2, to maximize the light harvesting efficiency over a wide range of the solar spectrum and enhancing the charge transfer efficiency by minimizing charge recombinations, polymer/TiO2 heterojunction photocatalyst has been synthesized. To find an ideal combination, and to construct an efficient heterojunction between poly (fluorene-benzothiadiazole) (PFBT) based polymeric photocatalyst and TiO2, three different fluorine substituted PFBT polymer (PFBT, PFFBT, and PF2FBT) have been synthesized and tested. Compared to bare TiO2, polymer/TiO2 heterojunction (polymer having 1 or 2 fluorine atom) generates nearly 80 times higher H2O2. The theoretical calculations corroborated by experimental results demonstrate that the hydrophobic character of the polymeric photocatalysts plays a key role in maximizing the performance of polymer/TiO2 hybrid catalyst. The hydrophobic character of fluorinated PFBT polymeric photocatalysts restricts the surface adsorption of photogenerated H2O2, which prevent the photodegradation of the same. Contrary to polymer/TiO2 heterojunction polymer, bare TiO2 show a high degree of H2O2 photodegradation which highlights its inability of H2O2 production in the absence of surface shielding. The hydrophobic nature of pristine polymeric, as well as hybrid photocatalysts, have been substantiated by contact angle measurement studies. As we highlighted earlier, one of the main challenges for scale up the photochemical H2O2 generation is the stabilization of photogenerated H2O2 in the reaction medium. In Chapter 3, the metal-organic framework (MOF) derived carbon encapsulated CdS (C@CdS) composite photocatalysts have been synthesized and tested for photochemical H2O2 generation. The C@CdS photocatalyst is synthesized by carbonization of cadmium and sulfur atoms containing MOF by annealing at the high temperature in different gases environment. The transformation of MOF structure to C@CdS photocatalyst has been confirmed by analysis-ray diffraction, and electron microscopic techniques. The carbon matrix on the surface of CdS photocatalyst act as a shield to inhibiting the H2O2 photodecomposition on its surface. The exciting feature of this reported work is the unassisted (i.e., in the absence of hole scavenger) H2O2 production under visible light irradiation. The encapsulation of CdS into the carbon matrix increases the H2O2 production (2 mM) by nearly 4 folds in comparison to commercialized CdS. The extended photochemical reaction studies demonstrate the absence of point of equilibration even after 24 h irradiation in the case of for C@CdS which further widened the difference in the photogenerated H2O2 concentration over C@CdS (2.09 mM) and commercial CdS (0.33 mM) after 24 h of visible light irradiation. In Chapter 4, organic photocatalyst graphitic carbon nitride (g-C3N4) has been utilized for photochemical production of H2O2. To carry out the photochemical reaction g-C3N4 was synthesized by ionothermal process. The optimized ionothermal method helps us in synthesizing triazine structured, highly photoactive g-C3N4. A comparison has also been drawn between the photochemical H2O2 generation efficiency of g-C3N4 sample synthesized by two different methods, i.e., thermal condensation, and ionothermal process. It was quite surprising that g-C3N4 synthesized by ionothermal process shows nearly 7 fold high H2O2 production (16 mM) after 24 h of visible light irradiation under ambient conditions. The H2O2 production rate over as-synthesized g-C3N4 is far higher than any of the photochemical processes reported so far, and even comparable to the electrochemical processes for H2O2 production.
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Department of Chemical Engineering
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