Iron Based Metal-Organic Frameworks (Fe-MOFs) for Photocatalytic Energy and Environmental Applications

Abstract

This thesis presents the synthesis and photocatalytic application of iron based MOF (MIL-53) and its heterostructure materials for the photocatalytic decontamination of environmental contaminants under visible light and the production of green and sustainable renewable energy by generating hydrogen from water splitting, thereby addressing the global environmental and energy crisis. Initially, a simple solvothermal technique was used to create MIL-53 (Fe-MOF) and MIL-53 based heterojunctions. The MIL-53 were then combined with metal carbonate, metal nanoparticles, graphitic carbon nitride (g-C3N4) and metal oxide to create novel heterostructure materials that have better optical absorbance and photoelectrochemical properties. The photocatalytic use of the synthesized heterostructure materials was tested for environmental purification process and hydrogen evolution reaction (HER) reactions. To comprehend the mechanism behind photocatalytic activity, a thorough mechanistic research and band position analysis of the composites were carried out. The photocatalytic activity of MIL-53 (Fe-MOF) was primarily checked by making a heterojunction with Ag2CO3 using a simple solvothermal method in which the Ag+ ion of Ag2CO3 self-reduces to Ag NPs, which function as a bridge in the heterojunction system. The Ag2CO3 was incorporated within an MIL-53 matrix to synthesize MAC-X hybrid p-n heterostructure that had remarkable photocatalytic activity towards RhB degradation and hydrogen evolution reaction. The optimal photocatalyst MAC-30 showed the highest photocatalytic efficiency towards RhB degradation with rate constant (0.025 min−1) which was 3.57 and 4.16 times more than pristine Ag2CO3 and MIL-53 respectively. The MAC- 30 photocatalyst also exhibited superior photocatalytic H2 evolution (1346 μmol.h−1.g−1) with apparent conversion efficiency of 8.59%. Self-reduced Ag nanoparticle in MAC-X composites acts as an electron mediator in the Z-scheme mechanism path to improve the photocatalytic performances. Ag nanoparticles adorned g-C3N4 modified with MIL-53 was synthesized using a simple solvothermal method. The finely distributed silver nanoparticles (Ag NPs) serve as an electron channelizing bridge for the photo-generated electrons in the composite systems. The hierarchical materials exhibited unique structural, compositional as well as opto-electrical properties, which include high crystallinity, surface exposed reactive site, nanosized interfacial contact, strong absorption in visible region, rapid migration of charge carriers and high resistance to recombination. The optimal 15% of ACN-20 modified wit MIL-53 (MACN-15) photocatalyst demonstrated outstanding photocatalytic activity for the degradation of rhodamine B (RhB) (98%), and generation of H2 (2.891 mmolg−1 h−1) from the water splitting with an apparent conversion efficiency of (14.8%). Further, to increase the ability to harness visible light and photoactivity, MIL-53 was constructed heterojunction with boron and sulfur co-doped hollow g-C3N4 which was implemented in photodegradation of Bisphenol A (BPA) and photocatalytic hydrogen production. The electrical and molecular structure of g-C3N4 can be altered by boron and sulfur elemental doping to enhance its photocatalytic capabilities. Large particular surface area and low interfacial resistance are provided by the boron sulfur co-doped g-C3N4 (CNBS)@MIL-53 (M-CNBS heterojunction) for quick electron transport. The M-CNBS heterostructure shows excellent photocatalytic activity towards BPA degradation with rate constant of 0.03873 min-1 and H2 evolution (2.80 mmol. g-1). The significant photoactivity of M-CNBS heterojunction attributes to the Z-scheme charge transfer dynamics. The degradation pathway and the molecular structures of the intermediates of BPA were analyzed by LCMS technique and a possible degradation pathway has also been predicted. Finally, the heterojunction between MIL-53 (Fe-MOF) and defect induced CeO2 was fabricated for the photodegradation of Bisphenol A (BPA) degradation and H2 evolution. A simple chemical redox etching methodology was adopted to narrow the band gap of pristine CeO2 through oxygen vacancy engineering. The optimal photocatalyst 30% of defect induced CeO2/ MIL-53(MCO-30) displayed the highest photocatalytic BPA degradation with rate constant (0.045 min−1) and H2 evolution (3286.2 μmol.h−1.g−1) respectively. The significantly improved photocatalytic application of MCO-X heterojunction could be attributed to the switching of charge dynamics mechanism from Type-1 to Type-II due to defect formation in the pristine CeO2. This study provides a comprehensive analysis on how defect in pristine CeO2 in MCO-X heterojunction can switch the charge transfer mechanism from Type-1 to Type-II to achieve remarkable visible light harnessing capacity and photocatalytic activity.