DEVELOPING FUNCTIONAL HYBRID MATERIALS FOR ENVIRONMENTAL APPLICATIONS

Abstract

The global energy crisis refers to the growing imbalance between the demand for energy and the supply of traditional fossil fuels, such as oil, coal, and natural gas. This crisis is aggravated by the finite nature of these resources and the environmental impact of their extraction and use, including air pollution and climate change. As a result, there is an urgent need to find sustainable solutions to meet the world's energy needs. Renewable energy offers a promising solution to the energy crisis. Sources like solar, wind, hydro, and geothermal energy are abundant, sustainable, and have a lower environmental impact compared to fossil fuels. The electrochemical oxygen evolution reaction (OER) is a critical process in various renewable energy technologies, including water splitting for hydrogen production, CO2 reduction, and metal-air batteries. OER involves the four-electron oxidation of water to produce oxygen gas, which is essential for the efficient generation and storage of clean energy. Despite its importance, OER faces significant challenges due to its inherently sluggish kinetics, requiring high overpotentials to drive the reaction efficiently. These shortcomings highlight the need for developing alternative, cost-effective, and abundant catalysts. RuO2 and IrO2 are currently regarded as the most effective electrocatalysts for OER. However, their widespread application is constrained by the high cost, scarcity and instability under high anodic potentials of OER. This thesis emphasizes on developing functional hybrid solids and investigating their applications in water splitting and visible-light-assisted CO2-mediated N-formylation of amines. Polyoxometalates (POMs) are a class of inorganic compounds composed of metal oxygen clusters with distinct structural and electronic properties. The first work focuses on synthesizing a novel POM-cluster based solid (C5H7N2)6[NiW12O44]. [NiW12O44]14− cluster bridges the missing gap of 1: 12 hetero-POMs of Keggin and Silverton together with a coordination number of 8 of the central heteroatom (Ni). The material was explored as an efficient and highly sustained oxygen evolution pre-catalyst in alkaline medium with an overpotential of 347 mV to attain a current density of 10 mA cm−2 and long-term stability up to 96 hours. Mechanistic investigation showed that in situ generated NiO and WOx (x = 1, 2) species acted as active species for the oxygen evolution reaction. The second study reported a POM-derived noble metal-free electrocatalyst for efficient oxygen evolution in acidic medium for the first time. An octamolybdate cluster-based solid, [(Cu(pic)2)2(Mo8O26)]·8H2O (POM), served as the precursor to obtain the nanosheet-like electrocatalyst, [Cu-MoO2] which offered the overpotential of 374 mV to reach 10 mA cm−2 current density and a tafel slope of 193 mV dec−1 , with the stability of 18 h. Further, density functional theory analysis revealed Mo as a prominent active site in [Cu-MoO2]. The third project focused on utilizing POMs as efficient catalysts for photocatalytic N-formylaytion of amines using CO2. Two novel POM-based solids were synthesized, (C5H7N2)5[CoW12O40] and (C5H7N2)5[CuW12O40], of which latter was active for effective photocatalytic N-formylation of various substituted anilines and morpholine with CO2 using phenyl silane as a reducing agent, under ambient conditions. Out of the array of amines tested, p-toluidine demonstrated the best conversion and yield of 83% and 96%, respectively. The catalyst exhibited robust recyclability, maintaining catalytic activity across 5 successive cycles without notable degradation. It was the first attempt in utilizing POM-based materials for this application. The final study concentrated on Metal Organic Frameworks (MOFs) and explored an ultrathin 2D Cobalt MOF, [Co2(bpe)2.5(NO3)4(CH3O)] for electrocatalytic oxygen evolution. The Co-MOF showed significant OER activity, competing well with RuO2, with overpotential of 267 mV at 10 mAcm-2, Tafel slope of 104 mVdec-1, and unprecedented 112 hours stability. The excellent results may be accredited to the fact that 2D MOFs offer better active sites exposure and structural flexibility over 3D MOFs.