Metal-organic frameworks (MOFs) are porous crystalline materials made up of metal ions and organic linkers. What makes MOFs more attractive to scientific community is “control over the porosity, particle size and shape at nano-level. This helps us to design and synthesize new nanostructured materials for various functional applications. One of them is “heterogeneous catalysis.” Among various catalytic applications, one of the most important use of MOFs is organic transformation. Using a chemically and thermally stable functional MOF can provide high product yields with high product selectivity. In the present thesis, we present a very new strategy to design MOF-based heterogeneous catalysts by immobilizing homogeneous metal catalysts within MOF cavities. Extending MOF to MOF-derived materials provide more research opportunities in heterogeneous catalysis, as a very few MOFs are known for stability under harsh reaction conditions. As MOFs are made up of metal and organic moieties, several nanostructured materials such as porous carbon or metal oxides can be designed and synthesized at higher calcination temperatures, depending upon the calcination environment. MOF-derived carbon materials are found more suitable in electrocatalysis as these are graphitic and conductive in nature. Electrocatalysis is one of the areas where studying the conversion between electricity and energy stored in chemical bonds, can offer solutions to clean energy challenges. In practice, energy conversion in electrochemical processes is often limited by high activation barriers requiring additional energy. Electrocatalysts need to be applied to modify the electrodes to lower the activation energies resulting in higher reaction rates. Precious-metal-based electrocatalysts, composed of metal nanoparticles and high surface area carbon materials, are a dominant class for important energy-related electrochemical reactions such as Oxygen Reduction (ORR), Oxygen Evolution (OER), Hydrogen evolution (HER) and Hydrogen Oxidation (HOR) reactions. For long-term application, electrocatalysts should be efficient, durable, low-cost, and sustainable. However, most of the existing electrocatalysts fall short in one or more of these requirements. Strategies to design new electrocatalysts that are highly stable and efficient in these important electrochemical reactions have been demonstrated and studied the structure-property relationship at atomic level.
Chapter 1 gives a brief introduction of MOFs and MOF-derived nanostructured material. The various MOF based heterogeneous catalysis for organic transformation is described. Electrocatalytic reactions such as water splitting, and ORR is elaborated and their direct applications in electrochemical devices is discussed. The importance of atomic structure and property is further included in the discussion. A detailed review on MOF, MOF-derived materials, nanostructures, electrocatalysis, and structure-property relationship is discussed.
In Chapter 2, we have synthesized and heterogenized a homogeneous Ni-Phenanthroline complex into the nanoconfined pores of Zeolitic MOF (ZIF-8) via in-situ method, denoted as Ni-Phen@ZIF. The catalyst catalyzed dehydrogenating coupling reaction of aromatic diamine and primary alcohol for selective synthesis of mono- and 1,2-disubstituted benzimidazoles. Transmission electron microscopy (TEM), NH3-TPD (temperature-programmed desorption) and kinetic experiments revealed that structural defects are generated during the reaction which are reasonably responsible for the catalytic activity and selectivity by permitting the reactants to the Ni-catalyst. During the reaction H2 produced and followed hydrogen borrowing strategy to convert alcohol to reactive carbonyl compounds such as aldehyde. Notably, the catalyst survived under harsh basic conditions and recyclable.
In Chapter 3, we present a nonprecious HER electrocatalyst which is composed of MOF-derived Co/carbon nanostructure and semi-crystalline ultrathin nanosheets of MoS2 or WS2 naosheets denoted as MoS2/Co@NC and WS2/Co@NC, respectively. Both the catalysts are stable in 0.5 M H2SO4 and delivered 25 mA cm-2 current density at an overpotential of 0.25 and 0.28 V (vs RHE), respectively, with low Tafel slope values. Density functional theory (DFT) suggested that doping of cobalt atom at Mo site improve the Gibb’s free energy (G) for HER activity. Interestingly, the interface between cobalt cluster and MoS2 layer is also in good agreement for HER activity due to the localization of electrons. Moreover, we have also demonstrated an acid-base water electrolyzer using MoS2/Co@NC and WS2/Co@NC as cathodes, producing 10 mA cm-2 current density for ~ 25 hours at cell voltage of only ~0.89 V.
In Chapter 4, we introduce a facile synthesis of MOF-derived unique N-doped hollow carbon structure, composed of atomically dispersed single-Ni-atoms (Ni-N4) and small NiCo nanoparticles (NPs) for highly efficient and durable ORR electrocatalyst. X-ray absorption spectroscopy (XAS) measurement confirmed Ni-N4 moieties and bimetallic NiCo nanoparticles are present. Density functional theory (DFT) calculations suggested that strong synergistic coupling between Ni-N4-site and NiCo nanoparticles. It can boost the activity of single-Ni-atoms and moderate the Gibb’s free energy of ORR intermediates, advocating the direct 4e- transfer process along with lengthening the O-O bond distance. Furthermore, NiCo/hNC as a cathode catalyst in PEM fuel cell delivers a stable performance with a peak power density ~ 355 mW cm-2. Our findings not only furnish the fundamental understanding of structure-activity relationship but also shed light on designing advanced ORR catalysts.
In Chapter 5, we present carbon nanotubes (CNTs)-based catalyst, CrCo@CNT, synthesized by calcination of CrCo-MOF under diluted H2 environment. The effect of Cr on robustness of catalyst is studied. HR-TEM and line-scan analysis suggested Co is atomically dispersed as Co-N4-site along with small CrCo alloy nanoparticles are present in N-doped CNTs. CrCo@CNT is found suitable and highly active tri-functional electrocatalyst for HER, OER and ORR in both alkaline and acidic media. DFT calculations revealed that, Co@CNT exhibited covalent bonding with adsorbed H* resulted unfavorable Gibb’s free energy (G), while addition of Cr boosts the G value. Only the Cr is responsible to activate the Co atoms and favoring the HER intermediates. Using the as-synthesized catalyst, we have also demonstrated an electrolyzer for overall water splitting with good faradaic efficiency and Zn air battery with superior cycling stability for clean energy application.
In chapter 6, we have derived the conclusion based upon the present studies. Important suggestions, and future scope and prospects of the work is also discussed.