Inorganic and Metal-Organic Heterogeneous Catalysts for Hydrogen Energy Applications

The main focus of this thesis is the exploration of inorganic and metal-organic heterogeneous catalysts for hydrogen (H2) energy applications. In particular, we investigated metal nanoparticles (NPs) and metal-organic frameworks (MOFs) for their employment in different perspectives of H2 energy. Our investigation includes; (i) H2 generation from water by MOFs, (ii) H2 release from a chemical storage medium (ammonia borane (AB)), and (iii) utilization of H2 for hydrogenation reactions. Our research was aimed at understanding the catalytic behavior of these materials and proposing approaches for improving their efficiency and sustainability.

MOFs have been promising candidates for the photocatalytic H2 evolution reaction (HER) from water. As yet, the main criteria for MOFs to be considered as photocatalyst have mostly been their light absorption capability, optical band gap and the alignment of their band edges with respect to the relevant redox levels. In Chapter 2, we present the synergy between the experiments and computations, to show that a deeper understanding is needed for evaluating their potential. We investigated isostructural pyrene-based MOFs (M-TBAPy, where M = Sc, Al, and In), which have similar band gap energies. Despite all being isostructural MOFs, Sc-TBAPy performed better in H2 generation compared to its Al and In counterparts. Our investigation allowed us to identify that, in addition to optical and electronic properties, the chemical characteristics of the metal nodes and the morphology of the structure also play an important role on the photocatalytic HER rate. We conclude that all these key factors should be studied together for the optimization of photocatalytic activity. The knowledge obtained in this study can be transferred to other MOF photocatalysts.

AB has been considered as a promising H2 storage medium owing to its high gravimetric capacity. Although non-noble metal NPs have been used for the release of H2 from AB, they might suffer from deactivation during cycles. In Chapter 3, we demonstrate that the in-situ formation of copper NPs (Cu0 NPs) upon reduction of the earth-abundant Jacquesdietrichite mineral (Cu2(BO)(OH)23) with AB can provide an alternative solution for stability issues. The mineral catalyzes the release of almost all H2 from AB. During the reaction, the CuII ions in mineral are in-situ reduced to Cu0 NPs, and after the reaction, following exposure to air, they re-form the fresh mineral. As a consequence, the catalytic activity of mineral remains unchanged over cycles. The regeneration of synthetic mineral gives a new perspective in heterogeneous catalysis.

In Chapter 4, we present that our approach presented in Chapter 3 can be applied in industrial hydrogenation reactions. We demonstrated that during the in-situ formation of Cu0 NPs, the released H2 gas can be utilized for the reduction of nitroarenes to their corresponding anilines, at room temperature and under ambient pressure. After the