Fundamental study of catalytic mechanism in complex metal hydride and METAL-BN-FRAMEWORK for hydrogen stotage

Abstract

Hydrogen is considered as a future energy source for automobile industry. Hydrogen storage is the main hurdle in achieving this goal. This thesis is directed towards the fundamental study of exploring new hydrogen storage material. This study has two main objectives. First is to understand the catalytic mechanism in enhancing the hydrogen storage properties in complex metal hydride in presence of metal salts and secondly, to design new potential hydrogen storage materials such as metal decorated BN system and functionalized metal-BN-framework. Density functional theory (DFT) is applied to calculate physicochemical properties of the systems by using different DFT methods like generalized gradient approximation, hybrid, and meta hybrid based on the system size. For larger system, dispersion corrected DFT has been used. The structural, energetic, and electronic properties have been obtained using B3LYP, M05-2X, and PBE functionals. Experimental studies have indicated the formation of MAl3 phase (M = Sc, Ti, Zr) when complex metal hydride is ball milled in the presence of transition metal halide which results in improved kinetics of H2 uptake and release. But the exact role of these additives is not known. In this study, hydrogen interaction and saturation on stable tetrahedral MAl3 clusters (M = Li, Sc, Ti, and Zr) has been performed by employing spin-polarized hybrid B3LYP and non-local density functionals. Kinetic, thermodynamic and structural parameters have been determined to obtain mechanistic insight into the hydrogenation process. Catalytic mechanism has been proposed which offer an explanation for catalyst role in improving kinetics in complex metal hydride. A conceptual DFT has been applied to investigate the hydrogen trapping efficiency of metal functionalized BN system at various sites by employing the M05-2X/6-311G+(d) level of theory. Metals are functionalized at three regions of the BN system, namely borazine, having high nucleus independent chemical shift values. H2 is trapped on the metal sites resulting in B3N3HXMiHm clusters [M = Li, Sc, Ti, V; X = 3, 6; i = 1-3; m up to 30]. Global reactivity attributes have been computed which obey maximum hardness and minimum electrophilicity principles. The adsorption energy for physisorbed hydrogen is found to be low with Kubas-Niu bonding. Sc and Ti functionalized systems show promising hydrogen storage capacity of 11.0 − 13.2 wt %. Metal-organic framework with organic linker is replaced with inorganic linker, borazocine (B4N4H8) for the first time and is functionalized with M = Ti, Li. Electronic structure calculations using spin polarized generalized gradient approximation with PerdewBurke-Ernzerhof functional, structural optimization and molecular dynamics simulations have been performed for hydrogen sorption efficiency of M functionalized Mg4O-BN framework (MBF). Low adsorption and desorption energies suggest high reversibility of the system. BN ring coordinates strongly with the M atom with high binding energy while each Ti atom adsorb hydrogen with Kubas bonding. At room temperature, 75 % of physisorbed H2 desorbs in Ti while all H2 desorb in Li with framework remaining stable at elevated temperature. Storage capacity for both system is found to be high. Ti functionalized MBF has been found to be better with respect to storage, stability, and reversibility, making it a potential hydrogen storage material. A summary of the present study is given in the end along with the conclusions and the future directions of research.