The active role of carbon dioxide in the thermolysis of ammonia borane complex

The aim of this thesis is to understand the active role of carbon dioxide (CO2) in the thermolysis of ammonia borane (AB, NH3BH3) complex. AB is a promising hydrogen storage material with storage capacity of 19.6 wt%. Dehydrogenation of AB can be achieved via thermolysis and up to 3 mole of hydrogen per mole AB can be released throughout the heating process. However, hydrogen release from direct thermolysis of AB is very slow at temperature below 100 °C. Our research shows that gaseous CO2 can dramatically improve the kinetics of AB thermolysis under mild conditions.;In chapter 2, a two-step process is proposed to accelerate dehydrogenation of AB: AB is first pretreated under 1.38 MPa of CO2 at 70 °C for 38 min, and then the treated AB is held at 85 °C for thermolysis. The results indicate 10.1 wt.% of hydrogen with regard to pristine AB (1.5 mol H2 per mol of AB) rapidly desorbs at 85 °C in 1 h. ATR-FTIR spectrometric and elemental analyses further reveals that CO2 is chemically incorporated into a solid product. Based on the hydrogen measurement and solid product analysis, we have deduced that the predominant solid product of spent fuel is a polymer with an empirical formula of (NBH3)n .;In chapter 3, we discuss the effect of a gaseous CO2 environment on the thermolytic dehydrogenation of polyaminoborane (PAB) at 85 - 90 °C and 5.48 wt.% hydrogen (referred to the mass of AB) released within 1 h for PAB thermolysis under 2.07 MPa CO2. The total hydrogen desorbed from the initial AB decomposition was 11.38 wt.%, considering the 5.90 wt. % hydrogen release during the transition from AB to PAB. The promoting effect of newly formed formic acid and the exothermic effect of the reaction between PAB and CO2 are believed to be the main contributing factors to this facile dehydrogenation. An empirical formula of BN0.86H 2.34(CO2)0.17 can be proposed based on spectroscopic and elemental analyses of the spent PAB.;In chapter 4, a discussion is provided at the preparation of nitrogen-doped graphite from the reaction of gaseous carbon dioxide (CO2) under moderate conditions with polyaminoborane (PAB). This approach involved two steps: CO2 fixation by PAB under mild temperature (<115°C) and low CO2 pressure (<3.10MPa); carbonization of solid product at the temperature up to 750°C under an inert N2 atmosphere. Formate (-OOCH), methoxy (-OCH3), and aliphatic groups were produced in the first step and a black carbon material was formed in the second step. A combination of Raman spectroscopy and high-resolution scanning electron microscopy demonstrated that a multi-layer graphitic carbon material was formed. Incorporation of nitrogen atoms into carbon lattice was further confirmed by X-ray photoelectron spectroscopy (XPS).