Preparation and characterization of zeolite confined cobalt(0) nanoclusters as catalyst for hydrogen generation from the hydrolysis of sodium borohydride and ammonia borane

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2011
Rakap, Murat
Because of the growing concerns over the depletion of fossil fuel supplies, environmental pollution and global warming caused by a steep increase in carbon dioxide and other greenhouse gases in the atmosphere, much attention has been given to the development of renewable energy sources that are the only long-term solution to the energy requirements of the world’s population, on the way towards a sustainable energy future. Hydrogen has been considered as a clean and environmentally benign new energy carrier for heating, transportation, mechanical power and electricity generation. However, the lack of effective, safe, and low-cost hydrogen storage materials for mobile, portable, and stationary applications is one of the major hurdles to be overcome for the implementation of hydrogen economy. Among various solid state hydrogen storage materials, chemical hydrogen storage materials such as sodium borohydride (NaBH4) and ammonia borane (H3NBH3) have received much attention as promising candidates for fuel cell applications under ambient conditions due to their high gravimetric and volumetric hydrogen storage capacities. Both sodium borohydride and ammonia borane generate hydrogen upon hydrolysis in the presence of suitable metal catalysts. Transition metal nanoclusters can be used as active catalysts to catalyze the hydrolysis reactions of sodium borohydride and ammonia borane for hydrogen generation since they exhibit unique properties that differ from their bulk counterparts. Although the catalytic activity of metal nanoclusters increases with decreasing particle size, they are unstable with respect to agglomeration into the bulk metal leading to a significant decrease in activity in their catalytic applications. Therefore, the exploitation of microporous and mesoporous materials with ordered porous structures as hosts to encapsulate metal nanoclusters has attracted great interest since the pore size restriction of these host materials could limit the growth of nanoclusters leading to an increase in the percentage of the catalytically active surface atoms. In this dissertation, we report the preparation, characterization and the investigation of the catalytic activities of zeolite confined cobalt(0) nanoclusters in the hydrolysis of sodium borohydride and ammonia borane. The zeolite confined cobalt(0) nanoclusters were prepared by the reduction of cobalt(II)-exchanged zeolite-Y by sodium borohydride in aqueous solution at room temperature with no alteration in the framework lattice or loss in the crystallinity. The characterization of zeolite confined cobalt(0) nanoclusters were done by using inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-visible spectroscopy (DR-UV-Vis), infrared spectroscopy (IR), Raman spectroscopy, and N2 adsorption-desorption technique. The catalytic activity of zeolite confined cobalt(0) nanoclusters and the kinetics of hydrogen generation from the hydrolysis of sodium borohydride and ammonia borane were studied depending on catalyst concentration, substrate concentration and temperature. The rate laws and the activation parameters (Arrhenius activation energy, Ea; activation enthalpy, ΔH#; and activation entropy, ΔS#) for both catalytic hydrolysis reactions were calculated from the obtained kinetic data.