Effects of Surface Decorated MWCNTs on the Dehydrogenation Behavior of Complex Metal Hydride

  • 譚 家彥

Student thesis: Doctoral Thesis

Abstract

Complex metal hydride such as LiAlH4 or NaAlH4 is a promising solid hydrogen storage material that possess high volumetric hydrogen storage density up to 10 5 wt% In addition the purity of the released hydrogen from complex metal hydrides is high enough which makes it suitable for fuel cell in light duty vehicles However the onset dehydrogenation temperature of complex metal hydrides is still at the temperature around 170 to 300 oC which is unsatisfactory for fuel cell application Therefore this study is dedicated on developing nano scale transition metal particles or titanium chloride (Ⅲ) decorated multi-walled carbon nanotubes (MWCNTs) as novel catalysts for improving the dehydrogenation properties of complex metal hydrides and discussing its effect on dehydrogenation reaction of complex metal hydrides First by chemical reduction and impregnation process the transition metal particles such as Ni Co Cu and V noble metal such as Pt Pd and TiCl3 compounds were able to decorate on the surface of MWCNTs The morphology and chemical composition of the surface decorated MWCNTs was examined by using scanning electron microscope (SEM) transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) The different percentage of the as prepared surface decorated MWCNTs were admixed with complex metal hydrides such as LiAlH4 or NaAlH4 by ball milling process The morphology of the admixed powders was examined by transmission X-ray microscopy (TXM) The dehydrogenation properties of the surface decorated MWCNTs admixed LiAlH4 or NaAlH4 was investigated by using thermo-gravimetric analysis (TGA) and the in-situ X-ray diffraction (in-situ XRD) in order to understand whether the catalysts would alter the dehydrogenation reaction For LiAlH4 the TGA results showed that the addition of 20 wt% pure MWCNTs could lower the onset dehydrogenation temperature of LiAlH4 from 170 oC to 140 oC In addition by decorating different transition metal particles the catalytic power of MWCNTs could be improved or deteriorated accorded to the decorated transition metal element Among all the tested catalysts the TiCl3 or Ni decorated MWCNTs have superior catalytic power in catalyzing the dehydrogenation of LiAlH4 By addition of 20 wt% TiCl3 or Ni decorated MWCNTs the onset dehydrogenation temperature of LiAlH4 could be reduced to as low as 70 oC Meanwhile the hydrogen desorption kinetic of LiAlH4 at 100 oC was significantly improved Although the TiCl3 or Ni decorated MWCNTs could effectively lower the dehydrogenation temperature of LiAlH4 the in-situ XRD analysis showed that the TiCl3 or Ni decorated MWCNTs catalysts would not alter the dehydrogenation reaction pathways of LiAlH4 Meanwhile the XRD results implied that the TiCl3 or Ni decorated MWCNTs admixed LiAlH4 would gradually decompose at room temperature which also verified the superior catalytic power of TiCl3 or Ni decorated MWCNTs On the other hand the Pt or Pd decorated MWCNTs exhibited both catalytic and inhibitive effect on the dehydrogenation of LiAlH4 The TGA and FTIR results revealed the superior capability of Pt or Pd decorated MWCNTs to absorb hydrogen under hydrogen atmosphere Addition of Pt or Pd decorated MWCNTs could absorb the hydrogen released from the LiAlH4 therefore delaying the hydrogen desorption rate during the TGA tests Meanwhile the in-situ XRD analysis showed that Pt and Pd particles would react with the residue of dehydrogenation reaction of LiAlH4 and therefore altering its dehydrogenation reaction pathways The detailed data would be discussed later in this thesis For NaAlH4 the effect of surface decorated MWCNTs on NaAlH4 is similar to that on LiAlH4 The TGA results showed that addition of TiCl3 and Ni decorated MWCNTs could lower the onset dehydrogenation temperature from 210 oC to 150 and 170 oC At the same time the hydrogen desorption kinetic of NaAlH4 at 160 oC was significantly improved by addition of TiCl3 and Ni decorated MWCNTs addition On the other hand the Pt and Pd decorated MWCNTs also exhibited the inhibitive effect on hydrogen desorption of NaAlH4 where the onset dehydrogenation temperature of NaAlH4 increased slightly from 210 oC to 220 oC The detailed discussion of TGA and in-situ XRD resulted was be presented later in this thesis Other than TGA and in-situ XRD analysis on the effect of surface decorated MWCNTs on the dehydrogenation behavior of complex metal hydrides the first principle simulation was also adopted in order to better understand the effect of the doping of transition metal elements on the crystal stability and the dehydrogenation behavior of LiAlH4 Previous reports implied that the doped transition metal elements such as Ti Ni Co Cu V would tend to occupy the interstitial site of LiAlH4 forming the interstitial doping The d orbitals of doped transition metal elements would interact with the electronic orbital of Al and weaken the bonding between Aland H as well as Li+ and [AlH4]- tetrahedral The hydrogen removal energy of transition metal doped LiAlH4 is therefore lower than the undoped LiAlH4 The simulation results could indirectly support the experiment results where the interaction between the decorated transition metal particles on MWCNTs and the LiAlH4 is the reason to explain the different catalytic power of surface decorated MWCNTs
Date of Award2015 Jul 9
Original languageEnglish
SupervisorWen-Ta Tsai (Supervisor)

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