Supporting metal catalysts on modified carbon nanocones to optimize dispersion and particle size
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Supporting metal catalysts on modified carbon nanocones to optimize dispersion and particle size P. Matelloni1, D.M. Grant1 and G.S. Walker1*
1
Fuel and Power Technology Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK, Tel: +44 (0)115 9513752 * Corresponding author contacts: University of Nottingham, University Park, Nottingham NG7 2RD,
Tel: +44 (0)115 9513752; Fax: +44 (0)115 9513800; E-mail: [email protected] ABSTRACT Carbon nanocones are the fifth allotropic form of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this work was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. XRD confirmed the complete metal reduction and TEM showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt.% were achieved and high dispersions were retained for loadings as high as 15 wt.%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed. INTRODUCTION Carbon nanostructures show good potential for solid state storage of hydrogen. Nanoporous carbons with high surface areas have been shown to have high excess adsorption of hydrogen, 6-7 wt.%, at 77 K and 20 bar pressure [1]. However, there is a desire to have equally high uptakes but at temperatures closer to RT. Due to the weak physisorption interaction of hydrogen on carbon materials, ca. 6-9 kJ mol-1, the capacity at RT and 100 bar is < 0.5 wt.% [2]. Therefore, to use carbon nanostructures as RT hydrogen storage media, an activation mechanism is needed to enhance the hydrogen interaction with the substrate. There has been a lot of interest in supporting catalysts on these carbon materials to dissociatively adsorb dihydrogen leading to a spillover of hydrogen atoms onto the support. There are a number of reports of surprisingly high uptakes of 10 wt.% at RT and 120 atm for graphitic nanofibers [3] and 4.2 wt.% for single walled carbon nanotubes at RT and 100 MPa [4]. On the other hand, there are other reports where the hydrogen uptake only outperforms the theoretical contribution from the metal hydrogenation plus hydrogen physisorption onto the carbon surface, e.g. graphite nanofibers decorated with palladium stored 1.5 wt.% of hydrogen at 10MPa [5]. It has been suggested that the presence of defects in the surface structure, created during a pre-treatment, m
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