Improvement of Hydrogen Adsorption on Carbon Nanostructures
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1018-EE14-21
Improvement of Hydrogen Adsorption on Carbon Nanostructures Lyubov F. Lafi, Richard Chahine, Pierre BÈnard, and …ric Poirier IRH, UQTR, 3351 boul. des Forges, P.O. Box 500, Trois-Rivieres, G9A 5H7, Canada ABSTRACT We present results of a study of the effect of various surface treatments on the adsorption properties of hydrogen on single-walled carbon nanotubes (SWNTs) and activated carbon. The alteration of the surface area by thermal and chemical treatments of SWNTs was studied by Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) and nitrogen adsorption. The Horwath-Kawazoe (HK) method was applied to describe the increase of the surface area and of the pore volume of the SWNT samples. The surface treatments lead to substantially improvement of hydrogen adsorption at 77 K and 1 atm. The averaged heat of adsorption (enthalpy), estimated from a Dubinin-Astakhov (DA) analysis, was found to be about 3.6 and 4.2 kJ/mol for heat-treated and as-received SWNTs, respectively.
INTRODUCTION Single-wall carbon nanotubes (SWNTs) have a high specific surface area and a substantial micro- and mesoporosity volumes [1,2]. These structural parameters of SWNTs constitute key parameters of the sorption process. The synthesis and preparation of the samples appear to influence substantially hydrogen adsorption on SWNTs. The objective of the present work is to optimize the treatment conditions in order to improve the hydrogen adsorption properties of SWNTs from excess adsorption measurements at 77 K and pressure up to 50 atm using a volumetric approach. The Dubinin-Astakhov (DA) approach is used to model and to compare some energetic and structural parameters.
EXPERIMENT The SWNTs used in the experiments were purchased from Carbon Nanotechnologies Inc. (CNI). They were studied in their original condition (referred to in this paper as SWNT-Pr) and after undergoing chemical or heat treatments. The pristine samples were oxidized in an oxygen flow for 2 hours at 200∞C. They were then dispersed in an acidic solution and soaked at 80100∞C for 2-20 hours. Afterwards, the SWNTs were filtered through a Teflon membrane (pore size 0.2 µm). They were then washed with distilled water and dried at 50∞C in air. The heat treatments were carried out at 600, 700 and 1000oC for 6 hours in an argon flow. ACS grade chemical treatment products from Fisher Scientific (hydrochloric acid (38%), sulphuric acid (98%)) were used in this process. Solutions were prepared with bi-distilled water at the following concentrations: 6M HCl, 3M H2SO4. The SWNT samples treated using this method will be denoted as SWNT-O2-HCl and SWNT-O2-H2SO4, respectively. For comparison purposes, the hydrogen uptake on a large specific surface area activated carbon (IRH-33) and another SWNTs sample denoted as CNI-3P was measured. The two latter samples were round-robins previously tested in other laboratories. The SWNT structures were analysed with Raman spectroscopy,
nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). XPS was performed on several samp
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