The Role of Metal Catalyst in Near Ambient Hydrogen Adsorption on Multi-walled Carbon Nanotubes
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The Role of Metal Catalyst in Near Ambient Hydrogen Adsorption on Multi-walled Carbon Nanotubes Yong-Won Lee1, Rohit Deshpande2, Anne C. Dillon3, Michael J. Heben3, Hongjie Dai4 and Bruce M. Clemens1 1
Department of Materials science and engineering, Stanford University, Stanford, CA 94305 Department of Chemical Engineering, University of Tulsa, Tulsa, OK 74104 3 Center for Basic Sciences, National Renewable Energy Laboratory, Golden, CO 80401 4 Department of Chemistry, Stanford University, Stanford, CA 94305 2
ABSTRACT Multiwalled carbon nanotubes (MWNTs) were continuously synthesized by hot wire chemical vapor deposition (HWCVD) using a methane source catalyzed by metal-organic ferrocene. The microstructure of the MWNTs and the catalyst particles were subsequently characterized with transmission electron microscopy which identified three different phases, i.e., bcc α-Fe, fcc γ-Fe and orthorhombic Fe3C. The hydrogen storage capacity of MWNTs was determined with temperature-programmed desorption (TPD) technique. Hydrogen adsorption at near ambient conditions was observed only in as-synthesized MWNTs containing iron particles and was dramatically increased after hydrogen reducing treatment. Possible adsorption mechanism was also discussed. INTRODUCTION Since their discovery carbon nanotubes (CNTs) have been extensively studied in many research areas. One promising application is their use in hydrogen storage nanostructures, due to their light mass, large surface area, and favorable adsorption and desorption conditions. However, the study of hydrogen storage in CNTs has been characterized by a wide range of reported storage densities. The role of metal catalysts in hydrogen adsorption in CNTs has not been clearly understood. For example, it has been reported that a small weight fraction of hydrogen can be stored in as-synthesized multi-walled nanotubes (MWNTs) with metal catalyst particles at near ambient temperatures and pressures [1] or at high pressures [2]. In contrast, purified nanotubes, from which the iron has been completely removed, exhibit only weak hydrogen physisorption by van der Waals attractive forces. Single-walled nanotubes (SWNTs) show a similar dependence on the presence of the catalyst particles in their hydrogen adsorption behavior [3]. In order to investigate the possible role of metal catalyst in hydrogen adsorption at near ambient conditions, the microstructure of Fe catalyst particles was characterized and purification of the metal catalyst was performed. Here the hydrogen adsorption capacity of MWNTs grown by hot wire chemical vapor deposition was measured using TPD before and after a hydrogen reducing treatment. In order to illuminate the effect of the catalyst and the nanotube structure, hydrogen absorption capacity was also measured in pure iron nanoparticles and in a graphite Fe nanoparticle mixture.
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EXPERIMENT MWNTs were grown by the hot-wire CVD (HWCVD) using ferrocene as the catalyst source and methane as the carbon source [4]. This synthesis method allows us to o
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