Irreversible hydrogenation of solid C 60 with and without catalytic metals
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Irreversible hydrogenation of solid C60 with and without catalytic metals Eric L. Brosha, John Davey, Fernando H. Garzon, and Shimshon Gottesfeld Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 26 January 1998; accepted 3 November 1998)
The dehydrogenation of C60 ? H18.7 was studied using thermogravimetric and powder x-ray diffraction analysis. C60 ? H18.7 was found to be stable up to 430 ±C in Ar at which point the release of hydrogen initiated the collapse of a fraction of fullerene molecules. X-ray diffraction analysis performed on C60 ? H18.7 samples dehydrogenated at 454, 475, and 600 ±C displayed an increasing volume fraction of amorphous material. The decomposition product comprises randomly oriented, single-layer graphite sheets. Evolved gas analysis using gas chromatograph (GC) mass spectroscopy confirmed the presence of both H2 and methane upon dehydrogenation. Attempts to improve reversibility or reduce hydrogenation/ dehydrogenation temperatures by addition of Ru and Pt catalysts were unsuccessful.
I. INTRODUCTION
One of the daunting engineering challenges that needs to be solved to enable wider use of polymer electrolyte membrane (PEM)-type fuel cells is safe, volume and weight-efficient hydrogen storage, preferably in the solid state and at a weight percent well exceeding the 1–2% level offered by metal hydrides that exhibit enthalpies of hydrogen uptake/release suitable for vehicular use. Low enthalpies and temperatures of hydrogenation/dehydrogenation ensure that the waste heat from the fuel cell can be used to drive hydrogen release. In addition to relatively low enthalpies, the other desirable feature is reversibility of multicharge/discharge cycles. Recently developed hydrogen fuel cell vehicle prototypes have used hydrogen gas stored in high-pressure gas cylinders (3000–5000 psig); this is the only practical way presently known to achieve storage levels of 5% hydrogen by weight.1 Pressurized cylinder safety concerns may impede the use of these gas storage systems in passenger vehicles.2 Safer hydrogen storage technologies such as metal hydrides possess specific energy densities which are too low for transportation applications and, hence, limit the single fill range of hydrogen-powered vehicles to a small fraction of that of conventional, gasoline-fueled vehicles. The unavailability of a solidstate, reversible mode of hydrogen storage at hydrogen densities in large excess of 1% is one of the most important reasons for having to resort to complex, on-board liquid fuel processing for the generation of hydrogen-rich gas mixtures as a feed stream to the fuel cell. Promising results have been reported for the possible use of carbon materials as hydrogen storage media. Recent experimental data for single-walled nanotubes (SWNT) have predicted hydrogen uptake in these materials of about 5–10 wt% per SWNT calculated from TPD data for hydrogen exposure conditions of 0.4 bar 2138
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and at 2140 and 0 ±C.3 A
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