Adsorption of spillover hydrogen atoms on single-wall carbon nanotubes.

Spillover of hydrogen on nanostructured carbons is a phenomenon that is critical to understand in order to produce efficient hydrogen storage adsorbents for fuel cell applications. The spillover and interaction of atomic hydrogen with single-walled carbon nanotubes (SWNTs) is the focus of this combined theoretical and experimental work. To understand the spillover mechanism, very low occupancies (i.e., 1 and 2 H atoms adsorbed) on (5,0), (7,0), (9,0) zigzag (semiconducting) SWNTs and a (5,5) armchair (metallic) SWNT, with corresponding diameters of 3.9, 5.5, 7.0, and 6.8 A, were investigated. The adsorption binding energy of H atoms depends on H occupancy, tube diameter, and helicity (or chirality), as well as endohedral (interior) vs exohedral (exterior) binding. Exohedral binding energies are substantially higher than endohedral binding energies due to easier sp(2)-sp(3) transition in hybridization of carbon on exterior walls upon binding. A binding energy as low as -8.9 kcal/mol is obtained for 2H atoms on the exterior wall of a (5, 0) SWNT. The binding energies of H atoms on the metallic SWNT are significantly weaker (about 23 kcal/mol weaker) than that on the semiconductor SWNT, for both endohedral and exohedral adsorption. The binding energy is generally higher on SWNTs of larger diameters, while its dependence on H occupancy is relatively weak except at very low occupancies. Experimental results at 298 K and for pressures up to 10 MPa with a carbon-bridged composite material containing SWNTs demonstrate the presence of multiple adsorption sites based on desorption hysteresis for the spiltover H on SWNTs, and the experimental results were in qualitative agreement with the molecular orbital calculation results.

Desulfurization of transportation fuels with zeolites under ambient conditions.

Deep desulfurization of transportation fuels (gasoline, diesel, and jet fuels) is being mandated by U.S. and foreign governments and is also needed for future fuel cell applications. However, it is extremely difficult and costly to achieve with current technology, which requires catalytic reactors operated at high pressure and temperature. We show that Cu+ and Ag+ zeolite Y can adsorb sulfur compounds from commercial fuels selectively and with high sulfur capacities (by pi complexation) at ambient temperature and pressure. Thus, the sulfur content was reduced from 430 to <0.2 parts per million by weight in a commercial diesel at a sorbent capacity of 34 cubic centimeters of clean diesel produced per gram of sorbent. This sulfur selectivity and capacity are orders of magnitude higher than those obtained by previously known sorbents.