Combination volumetric and gravimetric sorption instrument for high accuracy measurements of methane adsorption.

The accurate measurement of adsorbed gas up to high pressures (∼100 bars) is critical for the development of new materials for adsorbed gas storage. The typical Sievert-type volumetric method introduces accumulating errors that can become large at maximum pressures. Alternatively, gravimetric methods employing microbalances require careful buoyancy corrections. In this paper, we present a combination gravimetric and volumetric system for methane sorption measurements on samples between ∼0.5 and 1 g. The gravimetric method described requires no buoyancy corrections. The tandem use of the gravimetric method allows for a check on the highest uncertainty volumetric measurements. The sources and proper calculation of uncertainties are discussed. Results from methane measurements on activated carbon MSC-30 and metal-organic framework HKUST-1 are compared across methods and within the literature.

Interactions of NO2 at ambient temperature with cerium-zirconium mixed oxides supported on SBA-15.

New silica-based composites were obtained using a slow precipitation of mixed oxide Ce(1-x)Zr(x)O(2) on the surface of mesoporous silica, SBA-15. The samples were tested as NO(2) adsorbents in dynamic conditions at room temperature. The surface of the initial and exhausted materials was characterized using N(2) sorption, XRD, TEM, potentiometric titration, and thermal analysis before and after exposure to NO(2). In comparison with unsupported Ce(1-x)Zr(x)O(2) mixed oxides, a significant increase in the NO(2) adsorption capacity was observed. This is due to the high dispersion of active oxide phase on the surface of SBA-15. A linear trend was found between the NO(2) adsorption capacity and the amount of Zr(OH)(4) added to the structure. Introduction of Zr(4+) cations to ceria contributes to an increase in the amount of Ce(3+), which is the active center for the NO(2) adsorption, and to an increase in the density of -OH groups. These groups are found to be involved in the retention of both NO(2) and NO on the surface. After exposure to NO(2), an acidification of the surface caused by the oxidation of the cerium as well as the formation of nitrite and nitrates took place. The structure of the composites appears not to be affected by reactive adsorption of NO(2).

Hydrogen storage in engineered carbon nanospaces.

It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (approximately 80 g H2/kg carbon, approximately 50 g H2/liter carbon, at 50 bar and 77 K). Nanopores generate high storage capacities (a) by having high surface area to volume ratios, and (b) by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, giving rise to a binding energy nearly twice the binding energy in wide pores. Experimental case studies are presented with surface areas as high as 3100 m(2) g(-1), in which 40% of all surface sites reside in pores of width approximately 0.7 nm and binding energy approximately 9 kJ mol(-1), and 60% of sites in pores of width>1.0 nm and binding energy approximately 5 kJ mol(-1). The findings, including the prevalence of just two distinct binding energies, are in excellent agreement with results from molecular dynamics simulations. It is also shown, from statistical mechanical models, that one can experimentally distinguish between the situation in which molecules do (mobile adsorption) and do not (localized adsorption) move parallel to the surface, how such lateral dynamics affects the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface: in the samples presented, adsorption is mobile at 293 K, and localized at 77 K. These findings make a strong case for it being possible to significantly increase hydrogen storage capacities in nanoporous carbons by suitable engineering of the nanopore space.