Sasidhar Gumma

Net Adsorption: A Thermodynamic Framework for Supercritical Gas Adsorption and Storage in Porous Solids

The thermodynamic treatment of adsorption phenomena is based on the Gibbs dividing surface, which is conceptually clear for a flat surface. On a flat surface, the primary extensive property is the area of the solid. As applications became more significant, necessitating microporous solids, early researchers such as McBain and Coolidge implemented the Gibbs definition by invoking a reference state for microporous solids. The mass of solid is used as a primary extensive property because surface area loses its physical meaning for microporous solids. A reference state is used to fix the hypothetical hyperdividing surface typically using helium as a probe molecule, resulting in the commonly used excess adsorption; experimentalists measure this reference state for each new sample. Molecular simulations, however, provide absolute adsorption. Theoreticians perform helium simulations to convert absolute to excess adsorption, mimicking experiments for comparison. This current structure of adsorption thermodynamics is rigorous (if the conditions for reference state helium measurements are completely disclosed) but laborious. In addition, many studies show that helium, or any other probe molecule for that matter, does adsorb, albeit to a small extent. We propose a novel thermodynamic framework, net adsorption, which completely circumvents the use of probe molecules to fix the reference state for each microporous sample. Using net adsorption, experimentalists calibrate their apparatus only once without any sample in the system. Theoreticians can directly calculate net adsorption; no additional simulations with a probe gas are necessary. Net adsorption also provides a direct indication of the density enhancement achieved (by using an adsorbent) over simple compression for gas (e.g., hydrogen) storage applications.

Effect of Adsorbent History on Adsorption Characteristics of MIL-53(Al) Metal Organic Framework

Structural transformation of MIL-53(Al) metal organic framework from large pore to narrow pore form (lp → np) or vice versa is known to occur by adsorption of certain guest molecules, by temperature change or by applying mechanical pressure. In this work, we perform a systematic investigation to demonstrate that adsorbent history also plays a decisive role in the structural transitions of this material (and hence on its adsorption characteristics). By changing the adsorbent history, parent MIL-53(Al) is tuned into its np domain at ambient temperature such that it not only exhibits a significant increase in CO2 capacity, but also shows negligible uptake for CH4, N2, CO, and O2 at subatmospheric pressure. In addition, for the high pressure region (1-8 bar), we propose a method to retain the lp form of the sample to enhance its CO2 uptake.

THE NET ADSORPTION OF HYDROGEN ON PALLADIUM NANOPARTICLES

In this paper, we report the synthesis of polymer coated palladium (Pd) nanoparticles through a single stage reduction of Pd2+ ions by ethylene glycol. Polyvinyl pyrrolidone (PVP, MW 25,000) is used as a stabilizer. Self-assembled Pd nanoparticles (10–40 nm) were used in hydrogen adsorption studies. Gravimetric adsorption measurements were carried out in a pressure range of 0–26 bar at 293, 324, 364 and 392 K. Saturation for all isotherms was obtained within a few bars of pressure at all temperatures. Maximum hydrogen storage capacity observed was 0.58 wt.% at 324 K and 20 bar. Net adsorption calculations indicated that required tank volume (for storing a particular amount of hydrogen) can be significantly reduced by using a tank filled with Pd nanoparticle.