ABSTRACT
We have seen how hydrogen atoms can be bound within the interstitial sites of intermetallic metal hydrides (Chapter 5) and in the covalent chemical bonds of complex metal hydrides (Chapter 6). Here, we discuss the phenomenon whereby hydrogen gas is introduced in molecular form, adsorbs onto a surface as a molecule, and is desorbed as a molecule. The generally low activation energy associated with this phenomenon results in a conceptually and technologically simple means for storage that is not limited by cycle life, as the adsorbed molecule and the adsorbent remain relatively unchanged during the adsorption/ desorption cycle. This weak interaction, however, results in the requirement of low temperatures if reasonably large uptake values are to be expected. In adsorption, gas molecules can generally be found in greater concentration at the
surface of a substrate than in the free gas volume as determined by real (vs. ideal) gas law behavior. This physical adsorption (or physisorption) can provide the rationale for the use of adsorbents for hydrogen storage systems, provided that the so-called surface
CONTENTS
Introduction ................................................................................................................................. 213 Definitions .................................................................................................................................... 214 The Need to Consider Sorbents ................................................................................................ 215 Heats of Adsorption.................................................................................................................... 215 Absolute Uptake and the Langmuir Model ............................................................................ 217 The Gibbs Surface Excess ........................................................................................................... 220 The Mechanism of Physisorption .............................................................................................222 Electrostatic Interactions ............................................................................................................222 Orbital Interactions .....................................................................................................................222 Size of Molecular Hydrogen ......................................................................................................223 Adsorbents ...................................................................................................................................223
