Porous Compounds Probed With Positrons

Chemically and thermally induced changes in the pore structure of nanoporous metal-organic framework (MOF) compounds being investigated for gas-storage and other applications can be probed in detail with positrons, according to a Michigan-based research team (Adv. Mater., DOI: 10.1002/adma.200903618). Collisions between positrons (positively charged electrons) and electrons lead to annihilation events that generate gamma rays. The lifetimes of positrons injected into a solid depend on how quickly the particles encounter electrons, which in turn depends on the material’s porosity—its distribution of void spaces. Ming Liu, David W. Gidley, and Adam J. Matzger of the University of Michigan and coworkers exploited those relationships to investigate carbon dioxide uptake and the effects of temperature on MOF-5, a well-studied porous compound. Among the team’s observations is the finding that 20–30% of MOF-5’s open volume remains unfilled with CO₂, even at 400 psi, and that the crystals are unexpectedly riddled with 6-nm-long defects. In addition, the researchers found that heat treatments degrade the crystal structure and form pores of a broad range of sizes. These types of defects have not been reported previously in MOF studies based on X-ray structure methods and gas-adsorption techniques, which provide only structurally averaged results, the team says.