Characterisation of microporous carbons by positron annihilation lifetime spectroscopy
Gasification of coal and other organic materials using oxidative gases such as oxygen, carbon dioxide or steam produces porous carbon materials. The uncertainty in porosity information can be inadequate for a number of potentially applications of microporous carbons. Positron annihilation lifetime spectroscopy (PALS) senses the electronic properties of condensed matter. A PALS experiment implies the measurement of the lifetime, which is the inverse of the annihilation rate. In the present work PALS has been applied to characterise the porosity of gasified carbon materials. In the case of active carbon fibers (ACFs) the characterization by gas adsorption showed that these materials are essentially microporous with narrow pore size distribution. The PALS analysis showed two lifetime components. There is a predominant lifetime component (> 84%) which ranges from 376 to 393 ps and increases its value with the burn-off. The annihilation mechanism apparently depends on the activation agent. Saran based active carbons (ACs) were prepared by pyrolysis and activation using carbon dioxide, air and steam. Gas adsorption and small angle scattering (SAS) showed that these carbons are essentially microporous materials with wide pore size distribution. PALS showed only one lifetime component. This lifetime component (> 99%) ranges from 382 to 401 ps. The lifetime value increases with the burn-off for the C0₂ gasification process. Coal chars from Argonne Premium Coals were gasified using air, carbon dioxide and steam. The gas adsorption and SAS characterization of these activated chars indicated that they are a mixture of micro and mesoporous with a broad pore size distribution. For these samples PALS showed only one lifetime component this ranges from 352 to 367 ps. A good correlation between gas adsorption, SAS and PALS analyses permits to associate the main lifetime with the mean micropore size to ACF and AC materials.