Soot surface area evolution during air oxidation as evaluated by small angle X-ray scattering and CO2 adsorption

The total surface area of two diesel engine produced soots, a high volatile content NIST standard (termed NIST) and a low volatile content soot (termed NEU), were determined with CO2 adsorption and small angle X-ray scattering (SANS), as a function of the extent of oxidation. During initial volatilization of condensables of the NIST and NEU soots in a thermogravimetric analyzer, in helium at 1073K, their CO2 surface areas increased sharply from 49m(2)/g to 273m(2)/g and from 96m(2)/g to 367m(2)/g, respectively. During oxidation, the CO2 surface area increased by an additional 100-150m(2/g) , until 50% conversion was reached. Thereafter, the CO, surface area was relatively constant with conversion for the NIST soot, but decreased to 150 m(2)/g for the NEU soot. Three porosity regimes were assumed for the calculation of SAXS areas; they were based on (a) constant density (shrinking core), (b) constant diameter, and (c) an observed (with a TEM) diameter variation. The best agreement between the CO2 and SAXS surfaces area occurred for the constant density assumption, in contrast to the actual measured diameter variation. By applying fractal surface analysis to the SAXS data, this discrepancy is ascribed to the opening up of internal volume to reaction volatilization of condensables and oxidation. (C) 2004 Elsevier Ltd. All rights reserved.