A core-shell model for oxidation-driven evolution of porous carbon nanoparticles

The gasification of soot and carbon black nanoparticles is a fluid-solid reaction in which the solid phase undergoes structural changes. As the particles are permeable to the gas phase, oxidation takes place at the outer surface as well as in the interior. During oxidation progress, the internal porosity increases, resulting in an extreme increase in surface area. Experimental findings have recently been described by a model that treats oxidation by random removal of the solid phase. While these statistical models are computationally expensive and rely on tessellation of the solid, we propose a simple analytical expression to describe the gasification on the nanoparticle scale in the kinetically controlled regime. This novel model accounts for simultaneous internal and external oxidation and considers the heterogeneity of the particle, which consists of a core and a shell region that differ in their initial internal porosity. The model can explain the experimental observations found in literature, which report a sharp increase in specific surface area with subsequent flattening as oxidation progresses. Also, the experimental observation that the absolute surface undergoes a maximum can be well reproduced. An extension of our analytical model allows to account for the shift between the maximum reaction rate and the maximum surface area, often observed in experiments.