Can carbon surface oxidation shift the pore size distribution curve calculated from Ar, N2 and CO2 adsorption isotherms? Simulation results for a realistic carbon model

Abstract

Using the virtual porous carbon model proposed by Harris et al, we study the effect of carbonsurface oxidation on the pore size distribution (PSD) curve determined from simulated Ar, N2and CO2 isotherms. It is assumed that surface oxidation is not destructive for the carbonskeleton, and that all pores are accessible for studied molecules (i.e., only the effect of thechange of surface chemical composition is studied). The results obtained show two importantthings, i.e., oxidation of the carbon surface very slightly changes the absolute porosity(calculated from the geometric method of Bhattacharya and Gubbins (BG)); however, PSDcurves calculated from simulated isotherms are to a greater or lesser extent affected by thepresence of surface oxides. The most reliable results are obtained from Ar adsorption data. Notonly is adsorption of this adsorbate practically independent from the presence of surface oxides,but, more importantly, for this molecule one can apply the slit-like model of pores as the firstapproach to recover the average pore diameter of a real carbon structure. For nitrogen, the effectof carbon surface chemical composition is observed due to the quadrupole moment of thismolecule, and this effect shifts the PSD curves compared to Ar. The largest differences are seenfor CO2, and it is clearly demonstrated that the PSD curves obtained from adsorption isothermsof this molecule contain artificial peaks and the average pore diameter is strongly influenced bythe presence of electrostatic adsorbate–adsorbate as well as adsorbate–adsorbent interactions.