Control of the pore size distribution and its spatial homogeneity in particulate activated carbon

There are circumstances where it is desirable to achieve a particular, optimal, pore size distribution (PSD) in a carbon, including in the molecular sieving, gas storage, CO₂-capture and electrochemical energy storage. Activation protocols that cycle a carbon a number of times between a low-temperature oxygen chemisorption process and a higher temperature pyrolysis process have been proposed as a means of yielding such desired PSDs. However, it is shown here that for PFA-based char particles of ∼100 μm in size, only the super-micropores are substantially developed under such an activation protocol, with the ultra-micropores being substantially un-touched. It is also shown that a typical CO₂-activation process yields similar control over PSD development. As this process is nearly 15 times faster than the cyclic-O₂ protocol and yields larger pore volumes and areas for a given level of conversion, it is to be preferred unless spatial homogeneous porosity within the particles is also desired. If such homogeneity is desired, it is shown here that CO₂ activation should continue to be used but at a rate of around one-tenth the typical; this slow rate also has the advantage of producing pore volumes and areas substantially greater than those obtained using the other activation protocols.