Activated carbon pores
A previous article explained that activated carbon can be defined as a crude form of graphite with a random or amorphous structure, which is highly porous over a broad range of pore sizes, from visible cracks and crevices to cracks and crevices of molecular dimensions.
Activated carbon pores can be classified by their size, but also by their function. Below, we will focus on these two classification methods to distinguish activated carbon pore types.
Activated carbon pores by their function
Activated carbon pores for adsorption
Adsorption pores are the only regions within an active carbon particle with sufficient adsorption forces to adsorb impurities. They are the smallest pores within the particle, consisting of gaps between the graphite plates of about 1 to 5 molecular diameters in size. This can be explained by the fact that London dispersion forces are short ranged and distance sensitive.
All pores with any adsorption forces present will be filled with adsorbate molecules (molecules that can be adsorbed) at conditions near saturation (pressure or concentration). Therefore, the volume of all the adsorption pores for a given activated carbon particle can be measured by exposing the carbon to a gas or solution near saturated conditions. Standard tests applied in the activated carbon industry, like the iodine number, carbon tetrachloride number, or butane number, basically measure the total volume of the adsorption pores.
The second activated carbon pore type are transport pores. Transport pores are the largest pores within the particle. They vary from pores greater than 5 molecular diameters to visible cracks and crevices. Transport pores consist of a wide variety of different sizes and shapes of structures over five orders of magnitude within a single activated carbon particle.
Transport pores are too large to adsorb and therefore act simply as diffusion paths to transport the adsorbate to the adsorption sites. They are thus related to the adsorption kinetics or diffusion rate within the activated carbon particle. Transport pore volume is measured directly by mercury-intrusion porosimetry, but can be estimated by subtracting the void, skeletal, and adsorption volume from the total bed volume.
Activated carbon pores by their size: the IUPAC categorisation
According to the IUPAC (International Union of Pure and Applied Chemistry), three groups of pores are distinguished, according to the pore size:
- Macropores (> 50 nm diameter)
- Mesopores (2-50 nm diameter)
- Micropores (< 2 nm diameter)
Micropores generally contribute to the major part of the internal surface area. Macro and mesopores can generally be regarded as the highways into the carbon particle, and are crucial for kinetics.
The DESOTEC activated carbon research and development is equipped with a Belsorp high precision surface area and pore size analyser. This way, DESOTEC can assure you that activated carbon of only the highest quality (with the best adsorption capacity) is being selected for our industrial purification solutions.