Activated Carbon Preparation

Activated Carbon Preparation

  Depending on the carbon source, different pre-treatment steps are required before carbonization/pyrolysis and activation of the precursor material. In addition to the most commonly used physical or chemical activation methods, special methods such as the salt template method and ultrasonic spray pyrolysis are introduced.

(i) Pretreatment

Different pretreatment steps are required before carbonization or activation of carbonaceous precursors. The use of biomass as carbon precursor usually requires an additional washing step to remove impurities. Drying for a certain time at about 100°C removes free moisture from the material, which may interfere with the carbonization step. Obtaining a standardized starting material particle size or appearance size of the raw material is essential for the activated carbon production process, which is achieved by grinding and sieving the carbon precursors.

(ii) Ash removal / demineralization

Activated carbon varies in ash content depending on the raw material, which affects the chemical properties of the prepared material. For catalytic applications, only activated carbon with the lowest possible ash content should be used. Prior to the production of activated carbon from precursor materials, the ash and mineral content of the material is reduced by leaching in acidic or alkaline solutions. The samples are mostly demineralized by concentrated hydrochloric and hydrofluoric acids, e.g. a stepwise treatment with hydrochloric acid, hydrofluoric acid and again with hydrochloric acid removes metal oxides and silica from the samples, and is a relatively gentle method for carbon materials. Minerals can also be removed using HNO3 impregnation, but this method leads to oxidation of the material, which creates new oxygen-containing surface functional groups.

(iii) Activation - physical activation

Physical activation of carbon materials is a two-step process. First, the precursor material is carbonized at a certain temperature for a certain period of time under an inert gas atmosphere to prepare the carbonaceous material, which is then activated with steam, air, or CO2 at a higher temperature (800-1000°C) to form a porous structure. In order to form a well-developed porous carbon structure, a large number of internal carbon atoms need to be reacted away. In general, the advantage of physical activation over chemical activation is that the introduction of activating substances is avoided, thus avoiding the presence of impurities or additives in the final material.

C + O2 → CO2

C + CO2 → 2CO

C + 2H2O → CO2 + 2H2

Steam as a mild oxidizing agent has a higher reactivity than carbon dioxide. However, no clear trend has been found regarding pore development, and thus pore property regulation by the steam activation method is one of the difficulties in high-end activated carbon manufacturing. A team investigated the effect of different physical activation gases on the pore development of olive kernel carbon. They concluded that the activated carbon materials obtained by steam activation have lower microporous volume and wider pore size distribution (more mesopores and macropores are formed) compared to carbon dioxide. In addition, it is well known that the surface area obtained by physical activation is significantly lower compared to that obtained by chemical activation with zinc chloride at the same temperature.