46
A. Geethakarthi
650 and 950 °C for different pyrolysis times such as 30, 60 or 120 min, respectively.
The long holding time and high temperature of pyrolysis stabilized both, organic
and inorganic, phases. These phases would undergo aromatization, carbonization,
thermal decomposition, incorporation of nitrogen and undefined solid-state reactions. These processes not only change surface chemistry but also significantly affect
the porosity and texture of the adsorbents.
3.1.2 Chemical Activation and Characterization
The precursors are impregnated with activating agents such as ZnCl 2 , NaCl, Na 2 CO 3 ,
K 2 CO 3 , KOH, NaOH, Al 2 O 3 , H 3 PO 4 , H 2 SO 4 , NH 4 Cl. Orthophosporic acids [81,
Solum et al. 1995; 52, 65], potassium hydroxide [91, 94, 81], zinc chloride [49,
51] and sulphuric acid [17, 101] are the most commonly used chemical activating
agents used before incineration of the adsorbent precursors. Best results are obtained
in an inert atmospheric condition in the absence of air or oxygen (N 2 , Ar and CO 2
atmosphere). Types of activating agent, impregnation ratio, activation temperature
and activation time are related to different physical and chemical characteristics of
the products.
Alkali and Zinc Chloride Activation
The most commonly used activating agents are KOH and ZnCl 2, where the prepared
carbon has high surface area. These alkali agents are effective and influence the
structure of activated carbon due to their strong chemical base. Alkali activation is
advantageous because of their higher carbon yield at low activation temperatures
(500–800 °C) and less activation time. At higher temperature, the development of
micropores could be obtained due to steam activation. The corrosive nature of KOH
and higher cost are the setbacks of using this activating alkali agent [117].
Acid Activation
The acid activation can be carried out with strong and weak inorganic acids. Studies
conducted by Yanping and Rockstraw [120] used orthophosphoric acid as a activation
reagent for activating pecan shell. The characterized carbon showed well-developed
pore structure at temperatures ≥250 °C and reached 1130 m
2 /g and 0.34 cm
3 /g,
respectively, at 500 °C. The pore development was influenced by the impregnation
ratio and soaking time [19, 33]. The data indicated that reactions between the activation agent (existing as phosphorus pentoxide, a strong acid and dehydration agent,
at 400 °C) and precursor matrix also contributed the development of porosity.
A. Geethakarthi
650 and 950 °C for different pyrolysis times such as 30, 60 or 120 min, respectively.
The long holding time and high temperature of pyrolysis stabilized both, organic
and inorganic, phases. These phases would undergo aromatization, carbonization,
thermal decomposition, incorporation of nitrogen and undefined solid-state reactions. These processes not only change surface chemistry but also significantly affect
the porosity and texture of the adsorbents.
3.1.2 Chemical Activation and Characterization
The precursors are impregnated with activating agents such as ZnCl 2 , NaCl, Na 2 CO 3 ,
K 2 CO 3 , KOH, NaOH, Al 2 O 3 , H 3 PO 4 , H 2 SO 4 , NH 4 Cl. Orthophosporic acids [81,
Solum et al. 1995; 52, 65], potassium hydroxide [91, 94, 81], zinc chloride [49,
51] and sulphuric acid [17, 101] are the most commonly used chemical activating
agents used before incineration of the adsorbent precursors. Best results are obtained
in an inert atmospheric condition in the absence of air or oxygen (N 2 , Ar and CO 2
atmosphere). Types of activating agent, impregnation ratio, activation temperature
and activation time are related to different physical and chemical characteristics of
the products.
Alkali and Zinc Chloride Activation
The most commonly used activating agents are KOH and ZnCl 2, where the prepared
carbon has high surface area. These alkali agents are effective and influence the
structure of activated carbon due to their strong chemical base. Alkali activation is
advantageous because of their higher carbon yield at low activation temperatures
(500–800 °C) and less activation time. At higher temperature, the development of
micropores could be obtained due to steam activation. The corrosive nature of KOH
and higher cost are the setbacks of using this activating alkali agent [117].
Acid Activation
The acid activation can be carried out with strong and weak inorganic acids. Studies
conducted by Yanping and Rockstraw [120] used orthophosphoric acid as a activation
reagent for activating pecan shell. The characterized carbon showed well-developed
pore structure at temperatures ≥250 °C and reached 1130 m
2 /g and 0.34 cm
3 /g,
respectively, at 500 °C. The pore development was influenced by the impregnation
ratio and soaking time [19, 33]. The data indicated that reactions between the activation agent (existing as phosphorus pentoxide, a strong acid and dehydration agent,
at 400 °C) and precursor matrix also contributed the development of porosity.