N-functional groups for biochar activation

https://doi.org/10.1016/j.ccst.2021.100018

“Usually, ammonia, amine groups (including primary amine, secondary amine and tertiary amine), and polymer-based amine are used to introduce N into biochar. Amine groups strongly and selectively bind CO₂ via chemisorptive interactions with CO₂ molecules by forming a carbamate (Xiong et al., 2013). Ammonia is widely used as the N functionalities. Zhang et al. used black locust to produce ammonia-doped biochar, AC-KOH-N, with activation of KOH and ammonia (NH₃) under N₂, at 830°C (Zhang et al., 2016). At 1 bar, CO₂ adsorption reached 5.05 mmol g⁻¹ at 25 °C and 3.37 mmol g⁻¹ at 50 °C. CO₂/N₂ selectivity of AC-KOH-N calculated by the IAST method was 30.75 at 25 °C and 1 bar, and CO₂ adsorption of AC-KOH-N was fully reversible and stable after 10 cycles.

As urea is a cheap and non-toxic amine group, it is also commonly used to introduce nitrogen into porous material. Cansado et al. has compared activated carbon with the treatment of sodium hydroxide and urea, and the results showed that the urea treatment could lead to better porosity and a basic character; whereas, the treatment with sodium hydroxide showed little influence on the nature of activated carbon (Cansado et al., 2012). Rouzitalab et al. used walnut shell, activated by urea and following KOH, to produce biochar, which shows CO₂ adsorption capacity of 7.42 mmol g⁻¹, and CO₂/N₂ selectivity of 12.7 at 25 °C and 1 bar (Rouzitalab et al., 2018).

Melamine has also been used as a nitrogen source to modify a water chestnut for the prepare N-rich biochar through KOH activation at 600 °C-900 °C (Wei et al., 2018). The resulting samples showed highly developed micropores, outstanding surface area (3401 m² g⁻¹) and a high nitrogen content (4.89 at.%), leading to CO₂ adsorption capacity up to 6.0 mmol g⁻¹ at 0 °C and 1 bar and 4.7 mmol g⁻¹ at 25 °C and 1 bar.”