Mechanism of CO2 adsorption on N modified biochar

“Nitrogen atoms can replace carbon atoms in carbon materials as nitrogen atoms are close to C atoms in the periodic table. The covalent radius of carbon is 0.77 Å, while the nitrogen is 0.74 Å (Lv et al., 2018).

When the heteroatomic nitrogen is introduced into a porous carbon, the surface conjugate structure and electron distribution of the carbon carrier, as well as the electrical neutrality of carbon materials, are changed (Wu et al., 2019). N-containing groups were reported to perform well in CO2 adsorption, CO2/N2 selectivity, and initial isosteric heats of CO2 adsorption (Qst) with improved surface properties (Wickramaratne et al., 2014).

The nitrogen in porous carbon mainly contributes to the chemical adsorption of CO2 instead of the physical adsorption of CO2 (Chen et al., 2013). Primary and secondary amines formed various urea linkages in the reactions between the N-containing groups and CO2 (Sayari et al., 2012Sethia and Sayari, 2014). As shown in Fig. 2, primary amines form open-chain urea with CO2 (Route a, Fig. 2), which is regarded as the main reaction. As for adsorbents containing ethylenediamine units, the mechanism will be the reaction between secondary amines and CO2 to obtain cyclic urea (Route b, Fig. 2) (Sayari et al., 2012).”

“For example, the enhancement of CO2 adsorption by ammonia treatment depended on the C–N and C=N groups introduced by modifications (Przepiórski et al., 2004). In detail, NH3 decomposes to produce various free radicals including NH2*, NH*, and H* when the temperature is around 500 °C. Then these radicals react with the active sites on the surface of biochar to form N- functionalities such as amines, amides, imides, lactams, nitriles, or pyridine- or pyrrole-like functional groups as shown in the following routes (Scheme 1) (Li et al., 2019).”

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