https://doi.org/10.1016/j.ccst.2022.100039
“N doping can modify the pore textural properties; more crucially, the interaction with CO2 on the carbon surface will be strengthened by the Lewis acid‒base interaction (pyridinic N) and hydrogen bonding (amine, pyrrole-N and pyridine-N). For example, the promoted CO2 adsorption in the presence of pyridinic N could be indicated by the larger distortion angle of C‒O‒C and shorter bond distance of C(CO2)‒N (Zhao et al., 2012). There are two ways to obtain N-doped AC: pyrolysis of N-rich precursors (Gao et al., 2016) and pretreatment of carbon precursors with N-rich compounds (Li et al., 2017; Han et al., 2019). In terms of the N-rich precursors, N-containing polymer have been reported as the precursors to synthesize ACs in industry Hong et al. (2019). prepared fishnet-like carbon with polybenzoxazine (PBZ), a high-performance thermosetting resin containing N and O heteroatoms and high thermal stability. Owing to the abundant ultramicropores, high surface area, functionalized surface and strong affinity to CO2, a high CO2 adsorption capacity of 8.44 mmol/g and selectivity of CO2 to N2 (56) were obtained. In addition, amine compounds such as polyetherimide (PEI), diethylenetriamine (DETA) and triethylenephosphoramide (TEPA) are commonly used as N dopants to decorate carbon materials Liu et al. (2019). adopted PEI as N source and pressed the treated samples into particles under predetermined pressure. It was found that PEI could enhance the affinity of carbon to CO2 molecules and generate ultramicropores on the compacted particles at high temperatures. Although some of these N dopants exert positive effects in the preparation process, the corrosive and toxic hazards will damage the equipment and organisms. Besides, these methods suffers the high cost and low carbon yield (Geng et al., 2017). In order to improve economic benefits and prevent environmental issues, a large number of biological wastes or plants, such as coconut shell, nut shell, rice husk, almond shell, sawdust, waste tobacco stalk, microalgae, olive core and corn straw, have been studied as AC precursors (Zhang et al., 2020; Kamran and Park, 2020; Balou et al., 2020). Based on the specific elemental composition for each biomass molecule, a wise selection can be possibly made according to the needs. For example, soybean rich in vegetable proteins and nitrogen was carbonized and activated in KOH to prepare a N-doped AC for CO2 uptake (Song et al., 2020). The existence of pyridinic N and pyrrolic/pyridonic N enhanced the interaction with CO2 via Lewis acid‒base interaction (possibly also hydrogen bonding) (Xia et al., 2011). As a result, 1.28 mmol/g CO2 was adsorbed and isosteric heats of adsorption increased with the increase of N contents (Fan et al., 2013). As protein-rich organisms with relatively a high N content and short growth cycle, microalgae can also be excellent precursors for the production of N-doped AC Sevilla et al. (2012a). used these microalgae to prepare AC with high micropore volume via simple hydrothermal carbonization and activation. The resultant carbon adsorbent exhibited a CO2 adsorption capacity of up to 7.4 mmol/g at 0°C and atmospheric pressure. Similarly, Balou et al. (2020) applied microalgae as N source to impregnate the N-containing groups in the carbon skeleton, which increased the N content by 17.39% and CO2 uptake up to 8.43 mmol/g. N-containing functional groups not only enhance the adsorption capacity, but also positively affect the selectivity towards CO2 (Table 1). When N-doped AC was tested for CO2 and N2 competitive adsorptions, results showed that basic N sites favored the acidic CO2 rather than the neutral N2, thus realizing a highly selective CO2 capture (Sethia and Sayari, 2015).
Although AC doped with N-containing functional groups present admirable adsorption performances in many works, however, the CO2 adsorption capacity might not be solely determined by N contents. For example, the porous AC derived from industrial biomass waste with a higher N content possessed a lower CO2 uptake amount; in this scenario, pore structure became the dominant factor based on the large linear correlation coefficient of narrow micropore volume (< 1 nm) and CO2 adsorption capacity (6.61 mmol/g at 0°C and 1 bar) (Song et al., 2020).
It was also reported that there could be a trade-off between the N contents and porosity since the harsh condition usually required for high porosity would reduce the N-containing functionalities. For instance, Cui et al. (2019) found that with the increasing amount of nitrogen dopant in the form of urea, the content of N in the obtained AC also increased; however, the porosity decreased considerably. This was because the carbonization temperature was not high enough and the nitrogen groups partially blocked the pores through steric hindrance (Bagreev et al., 2004). Similarly, Li et al. (2019) observed the decrease of specific surface area and pore volume of carbon derived from urea as the nitrogen source, which probably resulted from the coverage of pores with the addition of N-containing group. Many N-doped carbon adsorbents showed that the CO2 adsorption capacity first increased and then decreased with the increasing amine loading, indicating that N contents could enhance the chemical adsorption capacity for CO2, but an excessive amount would block the pore sites and eventually lower the uptake (Wang et al., 2017; Ochedi et al., 2020; Zhu et al., 2010). Apart from the coverage of pores by the N functionalities themselves, more H2O molecules will be adsorbed onto the surface via hydrogen bonding with electronegative N and the water stacking further blocked the narrow micropores (Fig. 6a) (Liu and Monson, 2005). The enhancement of hydrophilicity especially with an excessive doping of N proven by the much smaller contact angles (Fig. 6b) might not benefit the CO2 adsorption but intensify the competitive H2O uptake.”