O/S/F doping biochar for CO2 capture

https://doi.org/10.1016/j.ccst.2022.100039

“Similar to N, other electronegative atoms (O, S and F) might also possess the potential as the doping sources to modify the carbon adsorbent surface. Owing to the high polarity, ‒OH or –OK bonds could form hydrogen bonding with CO₂ or promote the charge transfer and increase the adsorption energy (Yoshizawa et al., 2002; Zhao et al., 2012; Wu et al., 2020). In carboxylic group, the negatively charged oxygen atom in -COOH could capture electrons from the nearby H and C atoms to further enhance the electronegativity, which strengthened the electrostatic interaction between carbon surface and CO₂ molecules, thus showing a strong attraction to CO₂ (Ma et al., 2020b). As another Group VI element, S doping also exhibits a promotional effect on enhancing the CO₂ adsorption on the modified carbon surface. It was found that S-doped mesoporous carbon exhibited a higher uptake of CO₂ than the commercial AC (Saha et al., 2017) Sun et al. (2017). further confirmed the improvement of CO₂ interaction with the addition of oxidized S species by comparing with the AC without S moieties. Despite the very similar porosity, S-doped carbon exhibited a much higher CO₂ uptake of 4.21 mmol/g at 25°C with a high adsorption heat, indicating the dominating impact of an enhanced interaction between S and CO₂ (Seema et al., 2014). However, at a higher pressure (i.e., 8 bar), around 40% reduction was demonstrated, suggesting the suitability of S-doped carbon adsorbent at moderate instead of high pressure conditions. On the other hand, a trade-off effect was seen in post activation process where porosity became lower at a higher S content. To realize a simultaneous high S concentration and microporosity, in-situ templating method might be a reasonable option (Saha and Kienbaum, 2019). Besides, out of various S-containing groups, thiophene and oxidized S species have been proven effective while the role of other S functionalities are still waiting to be explored, such as the impact on the hydrophobility (Seredych et al., 2015; Seredych et al., 2016). In terms of the F doping, the separation efficiency of CO₂ against C₂H₂ was improved in the presence of F functionalities (Li et al., 2017); however in another work (Shahtalebi et al., 2016), both the CO₂ adsorption capacity and heat were decreased with the increase of F contents, which might be ascribed to the lower microporosity though the exact role was not well established.”