O/S/F doping biochar for CO2 capture

“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., 2002Zhao et al., 2012Wu 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., 2017Sun 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., 2015Seredych 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.”

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