Metal oxide modified biochar for CO2 capture

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

“In addition to the surface modification by heteroatom doping like pyridinic N, the surface basicity can be adjusted via the loading of metal compounds (e.g., MgO, CaO, Ba(OH)₂, K₂CO₃) (Przepiórski et al., 2013; Wan et al., 2013; Bae, 2018). By forming the AC/CaO heterostructure, 1.79 mmol/g CO₂ uptake was realized at 1 bar and 25°C due to the promoted acid‒base interaction with CO₂ (Song and Lee, 1998). Similarly, MgO featured with the strong basicity exhibits a great potential to integrate with AC, especially with the O^2‒ at the corners and edges of crystal lattice (Shahkarami et al., 2016). Compared with the pristine AC, MgO-loaded AC delivered a 6.5 mmol/g CO₂ uptake amount, 112% higher than the AC alone. Besides the basicity, the high dispersion of MgO was also responsible for the active adsorption sites (Wan et al., 2013). In a further comparative study regarding the preparation method, AC/MgO heterostructure synthesized by two-step activation enjoyed a poorer textural property but a higher Mg content, delivering a higher CO₂ adsorption (1.12 vs. 1.07 mmol/g), which suggested the dominating impact of MgO (Shahkarami et al., 2016). Besides the metal oxides, the heterostructure consisting of metal hydroxides and AC also exhibit a good adsorption performance towards CO₂ (Creamer et al., 2016). When Ba(OH)₂ and KOH were co-loaded onto the AC, a superior uptake of CO₂ (5.07 mmol/g) was approached at ambient conditions (Bae, 2018). Compared with hydroxides, the incorporation of alkali metal carbonates enables an improved capacity with a low cost. When K₂CO₃, KHCO₃ and K₂CO₃•1.5H₂O were compared by impregnating each compound onto the AC, KHCO₃ was found highly dispersed in the pore without blockage while the other two presented an aggregation (Zhao et al., 2011). To further enhance the AC/K₂CO₃ performances, polyethylenimine (PEI) was added to form a AC/PEI/K₂CO₃ heterostructure (Guo et al., 2014). In the presence of 10% water, the dual interactions with CO₂ by PEI and K₂CO₃ were responsible for the enhanced adsorption capacity of 3.6 mmol/g at 60°C and 1 bar.

Other than basic metals, transition and rare earth metal compounds have also been applied to construct an AC-based heterostructure (Boruban and Esenturk, 2018; Nowrouzi et al., 2018; Li et al., 2010). Owing to the low temperature needed for CO₂ capture and regeneration, CuO was deposited on the AC with a high dispersion. At 25°C and 1 bar, 6.72 mmol/g CO₂ uptake was delivered, 68% higher than AC alone (Boruban and Esenturk, 2018). Interestingly, when NiO was added into AC/CuO, a slightly lower capacity (6.27 mmol/g) was obtained for the binary metal oxide/AC than the single oxide-loaded carbons (6.78 and 6.48 mmol/g for AC/CuO and AC/NiO respectively), mainly attributed to the coverage of micropores (Nowrouzi et al., 2018). Different from NiO, the incorporation of CeO₂ in the AC/CuO could promote the CO₂ adsorption at a lower temperature (e.g., 20 and 35°C); but the impact turned to be negative when the temperature increased to 50 and 65°C (Li et al., 2010). Apart from CuO, in a comparative investigation on Fe₂O₃ and Cr₂O₃, a higher CO₂ uptake (1.45 mmol/g) was achieved for AC/Cr₂O₃ than the AC alone (1.19 mmol/g); on the contrary, the impregnation of Fe₂O₃ exerted negligible effect on the AC adsorption performance for CO₂ (Somy et al., 2009).”