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Alkalis activated biochar for CO2 capture

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

“KOH and NaOH are widely applied in chemical activation of biomass precursors to generate pores with various shapes and sizes, thus modifying the porosity and enhancing the CO2 adsorption capacity (Ma et al., 2020bMathangi et al., 2018Pezoti et al., 2016). The main reaction in the activation mechanisms for both alkalis is similar, that is, the carbon matrix is consumed by reaction with the alkalis to produce metals, metal carbonates and H2 (Gong et al., 2015Pezoti et al., 2016). At a higher temperature (i.e., above 700°C), metal carbonates are converted to metal oxides, CO and CO2 either by decomposition or through the reaction with carbon. By further increasing the temperature to 800°C, metal oxides are reduced by carbon to form metals and CO. The consumption of C in alkali activation benefits the pore formation, especially micropores for CO2 uptake. For instance, the unactivated biomass carbon derived from pine sawdust possessed nearly zero micropores (0.001 cm3/g); in sharp comparison, regardless of the activation temperature (700, 800 and 900°C), all KOH-treated AC adsorbents presented abundant micropores (0.671‒0.975 cm3/g), leading to a higher CO2 uptake (2.45‒4.21 mmol/g vs. 2.11 mmol/g) (Quan et al., 2020). To further study the temperature impact, KOH and carbonized polybenzoxazine were mixed and activated in a series of temperatures (600, 700 and 800°C). Results showed that the 700°C and 800°C-treated AC possessed a higher surface area and pore volume (Jin et al., 2018). However, the largest CO2 adsorption capacity was achieved by the 600°C-activated carbon, which probably resulted from the largest micropore volume (pore size less than 0.8 nm), promoting the CO2 uptake at a low pressure (kinetic diameter of CO2 was 0.33 nm) (Kou and Sun, 2016). Although a higher temperature may present a lower content of micropores, the formation of larger pores due to the evaporation enables a quick acid washing time to ensure a complete removal of metallic residues and more pore formations (Zhang et al., 2016Singh et al., 2017a).

Apart from the temperature, mass ratio of the activation agents and carbons considerably affects the pore structures (Sun et al., 2016). Among three feed ratios of KOH and hydrothermal treated carbons (1:1, 3:1 and 5:1), the highest micropore contents (97.9%) and largest specific surface area (2879 m2/g) were obtained for 3:1 sample (Fig. 5). On the contrary, 5:1 sample only exhibited 57.8% micropore contents due to the intensive reaction with carbon matrix while 1:1 sample possessed a poorly-developed pore structure possibly owing to the insufficient KOH amount. Similar to the solid activation, mixing the carbonized biomass with K2CO3 or KOH solutions demonstrated a strong dependence on the impregnation ratios (Okman et al., 2014). At 800°C, the highest BET area was achieved for 50 wt% K2CO3 (1238 m2/g) and 25% wt% KOH (1222 m2/g). More K2CO3 was necessary to reach a high surface area might be a result of the larger molecular mass than KOH.”

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