Pine cone shell-based activated carbon used for CO₂ adsorption

In this study, pine cone shell-based activated carbons were used to adsorb CO₂. After a carbonization process at 500 °C, the resulting preliminary activated carbons (Non-PAC) were activated under different conditions. The results indicated good CO₂ adsorption performance of pine cone shell-based activated carbons. For example, after activation at 650 °C and with a KOH : Non-PAC ratio of 2, the activated carbon (named as PAC-650/2) achieved a high CO₂ adsorption capacity of 7.63 mmol g⁻¹ and 2.35 mmol g⁻¹ at 0 °C under 1 and 0.15 bar pressure, respectively. To determine the potential correlation between the amount of CO₂ adsorbed and micropore distribution, linear correlations between cumulative pore volume over different ranges and amount of CO₂ adsorbed were analyzed. Results showed that pores <0.70 nm played an important role in the CO₂ adsorption process at 0 °C and 0.1 bar, and in contrast to previous research, pore volumes <0.80 nm or 0.82 nm did not show good linear correlation with the amount of CO₂ adsorbed at 0 °C and 1 bar, and we inferred that this was most likely due to the unique pore structure of pine cone shell-based activated carbons. The highest Brunauer–Emmett–Teller (BET) surface area of 3931 m² g⁻¹ was obtained after activation at 800 °C and with a KOH : Non-PAC ratio of 2, but the highest BET surface area did not result in the highest CO₂ adsorption capacity. This is mainly due to the BET surface area having regions unavailable for CO₂ adsorption. X-ray photoelectron spectroscopy (XPS) analysis results for all activated carbons indicated a higher stability of pyridonic-N than pyridinic-N. Furthermore, in order to better understand the interaction between CO₂ and pine cone shell-based activated carbons, we analyzed the isosteric heat of adsorption (Q_st). Q_st was higher than 22 kJ mol⁻¹ for all activated carbons, and the highest initial isosteric heat of adsorption of 32.9 kJ mol⁻¹ was obtained for the carbon activated at 500 °C and a KOH : Non-PAC ratio of 1. The optimal Q_st (Q_st,opt) under the conditions of a vacuum swing adsorption (at 25 °C, adsorption under 1 bar and desorption under 0.1 bar) process was 30 kJ mol⁻¹.