Excellent CO2 adsorption performance of nitrogen-doped waste biocarbon prepared with different activators

Abstract

A series of N-doped CO2 adsorbents were prepared by using waste walnut shell as the raw material, urea as nitrogen source, H3PO4 as a pretreatment agent and KOH, K2CO3 and ZnCl2 as the activators. Result indicates that both the pore structure and nitrogen content of the adsorbent together affect the CO2 adsorption capacity. HAC-850-KOH showed excellent CO2 adsorption capacity (5.13 mmol/g) at 0 °C and atmospheric pressure probably because of its large specific surface area (SBET 2354 m2/g). HAC-750-ZnCl2 has the higher CO2 adsorption capacity (0.6 mmol/g) at 25 °C and 0.15 bar due to its highest nitrogen content (9.32 wt%). HAC-850-K2CO3 has a good selectivity of 21.5 and cycle performance may be somewhat associated with its pore structure and nitrogen content, and the initial heat of adsorption is 44 kJ/mol. The result indicates that the pore structure of the adsorbent has a great influence on the adsorption performance under high pressure; the nitrogen content of the adsorbent has a decisive influence on the adsorption performance of the adsorbent under low pressure. At the same time, the pore structure properties of the prepared adsorbent have a more significant effect on the adsorption performance at lower temperatures. This work provides an effective guide to synthesize biomass-based adsorbent for different application conditions, especially in the selection of activators.