Synergistic adsorption mechanism of NO/CO₂ by CaO coupled nitrogen-doped biochar

High efficiency adsorbents are one of the effective ways to reduce NO_x and CO₂ emissions. Rice husks were used as raw materials, modified and activated by urea and CaO, and the effects of different nitro-containing biochar and CaO-coupled nitro-doped biochar on NO, CO₂ and their synergistic adsorption properties were investigated by combining experiments and density functional theory calculations. The experimental results show that nitrogen-containing porous carbon is mainly microporous, and its urea mixture can promote the formation of biochar pores. The specific surface area of BC1−2 is 1491.30 m²/g, and the microporosity is 72.5%. Theoretical calculation results show that introducing nitrogen-containing groups into biochar can improve the adsorption of NO and CO₂. Among all nitrogen-containing functional groups, N−5 functional group has the most obvious adsorption effect on NO and CO₂. Compared with pure biomass carbon, the adsorption energy of pyrrole-nitrogen-containing biochar （CN−5） for NO and CO₂ increased by 143.88% and 13.75% respectively. The coupling of CaO further increases the influence of N−5 groups on gas adsorption characteristics, and the adsorption energy of CaO/CN−5 is more than 5 times higher than that of CN−5. The coupling of CaO and CN−5 can promote NO/CO₂ co-adsorption. The co-adsorption energy of CaO/CN−5 for NO/CO₂ is 514.97, 502.58 and 35.7 kJ/mol higher than that of BC, CN−5 and CaO/BC, respectively. The results of adsorption capacity also show that adding pyrrole snitrogen-containing biochar （CN−5） is more beneficial to gas adsorption, and coupling CaO with CN−5 can further increase the gas adsorption capacity of biochar. At 273 K, the adsorption capacity of NO and CO₂ by CaO/CN−5 system reached 6.342 and 7.666 mmol/g, respectively, which was 2.16% and 23.49% higher than that by CN−5 system.