Heteroatom-doped biochar devised from cellulose for CO2 adsorption: a new vision on competitive behavior and interactions of N and S

Biochar, as a potential CO2 adsorbent, is of great significance in addressing the problem of global warming. Previous studies have demonstrated that the CO2 adsorption performance of biochar can be improved by nitrogen and sulfur doping. Co-doping can integrate the structure and function of two elements. However, the physicochemical interaction of nitrogen and sulfur during doping and the CO2 adsorption process remains unclear in co-doped biochar. In this study, the heteroatom-doped biochar was prepared with different additives (urea, sodium thiosulfate, and thiourea) via hydrothermal carbonization, and the physicochemical interaction of nitrogen and sulfur in co-doped biochar was investigated extensively. The findings revealed that nitrogen and sulfur competed for limited doped active sites on the carbon skeleton during the co-doping process. Interestingly, thiourea retained the amino group on the surface of biochar to a great extent due to carbon-sulfur double bond breaking and bonding, which facilitated the formation of pore in the activation process. Significantly, co-doping had no significant improvement effect although nitrogen and sulfur doping separately enhanced the CO2 adsorption performance of biochar by 11.9% and 8.5%. The nitrogen-containing and sulfur-containing functional groups in co-doped biochar exhibited mutual inhibition in the process of CO2 adsorption. The findings of this study will have pertinent implications in the application of N/S co-doped biochar for CO2 adsorption.