2.2 Green S doping process with SO2 cycling units
The green S doping system mainly contains a quartz tube reactor for SO2 generation, a rotary quartz tube reactor for S doping treatment, and an absorption column for SO2removal. In a typical synthesis of SCNTs, CaSO3 (250 g) was heated to 715 ℃ and held for 15 min in the horizontal quartz tube reactor with a N2 flow (300 mL/min). At the same time, CNTs (2 g) were heated to 900 ℃ in the rotary quartz tube reactor in N2 atmosphere. Then, the CaSO3calcination temperature rose from 715 ℃ to 800 ℃ in 10 min and held for 10 min. The resulted SO2-N2 mixture was introduced to the CNTs, and the S doping treatment time was around 35 min. The outlet gas from the S-doping unit was desulfurized by a packing column filled with CaO powder (20 g). A flue gas analyzer (Ecom-EN2) was used to detect the SO2 concentration of the final outlet gas after CaO absorption. The as-obtained SCNTs were labeled as SCNT-S. To produce SNCNTs, NCNTs prepared by a CVD process using ammonia as nitrogen source25 were used as the starting material in the green S doping process.