Waste tire heat treatment to prepare sulfur self-doped char via pyrolysis and K2FeO4-assisted activation methods

Abstract

Waste tire was heat-treated to prepare sulfur self-doped chars via pyrolysis and activation processes. Pyrolytic waste tire chars were activated at different temperatures (600 °C, 800 °C, 1000 °C, and 1200 °C) with K2FeO4 additive ratios (mass ratio of K2FeO4 to char) being 0.5, 1, 2, and 3, respectively. The effective activation occurred over 600 °C with K2FeO4 additive ratios over 0.5. The strongest activation occurred at 1000 °C with K2FeO4 additive ratio of 3, and the specific capacitance increased to 129.5 F/g at 1 A/g, which was six times higher than that without K2FeO4. The activation mechanism revealed that higher K2FeO4 additive ratio promoted the transformation of large aromatic ring systems (≥6 rings) to small ones and smaller pores formation. When K2FeO4 additive ratio was less than 2, high ratio not only promoted alkyl-aryl C-C bonds formation, but also inhibited sulfur enrichment with S 2p3/2 (sulphide bridge) converting to S 2p5/2 (sulphone bridge). But when the ratio was further increased, slight decomposition of alkyl-aryl C-C bonds with the promoted conversion of S 2p5/2 to S 2p3/2 was witnessed. Furthermore, higher activation temperature promoted the conversion of aromatic ring systems and alkyl-aryl C-C bonds to form ordered graphitic structures. S 2p3/2 was enriched before 800 °C, but both S 2p3/2 and S 2p5/2 were released at higher temperature. Formation of smaller pores was promoted before 1000 °C, but the char structure was then destroyed to form larger pores when temperature was further increased.