The DCR-EF Loop: Directed Self-Optimization in FeCo/Biochar-Catalyzed CNT Growth from Polystyrene

The catalytic upcycling of waste plastics into carbon nanotubes (CNTs) is often interpreted primarily in terms of metal composition and carbon deposition, while the dynamic participation of carbonaceous catalyst supports remains poorly understood. Here, biochar-supported FeCo catalysts with systematically varied Fe/Co ratios were developed for the catalytic conversion of waste polystyrene (PS) into CNTs, and the coupled evolution of the metal phase, biochar support, and carbon product was elucidated. The equimolar Fe5Co5/BC catalyst exhibited the most favorable Fe–Co alloy structure and pronounced electronic reconstruction, as evidenced by composition-dependent PXRD behavior and positive Fe2p3/2 and Co2p3/2 binding-energy shifts of 1.04 and 1.05 eV, respectively. Fe5Co5/BC simultaneously delivered the highest CNT yield and lowest Raman (ID/IG) ratio, while favoring C2H4 formation (∼70%) and suppressing CH4 (∼22%), indicating improved utilization of reactive carbon intermediates. Mechanistic control experiments revealed a directed carbon removal (DCR) process in which FeCo preferentially consumed and restructured defect-rich biochar carbon, producing an 11.7% support mass loss compared with only 3.59% for metal-free biochar. CNT growth subsequently generated an encapsulation-feedback effect: graphitic layers stabilized metallic FeCo, decreased the CoFe2O4/FeCo peak-area ratio from ∼38 to ∼5%, modified the interfacial electronic environment, and redirected carbon transformation toward less extensive fragmentation. Accounting for DCR-induced catalyst mass loss increased the calculated CNT yield across the FeCo series, with Fe5Co5/BC retaining the highest corrected yield, approximately 32% above its apparent value, while exhibiting the lowest support loss. These findings establish a directed carbon removal–encapsulation feedback (DCR–EF) loop, in which selective support restructuring maintains accessible catalytic interfaces and CNT encapsulation subsequently stabilizes and regulates the FeCo phase. The work therefore identifies catalyst–support–product co-evolution as a mechanistic design principle for efficient plastic-to-CNT upcycling.