Effects of cobalt salts on biomass conversion to functional carbon-based catalysts for peroxymonosulfate activation

Metal functionalization is an effective structure-engineering strategy for preparing biomass-derived carbon materials, whereas the influences of different metal precursors on the structure and catalytic performance remain unclear. Herein, an investigation of four typical cobalt salts for pyrolytic biomass conversion is performed. The melting points and anion species of cobalt salts are found to be critical factors. Co-salts (cobalt nitrate (Co(NO₃)₂), cobalt acetate (Co(OAc)₂), cobalt acetylacetonate (Co(acac)₂)) melted at low temperatures (≤165 °C) could promote mesopore formation and catalyze the graphitization process of a biomass flower during the carbonization, finally forming mesoporous graphitic carbon matrixes with Co@graphitic-C nanoparticles and trace isolated Co atoms as active catalytic sites (denoted as Co@C-NO₃, -Ac, -acac). By comparison, high-melting-point (735 °C) CoSO₄/biomass pyrolysis produces an amorphous carbon/Co₉S₈ nanoparticle composite (denoted as Co₉S₈@C-SO₄), with Co₉S₈ as active sites. Co@C-NO₃ and Co₉S₈@C-SO₄ demonstrated excellent activities with the reaction rates of 0.21 and 0.29 min⁻¹, respectively, in peroxymonosulfate (PMS) activation for bisphenol A (BPA) degradation with distinct catalytic mechanisms. Co@C-NO₃/PMS shows multiple nonradical/radical pathways with 40.9% mineralization of BPA, while Co₉S₈@C-SO₄/PMS demonstrates a selective sulfate radical-based reaction pathway to achieve 99.8% mineralization of BPA. Co@C-NO₃ and Co₉S₈@C-SO₄ presented excellent performance for multiple organic pollutant removal (100%) in real water and good regeneration ability by thermal treatment of the reclaimed samples at 400 °C. This study provided a novel insight into rational design of biomass-derived carbon-based catalysts with desired active sites to meet a different catalytic demand.