Preparation of Cobalt and Nitrogen-Doped Porous Carbon Composite Catalysts From ZIF-9 and Their Outstanding Fenton-like Catalytic Properties Towards Methylene Blue

To activate peroxymonosulfate (PMS) and degrade methylene blue (MB), cobalt and nitrogen-doped porous carbon composite catalysts were prepared through pyrolysing zeolitic imidazolate framework ZIF-9 and dicyandiamide (DCDA) at 700–900 °C with various mass ratios. Powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller results showed that Co, porous carbon and carbon nanotubes (CNTs) co-existed in the catalyst prepared at 800 °C with a ZIF-9/DCDA ratio of 1 : 3. Thermogravimetric analysis suggested that CNT@ZIF-800 had a higher ratio of graphitic to turbostratic carbon than CNT@ZIF-700. It had degraded 94.8 % MB within 30 min, which was about 1.9 times faster than its counterpart C-ZIF-800. It also showed faster degradation rates towards Congo red (98.5 %, 5 min) and slower rates towards rhodamine B (90.7 %, 30 min) than MB. Furthermore, XPS revealed that it had 14.5 % more Co−Nx/pyridinic-N active sites than C-ZIF-800. EPR suggested that ¹O₂ might be the primary catalytic species whereas the SO₄⁻⋅ and ⋅OH the secondary ones. The formation mechanism of such active species might be originated from the synergistic activation of PMS by CNTs, Co−Nx/pyridinic-N and abundant Co sites in the CNT@ZIF-800. This work not only provided efficient Fenton-like catalysts for MB degradation, but also clarified the catalytic mechanism.