Catalytic Reduction of Bromate Using ZIF-Derived Nanoscale Cobalt/Carbon Cages in the Presence of Sodium Borohydride

Conventional catalytic hydrogenation of bromate involves continuous feeding of H₂ gas, which leads to excessive consumption of H₂ because of limited solubility of H₂ gas in water. Herein, we propose to employ borohydride as a solid-phase H₂ source which releases H₂ with controllability depending on the presence of catalysts. A catalyst, possessing immobilized cobalt, porosity, and magnetic properties, was prepared via a one-step carbonization of a cobalt-based zeolitic imidazole framework. The resultant nanoscale cobalt/carbon cage (NCC) was used to activate NaBH₄ to release H₂. The presence of H₂ might also allow the variation of oxidative states of cobalt in NCC to act as a reducing agent to reduce bromate. Thus, NCC/NaBH₄ was found to completely convert bromate to bromide in water. Parameters affecting the bromate reduction were also investigated including NCC loading, borohydride dosage, temperature, pH, and competing anions. Hydrogen production of NCC/NaBH₄ was also evaluated, and a mechanism for bromate reduction by NCC/NaBH₄ was proposed. The multiple-cycle usability of the NCC/NaBH₄ system was also demonstrated, and the catalytic activity of NCC remained almost the same even though no regeneration treatments were applied on the used NCC. These features enable NCC as an effective and promising catalyst to activate NaBH₄ for bromate reduction.