Sequential hydrothermal carbonization-pyrolysis of Jatropha fruit husk for cobalt-carbon catalysts enabling efficient hydrogen generation

Cobalt-modified carbonaceous catalysts were prepared via a sequential hydrothermal carbonization (HTC)-pyrolysis scheme using Jatropha fruit husk (JFH) towards hydrogen generation from NaBH₄ hydrolysis. The synthesis method involves adding a cobalt precursor to the HTC treatment, followed by pyrolysis, which produces porous and magnetic materials. Four temperature levels were tested during HTC (i.e., 190, 220, 250, and 280 °C) to investigate thermal effects on the cobalt species fixation within the carbonaceous structure. Hydrochars exhibited hydrogen generation rates (HGR) between 70–85 mL min⁻¹ g⁻¹. In contrast, pyrolyzed hydrochars reached 336 mL min⁻¹ g⁻¹ and a relatively low activation energy of 58.6 kJ mol⁻¹ (∼23 % decrease compared to the non-catalyzed reaction). The materials showed stable catalytic activity after six consecutive cycles. Physicochemical characterization (via XRD, FTIR, SEM/EDS, BET, VSM, and XPS) confirmed the coexistence of well-dispersed active sites containing metallic Co⁰ and cobalt oxides (CoO/Co₃O₄) within a micro/mesoporous carbon matrix. BET surface areas for pyrolyzed hydrochars ranged from 245.61 to 463.13 m² g⁻¹, with a decreasing trend at higher HTC temperatures. On the other hand, stronger ferromagnetic behavior was observed at higher HTC temperatures, as magnetic saturation increased from 4.9 to 49.7 emu g⁻¹. Moreover, DFT calculations clarified how subcritical water promotes the aggregation of cobalt-related species during HTC, which are subsequently converted upon pyrolysis. Therefore, the combined HTC-pyrolysis route demonstrates a synergistic pathway for producing efficient, magnetically recoverable Co-based catalysts from agricultural waste, thereby facilitating sustainable hydrogen generation.