Green synthesis of Co–Ag biochar catalysts from rice husk using hydrothermal carbonization concentrate for efficient NaBH₄ hydrolysis

Here, we report a sustainable strategy for incorporating magnetic cobalt nanoparticles and non-magnetic silver nanoparticles into rice husk-derived biochar (RHB) using a waste concentrate derived from the hydrothermal carbonization of Jatropha fruit husk. The concentrate serves as both a reducing and stabilizing agent, enabling the formation of small, uniformly dispersed metal nanoparticles without harsh chemicals. The optimized material, RHB-0.09Co-Ag, exhibits excellent magnetic properties (20.4 emu g⁻¹) and catalytic activity in the hydrolysis of sodium borohydride (NaBH₄), achieving a hydrogen generation rate of 60 mL min⁻¹ g⁻¹ and an apparent activation energy of 59 kJ mol⁻¹ . The catalyst maintained full performance over five reuse cycles , confirming its structural stability. This behavior arises from a synergistic interaction between cobalt and silver that enhances the catalyst’s electronic and structural features. The feasibility of NaBH₄ hydrolysis in the presence of cobalt sites was further examined using density functional theory (DFT) calculations. Comprehensive characterization by SEM, FTIR, XRD, BET, XPS, and VSM analyses confirmed the successful incorporation of metallic Co and Ag, improved surface area, and retention of mesoporosity. These findings demonstrate the potential of biomass-derived concentrates as sustainable alternatives to conventional reducing agents for the design of efficient, recyclable, and dual-metal catalytic materials for clean hydrogen generation.