Ultrasonic Spray Nozzle-Mediated Green Activation for Hierarchical Pore-Structured Carbon Beads

This study focuses on enhancing sustainability through energy-efficient methods in producing hierarchically structured porous carbons. A novel approach, utilizing an ultrasonic spray nozzle-quartz tube reactor (USN-QTR), is introduced for fabricating carbon beads with customizable ultra-, super-, and mesopores. This study showcases noteworthy results from subjecting spherical char particles to activation processes involving carbon dioxide, a mixture of carbon dioxide and micron-sized water droplets, and highly concentrated supercritical steam at a temperature of 1173 K for durations of 3 and 5 h. Through pulse-field gradient nuclear magnetic resonance measurements, it was noted that carbon beads produced using USN-generated highly concentrated supercritical steam displayed remarkably elevated intrabead self-diffusivity of n-hexane. Inductively coupled plasma-optical emission spectroscopy demonstrates superior gold recovery kinetics from cyanide solutions compared to that from an industrial benchmark. The energy expenditure for USN-generated steam, producing carbon beads with an apparent surface area of 2691 m²/g, is estimated at 97 J per 1 m² of carbon. This contrasts with the traditional steam generation method requiring approximately the energy of 190 J/m² for activated carbon with an S_BET of 2130 m²/g, making the USN-assisted activation method a more environmentally friendly and sustainable option with nearly half the energy consumption.