Modification of Rice Husk with Ultrasound-Assisted Inorganic Treatment and Its Application for Catalytic Hydrogen Production

Rice husk is a massive byproduct of the agricultural sector, but less effort has been paid for its recycling. In this study, ultrasound-assisted inorganic treatment with HNO3, H2SO4, and H2O2 was adopted to modify rice husk as a biosynthesized catalyst. The activity of produced biosynthesized catalyst was assessed in hydrogen production through the hydrolysis of sodium borohydride in an alkaline solution. The characterization of as-prepared biosynthesized catalysts revealed that HNO3 was an efficient agent to protonate the surface of rice husk and make active sites available for the hydrolysis reactions. In addition, ultrasound-assisted HNO3 treatment caused a constructive effect on the structural property of rice husk and increased the surface area from 1.9 m2/g to 17 m2/g and pore volume from 0.45 cm3/g to 3.9 cm3/g. According to optimum synthesizing conditions (45 wt% HNO3 and 10 min ultrasonication), ARH-N45-10 could produce 745 mL/g hydrogen from the alkaline solution of NaBH4 at ambient conditions. Based on the kinetic study, the catalytic hydrolysis of NaBH4 by modified rice husk followed first-order kinetic concerning the NaBH4 per fixed NaOH ratio. Moreover, the thermodynamic analysis indicated that hydrolysis of NaBH4 and formation of hydrogen on the modified rice husk is an endothermic and spontaneous reaction, where the hydrogen generated at 55 °C (5280 mL/g) was seven times more than that at ambient temperature, and also activation energy was calculated 57.68 kJ/mol from Arrhenius plot.