Ultrasound-assisted catalytic valorization of nitric acid waste to ammonia using hydrogen nanobubbles

Abstract

The sustainable conversion of nitric acid waste into ammonia offers a dual benefit of wastewater remediation and green ammonia synthesis. Here we report an ultrasound-assisted catalytic process that efficiently converts nitric acid waste into ammonia through the synergistic coupling of ultrasonic irradiation, hydrogen nanobubbles (H2 NBs), and a hydrogen-bearing ruthenium oxide (HxRuO2) catalyst. Ultrasonic irradiation (2.4 MHz) generates stable H2 NBs that act as in situ hydrogen reservoirs, promoting nitrate reduction under mild conditions. The coupled H2 NB–HxRuO2 system achieves over 90 % nitrate-to-ammonia conversion at 60 °C within 4 hours using pressurized H2 gas. The process exhibits high ammonia selectivity (approximately 85 %) and excellent catalyst recyclability across multiple cycles. Mechanistic analysis reveals that acoustic cavitation enhances catalyst dispersion and interfacial hydrogen transfer between H2 NBs and HxRuO2, accelerating nitrate hydrogenation. This ultrasound-assisted approach exemplifies the integration of sonochemistry and heterogeneous catalysis for environmentally benign nitrogen recycling and sustainable ammonia production.