Relay Cascade Catalysis on Ru-Co Dual Sites Enables Selective Nitrate-to-Ammonia Conversion in Wastewater

Area of Science:

• Materials Science
• Electrochemistry
• Environmental Engineering

Background:

• Electrocatalytic nitrate reduction (NO3RR) offers simultaneous wastewater treatment and ammonia recovery.
• Kinetic limitations in sequential hydrogenation steps hinder ammonia selectivity.

Purpose of the Study:

• To develop an atomic-scale relay cascade catalyst for efficient and selective ammonia synthesis via NO3RR.
• To investigate the catalyst's performance in low-concentration nitrate wastewater and its economic viability.

Main Methods:

• Fabrication of a Ru-Co dual-site catalyst (Co3Ru4-CeOx) on a CeOx support.
• In situ spectroscopy and theoretical calculations to elucidate reaction mechanisms.
• Integration into an electrocatalytic membrane reactor for wastewater treatment.
• Techno-economic analysis for ammonia production cost.

Main Results:

• The Co3Ru4-CeOx catalyst spatially coordinates hydrogenation steps, establishing a kinetically favorable reaction regime (R2 > R1).
• Achieved high ammonia selectivity (97.1% Faradaic efficiency) by suppressing intermediate nitrite accumulation.
• Demonstrated sustained high selectivity in low-concentration nitrate wastewater (1-10 mM).

Conclusions:

• The atomic-scale relay cascade catalyst effectively overcomes kinetic mismatches in NO3RR for selective ammonia synthesis.
• The integrated electrocatalytic membrane reactor shows promise for treating dilute nitrate wastewater.
• The process presents a scalable and economically feasible method for nitrogen recovery from wastewater, with an estimated production cost of $2.08 per kg NH3.