Green ammonia catalyst reaches 98.95% efficiency turning nitrate into ammonia

A team led by Rongbin Zhang reports in the Proceedings of the National Academy of Sciences a covalent organic framework that turns nitrate into ammonia with very little wasted electricity. The work targets a bottleneck in electrocatalytic nitrate reduction (NO₃RR), a route to ammonia that avoids the high-temperature, high-pressure conditions of the Haber-Bosch process. Haber-Bosch consumes about 1% of global energy and produces more than 1% of annual human CO₂ emissions, according to Phys.org.

What the catalyst is

The team built hydroxyl-functionalized copper phthalocyanine covalent organic frameworks, porous net-like materials that link organic building blocks. They made three versions with zero, two or four hydroxyl groups. The four-hydroxyl version, Cu-COF-4OH, performed best.

Why hydroxyl groups matter

Nitrate reduction needs nine protons for each nitrate molecule. In standard catalysts, protons move slowly and at random through water, so too few reach the active sites. The hydroxyl groups reorganize nearby water into continuous hydrogen-bonded chains, which the researchers describe as proton expressways. The same groups help copper capture nitrate. The design also suppresses hydrogen evolution, a competing reaction that wastes electricity and can leave behind toxic nitrite. The published paper gives the full mechanism.

Performance in tests

Cu-COF-4OH reached a peak Faradaic efficiency of 98.95%, and 93.20% under standard testing conditions. In a membrane electrode assembly flow cell, it produced ammonia at 65.84 mg per hour with 93.20% efficiency. A zinc-nitrate battery built by the team removed nitrate while producing ammonia at 263.2 micrograms per square centimeter per hour at 6 mA/cm², with a peak power density of 1.4 mW/cm². The researchers also trapped the ammonia in acid and evaporated the liquid to make ammonium chloride powder suitable for fertilizer.

The limits

The catalyst has only been tested in prepared nitrate solutions. Real wastewater is more complex, and the researchers say further work is needed to show whether the efficiency holds. The coverage does not report scale-up, long-term durability or cost.

What to watch

Tests with real effluent, longer stability runs and cost estimates would show whether the approach can compete with Haber-Bosch for fertilizer production.

Source: Phys.org