Producing artificial fertiliser with a copper-zinc catalyst

Urea is a widely used artificial fertiliser that is typically produced from ammonia and CO₂ via thermal catalysis. However, the high temperatures required mean this process also generates significant CO₂ emissions. An alternative method is electrocatalysis. A team of researchers from Eindhoven University of Technology is now using electrocatalysis to combine the waste products nitrate and CO₂ to produce urea, a complex molecule of greater industrial value than ammonia. They published their results in Communications Chemistry.

The researchers studied a bimetallic system of CuO_xZnO_y with varying CuO and ZnO ratios. A 50-50 ratio of CuO and ZnO proved to yield the highest activity and selectivity for urea production. According to author Dimitra Anastasiadou, analysing the urea generated presented a considerable challenge. ‘It was an aqueous medium containing carbon compounds and nitrogen compounds, which I studied using UV-Vis. To analyse the quantity of urea, I first had to determine the amount of ammonia in the system, then use an enzyme to convert the urea back into ammonia, and subsequently perform the same determination again, so that I could calculate the quantity of product from the difference. It would have been more efficient had I been able to use a more powerful NMR, but I had to make do with that approach.’

According to Anastasiadou, their process is not intended to replace the Haber-Bosch process for ammonia production. ‘This is better suited to specific locations where arable farming is situated close to industry or wastewater with a high nitrate concentration. The challenge there is to filter the wastewater for other metals or nutrients, as these can poison the catalyst. In addition, an electrochemical cell is needed for industrial applications in order to improve efficiency.’ Before the researchers can look at such practical applications, they must carry out stability tests and select a separation process for the final mixture of urea and ammonia.

For the time being, the researchers are focusing on understanding the catalyst. ‘We are concentrating on why and how the copper and zinc oxide catalyse the reaction. Does the combination work better simply because we are combining two metals, or is there a synergistic effect? What is the active phase and what are the intermediates? We are currently working on a follow-up publication in which we explore in greater depth why this catalyst works and what the reaction pathway is.’

Anastasiadou et al. (2023) Communications Chemistry https://doi.org/10.1038/s42004-023-01001-5