A better catalyst

Research by scientists at the US Department of Energy’s Brookhaven National Laboratory may help lead to a more efficient catalyst for ammonia production, one of the USA’s largest industries.

“Ammonia is the fifth most abundantly produced chemical in the US and ranks number two on the list of chemicals requiring the most energy to produce,” said Zhen Song, the study’s lead scientist. “Determining a more efficient way to synthesise ammonia could have a major impact on the ammonia production industry.”

The Brookhaven scientists have uncovered details about the structure and reactivity of tiny particles of the metal ruthenium, which lead them to believe this metal could be more efficient in ammonia production than the catalysts currently used.

Ammonia, a nitrogen/hydrogen compound used to make fertilisers, textiles, explosives, and many other products, is produced by combining nitrogen and hydrogen under high temperatures and pressures in the presence of a catalyst, which is usually made from iron. Ruthenium catalysts display five to 10 times higher reactivity rates under the same temperature and half the pressure, but are rarely used because they do not remain active long enough.

This problem is due to the surface, or “support,” that ruthenium sits upon. Currently, the best type of support is “activated” carbon, a porous graphite with a layered, crystalline structure. However, the graphite is unstable during catalysis, tending to react with hydrogen to form methane gas, thus becoming slowly consumed.

Song and her colleagues studied how a graphite support affects the structure and reactivity of the ruthenium particles to try to understand why activated carbon – despite its drawbacks – is superior to other materials scientists have tested as ruthenium supports. Understanding these interactions may help them find a better support material.

First, they discovered that ruthenium can grow on a graphite surface “epitaxally” – mimicking the ordered structure of the surface below, forming flat crystalline particles. This makes graphite unique, Song said, since other surfaces don’t encourage the same epitaxial growth.

They also determined the atomic structure of the flat ruthenium particles. The structure shows that the ruthenium particles have a high density of “active sites” – locations that allow them to participate in ammonia synthesis. This makes the flat particles better performers than the round ruthenium particles often grown on other types of supports.

The flat particles have another key feature: They are built in layers. These layers form steps and terraces that are able to hold nitrogen, one of ammonia’s components. From there, the nitrogen can participate in the ammonia synthesis reaction.

“These results tell us that scientists should look for a support that encourages the epitaxial growth of flat, layered ruthenium particles like these, in order to find an alternative to activated carbon,” said Song. “Particles with these features are efficient catalysts.”

The research group, led by Brookhaven chemist Jan Hrbek, plans to continue studying ruthenium catalysts. For example, they may investigate how to use additives, called promoters, to boost the ruthenium’s effectiveness.