Phosphorus atoms help stabilise chromium catalyst

RESEARCHERS have designed, created and tested a new chromium catalyst, partially surrounded by a ring structure containing phosphorus atoms that stabilises the metal to drive ammonia production.

The team at the Center for Molecular Electrocatalysis, a partner of the US’ Department of Energy (DOE) and the Pacific Northwest National Laboratory (PNNL) has been looking at chromium supported by phosphorus ligand catalysts that have previously failed to react with nitrogen gas (N₂) in producing ammonia.

Now, they have created a 12-atom phosphorous-containing ligand that partially surrounds the chromium and controls its reactivity, causing the normally unreactive nitrogen gas to become more reactive when it binds to the chromium.

The ring-shaped ligand, is found to have a stabilising effect. Every fourth atom in the ring is phosphorus, which bonds with the chromium. This results in a very electron rich chromium atom, which can bind to N₂, resulting in weakening the N₂ triple bond.

The team showed that the specific surroundings enhance chromium's ability to bind and activate N₂, and in this case has greater activity than in similar complexes with heavier metals such as molybdenum and tungsten.

Producing ammonia from fertiliser requires vast quantities of energy. The team hopes this catalyst will reduce the amount of energy required in the process to help with the issue.

The team is also investigating the potential of using the catalyst to store solar energy for times when weather conditions are not favourable. If the electrons produced from solar panels and wind turbines could be stored in the chemical bond similar to nitrogen when it binds to the metal, the stabilising environment created by the ligand could lead to the development of a storage system.

The team found there have been problems with the catalyst’s development, as the reactions must be rigorously managed. They ran the reactions with acid at -50°C to prevent intermediate products from falling apart. However, hydrogen ions from the acid surrounding the complex only formed a small quantity amount of ammonia. Adding the acid caused protons to favour binding with the metal. Additional optimisation of the catalyst and the conditions is required to control the formation of ammonia.

Roger Rousseau, computational catalyst scientist at PNNL said, “This research shows how important it is to move six electrons and six protons in the right order. It is rather like herding cats, and very difficult cats at that.”

The next stage for the team will be to investigate why the specific structure supports chromium activity, and what factors determine this formation. They will also look to refine the experimental conditions in order to control the protons from the complex form interacting with the chromium to produce more ammonia. The team will need to address these issues in order to scale up its process.

Inorganic Chemistry, DOI: 10.1021/acs.inorgchem.5b00351