Catalyst ammonia formation at lower temperatures with ruthenium

Nitrogen is an essential nutrient for plant growth. Although about 80% of the earth is made up of nitrogen, it is mostly present as a gas in the atmosphere and therefore inaccessible to plants. Chemical nitrogen fertilizers are therefore needed to stimulate plant growth, especially in agricultural environments. A crucial step in the production of these fertilizers is the synthesis of ammonia, which involves a reaction between hydrogen and nitrogen in the presence of a catalyst.

Traditionally, ammonia production has been carried out through the “Haber-Bosch” process, which, despite being effective, requires high temperature conditions (400-500 ° C), making the process expensive. Therefore, scientists have been trying to find a way to lower reaction temperatures of ammonia synthesis.

Recently, scientists have stated that ruthenium – a transition metal – is an efficient “catalyst” for ammonia synthesis, as it operates under milder conditions than traditional iron-based catalysts. There is one caveat, however: nitrogen molecules must stick to the catalyst surface to undergo dissociation into atoms before reacting with hydrogen to form ammonia. However, with ruthenium, the low temperature often causes hydrogen molecules to stick to the surface – a process called hydrogen poisoning – hindering ammonia production. To work with ruthenium, it is therefore necessary to suppress hydrogen poisoning.

Fortunately, certain materials can enhance ruthenium’s catalytic activity when used as a “catalyst support”. A team of scientists from Tokyo Tech, Japan, recently revealed that lanthanide hydride materials in the form LnH_{2 + x} is one such group of supporting materials. “The enhanced catalytic performance is realized by two unique properties of the support material. First, they donate electrons, which direct the dissociation of nitrogen on the ruthenium surface. Second, these electrons combine with hydrogen on the surface to form hydride ions, which readily react with nitrogen to form ammonia and release the electrons, thereby suppressing hydrogen poisoning from ruthenium, ”explains Associate Prof. Maasaki Kitano, who led the study.

The team suspects that the mobility of hydride ions may play a role in ammonia synthesis, in a new study published in Advanced energy materials, investigated the performance of lanthanide oxyhydrides (LaH_3-2xOx) – reportedly fast hydride ion conductors at 100-400 ° C – as a support material for ruthenium, with the aim of revealing the link between ammonia synthesis and hydride ion mobility.

They found that while the ‘bulk’ conductivity of hydride ions had little influence on the activation of ammonia synthesis, the surface or ‘local’ mobility of hydride ions played a critical role in catalysis by helping to generate strong resistance. build against hydrogen poisoning from ruthenium. They also found that, compared to other support materials, lanthanum oxyhydrides required a lower initial temperature for ammonia formation (160 ° C) and showed higher catalytic activity.

Furthermore, the team noted that the presence of oxygen stabilizes the oxyhydride framework and hydride ions against nitridation – the conversion of lanthanum oxyhydride to lanthanum nitride and subsequent deactivation – which often hinders catalysis and is a major disadvantage when using hydride support materials. “The resistance to nitridation is a huge advantage, as it helps to maintain the electron-donating capacity of the hydride ions over a longer reaction time,” says Prof. Kitano.

Thus, the superior catalytic performance and lower synthesis initial temperature achieved with lanthanide oxyhydrides could be the most desirable solution to lower the heat of ammonia production.