Catalyzing the formation of ammonia at lower temperatures with ruthenium

Nitrogen is an essential nutrient for plant growth. While about 80% of the earth is nitrogen, it is mostly contained in the atmosphere as a gas and therefore inaccessible to plants. Chemical fertilizers with nitrogen are 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 achieved through the ‘Haber-Bosch’ process, which, although efficient, requires high temperature conditions (400-500 ° C), which makes the process expensive. Consequently, scientists have tried to find a way to reduce the reaction temperatures of ammonia synthesis.

Recently, scientists have reported ruthenium – a transition metal – as an effective “catalyst” for ammonia synthesis, as it works in milder conditions than traditional iron-based catalysts. However, there is a caveat: nitrogen molecules must stick to the surface of the catalyst to undergo dissociation into atoms before reacting with hydrogen to form ammonia. However, for ruthenium, the low temperature causes hydrogen molecules to stick to its surface – a process called hydrogen poisoning – which prevents the production of ammonia. Therefore, in order to work with ruthenium, it is necessary to suppress hydrogen poisoning.

Fortunately, certain materials can stimulate the catalytic activity of ruthenium when used as a “catalyst support”. A team of scientists from Tokyo Tech, Japan, recently revealed that LnH-shaped lanthanide hydride materials_{2 + x} is such a group of support materials. “Improved catalytic performance is achieved by two unique properties of the support material. First, it donates electrons, which guide the dissociation of nitrogen on the surface of ruthenium. Second, these electrons combine with surface hydrogen to form hydride ions, which easily react with nitrogen to form ammonia and release electrons, suppressing hydrogen poisoning of ruthenium, “explains Associate Professor Maasaki Kitano, who led the study.

Suspecting that the mobility of hydride ions could play a role in ammonia synthesis, the team, in a new study published in Advanced energy materials, investigated the performance of lanthanide oxyanhydrates (LaH_3-2xOx) —fast hydride ion conductors at 100-400 ° C – as a support material for ruthenium, in order to discover the relationship between ammonia synthesis and hydride ion mobility.

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

In addition, the team noted that the presence of oxygen stabilized the framework of hydride and hydride ions against nitriding – the conversion of lanthanum hydride to lanthanum nitride and its subsequent deactivation – which tends to prevent catalysis and is a major disadvantage in the use of hydride support materials. . “Resistance to nitriding is an extraordinary advantage, because it helps to preserve the electron cloning capacity of hydride ions for a longer reaction time,” says Prof. Kitano.

Higher catalytic performance and lower onset temperature of the synthesis obtained using lanthanide oxyhydrides could thus be the much sought after solution to reduce the heat of ammonia production.