Catalysis of ammonia formation at lower temperatures with ruthenium

Nitrogen is an essential nutrient for plant growth. While around 80% of the earth is nitrogen, it is mostly contained as a gas in the atmosphere and is therefore inaccessible to plants. Chemical nitrogen fertilizers are therefore needed to promote plant growth, especially in agricultural environments. A crucial step in the production of these fertilizers is the synthesis of ammonia, in which hydrogen and nitrogen are reacted in the presence of a catalyst.

Traditionally, ammonia production has been carried out by the “Haber-Bosch” process which, despite its effectiveness, requires high temperature conditions (400-500 ° C), which makes the process expensive. As a result, scientists have tried to lower the reaction temperatures of ammonia synthesis.

Recently, scientists have reported that ruthenium – a transition metal – is an efficient “catalyst” for ammonia synthesis because it works under milder conditions than traditional iron-based catalysts. There is one caveat, however: nitrogen molecules must adhere to the catalyst surface in order to dissociate into atoms before reacting with hydrogen to form ammonia. With ruthenium, however, the low temperature often causes hydrogen molecules to stick to its surface – a process known as hydrogen poisoning – and hinder the production of ammonia. In order to work with ruthenium, it is therefore necessary to suppress its hydrogen poisoning.

Fortunately, certain materials can increase the catalytic activity of ruthenium when used as “catalyst supports”. A team of scientists from Tokyo Tech, Japan recently found that lanthanide hydride materials are of the form LnH_{2 + x} is one such group of carrier materials. “The improved catalytic performance is realized by two unique properties of the support material. First, they donate electrons that control the dissociation of nitrogen on the ruthenium surface. Second, these electrons combine with hydrogen on the surface to easily form hydride ions, which react with nitrogen below Formation of ammonia and release the electrons, which suppresses hydrogen poisoning by ruthenium, “explains Associate Prof. Maasaki Kitano, who led the study.

In a new study published in, the team suggested that hydride ion mobility may play a role in ammonia synthesis Advanced Energy Materials, studied 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 uncovering the relationship between ammonia synthesis and hydride ion mobility.

They found that the conductivity of the “bulk” hydride ions had little influence on the activation of ammonia synthesis, but the surface or “local” mobility of the hydride ions played a crucial role in catalysis by contributing to a strong resistance to it Hydrogen poisoning build up ruthenium. They also found that lanthanum oxyhydrides required a lower onset temperature for ammonia formation (160 ° C) and exhibited higher catalytic activity compared to other support materials.

In addition, the team observed that the presence of oxygen stabilized the oxyhydride backbone and hydride ions against nitridation – the conversion of lanthanum oxyhydride to lanthanum nitride and its subsequent deactivation – which tends to hinder catalysis and is a major disadvantage of using hydride support materials. “The resistance to nitridation is an enormous advantage, since it helps to maintain the electron-donating ability of the hydride ions for a longer reaction time”, comments Prof. Kitano.

The superior catalytic performance and lower synthesis temperature achieved using lanthanide oxyhydrides could therefore be the coveted solution to reduce the heat on ammonia production.