A new catalyst will allow hydrogen to be used as a clean fuel

Scientists from the US Department of Energy combined theoretical and experimental studies to identify the properties of the catalyst - an accumulation of rhodium atoms, boron and other elements that allows hydrogen to be released for use in fuel cells

In order to use hydrogen as a clean energy source, some engineers are interested in embedding hydrogen inside larger cells, instead of compressing the gas into a tank. The gas is released out of the tank easily, but in order to release the hydrogen from the larger pod, a catalyst is needed. Now, researchers have discovered new details about one such catalyst. The findings are another step in advancing the development of catalysts for hydrogen energy applications, such as fuel cells.

Scientists from the US Department of Energy combined theoretical and experimental studies to identify the characteristics of the catalyst - an accumulation of rhodium atoms, boron and other elements. The catalyst chemically reacts with the ammonia borane substance, a compound that stores hydrogen in a dense manner, to release hydrogen gas. Their findings, which reveal many molecular details about this catalytic reaction, were published in the August issue of the Journal of the American Chemical Society.

"These experiments reveal to us what the most complex stage of the chemical reaction is," says chemist and author of the article Roger Rousseau. "If we manage to find a way to change this problematic step, that is - to release the hydrogen more easily - then we can improve this catalyst."

Researchers and engineers are trying to create a hydrogen fuel system that stores hydrogen safely and releases it easily, a system that could be used in fuel cells or similar applications.

One way to achieve such a fuel system is by storing hydrogen as part of a larger fuel cell. The part containing the hydrogen atoms, in our case, ammonia borane, serves as a kind of structural support. The catalyst releases the hydrogen from the ammonia borane when needed to use the energy stored within it.

The researchers tested a rhodium-based catalyst that performs this activity fairly easily, but it still has room for improvement. Their initial work showed that the catalyst acts as a ferrode consisting of a core of four rhodium atoms arranged in a tetrahedral structure, or triangular pyramid, with each corner containing a boron atom and another atom. However, the atoms of rhodium and the other elements can arrange themselves in dozens of different configurations in an atom.

These details were not enough to make improvements - the researchers wanted to know the exact structure, out of all the possible structures, of the catalyst, as well as how the various atoms work together to "tear off" the hydrogen atoms from the ammonia borane. For this, the researchers were required to combine experimental and theoretical results, since no single method was successful in isolation.

Initially, the researchers monitored the reaction of the catalyst with ammonia borane using several methods. One of the most important of them was a lesser-known method called XAFS, in which X-ray images of the catalyst can be obtained during its activity. The information from the different types of experiments were like pieces of a puzzle that the researchers had to fit together. To this end, the researchers used computer models to build a theoretical molecular configuration that fits all the information gathered. From these models, the most appropriate structure was obtained for all the information collected. In order to examine the reliability of the obtained structure, the researchers performed computer simulations using the XAFS method of the reaction of this structure with ammonia borane. In the next step they compared the information obtained from these simulations with experimental information collected for the activity of the catalyst. The two datasets matched to a large extent, suggesting that the structure they described closely matched the true structure.

The chemical nature of the structure, together with additional experimental information, allowed the researchers to describe the mechanism of the chemical reaction occurring between the catalyst and the ammonia borane. The findings imply that the active catalyst "harvests" hydrogen from precise sites in the ammonia borane product - nitrogen atoms that carry two hydrogen atoms on them. First, the catalyst abstracts one hydrogen. This is the most complex part of the reaction, notes the lead researcher, and it makes the bonds between the remaining hydrogen and boron atoms unstable. As a result, the atom releases another hydrogen atom to stabilize it, and this atom joins the hydrogen atom that has already been released to obtain hydrogen atom (H2) which is released as a gas, and can be used in engines or fuel cells in the future.

There are still more details that the researchers need to gather, says the lead researcher, but already this research is an important step towards the development of an efficient and inexpensive catalyst.

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