The researchers from Kumamoto University and National Institute of Technology from Japan together with their colleagues from North-West University in South Africa (HySA Infrastructure) performed a computer study of the processes underlying the hydrogen safety. To do that they used a modern simulation method based on machine learning – a universal neural network potential. It allows for describing chemical reactions on the surface of metals almost with quantum-chemical accuracy, but much quicker and with lower calculation costs.
Hydrogen is believed to be one of the most progressive eco-friendly fuels, but it has its own drawbacks. Он is readily ignited fuel and in case of a leak it can form explosive mixes with ambient air. To prevent explosions in the premises where hydrogen is used or stored (e.g., in garages with fuel cell vehicles), specialized equipment is used – passive autocatalytic recombiners (PARs). They contain catalyst (and most often it is expensive platinum), which assures safe combination reaction between hydrogen and oxygen from the ambient air forming water. The task for the scientists was to better understand the mechanisms of this reaction at the atomic level and to find cheaper alternatives to platinum without compromising the performance.
The researchers decided to simulate the key reaction – recombining hydrogen and oxygen – on the surface of nano particles of five metals: palladium, platinum, copper, silver and gold. Instead of studying static “frozen” structures, as it is often down in calculations, they used the universal neural network potential-driven molecular dynamics (UNNP-MD). This allowed for observing in the real-time mode under the temperature of circa 500 К (about 230 °C) and high pressure the interaction of gas molecules with the catalyst surface, their absorption, movement and participation in chemical reactions.
The simulation identified the differences in the behavior of metals and helped to understand why platinum is still considered “the golden standard”. Oxygen molecules are fixed rather firmly on its surface, but not very strongly, hence, they do not dissociate into atoms right away. At the same time, hydrogen atoms emerging from H₂ molecules remain labile and can quickly interact with oxygen. In the process of the calculations the scientists succeeded in tracking the main route of the reaction on the platinum surface: O₂ molecule is combined with hydrogen atom forming an intermediate (OOH), which then turns into water (H₂O). The scientists were able to observe this process directly during the simulation.
Other metals turned out less effective due to non-optimal combination of properties. Palladium surface captures both hydrogen and oxygen too strongly, hence, it is more difficult for them to interact. On the contrary, copper is too active in breaking down oxygen molecules and gets quickly covered by atomic oxygen, which inhibits the reaction with hydrogen. Silver and gold in general demonstrate inert behavior and poorly hold chemical agents on their surface.
Thus, the team of researchers proved that efficient catalyst for safe “burning” of hydrogen needs to combine two properties: firm retention of molecular oxygen and high mobility of hydrogen atoms on the surface. Platinum has exactly such balance, which makes it the most efficient. At the same time, understanding such mechanism helps to create new materials. For example, in order to decrease the cost it is possible to develop alloys combing small amount of platinum with more affordable materials, but maintaining the required interaction with chemical agents. Another way is to modify palladium adding inert elements to make its fixation of hydrogen not as strong.
