The researchers of North-West University in South Africa, Durban University of Technology, and Nelson Mandela University have presented an advanced platinum catalyst for hydrogen safety applications. It is designed for passive autocatalytic recombiners, which automatically and without an external power convert hazardous hydrogen into a harmless water vapor, preventing the risk of explosion. This is especially important for the locations with possible hydrogen leaks, such as hydrogen car garages, underground parking lots, and coal mines.
The primary goal of the scientists was improvement of the classic platinum catalyst based on aluminum oxide (Pt/Al₂O₃). Platinum effectively initiates the hydrogen-oxygen reaction but over time, its tiny particles can clump together and lose activity, especially at high temperatures. To address this issue, the team proposed to modify the catalyst support by zinc doping. The zinc was supposed to alter the surface microstructure, improve platinum distribution and slow its degradation.
To test their hypothesis, the researchers prepared several catalyst variants: a standard Pt/Al₂O₃ catalyst and samples with 1, 3, and 10wt% of zinc. First, they studied how the temperature of treatment in a hydrogen atmosphere affected the size of the platinum particles. The experiments showed that a temperature of approximately 350°C was optimal. At this temperature, platinum formed very small particles, approximately 1 nanometer in size, uniformly distributed over the support surface. At higher temperatures, especially above 800°C, the particles began to actively grow and aggregate, leading to a decrease in catalytic activity.
Then the scientists moved on to analyzing the role of zinc. Using electron microscopy, X-ray diffraction analysis, and chemisorption methods, they studied, in detail, how the catalyst structure changed with varying doping. It was found out that the changes were minimal at 1% zinc: the support surface barely changed, and platinum dispersion even decreased slightly. A completely different picture is observed with a zinc content of approximately 3%. In this case, the zinc is uniformly distributed over an aluminum oxide surface, forming a more developed and stable structure. This not only improves adhesion of platinum nanoparticles but also increases the specific surface area of the catalyst. When 10% zinc is added, the effect becomes negative: the zinc begins to form clusters and separate phases, which degrades the support structure and leads to an uneven distribution of platinum.
A decisive test of catalytic activity and stability during a 500-hour continuous reaction was crucial. The catalyst with 3% zinc demonstrated the best results: it reached the reaction temperature of approximately 275°C and maintained this temperature throughout the test, without any signs of deactivation. Moreover, even after one-month storage in the air, which simulates real pauses in equipment operation, this catalyst remained fully active. At the same time, the standard samples Pt/Al₂O₃ and the samples with 1% and 10% zinc were gradually losing efficiency, and the platinum particles in them became noticeably larger.
Finally, the researchers tested the optimal Pt/Zn-Al₂O₃ catalyst with 3% zinc as part of a prototype of a real passive recombiner. The device operation was stable with hydrogen-air mixtures of varying concentrations at high gas flow rates. Hydrogen conversion reached 90%, significantly higher than the typical values for conventional platinum catalysts, which typically achieve around 70-75%.
As a result, the researchers have demonstrated how fine-tuning of the nanostructure can fundamentally alter the catalyst behavior and make it a reliable element of the safety applications, bringing us closer to the point where hydrogen technologies can become a part of everyday infrastructure, without increased hazards.
