Multi-Stage Catalyst to Prevent Hydrogen Explosions in Liquefied Hydrogen Leakage and Self-Ignition by Reaction Heat

Hydrogen is gaining global attention as a clean energy carrier due to its potential to contribute to carbon neutrality. Among various forms, liquefied hydrogen (LH2) offers significant advantages in energy density and storage efficiency. In contrast, when LH2 leaks, extremely low-temperature and high-velocity hydrogen is released, posing serious safety challenges. One promising safety measure is the passive autocatalytic recombiner (PAR), which catalytically converts leaking hydrogen and atmospheric oxygen into water without external power. This study aims to enhance the safety performance of PAR systems. To achieve this, the thermal behavior of multilayer hydrogen oxidation catalysts was investigated under demanding forced-flow hydrogen release conditions at room temperature. Since hydrogen oxidation is a strongly exothermic reaction, localized reactions can cause rapid temperature increases, potentially leading to self-ignition. Here, self-ignition is also called auto-ignition or spontaneous ignition. At temperatures above 500°C, ignition occurs without an external source of ignition, such as a flame or spark. To mitigate this risk, it is essential to spatially distribute the reaction and control heat generation. As a countermeasure, catalyst configurations were designed with variations in precious metal loading and material properties across layers. The rear layer was loaded with a higher concentration of precious metals to respond to low hydrogen concentrations during the early stages. In contrast, the front layer was designed with reduced precious metal content or iron-based materials, allowing it to react only under higher hydrogen concentrations. Configurations where Fe was selectively placed in the middle or front catalysts showed favorable behavior. The Fe was activated under high-temperature conditions by receiving reaction heat from the rear layer. This contributed to sustaining the reaction and controlling heat generation, even without using precious metals. Furthermore, in configurations where Pt and Fe were supported sequentially and separated as distinct phases, Fe effectively acted as a reaction promoter. This structure facilitated stepwise reaction progress and contributed to better heat distribution. These findings indicate that spatial control of reactivity and appropriate material selection are key to preventing thermal runaway.