Decoupling Kinetically Coupled Steps via Hierarchical Relay Catalysis on High-Entropy Alloy for Efficient Ammonia...

Ammonia (NH₃) is a promising hydrogen carrier owing to its high gravimetric hydrogen storage density (17.6 wt%), but its practical utilization is hindered by sluggish decomposition kinetics at moderate temperatures. For commonly used Ru-based catalysts, the reaction remains constrained by kinetically coupled elementary steps, including both N-H bond activation and inhibited H desorption. Herein, we propose a hierarchical relay catalysis strategy enabled by a RuNiCoFeMo high-entropy-alloy (HEA) catalyst, wherein distinct metallic centers are rationally integrated to regulate individual elementary steps within a single catalytic framework. Specifically, NiCoFe sites preferentially promote sequential N-H bond scission, while Mo incorporation electronically modulates the alloy to weaken metal-H interactions and suppress hydrogen poisoning, thereby breaking scaling constraints across multiple elementary steps. Consequently, the optimized catalyst exhibits ca. 56% and 54% higher NH₃ and H₂ reaction orders relative to the monometallic Ru, respectively, indicating enhanced NH₃ activation and alleviated hydrogen poisoning. These kinetic benefits translate into a threefold increase in NH₃ conversion and H₂ production, reaching ca. 80% NH₃ conversion at 450°C, which is among the highest activities reported for Ru-based catalysts. This work provides a general design principle for catalyst design in complex multistep reactions beyond NH₃ decomposition.