Construction of Active Centres and their Contributions to Mechanisms during Thermocatalytic Ammonia Decomposition to Generate Hydrogen

The global transition from a coal-based energy economy to a green hydrogen economy, together with the demand for energy utilization efficiency, has intensified interest in ammonia as a carbon-free hydrogen energy carrier which is capable of storing and transporting renewable energy at scale. Efficient regeneration of hydrogen through thermal ammonia decomposition, enabled by advanced downstream technologies such as membrane reactors and purification systems, offers a techno-economically mature and evolving pathway. However, achieving low-temperature, energy-efficient ammonia decomposition remains a fundamental challenge. At the heart of this challenge lies the rational design of heterogeneous thermocatalysts capable of overcoming intrinsic kinetic limitations. Through critical examination of the vast research on catalyst systems and activity studies, we identify that tailored metal-support systems often give rise to multiple reaction pathways that govern the overall kinetics. To scientifically elucidate the origins of enhanced catalytic performance, the precise understandings on the nature of active sites and their coordination environments is the core. This requires precise identification of catalytically relevant sites, rigorous correlation between structure and reactivity, and operando-level insights into dynamic phase evolution. In this review, we reframe ammonia decomposition catalysis through the lens of active-phase chemistry. Based on our current understanding of active centres across different catalyst categories, we highlight strategies for rational catalyst design grounded in active phases and coordination environment. We further discuss advanced characterization methodologies capable of tracking active sites and unravelling their specific mechanistic contributions.