Catalytic properties and structural evolution of TiFe2 intermetallic catalysts for ammonia decomposition and effects of third-element substitution

Ammonia is a promising hydrogen carrier. However, the development of noble-metal-free catalysts for ammonia decomposition remains a major challenge. In this study, we found that TiFe 2 exhibited the highest activity among various intermetallic compounds consisting of 3d transition metals. The catalytic behavior was closely related to reaction-induced nitridation and associated structural evolution under an ammonia atmosphere. TiFe 2 was gradually nitrided, forming TiN and iron nitride phases. Significant deactivation occurred at low temperatures due to excessive nitridation of iron, whereas it reached an active and stable state at higher temperatures. Partial substitution of Fe with Mn enhanced the catalytic performance. Mn substitution suppressed the formation of highly nitrided and less active Fe 3 N and stabilized a composite nitrided state consisting of Fe 4 N and (Ti,Mn)N. These findings demonstrate that catalytic performance can be enhanced by controlling reaction-induced nitridation and structural evolution through third-element substitution. • TiFe 2 was the most active among 3d-transition-metal intermetallic catalysts. • Catalytic properties were governed by structural evolution via nitridation. • Catalytic activity reached a steady state after completion of nitridation. • Mn substitution enhanced activity by suppressing excessive nitridation. • A Fe 4 N/(Ti,Mn)N composite state resulted in high activity and durability.