<Graphical abstract>

The catalytic decomposition of ammonia (NH₃) offers a carbon-free route for on-demand hydrogen production; however, achieving high activity and durability under moderate temperatures remains challenging. Ru-based catalysts supported on CeO₂ exhibit excellent intrinsic activity, but their performance is limited by insufficient defect density and moderate metal–support interaction. Here, we establish a structure–defect–interaction–performance relationship for Ru catalysts supported on yttria–ceria mixed oxides (Ru/aY-CeO_x) by systematically varying the Y content of the support. Incorporating Y³⁺ into the CeO₂ lattice induces lattice distortion and oxygen vacancies while preserving the fluorite structure up to moderate Y contents. These structural defects strengthen the Ru–O–Ce interfacial bonding, facilitate electron transfer, and enhance the surface basicity of the support. The Ru/50Y-CeO_x catalyst achieves 80% NH₃ conversion at 450 °C with a hydrogen formation rate of 10.8 mmol∙gcat⁻¹∙min⁻¹ and maintains stable operation for 168 h without deactivation. This superior performance is attributed to the optimal defect density that maximizes Ru–support coupling, accelerates recombinative N₂ desorption, and mitigates H₂ poisoning. These findings highlight that tailoring lattice defects in rare-earth oxide supports provides a rational strategy for designing efficient and durable Ru-based catalysts for carbon-free hydrogen production.