Activating mild-condition ammonia synthesis via oxygen vacancy engineering in nitrogen-doped praseodymium oxide

Ruthenium (Ru)-based catalysts are among the most promising systems for ammonia (NH₃) synthesis under mild conditions, where the catalyst support plays a decisive role in modulating Ru activity. In this work, we report nitrogen-doped praseodymium oxide (PrO_xN_y), synthesized through a urea-assisted hydrothermal method, as a highly efficient support for Ru in atmospheric-pressure NH₃ synthesis. Structural, spectroscopic, and isotopic tracing analyses reveal that, unlike reactive rare-earth nitrides or hydronitrides that participate in ammonia synthesis via chemical looping, the incorporated nitrogen atoms in PrO_xN_y do not directly engage in the NH₃ synthesis cycle. Instead, nitrogen functions as a “defect inducer”—promoting the formation of abundant oxygen vacancies (OVs). These OVs act as strong electron donors to Ru nanoparticles, enhancing Ru electron density to promote the N₂ activation and helps migrate hydrogen poisoning. Compared to nitrogen-free Ru/PrO_x, Ru/PrO_xN_y reduces the activation energy from 91 to 84 kJ/mol, achieving an NH₃ yield of 4195 μmol g⁻¹ h⁻¹ (twice higher) at 0.1 MPa and 400 °C, with long-term stability over 100 h. Notably, Pr-based oxides exhibit unique advantages in vacancy engineering due to their favorable OV formation energetics, outperforming Ce- and La-based analogues. This study demonstrates a generalizable strategy for activating rare-earth oxide supports via heteroatom-induced vacancy engineering, clarifying the role of nitrogen in oxide supports and offering new insights into the rational design of high-performance Ru catalysts for sustainable ammonia synthesis.