Ru/La_0.5Pr_0.5O_1.75 Catalyst for Low-Temperature Ammonia Synthesis

To exploit the use of hydrogen as a source of sustainable energy, development of an efficient process for synthesizing an energy carrier such as ammonia under mild conditions will be necessary. Here, we show that Ru/La_0.5Pr_0.5O_1.75 prereduced at an extraordinary high temperature of 650 °C catalyzes high NH₃-synthesis rates under mild conditions. At 400 °C under 1.0 MPa, the synthesis rate was comparable with that of most active oxide-supported Ru catalysts. Kinetic analysis revealed that hydrogen poisoning, which is a typical drawback for oxide-supported Ru catalysts such as Cs⁺/Ru/MgO, was effectively suppressed over Ru/La_0.5Pr_0.5O_1.75. The high activity induced by high-temperature reduction was attributable to the good thermal stability of the support and a phase change of the La_0.5Pr_0.5O_1.75 support during prereduction. Fourier transform-infrared spectroscopy measurements after N₂ adsorption on the catalyst revealed that electrons were efficiently donated from trigonal La_0.5Pr_0.5O_1.5 to the antibonding π orbital of the N≡N bond of N₂ via Ru atoms. Cleavage of the N≡N bond, the rate-determining step for ammonia synthesis, was thus accelerated. Our results expand the range of possibilities for developing more effective ammonia synthesis catalysts under mild conditions. Such catalysts will be needed to enable development of hydrogen-based sustainable energy resources.