Plasma-derived hydrogen radical-mediated N2 activation for mild ammonia synthesis: insights into the importance of oxygen vacancies in the reaction mechanism

Abstract

Plasma-catalytic NH₃ synthesis has recently been recognized as a complementary route to the Haber–Bosch process for decentralized NH₃ production. However, the activation of N₂ in current plasma catalysis studies is still occurs through the conventional heterogeneous reaction mechanism on catalyst surfaces, which does not take full advantage of the highly reactive species generated in the plasma phase, resulting in high energy consumption and low reaction rate. Here, we present a distinctive hydrogen radical-mediated N₂ activation pathway that extricates itself from the conventional catalytic pathways. In this work, a considerable number of gaseous hydrogen radicals were generated on an oxygen-deficient CeO₂/CuO catalyst via the excitation of plasma and served as strong reducing agents and immediate hydrogen sources to reduce N₂ molecules with an ultralow energy barrier of 0.123 eV. Oxygen vacancies on catalysts can further accelerate the catalytic cycling of adsorbed H₂ to desorbed hydrogen radicals. With these strategies, we achieved a superhigh NH₃ yield of 196.2 mg h⁻¹ g_cat.⁻¹ under mild conditions. Our results illustrate the potential of exploiting plasma-derived hydrogen radicals as ideal and homogeneous activation agents for inert gas molecules and introduce a design strategy for catalysts that utilize oxygen vacancies to assist hydrogen radical generation.