Self-Promoting NO electrochemical reduction via N-N coupling by Surface-Adsorbed *NH intermediates on Mo2C nanosheets

Electrochemical reduction of NO (NORR) to ammonia has gained significant attention due to its potential in both electrocatalytic denitrification and ammonia synthesis. Transition-metal carbides (TMCs), with their high reactivity toward NO activation, have emerged as promising candidates for NORR electrocatalysts. However, their exceptional catalytic performance in the hydrogen evolution reaction (HER)—a major competing process—may hinder the Faradaic efficiency of NH₃ production. Interestingly, recent experimental studies have shown that NORR almost completely suppresses HER on Mo₂C nanosheets under typical HER operating conditions, but the underlying mechanism remains unclear. Here, using state-of-the-art grand canonical density functional theory calculations, we reveal the atomic-level reaction mechanism of NORR on TMCs by taking experimentally reported Mo₂C as a prototype. Our findings show that within the potential range of −0.3 V to 0 V vs. RHE, the Mo₂C surface becomes nitrogenated by a monolayer of chemisorbed *NH intermediates, leading to poisoning of the surface. However, these adsorbed surface *NH intermediates can effectively adsorb NO molecules through an N-N coupling mechanism, facilitating their electrochemical reduction to NH₃ with fast reaction kinetics and favorable thermodynamics, thereby showing a self-promoting catalytic mechanism. Importantly, these *NH intermediates significantly suppress HER through strong electrostatic repulsion with incoming protons (or adjacent *NH species), leading to barriers exceeding 2 eV for both Heyrovsky and Tafel reactions. Our calculations provide crucial insights into the decisive role of *NH intermediates in promoting NH₃ production while suppressing H₂ generation, thus providing valuable guidance for the rational design of TMC-based NORR electrocatalysts.