Ammonia plays a crucial role in agriculture and chemical engineering, and acts as a promising carbon-free transportation fuel. Catalysts design is deemed as a key to solve the restriction of energy-intensive Haber–Bosch process of ammonia production. With the development of computational modeling, computation-aided catalyst design serves as one important driving force for material innovation, saving a lot of experimental efforts based on trial and error. Computational modeling not only provides fundamental mechanistic insights into the reaction with great details regarding adsorbate geometries, electronic structures, and elementary-step energies, but also expedites the material discovery with descriptor-based catalyst design, core of which is the establishment of thermo/kinetic scaling relations. In this review, we present firstly the mechanistic understanding of ammonia synthesis and transition state scaling relations developed on pure transition-metal catalysts. We then summarize catalysts design strategies guided by alloy, size, and magnetic effects with the goal of breaking the limitations set by scaling relations to achieve better catalytic performance. Finally, future opportunities and challenges associated with computation design of optimal catalysts for ammonia synthesis are outlined.

Designing catalyst to achieve ammonia synthesis at mild conditions is a meaningful challenge in catalysis community. Defective g-C₃N₄ nanosheet supported single-cluster ruthenium and iron catalysts were investigated for their ammonia synthesis performance. Based on density functional theory (DFT) calculations and microkinetic simulations, Ru₃ single-cluster anchored on defective g-C₃N₄ nanosheet (Ru₃/N_v-g-C₃N₄) has a turnover frequency (TOF) 5.8 times higher than the Ru(0001) step surface at industrial reaction conditions of 673 K and 100 bar for ammonia synthesis. In other words, similar TOFs could be achieved on Ru₃/N_v-g-C₃N₄ at much milder conditions (623 K, 30 bar) than on Ru(0001) (673 K, 100 bar). Our computations reveal the reaction proceeds parallelly on Ru₃/N_v-g-C₃N₄ through both dissociative and alternative associative mechanisms at typical reaction conditions (600–700 K, 10–100 bar); N–N bond cleavage of *N₂ and *NNH from the two respective pathways controls the reaction collectively. With increasing temperatures or decreasing pressures, the dissociative mechanism gradually prevails and associative mechanism recedes. In comparison, Fe₃/N_v-g-C₃N₄ catalyst shows a much lower catalytic activity than Ru₃/N_v-g-C₃N₄ by two orders of magnitude and the reaction occurs solely through the dissociative pathway. The finding provides a prospective candidate and deepens the mechanistic understanding for ammonia synthesis catalyzed by single-cluster catalysts (SCCs).