Defective g-C3N4 supported Ru3 single-cluster catalyst for ammonia synthesis through parallel reaction pathways

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/Nv-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/Nv-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/Nv-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/Nv-g-C3N4 catalyst shows a much lower catalytic activity than Ru3/Nv-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).