Mechanistic Study on Enhanced Electrocatalytic Nitrogen Reduction Reaction by Mo Single Clusters Supported on MoS2

The electrocatalytic N₂ reduction reaction (eNRR) at ambient conditions is an appealing method for NH₃ synthesis. It has attracted broad research interest in eNRR catalysts. In this work, by a theoretical study based on density functional calculations, we attributed the higher eNRR activity of defective MoS₂ than pure MoS₂ to the exposed Mo atom with unsaturated coordination sites in the interlayer of defective MoS₂. The finding inspired us to explore the eNRR performance of Mo single atom/clusters with one/more active Mo sites supported on MoS₂ [Mo_n@MoS₂ (n = 1∼11)] and the corresponding catalytic mechanism. All considered Mo_n@MoS₂ irrespective of N₂ or H adsorption selectivity can achieve higher eNRR activity with lower overpotential and lower NH₃ desorption free energy than defective MoS₂. The competitive hydrogen evolution reaction can be well suppressed on Mo_n@MoS₂ when n = 2∼10. In particular, Mo₉@MoS₂ with N₂ adsorption selectivity exhibits excellent eNRR activity (η = 0.19 V) and high eNRR selectivity, and it can efficiently desorb the produced NH₃ with a low desorption free energy (0.50 eV) to achieve a high ammonia yield with the aid of the produced ammonia molecule in the first eNRR process, which is coadsorbed on the Mo₉ single cluster during the later eNRR process. The high eNRR activity of Mo_n@MoS₂ can be attributed to its inherent properties of excellent electrical conductivity, electron accessibility, and multiple exposed Mo active sites available for N-containing species coadsorption. The results demonstrate the significance of H preadsorption, the additional N₂ adsorption, and the adsorbed product ammonia in the prior eNRR process in enhancing the overall eNRR performance of different-size single-cluster catalysts. Our work provides a guidance for future study of single-cluster catalysts.