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 [Mon@MoS2 (n = 1∼11)] and the corresponding catalytic mechanism. All considered Mon@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 Mon@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 Mon@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.