Insight into the critical role of strong interaction between Ru and Co in RuCo single-atom alloy structure for significant enhancement of ammonia synthesis performance

Rational design of efficient catalysts for ammonia (NH3) synthesis at mild conditions is of paramount importance for the development of electrolysis driven Haber-Bosch (eHB) process. Ammonia synthesis is a structure sensitive reaction, and any small change of electronic structure could result in dramatic change of catalytic activity, especially for the Ru-based NH3 synthesis catalysts. Electronic structure modulation of the Ru-based catalysts could provide desirable catalytic activity. Herein, we explore a class of Ru-Co single-atom alloy catalysts (marked as RuxCo1 SAA, x stands for Ru/Co molar ratio), in which Co single atoms are located on Ru nanoclusters to form Co-Ru coordination, and then electronic structure of RuxCo1 SAA could be effectively tuned by regulating the Ru/Co molar ratio. Our studies show that a moderate ratio of Ru/Co (1.7:1) favors superior NH3 synthesis rate and low activation energy. Using a suite of elaborate characterizations, we have observed that the outstanding performance over Ru1.7Co1 SAA can be attributed to the electron redistribution between Ru and Co atom, resulting in an upshift of d band center and lowering of work function. However, there is decrease of NH3 synthesis rate when Ru/Co molar ratio is 2, due to the partial detachment of Ru entities from SAA structure, and the formed individual Ru nanoparticles (NPs) overlay the surface of catalyst. In such a case, N2 activation behavior over Ru2Co1 is alike that of Ru NPs catalysts. The present study demonstrates that the Ru/Co ratio in SAA structure can effectively regulate the electronic structure of catalyst for NH3 synthesis rate enhancement.