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[Display omitted] • A class of Ru-Co single-atom alloy catalysts (marked as Ru x Co 1 SAA, x stands for Ru/Co molar ratio) were prepared. • The electronic structure of Ru x Co 1 SAA could be effectively tuned by regulating the Ru/Co molar ratio. • A moderate ratio of Ru/Co (1.7:1) favors superior NH 3 synthesis rate. • The excess Ru destroys the SAA structure, resulting in the diminution of SAA advantages. Rational design of efficient catalysts for ammonia (NH 3) synthesis at mild conditions is of paramount importance for the development of electrolysis driven Haber-Bosch (e HB) 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 NH 3 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 Ru x Co 1 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 Ru x Co 1 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 NH 3 synthesis rate and low activation energy. Using a suite of elaborate characterizations, we have observed that the outstanding performance over Ru 1.7 Co 1 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 NH 3 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, N 2 activation behavior over Ru 2 Co 1 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 NH 3 synthesis rate enhancement. [ABSTRACT FROM AUTHOR]

• A dynamic N- and H-involved cyclic reaction pathway proposed for NH 3 production. • A dual-path mechanism including chemical looping and thermal catalysis illustrated. • Bottleneck of direct N N dissociation bypassed. The synthesis of ammonia via the Haber-Bosch process using Fe-derived catalysts requires harsh reaction conditions. It is hence meaningful to develop catalysts for low-temperature synthesis of ammonia for industrial application. Herein for the first time, we report that with the synergy between LaN and Ru/ZrH 2 , NH 3 can be synthesized via a dual-path mechanism. The LaN-promoted Ru/ZrH 2 catalyst shows exceptionally high NH 3 synthesis rate (up to 305 mmol NH3 g Ru −1 h−1) at 350 °C under 1 MPa and remarkable durability (tested for 200 h). The outcomes of isothermal surface reaction and a suite of 15N 2 and D 2 isotopic labeling experiments reveal that the N3− of LaN reacts with H− ions to produce NH 3 , leaving behind N and H vacancies. The initial state of Ru/xLaN/ZrH 2 can be restored by having the N and H entities replenished under the atmosphere adopted for NH 3 synthesis. Moreover, based on the results of N 2 reaction order, nitrogen K-edge NEXAFS, in situ XPS as well as in situ DRIFTS analyses under a 25%N 2 -75%D 2 atmosphere, it is reckoned that the direct dissociation of N 2 does not occur on the LaN-promoted Ru/ZrH 2 catalyst while N 2 hydrogenation takes place via an associative pathway under mild conditions. For the hydrogenation of N 2 , an appropriate amount of LaN would induce a synergic effect on the Ru active sites, leading to facile activation and hydrogenation of N 2 to *N 2 H 2. Nonetheless, an excess amount of LaN would result in blocking of Ru sites, consequently hindering the formation of *N 2 H 2 and decreasing the catalytic activity. Following the associative and chemical looping pathways, the LaN-promoted Ru/ZrH 2 catalyst bypasses the bottleneck of N 2 direct dissociation, making the synthesis of NH 3 at mild conditions possible. [ABSTRACT FROM AUTHOR]