Operando spectroscopic and isotopic-label-directed observation of LaN-promoted Ru/ZrH2 catalyst for ammonia synthesis via associative and chemical looping route.

• 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]