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

Abstract

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/ZrH2, NH3 can be synthesized via a dual-path mechanism. The LaN-promoted Ru/ZrH2 catalyst shows exceptionally high NH3 synthesis rate (up to 305 mmolNH3 gRu−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 15N2 and D2 isotopic labeling experiments reveal that the N3− of LaN reacts with H− ions to produce NH3, leaving behind N and H vacancies. The initial state of Ru/xLaN/ZrH2 can be restored by having the N and H entities replenished under the atmosphere adopted for NH3 synthesis. Moreover, based on the results of N2 reaction order, nitrogen K-edge NEXAFS, in situ XPS as well as in situ DRIFTS analyses under a 25%N2-75%D2 atmosphere, it is reckoned that the direct dissociation of N2 does not occur on the LaN-promoted Ru/ZrH2 catalyst while N2 hydrogenation takes place via an associative pathway under mild conditions. For the hydrogenation of N2, an appropriate amount of LaN would induce a synergic effect on the Ru active sites, leading to facile activation and hydrogenation of N2 to *N2H2. Nonetheless, an excess amount of LaN would result in blocking of Ru sites, consequently hindering the formation of *N2H2 and decreasing the catalytic activity. Following the associative and chemical looping pathways, the LaN-promoted Ru/ZrH2 catalyst bypasses the bottleneck of N2 direct dissociation, making the synthesis of NH3 at mild conditions possible.