Atomically Dispersed Ru Catalyst for Low-Temperature Nitrogen Activation to Ammonia via an Associative Mechanism

The industrial synthesis of NH3 using Fe- or Ru-based catalysts usually requires harsh reaction conditions. It is desirable to develop catalysts that perform well at low temperature and pressure (250-400 degrees C, <2 MPa). The main challenge of low-temperature NH3 synthesis is the dissociation of the extremely stable N N triple bond. Herein, we report the design of homogeneous single-atom Ru centers on an H-ZMS-5 (HZ) support with the Ru atoms individually anchored in the micropores of HZ, effectively boosting NH3 synthesis under mild conditions via an associative pathway. Synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and in situ DRIFTS analyses show that =N- groups are the primary intermediates, and DFT calculations further show that, unlike Ru nanoclusters, the cooperation of a single Ru atom and hydrogen species in HZ leads to N-2 hydrogenation rather than direct N-2 dissociation, and the indirect N-N bond dissociation occurs much more easily via the formation of the NHNH3* intermediate; the energy barrier for breaking the N-N bond keeps falling from 2.90 eV for *N-2 to 0.04 eV for *NHNH3, showing that N-2 hydrogenation is an effective way for sharp weakening of N-N bonds. Moreover, the rate-determining step is shifted from the dissociation of the N N triple bond to the formation of *N2H2. As a consequence, the single-atom 0.2 wt % Ru/H-ZSM-5 catalyst shows the highest NH3 synthesis rate per gram of Ru (1.26 mol(NH3) gRu(-1) h(-1) at 300 degrees C and 1 MPa) among the Ru-based catalysts ever reported.