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

Abstract

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 °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 N2 hydrogenation rather than direct N2 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 *N2 to 0.04 eV for *NHNH3, showing that N2 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 molNH3 gRu–1 h–1 at 300 °C and 1 MPa) among the Ru-based catalysts ever reported.