Robust Ru Single-Atom Alloy Catalysts Coupled with Adjacent Fe-Site for Highly Stable Ammonia Synthesis Under Mild Conditions

Abstract

In our pursuit of an efficient catalyst for ammonia production, we developed ruthenium based single atom alloy catalysts on a layered double hydroxide-derived support. The extended X-ray absorption fine structure studies provided evidence of single Ru atoms as a Fe-Ru alloy. High-resolution transmission electron microscopy showcased a larger particle size with higher Ru loading, emphasizing the role of Ru site geometry in catalytic activity. The MgFeOx-0.1Ru catalyst, with optimal Ru dispersion and smaller Fe-Ru particle size (1.6 nm), outperformed other catalysts in NH3 synthesis and demonstrated exceptional stability. Remarkably, the catalyst with 0.1 wt.% Ru exhibited superior performance, achieving an exceptional NH3 formation rate of 17,897 µmol g−1 h−1 (at 400 °C, and 5 MPa) along with maintaining a consistent NH3 synthesis rate of 7,217 µmol g−1 h−1 for a notable duration of 150 h. Our first-principles calculations show that Ru weakened the binding of both molecular and atomic nitrogen on the catalyst's surface, facilitating the desorption of N-intermediates. The optimized MgFeOx-0.1Ru catalyst composition with characteristics such as small Fe-Ru alloy particle size and the presence of all active Ru sites on the surface improves lifetime, reducing costs and marking a significant stride towards sustainable and economically viable NH3 production.

Keywords: single atom alloy catalyst, layered double hydroxide-derived support, ammonia synthesis, Mössbauer spectroscopy, Synchrotron EXAFS, DFT