Abstract:

The development of highly efficient and stable non-precious metal catalysts under mild conditions is highly desirable for NH₃ synthesis. However, the competitive adsorption of N₂ and H₂ on the single site and strong NH₃ adsorption greatly hinder the catalytic efficiency of catalysts under mild conditions. Herein, we propose the use of responsive alkali metal perrhenates (AMReO₄, AM = K, Na, or Cs) supports with scheelite-type structure that contains active Re metal and promoters to disperse cobalt (Co) species, constructing highly efficient catalysts by regulating the competitive reactant adsorption-activation pattern to a non-competitive mechanism. Our studies demonstrate that Co/KReO₄ catalyst shows excellent catalytic performance for NH₃ synthesis. Co and Re sites synergistically to promote the activation of N₂ molecules, while the adsorption and activation of H₂ primarily occur on Re sites of KReO₄. The presence of Co species facilitates H-spillover that enables the migration of *H species from Re to Co sites, then cascade catalysis of hydrogen and dissociated nitrogen species to form NH₃. Accordingly, the NH₃ synthesis rate of Co/KReO₄ (11.48 mmol_NH3 g_cat^-1 h^-1) is 3.2-fold higher than that of KReO₄ (3.58 mmol_NH3 g_cat^-1 h^-1) at 400 °C and 1 MPa. This work emphasizes the significance of employing reactive supports containing promoters and active metals, in collaboration with non-precious Co sites, to enhance NH₃ synthesis performance under mild conditions.

Key words: Ammonia synthesis, Responsive support, Hydrogen spillover, N₂ activation, Alkali metal perrhenates