Barium-Promoted Ruthenium Catalysts on Yittria-Stabilized Zirconia Supports for Ammonia Synthesis

The cost-effective, small-scale, distributed synthesis of ammonia depends on effective catalysts and processes that operate under modest elevated pressure (i.e., p < 20 bar) conditions. The present paper considers Ru as the active catalyst supported on yittria-stabilized zirconia (YSZ). The addition of alkali and alkaline-earth metal promoters is found to increase synthesis rates by an order of magnitude. The rate enhancement is largely insensitive to the promoter concentration, with Cs outperforming Ba and K by a factor of 2. However, Ba is found to be stable whereas Cs degrades more rapidly, which is attributed to the low melting point of its oxide. At 400 °C and 1.0 MPa, the specific synthesis rate over Ba–Ru is measured to be approximately 1410 mmol g_Ru^–1 h^–1, higher than the most active oxide-supported Ru catalysts reported in the literature. The rate becomes inhibited by H₂ absorption at low temperature (below 350 °C), but lower H₂/N₂ ratios enable the rate to remain comparable to what is observed in stoichiometric mixtures at temperatures below 400 °C. The paper reports a new detailed microkinetic model that accurately captures the observed behavior, revealing that adsorption is coverage dependent. These results provide insight and direction into developing alternatives to Haber–Bosch for distributed synthesis of green ammonia.