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 gRu-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.