Barium hexaaluminate (BHA) with large surface area (49 m(2) g(-1)) is synthesized by microemulsion-mediated synthesis technique and examined as a support promoting ammonia synthesis by particulate Ru. The Ru/BHA catalyst exhibits higher activity and stability than Ba-promoted Ru/MgO, the previously most active oxide-supported catalyst system for ammonia synthesis. The rate of ammonia synthesis over Ru/BHA is 5426 mu mol g(cat)(-1) h(-1) (653 K, 1.1 MPa), more than twice of that of Ba-Ru/MgO, and no decrease in the rate is observed over reaction for 80 h at 653 K and 0.1 MPa. Characterization of this catalyst system by various techniques reveals that the electronic and structural interactions between BHA and Ru particles are key factors responsible for the elevated activity and stability of this catalyst. (C) 2007 Elsevier Inc. All rights reserved.