On Optimal Barium Promoter Content in a Cobalt Catalyst for Ammonia Synthesis

High priority in developing an efficient cobalt catalyst for ammonia synthesis involves optimizing its composition in terms of the content of promoters. In this work, a series of cobalt catalysts doubly promoted with cerium and barium was prepared and tested in ammonia synthesis (H-2/N-2 = 3, 6.3 MPa, 400 degrees C). Barium content was studied in the range of 0-2.6 mmol g(Co)(-1). Detailed characterization studies by nitrogen physisorption, SEM-EDX, XRPD, H-2-TPR, and H-2-TPD showed the impact of barium loading in CoCeBa catalysts on the physicochemical properties and activity of the catalysts. The most pronounced effect was observed in the development of the active phase surface, a differentiation of weakly and strongly binding sites on the catalyst surface and changes in cobalt surface activity (TOF). Barium content in the range of 1.1-1.6 mmol g(Co)(-1) leads to obtaining a catalyst with the most favorable properties. Its excellent catalytic performance is ascribed to the appropriate Ba/Ce molar ratio, i.e., greater than unity, which results in not only a structural promotion of barium, but also a modifying action associated with the in-situ formation of the BaCeO3 phase.