The Kinetics of Ammonia Synthesis over Ruthenium-Based Catalysts: The Role of Barium and Cesium

The effect of barium and cesium on the kinetic behavior of Ru/MgO in ammonia synthesis was studied. The activity measurements were performed in a differential reactor at 400°C under a pressure of 6.3 MPa and supplemented with chemisorption measurements and temperature-programmed surface reaction experiments (TPSR). The latter were performed by titrating preadsorbed atomic nitrogen (Nads) with hydrogen. Both promoted systems proved to be much more active in NH3 synthesis than the unpromoted one: Ba-Ru/MgO>Cs-Ru/MgO»Ru/MgO. The kinetic behavior of Ba-Ru/MgO was found to be different from that of Cs-Ru/MgO which was much less sensitive to changes in the ammonia content (xNH3). The dependence of the turnover frequencies (TOF) on xNH3 for Ba-Ru/MgO and Cs-Ru/MgO was found to be analogous to that for Ba-Ru/C and Cs-Ru/C, respectively. For Ba-Ru/MgO and Ba-Ru/C the differences in TOF did not exceed 10 to 30% over the range of xNH3 studied. It is therefore suggested that cesium acts as an electronic promoter, whereas barium plays the role of a structural promoter that controls the concentration of active sites which are most likely B5-type sites, the effect of the support being negligible. The same onset temperature of ammonia formation during the TPSR experiment observed for Ru/MgO and Ba-Ru/MgO supports this hypothesis. Furthermore, a strong increase in activity was observed for Ru/MgO and Ba-Ru/MgO when heating in synthesis gas up to 520°C. This can be attributed to the sintering of very small Ru particles and to the removal of water traces from both the MgO and the Ba + O adlayer. In contrast, the activity of Cs-Ru/MgO decreased significantly after heating at 520°C. Thus, due to the very high activity, very high thermal stability, and absence of methanation problems, barium-promoted ruthenium catalysts supported on magnesia are considered excellent ammonia synthesis catalysts.