The catalytic activities of three supported, barium-promoted ruthenium catalysts for ammonia synthesis are reported. The three supports are silicon nitride (Si3N4), magnesium aluminum spinel (MgAl2O4), and graphitized carbon (C). The effect of the promoter on the activity is strongly dependent on the choice of support material in accordance with several previous observations. Here, this dependence is ascribed to a difference in the affinity of the promoter for the different supports. It is shown how it is possible to image the barium promoter present on the surface of ruthenium crystals in passivated catalysts by conventional high-resolution transmission electron microscopy (HRTEM). By comparison with in situ HRTEM images obtained lately from similar catalysts, and with reference to recent density functional theory (DFT) calculations, we suggest that active B-5-type sites on the surfaces of the ruthenium crystals are promoted by nearby promoter atoms via electrostatic interactions.