Effect of the support on the nature of metal-promoter interactions in Ru-Cs+/MgO and Ru-Cs+-Al2O3 catalysts for ammonia synthesis

The Ru-Cs+/MgO and Ru-Cs+/gamma-Al2O3 Catalysts, which were prepared by an impregnation method using RuOHCl3 and CS2CO3 as precursor compounds and reduced with H-2 at 450 degrees C, are characterized by X-ray diffraction, high-resolution transmission electron microscopy (with X-ray microanalysis), and X-ray photoelectron spectroscopy (XPS). The Cs+/MgO(Al2O3) Systems, Ru-Cs+ black, and model systems prepared by cesium sputtering onto polycrystalline ruthenium foil are studied as reference samples. It is found that, in the RuCs+/MgO sample, cesium is present as a CS2+xO Cesium suboxide, which weakly interacts with the support, localized on the surface of Ru particles or near them. In the case of Ru-Cs+/gamma-Al2O3, cesium occurs as a species that is tightly bound to the support; this is likely surface cesium aluminate, which prevents promoter migration to Ru particles. The Ru-Cs/MgO sample exhibits a considerable shift of the Ru3d line in the XPS spectra toward lower binding energies, as compared to the bulk metal. It is hypothesized that this shift is due to a decrease in the electron work function from the surface of ruthenium because of the polarizing effect of Cs+ ions in contact with Ru particles. Based on the experimental results, the great difference between the catalytic activities of the Ru-Cs+/MgO and Ru-Cs+/gamma-Al2O3 systems in ammonia synthesis at 250-400 degrees C and atmospheric pressure is explained.