The chemical state of ruthenium in the Ru/MgO and Ru-Cs
+/MgO catalysts prepared by the incipient wetnesstechnique with the use of Ru(OH)Cl
3 and Cs
2CO
3 as the catalyst and promoter precursors, respectively, ischaracterized by X-ray photoelectron spectroscopy (XPS). The influence of the final state effects (the differentialcharging and variations of the relaxation energy) on the binding energy of Ru 3d
5/2 core level measured forsupported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parametersdetermined for the Ru/MgO and Ru-Cs
+/MgO catalysts with the corresponding characteristics ofpolycrystalline Ru foil. High-resolution transmission electron microscopy (HRTEM) in combination withenergy dispersive X-ray (EDX) microanalysis reveals that Ru particles in the Ru-Cs
+/MgO catalyst arecovered with an amorphous layer containing cesium. XPS data show that the layer subjected to the prolongedtreatment with H
2 at 450
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C consists of cesium suboxide Cs
2+XO. The shift of the Ru 3d
5/2 binding energytoward lower values is found for the Ru-Cs
+/MgO catalyst (279.7 eV) with respect to metallic Ru (280.2eV) and the Ru/MgO catalyst (280.5 eV). It is assumed that the shift results from a decrease in the workfunction of ruthenium under the action of Cs
+ cations located on the Ru surface. The data obtained are usedto explain the sharp difference in the activities of the Ru/MgO and Ru-Cs
+/MgO catalysts for ammoniasynthesis at 250-400
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C.