0023-1584/05/4606- © 2005 MAIK “Nauka /Interperiodica” 0891 Kinetics and Catalysis, Vol. 46, No. 6, 2005, pp. 891–899. Translated from Kinetika i Kataliz, Vol. 46, No. 6, 2005, pp. 940–948. Original Russian Text Copyright © 2005 by Larichev, B. Moroz, E. Moroz, Zaikovskii, Yunusov, Kalyuzhnaya, Shur, Bukhtiyarov. INTRODUCTION Ruthenium catalysts promoted with alkali metal compounds are highly active in ammonia synthesis at low pressures (0.1–3.0 MPa) and temperatures (250– 350°ë) [1]. The most active catalysts were prepared with the use of magnesium oxide as a support [2]. The replacement of MgO by alumina, a more practical sub- stance, or various carbon materials noticeably decreased the low-temperature activity of ruthenium in the synthesis of ammonia [3, 4]. It was found that cata- lytic activity is related to the basicity of the support: the more basic the support, the higher the activity of the catalyst in the reaction of ammonia synthesis [4, 5]. Aika et al. [4] explained this correlation by electron- density transfer from the basic support (MgO) to ruthe- nium particles. Because of the appearance of a negative charge on metal particles, the dissociation of N 2 mole- cules is facilitated. This dissociation is the rate-limiting step of ammonia synthesis. This hypothesis was sup- ported experimentally based on the decreased binding energies (E b ) of electrons in the Ru3d core level in Ru/MgO samples (279.2–279.5 eV), as compared with that of the bulk metal. In contrast to this, normal values of the binding energy of ruthenium metal (280.2–280.5 eV) were found in Ru/Al 2 O 3 (SiO 2 ) and Ru/C samples [4, 6, 7]. Although a negative shift of XPS spectra can in fact suggest an excessive electron density on the supported metal because of the interaction of the support with the active component, other reasons, including instrumen- tal factors, can also be responsible for this effect. More- over, this explanation is inconsistent with the low activ- ity of samples based on aluminum oxide promoted with alkalis, although this modified support is highly basic [5, 8]. It is also well known that the activity of ruthenium catalysts for ammonia synthesis depends strongly on the basicity of the promoter and increases in the order Na < K < Rb < Cs [1, 9]. However, there is no direct experimental data on the electron effect of the pro- moter. The occurrence of an interaction between the promoter and the support and the nature of this interac- tion are not understood, although it is obvious that such an interaction can also affect the activity of catalysts. The aim of this work was to study the chemical state of a promoter (a Cs + compound) and the interaction of this promoter with a catalytically active metal (Ru) and a support in the Ru–Cs + /MgO and Ru–Cs + /Al 2 O 3 sys- tems with the use of X-ray diffraction (XRD), transmis- sion electron microscopy (TEM), and X-ray photoelec- tron spectroscopy (XPS). Effect of the Support on the Nature of Metal–Promoter Interactions in Ru–Cs + /MgO and Ru–Cs + –Al 2 O 3 Catalysts for Ammonia Synthesis Yu. V. Larichev*, B. L. Moroz*, E. M. Moroz*, V. I. Zaikovskii*, S. M. Yunusov**, E. S. Kalyuzhnaya**, V. B. Shur**, and V. I. Bukhtiyarov* * Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia ** Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 119991 Russia Received January 26, 2005 Abstract—The Ru–Cs + /MgO and Ru–Cs + /γ-Al 2 O 3 catalysts, which were prepared by an impregnation method using RuOHCl 3 and Cs 2 CO 3 as precursor compounds and reduced with ç 2 at 450°ë, are characterized by X-ray diffraction, high-resolution transmission electron microscopy (with X-ray microanalysis), and X-ray photoelec- tron spectroscopy (XPS). The Cs + /MgO(Al 2 O 3 ) 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 Ru– Cs + /MgO sample, cesium is present as a ës 2 + x O cesium suboxide, which weakly interacts with the support, localized on the surface of Ru particles or near them. In the case of Ru–Cs + /γ-Al 2 O 3 , 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 + /γ-Al 2 O 3 systems in ammonia synthesis at 250–400°ë and atmo- spheric pressure is explained.