XPS and TEM Studies on the Role of the Support and Alkali Promoter in Ru/MgO and Ru−Cs⁺/MgO Catalysts for Ammonia Synthesis

Abstract

The chemical state of ruthenium in the Ru/MgO and Ru−Cs+/MgO catalysts prepared by the incipient wetness technique with the use of Ru(OH)Cl3 and Cs2CO3 as the catalyst and promoter precursors, respectively, is characterized by X-ray photoelectron spectroscopy (XPS). The influence of the final state effects (the differential charging and variations of the relaxation energy) on the binding energy of Ru 3d5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/MgO and Ru−Cs+/MgO catalysts with the corresponding characteristics of polycrystalline Ru foil. High-resolution transmission electron microscopy (HRTEM) in combination with energy dispersive X-ray (EDX) microanalysis reveals that Ru particles in the Ru−Cs+/MgO catalyst are covered with an amorphous layer containing cesium. XPS data show that the layer subjected to the prolonged treatment with H2 at 450 °C consists of cesium suboxide Cs2+XO. The shift of the Ru 3d5/2 binding energy toward lower values is found for the Ru−Cs+/MgO catalyst (279.7 eV) with respect to metallic Ru (280.2 eV) and the Ru/MgO catalyst (280.5 eV). It is assumed that the shift results from a decrease in the work function of ruthenium under the action of Cs+ cations located on the Ru surface. The data obtained are used to explain the sharp difference in the activities of the Ru/MgO and Ru−Cs+/MgO catalysts for ammonia synthesis at 250−400 °C.