Dehydrogenation of ethylbenzene over Fe-Ce-Rb and Fe-Ce-Cs mixed oxide catalysts

Fe-Ce mixed oxide catalysts promoted by Rb and Cs have been investigated for the dehydrogenation of ethylbenzene. Fe-Ce-Rb and Fe-Ce-Cs were found to be highly active. X-ray diffraction (XRD) analysis confirmed the formation of RbFeO2 and CsFeO2 in the catalyst. X-ray photoelectron spectroscopy (XPS) analysis suggests a surface composition of the catalysts with an atomic ratio of Rb/Fe = 1, Cs/Fe = 1 and O/Fe = 2, meaning the surface is composed of the binary oxides, RbFeO2 and CsFeO2. We conclude that the active phases of these catalysts are RbFeO2 and CsFeO2, similar to KFeO2 in the case of K-Fe and Fe-Ce-K mixed oxide catalysts. The rate equation of the reaction was determined supposing a Langmuir-Hinshelwood mechanism, in which the competitive adsorption of ethylbenzene and styrene dominate the reaction rate. However, the relative adsorption constant of styrene, Z (ST), did not change significantly with the addition of Rb or Cs. The rate constant k increased with the addition of Rb or Cs. The activation energy increased with the addition of Rb or Cs compared with Fe-Ce, and the frequency factor also increased remarkably. Because the strong base points are most likely the active sites, addition of Rb and Cs increased the number of basic sites on the catalyst surface, resulting in higher activity. The kinetic factors, k and Z (ST), were applied for the rate equation. The experimental results were in very good agreement with the derived rate equation.