EPR Characterization of Oxide Supported Transition Metal Ions: Relevance to Catalysis

Abstract

EPR spectroscopy is a powerful tool to identify at a molecular level, the different steps of catalyst preparation, and of catalytic reactions: (i) Deposition of paramagnetic transition metal ions onto a support is monitored, and the coordination sphere of the metallic center is characterized by EPR. (ii) The catalyst is also characterized after activation (thermal oxidation or reduction): - the distribution among the different sites in zeolites can be determined; - the dispersion of the active phase may be appreciated; - the unsaturation degree of the active site may be evaluated using probe molecules such as water or13C enriched carbon monoxide. (iii) The catalytic mechanisms can be investigated by studying the elementary steps of the catalytic reaction, as illustrated for methanol oxidation over Mo/SiO2 catalysts whose EPR results have extended the reaction mechanism proposed on the basis of kinetic data. In addition, reaction intermediates may be isolated inquasi-in situ conditions as in the case of olefin oligomerization catalyzed by Ni/SiO2 systems.