EPR Methods in Heterogeneous Catalysis

Electron paramagnetic resonance (EPR) spectroscopy has been used in heterogeneous catalysis to explore the nature of the active sites and the identification of intermediates, both on the surface and in the gas phase. Although the number of systems which have been studied are limited because of constraints inherent in the technique, the sensitivity and in many cases the definite identification of paramagnetic species make EPR one of the more valuable types of spectroscopy available to the catalytic chemist. In this review the application of EPR spectroscopy to eleven types of catalytic reactions is described. Evidence is given for the role of paramagnetic oxygen ions, mainly O−, in such diverse reactions as H₂ — D₂ exchange, the oxidation of carbon monoxide and the partial oxidation of methane to methanol and formaldehyde. Electron transfer reactions give rise to anion radicals which have been used to study active sites in butene isomerization reactions. Similarly, the EPR spectrum of adsorbed nitric oxide has been employed as a probe molecule to determine the role of exposed aluminum in catalytic cracking reactions and in butene isomerization. The active oxidation state of chromium for ethylene polymerization and the redox behavior of copper in nitric oxide reduction has been followed by EPR. This technique has also been used to study gas phase allyl radicals which were formed at the surface of bismuth oxide.