Evidence for Exchange Coupled Electrons and Holes in MgO after Oxidative Activation of CH4: A Multifrequency Transient Nutation EPR Study

The effective spin of paramagnetic centers generated during the reaction of a mixture of CH4 and O2 on activated MgO was determined with transient nutation EPR (TN-EPR). For the first time it could be shown that apart from the generation of surface-trapped superoxide radicals O2–, a significant number of paramagnetic centers are formed with an effective spin larger than 1/2. Results from HYSCORE and other pulsed EPR experiments further give evidence that these centers are localized at the surface of the MgO catalyst. These centers are proposed to be exchange coupled clusters of holes and electrons, respectively, some of them formed at specific exposed sites of the highly structured MgO surface. Judging from the relative high concentration of the centers ascribed to hole clusters, it is tempting to assume that they are involved in the catalytic reaction network of the oxidative coupling of methane.