Ordered Structures, Spin Bags and Ferromagnetic Domains in Spinel-like Catalysts, as observed by EPR Spectroscopy

EPR systematic examination of Spinel-like and Perovskite-like catalysts is very useful to investigate their physical properties, even in the rather common case in which the spectrum is composed by a single, Lorentzian shaped, line. In this case, in particular, the line-width vs. temperature and chemical composition can provide interesting information on sample structure and physical properties, sometimes also related to catalytic activity. In particular, a line broadening with increasing temperature indicates an ordered structure (simply layer-like as with Cobaltites, or involving more complicated phenomena as with Manganites). Ion substitution induces changes in the sample structure and catalytic properties. During catalytic processes in many cases the crystal order decreases. Sometimes all these sample modifications cause changes in the EPR patterns, and in their trend vs. temperature. In few cases, a temperature dependent EPR feature adds, at lower magnetic fields, to the main pattern. This can be due to the formation of ferromagnetic domains, or to the generation of oxygen-based paramagnetic species like O-, sometimes interacting with the metal ions present in the sample, or grouping into spin bags. The formation of these new magnetic species, when favoured by the catalytic use of the sample, can provide further information on the mechanisms active during the reaction. At higher calcination temperatures more ordered structures can be produced, generally disfavouring the formation of spin bags. However, this situation tipically corresponds to smaller surface areas, and hence to worse catalysts.