EPR analysis of La(1-x)Me(x)MnO(3+y) (Me=Sr, Ce, Eu) perovskitic catalysts for methane oxidation

We have analyzed a series of catalysts before and after their use for methane oxidation with air. With most of them, a g  2, temperature-broadened, Lorentzian-shaped EPR line was observed at T > Tmin, where Tmin was a temperature value ranging between 244 and 392 K (see for example Fig.1). Tmin decreased with increasing concentration of Eu3+ as dopant, whilst it was independent of concentration of Ce3+ and increased with concentration of Sr2. This has been interpreted on the base of temperature-dependent interactions between phonons and conduction electrons (i.e. of polarons formation), influenced by the radius r(M) of the ions M substituting for La3+. The peak-to peak linewidth Hpp of the line increased also for Tsec < T < Tmin , where Tsec was about 20 K lower than Tmin . However, in the latter temperature range the signal was no more Lorentzian-shaped, becoming more asymmetric when T was closer to Tsec. At last, a second broader EPR feature added when T < Tsec , moving towards higher g values and becoming even broader and more intense at the lower temperatures. This has been attributed to the formation of ferromagnetic domains, causing an internal field Hint in the sample, more intense at lower temperatures. A more detailed analysis showed that on the sample surface the ferromagnetic order occurs at a temperature higher than in the bulk. However, the difference between these two temperatures was lower at higher concentrations of M (= Ce, Sr or Eu) substituting for La. With all the samples, Tmin increased after catalytic use for methane oxidation. This has been attributed to a variation of the lattice parameters, accompanying a chemical reduction from Mn3+,4+ to Mn2+,3+. This reduction accompanies the catalytic process. Indeed, as already reported, the oxidation of methane requires oxygen coming from the catalyst, even in the presence of gaseous oxygen. Therefore, the presence of Mn4+ instead of Mn3+ is expected to improve the catalyst activity. Indeed, lower light-off temperatures of the catalysts are obtained when higher amounts of Sr2+ substitute for La3+ , favouring the oxidation of the Mn ions in the sample.