Effects of thermal treatment on physico-chemical and catalytic properties of lanthanum manganite LaMnO3+y

Lanthanum manganite oxides were prepared by citric acid gel process. The precursor gel was thermally decomposed in four steps allowing high composition homogeneity and textural stabilisation. As the calcination temperature increased, the lattice constants increased whereas the average oxidation state of manganese decreased resulting in lower cation vacancies (lower y values in LaMnO3+y). The specific surface area (SSA) was found to decrease from 13 to 1.5 m2 g−1 when the sintering temperature was raised from 600 to 1000 °C. Well characterized (XRD, ATD-ATG, chemical analysis, surface area, SEM, XPS) perovskite phase LaMnO3+y synthetized after calcination at 700–1000 °C was tested for total methane oxidation in a fixed bed reactor with feed containing 1% methane, 4% oxygen and the balance helium. The catalytic activity was found to decrease with increasing calcination temperature and correlates not only with the bulk structure of the perovskite phase and consequently oxygen mobility but also with the content of manganese and its surface concentration. At low temperature (<600 °C), the decrease of catalytic activity with the increase of calcination temperature is correlated to the increase of surface Mn deficit. At higher temperature (>600 °C), the redox system is assumed to occur with the oxygen ions being transported through the lattice to the reduction site for reaction. This mechanism seems to be favoured by the increase of bulk cation vacancy amount.

Graphical abstractLanthanum manganite oxides were prepared by citric acid gel process. The precursor gel was thermally decomposed in four steps allowing high composition homogeneity and textural stabilisation. As the calcination temperature increased, the average oxidation state of manganese decreased resulting in lower cation vacancies (lower y values in LaMnO3+y). The surface area was found to decrease from 13 to 1.5 m2 g−1 when the sintering temperature was raised from 600 to 1000 °C.The catalytic activity in total methane oxidation was found to decrease with increasing calcination temperature which correlates well with the surface compositions of Mn as ascertained by XPS results and cation vacancies concentrations. The determined values of specific (per g) and areal (per m2) activities show that there is no direct linear proportionality between these characteristics and surface area.Figure optionsDownload full-size imageDownload as PowerPoint slide