LaAl1-xCrxO3 perovskite-type solid solutions:: Structural, electronic, magnetic properties and catalytic activity towards CO oxidation

Perovskite-type LaAl1-xCrxO3 samples were prepared from citrate precursors and calcined at several temperatures up to 1073 K. The materials were characterised by XRD, diffuse reflectance spectroscopy (DRS), EPR and magnetic susceptibility measurements. At 773 K all samples are amorphous, at 923 K the perovskite structure starts to form in addition to other La-Cr oxides such as LaCrO4 and La2CrO6, whereas at 1073 K all samples are single phase LaAl1-xCrxO3 perovskite-type solid solutions. EPR analysis confirmed the presence of both Cr3+ and Cr5+ in the materials red at 923 K. DRS patterns, magnetic susceptibility measurements and EPR analysis confirmed the presence of Cr3+ alone in all the perovskite-type solid solutions obtained at 1073 K. For these samples: (i) the particle sizes and surface areas were found to be in the 610 to > 1000 Angstrom and 3-7 m(2) g(-1) ranges, respectively; (ii) a linear increase of the unit cell volume, V, with x was observed, in agreement with expectations based on the ionic size of interchanging Al3+ and Cr3+ ions. The perovskite samples calcined at 1073 K were tested for CO oxidation. All Cr-containing materials are much more active than LaAlO3 pointing to the essential role of the transition metal ion in developing highly active catalysts. The activity trend and the variation of the activation energy with the increase in chromium content suggest a change in the active site from mononuclear to polynuclear.