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

Abstract

Perovskite-type LaAl_1−xCr_xO₃ 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 LaAl_1−xCr_xO₃ perovskite-type solid solutions. EPR analysis confirmed the presence of both Cr³⁺ and Cr⁵⁺ in the materials fired 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 Å and 3–7 m² 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.