Unveiling the self-activation of exsolved LaFe0.9Ru0.1O3 perovskite during the catalytic total oxidation of propane

Abstract

The exsolution process enables to produce and control the formation of stable and catalytically active nano particles via reductive extraction of uniformly incorporated precious metal ions from a solid oxide solution. Here we consider the simple and stable perovskite LaFeO3 (LFO) where 10% of Fe on B sites are substituted by ruthenium (LFRO). Hydrogen reduction of LFRO at 800 °C leads to the formation of socketed ruthenium particles whose low-temperature activity in the total propane oxidation reaction at 210 °C is substantially lower than that of the original LFRO. Upon increasing the reaction temperature once to 400 °C, the exsolved catalyst undergoes self-activation so that the activity at 210 °C turns out to be five times higher than that of the original LFRO. High-resolution transmission electron microscopy and nanometer-resolved element mapping, together with averaging characterization methods, including X-ray diffraction and X-ray photoelectron spectroscopy, Raman spectroscopy, and diffuse infrared spectroscopy, unveil that after reduction at 800 °C the exsolved Ru particles are slightly alloyed with Fe and encapsulated by an inert and protecting LaOx layer. Mild oxidative treatment at 400 °C leads to the removal of the conforming LaOx layer, while the uncovered RuFe alloy particle transforms to catalytically active oxidic Ru species, with no indication of a separate FeOx phase. We exemplify with our case study of LaFe0.9Ru0.1O3 that careful redox treatment enables to control the exsolution process and to avoid deactivation. This may be of importance for the whole class of exsolvable materials.