Microscopic Insight into Ruthenium Exsolution from LaFe0.9Ru0.1O3 Perovskite

Abstract

Metal exsolution is a smart strategy that allows modification and enrichment of a material’s surface with highly active catalytic phases, thus offering the possibility to fine-tune the surface chemical composition. We study the exsolution of Ru from a perovskite solid solution LaFe_0.9Ru_0.1O₃ (LFRO) to form Ru nanoparticles and their passivation by a conforming LaO_x layer by applying a variety of in situ techniques, including TEM and XPS, in combination with ex situ infrared and Raman spectroscopy, but most notably by utilizing the catalytic propane combustion to probe the formation of the passivating LaO_x layer. During the Ru exsolution process, Ru³⁺ in LFRO is reduced first to the Ru^β species and subsequently into a Ru⁰ species, evidencing the exsolution of Ru particle. The transformation of Ru³⁺ → Ru^β proceeds already below 300 °C and is correlated with the formation of oxygen vacancies under a reductive atmosphere. The subsequent transformation of Ru^β toward Ru⁰ needs at least a reduction temperature of 400 °C that is likely to be determined by the diffusion of Ru³⁺ from the near-surface region of LFRO toward the surface. Only above 600 °C ruthenium cations from the bulk of LFRO are exsolved, leading to the further growth of Ru particles. Around 600 °C, the exsolution of Ru particles is accompanied by the formation of a covering LaO_x layer. We propose that La segregation and precipitation as surface LaO_x are driven by the overstoichiometry of La in LFRO after Ru exsolution.