Understanding Ru Exsolution Process from LaFe0.9Ru0.1O3 and its Impact on Catalytic Propane Oxidation Reaction

Abstract

The catalytic transformation of hydrocarbons, particularly light alkanes such as propane, is of critical importance for both environmental and industrial applications. Catalysts that support precious metal nanoparticles (NPs) on oxides have shown significant promise due to their high surface area. However, these systems often face challenges such as NP sintering under reaction conditions, which can diminish catalytic performance over time. To address this, the development of catalysts that present high activity and improved stability is essential. Reductive exsolution is a facile way to produce homogenously distributed nanoparticles through the extraction of uniformly incorporated precious metal ions from a solid oxide solution. Nanoparticles originated via exsolution anchor in the surface of the parent backbone, delivering improved thermal stability, which makes it an ideal catalyst for reactions conducted at elevated temperatures, such as hydrocarbons total oxidations. The material studied in this work is the perovskite LaFe0.9Ru0.1O3 (LFRO), in which 10% Fe in the B site is replaced by Ru. Reduction in hydrogen at 800 ℃ (a typical temperature for the exsolution) leads to the formation of socketed nanoparticles (LFRO-800R). These particles are revealed to be a core-shell structure in which the active Ru is alloyed with a slight amount of Fe, encapsulated by an inert LaOx coating, thus exhibiting a lower catalytic activity compared with the pristine LFRO for the propane oxidation. However, LFRO_800R can be activated by an oxidative treatment at 400 ℃, leading to an improved activity which is five times higher than the activity of LFRO at 210 ℃. Detailed characterizations including TEM and CODRIFTS indicate that the inert LaOx can be selectively removed by oxidative treatment at 400 ℃. Meanwhile, the exposed RuFe alloy particle transforms into catalytically active oxidic Ru species, without any indication of a separate FeOx phase. The structure evolution of LFRO in each reduction temperature is systematically investigated to uncover the RuFe-LaOx formation process. When heating LFRO in the reducing atmosphere, Ru exsolves to the surface processing through Ru3+ → Ruβ → Ru0 and Ru is likely to diffuse in the form of Ruβ which serves as the intermediate species during this process. The transformation of Ru3+ initiates already at 300 °C while subsequent reduction to Ru0 which is the sign for the exsolution requires 400 °C. When the reduction temperature is higher than 500 ℃, Ru in the bulk starts to be exsolved and accompanied by the co-segregation of LaOx. Further enhancing of the reduction temperature leads to the continuous growth of Ru particles while the segregated LaOx eventually encapsulates the exsolved Ru, forming the core-shell structured nanoparticles.The information on structure evolution during Ru exsolution from LFRO and following change in reaction condition provides insights into the rational design of precious metal-based catalysts via the exsolution strategy for various application scenarios.