Atomic-scale elucidation of formation and structure in high-performance Re-Ge nanocatalysts

Rational design of high-performance bimetallic nanocatalysts requires an understanding of the unique atomic structures governing their performance. This study focused on the Re-Ge/TiO₂ catalyst, which exhibits high performance for carboxylic acid hydrogenation. We analyzed structural and electronic state changes occurring during its multistep preparation involving calcination in air, hydrogen reduction, and oxidative stabilization to elucidate the key structural factors responsible for its high performance. To this end, a complementary approach combining in situ X-ray absorption fine structure analysis and ex situ scanning transmission electron microscopy (STEM) was employed. In particular, the STEM measurements utilized an air-free transfer holder to track the same individual nanoparticles throughout all preparation steps, thereby addressing two issues: the challenge of distinguishing true structural changes from particle-to-particle variations among different particles and electron beam damage associated with long exposure times. It was revealed that the highly active catalytic state after hydrogen reduction is associated with approximately 1 nm crystalline Re-Ge alloy nanoparticles. The crystal structure of the nanoparticles was a unique, low-energy, and face-centered cubic fragment, verified by density functional theory calculations. Furthermore, quantitative STEM image analysis demonstrated that the nanoparticles formed a Re-Ge random alloy. This atomic-level mixing, as detected by the spatially averaged X-ray absorption, is considered to stabilize the metallic Re(0) state, rendering Re electron-rich. These findings not only provide a new strategy for designing high-performance nanocatalysts but also establish a correlative methodology as a way to identify performance-determining factors in complex nanomaterials.