Atomic-Scale Elucidation of Formation and Structure in High-Performance Re-Ge Nanocatalysts

Abstract

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/TiO₂catalyst, which exhibits high performance for carboxylic acid hydrogenation. We analyzed structural and electronic-state changes during its multistep preparation involving calcination in air, hydrogen reduction, and oxidative stabilization, to elucidate structural factors responsible for the high performance. Therefore, we used complementary in-situ X-ray absorption fine structure analysis and ex-situ scanning transmission electron microscopy (STEM). The STEM imaging utilized an air-free transfer holder to track individual nanoparticles and address two issues: the challenge of distinguishing true structural changes from variations among different particles, and electron beam damage associated with long exposure times. It was revealed that the highly active state after hydrogen reduction originates from approximately 1-nm crystalline alloy nanoparticles. The nanoparticle structure was a unique, low-energy, face-centered cubic fragment, verified by density functional theory calculations. Furthermore, quantitative STEM analysis demonstrated that the nanoparticles were a random alloy of Re and Ge. This atomic-level mixing, as detected by the spatially averaged X-ray absorption, considered to stabilize the metallic state, resulting in electron-rich Re. These findings provide a new strategy for designing high-performance nanocatalysts and establishes the correlative methodology as a way to identifying performance-determining factors in complex nanomaterials.