Ammonia Catalyst Evolution Under Reactor Conditions Revealed by Environmental and Multimodal Electron Microscopy

TL;DR

This study uses advanced electron microscopy to reveal how AuRu bimetallic catalysts structurally evolve under realistic ammonia synthesis conditions, highlighting the roles of temperature, pressure, and chemistry.

Contribution

It introduces a combined in situ gas-cell and multimodal electron microscopy approach to study catalyst restructuring under atmospheric pressure, providing new insights into ammonia catalyst dynamics.

Findings

AuRu catalysts phase-segregate into Au- and Ru-rich domains at high temperature.

Hydrogen pressure induces faceting and nanovoid formation in catalysts.

Hydrogen enhances Au/Ru diffusion asymmetry, promoting nanovoid formation via Kirkendall mechanism.

Abstract

Bimetallic catalysts provide new routes toward sustainable ammonia synthesis, but the structural dynamics controlling their performance under real-world conditions remain poorly understood. Here, we combine in situ gas-cell and multimodal electron microscopy to disentangle the temperature-, pressure-, and chemistry-dependent restructuring of AuRu catalysts, revealing pathways accessible only at atmospheric pressure. As synthesized, AuRu nanocatalysts are polycrystalline face-centered-cubic alloys with Au/Ru intermixing that phase-segregate into Au- and Ru-rich domains with elevated temperature (>450 {\deg}C). Increased pressure (~1 atm in 3:1, hydrogen:nitrogen) unlocks pronounced faceting and internal nanovoid formation, which systematic gas-chemistry variation identifies as hydrogen-driven. Density functional theory-based interatomic potentials show that hydrogen can amplify Au/Ru diffusion asymmetry, promoting nanovoid formation via a gas-mediated Kirkendall mechanism. Together, these results bridge the pressure gap between traditional in situ electron microscopy and benchtop ammonia reactors, enabling resolution of distinct restructuring stimuli in multicomponent systems.