[Paper Review] Ammonia Catalyst Evolution Under Reactor Conditions Revealed by Environmental and Multimodal Electron Microscopy

Amy S. McKeown‐Green, Parivash Moradifar+8 more|arXiv (Cornell University)|Jan 31, 2026

Ammonia Synthesis and Nitrogen Reduction0 citations

TL;DR

The paper combines in situ gas-cell and multimodal electron microscopy with DFT-based simulations to reveal how AuRu bimetallic catalysts restructure under ammonia-relevant reactor conditions, highlighting hydrogen-driven diffusion, nanovoid formation, and pressure-dependent faceting.

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 °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.

Motivation & Objective

• Understand how AuRu bimetallic catalysts restructure under reactor-relevant conditions.
• Identify how temperature, pressure, and gas composition influence spatial redistribution and morphology.
• Link observed restructuring pathways to catalyst performance under ammonia synthesis conditions.

Proposed method

• Use in situ gas-cell electron microscopy to watch structural changes under controlled temperature, pressure, and gas composition.
• Employ multimodal imaging to distinguish phases and morphologies in AuRu nanocatalysts.
• Apply density functional theory-based interatomic potentials to model hydrogen-enhanced diffusion and diffusion asymmetry.
• Identify pathways of nanovoid formation via a gas-mediated Kirkendall mechanism.

Experimental results

Research questions

• RQ1 How do AuRu nanostructures evolve under varying temperature and gas environments relevant to ammonia synthesis?
• RQ2 What roles do hydrogen and pressure play in driving diffusion and phase segregation in AuRu catalysts?
• RQ3 What restructuring pathways are accessible at atmospheric pressure as opposed to traditional high-vacuum in situ conditions?
• RQ4 Can computational potentials reproduce observed diffusion and nanovoiding phenomena?

Key findings

• AuRu catalysts phase-segregate into Au- and Ru-rich domains at elevated temperature (>450 °C).
• Increasing pressure around 1 atm with a 3:1 hydrogen:nitrogen mixture promotes pronounced faceting and internal nanovoid formation; this is hydrogen-driven.
• Hydrogen amplifies Au/Ru diffusion asymmetry, promoting nanovoid formation via a gas-mediated Kirkendall mechanism.
• The results bridge the pressure gap between conventional in situ EM and benchtop ammonia reactors, enabling resolution of distinct restructuring stimuli in multicomponent systems.