Ammonia Catalyst Evolution Under Realistic Reactor Conditions Revealed by Multimodal, Multi-Stimuli In Situ Gas-Cell Electron Microscopy

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Sustaining the world’s current population of 8.3 billion hinges on the large-scale production of ammonia fertilizer via the Haber-Bosch process [1,2], in which ammonia is synthesized over Fe- or Ru-based catalysts under extreme pressures (150–300 bar) and temperatures (350–500 °C) [3]. Maintaining these conditions incurs substantial energy costs, accounting for nearly 2% of global CO2 emissions [1]. Recent success has been achieved in shifting ammonia synthesis towards more sustainable, ambient-pressure conditions by employing multicomponent photocatalysts, such as AuRu and CuFe [4,5]. However, the structural and chemical heterogeneity intrinsic to these materials gives rise to dynamic restructuring during catalysis, with marked consequences for chemical activity. In situ gas-phase transmission electron microscopy (TEM) enables direct visualization of catalyst evolution under reactive environments. Historically, ammonia synthesis environmental TEM studies have largely been performed at pressures of 0.01-0.13 mbar [6,7], well below the pressures of atmospheric (1 bar) bench-top reactors. Here, we employ a Protochips Atmosphere gas-cell TEM holder in a probe-corrected Thermo Fisher Spectra 300 to investigate alloyed, bimetallic AuRu nanocatalysts at atmospheric pressure and elevated temperatures, closing this pressure gap and revealing structural evolution pathways that are not visible at lower pressures (Fig. 1a,b) [8]. Optical excitation is then introduced via a Protochips Sol holder, providing an initial framework for examining how plasmonic excitation can modulate catalyst restructuring. We first isolate thermally driven catalyst evolution by vacuum annealing AuRu (1:0.2) nanocatalysts using a 20 °C min−¹ ramp with 10 min holds from room temperature to 750 °C, unblanking the beam only during imaging to minimize beam effects. Using correlative, multimodal scanning TEM high-angle annular dark-field (STEM-HAADF), X-ray energy-dispersive spectroscopy (X-EDS), four-dimensional (4D) STEM nanobeam electron diffraction, and monochromated low-loss electron energy-loss spectroscopy (EELS), we show that the initially face-centered cubic (FCC) AuRu alloy phase-segregates at elevated temperatures into Au-rich FCC and Ru-rich hexagonal close-packed (HCP) regions [9], each exhibiting distinct plasmonic responses. We then expose the catalysts to an H2/N2 (3:1) environment at pressures of 50 Torr, 350 Torr, and 760 Torr (∼1 bar). Under identical thermal ramping conditions, restructuring at 50 Torr and 350 Torr mirrors vacuum behavior. By contrast, annealing at atmospheric pressure (760 Torr) activates a distinct restructuring pathway, in which internal nanovoids nucleate at high temperature (Fig 1c). Post-reaction electron tomography of one such particle reveals a through-thickness cavity intersected by a Ru-rich internal channel, confirmed by X-EDS (Fig. 2a-e). To identify the chemical origin of void formation, we vary gas composition at atmospheric pressure. Annealing under atmospheric pressure in pure Ar or N2 reproduces vacuum-like restructuring, whereas atmospheric-pressure H2 uniquely induces nanovoid formation. DFT-trained machine-learning interatomic potentials indicate that hydrogen enhances interdiffusion asymmetry between Au and Ru, promoting vacancy accumulation consistent with a gas-assisted Kirkendall mechanism. Lastly, we introduce optical excitation and employ wavelength-dependent studies to probe plasmonic contributions to catalyst restructuring. Together, these results apply a multi-stimuli in situ TEM framework to disentangle thermal, pressure, and chemical drivers of catalyst restructuring under real-world conditions [10]. (a) Schematic of the gas-cell STEM platform used to probe AuRu nanocatalysts under H2/N2 gas environments. (b) HAADF-STEM image of a representative AuRu nanocatalyst. (b) In situ STEM image series of an AuRu nanocatalyst exhibiting internal nanovoid formation at elevated temperature under atmospheric-pressure H2/N2 (3:1). (a) HAADF-STEM image of an AuRu nanocatalyst with an internal nanovoid. (b) Corresponding X-EDS map showing Au and Ru distributions. (c–e) Tomography reconstructions at selected tilts reveal a Ru-rich internal channel (arrows) intersecting the nanovoid.

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