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Environmental transmission electron microscopy probes the local structure, composition, and chemistry of materials under gas environments, while ambient-pressure X-ray photoelectron spectroscopy provides ensemble chemical and electronic structure information in gaseous conditions. Both techniques utilize similar differential pumping schemes to mitigate electron scattering by the gas phase, allowing for unique opportunities to correlate gas–surface interactions across comparable pressure ranges. Their integration has advanced the understanding of various catalytic reactions, including the water–gas-shift reaction, CO oxidation, and surface passivation dynamics. This Mini-Review discusses their methodological advancements, challenges, and potential for further integration with other in situ techniques to address complex catalytic phenomena and guide catalyst design. 

This study combines density functional theory (DFT) and ab initio thermodynamics calculations with X-ray photoelectron spectroscopy (XPS) investigations to identify the reduction properties of Fe2O3 (001) surface with implications for corrosion resistance, hydrogen transport, and energy safety. Ab initio thermodynamics modeling predicts fully hydroxylated surface stability across a broad range of pressures (1x10-23 to 1x105 mbar) and temperatures below 700K, consistent with previous experimental studies. Above 800K, exposures to 1x10-4 mbar H2, 1x10-4 mbar O2, or 1x10-4 mbar H2 + 1x10-4 mbar O2 each yield unique XPS signals indicating a loss of -OH coverage, aligning with DFT predictions. Insight into the mechanism of reduction as function of H2 exposure is provided, as well as conditions that promote further reduction towards Fe3O4. Theoretical and experimental investigations indicate the ability to maintain the Fe2O3 protective layer of iron oxides that have been exposed to H2 environments by including trace amounts of O2. 

The smelting of metallic iron has played a significant role in human civilization; however, conventional ironmaking processes re- main a leading contributor to greenhouse gas emissions, exacerbating global warming. Hydrogen-driven direct reduction (HyDR) of iron ore offers a sustainable alternative, leveraging the clean combustion of H2 to slash CO2 emissions. Nevertheless, HyDR involves complex, multi-step gas-solid transformations dictated by temperature: above 570°C, the reduction follows α-Fe2O3 → Fe3O4 → FeO → Fe, while below 570°C, the thermodynamic instability of FeO truncates the pathway to α-Fe2O3 → Fe3O4 → Fe, in accordance with the bulk Fe-O phase diagram [1-3]. While macroscopic characterization techniques such as thermogravimetric analysis (TGA) and high-resolution X-ray diffraction (XRD) are capable of tracking bulk phase transitions [4-6], they lack the spatial resolution to probe localized reaction dynamics at defects, grain boundaries, and reaction interfaces—critical features that govern solid-state diffusion pathways, nucleation barriers, and kinetic bottlenecks. Bridging this gap demands advanced in- situ techniques to visualize transient transformations and defect-mediated reduction mechanisms. Such insights are critical for optimizing HyDR efficiency and scalability in decarbonized steelmaking. Environmental transmission electron microscopy (ETEM) provides a unique platform to investigate local structural and inter- facial dynamics [7]. By introducing a reducing gas into the sample region while simultaneously capturing the atomic-scale evo- lution from the outermost surface to deeper layers, ETEM enables direct visualization of the reduction process of iron oxides. In this study, we employ ETEM to dynamically monitor the atomic-scale surface and interfacial structural evolution during H2-induced reduction of Fe3O4 and FeO, revealing the formation of an intermediate amorphous Fe phase. Our results demon- strate the critical role of surfaces and interfaces in the initial metallization process, offering new atomic-scale insights into the re- duction mechanisms of iron oxides. The fundamental insight has significant implications for metallurgy, thin-film fabrication, catalysis, and corrosion science. 

Using in situ synchrotron X-ray diffraction, we interrogate the microstructural and phase evolution of polycrystalline nickel (Ni) during redox cycling in O2, H2, and H2O environments. Oxidation in O2 promotes strong (111) texturing in both the NiO overlayer and the underlying Ni substrate. However, this crystallographic alignment is lost following reduction in H2 and subsequent reoxidation, demonstrating irreversible microstructural changes. H2 exposure leads to proton dissolution into the Ni lattice, triggering a localized phase transition from face-centered cubic (FCC) to hexagonal close-packed (HCP) Ni in hydrogen-saturated regions. In H2O-containing atmospheres, dissociative H2O adsorption produces protons that permeate the NiO layer, forming γ-NiOOH within the NiO lattice and HCP Ni beneath the NiO overlayer as protons accumulate. Kinetic analysis via the Johnson-Mehl-Avrami–Kolmogorov model uncovers distinct growth mechanisms: preoxidized Ni surfaces follow one-dimensional (1D) kinetics for NiO, γ-NiOOH, and HCP growth, whereas pristine Ni exhibits three-dimensional (3D) kinetics due to island-like nucleation and growth of NiO. These results highlight the critical interplay between H2O dissociation, hydrogen permeation, and redox-driven phase transformations, with practical implications in engineering nickel-based catalysts and hydrogen storage systems through controlled microstructural and phase evolution. 

The transition to hydrogen as a green reductant in metal production is critical for decarbonizing the metallurgical industry, yet atomic-scale mechanisms governing reduction pathways and phase evolution remain unresolved. Using in-situ environmental transmission electron microscopy, we identify a hidden pathway that reveals dynamic formation of amorphous metallic iron (Fe) during the hydrogen-driven reduction of ferrous oxides of Fe3O4 and FeO. Real-time imaging uncovers three coexisting transformation routes: (i) Fe3O4 → FeO, (ii) Fe3O4 → amorphous Fe, and (iii) FeO → amorphous Fe. The resulting amorphous Fe exhibits fluid-like mobility, enabling its rapid aggregation and crystallization into core-shell nanostructures, with a crystalline core enveloped by an amorphous shell. Complementary ab initio molecular dynamics simulations trace the amorphous Fe formation to interfacial strain at the metal/oxide interfaces, where large lattice mismatches destabilize the metal lattice during initial metallization. This interplay between thermodynamics and kinetics governs phase evolution: thermodynamics favors a self-limiting amorphous Fe overlayer, while rapid oxide reduction kinetics drives amorphous overgrowth. Our findings demonstrate that amorphous intermediates bypass rate-limiting crystalline steps, providing mechanistic insights to optimize H2-based processes for sustainable steelmaking. These insights bridge the gap between macroscopic process engineering and atomic-scale dynamics, with broader implications for catalysis and nanostructured material synthesis, where oxide reduction pathways critically shape functional phases and microstructures. 

Understanding oxide reduction is critical for advancing metal production, catalysis and energy technologies. Although carbon monoxide (CO) and hydrogen (H2) are widely used reductants, the mechanisms by which they work are often presumed to be similar, both involving lattice oxygen removal. However, because of growing interest in replacing CO with H2 to lower CO2 emissions, distinguishing gas-specific reduction pathways is critical. Yet, capturing these atomic-scale processes under reactive gas and high-temperature conditions remains challenging. Here we use environmental transmission electron microscopy, which is capable of real-time, atomic-resolution imaging of gas–solid redox reactions10,11,12,13,14,15,16, to directly visualize the gas-dependent oxide reduction dynamics in NiO. We show that CO drives surface nucleation and the growth of metallic Ni islands, leading to self-limiting surface metallization. Conversely, H2 activates a coupled surface-to-bulk transformation, where protons from dissociated H2 infiltrate the oxide lattice to promote the inward migration of surface-generated oxygen vacancies and enabling bulk metallization. By contrast, oxygen vacancies formed by CO remain confined near the surface, where they rapidly form a metallic Ni layer that inhibits further reduction. These results reveal distinct atomistic pathways for CO and H2 and provide insights that may guide metallurgical processes and catalyst design. 

Abstract Understanding how point defects in the bulk govern redox transformations is essential for advancing hydrogen‐based metal production and designing high‐performance oxide materials. This study reveals the atomic‐scale mechanisms driving hydrogen‐induced reduction of γ‐Fe₂O₃to Fe₃O₄, focusing on how bulk vacancy dynamics dictate structural evolution and reaction kinetics. A key finding is the pronounced contrast in defect behavior between the two oxides: in γ‐Fe₂O₃, intrinsic Fe vacancies promote oxygen vacancy clustering, destabilizing the local lattice and driving nanopore formation. In contrast, Fe₃O₄exhibits a higher oxygen vacancy formation energy and lacks intrinsic Fe vacancies, suppressing vacancy aggregation and maintaining a dense, pore‐free structure. This divergence governs distinct reduction pathways—γ‐Fe₂O₃undergoes an interface‐reaction‐limited transformation confined to the γ‐Fe₂O₃/Fe₃O₄boundary, while Fe₃O₄supports a uniform increase in oxygen vacancy concentration, enabling bulk‐phase reduction to lower‐oxide FeO. Integrated in situ electron microscopy and density functional theory modeling uncover a vacancy‐mediated mechanism, where synergistic cation‐anion vacancy dynamics steer microstructure evolution and phase progression. These insights highlight the critical role of vacancy dynamics in controlling oxide reactivity and offer a pathway toward vacancy engineering to enhance reduction kinetics in hydrogen metallurgy and to tailor porosity, reactivity, and structural resilience in oxide‐based catalysts and energy materials.