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The reduction of Fe3O4 by hydrogen is a process of significant industrial relevance, particularly in catalysis and hydrogen-based metallurgical applications. To gain atom-level insights into the underlying mechanisms, we perform density functional theory calculations to investigate hydrogen adsorption and water formation on both perfect and oxygen-deficient Fe3O4(011) surfaces. Our results reveal a consistent site preference for hydrogen adsorption at twofold-coordinated oxygen sites over threefold-coordinated oxygen sites, driven by electronic structure differences as revealed by projected density of states analysis. We identify the most stable adsorption configurations across a range of hydrogen coverages, including configurations that lead to surface-bound H2O formation. Comparison of H2O desorption energies indicates that H2O is more readily released from O-deficient surfaces, highlighting the promoting role of oxygen vacancies in oxide reduction. Thermodynamic modeling incorporating the Gibbs free energy of adsorption reveals that twofold-coordinated oxygen vacancies suppress H incorporation, while threefold-coordinated oxygen vacancies enhance H uptake across a broad range of temperature and H2 pressure. Surface free energy analyses further show that under reducing conditions, surfaces containing threefold-coordinated oxygen vacancies become more thermodynamically stable than the defect-free surface. These findings underscore the environment-dependent reactivity of Fe₃O₄(011) and demonstrate how the type and coordination of oxygen vacancies govern H adsorption and surface reducibility. The results have practical implications for tailoring the redox behavior of iron oxides in hydrogen-rich environments. 

Hydrogen reduction of Fe3O4 plays a pivotal role in sustainable steelmaking, offering a low-carbon alternative to traditional carbothermic processes. In this study, we employ density functional theory to investigate the dissociative adsorption of molecular H2 and the subsequent adsorption behavior of atomic H on Fe3O4 (110) surfaces, considering both stoichiometric and O-deficient configurations. Our results reveal that the type and location of O vacancies critically influence both the thermodynamics and kinetics of H2 activation. Compared to the perfect surface, the presence of O vacancies increases the activation barrier for H2 dissociation. Twofold coordinated O sites, which are thermodynamically more favorable to form, reduce the reaction exothermicity. Conversely, threefold coordinated O vacancies, though less readily formed, stabilize the dissociated state more strongly but incur the highest activation barrier. For atomic H adsorption, adsorption is strongly favored at O sites over Fe, particularly at hollow sites adjacent to twofold O ions. While O vacancies themselves are not favorable adsorption sites, they alter the local electronic environment and change the H binding strength. Bonding strength, quantified via the integrated crystal orbital Hamiltonian population, shows a strong linear correlation with H adsorption energies across all surface types. This correlation underscores the critical role of H–O orbital hybridization in stabilizing adsorbed H species and provides a quantitative link between adsorption strength and the underlying surface–adsorbate bonding characteristics. Our results offer atomic-level insights into defect-mediated H2 activation and H adsorption on Fe3O4, with implications for advancing hydrogen-based processes in steel production, hydrogen storage, and heterogeneous catalysis. 

Surface Plasmon Polariton (SPP), as a novel information carrier, offers unprecedented opportunity for confining electromagnetic fields that carry orbital angular momentum (OAM) to subwavelength dimensions. In this thesis, I focus experimentally on the generation, manipulation, and spatio-temporal evolution—and theoretically on the analytical modeling—of plasmonic phase singularities, known as plasmonic vortices, at the silver (Ag)/vacuum interface. I image and study the dynamics of plasmonic vortices by interferometric time-resolved multi-photon photoemission electron microscopy (ITR-mP-PEEM). Firstly, I report on the generation, evolution, and topological properties of plasmonic vortices carrying pure geometrically induced orbital angular momentum (OAM), generated by illuminating Archimedean spiral coupling structures with normally incident, linearly polarized light. Next, I present an analytical model describing the generation and evolution of these plasmonic vortices, and based on this model, I further analyze their spatial structure and dynamics. I also derived the spin angular momentum (SAM) of plasmonic vortices, whose textures reveal transient plasmonic spin-Skyrmion topological quasiparticles. In parallel, I also record images of plasmonic vectoral vortex field evolution on the nanometer spatial and femtosecond temporal scale, from which I derive the plasmonic spin Skyrmion boundary and topological charge. The excellent agreement between analytical model and experimental results confirms the topological spin texture at surface plasmon polariton vortex core. To extend the understanding of ITR-PEEM imaging, I perform a simple experiment withv double line coupling structure at the silver/vacuum interface, which reveals an asymmetric cross term between the different components of the SPP field that also appear in the ITR-PEEM imaging. Finally, I approach a novel method to manipulate momentum transport between two plasmonic vortices analytically and experimentally. By tuning the relative distance between two vortex generator structures with same sign and sign of the geometric charge, a conveyor belt-like field could be observed at the center of the device, which can be applied to transport the field, momentum, and energy between two plasmonic vortices. 

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. 

We apply ultrafast nanoscale microscopic imaging and analytical modeling to investigate the coherent field and spin textures of dual plasmonic vortices as a means to design the momentum flow, and spin topology by interaction of their gyrating fields. The ultrafast laser normal incidence illumination by circularly polarized light of two vortex generator structures with variable separations in silver films launches structured surface plasmon polariton fields. Two distinct primary vortices and a third emergent vortex, generated by interaction of the primary vortices and tunable by design of their separation, form through the spin–orbit interaction of light. The gyration of plasmon fields and the consequent vectorial Poynting momentum flow is imaged with sub-optical cycle phase and spatial resolution by interferometric time-resolved two-photon photoemission electron microscopy (ITR-2P-PEEM). The ultrafast imaging and analytical modeling of the interaction of the dual plasmonic vortices examines the nanoscale control of plasmon spin topology and momentum driven transport. 

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.