Fossil-free ironmaking is indispensable for reducing massive anthropogenic CO ₂ emissions in the steel industry. Hydrogen-based direct reduction (HyDR) is among the most attractive solutions for green ironmaking, with high technology readiness. The underlying mechanisms governing this process are characterized by a complex interaction of several chemical (phase transformations), physical (transport), and mechanical (stresses) phenomena. Their interplay leads to rich microstructures, characterized by a hierarchy of defects ranging across several orders of magnitude in length, including vacancies, dislocations, internal interfaces, and free surfaces in the form of cracks and pores. These defects can all act as reaction, nucleation, and diffusion sites, shaping the overall reduction kinetics. A clear understanding of the roles and interactions of these dynamically-evolving nano-/microstructure features is missing. Gaining better insights into these effects could enable improved access to the microstructure-based design of more efficient HyDR methods, with potentially high impact on the urgently needed decarbonization in the steel industry.

The metal-support interaction plays a critical role in heterogeneous catalysis. Under reducing conditions, oxidic supports may interact with supported metal particles, by either forming an oxide overlayer or an alloy. The structure of both the support and the nanoparticle, as well as of the interface itself, changes in response to varying environmental conditions. Here, we present a fullyab initioapproach to predict the structures and energetics of such systems for a range of transition metals (Me = Cu, Ru, Pd, Ag, Rh, Os, Ir, Pt, Au) supported on titania surfaces as a function of gas atmosphere composition. The competing formation of a monolayer comprising fully oxidized titania (TiO₂), its reduced forms (Ti₂O₃, TiO), and the Ti-Me surface alloy, is investigated. The stability of each of these phases is found to be very sensitive to the environmental conditions and the supported metal. Encapsulation of metal, also known as classical strong metal-support interaction (SMSI), was predicted by thermodynamic driving force analysis. We show that a simple parameter, the Ti-Me alloy formation energy, is a good descriptor for the strength of the interaction between metal substrates and reduced titania monolayers and has predictive power towards the conditions under which an overlayer is stable. The presented thermochemical data and phase diagram analysis can be used to identify the structure and stability of supported metal catalysts under realistic conditions.

Catalytic systems based on supported noble metals are extensively studied because of their widespread application. Discussions remain about the nature of the active species, whether they are atomically dispersed or nanoparticles, and their reactivity. In this work, combining in situ/operando spectroscopy with theoretical modeling, we propose a phase diagram of atomically dispersed platinum on ceria, demonstrating that it reversibly changes from PtIVO2 to PtIIO as a function of temperature and oxygen partial pressure. The phase diagram helps identify the stability domain of each species, while spectroscopies provide a quantitative evaluation depending on the reaction conditions. Finally, our results show that high-temperature activation in the presence of steam of supported atomically dispersed platinum enhances the activity toward low-temperature carbon monoxide oxidation because it promotes aggregation into nanoparticles. This work highlights the structure-activity relationship in supported metal catalysts and proposes a suitable approach to determine the amount of each species before the investigation of the reaction mechanism.

Heterogeneous catalysts play a pivotal role in the chemical industry. The strong metal-support interaction (SMSI), which affects the catalytic activity, is a phenomenon researched for decades. However, detailed mechanistic understanding on real catalytic systems is lacking. Here, this surface phenomenon was studied on an actual platinum-titania catalyst by state-of-the-art in situ electron microscopy, in situ X-ray photoemission spectroscopy and in situ X-ray diffraction, aided by density functional theory calculations, providing a novel real time view on how the phenomenon occurs. The migration of reduced titanium oxide, limited in thickness, and the formation of an alloy are competing mechanisms during high temperature reduction. Subsequent exposure to oxygen segregates the titanium from the alloy, and a thicker titania overlayer forms. This role of oxygen in the formation process and stabilization of the overlayer was not recognized before. It provides new application potential in catalysis and materials science.