Catalytic reactions involve the migration of chemical species—including hydrogen and oxygen—between an active metal and its supporting material. This process, known as "spillover," modulates interactions among distinct active sites, adjusts their abundance, and governs overall catalytic performance.

Researchers have extensively studied spillover confined to the catalyst surface. However, it has remained unclear whether the interior of the catalyst—the catalyst bulk—participates in reactions via non-surface spillover pathways.

To resolve this knowledge gap, a research team led by Profs. ZHANG Tao and HUANG Yanqiang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with researchers from the Southern University of Science and Technology, explored oxygen spillover mechanisms at the microscopic level.

The study was published in Nature on April 15.

The research team selected titanium dioxide (TiO₂) for its reducibility, which enables efficient oxygen storage and release. Combined with its diverse crystal structures, these features make TiO₂ an ideal model system and practical support for exploring oxygen spillover mechanisms at the microscopic level.

Using environmental transmission electron microscopy, the team directly visualized the full oxygen spillover process on individual Ru/TiO₂ particles. This marks the first experimental observation of bulk oxygen spillover in Ru/rutile-TiO₂ (Ru/r-TiO₂) catalysts.

"We have uncovered a channel within the TiO₂ support that facilitates oxygen spillover, while the metal–support interface acts as an atomic-scale gatekeeper, regulating whether oxygen spillover can occur," explained Prof. LIU Wei of DICP. "This insight inspires a novel strategy for utilizing the catalyst bulk, which has long been considered catalytically inactive."

Challenging the conventional view that spillover occurs mainly on exposed surfaces, the team verified that oxygen species travel across the Ru/r-TiO₂ interface from three to five atomic layers beneath the TiO₂ surface to the Ru metal. This transport is driven by the oxygen chemical potential gradient.

The distinctive oxygen spillover identified in this work allows the catalyst bulk to contribute to mass transfer during catalysis, highlighting the vital role of interface engineering in regulating spillover behavior.

Under highly reducing conditions, metal particles can become encapsulated by reducible oxides like TiO₂, resulting in the loss of H₂ and CO adsorption capacity. Traditional models of metal–support interactions focus on mass transport between the outer surfaces of metals and supporting oxides, with the peripheral interface thought to drive catalytic activity.

The team has expanded this framework by demonstrating that unique bulk oxygen spillover activates the catalyst's interior interface—a region usually inaccessible to reactants—enabling it to participate in mass transfer during catalytic reactions.

The study underscores the significance of interface engineering in controlling spillover and emphasizes the potential of in situ, single-particle microscopic imaging in unraveling reaction pathways in catalytic transformations.