Metal oxide catalysts like CeO 2 exhibit promising prospects to replace the currently used noble metal catalysts in the catalytic oxidation of volatile organic compounds (VOCs). Although the oxygen vacancy is regarded as an active site of these novel catalysts in VOC elimination (e.g., toluene), its actual function and activity remain unclear due to the limitation in following its dynamic evolution during reactions. Meanwhile, because the oxygen vacancy involves the generation−consumption cycle of reactive oxygen species rather than being constant, this barrier causes a deficient understanding of oxygen species backfilling in rate-determining steps. Hence, CeO 2 -based catalysts with varying oxygen vacancy concentrations were synthesized for toluene oxidation through the tensile-strained lattice and electron deficit induced by cobalt doping. The evolutions of oxygen vacancies and oxygen species were directly followed by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure. Evidence was captured that oxygen vacancies directly participate in the subsurface storage and diffusion of reactive oxygen species besides their surface regeneration. This mechanism reflects the fundamentals of oxygen storage capacity in CeO 2 by providing additional oxygen species as a supplement when needed. Additionally, part of the oxygen vacancies was discerned to be not active enough in oxygen species backfilling, leaving this process as a crucial rate-determining step besides aromatic ring opening. The toluene degradation mechanism, including electrophilic adsorption sites, and how cobalt regulates oxygen vacancy activity were also revealed via in situ spectroscopy and density functional theory + U calculation.

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum–ceria catalyst. At low temperatures (25–100 °C and 1 mbar H2) surface O–H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O–H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.