Probing the Actual Role and Activity of Oxygen Vacancies in Toluene Catalytic Oxidation: Evidence from In Situ XPS/NEXAFS and DFT +UCalculation

Abstract

Metal oxide catalysts like CeO2 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, CeO2-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 CeO2 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.