Vanadium and gold drastically change ceria performance for toluene combustion. Modulation Excitation-DRIFT coupled with mass spectrometry was performed on gold and vanadium on ceria-based catalysts. Different reactive exchanges were performed, for instance, toluene to oxygen, and toluene+O₂ to He, in the range of 200 -300 °C. Selective identification of active surface species and their discrimination from spectators was carried out by analyzing the MES-PSD spectra. Chemometric multivariate curve resolution-alternating least squares (MCR-ALS) method identified five different spectral contributions. Our results, summarized in the Scheme, indicate that toluene is molecularly adsorbed on the ceria. Next, the methyl group of adsorbed toluene on the ceria surface is activated by subtraction of an H atom by a lattice O^-2, forming benzyl (C₆H₅CH₂^-), n(CH₂) @ 2800 cm^-1. This key intermediate was further characterized by experimental and theoretical means. Benzyl species can be stepwise oxidized to benzoxyl/ benzaldehyde, and benzoate species by lattice oxygen species, that is, in absence of gas phase oxygen. Subsequent decarboxylation of benzoate and oxidation of the aromatic ring produces formate and aldehyde-like species, which are finally oxidized to CO₂ and water. The activation of the aromatic ring seems to be the rate-limiting step, because once decarbonilated, oxidation proceeds swiftly. Acidic sites from VOx hamper the abstraction of H from the methyl group of toluene. Conversely, the role of metal gold particles seems to facilitate the activation of molecular oxygen. Complementary operando Raman experiments allow observing the formation of peroxide/superoxide surface species, some of which are related to the higher combustion activity of the gold-supported catalyst. This is relevant for indoor air quality, not only from chemical air quality, but also for microbiological air quality, as we are exploring in LaCaixa Foundation SafeAir project.

Scheme. Proposed reaction mechanism of toluene oxidation on surface CeO<sub>2</sub>

Scheme. Proposed reaction mechanism of toluene oxidation on surface CeO₂

Raquel Portela

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