In situ DRIFTS study of O₃ adsorption on CaO, γ-Al₂O₃, CuO, α-Fe₂O₃ and ZnO at room temperature for the catalytic ozonation of cinnamaldehyde

Highlights: • In situ DRIFTS study of O₃ adsorption on metal oxides at room temperature. • Using acidic probe molecules (DRIFTS) characterization of surface basicity. • Correlation between basic strength of metal oxides and O₃ adsorption. • Study on the competitive adsorption of O₃ and CO₂. • DRIFTS study of cinnamaldehyde ozonation and benzaldehyde excessive oxidation. - Abstract: In situ DRIFTS were conducted to identify adsorbed ozone and/or adsorbed oxygen species on CaO, ZnO, γ-Al₂O₃, CuO and α-Fe₂O₃ surfaces at room temperature. Samples were characterized by means of TG, XRD, N₂ adsorption–desorption, pyridine-IR, nitrobenzene-IR, chloroform-IR, and CO₂-TPD. Pyridine-DRIFTS measurements evidence two kinds of acid sites in all the samples. Nitrobenzene, chloroform-DRIFTS, and CO₂-TPD reveal that there are large amounts of medium-strength base sites on all the metal oxides, and only CaO, ZnO, and γ-Al₂O₃ have strong base sites. And the benzaldehyde selectivity was increased in the same order of the alkalinity of the metal oxides. With weaker sites, ozone molecules form coordinative complexes bound via the terminal oxygen atom, observed by vibrational frequencies at 2095–2122 and 1026–1054 cm⁻¹. The formation of ozonide O₃⁻ at 790 cm⁻¹, atomic oxygen at 1317 cm⁻¹, and superoxide O₂⁻ at 1124 cm⁻¹ was detected; these species are believed to be intermediates of O₃ decomposition on strong acid/base sites. The adsorption of ozone on metal oxides is a weak adsorption, and other gases, such as CO₂, will compete with O₃ adsorption. The mechanism of cinnamaldehyde ozonation at room temperature over CaO shows that cinnamaldehyde can not only be oxidized into cinnamic acid, but also be further oxidized into benzaldehyde, benzoic acid, maleic anhydride, and ultimately mineralized to CO₂ in the presence of O₃.