Adsorption and surface reactions of acetaldehyde on TiO2, CeO2 and Al2O3 Original Research Article

The adsorption and surface reactions of acetaldehyde at 300–673 K on TiO2, CeO2 and Al2O3 were investigated by Fourier transform infrared spectroscopy and mass spectroscopy. Acetaldehyde adsorbs molecularly in two forms on the surfaces: (i) in a less stable H-bridge bonded form and (ii) in a more stable form adsorbed on Lewis sites through one of the oxygen lone pairs. Both forms of molecularly adsorbed acetaldehyde transform into crotonaldehyde (CH3CHsingle bondCHCHO) by β-aldolization on the surfaces. The reaction of adsorbed acetaldehyde and crotonaldehyde resulted in the formation of benzene at higher temperature. The formation of crotonaldehyde and benzene depended on the nature and the pre-treatments of the oxides: the amount of crotonaldehyde was higher on H2-pre-treated, while the amount of benzene was higher on O2-pre-treated surfaces. Primarily the more strongly held acetaldehyde underwent dehydrogenation resulting in H2 and acetylene. The formation of ethane was interpreted by hydrogenation of the transitorily formed ethylene and/or by catalytic decomposition of ethanol, which formed from adsorbed ethoxy produced by the surface reduction of acetaldehyde. Acetaldehyde could be oxidized into acetate, the decomposition of which resulted in gas phase methane. No CO and CO2 was detected up to 673 K.