Support effects in iridium-catalyzed aerobic oxidation of benzyl alcohol studied by modulation-excitation attenuated total reflection IR spectroscopy

Abstract

Abstract The influence of the support (Al2O3, CeO2, TiO2) in the oxidation of benzyl alcohol (BA) to benzaldehyde on Ir-based catalysts was investigated by ATR-IR spectroscopy in tandem with modulation excitation spectroscopy (ATR-IR-MES) at working conditions of the catalysts. ATR-IR-MES unveiled the dissociative adsorption of BA and the formation of adsorbed transient alkoxy species (C6H5CH2O-) to be key for high catalytic performance. These species were detected on Ir/TiO2 and Ir/Al2O3 but not on Ir/CeO2, which exhibited poor catalytic performance. On Ir/TiO2 pretreated with hydrogen at 450°C about 84% of the Ir sites were blocked due to formation of a titania overlayer. However, this catalyst afforded a more than seven times higher activity than the corresponding one pretreated at 300°C, where about 68% of the Ir sites were blocked. The high activity of the titania overlayer is attributed to the formation of Ti cations with oxygen vacancies caused by the hydrogen pretreatment.