Nature of HCl oxidation Au anomalies and activation of non-carbon-material-supported Au catalyst

Abstract

Supported cationic gold (Au) has been recognized as an active site of acetylene hydrochlorination for decades; however, it remains ambiguous why its loading on carbon-based materials is active but that on metal oxides (e.g., titania, TiO2) is virtually inactive. For the inactive Au/TiO2 catalyst, the Au(I)–Au(III) redox couple cannot be constructed in the presence of oxidizing gas (NO, N2O, and NO2), which is thermally desorbed from the catalyst synthesis when aqua regia is used. Surprisingly, the inactive Au/TiO2 can be activated when it is impregnated with trichloromethane, CHCl3, which releases the radical (Cl) to trigger the Au(I)–Au(III) redox couple. Once Cl radicals are consumed, the reaction will be terminated, although a large amount of supported cationic Au remains. The terminated Au/TiO2 catalyst can be reactivated by adding Cl again. In contrast to the carbon-supported cationic Au system, the inability to build Au(I)–Au(III) redox couples is the dominant reason for the inactivity of metal-oxide-supported Au catalysts and provides a new perspective on activating metal-oxide-supported metal-based hydrochlorination catalysts.