Investigation of the active species in the carbon-supported gold catalyst for acetylene hydrochlorination

The nature of the active species in carbon-supported gold catalysts used for synthesis of vinyl chloride monomer from acetylene hydrochlorination has been investigated using X-ray photoelectron spectroscopy and electron microscopy. Catalysts prepared by impregnation of chloroauric acid dissolved in aqua regia are initially inactive. During the initial reaction they show a pronounced induction period and we have used this opportunity to examine the evolution of the active catalyst as it is transformed during acetylene hydrochlorination. The fresh catalyst comprises a Au(III) surface film which on reaction with acetylene and HCl transforms to a mixture of Au(I)/Au(III). Experiments in which the catalyst is exposed sequentially to HCl and acetylene show that high activity is associated with a catalyst containing significant amounts of cations with both oxidation states and that the Au(I) and Au(III) oxidation cycle is important in the activation of both molecules. These findings are discussed in relation to the nature of the active species.