Gold-on-carbon catalysts are analogs of homogeneous gold catalysts that use a redox couple of Au(I) and Au(III) species.

Au/C catalysts are effective materials for the gas phase hydrochlorination of acetylene to vinyl chloride monomer, and to date, the most effective catalyst preparation protocol make use of impregnation using aqua regia. In the present study, the effect of this solvent is evaluated and discussed in detail by modifying the ratio of HCl and HNO 3 and the temperature of the impregnation step. These factors are observed to affect the Au 3+ /Au 0 ratio of the final catalyst, in addition to the modification of the functional groups of the carbon used as support. The results can be rationalized by the oxidation effect of HNO 3 on both the gold nanoparticles and the functional groups on the carbon surface, as well as a nucleation effect of HCl towards gold over the carbon support. Kinetic parameters for the reduction of Au 3+ to Au 0 were also determined and these support the existence of a redox cycle between Au 3+ /Au 0 that could explain the overall catalytic activity.

The effect of the gold oxidation state and carbon structure on the activity of Au/C catalysts for the hydrochlorination of acetylene was investigated by a combined approach using TPR, XPS and porosimetry determinations. The activity of the catalyst in the synthesis of vinyl chloride monomer was found to be dependent on the presence of Au 3+ species in the catalyst. However, by preparing catalysts with different Au 3+ content it was possible to determine the existence of a threshold Au 3+ amount, beyond which the excess of Au 3+ was not active for the reaction. This was explained by the existence of active sites at the Au/C interface, and not just by the presence of Au 3+ species on top of Au nanoparticles, as explained by current models for these catalysts. It was also possible to determine the existence of a subset of Au nanoclusters which do not take part in the reaction, as well as changes in the textural properties of the carbon that can affect its long term reusability.

Single-site Au species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. The evolution of these single-site species has been monitored by Au L 3 X-ray absorption spectroscopy (XAS). Alternating between a standard reaction mixture of HCl/C 2 H 2 and the single reactants has provided insights into the reaction mechanism and catalyst deactivation processes. We demonstrate that oxidative addition of HCl across an Au(I) chloride species requires concerted addition with C 2 H 2 , in accordance with both the XAS measurements of Au oxidation state and the reaction kinetics being first order with respect to each reactant. Excess C 2 H 2 changes the Au speciation and results in the formation of oligomeric acetylene species which were detected by inelastic neutron scattering. Catalyst deactivation at extended reaction times can be correlated with the formation of metallic Au particles. These Au(0) species generated during the sequential gas experiments, or after prolonged reaction times, results in the analysis of the normalized near-edge white line intensity becoming an unsuitable guide for identifying the active Au species, affecting the strong correlation between normalized white line height and VCM productivity usually observed in the active catalyst. Thus, a combination of scanning transmission electron continued...

A set of Au/C catalysts for the gas phase hydrochlorination of acetylene to vinyl chloride monomer were prepared using a range of strong acids as impregnating solvents and varying the preparation drying temperature. The most active catalyst is the material prepared using aqua regia as solvent with an intermediate drying temperature of 140 o C. The effects of the catalyst preparation parameters on the catalytic activity are examined using XPS and TPR as analytical tools. In particular, the use of thermal reduction methods allows the determination of kinetic parameters for the reduction of Au 3+ to Au 0 by means of H 2 . These data support the existence of a redox cycle between Au 3+ /Au 0 when carbon is used as support.