Identification of active sites and influence of real structure of gold catalysts in the selective hydrogenation of acrolein to allyl alcohol

We have successfully employed oxide supported gold catalysts in the gas-phase hydrogenation of acrolein, the alpha,beta-unsaturated aldehyde being most difficult to hydrogenate at the carbonyl group. The relation of structural characteristics and surface state of these catalysts with respect to their activity and selectivity have been studied by surface analytical techniques and high-resolution transmission electron microscopy which was carried out to elucidate the real crystal structures of gold particles. The performance of gold catalysts formed by various preparation and pretreatment procedures was strongly dependent on the reduction time: Increasing the latter resulted in a high number of multiple twinned gold particles which decrease both the selectivity to the desired product, allyl alcohol, and the turnover frequency. Furthermore, it has been demonstrated for the first time that the addition of a second metal (indium) results in a selective decoration of the faces of nanosized gold particles leaving edges free. The active sites of supported gold catalysts, favoring the adsorption of C=O group of acrolein and subsequent reaction to allyl alcohol, have been identified as edges of gold nanoparticles. The zinc oxide supported bimetallic Au-In catalyst produced allyl alcohol with a selectivity of 63% which is comparable to the best hydrogenation catalysts for this reaction.