Au–Cu on Nb2O5 and Nb/MCF supports – Surface properties and catalytic activity in glycerol and methanol oxidation

Abstract

Nb2O5 and Nb/MCF were used as supports for gold and copper (Au–Cu system). The activity of these catalysts in the oxidation of glycerol in the liquid phase and methanol in the gas phase was compared with that of monometallic gold catalysts. The oxidation states of metals in the samples prepared were characterised by XRD, TEM, UV–Vis, XPS and H2-TPR. It was found that gold was present in metallic form, whereas copper in the form of CuO, oligonuclear [Cuδ+⋯Oδ−⋯Cuδ+]n clusters and isolated cations. The introduction of copper changed the electronic state of gold in Au–Nb2O5 and Au–Nb/MCF and the reduction of copper was found easier in bimetallic AuCu catalysts, which was interpreted as a result of the interaction between gold and copper. Nb2O5 supported catalysts were active in the glycerol oxidation. Monometallic catalysts show high selectivity to glyceric acid, whereas an increase in the selectivity to glycolic acid was found over bimetallic samples due to the presence of copper species active in the oxidative dehydrogenation of glyceric acid. Au–Cu catalysts exhibit very high activity in methanol oxidation towards CO2 as a result of the presence of basic oxygen from CuO-like species on the catalyst surface. Superior performance of bimetallic catalysts in oxidation reactions is due to the synergy between gold and copper.