Mesoporous cerium–zirconium oxides modified with gold and copper – synthesis, characterization and performance in selective oxidation of glycerol

Abstract

The main goal of this work was the synthesis of mesoporous ceria, zirconia and cerium–zirconium oxides, the application of these supports for the preparation of monometallic (copper or gold) and bimetallic (copper and gold) catalysts and characterisation of their performance in the oxidation of glycerol in basic aqueous solution in the presence of oxygen. The methods applied for the study included: adsorption/desorption of nitrogen, XRD, ICP-OES, XPS, UV-vis, STEM-EDXS, H₂-TPR. The presence of the following species Au⁰, Au^δ+, negatively charged gold Au⁻ or Au⁰ in bimetallic particles, Cu⁺, Cu²⁺, Ce³⁺ and Ce⁴⁺, Zr⁴⁺ was confirmed and their content was found to depend on the catalyst composition. The interaction between gold and copper species in bimetallic samples caused the formation of the negative charge on gold and the increase of the amount of Cu⁺ cations on the catalyst surface. This feature and the higher gold dispersion gave rise to a significant increase in the catalytic activity in glycerol oxidation of bimetallic catalysts with respect to that of monometallic samples. The high selectivity to glyceric acid (GLA) is an important advantage of the bimetallic catalysts (copper–gold), especially those loaded on mixed cerium–zirconium oxides. The bimetallic catalysts were stable and in the second run of the reaction they almost did not change glycerol conversion and increased the selectivity to GLA. The texture parameters of the catalysts had also impact on the selectivity of the reaction: the lower the microporosity the higher the selectivity to GLA. The role of the support is the interaction with copper and gold active species and enhancement of the reagents' chemisorption and electron transfers.