Catalytic Oxidation of Cellobiose over Gold-Based Bimetallic Nanoparticles Supported on TiO2

A series of bimetallic catalysts Au-M (M = Cu, Co, Ru, Pd and Ni) were supported on TiO₂ via deposition-precipitation (DP) method using urea as precipitating agent. The resulting catalysts were used in the catalytic oxidation of cellobiose into gluconic acid. The interactions between the metals in the bimetallic catalysts were carefully investigated in order to unravel the effect of the second metal on the chemical and electronic properties of the active sites and their impact on the catalytic performance. Cu and Ru were found to be promising promoters for the oxidation of cellobiose into gluconic acid, promoting oxygen activation in gold-catalyzed oxidation reactions, while Co and Pd were found to enhance the retro-aldol condensation of fructose into glycolic acid. In this case, the catalytic activity is dominated by the second metal added to Au. Hence, Au behaves more like a promoter of the second metal, preventing over oxidation and poisoning by the reaction intermediates and products. Ni specifically promoted the C-C scission, yielding a mixture of lower carbon-containing compounds and glycolic acid as the main products.

The major reaction products obtained were gluconic acid, glycolic acid, ethylene glycol and erythritol. The catalytic activity of the catalysts followed the order: Au-Cu/TiO₂ > Au-Pd/TiO₂ > Au-Ru/TiO₂ > Au-Co/TiO₂ > Au/TiO₂ > Au-Ni/TiO₂. Upon comparison, Au-Ru/TiO₂ was found to be an active and selective catalyst towards the formation of gluconic acid. The nature of the metal interactions in the bimetallic systems greatly influenced the product distribution.