Highly efficient and selective electrooxidation of glucose and xylose in alkaline medium at carbon supported alloyed PdAu nanocatalysts

The effect of the composition of carbon supported PdAu nanomaterials toward electrocatalytic oxidation of glucose and xylose was evaluated in alkaline medium. PdxAu10-x/C catalysts were synthesized by a water-in-oil microemulsion method and characterized by physical (TGA, AAS, TEM, XRD) and electrochemical methods. Bulk Pd/Au alloys were formed, but the surface composition was different than the bulk one. The surface of PdxAu10-x/C catalysts as determined by electrochemical method were enriched in Pd with respect to the nanoparticle bulk composition as determined by atomic absorption spectroscopy (AAS). Electrochemical experiments combined with in situ infrared spectroscopy measurements showed the dissociative adsorption of aldoses on Pd-rich catalysts at low potentials (appearance of the typical band of COads on Pd surface). For catalysts with Au at. % ≥70%, this band was not visible. Pd3Au7/C catalyst provided the lowest oxidation onset potential, the highest activity and the best selectivity towards gluconate and xylonate. Therefore, long term chronoamperometry measurements of glucose and xylose electrooxidation were performed on this catalyst in a 25 cm2 geometric active surface area electrolysis cell at a voltage of 0.4 V and at 293 K. Reaction products were analyzed by HPLC every hour, and by 1H NMR at the end of the experiments. The main products were gluconate and xylonate. Glucose oxidation led to selectivity in gluconate of 87% for 67% conversion, and higher for lower conversion. Xylose oxidation led to selectivity in xylonate of 100% after 1 and 2 h electrolysis and of 92% after 6 h (52% conversion). The conversion of glucose and xylose into gluconate and xylonate was performed with high selectivity and an energy cost below 36 € per ton of reaction products.