The effect of MnO2 loading on the glycerol electrooxidation activity of Au/MnO2/C catalysts

Abstract

Manganese oxides have shown promise as co-catalysts for platinum- and palladium-based alcohol electrooxidation catalysts. In this investigation, the effect of adding MnO2 to a gold-based catalyst system was explored for the glycerol electrooxidation reaction. It was found that modification of the carbon support with MnO2 had the effect of stabilising gold nanoparticles, giving rise to smaller gold particles with a more uniform particle size distribution than on the carbon alone. All gold-based catalysts gave very high currents for glycerol oxidation (1–5Amg−1cat) and catalysts with MnO2 contents of 5 and 9wt% yielded the highest mass activities (35–37Amg−1Au). This is believed to be partly due to MnO2 preventing gold agglomeration, creating a larger gold surface area for reaction. In addition, it is believed that MnO2 may act as an active co-catalyst, especially at higher potentials, by means of oxygen spillover onto gold – leading to higher activities and lower peak potentials. MnO2 was also found to activate the glycerol oxidation reaction earlier, giving lower onset potentials. Stability and impedance studies showed that MnO2-modified catalysts were less stable at a low potential (0V) than Au/C. This could be due to blocking of mixed-valence manganese oxide sites or poisoning of the smaller gold particles on these catalysts. However, at a higher potential of 0.2V, Au/5% MnO2/C demonstrated the best activity and stability overall. All our catalysts are more active than the palladium- and platinum-based catalysts reported in literature, with our best-performing catalyst on a mass activity basis (Au/9% MnO2/C) being at least 12 times more active. This effectively demonstrates the advantage of using a gold-based catalyst for glycerol oxidation, especially when supported by MnO2 which aids in lowering the overpotential for glycerol oxidation over gold.