Influence of particle size on the electrocatalytic oxidation of glycerol over carbon-supported gold nanoparticles

Abstract

The influence of gold particle size on glycerol oxidation was investigated using carbon-supported gold catalysts. Small gold particles of average diameter ≤ 4.7nm had higher activities (on a mass basis) than medium-sized (14.7nm) particles and were at least twice as active as catalysts containing large (≥ 43nm) gold particles. The small gold particles were also activated at lower potentials, resulting in lower onset potentials for glycerol oxidation. On the other hand, large gold particles had much higher onset potentials and were less stable during glycerol oxidation. While small gold particles were more active on a mass basis (primarily due to surface area effects), it was found that the specific activity (per unit real surface area) increased with increasing gold particle size. Based on evidence from Pb underpotential deposition experiments, it is proposed that this increase in specific activity can be related to an changes in the proportion of Au(111) and Au(110) surfaces as a function of particle size, with the Au(111) plane (dominant on large particles) having higher intrinsic activity compared to the Au(110) surface. This analysis also suggests that the lower onset potentials for glycerol oxidation on small nanoparticles can be attributed to the larger fraction of Au(110) facets at the surface of the small particles, and that the deactivation of larger particles is related to the high proportion of Au(111) facets on the surface of these particles. This analysis provides an important structural rationale for understanding the electrocatalytic behaviour of Au nanoparticles towards glycerol oxidation–independent of inherent size effects. As the important requirements for fuel cell catalysts are high mass activity, low overpotentials and high stability, our investigation demonstrates that all these conditions are met by the catalysts containing small gold particles defined by predominantly Au(110) facets.