Particle Size Effect of Unsupported Pt/SnO x Nanoparticles for Ethanol Electro-Oxidation

摘要

Unsupported colloidal Pt/SnO x nanoparticles of three well-defined mean sizes: 2.5, 4.5, and 6.5 nm and Pt/Sn nominal composition of 70/30 at.% are synthesized and tested for ethanol electro-oxidation at room temperature. Studying unsupported particles allowed investigation of their intrinsic electrocatalytic behavior thus avoiding the metal support interaction effect. Change in particle size influenced the extent of tin surface segregation. For larger Pt/SnO x nanoparticles of 4.5 and 6.5 nm, the higher surface enrichment by tin is observed, as well as the amount of Pt surface active sites decreases with increasing the particle size. Despite the low amount of surface active sites on larger colloids, their catalytic activity towards ethanol electro-oxidation is superior at low overpotentials if compared to smaller ones. Thus, at 0.4 V, particles of 6.5 and 4.5 nm mean size show higher current density as per geometric, as well as active surface area than 2.5 nm colloids. This confirms that large ensembles of Pt surface atoms are required in order to dissociatively adsorb ethanol molecules and efficiently break C–C bond. Another benefit in having larger Pt sites is associated with the fact that poisonous intermediates formed during ethanol oxidation, e.g., COads are less strongly adsorbed on larger particles, which have a greater amount of terrace sites and therefore can be efficiently removed by OHads species provided by Sn.