CO oxidation and oxygen reduction activity of bimetallic Sn–Pt electrocatalysts on carbon: effect of the microstructure and the exclusive formation of the Pt3Sn alloy

Abstract

Alloy-type Sn–Pt/C electrocatalysts with the desired Pt/Sn = 3.0 ratio have been prepared by controlled surface reactions using home-made 20 wt% Pt/C (20Pt/C) catalysts with different Pt dispersion. Reaction conditions were found for the preparation of highly dispersed 20Pt/C catalysts by a modified NaBH4-assisted ethylene glycol reduction method using ethanol as a solvent. It has been demonstrated that the increase of the heating time in ethanol up to 2 h results in decreasing dispersion of Pt. Upon using highly dispersed 20Pt/C catalyst, the exclusive incorporation of Sn onto the Pt sites was achieved resulting in the exclusive formation of the Pt–Sn alloy phase. According to in situ XPS studies, the pre-treatment of the air exposed catalyst in H2 even at 170 °C resulted in the complete reduction of the ionic tin to Sn0, suggesting alloy formation. In contrast, the catalyst with lower Pt dispersion cannot be completely reduced even at 350 °C, as 10% of tin still remains in the form of Sn4+ surface species. The electrocatalytic performance of both Sn-20Pt/C catalysts in the CO electrooxidation and the oxygen reduction reaction is superior to that of the parent 20Pt/C catalysts. Our data obtained for the oxygen reduction indicate that the small size of the bimetallic nanoparticles in the highly dispersed Sn-20Pt/C catalyst, along with their optimal surface composition, result in increased activity compared to the catalyst with lower dispersion.