The function of ruthenium oxides in Pt-Ru catalysts for methanol electro-oxidation at low temperatures

The electrocatalytic activities of different binary Pt-Ru(ox) catalysts have been investigated in half-cell experiments by cyclic voltammetry and stationary current–potential measurements. The materials have been prepared using a modification of the Adams method. X-ray analytical methods (X-ray diffraction, XRD, and energy dispersive X-ray spectroscopy, EDX) as well as thermogravimetric analysis (TGA) have been used to characterize the composition and the catalysts' content of the crystalline phases, and their surface areas have been determined by the BET method. It is found that the composition of the catalyst is strongly influenced by the synthesis temperature, which is varied between 400 and 600 °C. In contrast, the particle size of the metallic phases of the catalysts is not significantly affected for synthesis temperatures below 600 °C, as investigated by transmission electron microscopy. Synthesis temperatures of ≥500 °C favor the formation of crystalline RuO2 phases, whereas at synthesis temperatures below 500 °C the presence of metallic alloy and of hydrous oxides was derived by the combined XRD and EDX measurements. The stationary current–potential curves show a correlation with the different synthesis temperatures. It can be concluded that both the presence of an alloyed metallic Pt-Ru phase as well as the presence of amorphous hydrated Ru oxides are favorable for the electrocatalytic oxidation of methanol.