Activity-Structure Correlation of Pt/Ru Catalysts for the Electrodecomposition of Methanol: The Importance of RuO2 and PtRu Alloying

Herein, we study the effect of both PtRu alloying and the presence of RuO2 species on the promotion of the methanol oxidation activity of PtRu/C catalysts. Bimetallic catalysts composed of 15wt% PtRu/CexC (x=0 or 10) are prepared by using the precipitation-deposition method and activated through hydrogen reduction at 470 K. Different heat treatments, utilizing either N-2 or air, are applied to the as-prepared catalysts to enhance the degree of alloying or produce RuO2, respectively. The electrocatalytic properties, the structure, and the surface composition of the alloys are investigated systematically by means of electrochemical measurements coupled with X-ray diffraction (XRD) and temperature-programmed reduction (TPR) experiments. We find that the N2 heat treatment improves the catalytic activity of the alloys more significantly than the air heat treatment. Also, the current density and long-term durability toward methanol oxidation can be significantly enhanced by combining a loading of 10% CeO2 and N-2 with a heat treatment at 570 K. Physical characterization performed by means of TPR reveals that the surface of the N-2-treated sample is covered with Pt, thereby presenting a higher methanol oxidation current than the air-treated sample whose surface is composed of RuO2 and some alloy species. Moreover, a model for describing the physical structures of the deposited bimetallic crystallites obtained after the N-2 and air treatments is proposed.-This model suggests that the catalysts with the best performance should have a small particle size and exhibit a structure characterized by a high degree of PtRu alloying and a Pt-related surface species. Therefore, we can conclude that the effect of PtRu alloying on the electro-oxidation activity of the catalysts is superior to that of the presence of RuO2 species under practical conditions.