Effects of Alloyed and Oxide Phases on Methanol Oxidation of Pt-Ru/C Nanocatalysts of the Same Particle Size Denis R. M. Godoi, Joelma Perez, and H. Mercedes Villullas* Departamento de Fı ´sico-Quı ´mica, Instituto de Quı ´mica, UniVersidade Estadual Paulista (UNESP), 14801-970 CP 355, Araraquara, Brazil ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: March 30, 2009 In this work, methanol oxidation was studied on carbon-supported Pt-Ru nanocatalysts, where the amounts of alloyed and oxide phases were modified by heat treatments in different atmospheres. Because particle growth was avoided using mild temperature conditions, the study reported here was conducted in the absence of particle size effects. All samples were characterized by X-ray diffraction and transmission electron microscopy. The general electrochemical behavior of the nanocatalysts was evaluated by cyclic voltammetry, and the electrocatalytic activity for the oxidation of methanol was studied in 0.5 mol L -1 methanol acid solutions by linear potential sweeps and chronoamperometry. The results obtained clearly evidence that the presence of oxide species is necessary to enhance the electrocatalytic activity for methanol oxidation. Oxidation of adsorbed CO was also measured. Both reactions, methanol and adsorbed CO oxidation, were found to be very sensitive to the surface changes produced by the heat treatments. Interestingly, the best catalyst for methanol oxidation was not found to be the most efficient for the oxidation of adsorbed CO. Electrocatalytic activities correlate well with oxidation states and electronic properties analyzed by X-ray photoelectron spectroscopy and in situ dispersive X-ray absorption spectroscopy. Introduction The use of fuel cells for the production of electric power in a clean, silent, and efficient way comes as an excellent alternative for supplying part of the current needs of energy production. 1,2 Proton exchange membrane fuel cells (PEMFC) are among the most promising of those devices, and the most efficient ones generate electric power from the oxidation of hydrogen in the anode and reduction of oxygen in the cathode. While direct methanol fuel cells (DMFC) are alternative systems for replacing hydrogen by a liquid fuel because they minimize storage and transport problems and eliminate the need of a reformer, several problems remain to be solved to make DMFC performance sufficient for many practical applications. Among them, the performance of the catalysts still needs to be improved. Even though Pt is the most active metal for the electrochemical oxidation of methanol, 3 the slow kinetics of the reaction and the formation of strongly adsorbed intermediates such as carbon monoxide produce substantial losses in operation potentials. The most common approach to this problem has been the development of Pt-based catalysts such as Pt-Ru, Pt-Mo, Pt-Ni, etc. 3-15 The enhanced activity of these materials is generally interpreted in terms of the ability of the second metal to supply, in lower potentials, the oxygenated species necessary for the complete oxidation of the alcohol and for the removal of the poisoning species adsorbed onto the catalyst surface. 16 Additionally, electronic effects have also been attributed to the second metal, which would result in weakening the bond between the poisoning species and catalyst surface. 17 Published results show that methanol oxidation on Pt-Ru is greatly influenced by a number of physical properties such as composition, structure, morphology, particle size, and degree of alloying. It was also shown that those properties depend on synthesis conditions, and therefore, it is not surprising to find that the electrocatalytic activity is also strongly dependent on the preparation methodology adopted. 18 In a recent work by our group, 19 properties such as the degree of alloying and amount of oxides were determined for Pt-Ru/C nanocatalysts, which were synthesized in water/n-heptane/AOT microemulsions with their average particle size carefully controlled, were found to be size-dependent. Thus, a better understanding of the effects of structural and chemical properties seems essential to improve the electrocatalysis of methanol oxidation and as a basis for developing new and more efficient materials. In this work, methanol oxidation was studied on Pt-Ru/C catalysts heat treated in mild temperature conditions, which did not cause any significant particle growth, and under different atmospheres that promoted changes in the amount of oxides and degree of alloying. Considerable differences in electrocata- lytic activities toward methanol and adsorbed CO oxidation were observed on these Pt-Ru/C catalysts of the same particle size; they are discussed here in terms of surface composition (oxide and alloy). Experimental Section Electrocatalysts Preparation. Pt-Ru nanoparticles were first obtained in water/n-heptane/AOT microemulsions as described elsewhere. 19 Briefly, an aqueous solution of H 2 PtCl 6 and RuCl 3 (0.5 wt % metal, Pt:Ru atomic ratio 1:1) was added to a mixture of n-heptane and AOT (15 wt %) under constant stirring to form a microemulsion with a water to surfactant molar ratio (w) equal to 8. Reduction of metallic precursors was done by adding NaBH 4 as a solid in a molar ratio of 10:1 to metals. The mixture was kept under constant stirring for 2 h, and then an appropriate amount of high surface area carbon (Vulcan XC-72, Cabot) was added to obtain the supported catalysts with a metal load (Pt + Ru) of 20 wt %. The suspension obtained was stirred overnight. * To whom correspondence should be addressed. E-mail: mercedes@ iq.unesp.br. Fax: +55 16 3301 6692. J. Phys. Chem. C 2009, 113, 8518–8525 8518 10.1021/jp8108804 CCC: $40.75  2009 American Chemical Society Published on Web 04/15/2009