Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst

Heat-Induced Alterations in the Surface Population of Metal Sites in Bimetallic Nanoparticles

The ability to alter the surface population of metal sites in bimetallic nanoparticles (NPs) is of great interest in the context of heterogeneous catalysis. Here, we report findings of surface alterations of Pt and Ru metallic sites in bimetallic carbon-supported (PtRu/C) NPs that were induced by employing a controlled thermal-treatment strategy. The thermal-treatment procedure was designed in such a way that the particle size of the initial NPs was not altered and only the surface population of Pt and Ru was changed, thus allowing us to deduce structural information independent of particle-size effects. X-ray absorption spectroscopy (XAS) was utilized to deduce the structural parameters that can provide information on atomic distribution and/or extent of alloying as well as the surface population of Pt and Ru in PtRu/C NPs. The PtRu/C catalyst sample was obtained from Johnson Matthey, and first the as-received catalyst was reduced in 2 % H2 and 98 % Ar gas mixture at 300 degrees C for 4 h (PtRu/C as-reduced). Later this sample was subjected to thermal treatment in either oxygen (PtRu/C-O2-300) or hydrogen (PtRu/C-H2-350). The XAS results reveal that when the as-reduced PtRu/C catalyst was exposed to the O2 thermal-treatment strategy, a considerable amount of Ru was moved to the catalyst surface. In contrast, the H2 thermal-treatment strategy led to a higher population of Pt on the PtRu/C surface. Characterization of the heat-treated PtRu/C samples by X-ray diffraction and transmission electron microscopy reveals that there is no significant change in the particle size of thermally treated samples when compared to the as-received PtRu/C sample. The electrochemical properties of the as-reduced and heat-treated PtRu/C catalyst samples were confirmed by cyclic voltammetry, CO-adsorption stripping voltammetry, and linear sweep voltammetry. Both XAS and electrochemical investigations concluded that the PtRu/C-H2-350 sample exhibits significant enhancement in reactivity toward methanol oxidation as a result of the increased surface population of the Pt when compared to the PtRu/C-O2-300 and PtRu/C as-reduced samples.

Kinetic study of methanol oxidation on Pt2Ru3/C catalyst in the alkaline media

The electrochemical oxidation of methanol in NaOH solution was examined on a thin film Pt2Ru3/C electrode. The XRD pattern revealed that the Pt2Ru3 alloy consisted of a solid solution of Ru in Pt and a small amount of Ru or a solid solution of Pt in Ru. It was shown that in alkaline solution, the difference in activity between Pt/C and Pt2Ru3/C is significantly smaller than in acid solution. It is proposed that the reaction follows a quasi bifunctional mechanism. The kinetic parameters indicated that the chemical reaction between adsorbed COad and OHad species could be the rate limiting step.

Mixtures of methanol and 2-propanol as a potential fuel for direct alcohol fuel cells

The electrochemical oxidation of methanol, 2-propanol, and their mixtures was investigated on a Pt/C thin film electrode in acid solution. It was confirmed that the oxidation of 2-propanol commences at less positive potentials than that of methanol and exhibits significantly higher oxidation current densities at low potentials. When both methanol and 2-propanol were present in the solution, the onset of the oxidation current was the same as for the oxidation of pure 2-propanol. Although both alcohols inhibit the oxidation reaction of each other to a certain extent, steady-state polarization measurements showed that their mixture provides higher current densities than single alcohols over the entire potential region from the hydrogen region to oxide formation on the Pt surface. This implies that the addition of 2-propanol into the fuel may extend the operational range of direct methanol fuel cells.

Novel Method for the Synthesis of Hydrophobic Pt−Ru Nanoparticles and Its Application to Preparing a Nafion-Free Anode for the Direct Methanol Fuel Cell

Pt-Ru alloy is a bimetallic catalyst most commonly used in the direct methanol fuel cell (DMFC). In this paper, a new process to synthesize an unsupported Pt-Ru colloid has been introduced. The characteristics of synthesized nanoparticles were identified by XRD, TEM/EDX, and SEM, and it shows that Ru atoms are incorporated into the Pt fcc structure and the well-dispersed particles (diameter approximately 4 nm) possess a Pt-rich feature. This catalyst shows a hydrophobic characteristic which can adsorb very well on the hydrophobic-treated carbon paper or carbon cloth without the need of Nafion. Accordingly, this method can avoid particle agglomeration, and the synthesized catalyst demonstrates strong adsorption with carbon paper. In addition, this colloid-type Nafion-free catalyst was measured via linear sweep voltammetry (LSV) and exhibited electrochemical activity for methanol oxidation comparable to the commercial one with Nafion binding.

Preparation and characterization of aqueous colloids of Pt–Ru nanoparticles

A synthetic method for platinum-ruthenium (PtRu) nanoparticles in aqueous media is proposed. This method employs citric acid as a capping agent and NaBH(4) as a reducing agent with the aid of pH control. The number-averaged size of the PtRu nanoparticles was ca. 2 nm. The crystal phase and chemical composition of the nanoparticles was investigated by X-ray diffraction measurement and scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis, which indicated that the nanoparticles mainly consisted of an alloy of Pt and Ru. Electrochemical measurement showed that the PtRu nanoparticles had catalytic activity for methanol oxidation.

Ru-Decorated Pt Surfaces as Model Fuel Cell Electrocatalysts for CO Electrooxidation

This feature article concerns Pt surfaces modified (decorated) by ruthenium as model fuel cell electrocatalysts for electrooxidation processes. This work reveals the role of ruthenium promoters in enhancing electrocatalytic activity toward organic fuels for fuel cells, and it particularly concerns the methanol decomposition product, surface CO. A special focus is on surface mobility of the CO as it is catalytically oxidized to CO(2). Different methods used to prepare Ru-decorated Pt single crystal surfaces as well as Ru-decorated Pt nanoparticles are reviewed, and the methods of characterization and testing of their activity are discussed. The focus is on the origin of peak splitting involved in the voltammetric electrooxidation of CO on Ru-decorated Pt surfaces, and on the interpretative consequences of the splitting for single crystal and nanoparticle Pt/Ru bimetallic surfaces. Apparently, screening through the literature allows formulating several models of the CO stripping reaction, and the validity of these models is discussed. Major efforts are made in this article to compare the results reported by the Urbana-Champaign group and the Munich group, but also by other groups. As electrocatalysis is progressively more and more driven by theory, our review of the experimental findings may serve to summarize the state of the art and clarify the roads ahead. Future studies will deal with highly dispersed and reactive nanoscale surfaces and other more advanced catalytic materials for fuel cell catalysis and related energy applications. It is expected that the metal/metal and metal/substrate interactions will be increasingly investigated on atomic and electronic levels, with likewise increasing participation of theory, and the structure and reactivity of various monolayer catalytic systems involving more than two metals (that is ternary and quaternary systems) will be interrogated.

Supplement to the Theory of Normal Pulse Voltammetry and Its Application to the Kinetic Study of Methanol Oxidation on a Polycrystalline Platinum Electrode

The theory of normal pulse voltammetry (NPV) for complex multistep multielectron transfer processes on a plane electrode was advanced and applied to the completely irreversible process of methanol oxidation to formic acid in the potential range from 0.3 to 0.8 V versus Ag/AgCl. The kinetic parameters for this process, such as the standard rate constant (k0) and anodic transfer coefficient (alpha) for this irreversible heterogeneous electron transfer process at the electrode/solution interface and apparent diffusion coefficient (D(app)) for the homogeneous charge transfer process within liquid film near the electrode surface, were obtained with NPV theory from analyzing the dependence of current-potential curves upon the sampling times. The results showed that this process is truly a very slow, completely irreversible kinetic process, as k0 is in the order of 10(-9) cm/s for the rate-determining step. The values of k0 and D(app) decreased with the increase of methanol concentration, while alpha was independent of the concentration of methanol and its value was 0.35 +/- 0.05. Theoretical fitting is very consistent with the experimental data.

Remarkable dynamical opening and closing of platinum and palladium pentaruthenium carbido carbonyl cluster complexes

The reaction of Ru(5)(CO)(15)(mu(5)-C), 1, with Pt(PBu(t)(3))(2) at room temperature yielded the mixed-metal cluster complex PtRu(5)(CO)(15)(PBu(t)(3))(C), 2, in 52% yield. Compound 2 consists of a mixture of two interconverting isomers in solution. One isomer, 2A, can be isolated by crystallization from benzene/octane solvent. The second isomer, 2B, can be isolated by crystallization from diethyl ether. Both were characterized crystallographically. Isomer 2A consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom spanning the square base. Isomer 2B consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom on an edge on the square base. The two isomers interconvert rapidly on the NMR time scale at 40 degrees C, deltaG(313)++ = 11.4(8) kcal mol(-1), deltaH++ = 8.8(5) kcal mol(-1), deltaS++ = -8.4(9) cal mol(-1) K(-1). The reaction of Pd(PBu(t)(3))(2) with compound 1 yielded two new cluster complexes: PdRu(5)(CO)(15)(PBu(t)(3))(mu(6)-C), 3, in 50% yield and Pd(2)Ru(5)(CO)(15)(PBu(t)(3))(2)(mu(6)-C), 4, in 6% yield. The yield of 4 was increased to 47% when an excess of Pd(PBu(t)(3))(2) was used. In the solid state compound 3 is structurally analogous to 2A, but in solution it also exists as a mixture of interconverting isomers; deltaG(298)++ = 10.6(6) kcal mol(-1), deltaH++ = 9.7(3) kcal mol(-1), and deltaS++ = -3(1) cal mol(-1) K(-1) for 3. Compound 4 contains an octahedral cluster consisting of one palladium atom and five ruthenium atoms with an interstitial carbido ligand in the center of the octahedron, but it also has one additional Pd(PBu(t)(3)) grouping that is capping a triangular face of the ruthenium cluster. The Pd(PBu(t)(3)) groups in 4 also undergo dynamical interchange that is rapid on the NMR time scale at 25 degrees C; deltaG(298)++ = 11(1) kcal mol(-1), deltaH++ = 10.2(4) kcal mol(-1), and deltaS++ = -3(2) cal mol(-1) K(-1) for 4.