High-loading Pt nanoparticles on mesoporous carbon with large mesopores for highly active methanol electro-oxidation

High metal-loading Pt/C electrocatalysts are important for the fabrication of thin-layered membrane electrode assemblies (MEAs). However, the preparation of high-loading Pt catalysts with a narrow size distribution of nanoparticles remains a challenge. Herein, ordered mesoporous carbon (OMC) with large mesopores (~15 nm) and a high surface area (1316.0 m² g^?1) was fabricated using a SiO₂ nanosphere array as a template. This material was developed to support a high loading of Pt nanoparticles (60 wt%) and was then used as an electrocatalyst for the methanol oxidation reaction (MOR). The prepared Pt/OMC contains Pt nanoparticles with an average size of ~1.9 nm that are uniformly dispersed on the mesoporous walls of the OMC. The Pt/OMC catalyst exhibits smaller Pt nanoparticle size, greater Pt dispersion, larger specific electrochemically active surface area (ECSA), and higher electrocatalytic activity for the MOR than the carbon black (Vulcan XC-72R)-supported Pt and the commercial Pt/C catalysts.     

碳黑预处理对Pt/C催化剂对甲醇氧化电催化活性的影响

直接甲醇燃料电池(DMFC)由于具有较多的优点而受到广泛的关注. 但是碳载Pt (Pt/C)阳极催化剂电催化活性低是限制其应用的一个主要问题. 为了提高Pt/C催化剂对甲醇氧化的电催化性能, 分别用CO₂, 空气, H₂O₂或HNO₃对常用作为载体的Vulcan XC-72碳黑进行预处理. 结果表明, 在用CO₂, 空气, HNO₃, H₂O₂处理的及未处理的碳黑作载体制得的Pt/C催化剂电极上, 甲醇氧化峰的峰电流密度顺序为39, 33, 32, 20和18 mA•cm^－2, 表明用CO₂处理的碳载体制备的Pt/C催化剂对甲醇氧化有最好的电催化活性和稳定性. 其主要原因是用CO₂处理能减少碳黑表面的含氧基团和增加石墨化程度, 而使碳黑的电阻降低及Pt粒子在碳黑上的分散性变好.     

High loading of Pt–Ru nanocatalysts by pentagon defects introduced in a bamboo-shaped carbon nanotube support for high performance anode of direct methanol fuel cells

Bamboo-shaped carbon nanotubes (BCNTs), with a large amount of pentagon defects introduced in the walls, were explored as the support of high loaded Pt–Ru catalysts for the anode of direct methanol fuel cells (DMFCs) in comparison with conventional carbon nanotubes (CNTs) and Vulcan XC carbon black. By ethylene glycol reduction, Pt–Ru catalysts with a high loading (60 wt%) and uniform particle size of 2–3 nm were uniformly deposited on BCNTs; while 60 wt% Pt–Ru catalysts on CNTs resulted in significant agglomeration. The Pt–Ru/BCNT catalyst showed the highest activity on methanol oxidation in cyclic voltammetry and highest performance as the anode in a DMFC single cell. Such an enhancement was largely ascribed to an enhanced interaction of the introduced pentagon defects with Pt–Ru, which could promote a high loading and well dispersion of Pt–Ru catalysts and the charge transfer from Pt–Ru to the tubes.     

Metal Oxide Promoted Electrocatalysts for Methanol Oxidation

An important research target in DMFCs is to find better catalyst materials that are cheaper, less-prone to poisoning and more catalytically active. In this context, metal oxides with good catalytic properties and stronger interaction with Pt nanoparticles can generate active interfacial regions for electrocatalysis. Pt catalysts promoted by certain metal oxides show enhanced methanol electro-oxidation activity and CO tolerance behavior. In this paper we summarize the recent progress from our laboratory which explored the possibility of developing Pt–MoO₃/C and Pt–Nb₂O₅/C electrocatalysts in acidic media, and Pt–V₂O₅/C electrocatalyst in alkaline media for direct electro-oxidation of methanol. The oxide electrocatalysts have been prepared by a fast and efficient method of loading the metal oxide on carbon black (Vulcan XC-72) employing an intermittent microwave heating (IMH) method. These materials are found to achieve higher activity and stability towards methanol electro-oxidation.     

Comparative characteristics of cathodes with different catalytic systems in hydrogen–oxygen and hydrogen–air fuel cells with proton-conducting polymer electrolyte

The characteristics of low-temperature hydrogen–oxygen (air) fuel cell (FC) with cathodes based on the 50 wt % PtCoCr/C and 40 wt % Pt/CNT catalysts synthesized on XC72 carbon black and carbon nanotubes (CNT) are compared with the characteristics of commercial monoplatinum systems 9100 60 wt % Pt/C and 13100 70% Pt/C HiSPEC. It is shown that the synthesized catalysts exhibit a high mass activity, which is not lower than that of commercial Pt/C catalysts, a high selectivity with respect to the oxygen reduction to water, and a significantly higher stability. The characteristics of PtCoCr/C and Pt/CNT were confirmed by testing in the hydrogen—oxygen FCs. However, when air was used at the cathode, especially in the absence of excessive pressure, a voltage of FC with the cathode based on PtCoCr/XC72 is lower as compared with the commercial systems. Probably, this is associated with the transport limitations in the structure of trimetallic catalyst synthesized on XC72 carbon black due to the absence of mesopores. This drawback was eliminated to a large extent by raising the volume of mesopores as a result of application of mixed support (XC72 + CNT) and the use of only CNT for the synthesis of the monoplatinum catalyst. However, this did not eliminate another drawback, namely, a low platinum utilization coefficient in the cathode active layer as compared with that measured under the model conditions in the 0.5 M Н₂SO₄ solution. Therefore, further research is required to improve the structure of the catalytic systems, which are synthesized both on carbon black and nanotubes, while maintaining their high stability and selectivity.     

Highly graphitized carbon-wrapped PtFeCo alloy with enhanced durability and activity toward methanol electro-oxidation

Exploring efficient strategies to construct durable and active Pt-based electrocatalysts toward methanol oxidation reaction (MOR) remains great significance for the application of direct methanol fuel cells (DMFCs). Here, we report a facile pyrolysis procedure for fabricating carbon layer wrapped PtFeCo alloy nanoparticles supported on nitrogen-doped carbon nanotubes (NCNT). Physical characterizations demonstrate that the nitrogen-doped carbon support is highly graphitized and the PtFeCo particles are firmly wrapped by the graphitized carbon. Since the wrapping of highly graphitized carbon effectively prevents PtFeCo alloy from metal dissolution, the durability of the synthesized PtFeCo/Co–NCNT_a catalyst has been substantially improved, remaining about 76% of its initial mass activity after 1000 cycles of durability test in acid condition. In addition, due to the strain and ligand effects caused by alloying Pt with Fe and Co, the PtFeCo/Co–NCNT_a catalyst exhibits a greatly enhanced mass activity of 4.2-fold and a specific activity of 6.3-fold higher than those of commercial Pt/C-JM catalyst. Consequently, this work may provide an effective route for preparing durable and active Pt-based catalysts for methanol electro-oxidation.     

Promoting effect of carbon functional groups in methanol oxidation on supported Pt catalyst

The contribution of carbon functional groups (CFG) to methanol oxidation at carbon-supported Pt catalysts is studied in this work. Platinum black, attached in a form of a thin layer to electrochemically treated glassy carbon (GC), is investigated as a model system. Impedance measurements reveal significant changes in capacitive response of GC anodically polarized in H₂SO₄ solution in comparison to unoxidized state. XPS analyses show the increased fraction of oxygen-containing CFGs upon electrochemical treatment of GC. The activity of Pt black attached to oxidized GC for methanol oxidation is more than two times larger in comparison to the catalyst attached to unoxidized GC. Increased activity is most likely due to the promotion of CO_ads removal by increased content of CFGs.     

Polymer-mediated synthesis of highly dispersed Pt nanoparticles on carbon black

A new method was developed to prepare highly dispersed Pt nanoparticles on carbon black to use as proton exchange membrane (PEM) fuel cell catalysts. This method involves using a polymer, poly(vinylpyrrolidone) (PVP), to prevent particle aggregation and thereby reduce nanoparticle sizes to achieve high dispersion. It was found that Pt nanoparticles mediated by PVP are smaller than those obtained without PVP and have a narrower size distribution. Well-dispersed Pt nanoparticles with metal loadings from 5 to 35 wt % were obtained on carbon black (Vulcan XC-72R). It was found that well-dispersed Pt nanoparticles on carbon black could be synthesized at a PVP monomers-to-Pt atoms ratio of 0.1 under our experimental conditions. Larger amounts of PVP did not produce smaller nanoparticles, but rather reduced the Pt mass loading on carbon black. The morphology of the Pt nanoparticles that were supported on carbon black was characterized with transmission electron microscopy and X-ray diffraction. Their active surface areas were determined using cyclic voltammetry in a sulfuric acid solution. High Pt dispersion was obtained for the catalysts synthesized with PVP mediation, even at Pt loadings up to 35 wt %. The catalysts prepared with PVP mediation generally showed larger active specific areas than did those prepared without PVP.     

One-step synthesis of carbon-supported Pd–Pt alloy electrocatalysts for methanol tolerant oxygen reduction

A facile, one-step reduction route was developed to synthesize Pd-rich carbon-supported Pd–Pt alloy electrocatalysts of different Pd/Pt atomic ratios. As-prepared Pd–Pt/C catalysts exhibit a single phase fcc structure and an expansion lattice parameter. Comparison of the oxygen reduction reaction (ORR) on the Pd–Pt/C alloy catalysts indicates that the Pd₃Pt₁/C bimetallic catalyst exhibits the highest ORR activity among all the Pd–Pt alloy catalysts and shows a comparative ORR activity with the commercial Pt/C catalyst. Moreover, all the Pd–Pt alloy catalysts exhibited much higher methanol tolerance during the ORR than the commercial Pt/C catalyst. High methanol tolerance of the Pd–Pt alloy catalysts could be attributed to the weak adsorption of methanol induced by the composition effect, to the presence of Pd atoms and to the formation of Pd-based alloys.     

Preparation of Multiwall Carbon Nanotubes-supported High Loading Platinum for Vehicular PEMFC Application

Multiwall carbon nanotube-supported Pt （Pt/MWNTs） catalysts with high dispersion and high loading of Pt were prepared by chemical reduction method and the loading of Pt got to 40 wt%. The average diameter of Pt nanoparticles on MWNTs was about 3.5 nm. When the hydrogen and air were used as reactant gases for PEMFC, Pt/MWNTs catalysts showed significantly higher performance than the Pt/XC-72 （carbon black） catalysts.     

分级结构氮掺杂碳纳米笼担载的铂基高效甲醇氧化电催化剂

高性能低成本的担载型铂基催化剂是直接甲醇燃料电池(DMFC)实用化过程中的一大挑战.利用高比表面积、高稳定性、容易负载金属的载体实现 Pt颗粒的高度分散,既可提高催化剂的催化性能,又可提高 Pt的利用率以降低成本,是担载型 Pt基催化剂实用化的有效途径.碳材料是一种常用的催化剂载体,近年来我们课题组发展了一种高性能的碳纳米笼材料,并可通过异原子掺杂调变其表面性能,提高其活性和负载能力.我们采用原位氧化镁模板法制备氮掺杂碳纳米笼：以具有多级结构的碱式碳酸镁作为氧化镁模板的前体,吡啶为碳源和氮源,经高温热解沉积,在原位形成的氧化镁模板表面形成氮掺杂的石墨化碳纳米薄层;经稀盐酸浸泡并洗涤,获得高纯度的氮掺杂碳纳米笼.氮掺杂碳纳米笼具有分等级的微纳米结构、高导电性、高比表面积和可调变的孔结构,结合表面氮原子的锚钉作用,氮掺杂碳纳米笼有望成为电化学催化剂 Pt的优良载体.
　　在前期研究基础上,本文探索多级结构氮掺杂碳纳米笼(hNCNC)作为新型载体负载 Pt的能力,并评价所构建的负载型催化剂 Pt/hNCNC的电催化性能.通过简便的微波辅助多元醇还原法,将氯铂酸还原成 Pt纳米粒子负载于 hNCNC的表面.为了揭示氮掺杂的效应,我们对比研究了具有相似分级结构但无掺杂的碳纳米笼(hCNC)以及商业化活性炭(Val-can XC-72)作为载体的情况.经热重(TG)和 X射线光电子能谱(XPS)分析,三种催化剂 Pt/hNCNC、Pt/hCNC和 Pt/XC-72的负载量均接近理论负载量(23.1 wt%),都主要以金属态存在.然而,扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明, Pt/hNCNC的 Pt分散状态优于 Pt/hCNC,更远优于 Pt/XC-72. Pt/hNCNC的平均 Pt粒径最小,仅约3.3 nm.这种良好的分散状态主要得益于氮原子掺杂,高负电性的氮原子改变了局域的表面极性,有利于 Pt颗粒的成核,也有利于固定 Pt颗粒.
　　由于 hNCNC对 Pt的优异分散能力, Pt/hNCNC表现出高的电化学活性面积.氢吸附和一氧化碳溶出伏安曲线表明, Pt/hNCNC的电化学活性面积高于 Pt/hCNC和 Pt/XC-72,这与显微观察和 X射线衍射(XRD)结果相吻合. Pt/hNCNC展现出优异的甲醇电催化氧化活性和高稳定性,其催化电流明显高于 Pt/hCNC和 Pt/XC-72,电流衰减亦慢于 Pt/hCNC和 Pt/XC-72. hNCNC的分级微纳米结构有利于孔内传质和电子输运,从而提高反应速度. hNCNC的氮掺杂有利于 Pt在载体表面的分散,增强了载体-金属相互作用,提高了电化学活性面积和催化活性.为了进一步考察 hNCNC对 Pt的负载能力,本文还考察了高负载量 Pt/hNCNC的性能.在负载量高达60 wt%时, Pt/hNCNC中的 Pt颗粒仍无明显聚集,其甲醇氧化电流增加了30%,可以有效提高 DMFC的输出电流密度.
　　综上可见, hNCNC可以有效分散并稳定 Pt颗粒,从而提高电化学活性面积和甲醇电催化氧化活性,优于未掺杂的碳纳米笼和传统碳材料,展示了 hNCNC高分散 Pt颗粒用作 DMFC的高效阳极催化剂的重要前景,也表明 hNCNC有望成为应用广泛的新型载体.     

Oxygen reduction reaction activity of monodispersed Pt nanoparticles-loaded carbon black catalyst prepared with a Pt carbonyl cluster anion

Pt nanoparticles-loaded carbon black (CB) was prepared from Pt carbonyl cluster complexes, and had much narrower size distribution than commercial Pt nanoparticles/CB. In the former the monodispersed Pt nanoparticles were highly dispersed on CB without aggregation even at high Pt loading of 80 wt.%. Hydrodynamic voltammograms in O₂-saturated 0.05 M H₂SO₄ solution at 30 °C showed that the onset potential of oxygen reduction reaction (ORR) current for the monodispersed Pt nanoparticles/CB electrode was more positive than that for a polycrystalline Pt electrode and similar to that for the commercial Pt nanoparticles/CB electrode. Moreover, the mass activity for ORR for the monodispersed Pt nanoparticles/CB electrode was ca. 4.9 times higher than that for the commercial Pt nanoparticles/CB electrode, clearly indicating that the control of size distribution of Pt nanoparticles is meaningful for reducing the Pt consumption.     

碳载体的O3处理对直接甲醇燃料电池Pt-Ru/C催化剂性能的影响

为了提高直接甲醇燃料电池阳极催化剂的催化活性, 利用O₃处理的Vulcan XC-72碳黑为载体, 制备Pt-Ru/C催化剂, 并与未经处理的Vulcan XC-72为载体制备的Pt-Ru/C催化剂的性能进行比较. 采用XPS和BET测试了O₃处理后碳粉表面的含氧浓度和比表面积, 结果表明, 随处理时间延长, 碳表面含氧浓度先减少后增加, 比表面积减小; 而随着处理温度升高, 比表面积增加, 含氧浓度先减少后增加. XRD, TEM方法对催化剂的结构及形貌的表征结果表明: O₃处理的碳黑为载体制备的Pt-Ru/C催化剂粒径均匀、分散性好. 在0.5 mol/L CH₃OH和0.5 mol/L H₂SO₄溶液中, 利用粉末微电极测试了循环伏安和稳态极化曲线, 结果表明: O₃处理碳粉为载体的催化剂比未经处理的碳粉为载体的催化剂的活性高. 研究了O₃处理碳粉的时间和温度对催化剂性能的影响, 电化学测试表明: 140 ℃处理6 min的碳粉为载体制备的催化剂对甲醇的电催化氧化活性最佳.     

Pt dendrimer nanocomposites for oxygen reduction reaction in direct methanol fuel cells

Dendrimer-encapsulated Pt nanoparticles (G4OHPt) were prepared by chemical reduction at room temperature. The G4OHPt, with average diameters of ca. 2.7 nm, were characterized by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. Electrocatalytic behavior for oxygen reduction reaction was investigated using a rotating disk electrode configuration in an acidic medium, with and without the presence of methanol (0.01, 0.1, and 1 M). Kinetic studies showed that electrodes based on Pt nanoparticles encapsulated inside the dendrimer display a higher selectivity for ORR in the presence of methanol than electrodes based on commercial Pt black catalysts. Also, the dendritic polymer confers a protective effect on the Pt in the presence of methanol, which allows its use as a cathode in a direct methanol fuel cell operating at different temperatures. Good performance was obtained at 90 °C and 2 bar of pressure with a low platinum loading on the electrode surface.     

Bimetallic Cluster Provides a Higher Activity Electrocatalyst for Methanol Oxidation

The cluster complex Pt₂Ru₄(CO)₁₈ was used as a precursor to prepare a 60 wt% 1:2 Pt:Ru nanoparticles on carbon (PtRu/C) for use as an electrocatalyst for methanol oxidation. This bimetallic carbonyl cluster complex was found to provide smaller, more uniform bimetallic nanoparticle that exhibited higher electrocatalytic activity than a 60 wt% 1:1 Pt:Ru commercial catalyst from E-Tek. Using bimetallic cluster precursors simplifies the synthetic procedures by reducing the need for high temperature reduction and assures a more intimate mixing of the two different metals. Transmission electron microscopy (TEM) images of the catalyst obtained from the cluster precursor showed bimetallic nanoparticles having a narrow size range of 2–3 nm that were dispersed uniformly over the surface of the support. Images of the commercial catalyst showed particles 3–4 nm in diameter that tended to agglomerate near the edges of the carbon support particles. Cyclic voltammograms of methanol oxidation from the two catalysts showed significantly higher activity for the cluster-derived catalyst. The onset potential for methanol oxidation for the cluster-derived catalyst was approximately 170 mV lower than that of the commercial catalyst at 100 A/g Pt, and approximately 250 mV lower at 400 A/g Pt. ^* This report is dedicated to Prof. Günter Schmid on the occasion of his 70th birthday.     

Porous Ni@Pt core-shell nanotube array electrocatalyst with high activity and stability for methanol oxidation

Bimetallic core-shell nanostructures are emerging as more important materials than monometallic nanostructures, and have much more interesting potential applications in various fields, including catalysis and electronics. In this work, we demonstrate the facile synthesis of core-shell nanotube array catalysts consisting of Pt thin layers as the shells and Ni nanotubes as the cores. The porous Ni@Pt core-shell nanotube arrays were fabricated by ZnO nanorod-array template-assisted electrodeposition, and they represent a new class of nanostructures with a high electrochemically active surface area of 50.08 m(2) (g Pt)(-1), which is close to the value of 59.44 m(2) (g Pt)(-1) for commercial Pt/C catalysts. The porous Ni@Pt core-shell nanotube arrays also show markedly enhanced electrocatalytic activity and stability for methanol oxidation compared with the commercial Pt/C catalysts. The attractive performances exhibited by these prepared porous Ni@Pt core-shell nanotube arrays make them promising candidates as future high-performance catalysts for methanol electrooxidation. The facile method described herein is suitable for large-scale, low-cost production, and significantly lowers the Pt loading, and thus, the cost of the catalysts.     

Hollow carbon spheres with wide size distribution as anode catalyst support for direct methanol fuel cells

Hollow carbonaceous composites (HCCs) possessing sphere and hemisphere shape, which had wide size distribution between several tens of nanometers and several micrometers, were prepared through a facile hydrothermal method using glucose as carbon source with the assistance of sodium dodecyl sulfate (SDS). Pyrolysis of these hollow carbonaceous composites at 900 °C under nitrogen flow produced carbonized hollow carbon spheres (HCSs) without changing their structures. Platinum (Pt) was directly deposited on the surface of the HCSs by incipient wet method, using the NaBH₄ as the reductant. TEM, SEM, powder XRD and FT-IR were utilized to characterize all these samples. It was found that Pt nanoparticles were uniformly anchored on the outer and the inner surface of HCSs. The electrocatalytic properties of the Pt/HCS electrode for methanol oxidation have been investigated through cyclic voltammetry and chronoamperometry. The Pt/HCS electrode showed significantly higher electrocatalytic activity and more stability for methanol oxidation compared with Pt supported carbon microspheres (Pt/CMs) and commercial carbon (Pt/XC-72) electrode. The excellent performance for the Pt/HCS might be attributed to the high dispersion of platinum catalysts and the particular hollow structure of HCSs.     

Ultrafine Pt Nanoclusters for the Direct Methanol Fuel Cell Reactions

Controlling the size of Pt nanomaterials can be an effective route to improve their catalytic activity on a mass basis for the reactions of direct methanol fuel cell (DMFC). In this work we demonstrated the synthesis of ultrafine platinum nanoclusters based on the replacement reaction between Ag nanoparticles and Pt(IV) precursors. All the reactions were carried out in the presence of commercial carbon powders. The Pt/C thus formed could be directly used for catalyzing the DMFC reactions. By comparing with the commercial Pt catalysts from E-Tek, the Pt nanoclusters exhibit higher catalytic activity toward the methanol oxidation and oxygen reduction, the two key reactions in DMFC, due to their relative smaller sizes.     

Electrocatalytic oxidation of methanol on Pt-Pd nanoparticles supported on honeycomb-like porous carbons in alkaline media

Honeycomb-like porous carbons (PCs) were synthesized using a facile self-assembly method with phenolic resin as the carbon source and tetraethyl orthosilicate (TEOS) as the silica source. The PCs were found to have a large BET surface area of 458 m² g^?1 and a partially graphitized structure. The obtained PCs were used as a support for various Pt-Pd bimetallic alloy catalysts employed for methanol oxidation in alkaline media. Compared with Pt supported on commercial Vulcan XC-72R carbon (Pt/C) and with the other Pt-Pd bimetallic alloy catalysts on PCs, Pt₃Pd₁ on PCs displayed the most negative onset potential for methanol oxidation and the highest steady-state current (2.04 mA cm^?2). This may be because the Pt₃Pd₁/PCs catalyst has the largest electrochemical active surface area (ESA), and because adding Pd to the catalyst improves the ability of the intermediate species to tolerate oxidation. The results show that the prepared Pt-Pd/PCs is a potential candidate for application as a catalyst in alkaline direct methanol fuel cells.     

In-situ X-ray absorption spectroscopy study of Pt and Ru chemistry during methanol electrooxidation

Methanol electrooxidation in a 0.5 M sulfuric acid electrolyte containing 1.0 M CH3OH was studied on 30% Pt/carbon and 30% PtRu/carbon (Pt/Ru = 1:1) catalysts using X-ray absorption spectroscopy (XAS). Absorption by Pt and Ru was measured at constant photon energy in the near edge region during linear potential sweeps of 10-50 mV/s between 0.01 and 1.36 V vs rhe. The absorption results were used to follow Pt and Ru oxidation and reduction under transient conditions as well as to monitor Ru dissolution. Both catalysts exhibited higher activity for methanol oxidation at high potential following multiple potential cycles. Correlation of XAS data with the potential sweeps indicates that Pt catalysts lose activity at high potentials due to Pt oxidation. The addition of Ru to Pt accelerates the rate of methanol oxidation at all potentials. Ru is more readily oxidized than Pt, but unlike Pt, its oxidation does not result in a decrease in catalytic activity. PtRu/carbon catalysts underwent significant changes during potential cycling due to Ru loss. Similar current density vs potential results were obtained using the same PtRu/carbon catalyst at the same loading in a membrane electrode assembly half cell with only a Nafion (DuPont) solid electrolyte. The results are interpreted in terms of a bifunctional catalyst mechanism in which Pt surface sites serve to chemisorb and dissociate methanol to protons and carbon monoxide, while Ru surface sites activate water and accelerate the oxidation of the chemisorbed CO intermediate. PtRu/carbon catalysts maintain their activity at very high potentials, which is attributed to the ability of the added Ru to keep Pt present in a reduced state, a necessary requirement for methanol chemisorption and dissociation.