Onset Potential For CO Oxidation Research Articles

Preparation of MOF-derived molybdenum-carbide-modified PtCu nano-alloy catalysts and their methanol oxidation performance

The catalytic performance of Pt0.10/Cu-MoC catalyst is higher than that of commercial Pt/C. After stability test, the performance has reserved to 79.2% of the initial value, and the onset potential for CO oxidation is lower than that of commercial Pt/C.

Plasma Synthesized Trilayered Rhodium−Platinum−Tin Oxide Nanostructures with Enhanced Tolerance to CO Poisoning and High Electroactivity for Ethanol Oxidation

The future of fuel cells technology will require porous or very organized multicomponent catalytic layers that can be prepared by thin film growth methods. Reducing the cost of these energy systems will also necessitate that the catalytic layers be binderless and contain low amount of the noble catalyst until efficient non‐noble catalysts are discovered. To address these requirements, monolayered SnO2, Pt and Rh, bi‐layered Pt/SnO2 and Rh/Pt and novel tri‐layered Rh (various thicknesses)/Pt/SnO2 catalysts supported on carbon paper are synthesized at room temperature via pulsed laser deposition. The catalysts are evaluated for their catalytic performance for the ethanol oxidation reaction (EOR), durability, and tolerance to CO‐poisoning. All the Rh/Pt/SnO2 catalysts produce high CO‐tolerance, high EOR catalytic activity and durability as compared to pure Pt. The possible mechanism by which SnO2 and Rh atoms enhanced the performance is also considered herein, and an optimal Rh/Pt/SnO2 structure having a 10 nm thickness of Rh layer offers a promising anode catalyst for ethanol fuel cells. Notably, the onset potential for CO oxidation is extraordinarily 430 mV lower than on Pt, and the mass activity for EOR and durability are 2.25 and 4.2 times higher than on Pt.

Assembling the PdCu/rGO catalysts for methanol oxidation reaction in alkaline media by tuning the electronic structure

Direct methanol fuel cells (DMFCs) have attracted extensive attention due to clean, efficient characteristics and portable applications. In this work, a series of PdxCuy/reduced graphene oxide (rGO) catalysts are assembled and the catalytic performances are investigated for methanol oxidation reaction (MOR) in alkaline media. The intrinsic surface electronic structures of the PdxCuy/rGO bimetallic catalysts are also studied to clarify the reaction mechanism. The results reveal that Pd1.2Cu0.2/rGO exhibits superior catalytic activity (1101.58 A gPd−1), 4.2 and 5.2 times higher than those of the commercial PtRu/C and Pd/C catalysts. CO stripping suggests that the onset potential for CO oxidation on the Pd1.2Cu0.2/rGO catalyst is lower than those on the commercial PtRu/C and Pd/C catalysts, implying that the anti-CO poisoning ability is enhanced. Combined with the analysis of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Raman, the enhanced catalytic activity and anti-CO poisoning ability are attributed to the charge transfer and lattice compression between Pd and Cu, which modify the d-band center and weaken the adsorption of CO. Furthermore, a volcano plot is found between the MOR activity and d-band center, indicating that the excellent MOR activity is attributed to the optimal d-band center with the moderate adsorption strength of CO-like intermediates.

Tungsten nitride decorated CNTs as efficient hybrid supports for PtRh alloys in electrocatalytic ethanol oxidation

Direct ethanol fuel cells (DEFCs) offer an attractive alternative to fossil fuel-powered devices due to their high energy density and environmental benignity. However, high cost and poor stability of catalysts are still the main obstacles for the commercialization of DEFCs. Herein, a novel catalyst comprising PtRh alloys anchored on carbon nanotubes that decorated with tungsten nitride (Pt9Rh-WN/CNTs) was synthesized via impregnation-reduction method and followed by thermal annealing in N2. The X-ray powder diffraction (XRD), scanning electron micrograph (SEM) and transmission electron microscopy (TEM) are employed to characterize the corresponding physico-chemical properties of the as-prepared catalysts. Electrocatalytic performance for ethanol oxidation is evaluated by cyclic voltammetry, linear scan voltammetry, CO-stripping voltammograms, chronoamperometry and chronopotentiometry. The current density on Pt9Rh-WN/CNTs is 484.8 mA mgPt−1, which is much higher than that of Pt9Rh/CNTs (305.7 mA mgPt−1) and Pt/CNTs (135.1 mA mgPt−1). Most importantly, the onset potential for CO oxidation on Pt9Rh-WN/CNTs is 0.27 V, which is more negative than that on Pt9Rh/CNTs (0.37 V) and Pt/CNTs (0.40 V). Therefore, the Pt9Rh-WN/CNTs catalyst displays both outstanding catalytic activity and excellent CO-poisoning tolerance for ethanol oxidation. Synergistic effects arising between WN and PtRh alloy along with nitrogen-doping effects of CNTs with ammonia are proposed to contribute to the outstanding performance of this catalyst in ethanol oxidation.

Ternary Pt9RhFex Nanoscale Alloys as Highly Efficient Catalysts with Enhanced Activity and Excellent CO-Poisoning Tolerance for Ethanol Oxidation.

To address the problems of high cost and poor stability of anode catalysts in direct ethanol fuel cells (DEFCs), ternary nanoparticles Pt9RhFex (x = 1, 3, 5, 7, and 9) supported on carbon powders (XC-72R) have been synthesized via a facile method involving reduction by sodium borohydride followed by thermal annealing in N2 at ambient pressure. The catalysts are physically characterized by X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy, and their catalytic performance for the ethanol oxidation reaction (EOR) is evaluated by cyclic and linear scan voltammetry, CO-stripping voltammograms, and chronopotentiometry. All the Pt9RhFex/C catalysts of different atomic ratios produce high EOR catalytic activity. The catalyst of atomic ratio composition 9:1:3 (Pt/Rh/Fe) has the highest activity and excellent CO-poisoning tolerance. Moreover, the enhanced EOR catalytic activity on Pt9RhFe3/C when compared to Pt9Rh/C, Pt3Fe/C, and Pt/C clearly demonstrates the presence of Fe improves catalytic performance. Notably, the onset potential for CO oxidation on Pt9RhFe3/C (0.271 V) is ∼55, 75, and 191 mV more negative than on Pt9Rh/C (0.326 V), Pt3Fe/C (0.346 V), and Pt/C (0.462 V), respectively, which implies the presence of Fe atoms dramatically improves CO-poisoning tolerance. Meanwhile, compared to the commercial PtRu/C catalyst, the peak potential on Pt9RhFe3/C for CO oxidation was just slightly changed after several thousand cycles, which shows high stability against the potential cycling. The possible mechanism by which Fe and Rh atoms facilitate the observed enhanced performance is also considered herein, and we conclude Pt9RhFe3/C offers a promising anode catalyst for direct ethanol fuel cells.

Efficient CO Oxidation Using Dendrimer-Encapsulated Pt Nanoparticles Activated with

In this paper, we show that the onset potential for CO oxidation electrocatalyzed by ∼2 nm dendrimer-encapsulated Pt nanoparticles (Pt DENs) is shifted negative by ∼300 mV in the presence of a small percentage (<2%) of Cu surface atoms. Theory and experiments suggest that the catalytic enhancement arises from a cocatalytic Langmuir-Hinshelwood mechanism in which the small number of Cu atoms selectively adsorb OH, thereby facilitating reaction with CO adsorbed to the dominant Pt surface. Theory suggests that these Cu atoms are present primarily on the (100) facets of the Pt DENs.

Design and Fabrication of Pt-Au Nanocatalyst with High Performance

Design and Fabrication of Pt-Au Nanocatalyst with High Performance Fanpeng Kong[a], Chunyu Du[a]*, Jinyu Ye[b], Lei Du[a], Guangyu Chen[a], Geping Yin[a]* [a] MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, Harbin Institute of Technology. Email: cydu@hit.edu.cn, yingphit@hit.edu.cn. [b] State Key Lab of Physical Chemistry of Solid Surfaces, Xiamen University. : Direct formic acid fuel cell (DFAFC), as a promising power source, can greatly improve energy conversion efficiency of small organic molecules and dramatically reduce the environment pollution.[1-2] Pt is a key catalytic component at both anode and cathode in DFAFC. However, the scarcity and high cost of Pt require the design and fabrication of Pt-based nanostructure with high performance and reduced usage of Pt. Here, we found that the formic acid oxidation (FAO) activity of Pt and Au nanoparticles supported on carbon (Pt1-Au1/C) increases more than 40 fold relative to that of Pt/C. Generally, ensemble and electronic effects are proposed to explain the enhanced FAO performance.[3-4] However, we have to exclude the ensemble effect since there is no Au adatoms on Pt. From X-ray photoelectron spectroscopy (XPS) patterns, the binding energy of Pt4f and Au4f peak in Pt1-Au1/C shifts positively and negatively compared to that in Pt/C and Au/C, respectively, indicating that Au modifies the electronic structure of adjacent Pt. This electronic modification is further confirmed by the higher onset potential of CO oxidation on Pt1-Au1/C than that on Pt/C in the CO stripping voltammetry, demonstrating stronger binding of CO on Pt1-Au1/C. Unfortunately, the pathway of formic acid oxidation on Pt-Au system is ambiguous because Pt and Au are closely coupled together, so that the in-situ information on the respective role of Au and Pt is difficult to obtain. To this end, a novel selective surface engineering method is proposed to probe the main active sites. We separately employed CO adsorption and selective under potential deposition to engineer respectively Pt and Au surfaces. However, the activity of Pt1-Au1/C with engineered surfaces is obviously lower than that of Pt1-Au1/C. Therefore, the coexistence of Pt and Au, rather than electronic effect, is indispensable for the high FAO activity. In-situ FTIR measurement of Pt/C, Au/C and Pt1-Au1/C is carried out to probe more detailed molecular information. Formate on Pt1-Au1/C is more than that on Pt/C and Au/C, which should result from the coverage of CO on Au, instead of Pt, at low potentials. Since Pt has higher affinity to H than Au, the increased formate is attributed to the accelerated rupture of O-H bond in formic acid on Au by Pt. However, formate is not an active intermediate but a spectator species, so that it cannot account for the increased activity. Recently, it is proved that –COOH is the active species during FAO. In view of strong affinity between Pt and H, we can rationally assume that Pt ruptures the C-H bond of formic acid on Au to produce –COOH, which decomposes to CO2 quickly, so that Pt1-Au1/C possesses a remarkable FAO activity. Reference: J. V. Perales-Rondon, A. Ferre-Vilaplana, J. M. Feliu, E. Herrero, J. Am. Chem. Soc. 2014, 136, 13110-13113.M. E. Scofield, C. Koenigsmann, L. Wang, H. Liu, S. S. Wong, Energy Environ. Sci. 2015, 8, 350-363.Q. S. Chen, Z. Y. Zhou, F. J. Vidal-Iglesias, J. Solla-Gullon, J. M. Feliu, S. G. Sun, J. Am. Chem. Soc. 2011, 133, 12930-12933.G. R. Zhang, D. Zhao, Y. Y. Feng, B. S. Zhang, D. S. Su, G. Liu, B. Q. Xu, ACS Nano 2012, 6, 2226-2236.

Ionic Liquid-Modified Electrocatalysts: The Interaction of [C1C2Im][OTf] with Pt(1 1 1) and its Influence on Methanol Oxidation Studied by Electrochemical IR Spectroscopy

Ionic liquids (ILs) have recently been proposed as modifying agents to tune the selectivity and the activity of electrocatalysts. In order to explore the origin of IL-induced effects on electrocatalytic reactions, we have investigated (i) the interaction of [C1C2Im][OTf]–water mixtures with Pt(111) single crystal electrodes and (ii) the influence of [C1C2Im][OTf] on methanol oxidation in acidic aqueous solution using cyclic voltammetry (CV) and electrochemical IR spectroscopy.We show that [C1C2Im][OTf] interacts strongly with the Pt(111) surface in the potential range from -0.2VAg/AgCl to 0.65VAg/AgCl, leading to partial blocking of the surface, partial suppression of hydrogen adsorption, and partial suppression of OHads formation. The interaction of the IL with Pt occurs in this potential range via specific adsorption of the [OTf]− anions which bind to Pt(111) via the SO3− group in a tilted adsorption geometry. The adsorption process is largely reversible as a function of the electrode potential, but shows a complex kinetics and becomes very slow at high IL coverage. Also, [C1C2Im][OTf] has a strong effect on the electro-oxidation of methanol in acidic solution. Adsorption of [OTf]− leads to partial blocking of the indirect oxidation pathway to CO. The onset potential for CO oxidation is found to be shifted to more positive values indicating hindered access of H2O to the electrode surface. CO and the [C1C2Im][OTf] form a mixed co-adsorption layer which is characterized by a strongly red-shifted CO stretching band. The effect is assigned to an IL-induced ligand effect in the mixed adsorbate layer and suggests that the direct modification of active centers by surface-bound IL species plays an important role in IL-modified electrocatalytic materials.

Enhanced Activity and Durability of Nanosized Pt-SnO2/IrO2/CNTs Catalyst for Methanol Electrooxidation.

Pt-SnO2/IrO2/CNTs anode catalyst for direct methanol fuel cell was designed and prepared with IrO2/CNTs as support for the subsequent immobilization of Pt and SnO2 at the same time. The structure of the catalysts and their catalytic performance in methanol electrooxidation were investigated and the roles of IrO2 and SnO2 in methanol electrooxidation were discussed as well. Results show that Pt-SnO2/IrO2/CNTs catalyst exhibits the best activity and durability for methanol electrooxidation when compared with Pt/CNTs, Pt/IrO2/CNTs and Pt-SnO2/CNTs. According to the results of electrochemical tests and physicochemical characterizations, the enhancements of Pt-SnO2/IrO2/CNTs were attributed to the special properties of IrO2 and SnO2, in which IrO2 mainly increases the methanol oxidation activity and SnO2 mainly improves the CO oxidation ability and durability. Therefore, Pt-SnO2/IrO2/CNTs exhibits excellent performance for methanol oxidation with higher electrocatalytic activity (I(f) of 1054 A g(Pt(-1)) and powerful anti-poisoning ability (the onset potential for CO oxidation of 0.3 V) and outstanding durability (the sustained time t in CP of 617 s), revealing a suitable anode catalyst for DMFCs.

Electrochemical Oxidation of the Carbon Support to Synthesize Pt(Cu) and Pt-Ru(Cu) Core-Shell Electrocatalysts for Low-Temperature Fuel Cells

The synthesis of core-shell Pt(Cu) and Pt-Ru(Cu) electrocatalysts allows for a reduction in the amount of precious metal and, as was previously shown, a better CO oxidation performance can be achieved when compared to the nanoparticulated Pt and Pt-Ru ones. In this paper, the carbon black used as the support was previously submitted to electrochemical oxidation and characterized by XPS. The new catalysts thus prepared were characterized by HRTEM, FFT, EDX, and electrochemical techniques. Cu nanoparticles were generated by electrodeposition and were further transformed into Pt(Cu) and Pt-Ru(Cu) core-shell nanoparticles by successive galvanic exchange with Pt and spontaneous deposition of Ru species, the smallest ones being 3.3 nm in mean size. The onset potential for CO oxidation was as good as that obtained for the untreated carbon, with CO stripping peak potentials about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C, respectively. Carbon oxidation yielded an additional improvement in the catalyst performance, because the ECSA values for hydrogen adsorption/desorption were much higher than those obtained for the non-oxidized carbon. This suggested a higher accessibility of the Pt sites in spite of having the same nanoparticle structure and mean size.

Niobium Dioxide Facilitating Methanol Electrooxidation on Pt/C Catalyst by Synergistic Effect

AbstractIn this work, Pt nanoparticles are deposited on NbO2‐modified carbon composites and evaluated as promising direct methanol fuel cell (DMFC) electrocatalysts. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) indicate that Pt nanoparticles (about 2.5 nm) are uniformly dispersed on NbO2‐modified carbon composites. Electrochemical measurements show that the mass activity toward methanol electrooxidation on Pt/NbO2‐C is as high as 3.0 times that of conventional Pt/C. Meanwhile, the onset potential of CO oxidation is negatively shifted by about 46 mV as compared with that of Pt/C, which means that the synergistic effect between NbO2 and Pt facilitates the feasible removal of poisoning intermediate CO during methanol electrooxidation. X‐ray photoelectron spectroscopy (XPS) characterizations reveal the electron transfer from Nb to Pt, which suppress the poisoning CO adsorption on Pt nanoparticles and facilitate methanol electrooxidation. NbO2 nanoparticles facilitate methanol electrooxidation on Pt/C catalyst by synergistic effect and electronic effect, which represents a step in the right direction for the development of excellent fuel cell anode electrocatalysts.

Synthesis, Characterization of Nanostructured Rhodium Films and their Electrochemical Behavior towards Carbon Monoxide Oxidation

Rhodium nanostructured thin film catalysts were prepared by pulsed laser deposition. The depositions were performed under vacuum and at different He background gas pressures ranging from 1 to 5 Torr. SEM, TEM, X-ray diffraction, and X-ray photoelectron spectroscopy were used to evaluate the morphology, the structure, the surface composition, and the electronic structure of the catalysts. Their electrochemical properties were studied towards CO stripping measurements in acidic solution; a reaction of critical issue to liquid fuel cells. It was discovered that rhodium (Rh) film deposited under 5 Torr of He background pressure showed the highest electroactive surface area, and the lowest onset potential of CO oxidation demonstrating an enhancement of the CO poisoning resistance. The reason for such enhanced electrocatalytic activity is ascribed to the high porosity and roughness of the Rh surface. Compared to Pt films made under similar processing conditions (5 Torr of He), Rh film displayed higher roughness factor of 58.5 (vs. 30 for Pt), a net charge due to oxidation and desorption of the CO adlayer of 385 μC cm−2 (vs. 350 μC cm−2 for Pt), and an onset potential for CO oxidation of 30 mV less anodic than that at Pt electrode.

Carbon Monoxide Oxidation on Nanostructured Pt Thin Films Synthesized by Pulsed Laser Deposition: Insights into the Morphology Effects

Nanostructured Pt thin film catalysts of various morphologies have been synthesized by pulsed laser deposition and studied towards enhancing their tolerance to CO poisoning, a reaction of critical issue to liquid fuel cells. It was discovered that Pt film deposited under 5 Torr of He background pressure showed the highest electroactive surface area and the lowest onset potential of CO oxidation demonstrating an enhancement of the CO poisoning resistance. The reason for such enhanced electrocatalytic activity is ascribed to the high roughness of Pt surface. This study further provides a methodology for the proper design of electrocatalysts that might be considered to be developed by the pulsed laser deposition technique.

Highly Stable and CO-Tolerant Pt/Ti0.7W0.3O2 Electrocatalyst for Proton-Exchange Membrane Fuel Cells

The current materials used in proton-exchange membrane fuel cells (PEMFCs) are not sufficiently durable for commercial deployment. One of the major challenges lies in the development of an inexpensive, efficient, and CO-tolerant anode catalyst. Here we report the unique CO-tolerant property of Pt nanoparticles supported on Ti(0.7)W(0.3)O(2). The Ti(0.7)W(0.3)O(2) nanoparticles (50 nm) were synthesized via a sol-gel process and platinized using an impregnation-reduction technique. Electrochemical studies of Pt/Ti(0.7)W(0.3)O(2) show unique CO-tolerant electrocatalytic activity for hydrogen oxidation compared to commercial E-TEK PtRu/C catalysts. Differential electrochemical mass spectrometry measurements show the onset potential for CO oxidation on Pt/Ti(0.7)W(0.3)O(2) to be below 0.1 V (vs RHE). Pt/Ti(0.7)W(0.3)O(2) is a promising new anode catalyst for PEMFC applications.

A first principles comparison of the mechanism and site requirements for the electrocatalytic oxidation of methanol and formic acid over Pt

First principles density functional theoretical calculations were carried out to examine and compare the reaction paths and ensembles for the electrocatalytic oxidation of methanol and formic acid in the presence of solution and applied electrochemical potential. Methanol proceeds via both direct and indirect pathways which are governed by the initial C-H and O-H bond activation, respectively. The primary path requires an ensemble size of between 3-4 Pt atoms, whereas the secondary path is much less structure sensitive, requiring only 1-2 metal atoms. The CO that forms inhibits the surface at potentials below 0.66 V NHE. The addition of Ru results in bifunctional as well as electronic effects that lower the onset potential for CO oxidation. In comparison, formic acid proceeds via direct, indirect and formate pathways. The direct path, which involves the activation of the C-H bond followed by the rapid activation of the O-H bond, was calculated to be the predominant path especially at potentials greater than 0.6 V. The activation of the O-H bond of formic acid has a very low barrier and readily proceeds to form surface formate intermediates as the first step of the indirect formate path. Adsorbed formate, however, was calculated to be very stable, and thus acts as a spectator species. At potentials below 0.6 V NHE, CO, which forms via the non-Faradaic hydrolytic splitting of the C-O bond over stepped or defect sites in the indirect path, can build up and poison the surface. The results indicate that the direct path only requires a single Pt atom whereas the indirect path requires a larger surface ensemble and stepped sites. This suggests that alloys will not have the same influence on formic acid oxidation as they do for methanol oxidation.

CO tolerant catalysts for PEM fuel cells: Spectroelectrochemical studies

CO tolerance of carbon-supported Pt 80Ru 20, Pt 80Os 20 and Pt 85Co 15 electrocatalysts has been studied in 0.1 M perchloric acid solution applying cyclic voltammetry and in situ Fourier transform infrared spectroscopy (FTIRS), and the results were compared with those for a commercial Pt ETEK catalyst. CO stripping is used for the normalization of the area of the electrodes. It is shown that FTIR spectra can be acquired during CO oxidation at these technical materials in an electrochemical cell, obtaining valuable information on the reactivity, which allows predicting their behaviour in a polymer electrolyte membrane fuel cell (PEMFC). Linear adsorbed CO is the main adsorbate, but also small amounts of bridge bonded CO are formed. Moreover, COH species seem to be also present for Pt 80Os 20 and Pt 85Co 15. On the other hand, the onset potential for CO oxidation to CO 2 is established and compared for the four catalysts. All bimetallic alloys are able to oxidise adsorbed CO at lower potentials than platinum and a shift of about 0.20 V is determined.

Efficient Electrochemical Conversion of Carbon Monoxide by Rhodium Octaethylporphyrin Adsorbed on Carbon Black

We have found efficient electrocatalytic removal of CO by rhodium octaethylporphyrin on carbon black at a wide potential range. Using carbon-supported rhodium octaethylporphyrin, we have separated the Rh(II) state participating reaction and the Rh(III) state participating reaction with CO. We have clearly demonstrated electrocatalytic CO oxidation by rhodium(III) porphyrin. The onset potential for CO oxidation is much lower than that for CO oxidation by conventional Pt/Ru catalysts and cobalt porphyrin.

Electrochemical oxidation of CO over tin oxide supported platinum catalysts

Tin oxide supported platinum catalysts (Pt/SnO x ) for use in polymer electrolyte fuel cells were prepared, and their electrochemical oxidation of CO was investigated. From the XPS measurements of Pt/SnO x catalysts, it was found that the binding energy of Pt showed no change compared to bulk Pt, regardless of tin oxide addition. In accordance with this result, CO-stripping voltammograms for Pt/SnO x catalysts revealed that the peak potential for the pre-adsorbed CO oxidation was almost the same value of Pt/C catalyst. However, the onset potential of CO oxidation shifted negatively compared to PtRu/C catalyst, indicating an enhancement of CO tolerance of Pt/SnO x catalysts.

CO adsorption and oxidation on a Pt(111) electrode modified by ruthenium deposition: an IR spectroscopic study

CO adsorption and electro-oxidation on a Pt(111) electrode modified by Ru deposition was studied by IR spectroscopy. The IR spectra in the region of the CO stretching vibration exhibit distinct bands which can be assigned to CO adsorbed on the Pt substrate (CO|Pt) and on Ru islands (CO|Ru). This is in contrast with IR studies of Pt  Ru alloy electrodes where only a single vibrational band of adsorbed CO is observed. The integrated band intensities of the CO|Ru and of CO|Pt bands on the Ru-modified Pt(111) electrode decrease simultaneously at the onset potential of CO oxidation. The results are interpreted within the framework of a bifunctional mechanism of CO electro-oxidation, postulating active Pt sites in close proximity to the Ru islands and a high mobility of the CO|Pt adsorbate layer. The differences in the IR spectra between the Ru|Pt(111) electrode and the Pt  Ru alloys are explained by disparate surface structures (Ru islands versus homogeneous distribution of Pt and Ru atoms).