Ru-decorated Pt surfaces

MaiUard F, Lu GQ, Wieckowski A, Stimnting U. 2005. Ru-decorated Pt surfaces as model fuel cell electrocatalysts for CO electrooxidation. J Phys Chem B 109 16230-16243. 

Ru-decorated Pt surfaces obtained by spontaneous deposition. These studies were reviewed recently here we give a brief summary. Several methods were used to obtain Ru-modified Pt surfaces, including electrodeposition of Ru ad-atoms,electrochemical, spontaneous deposition, UHV deposition" and organometallic chemistry. However, these various methodologies produce different amounts of metallic vs. oxidized Ru at-... 

Wieckowski et al. reported the spontaneous deposition Ru adlayers from RuO" solutions on three low-index Pt surfaces. The maximum coverage of Ru on these adlayer is about 20%, and potential must be applied to reduce the Ru adlayer to metallic Ru. The Ru-decorated Pt nanoparticles showed considerable catalytic activity in the methanol- oxidation reaction. We will discuss the catalytic properties of the Ru-decorated Pt nanoparticles in Section V.3. 

Pt has the highest adsorption of methanol on its surface, but its catalytic properties are low due to the formation of poison species (most notably CO) that can be oxidized only after the Pt is covered with OH. Platinum-based bimetallic electrocatalysts, such as Pt-Ru alloys and Ru-decorated Pt materials, are the most active ones. The bi-functional mechanism is to a large extent operative in these catalysts. Most commercial Pt-Ru catalysts are based on 1 1 Pt-Ru alloy. While the alloys typically show enhanced activity in comparison with pure Pt, there is significant Pt loading in the bulk of the alloy in which catalysis does not proceed because the sites are inaccessible for methanol adsorption hence, the need for reducing the Pt content. 

Figure 4.16. STM images of Ru-decorated Pt(lll) prepared by spontaneous deposition according to the method of Chrzanowski and Wieckowski [100]. Deposition times a) 10 s, b) 20 s, c) 40 s, and d) 90 s [99]. (Reproduced from Journal of Electroanalytical Chemistry, 500(1-2), Crown A, Moraes IR, Wieckowski A, Examination of Pt(l 11)/Ru and Pt(l 1 l)/Os surfaces STM imaging and methanol oxidation activity, 333 3, 2001, with permission from Elsevier.)...

The growth of Ru islands on Pt(hkl) was found to be substrate-dependent, so that the Ru layer is almost completely in the fonn of a monolayer on Pt(llO), whereas the two- and three-dimensional growth is facilitated on the other two low-index Pt surfaces, especially on Pt(lll). The Ru-Pt(lll) is more effective catalyst for methanol oxidation " than the other two surfaces decorated with Ru, because the edge of a Ru island is the active site in methanol oxidation therefore, controlling the extent of the multidimensional islands is of a particular importance for fuel cell catalysis. 2 ... 

As discussed above, Ru is an excellent promoter of Pt catalysts for methanol oxidation and Pt nanoparticles decorated with Ru show better performance in the oxidation process than do commercially available Pt/Ru (1 1) alloy nanoparticles. In order to compare the electronic properties of these two systems, we undertook further "C NMR investigations of the Pt/Ru alloy nanoparticles covered by CO, and have obtained new insights into the electronic alterations caused by Ru addition to Pt surfaces. 

Massong H, Wang H, Samjeske G, Baltruschat H. 2000. The co-catal3dic effect of Sn, Ru and Mo decorating steps of Pt(lll) vicinal electrode surfaces on the oxidation of CO. Electrochim Acta 46 701 -707. 

Although bulk- and surface-decorated samples agree broadly in terms of optimal Pt Ru surface ratios for MeOH oxidation, there is less agreement with practical PtRu catalysts, although the data are sparse. This would suggest that PtRu particles show Pt-segregated surfaces as predicted by theoretical calculations. 

Eigure 16a shows CO stripping on Pt(lll), Pt(lll)-Ru (following spontaneous deposition), and Pt(lll)-Ru (where the spontaneously deposited ruthenium has been reduced in hydrogen). Only a very small reduction in overpotential for CO electro-oxidation is observed for Pt(lll)-Ru . The overpotential for CO electro-oxidation on the Pt(l 11) surface has been reduced, however, on the Pt(lll)-Ru surface, and the latter exhibits a doublet structure. This CO stripping result on Pt(l 11)-Ru is nearly identical to that found on the Pt(l 11)-Ru surface where the ruthenium was MVD deposited (Eigure 15) It was concluded that Pt(lll)-Ru ° was decorated islands of Ru . 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Comparing the results of EC-NMR and IR investigations, we find that the potential dependence of C NMR shift and the vibrational frequency of adsorbed CO are primarily electronic in nature, and originate from changes in the f-LDOS. C NMR results show that CO adsorbed on Pt, either directly from CO gas or from methanol oxidation, have the same electronic properties. That is, the chemisorbed product (surface CO) from CO solutions and from methanol decomposition is the same. The electrode potential dependence of the C NMR spectra of CO adsorbed on Pt and Pd nanoparticles provide direct evidence for electric field induced alterations in the E/ -LDOS. In relation to fuel cell catalysis, EC-NMR investigations of Pt nanoparticles decorated with Ru show that there exist two different kinds of CO populations having markedly different electronic properties. COs... 

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

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