Oxides Ru and

Figure 10. Top Average cluster size and structure determined from EXAFS modelling for three different PtRu clusters. Light atoms Pt, oxidized Ru, and intermediate Ru atoms. The small PtRuW clusters are smaller with the Ru mostly in the metallic state, the Ru in much larger PtRuE clusters is heavily oxidized Bottom Estimated CO/Pt coverages and the indicated regions where each mechanism described above occurs. The brackets indicate the net amount of CO stripped via either the BF or Dsl mechanism due to mobile CO moving toward the Ru islands.

While the incorporation of transition metal oxides into complexes with materials such as alumina can lower their volatilities by factors from 10 (CuO) to 1000 (BaO) depending primarily upon the heat of reaction between the two oxides, it is also likely that formation of very stable complex metal oxides, such as aluminates, can also greatiy lower the chemical activity of the transition metal. As mentioned above, Mn, Ni, and Co may requite stabilization in complex oxides for long catalyst life, but the complex oxides generally have inferior activity. The most active transition metal oxides (Ru and Cu) may still have unacceptable volatility as relatively active complex oxides. As a consequence, there may be a technology-limiting trade-off between the catalytic activity of metals and metal oxides and their chemical and thermal stability in combustion environments. 

A few illustrative examples are the following. Photohydrogenation of acetylene and ethylene occurs on irradiation of Ti02 exposed to the gases, but only if TiOH surface groups are present as a source of hydrogen [319]. The pho-toinduced conversion of CO2 to CH4 in the presence of Ru and Os colloids has been reported [320]. Platinized Ti02 powder shows, in the presence of water, photochemical oxidation of hydrocarbons [321,322]. Some of the postulated reactions are ... 

Figure 25.2 Plot of volt-equivalent against oxidation state for Fe, Ru and Os in acidic aqueous solution.

The most interesting oxides of Ru and Os, however, are the volatile, yellow tetroxides, RUO4 (mp 25°C, bp 130°C< 3>) and OSO4 (mp 40°C, bp 130°C). They are tetrahedral molecules and the latter is perhaps the best-known compound of osmium. It is produced by aerial oxidation of the heated metal or by oxidizing other compounds of osmium with... 

By heating the metal with appropriate oxides or carbonates of alkali or alkaline earth metals, a number of mixed oxides of Ru and Os have been made. They include NasOs Og, LifiOs Og and the ruthenites , M Ru 03, in all of which the metal is situated in octahedral sites of an oxide lattice. Ru (octahedral) has now also been established by Ru Mdssbauer spectroscopy as a common stable oxidation state in mixed oxides such as Na3Ru 04, Na4Ru2 07, and the ordered perovskite-type phases M Ln Ru Og. 

Fluorides and 0x0 compounds of Ru and Os have already been mentioned, and salts such as (R4N)[Ru04l, (R = n-propyl, n-butyl) are useful reagents to oxidize a variety of organic materials without attacking double or allylic bonds,... 

The goal of Haber s research was to find a catalyst to synthesize ammonia at a reasonable rate without going to very high temperatures. These days two different catalysts are used. One consists of a mixture of iron, potassium oxide. K20, and aluminum oxide. Al203. The other, which uses finely divided ruthenium, Ru. metal on a graphite surface, is less susceptible to poisoning by impurities. Reaction takes place at 450°C and a pressure of 200 to 600 atm. The ammonia... 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

Crown A, Moraes IR, Wieckowski A. 2001. Examination of Pt(lll)/Ru and Pt(lll)/Os surfaces STM imaging and methanol oxidation activity. J Electroanal Chem 500 333-343. 

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. 

Krausa M, Vielstich W. 1994. Study of the electrocatalytic influence of Pt/Ru and Ru on the oxidation of residues of small organic-molecules. J Electroanal Chem 379 307-314. 

Isomer shift changes are sufficiently large for Ru atoms in different systems to distinguish between different oxidation states and different bond properties ... 

Carbon-supported Ru-Sn catalyst Ru and Sn Mossbauer measurements were performed to investigate catalysts of ruthenium and tin supported on activated carbon (Ru-Sn/C). The samples were subjected to different reducing and oxidizing treatments. The presence of tin leads to a substantial increase of the Lamb-Mossbauer factor of the metallic Ru-particles showing that tin strengthens the attachment of the particles to the support. The close contact between the two metals appears to be decisive for the formation of catalytically active sites (Ru-Sn and Ru-SnOj,-)... 

Isomer shift and quadmpole splitting of salts, [Ru(C5H5)X] Y (X = Cl, Br Y = PFg and X = I, Y = I3) are larger compared to those of ruthenocene. This indicates direct chemical bonding between Ru and Cl, Br and I and that the Ru ion in each salt is in an oxidation state higher than Ru(II) in ruthenocene... 

Various Ru-oxides, YBa2Cu307, c (I), Ba Ru2/3Gdi/303 (II) as well as Ru-doped a-Fe203 (III), to probe the local chemical structure around the Ru atoms. Compound (I) has interesting properties with x < 0.2 it is a superconductor and with x 1 a semiconductor. Ru oxidation state and coordination are discussed on the basis of measured isomer shifts and quadrupole splittings Ru(IV) ions exclusively occupy Cu-1 sites which form one-dimensional chains... 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Ruthenium. In contrast to Au and Pt, the monitoring of oxide formation on Ru and Ir by cyclic voltammetry is not straightforward. There is no sharp edge in the anodic... 

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