Ruthenium catalyst water oxidation

Dutta P K and Vaidyalingam A S (2003), Zeolite-supported ruthenium oxide catalysts for photochemical reduction of water to hydrogen , Micropor Mesopor Mater, 62, 107. 

In contrast the oxo-ruthenium complex c ,c -[ (bpy)2Runl(0H2) 2(//-0)]4+ and some of its derivatives are known to be active catalysts for the chemical or electrochemical oxidation of water to dioxygen.464-472 Many studies have been reported473 181 on the redox and structural chemistry of this complex for understanding the mechanism of water oxidation. Based on the results of pH-dependent electrochemical measurements, the basic structural unit is retained in the successive oxidation states from Rum-0 Ru111 to Ruv O Ruv.466... 

Table 19.1 Calculated values of k, rd and for various ammine ruthenium complexes as a water oxidation catalyst incorporated into Nation membrane...

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

It was concluded that by combining a hematite nanorod electrode with a suitable water oxidation catalysts, for instance platinum or ruthenium dioxide, the photoelectrochemical activity for direct water splitting applications should increase by a factor of 20. 

Recently, a ruthenium-catalysed oxidation in water was published by d Alessandro et al. [34]. Water can be regarded as an environmentally friendly solvent which, because it is inert, reduces the risk of explosions. The oxidation of cyclohexane directly to adipic acid was performed using ruthenium catalysts bearing water-soluble phthalocyanine ligands RuPcS (where PcS is tetra-sodium 2,3-tetrasulfophthalocyaninato) with KHSOs (Eq. 4). However we note that very low TONs were observed and the use of KHSOs as a primary oxidant is not viable for industrial-scale oxidations. 

The oxidation catalyst is believed to be ruthenium tetraoxide based on work by Engle,149 who showed that alkenes could be cleaved with stoichiometric amounts of ruthenium tetraoxide. Suitable solvents for the Ru/peracid systems are water and hexane, the alkene (if liquid) and aromatic compounds. Complex-ing solvents like dimethylformamide, acetonitrile and ethers, and the addition of nitrogen-complexing agents decrease the catalytic system s activity. It has also been found that the system has to be carefully buffered otherwise the yield of the resulting carboxylic acid drops drastically.150 The influence of various ruthenium compounds has also been studied, and generally most simple and complex ruthenium salts are active. The two exceptions are Ru-red and Ru-metal, which are both inferior to the others. Ruthenium to olefin molar ratios as low as 1/20000 will afford excellent cleavage yields (> 70%). vic-Diols are also... 

In contrast, Si, particularly in p-type form, has been examined for its H2 evolution efficacy under irradiation in several studies dating back to 1976. Thus, although H2 evolution was observed on heavily doped p-Si photocathodes in salt water, the efficiency was found to be poor.560 Interestingly, this was attributed to the presence of a surface oxide layer (see above). Subsequent studies have all focused on catalytic modification of p-Si surfaces so that the HER rate is enhanced a variety of metal catalysts have been examined in this regard.143,181484,561,562 An early study also considered n-type Si as a photoanode with a protective S11O2 layer to prevent it from undergoing corrosion.563 Ruthenium oxide layers have also been studied,564 this time on p-Si surfaces. 

Fig. 7. A schematic view of Nafion membrane showing the microheterogeneous environment. A hydrophobic fluorocarbon phase B hydrophilic sulfonate ionic clusters C interfacial region formed between A and B and Ru adsorbed ruthenium complex water oxidation catalyst...

As a general preparation procedure of the water oxidation catalyst of dinuclear ruthenium complex, [(bpy)2(H20)Ru0Ru(H20)(bpy)2], the corresponding mononuclear ruthenium complex ds-[Ru(bpy)2(H20)2] has been used as a precursor complex . However, other reports claimed that the mononuclear ruthenium complex itself can mediate water oxidation under... 

Polynuclear metal complexes are more suited for water oxidation catalyst because of their nature to act as multielectron transfer reagents in addition to the fact that charge delocalization can lead to stabilization of the catalyst rather than decomposition during the process. The trinuclear ruthenium complexes Ru-red and Ru-brown, [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)j] - (Ru "-Ru" -Ru" ) and [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)5] + (Ru -Ru" -Ru ), respectively, have been shown to be efficient water oxidation catalysts for oxygen evolution with high turnover numbers When Ru-red was dissolved in an acidic aqueous solution, it underwent one-electron oxidation with the formation of Ru-brown. When Ru-brown was dissolved in a basic solution, the complex underwent reduction to produce Ru-red. The one-electron oxidation and reduction of the Ru-red and Ru-brown has already been well established (Eq. 11) 6 65-6 )... 

The use of aqueous solutions with basic oxide supports such as magnesium oxide presents some difficulties because of the partial solubility of these oxides in water. To overcome this problem, highly dispersed Ru/MgO catalysts were prepared using ruthenium chloride dissolved in either anhydrous acetone or acetonitrile for the impregnation. ... 

The dual-state behaviour of RU-AI2O3 catalysts may also arise from metal-support interaction. In the oxidized state, the catalyst was more selective for nitrogen formation in NO reduction than when in the reduced state. It was also active for the water-gas shift reaction whereas the reduced form was rather inactive and differences were also observed for ammonia decomposition and the CO-H2 reaction. The more active form does not appear to contain ruthenium oxide the reduced catalyst may have been de-activated by reaction with the support and its transformation to the more active form by oxidation may involve surface reconstruction and/or destruction of the metal-support interaction. 

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