Lead ruthenate

Lead-Ruthenate Pyrochlore Modified Nafion Membrane for Tunable Heterogeneous Catalytic Oxidation... 

Lead-Ruthenate Pyrochlore Modified Nafion Membrane. 

S. Venkatesan, A. S. Kumar, J.-M. Zen, A Rugged Lead-ruthenate pyrochlore Membrane Catalyst for Highly Selective Oxidation of Alcohols, J. Mol. Catal. A Chem. 250 (2006) 87-93. 

Horowitz et al. [561] studied the oxidation of a number of organic compounds at high surface area lead ruthenate in aqueous alkali. The ruthenate used may be represented as Pb2 (Ru(1 I)PbI)20(7, ) where may have oxygen vacancies up to = 1. Electrode fabrication was carried out by mounting the ruthenate powder on gold screen using Teflon as binder. Horowitz et al. [561] observed that lead ruthenate catalyzes the electro-oxidation of primary... 

The reactants used were the more soluble cation sources, generally the nitrates. The aqueous solutions of these cation sources were combined in a post transition metal to noble metal ratio appropriate for the ultimately desired pyrochlore stoichiometry. For lead ruthenate syntheses the reactant Pb Ru ratio was required to be slightly higher than the intended final Pb Ru ratio because of the high solubility of lead relative to ruthenium. For bismuth ruthenate syntheses the Bi Ru ratio was required to be slightly lower than the intended final Bi Ru ratio because of the higher solubility of ruthenium. 

X-ray diffraction patterns of the lead-substituted lead ruthenates support the conclusion that they are pyrochlores of cubic symmetry. It must be noted, however, that since these materials were prepared at relatively low temperatures, the peaks in their x-ray spectra were broadened. X-ray line broadening was significant [half-height peak width at 50-60 (20) was equal to 0.5-0.8 (20)] thus any subtle evidence of distortion to lower symmetries would be difficult to... 

Figure 2. Transformation of amorphous lead ruthenate into crystalline pyrochlore.

Increasing the temperature of synthesis results in enhanced crystallinity as would be anticipated because of improved reaction kinetics. However, this observation is also consistent with a crystallization mechanism involving solubility. Furthermore, as the temperature increases so does the equilibrium concentration of lead in solution thus with all else held constant, increased temperature of reaction results in a smaller lattice parameter for the product lead ruthenate pyrochlore. 

One additional parameter that affects the solubility of lead ruthenate pyrochlores in alkali is the extent of lead substitution on the B-site. The greater the substitution (i.e. the larger x is in formula 1), the higher the solubility of the pyrochlores is in alkali 2). If it is assumed that a solution-reprecipitation mechanism of synthesis is operative, the stoichiometry-dependent solubility could explain why it becomes significantly more difficult to crystallize lead ruthenate directly out of alkaline solution when x <0.3. 

Synthesis of the bismuth-substituted bismuth ruthenates is, in most respects, similar to that of the lead ruthenate series. Precipitation/crystallization is effected in a relatively oxidizing. 

Figure 3. DTA Trace, run in air at 2O C/min., on amorphous lead ruthenate precipitate.

Figure 9 illustrates the O2 electroreduction activity of a number of lead ruthenate pyrochlores, Pb2(Ru2-xPbx)06.5> here 0 < x < 1.0. These data demonstrate that catalysts of roughly equivalent activity can be synthesized over the entire compositional range. In two examples where the activity was noticeably lower (x = 0.04 and x 0.98), the synthesis conditions were such that the surface areas of... 

While the oxygen electrocatalysis results reported here have been those obtained specifically on the lead ruthenate series, essentially equivalent results were obtained on the bismuth ruthenate series. 

Electro-Organic Oxidation Properties. Table I lists some results for the electro-oxidation of primary alcohols and propylene on leadsubstituted lead ruthenate. Propylene was cleaved with nearly 100% selectivity to acetic acid and CO2. In borate buffer at pH 9 the oxidation of propylene also occurred, and the selectivity to acetate and CO2, based on the amount of carbonate isolated, was also close to 100%. Dissolved ethanol and propanol were both converted with high selectivity to the corresponding carboxylic acid salts in alkaline electrolyte. In contrast, Pt black (also shown in Table I) oxidized ethanol to CO2 and then rapidly deactivated. 

LEAD RUTHENATES OXIDIZE ALCOHOLS AND CLEAVE PROPYLENE ... 

Table II shows results for the electro-oxidation of secondary alcohols and ketones. In alkaline electrolyte, secondary butanol was not oxidized to methyl ethyl ketone but was cleaved to acetate. Similarly methyl ethyl ketone was cleaved to acetate, although some CO2 and propionate formed, indicative of cleavage on the other side of the carbonyl group. Butanediol (2 ) went to acetate yielding less CO2. At pH 9 in borax buffer 2 Trtanol went exclusively to methyl ethyl ketone at 89% conversion, suggesting that enolization in alkali is a necessary part of the cleavage process. Cyclohexanol and cyclohexanone were both converted to adipic acid. Figure 12 summarizes the various types of electro-organic oxidations, thus far discussed, which are observed to occur on lead ruthenate in alkaline electrolyte.

In order to confirm the reactivity and selectivity of lead ruthenates for the oxidation of isolated double bonds, two soluble, unsaturated carboxylic acids were chosen that contain a double bond far removed from the solubilizing carboxylate group. The two olefinic compounds, 1-undecylenic acid and 2-cyclopentene-l-acetic acid were both cleaved at the double bond as shown in Figure 13. 

Figure 12. Typical Electro-Oxidations on Lead Ruthenate in Alkali.

Figure 14. Rates of Oxidation of Various Organics on Lead Ruthenate.

In the case of lead ruthenate, the oxygen non-stoichiometry concept can be developed further by combining it with the known reactions of the variable valence ruthenium. It has been shown in this work that these same catalysts can cleave carbon-carbon double bonds (3) in a manner analogous to that of osmium and ruthenium tetroxiHe (1,1). It is known (12) that OSO4 (and presumably RUO4) cleave olefins via complexes with the structure ... 

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