Oxide pyrochlore

J.-M. Zen, C.-B. Wang, Oxygen Reduction on Ruthenium Oxide Pyrochlore Produced in a Proton Exchange Membrane, J. Electrochem. Soc. 141 (1994) 38-39. 

J.-M. Zen, C.-W. Wang, Determination of Dissolved Oxygen by Catalytic Reduction on a Nafion /Ruthenium Oxide Pyrochlore Chemically Modified Electrode, J. Electroanal. Chem. 368 (1994) 251—256. 

J.-M. Zen, A. S. Kumar and J.-C. Chen, Electrochemical Behavior of Lead-Ruthenium Oxide Pyrochlore Catalyst Redox Characteristics in Comparison with that of Ruthenium Dioxide, J. Mol. Catal. A Chem. 165 (2001) 177-188. 

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. 

Subramanian, M. A., Aravamudan, G. Subba Rao, G. V. 1983. Oxide pyrochlores - a review. Progress in Solid State Chemistry, 15, 55-143. 

Oxide pyrochlores of the general formula A2B2O7 show interesting electronic properties (Subramanian et al, 1983). Ferromagnetic pyrochlores of rare earths have also been described (Subramanian et al., 1988). A composition-dependent metal-semiconductor transition has been found in A2(Ru2 (AJ07 j, where A = Bi or Pb (Beyerlein et al., 1988). 

J.M. Zen, R. Manoharan, and J.B. Goodenough. Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane. Journal of Applied Electrochemistry 22, 140-150 1992. 

M.P. Dijk, K.J. de Vries, A.J. Burggraaf, A.N. Cormack and C.R.A. Catlow, Defect structures and migration mechanisms in oxide pyrochlores. Solid State Ionics, 17 (1985) 159-167. 

Zen, J.M., Kumar, A.S., and Chen, J.C., Electrochemical behavior of lead-ruthenium oxide pyrochlore catalyst Redox characteristics in comparison with that of ruthenium dioxide, J. Mol. Catal. A, 165, 177, 2001. 

The data summarized in this paper have established that the oxide pyrochlores under discussion substantially reduce the activation energy overvoltages associated with oxygen electrocatalysis. Specifically, it is found that these catalysts, in aqueous alkaline media near ambient temperature, are superior to any other oxygen evolution catalyst and are equal in performance to the best known oxygen reduction catalysts. As bidirectional oxygen electrocatalysts, they appear to be unmatched. 

There are several physical and chemical characteristics of these oxide pyrochlores which may contribute to their high electrocatalytic activity. The previously described alkaline solution synthesis technique (6,7) provided these materials with surface areas typically ranging from 50 to 200 m /g. Thus, one of the basic requirements for an effective electrocatalyst has been satisfied the electrocatalytic activity is not limited by the unavailability of catalytically active surface sites, as is so often the case with metal and mixed metal oxides. 

O in similar way as reported for an oxidized pyrochlore-type CeZrOs.s+x sample [85]. 

It should be noted that the exact cation stoichiometry of the product is highly sensitive to the exact metal concentration of the ruthenium source solution and temperature and pH of the reaction medium (inadvertent increases in both of these parameters lead to increased solubility of lead in the alkaline reaction medium and consequently yield solid products of lower lead ruthenium ratios). While synthesis of a pure lead ruthenium oxide pyrochlore is relatively easy, the precise cation stoichiometry of the product is a property that is not always easy to control. A relatively quick check on the cation stoichiometry of the lead ruthenium oxide product can be obtained, however, by using the correlation between lattice parameter and composition that is displayed in Fig. 1. When lattice parameter and cation stoichiometry are independently determined, the relationship shown in Fig. 1 also provides an assessment of product purity since data points that show significant departures from the displayed linear correlation indicate the presence of impurity phases. The thermal stability of the lead ruthenium oxides decreases with increasing occupancy of tetravalent lead on the octahedrally coordinated site, but all of the ruthenium oxide pyro-chlores described are stable to at least 350° in oxygen. 

Enzyme-like behavior was observed for electrocatalyt-ic oxidation on Nafion/lead-ruthenium oxide pyrochlore chemically modified electrodes.Nafion is loaded with lead(II) and ruthenium(III) cations by ion exchange, and then their oxides are prepared by in situ precipitation in such a way that the catalytically active sites remain... 

Sohd electrodes modified with various compounds have been used to improve DNA oxidation response. Siontorou and coworkers [264] obtained peaks A° and of degraded DNA at GCE modified with self-assembled bdayer hpid membrane. Using GCE modified with Nafion-ruthenimn oxide pyrochlore, enhancement of oxidation peaks of both peaks, G° and A°, was achieved [145]. Thorp s group investigated DNA oxidation response at GC and indium-tin oxide (ITO) electrodes modified with self-assembled dicarboxylate monolayers [265], and with nitrocellulose and nylon membranes [266]. In these experiments, DNA was attached to the electrode either covalently or via adsorption forces in the modifier layer bare ITO surface did not adsorb DNA. Oxidation of DNA was mediated by a redox metal chelate [Ru(bipy)3], which shuttled electrons to the electrode surface from DNA in solution or attached at the modifier film [265, 266]. Electro-catalytic oxidation of DNA was observed also when a redox mediator was immobilized on the electrode surface, for example, on ITO modified with electropolymerized poly[Ru(bipy)3] film [146]. 

Barker, W.W., J. Graham, O. Knop and F. Brisse, 1970, Crystal chemistry of oxide pyrochlores, in The Chemistry of Extended Defects in Non-metal Solids, eds L. Eyring and M. O Keeffe (North Holland, Amsterdam). 

Tabira, Y., Withers, R.L., Minervini, L., and Grimes, R.W. (2000) Systematic structural change in selected rare earth oxide pyrochlores as determined by wide-angle CBED and a comparison with the results of atomistic computer simulation. /. Solid State Chem., 153 (1), 16-25. 

Dolgikh and Lavut (1991) have reported p5Tochlore-type phases with a composition close to R2Ti20s 5N (R=Sm, Dy, Y) after nitridation at 1000 C of the oxide pyrochlores Ln2Ti207. 

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