Pyrochlore alkaline solution synthesis

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. 

The synthesis method (6,7) involves reacting the appropriate cations to yield a pyrochlore oxide by precipitation, and subsequent crystallization of the precipitate in a liquid alkaline medium in the presence of oxygen. The alkaline solution serves both as a precipitating agent and as a reaction medium for crystallizing the amorphous precipitate to pyrochlore, thus obviating the need for subsequent heat treatment. 

The COH ] must be maintained at a certain minimum value (pH 10) in order to synthesize crystalline pyrochlore from solution. This implies that a minimum solubility is required for crystallization and that this crystallization of the pyrochlore directly out of alkaline solution may involve a solution-reprecipitation mechanism. Once the restriction of minimum pH has been satisfied, COH ] does not seem to have a significant effect on crystallinity as long as oxidizing conditions are maintained. The solubility of lead rapidly increases with hydroxide concentration therefore, when all else is held constant, the lattice parameter of the product pyrochlore decreases as the pH of the synthesis medium increases. 

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. 

Perhaps the most important synthesis parameter affecting the crystallization of pyrochlore from alkaline solution is the oxidizing potential within the reaction medium. It is observed that this parameter has dramatic effects on both crystallinity and extent of ruthenium substitution by Pb . For example, a synthesis in which O2 is bubbled into the reaction medium will yield a well crystallized pyrochlore 2-3 times faster than a synthesis where O2 sparging is not provided. Crystalline pyrochlores cannot be obtained under any synthesis conditions (except those that are electrochemically assisted) when N2 sparging is used. The necessity for relatively oxidizing conditions in order to yield crystalline expanded pyrochlores is consistent with the hypothesis that these pyrochlores do... 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

The combined cation solutions were stirred for 10 minutes and then added to the alkaline reaction medium (usually KOH or NaOH) where precipitation occurred. The pH of the reaction mixture was adjusted to be between 10.0 and 14.0. Crystallization of the precipitate to the pyrochlore structure was achieved in a period ranging from 8 hours to 5 days by maintaining the stirred, oxygen sparged reaction medium at a temperature of BO-SO C. The reactions were carried out in polymethyl pentene beakers to avoid corrosion by the alkaline synthesis medium. 

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. 

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