Mixed metal oxides pyrochlore structure

While metal oxides and mixed metal oxides have often been considered for various electrocatalytic applications, they are often limited by low electronic conductivity and/or low surface area. One series of mixed metal oxides with the pyrochlore structure has been discovered ( ) that has demonstrated high catalytic activity for electroreduction and electroevolution of oxygen 2) and the selective electrooxidation of certain organics 3). These materials, which are characterized by high electronic conductivity and can be prepared in high surface area form, are described by the general formula ... 

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. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

Mixed oxides of Ru" and are frequently metallic as are RuOj and Ir02-The more localized electron behavior observed when these cations are located in the pyrochlore structure is presumably due to structural considerations. The M-O-M bond angle in pyrochlores is not the most favorable for delocalization of t2g electrons. The most favorable angle would be 180°, whereas this angle tends to be about 130° in the pyrochlore structure. This is a significant deviation from 180° and is therefore presumably adequate to cause an activated hopping process in the absence of mixing with post-transition metal s or p bands. 

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