Metal oxide, mixed, with pyrochlore

Metal oxides are usually prepared by calcinations of suitable precursors such as hydroxides, nitrates, carbonates, carboxylates, etc. This process usually gives oxides with pseudomorphs of the starting materials. When large amounts of thermal energy are applied for the decomposition of the precursors, it facilities sintering of the product particles and therefore aggregated particles are obtained. When mixed oxides such as spinel, perovskite, and pyrochlore are the desired products, heat treatment at higher temperatures is required. 

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 ... 

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. 

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. 

Ceria-zirconia mixed oxide-supported metal nanoparticles are attractive catalysts in low-temperature WGS, CO preferential oxidation (PROX), or three-way catalysis. It was observed that after redox cycles (reduction at 1173K and then oxidation at 823 K), the reducibility of these catalysts was enhanced substantially, which greatly affected their catalytic performance. HAADF studies showed that the pyrochlore-type cation sublattice in the reduced Ce Zr O was retained in the fully oxidized mixed oxide with a Ce Zr Og stoichiometry as far as the oxidation temperature did not exceed 823 K. However, it is not clear whether compositional heterogeneity occurred at the atomic level in Ce Zr Og. Trasobares and coworkers addressed this issue by aberration-corrected STEM, atomic-resolution EELS mapping, and EELS image simulations [66], They synchronously acquired EELS and HAADE signals in... 

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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