Several new compounds with a pyrochlore-type crystal structure have been synthesized by a low-tempera ture hydrothermal process under N2 atmosphere using a mixture of nitrate salts. The chemical compositions of the new pyrochlores are represented as (a) Y2Sn1.8Cr0.2O7, . The Ba and Ca-containing pyrochlores exhibited excellent spherical morphology with a pure pyrochlore phase structure and were composed of nanometer-sized particles. A-and B-site doped pyrochlores maintained a fully ordered

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... al structure over a wide temperature range. The pyrochlore oxide powders were tested as catalysts for NOx absorption and reduction by CH4. NO could be almost completely converted into N2 using hydrothermally derived pyrochlore oxide catalysts in the absence of oxygen at high temperatures. Rapid absorption of NO into alkali-substituted pyrochlores was effective for NO removal from the gas phase, absorption being significantly accelerated by the presence of oxygen. [