Structural Characterization and Redox Catalytic Properties of Cerium(IV) Pyrochlore Oxides

Ce(IV) pyrochlore oxides have been prepared by hydrothermal synthesis, and the parent material, a sodium cerium titanate, has been studied using total neutron scattering. While analysis of Bragg diffraction is consistent with an average cubic pyroclore structure, the profile is broadened because of the crystal size of <10 nm. Analysis of the pair distribution function (PDF) produced by Fourier transformation of the total scattering yields a structural model consistent with formulation of the parent material as (Na0.33Ce0.53Ti0.14)(2)Ti2O7. This contains a proportion of A-site titanium, consistent with the measured bulk density of the material. The PDF also contains evidence that the short-range order of the pyrochlore structure is disordered, with oxide anions displaced from the positions of the ideal Fd<(3)over bar>m pyrochlore structure to give local symmetry F (4) over bar 3m. These observations are supported by static (broadline) solid state Ti-49 NMR measurements on a Ti-49 isotopically enriched sample, which showed a dominant, narrow resonance at an apparent shift of delta - 912 ppm and a second minor resonance consistent with A-site titanium. Sri(W) doping of the pyrochlore phase is possible by one-step hydrothermal synthesis: this gives a series of materials with a maximum tin content of Sn:Ti = 0.4:0.6, for which Sn-119 solid-state NMR confirms the presence of octahedral, B-site Sn(IV), and powder X-ray diffraction shows an associated expansion of the pyrochlore lattice. Temperature programmed reduction/oxidation studies of the materials reveal that after an activation cycle the parent pyrochlore shows a reversible low temperature reduction at <200 degrees C, more facile than ceria itself. The Sn-doped analogues also show a low temperature reduction, but on continued heating collapse irreversibly to yield a mixture of products that includes SnO. The parent pyrochlore has been tested as a support for gold in the water gas shift reaction and shows a lower temperature conversion of H2O and CO to H-2 and CO2 than a ceria sample of similar surface area.