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Redox Behavior of VI B Transition Metal Ions in Rutile TiO2 Solid Solutions: An XRD and EPR Study

2000; Elsevier BV; Volume: 152; Issue: 2 Linguagem: Inglês

10.1006/jssc.2000.8698

1095-726X

D. Cordischi, Delia Gazzoli, Manlio Occhiuzzi, M. Valigi,

Thermal and Kinetic Analysis

CrO2–TiO2 (TC), MoO2–TiO2 (TMo), and WO2–TiO2 (TW), prepared by heating in vacuo at 1173–1273 K mixtures of MO2 (M=Cr, Mo, or W) and TiO2, were characterized by XRD and EPR. The transition metal ions were incorporated as isolated and clustered CrIII, MoIV, and WIV and a small fraction as isolated MoV and WV in substitutional sites. After heating in air or in O2 up to 1273 K, in TC no isolated CrV formed, whereas in TMo and TW, MIV was oxidized to MV and MVI, and MO3 was segregated. The oxidation started at 673 K and gave isolated substitutional MoV and isolated interstitial and substitutional WV. After heating in H2 up to 1073 K, TMo and TW reduction process differed: MoIV gave isolated and clustered MoIII, and MoIV–MoIV pairs gave Mo7+2 species; WIV was not reduced to WIII; and temperatures >900 K caused the segregation of metallic molybdenum and tungsten.