Redox behavior of VIB transition metal ions in rutile TiO2 solid solutions:: An XRD and EPR study

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 Cr-III, Mo-IV, and W-IV and a small fraction as isolated Mo-V and W-V in substitutional sites. After heating in air or in O-2 up to 1273 K, in TC no isolated Cr-V formed, whereas in TMo and TW, M-IV was oxidized to M-V and M-VI, and MO3 was segregated. The oxidation started at 673 K and gave isolated substitutional MoV and isolated interstitial and substitutional WV. After heating in H-2 up to 1073 K, TMo and TW reduction process differed: Mo-IV gave isolated and clustered Mo-III, and Mo-IV-Mo-IV pairs gave Mo-2(7+) species; W-IV was not reduced to W-III; and temperatures > 900 K caused the segregation of metallic molybdenum and tungsten, (C) 2000 Academic Press.