Reduction process of the (NH4)3[CrMo6O24H6]•7H2O Anderson phase:: spectroscopic, structural and morphological properties

The reduction steps of the (NH4)(3)[CrMo6O24H6]. 7H(2)O Anderson phase with H-2 were analyzed by TPR and other techniques (XRD, SEM and EDAX, IRI Raman, DR and EPR spectroscopy). The behavior of Cr-2(MoO4)(3) was also studied for comparison. All the results are discussed in relation to the reduction process of other Anderson phases containing trivalent heteroatoms. The results suggested a complex scheme of reaction through the preliminary formation of Cr-2(MoO4)(3) and MoO3-x phases (similar to 673 K). Although, Mo-VI showed the typical Mo-VI --> Mo-IV --> Mo-0 reduction sequence, the temperatures of the step wise process differed from those observed for ammonium heptamolybdate, Cr-2(MoO4)(3) and other related Anderson phases. The presence, in an intermediate stage (similar to 773 K), of a disordered and non stoichiometric (Mo, Cr)O-2 phase, with a reduced rutile structure, is suggested, alpha-Cr2O3 was segregated only at 1273 K. In addition, some Cr-doped Al-Anderson phases were synthesized and analyzed by DR, Raman and EPR spectroscopies. (C) 2000 Elsevier Science S.A. All rights reserved.