Reduction process of the (NH4)(3)[CrMo6O24H6]center dot 7H(2)O 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 Rama n, 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 res ults suggested a complex scheme of reaction through the preliminary formati on of Cr-2(MoO4)(3) and MoO3-x phases (similar to 673 K). Although, Mo-VI s howed the typical Mo-VI --> Mo-IV --> Mo-0 reduction sequence, the temperat ures of the step wise process differed from those observed for ammonium hep tamolybdate, Cr-2(MoO4)(3) and other related Anderson phases. The presence, in an intermediate stage (similar to 773 K), of a disordered and non stoic hiometric (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-A nderson phases were synthesized and analyzed by DR, Raman and EPR spectrosc opies. (C) 2000 Elsevier Science S.A. All rights reserved.