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

The reduction steps of the (NH4)3[CrMo6O24H6]·7H2O Anderson phase with H2 were analyzed by TPR and other techniques (XRD, SEM and EDAX, IR, Raman, DR and EPR spectroscopy). The behavior of Cr2(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 Cr2(MoO4)3 and MoO3−x phases (∼673 K). Although, MoVI showed the typical MoVI → MoIV → Mo0 reduction sequence, the temperatures of the step wise process differed from those observed for ammonium heptamolybdate, Cr2(MoO4)3 and other related Anderson phases. The presence, in an intermediate stage (∼773 K), of a disordered and non stoichiometric (Mo, Cr)O2 phase, with a reduced rutile structure, is suggested. α-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.