In situ resonance Raman studies of molybdenum oxide based selective oxidation catalysts

Abstract

The preparation (aqueous chemistry and the thermal activation of polyoxometalates) and the structures of promoted molybdenum oxide catalysts are investigated under the condition of selective propene oxidation by in situ Raman spectroscopy. The influences of V and/or Wpromoters on the structures of mixed MoVW oxides and their catalytic properties are investigated by XRD, TEM, SEM, in situ Raman spectroscopy and TPRS. V addition causes high propene conversions and the formation of an oxide with Mo5O14 structure is observed. Minor amounts of W in the molybdenum oxides matrix inhibit structural reorganisation processes which is explained by preferred octahedral coordination of the redox stable W. The combined XRD, TEM and Raman spectroscopic identification of the oxide of Mo5O14 structure points to its relevant role for the selective propene oxidation. A resonance Raman effect is proven to be operative in oxygen defective molybdenum oxides. For an excitation wavelength of 632 nm (1.96 eV), the observed Raman cross section varies as a function of the degree of reduction of five different MoO3 x samples. A model of the electronic transitions in MoO3 x based on crystal field theory explains the electronic transitions observed by DR-UV/VIS spectroscopy. The observed resonant Raman scattering is coupled to the IVCT transition at about 2 eV arising from oxygen vacancies present in the materials. Due to the local nature of the absorption process, the developed model is valid for intermediate oxides too. Resonant Raman scattering was proven for Mo4O11 and MoO2 too. Hence, the experimentally observed Raman intensity bears information about the degree of reduction of the molybdenum oxide. In situ Raman spectroscopy of MoO3 x catalysts during propene partial oxidation indicates that the propene conversions and the selectivities are a function of the degree of reduction of the catalyst. It is attempted to control the spatial elemental distribution within the polyoxometalate catalyst precursor by the formation of molecularly defined species in solution by addition of acetate, oxalate or tartrate. Due to the low stability of the acetate complexes, the addition of acetate only leads to minor amounts of monomeric species beside mixed and pure polyoxometalates. The formation of stable monomeric oxometalate oxalate and tartrate complexes is observed, which are expected to lead to the formation of a catalyst precursor with a homogeneous elemental distribution.