Mechanically Activated MoO₃. 4. In Situ Characterization of Physical Mixtures with Al₂O₃

Abstract

Mechanical activation of solids leads to an increased internal energy caused by the introduction of defects. This can result for example in a reduced surface melting temperature of activated particles, which in turn may affect their sintering behavior. Analogously the spreading behavior of MoO3 over Al2O3 may depend on the mechanical treatment during physically mixing the solids. Differences in the spreading over Al2O3 of unmilled MoO3 and MoO3 that was mechanically activated for 600 min were investigated by SEM/TEM, XPS, ESR, and in situ high-temperature Raman spectroscopy. SEM and EDX analyses of these physical mixtures make surface melting during the calcination very probable. In addition, analysis of XPS spectra also shows that spreading occurs under these conditions. However, spreading in the mixture with milled MoO3 is more effective. ESR spectroscopy shows that Mo5+ centers are reoxidized after calcining the mixtures with unmilled MoO3 in moist oxygen. For the mixture with milled MoO3 an additionally observed Mo5+ species in C 2v distorted sixfold coordination is stable against oxygen for many hours, independent of the presence or absence of water. This higher stability of this defect species against reoxidation is attributed to an improved stabilizing effect of the Al2O3 support due to a pronounced spreading of mechanically activated MoO3. High-temperature Raman spectroscopy of pure, unmilled MoO3 reveals that a transformation into polymeric species occurs at temperatures above 948 K. At 1053 K, melting is observed and the Raman bands of crystalline MoO3 are lost. Calcination of an unmilled physical mixture of 9 wt % MoO3 and Al2O3 at the considerably lower temperature of 823 K in dry O2 leads to the observation of Raman bands of an amorphous polymeric Mo surface melt. Quenching this sample to room temperature results in a Raman spectrum which is attributed to a glassy surface MoO3 phase. Calcination of the physical mixture milled for 3 h at 823 K also leads to a Raman spectrum of the surface melt. Quenching to 298 K does not lead to a considerable change of the spectrum, this being explained by a more effective spreading of the Mo phase in the mechanically activated mixture. This surface Mo phase is highly reactive toward H2O during rehydration at 298 K, which leads to the formation of a polymeric surface species. A long time spreading experiment at 723 K reveals that this process is considerably slower and less effective at this lower temperature.

Spreading and Wetting

“…During grinding, several particles are simultaneously and repeatedly subjected to stress application in the grinding zone, and with each stress application several fractures may occur in each particle. When this milled MoO 3 was gently mixed with alumina, without applying additional mechanical stress, and then thermally treated in O 2 at 823 K (melting point of bulk MoO 3 is 1068 K), in-situ high-temperature Raman spectroscopy demonstrated the existence of a surface melt of molybdenum oxide which, on quenching to room temperature, transformed into a glassy MoO 3 surface phase [85]. The physical and chemical interaction between particles and the grinding environment, as well as the transport of material through the grinding zone, will also affect the nature of the product obtained.…”

EPR Monitoring of Redox Processes in Transition Metal Oxide Catalysts

“…[156,157] As pointed out earlier, the spectrum of reduced molybdena was very complex, and contained a number of other signals beside the already discussed C and G. Their EPR parameters fell into the range characteristic for reduced molybdenum species retaining the molybdenyl bond. [14,46,47,118,135,140,147,189,190] Analysis of the spectra led to the identification of hexacoordinated and pentacoordinated Mo 5 ions dispersed in the molybdena lattice (signals A and F, respectively), as well as hexacoordinated Mo 5 present in the shear structures formed upon deep reduction (signals E and B). [140,141,147,148] The two former species could be described by the orthorhombic Hamiltonian, Equation (25).…”

Defect structure of nanosized mechanically activated MoO3

“…Figure 5 illustrates fragments of Raman spectra for the initial and mechanically activated МоО 3 . The assignment of the bands in the Raman spectra of МоО 3 has been performed in [11,12,23]. The spectra comprise the bands attributed to the vibrations of indi vidual atoms relative to each other, small groups, and polymer chains.…”

“…The widening and shift of the bands seem to be due to the following factors: (1) a reduction in the sizes of crystallites, which leads to an increase in the number of atoms (groups, chains) that are located on a surface and in a near surface layer (surface factor) [11,12]; and (2) the appearance of local strains, point defects, stressed bonds, etc.…”

“…The primary data on morphological transforma tions and the formation of a defect structure in MoO 3 as a result of MA were obtained in works by Knözinger et al [8][9][10][11][12] and a number of other authors [13][14][15]. This work is distinguished by the study of the genesis of the defect structure in a very wide range of mechanical treatment doses rather than at the initial stage of MA alone, as it has been in the majority of the published works.…”