MECHANICALLY ACTIVATED MOO3 .4. IN-SITU CHARACTERIZATION OF PHYSICAL MIXTURES WITH AL2O3

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 b ehavior of MoO3 over Al2O3 may depend on the mechanical treatment duri ng physically mixing the solids. Differences in the spreading over Al2 O3 of unmilled MoO3 and MoO3 that was mechanically activated for 600 m in were investigated by SEM/TEM, XPS, ESR, and in situ high-temperatur e 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 mor e effective. ESR spectroscopy shows that Mo5+ centers are reoxidized a fter calcining the mixtures with unmilled MoO3 in moist oxygen. For th e mixture with milled MoO3 an additionally observed Mo5+ species in C- 2 upsilon distorted sixfold coordination is stable against oxygen for many hours, independent of the presence or absence of water. This high er stability of this defect species against reoxidation is attributed to an improved stabilizing effect of the Al2O3 support due to a pronou nced 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, melt ing is observed and the Raman bands of crystalline MoO3 are lost. Calc ination of an unmilled physical mixture of 9 wt % MoO3 and Al2O3 at th e considerably lower temperature of 823 K in dry O-2 leads to the obse rvation of Raman bands of an amorphous polymeric Mo surface melt. Quen ching this sample to room temperature results in a Raman spectrum whic h is attributed to a glassy surface MoO3 phase. Calcination of the phy sical mixture milled for 3 h at 823 K also leads to a Raman spectrum o f the surface melt. Quenching to 298 K does not lead to a considerable change of the spectrum, this being explained by a more effective spre ading of the Mo phase in the mechanically activated mixture. This surf ace 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 consid erably slower and less effective at this lower temperature.