Dynamic electron microscopy in controlled environments

An isotherm of C3H6 conversion on y is shown in figure 3.16, where the products are monitored at 450 °C. The figure shows various stages of activity and the activity begins to decrease after 1 hr. A drop in the selectivity to acrolein is also observed above 450 °C. Sample C, for example, shows the presence of y, and a structure with dimensions 8.4 A x 10.8 A. Sample D has more metallic Bi and reduced Mo-oxides. 

The superlattice can be indexed as the (101) -phase. The HRTEM in figure 3.18(a) shows a contrast (circled and arrowed) indicative of cation point defect clusters. 

These results suggest that the (101) superstructure observed on the (001) -phase at the catalyst s operating temperature is closely related to Bi2M02O9. A quantification of the microanalysis of the jS-preparation shows a Bi-deficiency. Similar results are observed in the reaction of the a-phase in propylene. In a C3 H6-O2 mixture under working conditions both phases show the presence of this superstructure similar to the jS-structure. The ETEM results are consistent with XPS and Raman data which show that the surface structure of the active bismuth molybdate is close to the jS-phase and that the jS-phase is more active (Matsurra et al 1980, Burrington et al 1983). In these studies dramatic increases in the activity 

Erom HRTEM studies, it is proposed that the majority of the bismuth molybdate phases can be derived from the fluorite structure, in which both the cation and anion vacancies can be accommodated within the fluorite framework (Buttrey et al 1987). Several industrial processes containing multicomponent bismuth molybdates may suffer loss of Mo oxides by volatilization under operating conditions, resulting in the loss of catalytic activity. Monitoring the catalyst microstructure using EM is therefore crucial to ensuring the continuity of these catalytic processes. 

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