Wavelet transform EXAFS analysis of mono- and dimolybdate model compounds and a Mo/HZSM-5 dehydroaromatization...

The local structure of molybdate ions in the crystalline materials MgMoO(4) and MgMo(2)O(7) and in Mo/HZSM-5 (2 wt% Mo, Si/Al = 15) was explored by EXAFS at the Mo K-edge, using both Fourier and wavelet transforms. For the two model compounds, curve fitting analysis of the FT-EXAFS reveals the need to include several single- and multiple-scattering paths in order to locate the Mo-Mo paths accurately. Some of these overlapping paths can be identified using WT-EXAFS analysis. Indeed, the positions of WT maxima can be predicted fairly accurately, allowing for qualitative identification of the paths required for curve fitting in samples of unknown structure and providing additional criteria for distinguishing statistically similar curve fits. The application of the WT analysis technique to the heterogeneous catalyst Mo/HZSM-5 clearly shows the presence of dimolybdate ions, although with structures perturbed by their interactions with the zeolite framework.