New frontiers in X-ray spectroscopy of FeZSM-5

Abstract

The aim of the research described in this work is two-fold. Firstly, new techniques based on X-ray spectroscopy have been introduced in the field of heterogeneous catalysis. As a consequence, a more advanced characterization of catalytic solids could be pursued. Secondly, the application of these new methodologies on differently prepared FeZSM-5 zeolites revealed a more detailed understanding of the Fe species present in these important catalytic materials. This thesis ‘New Frontiers in X-ray Spectroscopy of FeZSM-5’ intends to contribute to the application of novel X-ray spectroscopic techniques in the study of catalytic solids as well as to the understanding of the structure and function of FeZSM-5 zeolites. Throughout the thesis the description of the ‘New Frontiers in X-ray Spectroscopy of FeZSM-5’ followed five main themes: I) The application of in situ soft X-ray Absorption Spectroscopy (XAS) in the study of heterogeneous catalysts and a proposal for the detailed data analysis of transition metal spectra (chapters 3 and 4). II) The application of X-ray Magnetic Circular Dichroism to heterogeneous catalysts (chapter 5). III) The exploitation of X-ray Near Edge Absorption Structure (XANES) by benefiting from high-quality spectra gathered by selective detection methods in hard XAS (chapter 6). IV) The push of the detection limits in the Extended X-ray Absorption Fine Structure (EXAFS) to measure low-concentrated FeZSM-5 samples by selective detection (chapter 6 and 8). V) The refinement of the data analysis for EXAFS and XANES (chapters 7 and 8). It turned out that the knowledge about FeZSM-5 zeolites keeps up with the increasing possibilities afforded by novel X-ray Spectroscopic characterization methods. During the research we concentrated our attention on two types of FeZSM-5 zeolites: overexchanged Fe/ZSM-5 (4.4 wt % Fe in extraframework positions) and framework-substituted FeZSM-5 (0.3 wt % Fe initially in framework positions).

We demonstrated that the precise, quantitative measurements of key characteristics as valence and local symmetry of transition metal ions in catalysts is made possible by both (in situ) soft XAS and selective hard XAS techniques. Even for measurements performed at high temperature and for low-concentrated samples we present higher accuracy data and more detailed analysis procedures. In summary, the results that we described in this thesis show that the characterization of heterogeneous catalysts clearly benefits from the continuous developments in X-ray spectroscopy. In particular for the study of complex catalytic systems, as FeZSM-5, one should exploit the possibilities offered by advanced spectroscopic techniques to unravel the active site structures and reaction mechanisms. Also other fields engaged in the study of chemical and biological systems will take advantage of new X-ray spectroscopic techniques and methods. The frontiers as emerged in this work are today’s limits, but will certainly contribute to that of