Creation of α-oxygen during the N2>O decomposition over Mn-ZSM-5

Abstract

The main goals of this PhD-thesis are to answer two important questions regarding the generation of α-oxygen species in Mn-based zeolites: 1) What are the requirements that a zeolite material needs to possess in order to generate α-oxygen species? 2) What characterisation techniques can be used in order to reveal the properties of the active centre for N2O decomposition and the related formation of α-oxygen species? In order to address the first question, two types of materials were prepared presenting different properties from the geometric and electronic structure point of view. The ion-exchanged Mn-ZSM-5 materials are characterized by a dynamic geometric and electronic structure, as is evident from the changes in the coordination sphere and oxidation state of the manganese during the catalytic reactions. An opposite conclusion can be drawn for the Mn-ZSM-5 materials prepared using hydrothermal synthesis. Due to the presence of large manganese-rich domains that incorporate the majority of the present manganese, the overall system presents a static geometric and electronic behaviour. We can conclude that the generation of α-oxygen species is made possible by fulfilling a number of factors related to the Mn - cluster: a. Dynamic divalent and trivalent oxidation states with the possibility to easily exchange between them. b. The possibility to contain different coordination environments and the possibility to exchange between them c. The dispersion of the active phase should be high in order to hinder the formation of particles that can influence in a negative way the possibility to change the coordination state of the active centre. Concerning the characterization techniques, the challenge to identify the active species lies in the diversity of the species present in the catalyst materials, so discrimination between spectator and active species is necessary. In order to do that we need to identify the characterisation techniques capable of determining the characteristics presented as an answer to the first question. One possible technique is the HERFD-XAS technique, where the 3d transitions are used to determine the nature of neighbours, their number and their distances. The energy position of the K edge varies with the valence, while the pre-edge region gives information on the average valence and site symmetry of the metal sites. One of the advantages of the HERFD-XAS technique is the improved separation of the pre-edge region to the edge jump region, allowing for a detailed analysis of this region. The comparison of the pre-edge characteristics from the in-situ data with that of the references can give valuable information on the local coordination and valence of the active centre. Another technique that proved to be very helpful in understanding the morphology of the material is STEM-EELS. This technique combines the strength in spatial resolution of electron microscopy techniques with the possibility to quantitatively identify the chemical nature (i.e. element and valence state) of the measured phase given by EELS.