Operando chemical tomography of packed bed and membrane reactors for methane processing

Heterogeneous functional materials, like catalytic solids, batteries and fuel cells tend to usually possess complex structures where the 3D spatial distribution of the various components of these materials is rarely uniform. Such materials are known to change with time under operating conditions. In order to gain an insight into the structure-function relationships, it is essential to study them in situ with spatially-resolved techniques. The work presented in this thesis focuses on the development and application of synchrotron X-ray tomographic imaging methods to study various catalytic materials in real time and under real process conditions. The main X-ray tomographic imaging technique used in this study is X-ray diffraction computed tomography (XRD-CT) which couples powder diffraction with “pencil” beam computed tomography. Chapters 3 and 4 of this thesis outline some of the technical achievements accomplished in this work. More specifically, Chapter 3 outlines the development of a new data processing strategy used to remove line or “streak” artefacts generated in reconstructed XRD-CT images due to the presence of large crystallites in the sample; a common problem in XRD-CT measurements. Chapter 4 introduces a new data collection strategy, termed interlaced XRD-CT, which allows, post experiment, choice between temporal and spatial resolution. This data collection strategy can in principle be applied to all pencil beam CT techniques. The results from the first multi-length scale chemical imaging experiments of an unpromoted and a La-promoted Mn-Na-W/SiO2 catalyst for the oxidative coupling of methane are presented in Chapter 5. The spatially-resolved chemical signals obtained from these operando experiments provided new chemical information that can lead to the rational design of improved OCM catalysts. In Chapter 6, the results from, the first ever reported, XRD-CT experiments of working catalytic membrane reactors are presented. It is shown that the pertinent changes in the physicochemical state of these integrated reactor systems can be spatially-resolved. The results from Rietveld analysis of a 5D diffraction imaging (>106 diffraction patterns) redox experiment of a Ni-Pd/CeO2-ZrO2/Al2O3 catalyst and the first XRD-CT study of this catalyst during partial oxidation of methane are presented in Chapter 7