Multi-dimensional characterisation of solid catalysts is necessary to understand their complex and often inhomogeneous structure. Over the past decades, there has been a growing interest in designing and developing operando and in situ characterisation techniques in order to fully rationalise the structure-activity relationships of working catalysts. In particular, the combination of X-ray tomography with diffraction and/or spectroscopic techniques allows the extraction of chemical and physical information within the interiors of intact materials and reactors. The spatially-resolved signals obtained with the tomographic approach often reveal information that would be otherwise lost in bulk measurements. The work presented in this thesis focuses on the application of X-ray diffraction computed tomography (XRD-CT) to study various catalytic systems for methane upgrade processes, namely the oxidative coupling of methane (OCM) and the partial oxidation of methane (POX) reactions. A significant part of this work focused on developing routines for data handling and analysis of the thousands of diffraction patterns present in XRD-CT data. The application of the XRD-CT technique to study working catalytic reactors, in packed-bed and catalytic membrane rector configurations, provided unprecedented information about the spatio-temporal evolution of the solid-state chemistry in these material systems. As presented in Chapter 4, the Rietveld analysis of the spatially-resolved diffraction data obtained during the OCM reaction with a La-Sr/CaO catalyst allowed us to map the temperature and chemical gradients in a reactor cross section by tracking the presence of SrCO3 polymorphs and SrO. In Chapter 5, the application of the operando chemical imaging techniques allowed us to identify the possible reasons behind the deactivation of a Na-Mn-W/SiO2 catalyst in the OCM reaction; these investigations led to the design of a new catalyst pre-treatment protocol improving its stability. Catalytic membrane reactors with perovskite membrane and Na-Mn-W/SiO2 catalyst were studied in Chapter 6; the XRD-CT data revealed that the Ba0.5Sr0.5Co0.8Fe0.2O3-delta membrane did not exhibit long-term phase stability during the OCM process and also it chemically interacted with the volatile catalyst active components. Finally, Chapter 7 presents the results from the first 5D diffraction imaging experiment with a complex Ni-based catalyst during the partial oxidation of methane reaction, where we determined the role of the various catalyst components on the catalyst stability and performance.
|Date of Award||31 Dec 2019|
- The University of Manchester
|Supervisor||Robert Cernik (Supervisor) & Timothy Burnett (Supervisor)|
- X-ray diffraction computed tomography