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

Operando and Postreaction Diffraction Imaging of the La-Sr/CaO Catalyst in the Oxidative Coupling of Methane Reaction

A La−Sr/CaO catalyst was studied operando during the oxidative coupling of methane (OCM) reaction using the X-ray diffraction computed tomography technique. Full-pattern Rietveld analysis was performed in order to track the evolving solid-state chemistry during the temperature ramp, OCM reaction, as well as after cooling to room temperature. We observed a uniform distribution of the catalyst main components: La2O3, CaO−SrO mixed oxide, and the hightemperature rhombohedral polymorph of SrCO3. These were stable initially in the reaction; however, doubling the gas hourly space velocity resulted in the decomposition of SrCO3 to SrO, which subsequently led to the formation of a second CaO−SrO mixed oxide. These two mixed CaO−SrO oxides differed in terms of the extent of Sr incorporation into their unit cell. By applying Vegard’s law during the Rietveld refinement, it was possible to create maps showing the spatial variation of Sr occupancy in the mixed CaO−SrO oxides. The formation of the Srdoped CaO species is expected to have an important role in this system through the enhancement of the lattice oxygen diffusion as well as increased catalyst basicity

Effect of thermal treatment on the stability of Na–Mn–W/SiO2 catalyst for the oxidative coupling of methane

In this study, we investigate the effect of thermal treatment/calcination on the stability and activity of a Na-Mn-W/SiO2 catalyst for the oxidative coupling of methane. The catalyst performance and characterisation measurements suggest that the W species are directly involved in the catalyst active site responsible for CH4 conversion. Under operating conditions, the active components, present in the form of a Na-W-O-Mn molten state, are highly mobile and volatile. By varying the parameters of the calcination protocol, it was shown that these molten components can be partially stabilised, resulting in a catalyst with lower activity (due to loss of surface area) but higher stability even for long duration OCM reaction experiments

Supplementary Information from X-ray physico-chemical imaging during activation of cobalt-based Fischer–Tropsch synthesis catalysts

Additional reference spectra and 2D image