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Characterization of catalyst pellets using NMR and MRI: MRI, diffusion and relaxation measurements of liquid imbibed in alumina and titania extrudates



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Karsten, Vivian 


In this thesis a range of catalyst pellets are studied using Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques with the aim of characterizing mass transport in the pellets and exploring the presence of spatial heterogeneities formed in the manufacturing process. Measurements have been done both in bulk, i.e. spatially unresolved, and at high spatial resolution. Measurements have been used for comparison with standard bulk pellet characterization methods, in particular the estimation of the tortuosity via the bulk porosity, but also to provide information about whether these characteristics are uniform throughout the pellet or whether there are heterogeneities introduced by the manufacturing process. Using the insights gained into mass transport characteristics and spatial heterogeneities in catalyst pellets should lead to better understanding and optimisation of catalysts choice and catalyst production processes (mainly extrusion, drying and calcination) and models of catalytic systems.

Spatially unresolved Pulsed Field Gradient NMR (PFG NMR) methods have been used to measure the self diffusion coefficient of liquid contained within catalyst support pellets. Mass transport in catalyst pellets is often characterized through the tortuosity parameter, which impacts the catalyst effectiveness, though it is difficult to measure and simple relationships, such as tortuosity = 1/porosity, are often used. The tortuosity can be directly calculated from PFG NMR diffusion measurements and this work investigates the relationship between tortuosity and porosity for a range of alumina and titania pellets and shows that whilst a simple reciprocal relationship between tortuosity and porosity provides a reasonable estimate for the titania pellets, this relationship does not hold for the alumina pellets. This highlights the need of experimental techniques to measure the tortuosity.

PFG NMR tortuosity measurements have been extended to catalyst materials containing high concentrations of paramagnetic species. PFG NMR measurements are usually carried out at high magnetic field strengths (>1 T) but this is not possible when samples contain high concentrations of paramagnetic species as the NMR signal lifetime becomes very short. Short signal lifetimes are due to paramagnetic species causing large internal magnetic field gradients which scale with the external magnetic field strength. In this work it is shown that if PFG NMR is performed at low field (2 MHz), the tortuosity of catalyst pellets containing industrially relevant concentrations of paramagnetic species (20 wt.% Co3O4/TiO2 used for Fischer-Tropsch Synthesis) can be measured successfully.

Spatially resolved measurements of the NMR signal intensity have been obtained at high resolution (typically 10μm × 39 μm) of a range of titania pellets, some of which revealed significant spatial heterogeneity. These heterogeneities are attributed to spatial differences in the oxidation state of titanium. Ti4+ is diamagnetic, whereas Ti3+ is paramagnetic. These spatial variations could have been introduced by the catalyst production process and could be important in catalyst performance where the presence of Ti3+ can influence metal-support interactions.

Spatially resolved measurements of diffusion and NMR relaxation parameters (T1, T2) have been obtained at high resolution using MRI pulse sequences specifically designed to investigate spatial heterogeneities in catalyst pellets. The imaging methods have been applied to a range of both titania and alumina catalyst support pellets in the shape of trilobes. NMR relaxation measurements of liquid confined in porous material are sensitive towards the surface to volume ratio of the pores, the liquid-solid adsorption strength and the presence of relaxation sinks (including paramagnetic species) at the pore surface. T2 relaxation maps revealed significant heterogeneities. An increase in T2 values was for example observed at the pinch points of the trilobes, which can be attributed to a difference in solid-liquid interaction at the pore surface. These heterogeneities could be a result of imperfections in the extrusion or drying process. This thesis demonstrates how NMR methods can be used to gain a more complete and realistic understanding of catalyst pellets and to optimize the manufacturing processes of catalyst pellets.





Gladden, Lynn
Sederman, Andrew
Mantle, Michael


activity, Alumina, calcination, Catalysis, catalyst effectiveness, Catalyst pellets, Co/Ti, diffusion, diffusion map, drying, EPR, Extrusion, Fischer-Tropsch, imaging, k-space, Longitudinal relaxation, manufacturing process, mass transport, mass transport limitations, MRI, NMR, Paramagnetic species, PFG NMR, pore size, pore structure, porosity, porous material, pulse sequence, Relaxation, spatial heterogeneities, Spatially resolved, surface relaxivity, T1, T1 map, T2, T2 map, Titania, Titanium, tortuosity, Transverse relaxation, trilobe


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
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