Catalyst Pore Space Exploration using Fluorescence and X-Ray Microscopy Techniques
Authors:   , Mayorga Gonzalez Rafael 
Published: May 2024

Mass transport is a crucial aspect in heterogeneous catalysis as it can influence activity, selectivity, and the overall lifespan of a catalyst material. Therefore, rational pore space design has immense potential for improving catalytic performance. To investigate how synthesis parameters affect catalyst materials' pore networks, high-resolution and high-throughput porosity characterization techniques are required. However, current techniques face challenges: bulk analytical measurements ignore heterogeneity within and between catalyst particles, and high-resolution techniques are complex, expensive, and offer poor statistical representation. In this PhD thesis, new analytical methods to study the pore space of heterogeneous catalysts using X-ray and fluorescence microscopy were explored.
In Chapter 2, transmission X-ray microscopy (TXM) was used to study the macroporosity of MIL-47(V) metal-organic framework (MOF) crystals, revealing macropore defects up to the micron scale. These defects were confirmed by focused ion beam (FIB) cutting and scanning electron microscopy (SEM), showing localized and poorly connected macropore networks.
In Chapter 3, a cost-effective method to characterize individual porous particles was developed using conventional fluorescence microscopy and a PDMS-made microfluidic device. Here, the uptake of fluorophores into the solid particles over time was used to characterize them, showing significant heterogeneities among seemingly homogeneous particles. Electrostatic interactions, modulated by pH and ionic strength, significantly influenced mass transfer.
Chapters 4, 5, and 6 explore the possibility of using single-molecule (particle) localization microscopy (SMLM) and single particle tracking (SPT) to explore and map porous catalyst materials with sub-diffraction limit resolution.
In Chapter 4, we introduce a two-dimensional (2D) silica model pore system made with lithography and wet etching. We investigated the diffusion and trapping (immobilization) behavior of quantum dots (QDs) as local probes in confinement. The duration and frequency of trap events could be suppressed via the pH. Further, we demonstrated the use of QDs under nearly non-trapping conditions for pore-space mapping of a real-life polymerization catalyst support particle, with sub-diffraction-limit resolution.
In Chapter 5, we investigate whether we can probe pore sizes in three dimensions based on local diffusion coefficients measured with SPT. Therefore, we varied the 2D model pore depth and studied the motion behavior of individual quantum dots under non-trapping conditions. Surprisingly, the trajectories displayed virtually the same diffusion coefficients for all studied pore depths. This contradicted hydrodynamic drag simulations, where the diffusion coefficient was predicted to vary notably with the 2D pore depth.
In Chapter 6, we investigated the use of smaller carbon dot probes for pore space exploration. Their small size (~2 nm) allowed us to track them within porous silica particles that were inaccessible to the probes used in Chapters 4 and 5, showing their potential to map mesoporous materials. Furthermore, different trapping event durations were observed on unconfined surfaces with different chemical compositions. Potentially, this parameter could be used to map chemically heterogeneous surfaces. Finally, confocal laser scanning microscopy imaging of stained mesoporous silica particles suggested that the trapping behavior could also be tuned via the pH of the system.