Mapping Porous Solid Catalysts with Fluorescent Molecules and Nanoparticles
Authors:   , Maris Jacob Jan Erik 
Published: May 2023

In this PhD thesis, fluorescent molecules or nanoparticles were investigated to probe the pore space of heterogeneous catalysts and mass transport therein. The fluorescent emitters (guests) report on the properties of the catalyst (hosts) via guest–host interactions. These interactions can be a resistance on the movement of the probes by the pore space, reversible and irreversible adsorption of probes on the pore wall, or a chemical reaction between the guest and host. It is advantageous to follow reporters via their fluorescence because it can be detected with high sensitivity, e.g., to determine the location of the reporter, which makes it possible to probe individual molecules or nanoparticles. A second advantage is that the fluorescence emission spectrum is often dependent on the chemical identity of the fluorescent reporter. Thus, chemical reactions of the guest with the host can be followed directly via the guest’s emission spectrum.

Analysis of the trajectories obtained with single-molecule tracking in inorganic porous hosts is often challenging because trajectories are short and/or motion is heterogeneous. In Chapter 3, we presented the software DiffusionLab for motion analysis of such challenging datasets. This approach relies on the pooling of trajectories with a similar motion behaviour into a population. The average motion characteristics of a population can be computed and compared without losing information on the single-molecule level about, e.g., the spatial distribution of adsorption.

In Chapter 4, we introduced a microfluidic device designed for the characterization of fluorescent reporters in confinement. The device consists of a two-dimensional model pore with a height of 50 nm. This design allowed for the measurement of long trajectories, which facilitated detailed probe characterization. We investigated guest–host adsorption of single quantum-dot emitters to the pore wall and guest diffusion through the model pore. Building on the probe characterization, we defined conditions that allowed for mapping of the accessible pore space of a model-pore array and a real-life catalyst particle.

We studied molecular diffusion in the straight and the sinusoidal pores of ZSM-5 zeolites in Chapter 5. The pore types have a similar effective pore size but a different shape. Both the mobility, when the molecules were not adsorbed over hundreds of milliseconds, and the frequency of adsorption was different in the straight and sinusoidal pores. Additional meso- and macropores with a diameter ≥ 2 nm were etched into the zeolite material to promote diffusion. The molecular mobility increased and the adsorption frequency of the molecules decreased in the sinusoidal pores, which indicated the formation of an interconnected secondary pore network.

In Chapter 6, we studied the guest–host interactions of fluorescent resorufin molecules inside the micropores of ~10 micrometre-sized zeolite-β crystals. We found that resorufin molecules were protonated after entering the zeolite, which provided contrast between the resorufin species inside the zeolite micropores and in aqueous solution. Using this effect, it was shown that the micropores in the crystals were fully accessible. Moreover, we visualised that diffusion through the straight pores of zeolite-β was impeded when diffusing through the boundaries between the zeolite subunits.