Combined super-resolution fluorescence and scanning electron microscopy for catalysis research

The rational improvement of solid catalysts requires a thorough understanding of the structure-activity relationship down to the smallest possible length scales. Recently developed approaches that correlate fluorescence microscopy (FM) and scanning electron microscopy (SEM) allow such insights by direct linking of nanoscale catalytic activity maps, as recorded using fluorescence, to the local structural context. The goal of this PhD was to investigate structure-activity relationships at the nanoscale, by developing an integrated light and electron microscope (ILEM) that combines the power of single molecule sensitive FM to resolve reactivity at the nanoscale with a high-end SEM.

Initially, the ILEM was applied to visualize silver nanoparticle photodeposition from an aqueous silver(I) solution on individual ZnO crystals, in real time. This was enabled by the ability to simultaneously perform local UV irradiation using the integrated light microscope and SEM imaging at the same region of interest (ROI) while the sample was contained inside a specialized liquid cell. This research revealed that silver nanoparticle formation predominantly occurs at crystal edges. However, the contribution of the electron beam during silver nanoparticle deposition was found to be non-negligible.

Follow-up research was performed by applying the ILEM in a correlative fashion; i.e., by performing structural imaging before and after, and not during, UV induced silver photodeposition. As such, the facet dependent photocatalytic reactivity could be explored at the single particle level and, at the sub-particle level, variations were related to crystallographic structural features and defects.

The first correlative super-resolution fluorescence and electron microscopic investigation of zeolite catalysts was made possible after resolving several technical challenges encountered during the ZnO research and by further optimizing the fluorescence microscope to enable the detection of individual catalytic fluorogenic conversions. This improved setup made it possible to directly observe the effect of intercrystalline intergrowths on the overall catalytic performance of acid mordenite zeolites. By determining the orientation of the individual reaction products compared to the underlying zeolitic framework, it was found that shape-selectivity was maintained at the defect-rich intergrowth boundary. Hence, the intergrowth was identified as a void space that facilitates mass transport into these pores. Acid leaching did not dramatically change this, as activity increased on previously active regions while the molecular orientation was maintained.

A second correlative zeolite structure-activity investigation targeted individual ZSM-22 catalyst particles. The typical needle-shaped morphology of these particles results from a lateral fusion of elementary nanorods and indirect experimentation already suggested that during this lateral fusion, external, catalytically inactive, aluminum is converted into catalytically active internal aluminum. This was confirmed by the visualization of catalyst activity and shape-selectivity at the sub-particle level on the ILEM.

In summary, over the course of this PhD, a performant ILEM was developed that combined super-resolution fluorescence microscopy based on the localization of individual fluorescent molecules with high-end field emission SEM. Also the necessary experimental routines and software tools were developed, enabling the determination of the structure-activity relationship of heterogeneous catalysts at the nanoscale. This powerful research tool allows a direct correlation of catalyst structure and activity beyond the single particle level. It is anticipated that the further application and development of this ILEM will ultimately lead to a more rationalized catalyst optimization.