Development of Tip-enhanced Raman Spectroscopy and its Application to Heterogeneous Catalysis Research

Abstract

Catalysis is at the forefront of many developments to make our society more sustainable and less dependent on fossil resources, such as coal, gas and crude oil. To make this possible, it is important to obtain detailed insights into the working principles of solid catalysts. In the past decades, several powerful analytical techniques have been developed to investigate the working mechanisms of various catalytic materials. Raman spectroscopy is one of these methods, however, it suffers from the limitations of a relatively low sensitivity and poor spatial resolution. A way to counter this is to use tip-enhanced Raman spectroscopy (TERS), which dramatically improves the sensitivity of Raman spectroscopy and pushes its spatial resolution far beyond the optical diffraction limit. This PhD research has focussed on the further development of TERS as a robust and versatile analytical tool for heterogeneous catalysis research. Metallic probes are the very heart of a TERS experiment. Therefore, first, a better understanding into the degradation mechanism of silver-coated TERS probes was gained by conducting a time-series study under different environmental conditions. A significant improvement in the plasmonic lifetime of TERS probes, from a few hours to greater than five months, was obtained under < 1 ppm oxygen and moisture environment. This study allowed practical strategies to be developed for long term preservation of TERS probes. For in situ monitoring of catalytic reactions, TERS has to work in both air and liquid environments. However, current metal coated TERS probes are unsuitable for operation in liquids due to the lack of structural stability. This issue was addressed by developing TERS probes with a unique multi-layer metal coating, which allowed sub-30 nm resolution chemical imaging in a liquid environment using TERS for the first time. To monitor catalytic reactions effectively, TERS probes also need to be chemically inert. For this, we developed a unique method of protecting TERS probes with an ultrathin (1 - 5 nm) zirconia coating, thereby drastically improving their chemical inertness as well as structural stability within a liquid environment. Using these probes, we demonstrated spatially-resolved mapping of a photocatalytic reaction over a heterogeneous catalyst substrate within a liquid environment for the first time. Finally, we introduced tip-enhanced fluorescence (TEFL) microscopy as a new tool for heterogeneous catalysis research. Using selective staining of Brønsted acidity in single fluid cracking catalyst (FCC) particles we performed high‑resolution TEFL mapping of different regions, which revealed a hierarchical distribution of Brønsted acidity within individual zeolite domains. Comparison of TEFL measurements from different FCC particles showed significant intra- and inter-particle heterogeneities in zeolite domain size and activity. With a dramatic improvement in the lifetime and structural and chemical stability of Ag coated probes, this research has brought TERS a step closer to becoming a routine non-destructive and label‑free analytical tool for nanoscale molecular characterisation of almost any catalyst surface and within any environment. Furthermore, TEFL microscopy has been demonstrated to be a powerful and sensitive tool for nanoscale characterisation of real-life catalyst particles with direct industrial relevance.