Ultraviolet-Visible (UV-Vis) Spectroscopy

Ultraviolet-Visible (UV-Vis) spectroscopy is a versatile and powerful analytical method, which allows to investigate a wide variety of catalysts in both the liquid-phase and solid-state and their interfaces at elevated temperatures and pressures. In the case of solid catalysts, they can be studied in the form of powders (e.g., in diffuse reflectance mode) and as thin wafers (in transmission mode), and when combined with a microscope even in the form of catalyst bodies (e.g., extrudates) and single crystals. In the past two decades, UV-Vis spectroscopy has been increasingly used under in situ and operando conditions to shed light on/gain insight in the working principles of heterogeneous catalysts, homogeneous catalysts, electrocatalysts, as well as photocatalysts. One of the advantages of this method is that it can simultaneously measure, e.g., the electronic transitions of organic molecules (mainly via their n → π* and π → π* transitions) and transition metal oxides or ions (via their d-d and charge transfer transitions). Unfortunately, absorption bands in the UV-Vis range are often broad and overlapping and hence their interpretations are not always trivial. Advanced theoretical calculations are required to provide a proper foundation of their interpretation, while, e.g., chemometrics can help prevent biased analysis when many (time-resolved) spectra are collected. Finally, UV-Vis spectroscopy is often combined with other analytical methods to provide complementary information. Examples include X-ray absorption spectroscopy and diffraction, next to vibrational spectroscopy (i.e., infrared and Raman) and magnetic resonance (i.e., electron spin resonance and nuclear magnetic resonance) methods. The above-described scientific and instrumental developments will be illustrated by using a selection of showcase examples, covering the different areas of catalysis. The chapter concludes with some main observations as well as some future developments on what might become possible in the not-too-distant future.