Real-time in-situ monitoring of photocatalytic reactions by ATR infrared spectroscopy

The work described in this PhD thesis describes efforts to develop a real-time in-situ infrared spectroscopy technique with the aim of better understanding the chemistry behind the photocatalytic decomposition of methylene blue dye in water. Methylene blue is a commonly used proxy for several types kinds of toxic organic chemicals that can make their way into water systems. In the presence of the semiconductor photocatalyst titanium dioxide, and under ultraviolet illumination, methylene blue breaks down into simpler chemical components, including water and carbon dioxide. However, the reaction pathway is not well understood. Infrared spectroscopy provides a diagnostic tool to study the chemical bonding of molecules, but implementing this in liquid systems is challenging because of their high infrared opacity. Attenuated total internal reflection (ATR) offers a way to shrink the effective path length to allow the infrared light to traverse the liquid, and provide enough signal to make a spectroscopic measurement. An additional challenge is to find an infrared material for ATR crystals that is chemically stable in an aqueous environment. During my PhD, several potential ATR materials were studied and the best compromise solution was found to be single-crystal zinc sulphide. An ATR ow-cell based on zinc sulphide was constructed in a multi-bounce geometry. This ATR flowcell was used to study the photocatalytic decomposition of methylene blue under ultraviolet light in the presence of titanium dioxide. A bespoke peak fitting algorithm was designed to de-convolve the peaks in the infrared spectrum during the reaction, and the changes were tracked over time. The results are compared with a simultaneous measurement of the visible absorption of methylene blue