Operando Gas Imaging in Heterogeneous Catalysis

This thesis deals with the development and application of laser-based techniques, for gas phase
characterization/gas imaging in heterogeneous catalysis. Two techniques are presented: planar laser-induced fluorescence (PLIF) and degenerate four wave mixing (DFWM).
This thesis is mainly focused on PLIF for the visualization of CO₂ and CO in the vicinity of a catalyst during CO oxidation reaction at semi-realistic conditions, i.e. up to 300 mbar and 400 ^oC. The catalytic CO oxidation on a Pd(100) single crystal model catalyst is used as a showcase throughout this thesis. With its high temporal and spatial resolution, PLIF can deliver 2D spatially resolved gas phase measurements on a sub-second time scale, which allows to follow the dynamic changes in the gas phase above a sample during catalytic processes. The 2D spatially resolved gas phase data facilitates revealing the gas-surface correlation information which is otherwise hidden or difficult to obtain when conventional techniques, e.g. mass spectroscopy (MS), are used. As the gas flow and the reactor geometries have a strong effect on the gas phase above a highly active catalyst, the visualization of CO2 distribution by PLIF has been used for flow characterization and design of catalytic reactors
for studying model catalysts. Furthermore, by combining PLIF and surface sensitive techniques, surface optical reflectance (SOR) and high-energy surface X-ray diffraction (HESXRD), the gas phase and surface can be studied simultaneously. In this way, we can obtain new insights into the spatial and temporal correlations between the gas phase and surface change in operando studies by taking advantages of the high spatial and temporal resolution of PLIF, respectively.
For the other gas detection technique, DFWM, only a brief introduction with an example of the detection of CH₃OH is given, aiming at showing its potential application in catalysis.