A Planar Laser-induced Fluorescence Study of NO + CO oxidation at Pd(100)

Abstract: Catalysis is an essential tool in industry, and the developments in catalysis research during the last decades have led to higher efficiency in many important processes. CO oxidation is a common reaction to use in catalysis research both due to its applications in industry but also since it is a simple reaction where the knowledge gained about its reaction mechanism can be applied to important but more complicated reactions. There is an active debate within the catalysis field regarding the phase where late transition metals, such as Pd, are active. Thin layer surface oxides are by some considered inactive and by others equally or more active than the metallic surface. Lorentzi et al. performed a theoretical study of simultaneous CO and NO oxidation. The predictions of the study were that the NO under some gas conditions prevents the formation of surface oxide, and thus, under the assumption that the oxide is inactive, improves the activity both through this removal and by other synergistic effects. This thesis tries to experimentally determine the effects of NO in CO oxidation at a single crystal Pd(100) surface. This is further connected to the activity of the sample. Surface optical reflectance and planar laser-induced fluorescence are used to get comparable measurements of surface reflection intensity and gas composition in the vicinity of the surface for reactions with and without small amounts of NO for two different O2:CO ratios. The result was that the surface optical reflectance measurements were approximately constant during the entire reaction process when NO was present in the gas mixture, implying the lack of oxide formation. However, the CO oxidation activity, shown by the planar laser-induced fluorescence measurements, was limited by the presence of NO. The activity had a more significant decrease at higher O2:CO ratios. The limitation can be concluded to be caused by limitations in surface access for the CO caused by NO at the surface.