Planar Laser Induced Fluorescence Applied to Catalysis

Abstract

In this chapter we describe Planar Laser Induced Fluorescence (PLIF) to investigate the reactants or products in the vicinity of a catalyst at semi-realistic conditions. PLIF provides a 2D view of the gas-phase distribution of a pre-chosen gas. Here we present PLIF results from CO$_2$ and CO from the oxidation of CO into CO$_2$ by Pd single crystals and by various powder catalysts as well as from NH$_3$ from the oxidation of NH$_3$ above a Ag/Al$_2$O$_3$ powder catalyst. We describe our experimental set-up in detail, and the laser instrumentation needed to enable detectable gas fluorescence from CO$_2$, CO, and NH$_3$, respectively. Further, intensity corrections of the PLIF signal due to scattering and temperature effects are described. In the case of the CO oxidation, the results directly show the creation of a CO$_2$ boundary layer and thus a drastic change of the gas-phase composition close to the catalyst surface, illustrating the effect of gas diffusion and reaction speed, which in turn should affect the surface structure of the active catalyst. The 2D character of the PLIF images is used to investigate differences in catalyst activity by studying adjacent catalysts in the reaction cell during the reaction, and a solution to avoid spill-over effects between catalysts in the same reactor is presented. The results from PLIF images of CO of the same reaction show the corresponding depletion of the PLIF intensity above the catalyst, in accordance with observations from other techniques confirming the drastic difference between the gas composition close to the catalyst and at the inlet or outlet of the reactor. Finally we present NH$_3$ PLIF results from above a Ag/Al$_2$O$_3$ powder catalyst while the NH$_3$ is being oxidized in an oxidizing environment with the assistance of H$_2$.