Laser-induced fluorescence measurement of oxygen atoms above a catalytic combustor surface

Investigation of Parameters Influencing the Activation of a Pd/γ-Alumina Catalyst during Methane Combustion

The progressive activation of a Pd(2 wt %)/gamma-alumina catalyst under the reaction conditions of catalytic combustion of methane (CCM) was studied. The reasons of this activation were investigated by XPS, CO-chemisorption, and HR-TEM. The removal of carbon from the surface cannot explain the observed activation process. Sintering of the palladium particles was detected but this parameter alone does not fully explain the activation process of the catalyst. HR-TEM imaging evidences (i) that PdO is present both in the fresh and the active catalyst and (ii) that the PdO nanoparticles sinter and restructure (surface roughening) during the reaction. Development of preferential faces was not observed. It is suggested that this restructuring may be responsible for the activation process by facilitating the formation of an active oxygen layer on the PdO surface. CCM on Pd/gamma-Al(2)O(3) depends on the thermal history of the catalyst and is a structure-sensitive reaction.

Oxygen-atom concentrations measured in flames: a method to improve the accuracy of laser-induced fluorescence diagnostics

A procedure is proposed, denoted as the corrected laser-induced fluorescence (LIF) method, that reduces the error associated with the unavoidable photodissociation of O(2) molecules that has limited the measurement of oxygen-atom concentrations in the past. Two different laser intensities are employed, and the two signals that are obtained with two-photon LIF diagnostics are used to correct for the photolysis error. We measured oxygen-atom concentrations using this method at 33 locations in lean and rich flames. Results are compared with values determined by use of two independent techniques: the partial equilibrium method and equilibrium calculations. The measurements also quantify the shot noise, the photolysis errors, and the critical laser intensity required to avoid photolysis errors.

Avoidance of the photochemical production of oxygen atoms in one-dimensional, two-photon laser-induced fluorescence imaging

We demonstrate that one-dimensional, two-photon laser-induced fluorescence imaging of oxygen atoms in flames can be achieved with reasonable signal intensities and spatial resolution but without the interference caused by the photolysis of O(2), by the use of a beam telescope instead of a focusing lens. The increase in probed volume (and concomitantly the number of excitable atoms) caused by the larger beam diameter in the telescope experiments offsets, to a certain extent, the loss in signal that is due to the reduced laser fluence. Further, the results show that the "correct" dependence of the fluorescence intensity on laser fluence (in this case quadratic) does not guarantee the absence of photochemistry.