Characterization of catalytic reactions on surfaces from three-dimensional partial pressure distributions
Abstract: A chemical reaction taking place on a surface exposed to a viscous gas will give rise to a nonhomogeneous gas composition and temperature in the vicinity of the surface. This is due to the limited rate at which species and heat can be transported in the gas mixture by convection and diffusion. The phenomenon has important consequences both for heterogeneous catalysis and in the field of catalytically active gas sensors.In heterogeneous catalysis a situation where the reaction rate is determined by the rate at which species and heat can be transported in the gas phase, rather than by the processes on the catalytic surface, is referred to as a mass transfer limited chemical reaction. For fast catalytic reactions the partial pressures of reactants and products may vary more than a factor of ten over a few mm close to the catalyst surface. The temperature and gas composition can also be expected to vary along the catalytic surface. In order to measure the reaction rate as a function of the gas composition and temperature at the surface, it is thus necessary to do the measurements very close to the catalyst surface. This may be difficult in practise.An alternative approach is to utilize the partial pressure distributions over a catalytic surface to draw conclusions about the reaction rate as a function of the gas composition at the catalytic surface. A requirement for using this approach is that the transport processes in the gas phase can be modelled well.If the catalytic surface is part of a chemical sensor and if the molecule to be detected, the analyte,is consumed in a chemical reaction on the sensor surface, the lowering of the analyte concentration in the vicinity of the sensor will give rise to an "error" in the sensor signal. The spatial variation in gas composition in the vicinity of a catalytic gas sensor can also be used to an advantage, since it will increase the variation in the sensor response to gas mixtures. This has been utilized in the concept of response maps where the response can be measured locally on a catalytic sensing surface. Another possibility is to use separate catalysts to modify the gas mixture before it is analysed with a number of discrete chemical sensors. In order to be able to design such so-called distributed chemical sensing systems, it is necessary to have a good understanding of both the processes in the gas phase surrounding the sensor and of the chemical reactions taking place on the sensor surface.The scope of this work has been to increase the understanding of the chemical reactions occurring on the catalytic surfaces of gas sensors and to quantify the influence of the transport processes in the gas phase on the sensor response. Thus, a model for the transport of analyte in the gas phase over a catalytic surface has been de1ived and an equipment has been designed and built which is able to measure temperature and partial pressure distributions in a laminar gas flow over a flat catalytic surface with a resolution better than 0.5 mm. The measurements made with the equipment verified the model for analyte transport in the gas phase.Since it turned out to be possible to model the transport of hydrogen in the gas phase accurately, measured hydrogen pressure distributions could be used to determine the water formation rate as a function of the hydrogen pressure at the catalytic surface for palladium and platinum under atmospheric conditions. Such studies are important for the understanding of the response to hydrogen in presence of oxygen obtained from catalytic metal-insulator-semiconductor hydrogen sensors.In order to further investigate the response mechanism of palladium-insulator-semiconductor hydrogen sensors the amount of hydrogen present on a palladium surface during water formation was studied in permeation experiments with palladium membranes. The results of these studies indicate that the presently used model for the hydrogen response of the sensors has to be modified.A major conclusion of this work is that the response of a chemical sensor is, in general, not only a function of the composition of the gas fed to the sensor system, but is also influenced by the geometry of the gas volume surrounding the chemical sensor, the size and shape of the catalytic element of the sensor and the gas velocity at the sensor surface.The equipment for spatially resolved partial pressure measurements can be used for determination of reaction rates as a function of surface partial pressures for very general gas-surface reactions. Such data would be useful in order to increase the understanding of the processes occuring on the surface of real catalysts at high pressures. Data on reaction rates can also be used together with the model for the transport of analyte in the gas phase to design distributed sensing systems and to predict the "e1rnr" in response for discrete gas sensors.
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