Thermal effects on gas transport in catalytic membranes

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Diffusion effects in the continuum region are well described by the Navier-Stokes equations, which are based on measurable gas properties. In meso- and micropores, however, wall effects control gas flows and thus often catalytic reactions. Under dilute conditions, i.e. in small pores, additional thermal mass transport effects also come into play, such as thermal transpiration, Soret diffusion, and Dufour conduction, which could be summarized as additional Knudsen Pump Effect (KPE). We hypothesize that KPE, which increases with temperature differences, may have a significant impact on heterogeneous catalytic reactions. For catalytic porous materials, such temperature differences are determined by several parameters, namely the heat of reaction, the thermal conductivity of the solid, and the geometric dimensions. However, in reactor modeling, KPE has been mostly ignored or, in rare cases, mentioned but then neglected. Surprisingly, as far as we know, there is no experimental study showing parameter screening or quantification of KPE for catalytic reactions. On the other hand, this temperature driven pumping effect starts to be used in various applications as gas chromatography, gas separation and other microfluidic applications to pump a gas with a pump without moving parts. In this project we want to investigate whether mass transfer limitations in catalytic reactors can be reduced by additional temperature gradient driven mass flow (KPE), e.g., inside catalytic membranes. For a temperature gradient of 80 K/cm the effects of KPE on mass transport in a mesoporous system were shown to be in the same order of magnitude as those of the effective diffusion coefficients of species at Knudsen number (which is the ratio of the molecular mean free path to the characteristic pore size) around 0.2.