Thermal effects on gas transport in catalytic membranes

在连续区域中的扩散效应由Navier-Stokes方程很好地描述，该方程基于可测量的气体性质。然而，在介孔和微孔中，壁效应控制气体流动，因此通常控制催化反应。在稀释条件下，即在小孔隙中，额外的热质量传输效应也发挥作用，例如热蒸腾、Soret扩散和Dufour传导，其可以被概括为额外的Knudsen泵效应（KPE）。我们推测，KPE，这增加了温度差异，可能有显着的影响，多相催化反应。对于催化多孔材料，这样的温差由几个参数决定，即反应热、固体的热导率和几何尺寸。然而，在反应堆建模中，KPE大多被忽略，或者在极少数情况下，被提及但随后被忽略。令人惊讶的是，据我们所知，没有实验研究显示用于催化反应的KPE的参数筛选或定量。另一方面，这种温度驱动的泵送效应开始用于各种应用中，如气相色谱、气体分离和其他微流体应用，以在没有移动部件的情况下用泵泵送气体。在这个项目中，我们想研究是否可以通过额外的温度梯度驱动的质量流（KPE）来减少催化反应器中的传质限制，例如，在催化膜内。对于80 K/cm的温度梯度，KPE对中孔系统中的质量传输的影响被证明是在相同的数量级的有效扩散系数的物种在克努森数（这是分子平均自由程的比率的特征孔径）约0.2。

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.