Surface temperature excess in heterogeneous catalysis

Abstract

In this dissertation we study the surface temperature excess in heterogeneous catalysis. For heterogeneous reactions, such as gas-solid catalytic reactions, the reactions take place at the interfaces between the two phases: the gas and the solid catalyst. Large amount of reaction heats are released at the interface for strong exothermic reactions, which makes it possible that the catalyst surface temperatures are higher than those in the gas phases due to poor thermal conductivities of gases. These are very often the case for transport limited reactions based on the literature and the temperature differences can be a few hundred degrees. It is often found in kinetic studies that Arrhenius plots are gradually curved when measured catalyst temperatures or gas temperatures are applied. It seems that the activation energy changes gradually with increasing temperature. However, according to Arrhenius, the activation energies should be constant. Therefore, there is a possibility that the real reaction temperature in a 2-D reacting surface differs from the measured catalyst temperature or gas temperature and that the real reaction temperature Tr should be used in the Arrhenius plot. Using irreversible thermodynamics three distinct temperatures, gas temperature, solid catalyst surface temperature and reaction temperature in a 2-dimensional Gibbs surface, are modelled for two oxidation reactions. They are one transport limited reaction: hydrogen oxidation, and one kinetically controlled reactions: CO oxidation, respectively