The Kinetics of the Palladium-Catalyzed Vapor-Phase Thermal Decomposition of Ethanol

The rate of thermal decomposition (190−231 °C) of ethanol on Pd/Al₂O₃ to yield CH₄, CO, and H₂ has been studied in a tubular reactor at differential conversion and in a Berty reactor. Ethanal is a byproduct that is also present as an intermediate species formed in a dehydrogenation reaction. The presence of H₂ is necessary in the decomposition of ethanal to suppress coking, and under these conditions, decarbonylation is faster than dehydrogenation of ethanol. The differential rate data (206 °C) were fitted to a large number of alternative LHHW kinetic models. These effectively describe the relative strength of adsorption (C₂H₅OH ≈ CH₃CHO > CO > H₂ > CH₄), whereas discrimination between different kinetic pathways in the surface reaction is difficult. Nevertheless, models that describe the adsorption of ethanol or its surface dehydrogenation perform well. Desorption of H₂ is not rate-limiting. In the modeling, the goodness of the fit is dependent on the number of kinetic constants, and they can be increased by elaboration of the mechanistic steps.