Kinetic Analysis of CO2 Hydrogenation to Long-Chain Hydrocarbons on a Supported Iron Catalyst

Hydrogenation of CO2 to long-chain hydrocarbons via combined reverse water gas shift (RWGS) and Fischer-Tropsch (FT) gained much attention in the last years as a way to produce sustainable hydrocarbons for the chemical industry or fuel applications. Despite the large amount of interest in the reaction, so far only a few studies have been conducted regarding the kinetics. In this study we carefully investigated the kinetics of an alumina supported iron catalyst at 280-320 degrees C, 10-20 bar, 900-120 000 mLN h-1 g-1, and a H2/CO2 molar inlet ratio of 2-4. Special attention was focused toward the thermodynamic constraints under reaction conditions. Based on elementary reaction steps according to recent mechanistic investigations, we derived new Langmuir- Hinshelwood-Hougen-Watson type kinetic expressions which allow an excellent reproduction of the experimental data and outperform existent models. Possible model combinations were discriminated against each other, and the best fit was obtained for the assumption of H-assisted CO2 and H-assisted CO dissociation mechanisms for RWGS and FT, respectively. Model uncertainties that are introduced by the RWGS being close to equilibrium are discussed in detail and are possibly a reason for strongly varying results for activation energies between different studies. The detrimental effect of water vapor on the reaction progression is analyzed numerically and can be attributed to two parameters: kinetic inhibition via strong adsorption of oxygen containing species and thermodynamic constraints by shifting the equilibrium CO partial pressures to lower values.