Modelling the CO2 hydrogenation reaction over Co, Ni and Ru/Al2O3

The CO2 hydrogenation reaction was experimentally investigated over pristine Co, Ni and an Al2O3 supported Ru catalyst with 0.5 wt% Ru loading. We developed a reaction model which takes the kinetical, diffusional -and thermodynamic reaction regimes into account and enables the description of the reaction over a broad temperature range. The model is based on a fractional rate law with experimentally determined reaction orders. We found that the overall reaction orders on the different catalysts are in proximity to zero order (0.13 for Co, 0.14 for Ni and 0.38 for Ru/Al2O3), which leads to the interpretation that the reaction is limited by the available surface sites in the underlying reaction conditions. We demonstrate on the example of Ru/Al2O3 that the reaction rate strongly depends on the partial pressure of CO2 in the gas phase. Upon reducing the partial pressure of CO2 in the reaction gas stream via He dilution, the reaction approaches a higher reaction order. Furthermore, the supported Ru catalyst is less limited by pore-diffusion compared to pristine Co. With the derived model we can accurately calculate the CO2 conversion over a broad temperature range; the temperature of maximum conversion is predicted within 15 K and the deviation between simulation and experiment is mostly less than 20%. This enables the simple and rapid prediction of the influence of different reaction parameters such as the activation energy or the diffusional limitations on the CO2 conversion. (C) 2019 Elsevier Inc. All rights reserved.