Multiscale modelling from quantum level to reactor scale: An example of ethylene epoxidation on silver catalysts
Artikel i vetenskaplig tidskrift, 2019
Ethylene epoxidation is one of the most important selective chemical oxidations in industry. For a controlled transformation of ethylene (ethene) into epoxide, silver is the only commercially suitable catalyst. Although it is usually doped, even with pristine silver activity, selectivity, and stability vary strongly with facets. In this work, we use this reaction on Ag(111) and Ag(100) as a classical formation model to demonstrate the capabilities of physical multiscale modelling, to show why Ag(100) nanocubes offer superior catalysis, and to optimise reactivity. First, we describe the elementary reactions on pristine surfaces with the quantum chemistry calculations, using density functional theory (DFT). The free energies of all intermediates, kinetic rates from the transition state theory and adsorption/desorption equilibria are calculated from first principles. These results are applied to kinetic Monte Carlo (kMC) simulations, where the spatio-temporal evolution of the system on a meso-scale can be followed. The differences in activity, concentration, selectivity, and apparent activation energy are observed, investigated, and analysed. Lastly, mean-field concepts – micro-kinetics and computational fluid dynamics (CFD) – are used to simulate how the synthesis proceeds in a reactor. Mechanism, catalytic coverage and the effects of pressure, temperature, and particle composition, size and shape on the performance are evaluated. We show that multiscale modelling is a powerful instrumental approach for real unit engineering, while the level of detail required is dictated by the purpose of a representation and available resources.