Title : From electrons to reactors: AB initio multi-scale simulation of methane oxidation over palladium oxide
Abstract:
Heterogeneous catalysis, in practical applications, is an inherently multi-scale phenomenon. Multi-scale modelling aims to tackle the “grand challenge” of simulating catalytic processes by bridging atomic, molecular, pore, and reactor scales. However, it brings substantial challenges due to the complexity and diversity of phenomena involved, combined by the difficulties in aligning differences in temporal and spatial scales and handling the computational demands across each model layer. Within the ReaxPro H2020 project, Johnson Matthey developed a multiscale workflow for catalytic methane combustion over palladium oxide. This work describes the construction, application, and outcomes of a fully ab initio multiscale simulation of catalytic methane oxidation over palladium oxide and doped catalysts within a structured (coated monolith) reactor. The simulation workflow spans the electronic and atomic, up to reactor scales, integrating multiple simulation techniques and software. Electronic structure simulations were carried out using the GPAW[1] Density Functional Theory (DFT) package, with a plan-wave basis set and the BEEF-vdW functional. Atomic scale optimisation, transition state searching, and vibrational analysis applied routines in the Atomic Simulation Environment (ASE)[2]. The image-dependent pair potential method was applied for interpolation of intermediate images as input to climbing image nudged elastic band transition state searching algorithm. Vibrational analysis used a finite displacement method and the resulting modes derived by the method of Frederiksen[3]. Custom Python scripts were used to develop a microkinetic model based on DFT-derived data was processed, providing reaction rates for each elementary step of the methane oxidation reaction mechanism. Effective diffusivity for gas transport within the washcoat porous media was estimated from 3D porous structures obtained via FIB-SEM and digitally reconstructed. To build a continuum model of the monolith reactor, the kinetic model was coupled with heat and mass transport equations. This approach included both idealised plug-flow models and detailed reactive CFD simulations (using CatalyticFOAM[4]), to describe gas transport and reaction in the monolith under varying operating conditions. The developed models were validated against experimental data collected through different reactive characterisation techniques. Model predictions demonstrated reasonable agreement with experimental results without any experimental fitting of parameters, highlighting the workflow's potential for tackling the complexities inherent to industrial catalytic processes.
