Surface-access limitation in catalytic porous monoliths: Performance diagnosis using pore-resolved CFD
Porous monoliths are promising catalyst supports due to their high surface area, interconnected channels, thermal stability and mechanical robustness. However, their tunable topology complicates design: trade-offs between conversion and pressure drop are not reliably captured by macroscopic descriptors, such as porosity, specific surface area, or tortuosity. Pore-resolved computational fluid dynamics~(PRCFD) addresses this gap by resolving pore-scale flow and transport, enabling diagnostics and discrimination between macroscopically similar structures. We investigate surface-access-boundedness: a case where conversion is limited by flow maldistribution and incomplete utilisation of the catalytic surface, even at low Damköhler numbers (Da<1). Using palladium-nanoparticle-coated silicone monoliths for p-nitrophenol reduction, we perform reactive PRCFD in microcomputed-tomography-based geometries, calibrate a pseudo-heterogeneous eggshell reaction model, and validate transferability across samples and flow rates. We then diagnose surface-access-boundedness via the limited influence of diffusivity and reaction kinetics on conversion. Furthermore, we compare synthesised random monoliths with triply periodic minimal surface structures under matched porosity and surface area. Significantly, the required pumping power can decrease by up to an order of magnitude for the same molar production rate, depending on topology. These results show that, in heterogeneous systems affected by surface-access limitations, reactor performance is governed by structure-dependent surface accessibility rather than intrinsic kinetics or molecular diffusion alone, and that validated reactive PRCFD provides a practical framework to diagnose and compare porous reactor geometries under realistic operating conditions.