Hyperpolarised xenon MRI and time-resolved X-ray computed tomography studies of structure-transport relationships in hierarchical porous media

Catalysed diesel particulate filter (DPF) monoliths are hierarchical porous solids, as demonstrated by mercury porosimetry. Establishing structure-transport relationships, including assessing the general accessibility of the catalyst, is challenging, and, thus, a comprehensive approach is necessary. Contributions, from each porosity level, to transport have been established using hyperpolarised (hp) xenon-129 magnetic resonance imaging (MRI) of gas dispersion within DPF monoliths at variable water saturation, since X-ray Computerised-Tomography, and ¹H and ²H NMR methods, have shown that porosity levels dry out progressively. At high saturation, hp ¹²⁹Xe MRI showed gas transport between the channels of the monolith is predominantly taking place at channel wall intersections with high macroporosity. The walls themselves make a relatively small contribution to through transport due to the distribution of the micro-/meso-porous washcoat layer away from intersections. Only at low saturation, when the smallest pores are opened, do hp ¹²⁹Xe MR images became strongly affected by relaxation. This observation indicates accessibility of paramagnetic (catalytic) centres for gases arises only once the smallest pores are open.