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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.

The perovskite- and K2NiF4-structure group 5 oxynitrides SrTaO2N, BaTaO2N, Sr2TaO3N, and Ba2TaO3N, previously prepared by reacting oxide precursors with ammonia, may also be prepared by the reaction between the appropriate alkaline earth oxide and TaON at 1500°C under 1 atm of pure nitrogen gas for a few hours. The nitrogen atmosphere is essential to prevent reduction of TaV, and the method of inductive heating ensures that the nitrogen atmosphere above the sample is maintained free of oxygen or water. SrTaO2N crystallizes in I4/mcm with a = 5.694 11(7) Å, b = 8.0658(2) Å, and Z = 4. The oxide and nitride anions are almost completely disordered, as observed when the material is made by nitridation of a pure oxide precursor using ammonia at 900-1000°C, but contrasting with the full O/N order observed when such nitridation is mineralizer assisted. The detailed structure of Ba2TaO3N is reported for the first time: this compound adopts the K2NiF4 type structure with full O/N order in space group I4/mmm with a = 4.115 08(3) Å, c = 13.3778(1) Å, and Z = 2 and is isostructural with the strontium analogue (Sr2TaO3N: I4/mmm, a = 4.038 99(4) Å, c = 12.6007(3) Å, and Z = 2). O/N disorder is favored in perovskite-structure oxynitrides, but in layered K2NiF4-structure materials O/N ordering is the norm, with the ordering scheme dictated by the competition between cations of differing electronegativity for the anions.

The perovskite- and K2NiF4-structure group 5 oxynitrides SrTaO2N, BaTaO2N, Sr2TaO3N, and Ba2TaO3N, previously prepared by reacting oxide precursors with ammonia, may also be prepared by the reaction between the appropriate alkaline earth oxide and TaON at 1500 °C under 1 atm of pure nitrogen gas for a few hours. The nitrogen atmosphere is essential to prevent reduction of TaV, and the method of inductive heating ensures that the nitrogen atmosphere above the sample is maintained free of oxygen or water. SrTaO2N crystallizes in I4/mcm with a = 5.694 11(7) A, b = 8.0658(2) A, and Z = 4. The oxide and nitride anions are almost completely disordered, as observed when the material is made by nitridation of a pure oxide precursor using ammonia at 900−1000 °C, but contrasting with the full O/N order observed when such nitridation is mineralizer assisted. The detailed structure of Ba2TaO3N is reported for the first time: this compound adopts the K2NiF4 type structure with full O/N order in space group I4/mmm wit...