Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK.
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK.
Phys Chem Chem Phys. 2018 May 3;20(17):11976-11986. doi: 10.1039/c8cp01022f.
The diffusion of ammonia in the small pore zeolite and potential commercial NH3-SCR catalyst levynite (LEV) was measured and compared with its mobility in the chabazite (CHA) topology (more established in NOx abatement catalysis), using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations at 273, 323 and 373 K. The QENS experiments suggest that mobility in LEV is dominated by jump diffusion through the 8-ring windows between cages (as previously observed in CHA) which takes place at very similar rates in the two zeolites, yielding similar experimental self-diffusion coefficients (Ds). After confirming that the same characteristic motions are observed between the MD simulations and the QENS experiments on the picosecond scale, the simulations suggest that on the nanoscale, the diffusivity is higher by a factor of ∼2 in the CHA framework than in LEV. This difference between zeolites is primarily explained by the CHA cages having six 8-ring windows in the building unit, compared to only three such windows in the LEV cage building unit, thereby doubling the geometric opportunities to perform jump diffusion between cages (as characterised by the QENS experiments) leading to the corresponding increase in the MD calculated Ds. The techniques illustrate the importance of probing both pico- and nanoscale dynamics when studying intracrystalline diffusion in both NH3-SCR catalyst design, and in porous materials generally, where notable consistencies and differences may be found on either scale.
采用准弹性中子散射（QENS）和分子动力学（MD）模拟，在 273、323 和 373 K 下，测量并比较了氨在小孔沸石和潜在商用 NH3-SCR 催化剂莱文石（LEV）中的扩散以及在 CHA 拓扑结构（在 NOx 减排催化中更为成熟）中的迁移率。QENS 实验表明，LEV 中的迁移率主要由笼间 8 元环窗口的跳跃扩散主导（如在 CHA 中所观察到的），在两种沸石中，这种跳跃扩散的速率非常相似，导致实验自扩散系数（Ds）相似。在确认 MD 模拟与 QENS 实验在皮秒尺度上观察到相同的特征运动后，模拟表明，在纳米尺度上，CHA 骨架中的扩散系数比 LEV 中的高约 2 倍。沸石之间的这种差异主要归因于 CHA 笼在构建单元中具有六个 8 元环窗口，而 LEV 笼构建单元中只有三个这样的窗口，从而使笼间跳跃扩散的几何机会增加一倍（如 QENS 实验所表征的），从而导致 MD 计算的 Ds 相应增加。这些技术说明了在研究 NH3-SCR 催化剂设计和一般多孔材料中的晶内扩散时，探测微观和纳米尺度动力学的重要性，在这两个尺度上可能会发现明显的一致性和差异。
