Palladium/Beta zeolite passive NOx adsorbers (PNA): Clarification of PNA chemistry and the effects of CO and zeolite crystallite size on PNA performance
Palladium/Beta zeolite passive NOx adsorbers (PNA): Clarification of PNA chemistry and the effects of CO and zeolite crystallite size on PNA performance
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DOI：
10.1016/j.apcata.2018.10.021
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发表时间：
2019-01-05
影响因子：
5.5
通讯作者：
Szanyi, Janos
中科院分区：
文献类型：
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作者：
Khivantsev, Konstantin;Jaegers, Nicholas R.;Szanyi, Janos
模型0.3wt%和lwt % Pd/β用纳米尺寸的β晶体（平均尺寸< 50 nm）和无缺陷的大的微米尺寸的晶体合成，具有类似于13和15的类似Si/Al比。借助FTIR，我们揭示了在与PNA相关的条件下，NO吸附在这些材料上时形成至少两个Pd（II）-NO和一个Pd（I）-NO络合物。在较高的NO压力下，在室温下，我们检测到的第一个钯（II）二亚硝酰基Pd（NO）（2）配合物的形成，没有观察到以前的固体载体或在有机金属化合物。O-2在干燥的流促进NO存储通过额外的形成亚硝酰基（NO+）离子，它也导致稳定的显着量的N2 O3在多孔。在水的存在下，NO储存能力显著降低。然而，在存在CO（其总是存在于废气中）的情况下，性能提高。借助于光谱和PNA测量，我们表明，在CO的存在下，稳定的混合钯（II）羰基亚硝酰基配合物[Pd（II）（CO）（NO）]形成，负责增强NOx存储。这指出了一个一般的结论，即在水和CO的存在下，在各种沸石上负责NOx储存的物质实际上是Pd（II）（CO）（NO）Jd负载量降低至0.3wt%改善了Pd分散和NO/Pd储存比。我们还表明，不同的晶体尺寸的NOx的释放温度，以及NO/Pd比的显着变化。更大、无缺陷、更疏水的纳米晶体储存更多的NOx，并在更高的温度下释放。此外，更大的、无缺陷的和更疏水的纳米晶体在水热老化后对PNA保持显著更高的活性，与有缺陷的纳米晶体相比失去很少的活性。高场Al-27 NMR结果表明，较大的疏水性的PNA晶体在水热老化（HTA）时更耐脱铝，因此对于用Pd存储PNA更有吸引力。
Model 0.3 wt% and 1 wt% Pd/Beta was synthesized with nano-sized Beta crystals (average size < 50 nm) and defect-free large micrometer sized crystals with similar Si/Al ratios similar to 13 and 15. With the aid of FTIR we reveal the formation of at least two Pd(II)-NO and one Pd(I)-NO complexes upon NO adsorption on those materials under conditions relevant to PNA. Under higher NO pressures at room temperature, we detect the formation of the first palladium(II) dinitrosyl Pd(NO)(2) complex, not observed previously on the solid supports or in organometallic compounds.O-2 in dry streams promotes NO storage via additional formation of nitrosyl (NO+) ions; it also leads to stabilization of significant amounts of N2O3 in BEA pores. In the presence of water, NO storage capacity is dramatically lowered. However, in the presence of CO (which is always present in exhaust gas) the performance improves. With the aid of spectroscopy and PNA measurements we show that in the presence of CO, stable mixed palladium(II) carbonyl nitrosyl complex [Pd(II)(CO)(NO)] is formed and responsible for enhanced NOx storage. This points to a general conclusion that the species responsible for NOx storage on various zeolites in the presence of water and CO, are in fact, Pd(II)(CO)(NO).Decreasing the Pd loading to 0.3 wt% improves Pd dispersion and NO/Pd storage ratio. We also demonstrate that varying crystal sizes of BEA leads to significant changes in the NOx release temperature as well as NO/Pd ratio. Larger, defect free, more hydrophobic BEA crystals store more NOx and release it at higher temperature. Furthermore, larger, defect free and more hydrophobic BEA crystals stay significantly more active for PNA after hydrothermal aging, losing little activity compared with defective nanocrystals. High-field Al-27 NMR results show that larger, hydrophobic BEA crystals are more resistant to de-alumination upon hydrothermal aging (HTA), and thus are more attractive for PNA storage with Pd.