Braglia L, Borfecchia E, Martini A, Bugaev A L, Soldatov A V, Øien-Ødegaard S, Lønstad-Bleken B T, Olsbye U, Lillerud K P, Lomachenko K A, Agostini G, Manzoli M, Lamberti C
Department of Chemistry, NIS Interdepartmental Centre and INSRM Reference Centre, University of Turin, via Quarello 15A, I-10135 Turin, Italy.
IRC "Smart Materials", Southern Federal University, Zorge Street 5, 344090 Rostov-on-Don, Russia.
Phys Chem Chem Phys. 2017 Oct 18;19(40):27489-27507. doi: 10.1039/c7cp05185a.
An X-ray absorption spectroscopy study of the UiO-67 Pt functionalized metal organic frameworks (MOFs) demonstrates that under appropriate conditions, at least two types of catalytically active sites can be formed in the cavities of the MOF: isolated Pt-complexes and Pt nanoparticles (Pt-NPs). Both pre-made linker synthesis (PMLS) and post-synthesis functionalization (PSF) methods were adopted. XAS was used to monitor the temperature-dependent behaviour of UiO-67-Pt while heating from RT to 623 K, in different gas feeds (pure He, 3% H/He and 10% H/He). We collected static in situ Pt L XANES and EXAFS spectra at room temperature (RT) before and after the thermal treatment, as well as spectra acquired under operando conditions upon heating. Under 10% H/He thermal treatment, we unambiguously detected Pt-NP formation which has been followed by a parametric EXAFS analysis of the data collected during temperature programmed H-reduction (TPR), using the Einstein model to predict the temperature dependence of the Debye-Waller factors. Conversely, in pure He flow, the only significant change observed during TPR is the progressive decrease of the Pt-Cl single scattering contribution, leading to the conclusion that the Pt grafted to the bpydc-linkers remains naked. Advanced EXAFS/TEM analysis allowed us to quantify the fraction of Pt in the form of Pt-NPs, values that have been quantitatively confirmed by linear combination analysis of the XANES spectra. In situ XANES/EXAFS study was supported by ex situ XRPD and BET analyses, confirming the framework stability and testifying a loss of the internal volume after TPR due to the formation of Pt-NPs insides the MOF pores, more relevant in the sample where smaller Pt-NPs were formed.
对UiO - 67铂功能化金属有机框架（MOF）的X射线吸收光谱研究表明，在适当条件下，MOF的空腔中可形成至少两种类型的催化活性位点：孤立的铂配合物和铂纳米颗粒（Pt - NPs）。采用了预制连接体合成（PMLS）和后合成功能化（PSF）两种方法。利用X射线吸收光谱（XAS）监测在不同气体进料（纯氦气、3%氢气/氦气和10%氢气/氦气）中，UiO - 67 - Pt从室温（RT）加热到623 K时随温度变化的行为。我们在热处理前后的室温（RT）下收集了静态原位铂L边X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）光谱，以及加热过程中在操作条件下获取的光谱。在10%氢气/氦气热处理下，我们明确检测到了Pt - NP的形成，随后使用爱因斯坦模型预测德拜 - 瓦勒因子的温度依赖性，对程序升温氢气还原（TPR）过程中收集的数据进行了参数EXAFS分析。相反，在纯氦气流中，TPR过程中观察到的唯一显著变化是Pt - Cl单散射贡献的逐渐降低，得出的结论是接枝到bpydc - 连接体上的铂保持裸露状态。先进的EXAFS/透射电子显微镜（TEM）分析使我们能够量化以Pt - NPs形式存在的铂的比例，这些值已通过XANES光谱的线性组合分析得到定量确认。原位XANES/EXAFS研究得到了非原位X射线粉末衍射（XRPD）和比表面积与孔径分布（BET）分析的支持，证实了框架的稳定性，并证明TPR后由于MOF孔内形成Pt - NPs导致内部体积损失，在形成较小Pt - NPs的样品中更为明显。
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