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• 作者：Bettina Jee ; Petko St. Petkov ; Georgi N. Vayssilov ; Thomas Heine ; Martin Hartmann ; Andreas P枚ppl
• 刊名：The Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：April 25, 2013
• 年：2013
• 卷：117
• 期：16
• 页码：8231-8240
• 全文大小：464K
• 年卷期：v.117,no.16(April 25, 2013)
• ISSN：1932-7455

文摘

Cu₃(btc)₂ (btc = 1,3,5-benzenetricarboxylate), also called HKUST-1, is one of the well-known representatives of the metal鈥搊rganic framework (MOF) compounds. It exhibits a large surface area and a high pore volume. Due to the coordinatively unsaturated metal centers as preferential adsorption sites, Cu₃(btc)₂ is particularly interesting for the separation of CO₂ and CO in gaseous mixtures. We studied the interactions of ¹³C-enriched carbon dioxide (¹³CO₂) and carbon monoxide (¹³CO) with the Cu²⁺ centers in the zinc-substituted homologue Cu_2.97Zn_0.03(btc)₂ using continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy (Davies or Mims electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE)). Upon adsorption of ¹³CO₂ and ¹³CO, the coordination geometry of the Cu²⁺ centers changed from square planar to square pyramidal. The cupric ion g-tensor and the ^63/65Cu hyperfine coupling tensor A^Cu show the changes in the ligand field of Cu²⁺. Moreover, the interaction with the ¹³C nuclei of the gas molecules is reflected in the isotropic coupling constant A_iso^C and the dipolar coupling parameter T^C which are derived from the ¹³C hyperfine coupling tensor A^C obtained by the pulsed EPR experiments. From the experimentally obtained parameters, we derived a geometrical model for the adsorption of ¹³CO₂ and ¹³CO at the Cu²⁺ ions that is consistent with our DFT calculations. The ¹³CO molecule is found to coordinate linearly at the Cu²⁺ center via the ¹³C atom and perpendicular to the CuO₄ plane with a Cu鈥揅 distance of r_CuC = 2.57(10) 脜 (DFT, 2.42 脜). The ¹³CO₂ molecule is coordinated slightly tilted via the O atom with a Cu鈥揅 distance of r_CuC = 3.34(10) 脜 (DFT, 3.27 脜). The Cu鈥揙 distance for adsorbed ¹³CO₂ is not directly accessible to EPR measurements but could be estimated from geometrical considerations in the range of r_CuO = 2.53鈥?.73 脜 (DFT, 2.39 脜). The results provide detailed insight into the geometry of adsorbed CO₂ and CO in porous materials and show the potential of EPR spectroscopy for analyzing adsorption complexes.