H, D and HD adsorption upon the metal-organic framework [CuZn(btc)] studied by pulsed ENDOR and HYSCORE spectroscopy

Abstract

The adsorption of hydrogen has become interesting in terms of gas separation as well as safe and reversible storage of hydrogen as an energy carrier. In this regard, metal-organic framework compounds are potential candidates. The metal-organic framework [CuZn(btc)] as a partially Zn-substituted analogue of the well known compound HKUST-1 is well suited for studying adsorption geometries at cupric ions by electron paramagnetic resonance (EPR) methods due to the formation of few mixed Cu/Zn paddle wheel units with isolated S = 1/2 electron spins. The adsorption of hydrogen (H₂) as well as the deuterium (D₂) and HD molecules were investigated by continuous wave EPR and pulsed ENDOR and HYSCORE spectroscopy. The principal values of the proton and deuterium hyperfine coupling tensors and were determined by spectral simulations as well as of the deuterium nuclear quadrupole tensor for adsorbed HD and D₂. The results show a side-on coordination of HD and D₂ with identical Cu–H and Cu–D distances r_CuX = 2.8 Å with the tensors and aligned parallel to the C₄ symmetry axis of the paddle wheel unit. A thermodynamic non-equilibrium state with J = 1, m_J = ±1 is indicated by the experimental data with and averaged by rotation around C₄.

Keywords:

• orientation-selective pulsed EPR spectroscopy
• hydrogen adsorption
• HKUST-1
• deuterium nuclear quadrupole splitting

Acknowledgements

The authors thank Prof. Haase and Prof. Berger for valuable discussions, L. Moschkowitz for loading the samples and the Deutsche Forschungsgemeinschaft (DFG) for financial support within the Priority Program 1362.