T1-relaxation of 129Xe on metal single crystal surfaces-multilayer experiments on iridium and monolayer considerations

129Xe and 131Xe NMR of Gas Adsorption on Single- and Multi-Walled Carbon Nanotubes

129Xe and 131Xe nuclear magnetic resonance (NMR) spectroscopy was used to study the adsorption of xenon gas on as-produced single-walled and multi-walled carbon nanotubes. Overall, the adsorption was weak, with slightly stronger interaction between xenon and multi-walled nanotubes. Temperature-dependent spectra, relaxation times, line widths, and signal intensities provide evidence that xenon forms a multilayer bulk phase rather than a homogeneous surface coating. The estimated adsorption energy of 1.6 kJ/mol is significantly lower than 23 kJ/mol predicted for monolayer adsorption but is in keeping with the Xe-Xe attractive potential. Xenon preferentially adsorbs on metallic particles in single-walled tubes, while defects are the nucleation sites for the stronger adsorption on multi-walled tubes.

Substrate and Field Dependence of the SPINOE Transfer to Surface 13C from Hyperpolarized 129Xe

The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed.

129Xe on Ir(111): NMR study of xenon on a metal single crystal surface

NMR experiments of (129)Xe adsorbed on an iridium single crystal surface are reported. Very high nuclear polarization (P(z) approximately 0.7) makes the experiment possible. A coverage of less then one monolayer is investigated on the Ir(111) surface with an area of 0.8 cm(2). The observed resonance line shifts are very large and highly anisotropic. We find sigma(iso) = 1,032 +/- 11 ppm and sigma(an) = 291 +/- 33 ppm, which are far above the typical range of physisorption. The highly ordered substrate leads to homogeneous conditions for the xenon atoms, as seen in the narrow linewidth of 20 ppm. Chemical shifts under physisorption conditions are not large enough to totally explain the results. Knight shift can clearly be identified as the cause of the findings. This shift shows the presence of conduction electrons of the metallic substrate at the xenon nucleus and thus the mixing of metallic and atomic states at the Fermi level. Such mixing is in accordance with recent Hartree-Fock and density functional calculations of similar van der Waals adsorption systems. Quantitative comparisons, however, fail completely. The size and ratio of sigma(an) and sigma(iso) are pure ground-state properties in a structurally simple system. They are accessible to theory and provide detailed local information that can serve as a benchmark for theory.