Tanja Pietraß

Synthesis of dealuminated zeolites NaY and MOR and characterization by diverse methodologies: 27Al and 29Si MAS NMR, XRD, and temperature dependent 129Xe NMR

Zeolites NaY and mordenite have been dealuminated by SiCl4 and steaming methods and carefully characterized during and after the various synthetic stages by 27Al and 29Si magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) to ensure sample quality. 129Xe NMR spectra for xenon adsorbed in NaX, siliceous Y, mordenite, and dealuminated mordenite have been obtained for very low xenon loadings and as a function of temperature. Data obtained for faujasites indicate that 129Xe NMR chemical shifts are very sensitive to the Si/Al ratio reflecting the presence or absence of strong adsorption sites. 129Xe NMR chemical shift data for the mordenite samples show that Xe dwells preferentially in the side pockets which do not seem to be affected by the dealumination process. In addition, the 129Xe NMR data clearly reveal information not available from 29Si and 27Al NMR or XRD regarding structural integrity of the zeolites. The findings presented here point to the need for characterization of siliceous zeolites by several techniques before conclusions regarding zeolite structural integrity can be drawn.

NMR of molecules and surfaces using laser-polarized xenon

Abstract Practical aspects of the techniques currently in use for the generation of highly polarized xenon through optical pumping of rubidium vapor are compared. Optically polarized xenon has applications in NMR spectroscopy using direct 129 Xe NMR detection, and as a nuclear spin polarization source for polarization transfer experiments. Polarization transfer has been achieved using thermal mixing, Hartmann–Hahn cross polarization, and the nuclear Overhauser effect. The practicality of these techniques is compared, highlighting recent examples. The implementation of magic angle spinning and continuous-flow techniques in combination with spin induced nuclear Overhauser enhancement makes laser-polarized xenon a widely applicable tool for the study of surfaces.

2H nuclear magnetic resonance spectroscopy of deuterium adsorption on single-walled carbon nanotubes

2H nuclear magnetic resonance (NMR) spectroscopy was employed to study the interaction between deuterated hydrogen gas and single walled carbon nanotubes before and after purification. Transmission electron micrographs revealed strong bundling of the tubes. After purification, very little amorphous carbon and no graphitic particles were present, implying that the interactions observed are truly due to the nanotubes. In the parent material, the NMR signal is dominated by interaction of hydrogen with residual metal catalyst particles. For purified material, hydrogen in the gas phase is discernible from adsorbed hydrogen. The two phases do not exchange with each other on a ms time scale. The hydrogen molecules move among different adsorption sites, presumably outer tube surfaces and interstitial channels. This process is diffusion limited in the pressure range investigated.

Gas Adsorption on Carbon Nanotubes

Since their discovery in 1991 [1], carbon nanotubes (CNTs) have attracted much attention due to their unique mechanical and electronic properties. Single-walled carbon nanotubes (SWNTs) can be envisioned as a single sheet of graphite rolled into a seamless tube. In multiwalled carbon nanotubes (MWNTs), several of these tubes are stacked inside each other in a concentric fashion. The length and direction of the roll-up vector determine the diameter and chirality of the tube. Statistically, one-third of tubes should be metallic, while the other two-thirds are semi-conducting [2]. Potential applications of CNTs range from field-emitting devices to gas sensors [3]. In 1997, CNTs were reported to store large amounts of hydrogen gas with uptake capacities in the range of 5–10% by weight (wt%) [4]. This result sparked intense research activity due to the potential to exceed the benchmark set by the Department of Energy of 6.5 wt% uptake capacity for an economically viable hydrogen storage medium [5]. The ensuing and continuing controversy about the uptake capacity and sorption mechanism has not yet been settled [10,11]. Results from experimental and theoretical studies indicate hydrogen storage capacities from less than 1 wt% to about 14 wt% [6–9]. A possible origin for these discrepancies may be due to the fact that a large number of production and purification methods are used, resulting in a wide variety of samples. Experimental studies concerning gas adsorption on CNTs typically employ macroscopic techniques, such as temperature programmed desorption, thermal gravimetric analysis, or adsorption isotherms [6,9,12]. Spectroscopic tools, and in particular magnetic resonance techniques, have been rarely used for the study of gas adsorption. In this contribution, a summary of the literature will be provided together with a discussion of the data generated in our laboratory.