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Solid-State NMR of Composite Materials and Mesoporous Solids

Solid-state NMR experiments, coupled with powder X-ray diffraction and Raman spectroscopy are applied to study composite materials composed of mesoporous metal oxides (tantalum, niobium oxides) as the host material and a variety of organometallic complexes as guest materials.  This work is done in collaboration with Prof. David Antonelli at the University of Windsor.

B. Skadtchenko, M. Trudeau, R.W. Schurko, M.J. Willans and D.M. Antonelli, 2003.  Structural and Spectroscopic Studies on Mesoporous Ta Oxide-Na Fulleride Composites with Conducting Fulleride Columns in the Pores. Adv. Funct. Mater. 13, 671-681.

Mesoporous tantalum oxide–sodium fulleride composites were synthesized by solution impregnation and characterized by elemental analysis, X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption–desorption, X-ray electron photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID) magnetometry, variable-temperature electron transport measurements, and solid-state 13C and 23Na NMR spectroscopy. The room temperature conductivity pattern as a function of sodium reduction level displayed a minimum at n = 3.0 and a maximum at n = 4.5, where n is the formal charge on the fulleride. The variable-temperature conductivity measurements demonstrated that the n = 0.5 and n = 4.5 materials were semiconductors. Solid-state 23Na NMR spectroscopy of the n = 0.5 composite exhibited three Na environments in the composites: two associated with the tantalum oxide walls and a third associated with the fulleride. The n = 3.0 and n = 4.5 materials showed a large build-up of Na ions in the wall with no visible Na resonances associated with the fulleride, suggesting a structure in which the fulleride units exist as naked anions in one-dimensional chains surrounded first by a layer of Na ions and then by a layer of mesoporous tantalum oxide. Solid-state 13C NMR experiments showed more than one fulleride species in both the n = 0.5 and the n = 4.5 composites, as well as pure C60 in the n = 0.5 material, but almost exclusively C603– in the n = 3.0 material. The retention of carbon throughout reduction suggests that polymerization may have occurred, however this could not be verified by 13C NMR spectroscopy, because the region where sp3 fullerene resonances normally appear was obscured by solvent peaks.

 

(Right): Highly schematized drawings of pure Na3C60 and the buckminsterfulleride in the channels of mesoporous tantalum oxide

(Below): 23Na MAS NMR spectrum showing three unique Na environments in the composite material

X. He, A. Y. H. Lo, M. Trudeau, R. W. Schurko and D. Antonelli, 2003. Compositional and H-2 NMR studies of bis(benzene)chromium composites of mesoporous vanadium-niobium mixed oxides, Inorg. Chem. 42, 335-347.

Abstract: New mesoporous niobium oxides with 5, 10, and 15 mol % vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(I) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.

Top: Variable-temperature 2H static NMR spectra of bis(benzene)chromium (0) acquired at 9.4 T. Bottom: Variable-temperature 2H static NMR spectra of and simulations of mesoporous niobium oxides loaded with 0.7 equiv of bis(benzene)chromium(0).

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M. Vettraino, M. Trudeau, A. Y. H. Lo, R. W. Schurko and D. Antonelli, 2002. Room-temperature ammonia formation from dinitrogen on a reduced mesoporous titanium oxide surface with metallic properties, J. Am. Chem. Soc. 124, 9567-9573.

Abstract: Mesoporous titanium oxide was treated with bis(toluene) titanium under nitrogen at room temperature in toluene, leading to a new blue black material possessing conductivity values of up to 10 -2 ¿ -1 cm -1 . XRD and nitrogen adsorption showed that the mesostructure was fully retained. Elemental analysis indicated that the material absorbed Ti from the organometallic, without any incorporation of the toluene ligand. There was also an increase of nitrogen from below the detection limit to 1.16%. XPS studies showed that the Ti framework was reduced by the organometallic and that the material had reduced nitride on the surface. There was also an emission at the Fermi level, suggesting metallic behavior. This was confirmed by variable-temperature conductivity studies, which showed a gradual decrease of resistivity with temperature. SQUID magnetometer studies revealed spin glass behavior with a degree of temperature independent paramagnetism, consistent with metallic properties. Solid-state 15 N NMR studies on materials synthesized in the presence of labeled dinitrogen showed that the source of the nitride was the reaction atmosphere. IR and 15 N NMR demonstrated that this nitrogen species was surface ammonia, suggesting that the initially formed nitride species had reacted with moisture imbedded in the walls of the mesostructure.  The direct conversion of dinitrogen to ammonia is a very rare process and this work represents the first example of this process mediated by a molecular sieve.

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Figure: 15N{1H} CPMAS and MAS NMR spectra of mesoporous titanium oxide treated with 15N-labelled NH3 and *NH4NO3 at 9.4 T.

More coming soon!

Last Updated: September 1, 2003
Copyright Rob Schurko, 2003