Influence of cation nature on stabilization of gold nanospecies in mordenites.

Gold species deposited on NH4(+)- and H(+)-mordenites were studied by X-ray diffraction, xenon adsorption, NMR of 27Al and 129Xe, UV-Visible spectroscopy and TPR. Mordenite ion-exchange cations exert a significant effect on size distribution and electronic state of gold species supported on the zeolites. AuNH4M contains larger nanoparticles of gold on its external surface than AuHM because protonic form of zeolite has stronger sites for stabilization of gold nanospecies. In the case of NH4(+)-mordenite, Au clusters inside zeolite channels have bigger size and fill the side-pockets completely after gold deposition due to more complete reduction of gold entities, while in H(+)-mordenite only half the side-pockets are filled by gold species. H(+)-mordenite favors the stabilization of both small gold clusters and cations. Even after the reduction of gold by hydrogen flow at 100 degrees C, some gold species are not completely reduced and have certain effective charge (Au(n)delta+ clusters).

Preparation, structure and thermal stability of onium- and amino-functionalized silicas for the use as catalysts supports.

We explored and compared several synthetic methods of grafting silica with strong (alkyltriphenylphosphonium, tetralkylammonium, propylpyridinium and dialkylimidazolium) and weak (gamma-aminopropyl, gamma-(N-imidazolyl)propyl) anion-exchanging groups starting with commercially available chloroalkyl- and gamma-aminopropylsilanes. Structure of the intermediate and final materials was investigated by elemental analysis, titration, 13C, 29Si, 31P MAS NMR, DRIFT, and TPD MS. The derivatives of alkyltriphenylphosphonium, propylpyridinium and dialkylimidazolium cation can be prepared with satisfactory quaternisation yields (ca. 30-100%) via the nucleophilic substitution of gamma-chloropropyl groups either in the silane or in chloropropylsilica, resulting in bonded phases with moderate densities: 0.2-1.0 group nm(-2) (onium salts) and 0.2-1.5 group nm(-2) (amines). Parallel one-pot end-capping/hydrophobization can be done if a mixture of target silane with end-capping reagent or gamma-chloropropylsilane is used. The grafted layer is highly stable at the level of Si-C bonds and decomposes at ca. 400 degrees C, while the onium functions begin to decompose at ca. 250 degrees C, lowering the thermal stability of materials. Thus, anion-exchanging silicas can be envisaged for the use as catalyst supports at moderate temperatures.

129Xe NMR study of Xe adsorption on multiwall carbon nanotubes.

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Synthesis, structure, and acidic properties of MCM-41 functionalized with phosphate and titanium phosphate groups.

Mesoporous molecular sieves Si-MCM-41 (purely siliceous) and Ti-MCM-41 (partly covered with a surface layer of TiO2) were functionalized with phosphate groups by treatment with POCl3 (denoted -MCM-41(P)and Ti-MCM-41(P), respectively). With the use of TEM, X-ray diffraction, and N2 adsorption, it was shown that the initial hexagonal structure, the high specific surface area, and porosity are retained in the functionalized materials but are not as good as in the starting materials. 1H MAS NMR and 31P MAS NMR revealed that the surface of Si-MCM-41(P) consists of silicon phosphate and pyrophosphate species. That of Ti-MCM-41(P) additionally contains titanium dihydro-, hydro-, and pyrophosphate species, the latter being predominant. TPD of adsorbed ammonia for Si-MCM-41(P) and Ti-MCM-41(P) showed that functionalization leads to the creation of moderate and strong acid sites. A combination of mesoporous structure with acidic properties makes the MCM-41 functionalized with phosphate groups promising for use as solid acid catalysts.

ZSM-5 zeolite synthesis in the presence of a copper-phthalocyanine complex characterization by ESR and 129Xe NMR.

In the presence of a copper phthalocyanine complex (CuPc), it is possible to synthesize ZSM-5 zeolite containing the complex, and the crystallinity is good, provided that the amount of CuPc trapped is lower than the number of channel intersections where it can be located without too much distortion. The crystallinity is retained after decomposition of the CuPc in a stream of air at 500 degrees C. The copper-ZSM-5 system is characterized by the combined application of xenon adsorption, ESR and 129Xe NMR spectroscopy.

Spin-5/2 Hahn echoes in solids.

The Hahn echoes for a spin I = 5/2 system were derived by considering the first-order quadrupole interaction throughout the experiment. Consequently, our results are valid for any ratio of the quadrupole coupling, omega Q, to the amplitude of the RF pulse, omega RF. The use of simplified density operators reduced the computation time dramatically. Furthermore, this approach allows a better understanding of the origin of the Hahn echoes in half-integer quadrupole spins. The tau 4 = tau 2 Hahn echoes were illustrated with the 27Al nucleus in a polycrystalline sample of KAl(SO4)2.12H2O. The quadrupole coupling constant and the asymmetry parameter of 27Al were determined by two methods: from the lineshape of the powder pattern and from the variation of the central line integrated intensity as a function of the second pulse duration.

Application of 129Xe nuclear magnetic resonance to the study of Cu-Y zeolites: dehydration and redox effects.

Copper-exchanged sodium Y zeolites have been studied by using 129Xe nuclear magnetic resonance (NMR) spectroscopy. The oxidation state as well as the location of copper in the zeolite have been determined for different samples at various levels of exchange (lambda = 15, 35, 58%) and hydration, and at different stages of redox treatment. Information about the nature of Xe-Cu+ or Xe-Cu2+ interactions has been obtained by a combination of electron spin resonance (ESR) and 129Xe NMR measurements on Cu-Y and Cu-Rho zeolites.

Application of one-dimensional nutation nuclear magnetic resonance to 51V in ferroelastic BiVO4.

The central line intensity of a spin I = 7/2, excited by a radio frequency (rf) pulse, is calculated by taking into account the first-order quadrupolar interaction during excitation. Thus, the result is valid for any ratio of quadrupolar coupling to pulse amplitude. The quadrupolar coupling of the nuclei vanadium 51V in a single crystal of ferroelastic BiVO4 is determined using this one-dimensional (1D) mutation method.

1H and 129Xe nuclear magnetic resonance studies of hydrogen chemisorption on supported platinum. Application to the metal dispersion and spillover.

Chemisorption of hydrogen, electron microscopy and 1H nuclear magnetic resonance (NMR) have confirmed that the chemical shift of hydrogen chemisorbed on platinum is directly related to the size of the metal particles. The influence of the hydrogen adsorbate concentration and the chemisorption temperature on the chemical shift delta H and the distribution of the hydrogen chemisorbed on these particles have been determined. This study also shows how the dispersion can be deduced from the variation of delta H with the H2 concentration and clarifies the effect of temperature on the concentration of spillover H2. 129Xe NMR of adsorbed xenon used as a probe confirmed most of the previous results.