Evidence for a charge transfer from spilt-over hydrogen to platinum by means of ESR spectroscopy

An electron transfer from spilt-over hydrogen (H*) to platinum on a Pt/γ-Al2O3 catalyst has been investigated by ESR spectroscopy. Activated hydrogen is formed on Pt clusters and the resulting atomic H* species migrate onto the alumina surface where they can interact with single Pt atoms. The chemisorption of H* at 77 K leads to the appearance of ESR signals which are related to the formation of Ptn− species. This charge transfer is correlated to the formation of H+ ions from initially present radical hydrogen spillover species

Influence of cation exchange on the Al-27-NMR spectra of zeolites

The influence of cation exchange on the Al-27-NMR spectra of NaA-zeolites has been studied by Al-27-MAS- and MQ-MAS-Solid State-NMR. From the Al-27-spectra a characterization of the different At sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the Al-27-NMR-spectra than exchange with larger cations like Ba2+. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites

Characterization of Lewis-acid sites in zeolites by EPR using the NO molecule as a probe

EPR spectra of NO adsorbed on both Na-ZSM-5 and H-ZSM-5 zeolites have been recorded at 10 and 78 K. Using the experimental parameters, it was possible to simulate the spectra on the assumption that in the studied zeolites, an NO molecule is tightly bound to an Na+ ion, already at 78 K. In the case of a ‘true’ Lewis site in these zeolites, a tight binding of NO at 10 K, but a rotation of the molecule around its z axis at 78 K, was assumed. It was possible to estimate the Al–N distance and to propose a structure of the ‘true’ Lewis site

Influence of Cation Exchange on the ²⁷Al-NMR Spectra of Zeolites

The influence of cation exchange on the ²⁷Al-NMR spectra of NaA-zeolites has been studied by ²⁷Al-MAS- and MQ-MAS-Solid State-NMR. From the ²⁷Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the ²⁷Al-NMR-spectra than exchange with larger cations like Ba²⁺. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites

NMR study of role of the cross-relaxation effect in the cortex and the nucleus rabbit lens fragments

We have experimentally studied the coupling between the longitudinal magnetization of macromolecular protons and water protons in the cortex and the nucleus rabbit lens fragments (cross-relaxation). The NMR lines of water protons in the cortex and the nucleus of rabbit lens are inhomogenously broadened and relative narrow. We have assumed that the NMR line of crystalline protons in the lens is homogeneously broadened, very broad (the halfwidth order of 25 kHz) with small intensity. To detect the effect of cross-relaxation we adopted concept of the magnetization transfer used in magnetic resonance imaging. We irradiated the sample 20 kHz off resonance of the water protons with a train of low power RF pulses forming a selective saturation pulse and then after variable delay excited with a high-power 90 degrees, detection pulse. Finally we have measured the sample NMR line intensity as a function of duration of selective pulse for several fixed delays between selective and detection pulses. We have found the significant difference between time constants of decays for the cortex and the nucleus. We attributed this difference to longer time of hydration water molecules in the cortex spend within proximity of the protein surface than in the nucleus. We have interpreted these finding as an effect of chemical protons exchange between macromolecular and water protons rather then cross-relaxation effects, and it is discussed in terms of the Levy walk mechanism. (C) 1999 Elsevier Science B.V. All rights reserved

Surface diffusion of water molecules on proteins of rabbit lens by H-1 NMR relaxation measurements

In this work, we propose a relaxation model for the interpretation of NMR proton spin-lattice and spin-spin relaxation times of mammalian lenses. The framework for this model is based on nuclear magnetic spin-lattice relaxation measurements as a function of temperature at different Larmor frequencies for whole rabbit lenses and fragments of the lens. According to this model, two different dynamic processes of the water molecules determine the relaxation behaviour, namely rotational diffusion and translational surface diffusion. These dynamic processes in conjuction with a two site exchange model give a good explanation of all the measured relaxation data. From the experimental data, we were able to obtain the activation parameters fbr rotational and translational diffusion of bound lens water. Correlation times of 2.1x10(-11) sec and 2.5x10(-9) sec and activation energies of 20.5 kJ/mol and 22.5 kJ/mol respectively were found at 308 K. At low Larmor frequencies (less than or equal to 100 MHz) the longitudinal relaxation is mainly determined by translational surface diffusion of bound water with a mean square displacement of 1.5 nm, whereas at higher frequencies (greater than or equal to 300 MHz), rotational diffusion is the main relaxation mechanism. The spin-spin relaxation is determined by translational diffusion over the whole frequency range and therefore shows only a very small dispersion. By our model it is possible to explain: 1) the strikingly large difference between the T-1 value and the T-2A and T-2B values observed in the lens and 2) the different values of the activation energies measured at different fields for the lens

Evidence of Anisotropic Reorientations of Water-Molecules in the Cortex of the Rabbit Lens Detected by H-1-Nmr Spectroscopy

We studied the order of water molecules in different fragments of nucleus and cortex of lenses of 3 month old rabbits by investigating the H-1-NMR spin-spin relaxtion behaviour of the water protons at room temperature. The experiments were carried out using the Carr Purcell Meiboom Gill (CPMG) technique. The apparent relaxation rate was found to be dependent on the pulse spacing in the CPMG sequence in a different way for nucleus and cortex. While for the nucleus the pulse spacing dependence can be explained by chemical exchange of water protons, the pulse spacing dependence of the cortex protons suggests the existence of not fully averaged residual magnetic dipolar couplings among the protons of a water molecule. To support this interpretation, measurements of the ratio of the solid echo (90(x)-90(y)) to the Hahn echo (90(x)-180(x)) amplitude were carried out for the same samples. These experiments give the expected ratio of K = 0.5, characteristic of no residual couplings for the nucleus, but a ratio of K = 0.7 for the cortex, which is characteristic of residual dipolar couplings, caused by anisotropic reorientations of the water molecules. Thus, evidence for an ordered state of the water molecules in the cortex has been found