Showing papers by "Juergen Eckert published in 2002"

Breathers or Structural Instability in Solid L-Alanine: a New IR and Inelastic Neutron Scattering Vibrational Spectroscopic Study

Abstract: Incoherent Inelastic Neutron Scattering data and new infrared spectra were acquired in order to examine both the external and internal vibrations in crystalline L-alanine. For the first time we observe a splitting of the NH3+ torsional band below a temperature of approximately 220 K as well as an overtone of this band. The intensity of both of these bands is strongly dependent on temperature. Birefringence and depolarization measurements performed with single crystals reveal a subtle breaking of symmetry around 220 K perhaps involving the hydrogen bond networks. We show that this instability cannot, however, be the origin of the observed splitting. Instead, the anomalous temperature dependence of the observed intensity and frequency of the torsional mode and its overtone may be explained on the basis of a nonlinear coupling of the NH3+ oscillator with lattice phonons. This leads to localization of vibrational energy, a so-called “breather” or “vibrational polaron”.

Characterization of C2 (CxHy) Intermediates from Adsorption and Decomposition of Methane on Supported Metal Catalysts by in situ INS Vibrational Spectroscopy

Abstract: Conversion of methane into higher hydrocarbons is a process of enormous technological importance, which would benefit from a better understanding of the nature of the surface intermediates formed during methane activation. Currently methane is converted into hydrocarbons mainly by an indirect route[1, 2] via syn gas[3] as the intermediate. Alternative routes, such as oxidative coupling of methane[4±7] over metal oxide catalysts and methane homologation[8±10] on transition metal catalysts have also been extensively investigated. In a previous study we[11] have observed the formation of various CxHy surface intermediate species after methane decomposition on single-crystal Ru(0001) and Ru(1120) model catalysts, observed by high-resolution electron energy loss spectroscopy (HREELS). In this study we used INS to investigate the surface intermediate species formed during the decomposition of methane on Ru/Al2O3 and Ni/SiO2 catalysts. We relate these findings to our previous work on idealized single-crystal model catalysts. The present work represents a step in the effort to bridge the gap between surface science and real-world catalysts in terms of the type of material and pressure used. Inelastic neutron scattering (INS)[12±14] vibrational spectroscopy is a technique that can be directly applied to high surface-area catalysts from ambient to high pressures. This method can also provide accurate quantitative information, has high sensitivity to hydrogenous species, and is not limited by the selection rules of other vibrational spectroscopies. Herein we report the first experimental evidence of the formation of ethylidyne, vinylidene, and methylidyne (CxHy) species from methane on supported metal catalysts. Shown in Figure 1 is the difference INS vibrational spectrum after methane decomposition on Ru/Al2O3. The rather modest resolution, especially at higher frequencies, and poor statistics are the result of having to use a difference spectrum and of the relatively small amount of H in the sample. Nonetheless, the rate was defined as the number of doublings between day 5 and day 9 (determined as the logarithm in base 2 of the increase in the DNA amount) divided by the elapsed time (4 days).

Direct Observation of Host−Guest Hydrogen Bonding in the Zeolite NaY/Chloroform System by Neutron Scattering

Abstract: The differential H pair distribution function obtained from neutron scattering contrast measurements on hydrogenous and deuterated chloroform adsorbed into zeolite NaY (two molecules per supercage) reveals direct evidence for hydrogen bonding between hydrogen atoms on the sorbate molecule and oxygen atoms of the zeolite framework. A vector between the hydrogen and framework Si/Al atoms is also observed. The results confirm the conclusions drawn from previous vibrational spectroscopy and computer modeling studies.