Hydrogen adsorption kinetics on Pd/Ce0.8Zr0.2O2

Hydrogen adsorption on Pd/Ce(0.8)Zr(0.2)O(2) was studied by temperature-programmed reduction, volumetric measurements and IR spectroscopy. Hydrogen uptake and reduction rate at 353 K are strongly dependent on the hydrogen pressure. At relatively high hydrogen partial pressure, reduction involves PdO, the surface and a significant fraction of the bulk of the ceria based oxide. Formation of oxygen vacancies even at low temperature (<373 K) is observed. The hydrogen adsorption process is mainly irreversible, as is shown by an increase in the (2)F(5/2)-->(2)F(7/2) electronic transition of Ce(3+) with hydrogen pressure and surface dehydroxylation. This "severe" reduction has a negative effect on the subsequent hydrogen adsorption capability. The decrease of hydrogen uptake capacity and rate during adsorption can be associated with the partial loss of superficial OH and the presence of Ce(3+), which deactivates Pd electronically.     

Modulating and scanning the mode-lock frequency of an 800-MHz femtosecond Ti:sapphire laser

An 800-MHz self-mode-locked femtosecond Ti:sapphire laser has been developed whose mode-lock frequency can be robustly scanned and modulated. The laser is based on the three-element design of Ramaswamy-Paye and Fujimoto [Opt. Lett. 19, 1756 (1994)]. By translation and modulation of the position of the prismatic output coupler, the mode-lock frequency can be reliably scanned more than 1% and modulated at 80 Hz with a deviation of 2.5 kHz without interrupting the mode lock, changing the pulse length, or inducing significant amplitude modulation. An application in tunable high-resolution terahertz spectroscopy is also demonstrated.     

Visualizing the whispering gallery modes in a cylindrical optical microcavity

Whispering gallery modes in cylindrical integrated optics microcavities have, for what is to our knowledge the first time, been mapped with a photon scanning tunneling microscope. Optical images were obtained with a spatial resolution of 50 nm. By combination of information on the spatial optical distributions with wavelength-dependent measurements, an unexpectedly rich variety of intracavity phenomena, such as polarization conversion and interference of copropagating and counterpropagating modes, could be directly observed. A quantitative comparison of the experimental data with computer simulations results in a comprehensive understanding of the various whispering gallery modes inside the microcavity.     

Regeneration of poly-L-lysine modified carbon electrodes in the accumulation and cathodic stripping voltammetric determination of the cromoglycate anion

Cromoglycate is accumulated on a poly-l-lysine (PLL) modified carbon electrode best from pH 4 solution, where it is anionic and the PLL is cationic, and at which pH the cromoglycate gives a good reduction peak at −0.82 V. The PLL film can be regenerated readily by washing the electrode with 3 M sodium hydroxide solution, in which the PLL is deprotonated. Regeneration of the film is not required as frequently when larger amounts of PLL are incorporated into it. This allows standard addition procedures to be carried out without regenerating the electrode. Linear calibration graphs have been obtained typically in the range 0.1-1.5 μg ml⁻¹. Detection limits have been calculated to be 10 ng ml⁻¹. The standard addition method has been applied satisfactorily to diluted urine solutions.     

New insights into the reaction mechanism and the rate-determining step of n-butane hydroisomerisation over reduced MoO3 catalysts

The comparison of the proportions of the minor products formed during the hydroisomerisation of n-butane over reduced MoO3 with thermodynamic data demonstrates that a bifunctional mechanism operates and that the rate-determining step is the isomerisation of the linear butene intermediates to iso-butene.