Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary C?H Bonds

Achieving selective C H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C H bonds.     

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.     

High selectivity for primary C-H bond cleavage of propane σ-complexes on the PdO(101) surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.     

Individual and collective behavior of vibrating motors interacting through a resonant plate

We report on experiments of many small motors—cell phone vibrators—glued to and interacting through a resonant elastic plate. We find that the motors tend to avoid frequencies that are just higher than the resonances of a plate, preferring instead frequencies just below those resonances. As a result, motors interacting through a resonant plate exhibit hysteresis in their frequency versus driving voltage. We also find that the stability of a single motor near a resonance is different from the stability of a group of motors near a resonance. When the driving voltage is constant and the transient behavior of the system has passed, we find that the average frequency of all the motors is constant.     

From finitary to infinitary second‐order logic

A back and forth condition on interpretations for those second‐order languages without functional variables whose non‐logical vocabulary is finite and excludes functional constants is presented. It is shown that this condition is necessary and sufficient for the interpretations to be equivalent in the language. When applied to second‐order languages with an infinite non‐logical vocabulary, excluding functional constants, the back and forth condition is sufficient but not necessary. It is shown that there is a class of infinitary second‐order languages whose non‐logical vocabulary is infinite for which the back and forth condition is both necessary and sufficient. It is also shown that some applications of the back and forth construction for second‐order languages can be extended to the infinitary second‐order languages. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Shielding against noise interfering with quantitation of insect infestations by acoustic detection systems in grain elevators

Acoustic devices used to detect hidden insect infestations must be shielded from noise in most practical applications. One device developed specifically for use in a noisy environment, the Acoustic Location ‘Fingerprinting’ Insect Detector (ALFID), counts the numbers of insects present in grain samples from shipments being graded for export at commercial grain elevators. This report considers the performance of ALFID's noise-shielding components, which include an enclosure for passive reduction of ambient noise, and an electronic system for active detection and masking of sounds originating outside the grain sample container. Sound pressure levels (SPLs) of ambient noise are reduced inside the enclosure by 60–90 dB at frequencies between 1 and 10 kHz, with a reduction of ~6.5 dB per octave (frequency doubling). The active noise-masking system protects ALFID from loud ambient sounds not sufficiently attenuated by the enclosure. If the output from one of four sensors mounted on the outside of the grain sample container rises above a preset amplitude threshold, a signal is triggered that inhibits acquisition of insect sound data from sensors inside the container. In tests of the complete ALFID system at a grain elevator with ambient noise of 73 ± W dB re 20 μPa SPL, the mean rate of noise-mask triggering was 5.5 s⁻¹, inhibiting acquisition of insect sounds for only 3.9% of the total testing period. This level of performance is sufficient to enable successful operation of ALFID under such noise conditions.     

Electrosorptive detection in liquid chromatography of simple charged organic compounds

Summary The applicability of electrosorptive (ES) detection in liquid chromatography to charged organic species, specifically amines, alkanolamines, and alkylsulfonates is demonstrated. Separations can be carried out using ion-pair chromatography, and detection acieved by measuring changes in differential double-layer capacitance (DLC), C_d, brought about by analyte adsorption at potentials either well negative or positive of the potential of zero charge (pzc), depending on whether cationic or anionic forms, respectively, are being determined. The C_d-analyte concentration response curves are linear over a limited range. A welcome feature of the technique is that organic modifiers may be used in the mobile phase without deleterious consequence to the detection. Attempts were also made to detect these charged species as ion pairs by DLC at potentials close to the pzc. This approach, however, appears to have less practical utility.