Search results for "Density"
showing 10 items of 4402 documents
Kinetics of streptolysin O self-assembly.
1995
Streptolysin O is a member of a family of membrane-damaging toxins that bind to cell membranes containing cholesterol and then polymerize to form large pores. We have examined the kinetics of toxin action using 125I-labelled streptolysin O. Binding of toxin monomers to membranes displays first-order kinetics and is reversible; the rate of desorption from red cells shows a marked dependence on temperature. To study oligomerization, toxin was bound to erythrocytes at 0 degrees C. Oligomer formation was then triggered by a sudden temperature shift and stopped by solubilization of membranes with deoxycholate. While at moderately high streptolysin O concentrations oligomerization behaves as a re…
Toward an understanding of the hydrogenation reaction of MO2 gas-phase clusters (M = Ti, Zr, and Hf).
2013
A theoretical investigation using density functional theory (DFT) has been carried out in order to understand the molecular mechanism of dihydrogen activation by means of transition metal dioxides MO2 (M = Ti, Zr, and Hf) according to the following reaction: MO2 + H2 → MO + H2O. B3LYP/6-311++G(2df,2pd)/SDD methodology was employed considering two possible reaction pathways. As the first step hydrogen activation by M═O bonds yields to metal-oxo hydride intermediates O═MH(OH). This process is spontaneous for all metal dioxides, and the stability of the O═MH(OH) species depends on the transition metal center. Subsequently, the reaction mechanism splits into two paths: the first one takes place…
Oxygen-assisted hydroxymatairesinol dehydrogenation: a selective secondary-alcohol oxidation over a gold catalyst.
2013
Selective dehydrogenation of the biomass-derived lignan hydroxymatairesinol (HMR) to oxomatairesinol (oxoMAT) was studied over an Au/Al(2)O(3) catalyst. The reaction was carried out in a semi-batch glass reactor at 343 K under two different gas atmospheres, namely produced through synthetic air or nitrogen. The studied reaction is, in fact, an example of secondary-alcohol oxidation over an Au catalyst. Thus, the investigated reaction mechanism of HMR oxidative dehydrogenation is useful for the fundamental understanding of other secondary-alcohol dehydrogenation over Au surfaces. To investigate the elementary catalytic steps ruling both oxygen-free- and oxygen-assisted dehydrogenation of HMR…
Hydro-dimerization of Pt2Cl2(C2H4)2: model reaction to capture details on catalytic mechanisms
2005
Abstract The density functional theory was employed to mimic the homogeneous hydro-dimerization of the Zeise-similar dichloro-bis(ethene)-platinum(II), PtCl 2 ( C 2 H 4 ) 2 . Three energy minima and four transition-states were involved in the reaction mechanism, which was even characterized by a catalytic cycle. The minima were fully optimized at B3LYP level whereas the transition-states were first individuated at the same level, by the synchronous transit-guided quasi-Newton method, then fully optimized. The mechanistic hypotheses on the PtCl 2 ( C 2 H 4 ) 2 dimerization implied the displacement of two ethene or two ethane molecules, respectively, occurring in absence or in presence of mol…
Unraveling the Role of the Rh–ZrO2 Interface in the Water–Gas-Shift Reaction via a First-Principles Microkinetic Study
2018
The industrially important water–gas-shift (WGS) reaction is a complex network of competing elementary reactions in which the catalyst is a multicomponent system consisting of distinct domains. Herein, we have combined density functional theory calculations with microkinetic modeling to explore the active phase, kinetics, and reaction mechanism of the WGS over the Rh–ZrO2 interface. We have explicitly considered the support and metal and their interface and find that the Rh–ZrO2 interface is far more active toward WGS than Rh(111) facets, which are susceptible to CO poisoning. CO2 forming on the zirconia support rapidly transforms into formate. These findings demonstrate the central role of…
Understanding the mechanism of polar Diels–Alder reactions
2009
A good correlation between the activation energy and the polar character of Diels-Alder reactions measured as the charge transfer at the transition state structure has been found. This electronic parameter controls the reaction rate to an even greater extent than other recognized structural features. The proposed polar mechanism, which is characterized by the electrophilic/nucleophilic interactions at the transition state structure, can be easily predicted by analyzing the electrophilicity/nucleophilicity indices defined within the conceptual density functional theory. Due to the significance of the polarity of the reaction, Diels-Alder reactions should be classified as non-polar (N), polar…
Coadsorption of NRR and HER Intermediates Determines the Performance of Ru-N4 toward Electrocatalytic N2 Reduction
2021
Efficiency of the electrochemical N2 reduction reaction (NRR) to ammonia is seriously limited by the competing hydrogen evolution reaction (HER) but our current atomic-scale insight on the factors controlling HER/NRR competition are unknown. Herein we unveil the elementary mechanism, thermodynamics, and kinetics determining the HER/NRR selectivity on the state-of-the-art NRR electrocatalyst, Ru-N4 using constant potential density functional theory calculations (DFT). The calculations show that NRR and HER intermediates coadsorb on the catalyst where HER is greatly suppressed by the NRR intermediates. The first reaction step leading to either *NNH or *H determines the selectivity towards NRR…
Investigation of electrode material – redox couple systems for reverse electrodialysis processes. Part II: Experiments in a stack with 10–50 cell pai…
2013
Abstract The performances of reverse electrodialysis depend on several factors, including the nature of the electrode material and of the redox processes adopted to make possible the conversion between chemical potential and electric power. In this paper the possible utilization of various redox processes (reduction/oxidation of iron species, oxidation and reduction of water, oxidation of chlorine and reduction of water) was studied in a stack equipped with 10–50 cell pairs and by focused electrolyses in a three compartment cell. The effect of selected redox processes on power density output and eventual contamination of saline solutions flowing in the stack was evaluated in detail. The eff…
Density and activity patterns of Pallas's cats, Otocolobus manul, in central Mongolia
2021
Abstract Context. The ranges of many small, at-risk felid species occur almost entirely in unprotected areas, where research efforts are minimal; hence data on their density and activity patterns are scare. Aims. We estimated density and activity patterns of Pallas’s cats on unprotected lands in central Mongolia during two periods (May–August and September–November) in 2019. Methods. We used spatially explicit capture–recapture models to estimate population density at 15.2 ± 4.8 individuals per 100 km2. Key results. We obtained 484 Pallas’s cat images from 153 detections during 4266 camera-days. We identified Pallas’s cats using pelage markings and identified 16 individuals from 64 detectio…
Influence of oxygen vacancies on the photorefractive effect in barium titanate single crystals
1990
Abstract In this work the influence of the vacancies concentration in pure BaTiO3 single crystals is studied. It is shown that the oxidation and the reduction do not change the electrooptic coefficients. The reduction modifies the nature of the majority carriers. An evaluation of the trap density N is done.