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Lower Activation Energy (lower + activation_energy)
Selected AbstractsDesign of Ceramic Materials for Chemical Sensors: Effect of SmFeO3 Processing on Surface and Electrical PropertiesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2001Hiromichi Aono Perovskite-type SmFeO3 powders were prepared by the thermal decomposition of a heteronuclear complex, Sm(Fe(CN)6)·4H2O and by solid-state reaction between the corresponding single oxides, Sm2O3 and Fe2O3. The thermal decomposition behavior of the complex was studied by thermogravimetric analysis. X-ray diffractometry was used to investigate the structure of the products from the complex thermal decomposition and the formation of SmFeO3 from the oxide mixture. Powders prepared by both methods were used to deposit thick films onto alumina substrates with comb-type gold electrodes. The microstructure and chemical homogeneity of the film surfaces were investigated by scanning electron microscopy and Auger electron spectroscopy. Thick SmFeO3 single-phase films having a homogeneous elemental distribution on the surface were obtained when powder prepared by thermal decomposition of the complex was used for deposition, even when the powder was fired at low temperature (800°C). Surface chemical analysis was performed by X-ray photoelectron spectroscopy (XPS). The O 1s XPS line was deconvoluted into two peaks, attributed to adsorbed oxygen (Oad) and oxygen in the lattice (Olattice). Quantitative analysis showed that the surface coverage of iron, expressed as Fe/(Fe + Sm), was larger for the films prepared using the solid-state reacted powder. Although the Olattice/(Fe + Sm) atomic ratio was not influenced by the processing procedures (and, thus, by iron surface coverage), the amount of Oad decreased with increasing iron surface coverage. A model of the SmFeO3 surface was used to determine that the outermost layer of the perovskite-type SmFeO3 prepared from the complex consisted mainly of samarium ions that could each bond four adsorbed oxygen ions. A single oxygen ion could adsorb onto an iron ion, and therefore, the content of adsorbed oxygen was lower for the film prepared from the solid-state reacted powders, which showed larger iron surface coverage. Electrical conductance measurements, performed with increasing temperature in different gaseous environments, confirmed these findings. Higher conductances and lower activation energies were observed for the films with larger samarium surface coverage. [source] Perfluoroalkyl Phosphonic and Phosphinic Acids as Proton Conductors for Anhydrous Proton-Exchange MembranesCHEMPHYSCHEM, Issue 13 2010Mahesha B. Herath Abstract A study of proton-transport rates and mechanisms under anhydrous conditions using a series of acid model compounds, analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, sulfonic, and carboxylic acid protogenic groups, is reported. Model compounds are characterized with respect to proton conductivity, viscosity, proton, and anion (conjugate base) self-diffusion coefficients, and Hammett acidity. The highest conductivities, and also the highest viscosities, are observed for the phosphonic and phosphinic acid model compounds. Arrhenius analysis of conductivity and viscosity for these two acids reveals much lower activation energies for ion transport than for viscous flow. Additionally, the proton self-diffusion coefficients are much higher than the conjugate-base self-diffusion coefficients for these two acids. Taken together, these data suggest that anhydrous proton transport in the phosphonic and phosphinic acid model compounds occurs primarily by a structure-diffusion, hopping-based mechanism rather than a vehicle mechanism. Further analysis of ionic conductivity and ion self-diffusion rates by using the Nernst,Einstein equation reveals that the phosphonic and phosphinic acid model compounds are relatively highly dissociated even under anhydrous conditions. In contrast, sulfonic and carboxylic acid-based systems exhibit relatively low degrees of dissociation under anhydrous conditions. These findings suggest that fluoroalkyl phosphonic and phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors. [source] Mechanism of the Asymmetric Sulfoxidation in the Esomeprazole Process: Effects of the Imidazole Backbone for the EnantioselectionADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2009Muthu Seenivasaperumal Abstract The asymmetric sulfoxidation reaction of imidazole-based prochiral sulfides was studied to explore the mechanistic details of the highly efficient esomeprazole process, which is one of the few industrial scale catalytic asymmetric procedures. The synthetic studies revealed that the smallest subunit governing the selectivity in the esomeprazole process is an imidazole ring. Thus, by using the esomeprazole procedure methyl imidazole sulfide could be oxidized as efficiently as its several functionalized derivatives, including pyrmetazol. However, alkylation of the imidazole nitrogen led to a major drop of the enantioselectivity. Our atmospheric pressure chemical ionization-mass spectrometry (APCI/MS) studies indicate that addition of small amounts of water to the reaction mixture facilitates the formation of mononuclear titanium species, which are the active catalytic intermediates of the selective oxidation reaction. One of the most important features of the esomeprazole procedure is that amine additives increase the enantioselectivity of the oxidation process. The NMR studies of the presumed reaction intermediates show that under catalytic conditions the amines are able to coordinate to titanium and dissociate the coordinated imidazole substrate. The density functional theory (DFT) modelling studies provided new insights in the mechanism of the asymmetric induction. It was found that the oxidation requires a lower activation energy if the imidazole sulfide precursor does not coordinate to titanium. Two possible reaction paths were explored for this out of sphere oxidation mechanism. The most important interaction governing the enantioselection is hydrogen bonding between the NH of the imidazole ring and the chiral tartrate ligand on titanium. Furthermore, the oxidation reaction imposes an important structural constraint to the TS structure involving a linear arrangement of the peroxide oxygens and the sulfur atom. This constraint and the N coordination of imidazole leads to a very strained structure for the inner sphere mechanism of the oxidation, which leads to a much higher activation barrier than the corresponding out of sphere process, and therefore it is unlikely. [source] A hybrid density functional theory study of the low-temperature dimethyl ether combustion pathways.ISRAEL JOURNAL OF CHEMISTRY, Issue 2-3 2002I: Chain-propagation Dimethyl ether (DME) has been proposed to be a promising alternative to conventional diesel fuel because of its favorable compression ignition property (high cetane number) and its soot-free combustion. A radical chain mechanism for hydrocarbon autoignition has been proposed for DME at low temperatures. In this mechanism, the chain initiation step consists of DME undergoing hydrogen abstraction by a highly reactive species (typically ·OH). The CH3O·H2 created in the initiation step then combines with O2; the subsequent CH3OCH2OO· radical is involved in a Lindemann-type mechanism, which can lead to the production of formaldehyde (CH2 = O) and ·OH. This concludes the chain-propagating step: the one ·OH produced then sustains the chain-reaction by creating another CH3O·H2. A relatively stable intermediate (·CH2OCH2OOH), formed via isomerization of CH3OCH2OO· in the chain-propagation step, can combine with a second O2 to produce a radical (·OOCH2OCH2OOH) that can potentially decompose into two ·OH radical (and other products). This path leads to chain-branching and an exponential increase in the rate of DME oxidation. We have used spin-polarized density functional theory with the Becke-3-parameter Lee,Parr,Yang exchange-correlation functional to calculate the structures and energies of key reactants, intermediates, and products involved in (and competing with) the chain-propagating and chain-branching steps of low-temperature DME oxidation. In this article, Part I, we consider only the chain-propagation mechanism and its competing mechanisms for DME combustion. Here, we show that only certain conformers can undergo the isomerization to ·CH2OCH2OOH. A new transition state has been discovered for the disproportionation reaction ·CH2OCH2OOH , 2CH2O + ·OH in the chain-propagating step of DME autoignition that is much lower than previous barriers. The key to making this decomposition pathway facile is initial cleavage of the O,O rather than the C,O bond. This renders all transition states along the chain-propagation potential energy surface below the CH3O·H2 + O2 reactants. In contrast with the more well-studied CH3·H2 (ethyl radical) + O2 system, the H-transfer isomerization of CH3OCH2OO· to ·CH2OCH2OOH in low-temperature DME oxidation has a much lower activation energy. This is most likely due to the larger ring strain of the analogous transition state in ethane oxidation, which is a five-membered ring opposed to a six-membered ring in dimethyl ether oxidation. Thus low-temperature ethane oxidation is much less likely to form the ·ROOH (where R is a generic group) radicals necessary for chain-branching, which leads to autoignition. Three competing reactions are considered: CH3O·H2 , CH2O + ·CH3; ·CH2OCH2OOH , 1,3-dioxetane + ·OH; and ·CH2OCH2OOH , ethylene oxide + HOO·. The reaction barriers of all these competing paths are much higher in energy (7,10 kcal/mol) than the reactants CH3O·H2 + O2 and, therefore, are unlikely low-temperature paths. Interestingly, an analysis of the highest occupied molecular orbital along the CH3O·H2 decomposition path shows that electronically excited (1A2 or 3A2) CH2O can form; this can also be shown for ·CH2OCH2OOH, which forms two formaldehyde molecules. This may explain the luminosity of DME's low-temperature flames. [source] Anomeric information obtained from a series of synthetic trisaccharides using energy resolved mass spectraJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2007Shusaku Daikoku Abstract The majority of structural investigations of oligosaccharides based on mass spectrometry use naturally occurring oligosaccharides, which do not allow extracting any common feature associated with anomeric structures and linkage positions. In order to address the issue to find such characteristics possibly contained in oligosaccharide structure, a synthetic combinatorial trisaccharide library was analyzed. The trisaccharides used in the analysis consisted of L -fucose, D -galactose and D -glucose, in which individual glycosidic linkages existed in either ,- or ,-anomers. The analysis of energy-resolved mass spectra (ERMS) and the scattered plot analysis of some parameters obtained from ERMS for a series of trisaccharides revealed that lower activation energy was required for the dissociation of ,-glycosides of these sugars compared to those of the corresponding ,-anomers. It is suggested that this finding may be useful in structural analysis of natural oligosaccharides. Copyright © 2007 John Wiley & Sons, Ltd. [source] Modeling of Slurry Polymerization of Ethylene Using a Soluble Cp2ZrCl2/MAO Catalytic SystemMACROMOLECULAR THEORY AND SIMULATIONS, Issue 5 2007Mostafa Ahmadi Abstract The slurry homopolymerization of ethylene catalyzed by a Cp2ZrCl2/MAO catalytic system was studied. A simple kinetic model including initiation, propagation, transfer to monomer and cocatalyst, spontaneous transfer and spontaneous deactivation was developed to predict dynamic yield of polymerization and molecular weight of final products. Kinetic constants were estimated by numerical solution of polymerization kinetic model, combined with Nelder-Mead simplex method. The model predicts that the propagation reaction has the lower activation energy in relation to chain transfer reactions which leads to decrease of molecular weight at elevated temperatures. The initiation reaction has however, the highest activation energy that results in raising the peak of reaction rate at higher temperatures. [source] Determination of the Catalytic Pathway of a Manganese Arginase Enzyme Through Density Functional InvestigationCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2009Monica Leopoldini Dr. Abstract The catalytic mechanism of dimanganese-containing arginase enzyme has been investigated by DFT calculations. Two exchange-correlation functionals, B3,LYP and MPWB1,K, have been used to construct the potential energy profiles for the hydrolysis of an arginine substrate performed by an arginase active site model system. Two reaction mechanisms have been investigated, one involving a water molecule (mechanism,1) and the other involving a hydroxide ion (mechanism,2) as nucleophilic agent. Results obtained in the gas phase and in the protein environment have indicated that mechanism,1 involving a water molecule entails structural features as well as an activation energy for the rate-determining step that are inconsistent with experimental data available for the arginase enzyme. On the other hand, when a hydroxide ion is present at the Mn2 site, a lower activation energy and a structural arrangement closer to the experimental indication are obtained. [source] | |||||||||||||