A thermogravimetric study of the oxidative growth of Al2O3/Al alloy composites

The oxidation of liquid Al–Mg–Si alloys at 900–1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl2O4 allows the continuous formation of an Al2O3-matrix composite containing an interpenetrating network of metal microchannels at 1000–1350 °C. An initial incubation period precedes bulk oxidation, wherein Al2O3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ~1300 °C, with a nominal activation energy of 270 kJ/mole for an Al-2.85 wt. % Mg-5.4 wt. % Si alloy in O2 at furnace temperatures of 1000–1300 °C. An oscillatory rate regime observed at 1000–1075 °C resulted in a banded structure of varying Al2O3-to-metal volume fraction.

Lewis Acid-Lewis Base Mediated Metal-Free Hydrogen Activation and Catalytic Hydrogenation

Organocatalysis, the use of organic molecules as catalysts, is attracting increasing attention as one of the most modern and rapidly growing areas of organic chemistry, with countless research groups in both academia and the pharmaceutical industry around the world working on this subject. The literature review of this thesis mainly focuses on metal-free systems for hydrogen activation and organocatalytic reduction. Since these research topics are relatively new, the literature review also highlights the basic principles of the use of Lewis acid-Lewis base pairs, which do not react irreversibly with each other, as a trap for small molecules. The experimental section progresses from the first observation of the facile heterolytical cleavage of hydrogen gas by amines and B(C6F5)3 to highly active non-metal catalysts for both enantioselective and racemic hydrogenation of unsaturated nitrogen-containing compounds. Moreover, detailed studies of structure-reactivity relationships of these systems by X-ray, neutron diffraction, NMR methods and quantum chemical calculations were performed to gain further insight into the mechanism of hydrogen activation and hydrogenation by boron-nitrogen compounds.

Catalytic hydrogen combustion for treatment of combustible gases from fuel cell processors

Catalytic combustion of H-2 was carried out over combustion synthesized noble metal (Pd or Pt) ion-substituted CeO2 based catalysts using a feed stream that simulated exhaust gases from a fuel cell processor The catalysts showed a high activity for H-2-combustion and complete conversion was achieved below 200 C over all the catalysts when O-2 was used in a stoichiometric amount With higher amounts of O-2 the reaction rates Increased and complete conversions were possible below 100 C The reaction was also carried out over Pd-impregnated CeO2 The conversions of H-2 with stoichiometric amount of O-2 were found to be higher over Pd-substituted compound The mechanism of the reaction over noble metal-substituted compounds was proposed on the basis of X-ray photoelectron spectroscopy studies The redox couples between Ce and metal ions were established and a dual site redox mechanism was pi posed for the reaction (C) 2010 Elsevier B V All rights reserved

Functional Mimics of Glutathione Peroxidase: Bioinspired Synthetic Antioxidants

Oxidative stress is caused by an imbalance between the production of reactive oxygen species (ROS) and the biological system's ability to detoxify these reactive intermediates. Mammalian cells have elaborate antioxidant defense mechanisms to control the damaging effects of ROS. Glutathione peroxidase (GPx), a selenoenzyme, plays a key role in protecting the organism from oxidative damage by catalyzing the reduction of harmful hydroperoxides with thiol a ``catalytic triad'' with tryptophan and glutamine, which cofactors. The selenocysteine residue at the active site forms activates the selenium moiety for an efficient reduction of peroxides. After the discovery that ebselen, a synthetic organoselenium compound, mimics the catalytic activity of GPx both in vitro and in vivo, several research groups developed a number of small-molecule selenium compounds as functional mimics of GPx, either by modifying the basic structure of ebselen or by incorporating some structural features of the native enzyme. The synthetic mimics reported in the literature can be classified in three major categories: (i) cyclic selenenyl amides having a Se-N bond, (ii) diaryl diselenides, and (iii) aromatic or aliphatic monoselenides. Recent studies show that ebselen exhibits very poor GPx activity when aryl or benzylic thiols such as PhSH or BnSH are used as cosubstrates. Because the catalytic activity of each GPx mimic largely depends on the thiol cosubstrates used, the difference in the thiols causes the discrepancies observed in different studies. In this Account, we demonstrate the effect of amide and amine substituents on the GPx activity of various organoselenium compounds. The existence of strong Se ... O/N interactions in the selenenyl sulfide intermediates significantly reduces the GPx activity. These interactions facilitate an attack of thiol at selenium rather than at sulfur, leading to thiol exchange reactions that hamper the formation of catalytically active selenol. Therefore, any substituent capable of enhancing the nucleophilic attack of thiol at sulfur in the selenenyl sulfide state would enhance the antioxidant potency of organoselenium compounds. Interestingly, replacement of the sec-amide substituent by a tert-amide group leads to a weakening of Se ... 0 interactions in the selenenyl sulfide intermediates. This modification results in 10- to 20-fold enhancements in the catalytic activities. Another strategy involving the replacement of tert-amide moieties by tert-amino substituents further increases the activity by 3- to 4-fold. The most effective modification so far in benzylamine-based GPx mimics appears to be either the replacement of a tert-amino substituent by a sec-amino group or the introduction of an additional 6-methoxy group in the phenyl ring. These strategies can contribute to a remarkable enhancement in the GPx activity. In addition to enhancing catalytic activity, a change in the substituents near the selenium moiety alters the catalytic mechanisms. The mechanistic investigations of functional mimics are useful not only for understanding the complex chemistry at the active site of GPx but also for designing and synthesizing novel antioxidants and anti-inflammatory agents.

High-Surface Step Density on Dendritic Pd Leads to Exceptional Catalytic Activity for Formic Acid Oxidation

Dendrite Pd with corrugated surfaces, obtained by a novel AC technique, exhibits an exceptionally high catalytic activity for the oxidation of formic acid because of the presence of a high density of surface steps. The formation of twinned dendrites leads to a predominance of exposed 111 facets with a high density of surface steps as evident from high resolution electron microscopy investigations. These surface sites provide active sites for the absorption of the formic acid molecules, thereby enhancing the reaction rate. Control experiments by varying the time of deposition reveal the formation of partially grown dendrites at shorter times indicating that the dendrites were formed by growth rather than particle attachment. Our deposition method opens up interesting possibilities to produce artisotropic nanostructures with corrugated surfaces by exploiting the perturbations involved in the growth process.

Influence of cerebral ischemia and post-ischemic reperfusion on mitochondrial oxidative phosphorylation

Unilateral ischemia in the right cerebral hemisphere of the rat was induced by ligation of the right common carotid artery coupled with controlled hemorrhage to produce hypotension (25±8 mm/Hg). Where indicated after 30 min of ischemia, the withdrawn blood was reinfused to restore arterial pressure to normal. Mitochondria isolated from the ipsilateral hemisphere after 30 min of ischemia showed significantly lower respiratory rates than the organelles isolated from the contralateral side. Oxidation of NAD+-linked substrates was more sensitive to inhibition in ischemia (30%) than was of ferrocytochromec (12%), succinate oxidation being intermediate. The activities of membrane-bound dehydrogenases (both NADH and succinate-linked) were also significantly lowered. Ischemia did not affect the cytochrome content of mitochondria. Respiratory activity (NAD+-linked) of mitochondria isolated from the ipsilateral hemisphere was twice as sensitive to inhibition by fatty acid as was of preparations from the contralateral side. Mitochondria isolated from cerebral cortex after 90 min of post-ischemic reperfusion showed no significant improvement in the rate of substrate oxidation. Adenine nucleotide translocase activity and energy-dependent Ca2+ uptake, both of which decreased significantly in mitochondria isolated from the ischemic brain, showed little recovery, on reperfusion. These observations suggested the strong possibility that the deleterious effects of ischemia on mitochondrial respiratory function might be mediated by free fatty acids that are known to accumulate in large amounts in ischemic tissues. The pattern of inhibition of ATPase activity was consistent with this view.

Noble metal ionic sites for catalytic hydrogen combustion: spectroscopic insights

A catalytic hydrogen combustion reaction was carried out over noble metal catalysts substituted in ZrO2 and TiO2 in ionic form. The catalysts were synthesized by the solution combustion technique. The compounds showed high activity and CO tolerance for the reaction. The activity of Pd and Pt ion substituted TiO2 was comparable and was higher than Pd and Pt ion substituted ZrO2. The mechanisms of the reaction over the two supports were proposed by making use of the X-ray photoelectron spectroscopy and FT infrared spectroscopic observations. The reaction over ZrO2 supported catalysts was proposed to take place by the utilization of the surface hydroxyl groups while the reaction over TiO2 supported catalysts was hypothesized to be a hybrid mechanism utilizing surface hydroxyl groups and the lattice oxygen.

Oxidative copolymerization: microstructure analysis of the terpolymer of styrene, methyl methacrylate, and oxygen

This paper reports the first study of the microstructure of a copolyperoxide by nuclear magnetic resonance spectroscopy. The copolyperoxides of styrene and methyl methacrylate (MMA) of various compositions have been synthesized. An analysis of the resonance signal of the backbone methylene protons gave the diad sequence probabilities which led to the calculation of the oxidative copolymerization reactivity ratios for styrene and MMA and the microstructural parameters like average chain length of the repeat unit sequences, run number, etc. The results point to the tendency of the SO1 and MO:! units to alternate in the chain. Compared to poly(styrene peroxide), the aromatic C1 seems to be stereosensitive in the terpolymers.

Dynamic coupling between the LID and NMP domain motions in the catalytic conversion of ATP and AMP to ADP by adenylate kinase

The catalytic conversion of adenosine triphosphate (ATP) and adenosine monophosphate (AMP) to adenosine diphosphate (ADP) by adenylate kinase (ADK) involves large amplitude, ligand induced domain motions, involving the opening and the closing of ATP binding domain (LID) and AMP binding domain (NMP) domains, during the repeated catalytic cycle. We discover and analyze an interesting dynamical coupling between the motion of the two domains during the opening, using large scale atomistic molecular dynamics trajectory analysis, covariance analysis, and multidimensional free energy calculations with explicit water. Initially, the LID domain must open by a certain amount before the NMP domain can begin to open. Dynamical correlation map shows interesting cross-peak between LID and NMP domain which suggests the presence of correlated motion between them. This is also reflected in our calculated two-dimensional free energy surface contour diagram which has an interesting elliptic shape, revealing a strong correlation between the opening of the LID domain and that of the NMP domain. Our free energy surface of the LID domain motion is rugged due to interaction with water and the signature of ruggedness is evident in the observed root mean square deviation variation and its fluctuation time correlation functions. We develop a correlated dynamical disorder-type theoretical model to explain the observed dynamic coupling between the motion of the two domains in ADK. Our model correctly reproduces several features of the cross-correlation observed in simulations. (C) 2011 American Institute of Physics. doi:10.1063/1.3516588]