Cyclic Reduction Research Articles

Thermal analysis of the cyclic reduction and oxidation behaviour of SOFC anodes

The reduction and oxidation (redox) kinetics of a Ni/YSZ cermet were studied at temperatures between 400 and 850 °C by thermogravimetric analysis (TGA) and reduction and oxidation activation energies were determined. In addition, the volume change after redox cycling was examined by thermomechanical analysis (TMA). Samples with fine microstructure similar to a typical SOFC anode functional layer (AFL) and samples with coarse microstructure similar to a typical anode substrate were characterized. Coarse structured samples experienced no volume change or cracking upon redox cycling. Fine structured anode samples did not change in volume after reduction, but expanded between 0.9% and 2.5% after oxidation. The samples were significantly cracked after oxidation. The amount of expansion and cracking was reduced by lowering the Ni content of the anode and by reducing the oxidation temperature. The reduction of the NiO/YSZ samples followed linear kinetics with an activation energy of 78 kJ/mol. Ni/YSZ oxidation followed parabolic kinetics at temperatures lower than 700 °C. A divergence from parabolic kinetics was seen at higher temperatures (700–850 °C). This divergence has been seen in many other kinetic studies and is usually attributed to short-circuit diffusion mechanisms. An activation energy of 87 kJ/mol was calculated for oxidation.

Initial Testing of Solutions to Redox Problems with Anode-Supported Solid Oxide Fuel Cells

The most commonly used solid oxide fuel cell (SOFC) anode material is a two phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. During typical fuel cell operation, this material remains a cermet; however, in a commercial SOFC system seal leakage, fuel supply interruption, or other situations may cause the anode to reoxidize. The cyclic reduction and oxidation (redox) of nickel will result in large bulk volume changes, which may have a significant effect on the integrity of interfaces within the fuel cell and thus may cause significant performance degradation. Following initial study of the degradation mechanisms and redox kinetics, a number of solutions have been devised. These can be broadly categorised into system solutions and unit cell or materials solutions. The former includes purge gas, other means to produce a reducing gas, cell reversal and passive solutions such as hydrides, oxygen getters and steam purging. The latter includes alternative anode materials, microstructural modification of anode functional layers (AFLs), graded porosity and composition of AFL and redox barrier layers. This paper discusses the pros and cons of each approach and some initial feasibility testing in short stack repeat units.

Shift Techniques and Canonical Factorizations in the Solution of M/G/1-Type Markov Chains

ABSTRACT By using properties of canonical factorizations, we prove that under very mild assumptions, the shifted cyclic reduction method (SCR) can be applied for solving QBD problems with no breakdown and that it always converges. For general M/G/1 type Markov chains we prove that SCR always converges if no breakdown is encountered. Numerical experiments showing the acceleration provided by SCR versus cyclic reduction are presented.

Antiplatelet and Antithrombotic Activity of Cilostazol is Potentiated by Dipyridamole in Rabbits and Dissociated from Bleeding Time Prolongation

To determine the antiplatelet effect of cilostazol (Pletal) and its interaction with dipyridamole in in vitro and in vivo rabbit models, and to see if it can be dissociated from bleeding time prolongation. In vitro collagen-induced platelet aggregation was measured by an impedance-based aggregometer. The in vivo antithrombotic effect was evaluated in a rabbit carotid artery cyclic flow reduction (CFR) model, in which repetitive thrombosis was induced by mechanical injuries of the artery and stenosis. Template bleeding time was determined in rabbit ear arterioles and hindlimb nail cuticles. In vitro platelet aggregation was slightly inhibited by 4 microM cilostazol (22 +/- 6%), and modestly by 13 microM (57 +/- 3% of aggregation). While dipyridamole itself up to 13 microM had no significant inhibition, it potentiated the effect from cilostazol: in the presence of 4 microM dipyridamole, 4 microM cilostazol inhibited aggregation by 47 +/- 6%. Dipyridamole also potentiated the CFR reducing effect of cilostazol: combination of dipyridamole (no effect by itself) and cilostazol at 1 microM decreased CFRs to levels achieved by 3-4 microM cilostazol alone. Bleeding times were similar in controls and animals treated with cilostazol, or with cilostazol and dipyridamole. In contrast, aspirin (4 mg/kg), while reducing CFRs, significantly increased bleeding time. These results suggest that dipyridamole potentiates the antiplatelet effect of cilostazol without prolongation of the bleeding time, implying a potential novel combination antithrombotic therapy.

Al-Isopropoxydiisobutylalane: A Study of the Effect of Solvent on the Rate and Stereoselectivity of Cyclic Ketone Reduction

The effect of solvent on the rate and stereoselectivity of cyclic ketone reduction by Al-isopropoxydiisobutylalane (DIBA(i)OPr) has been investigated. In dichloromethane, DIBA(i)OPr behaves as a bulky reducing agent, approaching the carbonyl group along an equatorial trajectory to produce the axial alcohol with >10:1 stereoselectivity. In sharp contrast, reduction in toluene gives the complementary outcome, affording the thermodynamically more stable isomer with >99:1 stereoselectivity.

Temperature variations in the oxygen carrier particles during their reduction and oxidation in a chemical-looping combustion system

A particle reaction model including mass and heat transfer has been developed to know the temperature variations produced inside the oxygen carrier particles during the cyclic reduction and oxidation reactions taking place in a chemical-looping combustion (CLC) system. The reactions of the different oxygen carriers based on Cu, Co, Fe, Mn, and Ni during the reduction with fuel gas ( CH 4 , CO, and H 2 ) and oxidation ( O 2 ) have been considered. In these systems, the oxidation reaction is always exothermic with subsequent heat release; however, the reduction reaction can be exothermic or endothermic depending on the metal oxide and the fuel gas. The heat generated inside the oxygen carriers during the exothermic reactions increases the particle temperature, and could affect the particle structure if the temperature increase is near to the melting point of the active materials. Several variables that affect the reaction rate and the heat transport process have been analyzed to know their effect on the internal particle temperature. For a given oxygen carrier and reaction, the maximum temperature of the particles depended mainly on the particle size, the reaction rate, and the external heat transfer resistance, being lower than the effect of the oxygen carrier porosity, type of inert material, and metal oxide content. The highest temperature variations were reached for the oxidation reactions, with the maximum corresponding to the Ni and Co oxygen carriers with values of ∼ + 90 K for 1 mm particles. The highest temperature increase observed during the reduction reactions corresponded to the reaction of CuO with CO, with values of ∼ + 40 K for 1 mm particles. For the rest of the reactions and metals, the variations in the particle temperature were below 10 K for particle sizes below 1 mm . Under the typical operating conditions that exist in a CLC system, with particle sizes lower than 0.3 mm , 40 wt % of metal oxide content, and overall conversion times lower than 30 s , the increases of temperature with respect to the bulk conditions were lower than 15 K for any reaction of any oxygen carrier. Moreover, the temperature profiles inside the particles were near flat in most of the practical conditions, and no local points with high temperatures were found. Thus, changes in the solid porous structure of the carrier due to sintering during oxidation in fluidized bed reactors are not expected working at typical temperatures of CLC systems (1000– 1300 K ).

Optimal and Efficient Parallel Tridiagonal Solvers Using Direct Methods

The problem of solving tridiagonal linear systems on parallel distributed-memory environments is considered in this paper. In particular, two common direct methods for solving such systems are considered: odd-even cyclic reduction and prefix summing. For each method, a variety of lower bounds on execution time for solving tridiagonal linear systems are presented. Specifically, lower bounds are presented that (a) hold when the number of data items per processor is bounded, (b) are general lower bounds, and (c) for specific data layouts commonly used in designing parallel algorithms to solve tridiagonal linear systems. Furthermore, algorithms are presented that have running times within a constant factor of the lower bounds provided. Lastly, a comparison of bounds for odd-even cyclic reduction and prefix summing is given.

Activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction

We report here on the high activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction (WGS). These catalysts are reversible in cyclic reduction–oxidation treatment up to 400°C, are non-pyrophoric, and are thus potential candidates for application to hydrogen generation for fuel cell power production. Low-content (0.2–0.9at.%) gold–ceria samples were prepared by single-pot synthesis by the urea gelation/coprecipitation method; and by sodium cyanide leaching of high-content (2–8at.%) gold–ceria materials prepared by various techniques. The low-content gold–ceria catalysts are free of metallic gold nanoparticles. Gold is present in oxidized form, as verified by a variety of analytical techniques. However, these materials display the same WGS activity as the high-content gold ones, and remain free of gold nanoparticles after use in a reaction gas stream composed of 11% CO–26% H2O–26% H2–7% CO2–balance He up to 300°C. We show that the determining factor for the retention of active gold in ceria is the surface properties of the latter. Measurements of lattice constant expansion indicate gold ion substitution in the ceria lattice. The turnover frequency of WGS under the assumption of fully dispersed gold is the same for a variety of low-content gold–ceria preparations. The stability of gold–ceria in various gas compositions and temperatures was good. The most serious stability issue is formation of cerium hydroxycarbonate in shutdown operation.

Solving quadratic matrix equations and factoring polynomials: new fixed point iterations based on Schur complements of Toeplitz matrices

AbstractA new class of functional iterations is introduced for the numerical solution of the matrix equation C+AX+BX2=0 where A,B,C are given m × m matrices and X is the unknown matrix. Each iteration in this class is globally convergent, self‐correcting, and the local convergence is linear with an arbitrarily large speed. The new iterations, which rely on the properties of Schur complements of block tridiagonal block Toeplitz matrices, overcome the problems encountered by quadratically convergent methods, like cyclic reduction, in the case of numerical singularity situations. Copyright © 2004 John Wiley & Sons, Ltd.

Plastic deformation of Al-4.5 wt% Cu and Al-4.5 wt% Cu-0.1 wt% In alloys under the effect of cyclic stress reduction

The plastic deformation behavior of Al–4.5 wt% Cu and Al–4.5 wt% Cu–0.1 wt% In alloys under cyclic stress reduction was investigated by studying the creep behavior. The influence of cyclic stress reduction of increasing amplitudes and frequencies was systematically examined at various working temperatures. Both transient and steady-state creep were found to be higher under cyclic stress reduction conditions than under static conditions for the same peak stress. An enhancement of the creep rate was observed above either threshold stress or threshold frequency. A dislocation intersections mechanism was assumed as the rate-controlling mechanism for both creep stages. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of electrode materials on the kinetics of the electro-reduction of peroxyacetic acid

Electrochemical behavior of peroxyacetic acid (PAA) and hydrogen peroxide (H 2O 2) was examined at various metal and carbon electrodes (i.e., Au, Ag, Cu, Pt, Pd, Rh, Ti, W, Hg, Ni, Fe, glassy carbon (GC), and basal-plane pyrolytic graphite (BPG)) in 0.1 M acetate buffer solution (pH 5.5) using potentiostatic (i.e., cyclic voltammetry and rotating disk electrode voltammetry) and galvanostatic techniques. It was found that the electro-reductions of PAA and H 2O 2 are highly sensitive to electrode material. Both species were found to be electrochemically and separately reduced at Au, Ag, Cu, Pt, Pd, GC, and BPG electrodes. On the other hand, at Fe, Ni, Hg, Rh, Ti, and W electrodes, voltammetric response for the PAA reduction was not obviously observed. The kinetics of electro-reduction of PAA in 0.1 M acetate buffer solution was studied at Au, Ag, and GC electrodes in details, and the relevant kinetic parameters (i.e., the exchange current density, j 0, the standard rate constant, k 0, and cathodic transfer coefficient, α c) were estimated from the Tafel plots. The cyclic voltammetric reduction peak potentials obtained for the PAA reduction at Au, Ag, and GC electrodes were compared with those calculated using the kinetic and thermodynamic parameters obtained under the same experimental conditions. The measured and calculated reduction peak potentials at each electrode were found to be in agreement with each other, indicating that the evaluated values of kinetic parameters for the reduction of PAA at Au, Ag, and GC electrodes are reasonable.

Effect of electrode materials on the kinetics of the electro-reduction of peroxyacetic acid

Electrochemical behavior of peroxyacetic acid (PAA) and hydrogen peroxide (H2O2) was examined at various metal and carbon electrodes (i.e., Au, Ag, Cu, Pt, Pd, Rh, Ti, W, Hg, Ni, Fe, glassy carbon (GC), and basal-plane pyrolytic graphite (BPG)) in 0.1 M acetate buffer solution (pH 5.5) using potentiostatic (i.e., cyclic voltammetry and rotating disk electrode voltammetry) and galvanostatic techniques. It was found that the electro-reductions of PAA and H2O2 are highly sensitive to electrode material. Both species were found to be electrochemically and separately reduced at Au, Ag, Cu, Pt, Pd, GC, and BPG electrodes. On the other hand, at Fe, Ni, Hg, Rh, Ti, and W electrodes, voltammetric response for the PAA reduction was not obviously observed. The kinetics of electro-reduction of PAA in 0.1 M acetate buffer solution was studied at Au, Ag, and GC electrodes in details, and the relevant kinetic parameters (i.e., the exchange current density, j0, the standard rate constant, k0, and cathodic transfer coefficient, αc) were estimated from the Tafel plots. The cyclic voltammetric reduction peak potentials obtained for the PAA reduction at Au, Ag, and GC electrodes were compared with those calculated using the kinetic and thermodynamic parameters obtained under the same experimental conditions. The measured and calculated reduction peak potentials at each electrode were found to be in agreement with each other, indicating that the evaluated values of kinetic parameters for the reduction of PAA at Au, Ag, and GC electrodes are reasonable.

Graph Transformation and Designing Parallel Sparse Matrix Algorithms beyond Data Dependence Analysis

Algorithms are often parallelized based on data dependence analysis manually or by means of parallel compilers. Some vector/matrix computations such as the matrix‐vector products with simple data dependence structures (data parallelism) can be easily parallelized. For problems with more complicated data dependence structures, parallelization is less straightforward. The data dependence graph is a powerful means for designing and analyzing parallel algorithms. However, for sparse matrix computations, parallelization based on solely exploiting the existing parallelism in an algorithm does not always give satisfactory results. For example, the conventional Gaussian elimination algorithm for the solution of a tri‐diagonal system is inherently sequential, so algorithms specially for parallel computation has to be designed. After briefly reviewing different parallelization approaches, a powerful graph formalism for designing parallel algorithms is introduced. This formalism will be discussed using a tri‐diagonal system as an example. Its application to general matrix computations is also discussed. Its power in designing parallel algorithms beyond the ability of data dependence analysis is shown by means of a new algorithm called ACER (Alternating Cyclic Elimination and Reduction algorithm).

A note on computing the matrix square root

We revisit recent algorithms for computing matrix square roots and relate them to Newton iteration. New iterations are derived and a stability analysis is performed. A suitable scaling of an algorithm based on cyclic reduction is introduced, which removes the slow convergence of this iteration encountered in certain cases.

TRIDIAGONAL SOLVERS WITH MULTIPLE RIGHT HAND SIDES ON k-DIMENSIONAL MESH AND TORUS INTERCONNECTION NETWORKS

We consider the problem of designing optimal and efficient algorithms for solving tridiagonal linear systems with multiple right-hand side vectors on k-dimensional mesh and torus interconnection networks. We derive asymptotic upper and lower bounds for these solvers using odd-even cyclic reduction. We present various important bounds on execution time including general lower bounds which are independent of initial data assignment, and lower bounds based on classifying assignments via the proportion of initial data assigned amongst processors. Finally, different algorithms are designed in order to achieve running times that are within a small constant factor of the lower bounds provided.

Cyclic oxidation and reduction of methionine residues of proteins in antioxidant defense and cellular regulation

Methionine is one of the main targets for biological oxidants. Its reaction with the majority of oxidants generates only methionine sulfoxide. However, when N-terminal methionine reacts with hypohalous acids (HOCl and HOBr) or singlet molecular oxygen (1O2), it can also generate a cyclic product called dehydromethionine (DHM). Previously, DHM was suggested as a biomarker of oxidative stress induced by hypohalous acids. However, DHM can also be generated by 1O2 –oxidation of methionine, and the contribution of this pathway of DHM formation in a context of a site-specific redox imbalance in an organism is unknown. In this work, a through comparison of the reactions of hypohalous acids and 1O2 with methionine, either free or inserted in peptides and proteins was undertaken. In addition, we performed methionine photooxidation in heavy water (H218O) to determine the influence of the pH in the mechanism of DHM formation.We showed that for free methionine, or methionine-containing peptides, the yields of DHM formation in the reactions with 1O2 were close to those achieved by HOBr oxidation, but much higher than the yields obtained with HOCl as the oxidant. This was true for all pH tested (5, 7.4, and 9). Interestingly, for the protein ubiquitin, DHM yields after reaction with 1O2 were higher than those obtained with both hypohalous acids. Our results indicate that 1O2 may also be an important source of DHM in biological systems.

03/01638 Cyclic carbon monoxide combustion-carbon dioxide reduction for petroleum recovery by retorting of oil shale: Peterson, O. V. U.S. US 6,464,8611 (Cl. 208–400; C1OG1/00), 15 Oct 2002, Appl. 610,842

03/01638 Cyclic carbon monoxide combustion-carbon dioxide reduction for petroleum recovery by retorting of oil shale: Peterson, O. V. U.S. US 6,464,8611 (Cl. 208–400; C1OG1/00), 15 Oct 2002, Appl. 610,842

Functional split and crosslinking of the membrane domain of the beta subunit of proton-translocating transhydrogenase from Escherichia coli.

Proton pumping nicotinamide nucleotide transhydrogenase from Escherichia coli contains an alpha subunit with the NAD(H)-binding domain I and a beta subunit with the NADP(H)-binding domain III. The membrane domain (domain II) harbors the proton channel and is made up of the hydrophobic parts of the alpha and beta subunits. The interface in domain II between the alpha and the beta subunits has previously been investigated by cross-linking loops connecting the four transmembrane helices in the alpha subunit and loops connecting the nine transmembrane helices in the beta subunit. However, to investigate the organization of the nine transmembrane helices in the beta subunit, a split was introduced by creating a stop codon in the loop connecting transmembrane helices 9 and 10 by a single mutagenesis step, utilizing an existing downstream start codon. The resulting enzyme was composed of the wild-type alpha subunit and the two new peptides beta1 and beta2. As compared to other split membrane proteins, the new transhydrogenase was remarkably active and catalyzed activities for the reduction of 3-acetylpyridine-NAD(+) by NADPH, the cyclic reduction of 3-acetylpyridine-NAD(+) by NADH (mediated by bound NADP(H)), and proton pumping, amounting to about 50-107% of the corresponding wild-type activities. These high activities suggest that the alpha subunit was normally folded, followed by a concerted folding of beta1 + beta2. Cross-linking of a betaS105C-betaS237C double cysteine mutant in the functional split cysteine-free background, followed by SDS-PAGE analysis, showed that helices 9, 13, and 14 were in close proximity. This is the first time that cross-linking between helices in the same beta subunit has been demonstrated.

Free radical-mediated oxidation of free amino acids and amino acid residues in proteins.

We summarize here results of studies designed to elucidate basic mechanisms of reactive oxygen (ROS)-mediated oxidation of proteins and free amino acids. These studies have shown that oxidation of proteins can lead to hydroxylation of aromatic groups and aliphatic amino acid side chains, nitration of aromatic amino acid residues, nitrosylation of sulfhydryl groups, sulfoxidation of methionine residues, chlorination of aromatic groups and primary amino groups, and to conversion of some amino acid residues to carbonyl derivatives. Oxidation can lead also to cleavage of the polypeptide chain and to formation of cross-linked protein aggregates. Furthermore, functional groups of proteins can react with oxidation products of polyunsaturated fatty acids and with carbohydrate derivatives (glycation/glycoxidation) to produce inactive derivatives. Highly specific methods have been developed for the detection and assay of the various kinds of protein modifications. Because the generation of carbonyl derivatives occurs by many different mechanisms, the level of carbonyl groups in proteins is widely used as a marker of oxidative protein damage. The level of oxidized proteins increases with aging and in a number of age-related diseases. However, the accumulation of oxidized protein is a complex function of the rates of ROS formation, antioxidant levels, and the ability to proteolytically eliminate oxidized forms of proteins. Thus, the accumulation of oxidized proteins is also dependent upon genetic factors and individual life styles. It is noteworthy that surface-exposed methionine and cysteine residues of proteins are particularly sensitive to oxidation by almost all forms of ROS; however, unlike other kinds of oxidation the oxidation of these sulfur-containing amino acid residues is reversible. It is thus evident that the cyclic oxidation and reduction of the sulfur-containing amino acids may serve as an important antioxidant mechanism, and also that these reversible oxidations may provide an important mechanism for the regulation of some enzyme functions.

Cyclic carbon monoxide combustion-carbon dioxide reduction for petroleum recovery by retorting of oil shale : Peterson, O. V. U.S. US 6,464,860 (Cl. 208–400; C10GI/00), 15 Oct 2002, Appl. 610,842

Cyclic carbon monoxide combustion-carbon dioxide reduction for petroleum recovery by retorting of oil shale : Peterson, O. V. U.S. US 6,464,860 (Cl. 208–400; C10GI/00), 15 Oct 2002, Appl. 610,842