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In this work, a reliable kinetic reaction mechanism was revised to accurately reproduce the detailed reaction paths of steam reforming of methane over a Ni/Al2O3 catalyst. A steady-state fixed-bed reactor experiment and a 1D reactor catalyst model were utilized for this task. The distinctive feature of this experiment is the possibility to measure the axially resolved temperature profile of the catalyst bed, which makes the reaction kinetics inside the reactor visible. This allows for understanding the actual influence of the reaction kinetics on the system; while pure gas concentration measurements at the catalytic reactor outlet show near-equilibrium conditions, the inhere presented temperature profile shows that it is insufficient to base a reaction mechanism development on close equilibrium data. The new experimental data allow for achieving much higher quality in the modeling efforts. Additionally, by carefully controlling the available active surface via dilution in the experiment, it was possible to slow down the catalyst conversion rate, which helped during the adjustment of the reaction kinetics. To assess the accuracy of the revised mechanism, a monolith experiment from the literature was simulated. The results show that the fitted reaction mechanism was able to accurately predict the experimental outcomes for various inlet mass flows, temperatures, and steam-to-carbon ratios.

There exists much uncertainty surrounding interstellar grain-surface chemistry. One of the major reaction mechanisms is grain-surface diffusion for which the the binding energy parameter for each species needs to be known. However, these values vary significantly across the literature which can lead to debate as to whether or not a particular reaction takes place via diffusion. In this work we employ Bayesian inference to use available ice abundances to estimate the reaction rates of the reactions in a chemical network that produces glycine. Using this we estimate the binding energy of a variety of important species in the network, by assuming that the reactions take place via diffusion. We use our understanding of the diffusion mechanism to reduce the dimensionality of the inference problem from 49 to 14, by demonstrating that reactions can be separated into classes. This dimensionality reduction makes the problem computationally feasible. A neural network statistical emulator is used to also help accelerate the Bayesian inference process substantially. The binding energies of most of the diffusive species of interest are found to match some of the disparate literature values, with the exceptions of atomic and diatomic hydrogen. The discrepancies with these two species are related to limitations of the physical and chemical model. However, the use of a dummy reaction of the form H + X -> HX is found to somewhat reduce the discrepancy with the binding energy of atomic hydrogen. Using the inferred binding energies in the full gas-grain version of UCLCHEM results in almost all the molecular abundances being recovered., 21 pages, 7 figures, accepted for publication in ApJ

The distribution of rheologically strong cratons, and their weakening by metamorphic hydration reactions, is of fundamental importance for understanding first���order strength contrasts within the crust and the resulting controls on the tectonic evolution of the continents. In this study, the Douglas Harbor structural window within the Paleoproterozoic Trans���Hudson orogen of Canada is used to study the hydration of the footwall Archean Superior craton basement by water released from the overlying Paleoproterozoic Cape Smith thrust���fold belt. Phase equilibria modeling is applied to quantify the Archean and Paleoproterozoic metamorphic conditions, and to determine the effect of hydration on basement mineralogy. The amount of structurally bound water within the basement is calculated and shown to decrease as a function of distance below the basal d��collement of the thrust���fold belt. Applying a reactive fluid transport model to these results, the rate coefficient for fluid���rock reaction is constrained to be 10 ��� 19 m o l ��� 1 m 3 s ��� 1 , and the diffusivity of water through the grain boundary network to be 10 ��� 9 m 2 s ��� 1 at the ambient metamorphic conditions of 570 �� C and 7.5 kbar. This newly documented rate of water diffusion is three orders of magnitude slower than thermal diffusion, implying that hydration by diffusion may be the rate���limiting factor in the weakening of cratons, and therefore plays an important role in their geological persistence. This conclusion is consistent with field observations that Paleoproterozoic strain in the Douglas Harbor structural window is restricted to hydrated portions of the Archean Superior craton basement.

Recent astronomical observations of both isomers E and Z of imines such as cyanomethanimine, ethanimine and 2-propyn-1-imine, have revealed that the abundances in the ISM of these isomers differ by factors of ~3-10. Several theories have been proposed to explain the observed behavior, but none of them successfully explains the [E]/[Z] ratios. In this work we present a detailed study of the kinetics of the one-step E-Z isomerization reactions of cyanomethanimine, ethanimine and 2-propyn-1-imine under interstellar conditions (in the 10-400 K temperature range). This reaction was previously thought to be non-viable in the ISM due to its associated high-energy barrier (about 13,000 K). In this Letter, we show that considering the multidimensional small curvature tunneling approximation, the tunneling effect enables the isomerization even at low temperatures. This is due to the fact that the representative tunneling energy lies in the vibrational ground state of the least stable isomer up to approximately 150 K, making the reaction constants of the isomerization from the least stable to the most stable isomer basically constant. The predicted [E]/[Z] ratios are almost the same as those reported from the astronomical observations for all imines observed. This study demonstrates that the [E]/[Z] ratio of imines in the ISM strongly depends on their relative stability., Comment: 13 pages, 2 figures

The Big Bang Nucleosynthesis (BBN) model is a great success of nuclear astrophysics, because of the outstanding agreement between observational and predicted light elements abundances, except for the so-called "lithium problem". In this context, experimental efforts to measure the relevant reactions have brought to an increased level of accuracy in measuring primordial abundances and the introduction of indirect methods has allowed to overcome the natural limitations of direct measurements in the energy range of interest for BBN. Here we review the results obtained from the application of the Trojan Horse Method to some of the most influential reactions of the standard network, such as $^2$H(d,p)$^3$H, $^2$H(d,n)$^3$He, $^3$He(d,p)$^4$He, $^7$Li(p,$\alpha$)$^4$He and $^7$Be(n,$\alpha$)$^4$He. The relevant cross sections have been thus used as new inputs of a classical BBN code resulting in important constraints that advice on a possible solution of the lithium problem outside of nuclear physics.

Saturation flow is one of the fundamental parameter that is used to define the level of service, to assign signal timings and other traffic operations. In the conventional approaches (Highway Capacity Manual, Canadian Capacity Guide etc.) base value is adjusted by the parameters that represent field and traffic conditions. Although the results obtained are acceptable, the adjustment parameters may not reflect the field conditions properly in many countries and does not represent any effects on driver behavior. This paper presents a new formula for saturation flow based on driver behavior and some vehicle characteristics. In this formula, mean length, headway and acceleration rates of vehicles, saturation speeds of intersection and mean reaction time of vehicles in a queue are considered. The formula is tested with field data and compared to the values obtained by the HCM, CCG (Canadian Capacity Guide) and the Akcelik approaches. Based on these comparisons, it is commented that estimation of the new formula is promising. C1 [Murat, Yetis Sazi] Pamukkale Univ, Fac Engn, Dept Civil Engn, TR-20070 Denizli, Turkey. [Cetin, Murat] Headquarters Second Reg, Gen Directorate Highways, TR-35100 Izmir, Turkey.

Optical biosensors provide a platform for qualitatively and quantitatively analyzing various biomolecular interactions. In addition to advantages such as label-free and high-throughput detection, these devices are also capable of measuring real-time binding curves in response to changes in optical properties of biomolecules. These kinetic data may be fitted to models to extract binding affinities such as association rates, dissociation rates, and equilibrium dissociation constants. In these biosensors, one of the binding pair is usually immobilized on a solid substrate for capturing the other. Due to the nature of these surface-based methods, mass transport effects and immobilization heterogenetity may cause problems when fitting the kinetic curves with the simple one-to-one Langmuir model. Here real-time binding curves of various antibody-antigen reactions were obtained by using an ellipsometry-based biosensor, and the results were fitted to the simple one-to-one model as well as a more sophisticated approach. The results show that the one-to-two model fitted much better to the curves than the one-to-one model. The two-site model may be explained by assuming two immobilization configurations on the surface. In summary, in fitting real-time curves obtained from optical biosensors, more sophisticated models are usually required to take surface-related issues, such as immobilization heterogenetity and mass transport effects within targets, into account.

The effect of the preheating rate of the char particles from Kuznetsk bituminous coal on their specific reaction rate was determined. The experiments were carried out with char samples with mean diameters of 0.15 and 0.35 mm heated to temperatures of 900–1000°C. It was found that the specific surface area of the initial coal, measured by CO2 adsorption at 273 K, for rapid and slow heat treatments increased more that two times. According to the results of thermogravimetric analysis, it was established that rapid pyrolysis in a muffle furnace led to the production of char whose maximum specific reaction rate was higher than that of the samples after slow pyrolysis. © 2017, Allerton Press, Inc.

We discuss the design and implementation of StarRate, a modern-Fortran program for the calculation of chemical kinetics coupled to anharmonic vibrational perturbative treatments. The program is written in the F language, a carefully crafted subset of Fortran 95, and is conceived in an object-based programming paradigm, i.e. the set of object-oriented programming features supported by Fortran 90/95. StarRate is made up of three main modules handling the involved molecular species, the elementary reaction steps, and the whole reaction scheme. Input data are accessed through an XML interface based on a cross-code hierarchical data format granting interoperability with popular electronic-structure packages and with the graphical interface of the Virtual Multifrequency Spectrometer developed in our group. Data parsing is performed through versatile Python scripts. Test calculations on the isomerization reaction of C-cyanomethanimine using anharmonic densities of states obtained with a development version of Gaussian are reported together with an account of ongoing developments.

Rate coefficients have been measured for the reaction of CH radicals with formaldehyde, CH$_{2}$O, over the temperature range 31 - 133 K using a pulsed Laval nozzle apparatus combined with pulsed laser photolysis and laser induced fluorescence spectroscopy. The rate coefficients are very large and display a distinct decrease with decreasing temperature below 70 K, although classical collision rate theory fails to reproduce this temperature dependence. The measured rate coefficients have been parameterized and used as input for astrochemical models for both dark cloud and AGB stellar outflow scenarios. The models predict a distinct change (up to a factor of two) in the abundance of ketene, H$_{2}$CCO, which is the major expected molecular product of the CH + CH$_{2}$O reaction., 17 pages, 9 Figures, 5 Tables, Accepted for publication in The Astrophysical Journal

The importance of the activation energy of surface diffusion (E sd) of adsorbed molecules on amorphous solid water (ASW) has been widely discussed in terms of chemical reactions on ASW at low temperatures. However, in previous work, E sd has not been measured directly but estimated from indirect experiments. It has been assumed in chemical network calculations that E sd is between 0.3 and 0.8 of the desorption energies of a molecule. It remains important to obtain direct measurements of E sd. We performed in situ observations of the deposition process of CO and CO2 on ASW using transmission electron microscopy (TEM) and deduced the E sd of CO and CO2 on ASW to be 350 ± 50 and 1500 ± 100 K, respectively. The value of E sd of CO is approximately 0.3 of the total adsorption energy of CO on ASW, i.e., much smaller than assumed in chemical network calculations, where the corresponding figure is 575 K, assuming approximately 0.5 of the desorption energy. We demonstrated that TEM is very useful not only for the observation of ices but also for the measurement of some physical properties that are relevant in astrochemistry and astrophysics. Using the E sd of CO measured in the present study (350 K), we have updated the chemical network model of Furuya et al., confirming that CO2 could be efficiently formed by the reaction CO + OH → CO2 + H in the initial stages of the evolution of molecular clouds.

The work presents a study on effect of vapor-liquid equilibrium phase behavior on enhanced oil recovery by in situ combustion method. The physical model is compositional in nature and takes into account the advective-reactive mass conservation together with conductive and convective transport of heat through a porous oil reservoir. Peng-Robinson equation of state (PR EoS) approach is implemented in estimating the equilibrium compositions of crude oil components. From the numerical results, the phase behavior of oil is found to have an appreciable effect on reaction rates. The present numerical shows that the composition independent models overestimate a cumulative oil recovery of about 18% of original oil in place than PR EoS approach. Copyright � 2015 Taylor & Francis Group, LLC.

Mixtures of straight and defect-free multiwalled carbon nanotubes (MWCNTs), single walled carbon nanohorns (SWCNHs), and multiwalled carbon nanohorns (MWCNHs) were prepared by the arc discharge method. Functionalized Carbon Nanotubes (FCNTs) composed of MWCNTs, MWCNHS and SWCNHs were obtained using acid functionalization process. TiO2/FCNTs (CTP) nanohybrid photocatalysts were obtained with different amounts of FCNTs (2-20 wt%), by wet impregnation method. The photocatalytic activity of both pristine TiO2 as well as the nanohybrids were studied in aqueous glycerol solution under solar irradiation. Reaction parameters such as amount of catalyst and amount of FCNTs in nanohybrids were optimized to ensure high rates of H2 production. With a relatively low amount of pristine TiO2, a high rate of H2 production of 2134 ?mol h-1 gcat -1 has been obtained and is likely due to several factors such as well dispersed catalyst, reduced particle-particle agglomeration, resulting in enhanced light absorption (less light shielding), improved separation of charge carriers and utilization for red-ox reactions. Even better results were obtained with the use of nanohybrid catalysts. The enhanced photocatalytic activities of the nanohybrid catalysts are ascribed to the synergetic effects of FCNTs that minimize light reflection which promotes enhanced solar light sensitization, good dispersion of the catalysts in the reaction solution, and improved surface-interface reactions of photogenerated charge carriers. In the present study 10 wt% FCNTs in nanohybrid (CTP-10) exhibited superior photocatalytic activity for H2 generation, nearly 3 times higher than that of pristine TiO2 catalyst. The CTP-10 photocatalyst exhibited photostability under the experimental conditions studied. The nanohybrid photocatalysts were characterized in order to confirm the crystal structure, morphology, surface chemical composition and optical properties. The present work demonstrates the unique beneficial photocatalytic properties of carbon nanotubes and nanohorn mixtures in nanohybrid photocatalysts, for enhanced H2 production. � 2014 Hydrogen Energy Publications, LLC.

The main objective of this research was to study fuel bed combustion dynamics of a BioGrate boiler with a mechanistic model. First, the fuel specific pyrolysis reaction rates were experimentally determined for the model. Second, the model was validated and finally, it was used to investigate the effects of the primary air flows on drying, pyrolysis and char consumption rates occurring inside the fuel bed. The research results are presented and the role of the dynamic behavior of the reactions on the biomass combustion process discussed.

Heat extraction by cold water circulation disturbs the thermo-chemical equilibrium of a geothermal reservoir, activating the dissolution/precipitation of minerals in the fractures. Calcite being a more reactive mineral than other rock minerals composing the earth curst, we investigate the permeability alteration during geothermal heat production from carbonate reservoirs. In this study the simulations are performed using the code FEHM with coupled thermo-hydro-chemical (THC) capabilities for a three dimensional domain. The computational domain consists of a single fracture connecting the injection and production wells. For reactive alteration of aperture, the model considers that the kinetics of dissolution/precipitation is coupled to the equilibrium interactions among the aqueous species/ions. The reaction rate predominantly depends on the temperature dependent solubility and advective-dispersive solute transport in the fracture. Due to the nonuniform flow fields resulting from injection and production, the coupled thermo-hydro-chemical processes initiate significant variation of the aperture alteration rate over the fracture. We have considered different operating conditions such as different mass injection rate, injection temperature and concentration of minerals. Our simulations show that dissolution and precipitation can occur simultaneously at different locations in fracture. Furthermore the reaction rate varies with time and the reaction rate can also switch between dissolution and precipitation. To illustrate this interesting behavior, the variations of shape and size of zero reaction rate contours with time are shown. An interesting outcome is a non-monotonic evolution of the overall transmissivity between the wells. The alteration of overall transmissivity largely depends on the concentration of mineral in the injected water with respect to the solubility at the initial fracture temperature. For both dissolution and precipitation controlled cases, the rapid changes in transmissivity provide challenges for maintaining circulation of water at constant mass flow rate. � 2013 Elsevier Ltd.

Neutron spectrum analyses of spallation neutrons are conducted in the accelerator-driven system (ADS) facility at the Kyoto University Critical Assembly (KUCA). High-energy protons (100 MeV) obtained from the fixed field alternating gradient accelerator are injected onto a tungsten target, whereby the spallation neutrons are generated. For neutronic characteristics of spallation neutrons, the reaction rates and the continuous energy distribution of spallation neutrons are measured by the foil activation method and by an organic liquid scintillator, respectively. Numerical calculations are executed by MCNPX with JENDL/HE-2007 and ENDF/B-VI libraries to evaluate the reaction rates of activation foils (bismuth and indium) set at the target and the continuous energy distribution of spallation neutrons set in front of the target. For the reaction rates by the foil activation method, the C/E values between the experiments and the calculations are found around a relative difference of 10%, except for some reactions. For continuous energy distribution by the organic liquid scintillator, the spallation neutrons are observed up to 45 MeV. From these results, the neutron spectrum information on the spallation neutrons generated at the target are attained successfully in injecting 100 MeV protons onto the tungsten target.

The following dissertation is divided into two parts. Part I deals with the modeling of helium trapping at oxide-iron interfaces in nanostructured ferritic alloys (NFAs) using density functional theory (DFT). The modelling that has been performed serves to increase the knowledge and understanding of the theory underlying the prevention of helium embrittlement in materials. Although the focus is for nuclear reactor materials, the theory can be applied to any material that may be in an environment where helium embrittlement is of concern. In addition to an improved theoretical understanding of helium embrittlement, the following DFT models will provide valuable thermodynamic and kinetic information. This information can be utilized in the development of large-scale models (such as kinetic Monte Carlo simulations) of the microstructural evolution of reactor components. Accurate modelling is an essential tool for the development of new reactor materials, as experiments for components can span decades for the lifetime of the reactor. Part II of this dissertation deals with the development, and use of, kinetic Monte Carlo (KMC) simulations for improved efficiency in investigating catalytic chemical reactions on surfaces. An essential technique for the predictive development and discovery of catalysts relies on modelling of large-scale chemical reactions. This requires multi-scale modelling where a common sequence of techniques would require parameterization obtained from DFT, simulation of the chemical reactions for millions of conditions using KMC (requiring millions of separate simulations), and finally simulation of the large scale reactor environment using computational fluid dynamics. The tools that have been developed will aid in the predictive discovery, development and modelling of catalysts through the use of KMC simulations. The algorithms that have been developed are versatile and thus, they can be applied to nearly any KMC simulation that would seek to overcome similar challenges as those posed by investigating catalysis (such as the need for millions of simulations, long simulation time and large discrepancies in transition probabilities). Ph. D.

A 3D full-core heterogeneous model of the BFS-62-3A critical benchmark experiment was developed and validated using the Monte Carlo MCNPX-2.4.0 code.

Isothermal kinetic measurements of 15NO reduction with CO on Pd(111) surfaces were carried out under net-oxidizing conditions with 15NO + CO + O2, using a molecular beam instrument (MBI). Transient state (TS) and steady state (SS) kinetic details of the above reaction were obtained for a wide range of temperature and beam compositions, especially with O2-rich compositions. Increasing O2 content, generally, suppresses 15NO reduction in the SS; nonetheless, irrespective of O2 content, 15N2 was produced in TS, and to a significant extent under SS conditions too. Sustainable N 2 production between 450 and 600 K and with low to moderate amount of oxygen was observed, and the extent of NO decomposition was also quantified. The ratio of 15N2:15N2O was generally found to be around 8:1 under most of the reaction conditions. Maxima in the SS reaction rates of all products were observed between 500 and 600 K. Compared to other elementary reaction steps, a slow decay observed with N + N ? N2 step under SS beam oscillation conditions demonstrates its contribution to the rate limiting nature of the overall reaction. Fast beam switching experiments have been performed alternately between O2-lean and -rich conditions, thus highlighting the effectiveness of 15NO reduction in TS, irrespective of the beam composition. Possibly in a future technology initiative, this aspect could be exploited to manage more 15NO reduction on Pd-based catalysts. � 2011 American Chemical Society.

Investigations of structure and reaction zones of unconfined methane-hydrogen laminar jet diffusion flames are presented. In a lab-scale burner, experiments have been conducted using a mixture of pure methane and pure hydrogen in different volumetric proportions. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations outside the flame zone have been measured. Numerical simulations are carried out with a C2 chemical kinetics mechanism having 25 species and 121 reaction steps and an optically thin radiation sub-model. The numerical results are validated against the experimental data. Parametric studies have been carried out for a range of methane-hydrogen mixtures with volumetric proportion of hydrogen in the mixture varying from 0% to 80%. Variation of flame height, contours of temperature, mass fractions of product species and the net reaction rates of methane and hydrogen for various cases are presented and discussed in detail. Further, analysis of net reaction rates of important reactions involving methane and hydrogen with radicals such as O, H and OH are analyzed. The maximum temperature in the domain is seen to decrease for the fuel mixtures with higher hydrogen content. The overall flame length also decreases. For a fuel mixture having 40% hydrogen by volume, the net molar consumption rates of methane and hydrogen are found to be almost equal. Examination of individual reactions of fuel species with radicals shows that hydrogen is mainly consumed by its reaction with OH, whereas methane consumption is mainly through its reactions involving H as well as OH radicals. � 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The constrained Gibbs energy method has been developed for the use of immaterial entities in the formula conservation matrix of the Gibbs energy minimization problem. The new method enables the association of the conservation matrix with structural, physical, chemical, and energetic properties, and thus the scope of free energy calculations can be extended beyond the conventional studies of global chemical equilibria and phase diagrams. The use of immaterial constraints enables thermochemical calculations in partial equilibrium systems as well as in systems controlled by work factors. In addition, they allow the introduction of mechanistic reaction kinetics to the Gibbsian multiphase analysis. The constrained advancements of reactions are incorporated into the Gibbs energy calculation by using additional virtual phases in the conservation matrix. The virtual components are then utilized to meet the incremental consumption of reactants or the formation of products in the kinetically slow reactions. The respective thermodynamic properties for the intermediate states can be used in reaction rate formulations, e.g., by applying the reaction quotients.

A series of x wt.% Pt/CeO2 catalysts (x = 0.1-2.0) was prepared to investigate for the first time the effect of Pt particle size on important kinetic and mechanistic aspects of the water-gas shift (WGS) reaction, namely the concentration (μmol g-1) and chemical structure of active adsorbed reaction intermediates present in the "carbon-path" and "hydrogen-path" of the WGS reaction at 300 °C. For this, steady-state isotopic transient kinetic analysis (SSITKA) coupled with in situ DRIFTS and mass spectrometry experiments was performed using D2O and 13CO. α novel transient isotopic experiment performed allowed to quantify the initial transient rates of reactions of adsorbed formate (-COOH) and CO by water, based on which it was concluded that formate should not be considered as an important intermediate. According to the present work, it is proposed that the WGS reaction on ceria-supported Pt at 300 °C occurs largely via the "redox" mechanism, and to a lesser extent via the "associative formate with -OH group regeneration" mechanism. The TOF (s-1) of WGS was found to vary only slightly with Pt particle size (1.3-8.0 nm), while the specific reaction rate based on the length of periphery of Pt-CeO2 interface (μmol cm-1 s-1) was found to significantly increase with increasing platinum loading and mean particle size in the 250-300 °C range. © 2011 Elsevier Inc. All rights reserved. 279 2 287 300 Cited By :110

Fresh sardines were initially exposed to 0, 5 and 10 °C for 24 h and then, stored at either 4 or 8 °C for 12 and 5 d, respectively. The pH, total volatile basic nitrogen (TVBN), oxidation levels and putrescine, cadaverine and histamine concentration were measured. TVBN and histamine formation rates were calculated for the different storage conditions. Samples initially exposed to higher temperatures exhibited a faster development of TVBN and biogenic amines in both 4 and 8 °C storage temperatures. The initial handling temperature is a determining factor for the safety and quality of sardine.

Numerical investigation of laminar counterflow premixed methane-hydrogen-air flames at a low strain rate is presented. Simulations are carried out with a numerical model incorporated with C2 chemical mechanism having 25 species and 121 reaction steps, and with an optically thin radiation submodel. The numerical model is validated using the experimental data reported in the literature in terms of temperature and species concentrations in flames from opposed flow premixed methane-air and hydrogen-air streams. Parametric studies are performed for opposed flowing methanehydrogen and air mixtures. A premixed methane-hydrogen and air with a rich mixture equivalence ratio (1.75) flows from the top duct, and one having a lean mixture equivalence ratio (0.25) flows from the bottom duct. The volumetric fraction of hydrogen in the fuel mixture has been varied from 20 to 80%. The strain rate used in the present study is kept constant at 50 s -1. Variation of velocity, temperature, species concentrations, and net reaction rates of oxygen, methane and hydrogen along the axis for various cases are presented and discussed in detail. Double flame zones are observed for all the cases. The addition of hydrogen to the rich side stream is seen to be effective in modifying the extents of both reaction zones. The reaction rate of methane is seen to be enhanced with the addition of hydrogen. � 2011 by Begell House, Inc.

The transformation kinetics and the morphology of structures formed in the Cu-24 at % Ga alloy at a cooling rate of 1 to 4 × 105 K/s are studied. A massive transformation shows up in the Cu-24 at % Ga alloy at slow cooling rates, and this transformation can be replaced, upon sharp quenching, by a martensitic reaction. The transformation kinetics and the morphology of structures formed in the Cu-24 at % Ga alloy at cooling rates of up to 400 kK/s are analyzed in the study. © 2011 Pleiades Publishing, Ltd.

A variety of aromatic, aliphatic and conjugated alcohols were transformed into the corresponding carboxylic acids and ketones with aq 70% t-BuOOH in the presence of catalytic amounts of bismuth(III) oxide. This method possesses a wide range of capabilities, does not involve cumbersome work-up, exhibits chemoselectivity and proceeds under ambient conditions. The resulting products are obtained in good yields within reasonable time. The overall method is green. � 2010 Georg Thieme Verlag Stuttgart New York.

Kinetics of heterogeneous catalysed esterification of propionic acid with n-amyl alcohol was studied with a microporous cation-exchange resin catalyst, Dowex 50Wx4, in a stirred batch reactor to synthesise amyl propionate. Effects of various parameters such as speed of agitation, catalyst loading, and reaction temperature on reaction rate were investigated. The equilibrium conversion of propionic acid increased with in catalyst loading and reaction temperature. Stirrer speed had virtually no effect on the reaction rate under the experimental conditions. The apparent activation energy was found to be 43.167 kJmol−1 for the formation of amyl propionate and the equilibrium constant, which is independent of temperature ranging from 318 to 348 K, was found to be 4.05. It was also observed that the initial reaction rate decreased with water concentrations and increased with that of acid and increased with that of alcohol linearly. The reaction was found to occur between an adsorbed acid molecule and a molecule of alcohol in the bulk and it was concluded that the reaction mechanism can be represented by Eley-Rideal model.

A detailed kinetic and mechanistic study of the water-gas shift (WGS) reaction on a 0.5 wt% Pt/TiO2 catalyst has been carried out. The dependence of kinetic reaction rate on the partial pressures of reactants (CO, H2O) and products (H2, CO2), and the concentration and chemical structure of active and inactive reaction intermediates that are found in the "hydrogen-path" and "carbon-path" of the reaction have been investigated in the 200-270 °C range. The most likely mechanistic pathway of the WGS reaction over the present catalytic system is discussed. It has been found that the reaction rate increases with an increase in the concentration of CO or H2O in the feed stream, while it decreases significantly with the addition of H2 in the feed stream. On the contrary, the kinetic reaction rate was found to be practically independent on the concentration of CO2 in the feed stream. The experimental reaction rates that were estimated were fitted to an empirical power-law rate expression from which the kinetic reaction orders with respect to CO, H2O, CO2, and H2 were estimated to be 0.5, 1.0, ∼0.0, and -0.7, respectively. An apparent activation energy of 10.8 kcal/mol was also estimated. The formation of formate and carbonate surface species over the TiO2 support under WGS reaction conditions was proved via SSITKA-DRIFTS experiments. However, these reaction intermediates must be considered as inactive (spectator) species for the steady-state WGS reaction. Additional transient experiments that involved 18O-isotope provided strong support for the red-ox mechanism as the prevailing one on the present Pt/TiO2 catalyst, where labile oxygen and oxygen vacancies of TiO2 near the metal-support interface can participate in the reaction path of the WGS reaction. © 2009 Elsevier Ltd. All rights reserved. 264 2 117 129 Cited By :106

A heterogeneous one-dimensional model is used to simulate a three way catalytic converter (TWC). The mathematical model consists of mass and energy balance equations in the gas and solid phases and accounts for 15 heterogeneous chemical reactions. Langmuir-Hinshelwood type kinetics is used to represent most of the reaction rates. In this work, the kinetic parameters of the various reactions occurring in a TWC are estimated using vehicle data based on a genetic-algorithm-based optimization. The method uses the test data to estimate the reaction kinetic parameters by minimizing the deviation between the model predictions and experimental observations. It is found that the objective function based on the cumulative mass of a species that leaves the reactor until a certain time instant is a preferred performance measure for the optimization. The sensitivity of the exit concentration of various species to various kinetic parameters was found. This is used to identify the subset of reactions which have a significant effect on the various species concentrations in the effluent stream and is used to determine the set of kinetic parameters for optimizing the reactor performance. Single parameter and multiple parameters tuning using genetic algorithm have been demonstrated successfully for a TWC application. � 2009 American Chemical Society.

Au and Au-Pt alloy nanoparticles are prepared and patterned at room temperature within the PMMA polymer matrix by the action of 254 nm UV light or X-rays. The polymer matrix enables us to entangle the kinetics of the photochemical reduction from the nucleation and growth processes, when monitored by UV-vis spectroscopy. Accordingly, increase of the temperature to 50 degrees C of the reaction medium increases the nucleation and growth rates of the nanoparticle formation by more than one order of magnitude, due to enhanced diffusion and nucleation at the higher temperature, but has no effect on the photochemical reduction process. Presence of Pt ions also increases the same rate, but by a factor two only. Similar photochemical reduction and particle growth take also place within the PMMA matrix, when these metal ions are subjected to prolonged exposure to X-rays, as evidenced by XPS analysis. Both angle-resolved and charge-contrast measurements using XPS reveal that the resultant Au and Pt species are in close proximity to each other, indicating the Au-Pt alloy formation to be the most likely case. (C) 2008 Elsevier Ltd. All rights reserved.

Chemical species are advected by water and undergo mixing processes due to effects of local diffusion and/or dispersion. In turn, mixing causes reactions to take place so that the system can locally equilibrate. In general, a multicomponent reactive transport problem is described through a system of coupled non-linear partial differential equations. Under instantaneous chemical equilibrium, a complex geochemical problem can be highly simplified by fully defining the system in terms of conservative quantities, termed master species or components, and the space–time distribution of reaction rates. We investigate the parameters controlling reaction rates in a heterogeneous aquifer at short distances from the source. Hydraulic conductivity at this scale is modeled as a random process with highly anisotropic correlation structure. In the limit for very large horizontal integral scales, the medium can be considered as stratified. Upon modeling transport by means of an ADE (Advection Dispersion Equation), we derive closed-form analytical solutions for statistical moments of reaction rates for the particular case of negligible transverse dispersion. This allows obtaining an expression for an effective hydraulic conductivity, K eff R , as a representative parameter describing the mean behavior of the reactive system. The resulting K eff R is significantly smaller than the effective conductivity representative of the flow problem. Finally, we analyze numerically the effect of accounting for transverse local dispersion. We show that transverse dispersion causes no variation in the distribution of (ensemble) moments of local reaction rates at very short travel times, while it becomes the dominant effect for intermediate to large travel times.

We present a detailed microscopic study of the dynamics of the Michael addition reaction leading from 6'-deoxychalcone to the corresponding flavanone. The reaction dynamics are analyzed for both the uncatalyzed reaction in aqueous solution and the reaction catalyzed by Chalcone Isomerase. By means of rare event simulations of trajectories started at the transition state, we have computed the transmission coefficients, obtaining 0.76 +/- 0.04 and 0.87 +/- 0.03, in water and in the enzyme, respectively. According to these simulations, the Michael addition can be seen as a formation of a new intramolecular carbon-oxygen bond accompanied by a charge transfer essentially taking place from the nucleophilic oxygen to the carbon atom adjacent to the carbonyl group (C (alpha)). As for intermolecular interactions, we find a very significant difference in the evolving solvation pattern of the nucleophilic oxygen in water and in the enzyme. While in the former medium this atom suffers an important desolvation, the enzyme provides, through variations in the distances with some residues and water molecules, an essentially constant electric field on this atom along the reaction progress. Grote-Hynes (GH) theory provides a useful framework to systematically analyze all the couplings between the reaction coordinate and the remaining degrees of freedom. This theory provides transmission coefficients in excellent agreement with the Molecular Dynamics estimations. In contrast, neither the frozen environment approach nor Kramers theory gives results of similar quality, especially in the latter case, where the transmission coefficients are severely underestimated. The (unusual) failure of the frozen environment approach signals the importance of some dynamical motions. Within the context of GH theory, analysis of the friction spectrum obtained in the enzymatic environment, together with normal-mode analysis, is used to identify those motions, of both the substrate and the environment, strongly coupled to the reaction coordinate and to classify them as dynamically active or inactive.

Eyring theory employs the statistical mechanical theory of absolute reaction rates to analyse the transport mechanisms in fluids. A physicochemical methodology combining molecular dynamics (MD) and Eyring theory of reaction rates is proposed for investigating the liquid slip on a solid wall in the nanoscale domain. The method involves the determination of activation energy required for the flow process directly from the MD trajectory information and then calculate the important transport properties of the confined fluid from the activation energy. In order to demonstrate the universal applicability of the proposed methodology in nanofluidics, the slip flow behavior of argon, water and ionic liquid confined in various nanostructures has been investigated. The slip length is found to be size dependent in all the cases. The novelty of this method is that the variations in slip length are explained on the basis of molecular interactions and the subsequent changes in the activation energy. Graphical abstract: [Figure not available: see fulltext.] � 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

Quantum transition-state theory (QTST) and free-energy instanton theory (FEIT) are two closely related methods for estimating the quantum rate coefficient from the free-energy at the reaction barrier. In calculations on one-dimensional models, FEIT typically gives closer agreement than QTST with the exact quantum results at all temperatures below the crossover to deep tunneling, suggesting that FEIT is a better approximation than QTST in this regime. Here we show that this simple trend does not hold for systems of greater dimensionality. We report tests on several collinear and three-dimensional reactions, in which QTST outperforms FEIT over a range of temperatures below crossover, which can extend down to half the crossover temperature (below which FEIT outperforms QTST). This suggests that QTST-based methods such as ring-polymer molecular dynamics (RPMD) may often give closer agreement with the exact quantum results than FEIT.

In stoichiometric network analysis (SNA) the instability condition is calculated by the current rates. Recently, we have shown that in the final result the current rates can be substituted by reaction rates, which is a more appropriate value for the examination of instability from experimental point of view. Here, we elaborate the problem of whether the current rates are necessary parameters in the calculation, with the aim of obtaining the region of instability. All calculations are performed on a model for Belousov-Zhabotinsky (BZ) reaction, which has not been examined by SNA.

Platinum nanoparticles ( d Pt = 1.0–1.2 nm) supported on single CeO 2 and La 2 O 3 metal oxides and Ce 0.8 La 0.2 O 2− δ solid solution were prepared to investigate for the first time the effect of La 3+ -doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300 °C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D 2 O) and C-path (use of 13 CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce 1− x La x O 2− δ ( x = 0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol CO g −1 s −1 ) increased by a factor of 2.0 and 2.8 at 300 °C on 0.5 wt% Pt supported on Ce 0.8 La 0.2 O 2− δ compared to CeO 2 and La 2 O 3 , respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites ( k , s −1 ) responsible for CO 2 and H 2 formation on Pt/Ce 0.8 La 0.2 O 2− δ compared to Pt/CeO 2 and Pt/La 2 O 3 . Active OH groups is suggested to be formed on defect sites (Ce 3+ □ s ) of Ce 0.8 La 0.2 O 2− δ as a consequence of the introduction of La 3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

Pt nanoparticles (1.0-1.4nm size) supported on Ce1-xLaxO2-δ (x=0.0, 0.2, 05, 0.8 and 1.0) carriers, the latter prepared by the citrate sol-gel method, were tested toward the water-gas shift (WGS) reaction in the 200-400°C range. A deep insight into the effect of Ce/La atom ratio of support chemical composition on the catalytic performance (CO conversion vs. temperature and stability) and kinetic rates of Pt-loaded catalysts was realized after employing HAADF/STEM, in situ Raman and DRIFT spectroscopies under different gas atmospheres, temperature-programmed surface reaction (TPSR) in He and O2/He gas atmospheres following WGS reaction, CO-TPD, in situ UV-vis/DRS, oxygen storage capacity measurements, and transient 18O-isotopic exchange studies followed by WGS reaction. It was found that doping of ceria with 20at.% La3+ has increased significantly the catalytic activity of 0.5wt% Pt/Ce0.8La0.2O2-δ solid in the 250-350°C range, whereas addition of 50-80at.% La3+ in ceria caused a negative effect on the CO conversion with respect to pure ceria. It was found that the Ce/La atom ratio in Ce1-xLaxO2-δ influences the catalytic site reactivity (k) along the Pt-support interface. The optimum La3+-dopant concentration of 20at.% (Ce/La=4/1) used in Pt/Ce0.8La0.2O2 compared to the worst one of 80at.% (Pt/Ce0.2La0.8O2-δ, Ce/La=1/4) correlates with (i) the higher specific kinetic rate per length of Pt-support interface (μmol COcm-1s-1), (ii) the higher concentration of oxygen vacant sites, (iii) the lower amount (μmol/g-1) of "carbon" accumulated during WGS and best stability with time on stream, (iv) the lower apparent activation energy (kcalmol-1) of WGS reaction, (v) the lower degree toward Pt oxidation (largest Pt2+/Pt4+ ratio), (vi) the lower Ce1-xLaxO2-δ support energy band gap, and (vii) the lower mobility of surface lattice oxygen. © 2013 Elsevier B.V. 140-141 333 347 Cited By :39

The present work is focused on the sol-gel synthesis, characterization and catalytic performance evaluation of novel Mg-Ce-Zr-O-based mixed oxides supported Rh catalysts for the purification of H 2-rich gas streams from phenol through steam reforming in the 575-730°C range. Phenol is used as a model compound of tars, the former being one of the main constituents of tars formed during steam gasification and pyrolysis of biomass. Novel Rh catalysts (0.1wt%) supported on Mg-Ce-Zr-O-based mixed oxides modified by alkaline-earth, rare earth, and transition metal ions, having the general formula Mg-Ce-Zr-X-O (where X stands for La, Sr, Ba, Ca and Zn) were systematically studied. The physicochemical properties of the catalysts were evaluated using complementary bulk and surface characterization techniques, such as BET, XPS, in situ XRD, TEM/SAED and SEM/EDX. Transient techniques including H 2-TPR, TPD-CO 2 and TPD-NH 3 were employed in order to characterize redox behavior, surface basicity, and surface acidity of support, respectively. Among the catalysts examined, a 0.1wt% Rh/40Mg-20Ce-20Zr-20La-O was found to exhibit the highest hydrogen product yield, specific H 2 production rate (μmol/m 2min), and the lowest CO/CO 2 product ratio (575-730°C), even when compared to a Ni-based commercial steam reforming of tars catalyst. At 655°C, a H 2 yield of ∼95% and a specific reaction rate of 100μmolH 2/m 2min were obtained for a feed containing 0.5vol% phenol and 40vol% H 2O at a gas hourly space velocity of ∼80,000h -1. This activity behavior is correlated with the largest concentration of basic sites and labile oxygen species (μmol/m 2), and also with the largest acid/base site ratio present in the 40Mg-20Ce-20Zr-20La-O support composition. Steam was also found to be an efficient reagent for the removal of carbonaceous deposits (e.g., C xH y) formed on the catalyst surface during phenol steam reforming. The H/C atom compositional ratio of the carbonaceous deposits was found to depend on reaction temperature. © 2011 Elsevier B.V. 111-112 360 375 Cited By :23

The steam reforming of phenol in the low-temperature range of 350-550°C was studied over 0.5wt% Rh supported on Ce 0.15Zr 0.85O 2, Zr 0.93La 0.07O 2, and Ce 0.13Zr 0.83La 0.04O 2 mixed metal oxides. The reforming specific activity (per gram of catalyst basis) was found to depend on (a) Rh dispersion, (b) the concentration (μmolg -1) of labile oxygen species of support, as determined by " oxygen storage capacity" measurements, and (c) the promotion of the water-gas shift (WGS) reaction rate, all of which largely influenced by the support chemical composition. The addition of 4atom% La 3+ into the Ce 0.15Zr 0.85O 2 crystal structure was found to largely promote the dispersion of Rh, to increase significantly (by a factor of 1.75) the concentration of surface basic sites (μmolm -2) of Ce 0.13Zr 0.83La 0.04O 2 compared to Ce 0.15Zr 0.85O 2 support, to prevent to a large extent the inhibiting role of hydrogen in the rate of WGS reaction, and to influence significantly the concentration and structure of surface reaction intermediates of the WGS, namely: formate (HCOO-) and -OH groups formed on the support, and adsorbed CO formed on the Rh metal. The 0.5wt% Rh/Ce 0.13Zr 0.83La 0.04O 2 catalyst led to a significantly better performance towards steam reforming of phenol in terms of phenol conversion, H 2-yield, and CO/CO 2 product ratio in the 350-450°C range compared to a commercial Ni-based catalyst (44wt% Ni) used for steam reforming reactions. At 400°C, the 0.5wt% Rh/Ce 0.13Zr 0.83La 0.04O 2 catalyst exhibited approximately 54% and 50%, respectively, higher phenol conversion and hydrogen yield, compared to the Ni-based industrial catalyst. The significantly lower CO/CO 2 product ratio obtained in the case of Rh/Ce 0.13Zr 0.83La 0.04O 2 compared to that obtained in the other supported-Rh catalysts is in agreement with the higher kinetic rate for the WGS reaction observed in the former catalyst. © 2012 Elsevier B.V. 117-118 81 95 Cited By :21

[EN] Laser flash photolysis of supported gold nanopar-ticles exciting at the surface plasmon band (532 nm) has allowed in the case of Au/CeO 2 and Au/OH-npD (OH-npD: Fenton-treated diamond nanoparticles) detection of transients decaying in the microsecond time scale that have been attributed as indicating photoinduced electron ejection from gold based on N 2O quenching and the observation of the generation of methyl viologen radical cations. This photochemical behavior has led us to hypothesize that there could be assistance to the catalytic activity of these materials by irradiation in those cases wherein the mechanism involves electron transfer to or from a substrate to the gold. This hypothesis has been confirmed by observing that the catalytic activity of Au/OH-npD for the Fenton degradation of phenol with hydrogen peroxide can be increased over 1 order of magnitude by irradiation at 532 nm. Moreover, there is a linear relationship between the initial reaction rate and the incident photon flux. This photoenhancement allows promoting Fenton activity at pH 8 in which the catalytic activity of Au/OH-npD is negligible. The same photo enhancement activity for the Fenton degradation of phenol was observed for other supported gold catalysts including those that do not exhibit microsecond transients in the nanosecond laser flash photolysis (Au/TiO 2 and Au/SiO 2) due to their lifetime shorter than microseconds. It is proposed that the photo enhancement should be a general phenomenon in gold catalysis for those reaction mechanisms involving positive and/or negative gold species. © 2011 American Chemical Society., Financial support by Spanish MICINN (CTQ2009-11583) is grateful acknowledgment. S.N. thanks the Technical University of Valencia for an assistant professor contract (Programa Cantera). R.M. thanks Spanish minister of Science Innovation MICINN for a FPU postgraduate fellowship.

Presentned at The 2nd Conference on Environmental Applications of Advanced Oxidation Processes, 2009, Nicosia, Cyprus, September The selective catalytic reduction of nitrates (NO3) in pure water towards N2 formation by the use of gaseous H2 and in the presence of O2 (air) at 1 atm total pressure and 25◦C has been investigated over PdCu supported on TiO2-Al2O3 pellets (spherical shape). The effects of internal mass transport phenomena, of Pd and Cu loading (wt%), and of reducing feed gas composition on the reaction rate, N2-selectivity, and the kinetics of the overall reaction in terms of apparent reaction orders with respect to nitrate concentration (mg/L) and H2 partial pressure (bar) have been investigated. Apparent reaction orders of 0.32 and 1.04 were estimated, respectively with respect to NO3 − concentration and H2 partial pressure in the reducing feed gas stream (H2/He). However, when 20% air is added in the hydrogen reducing feed gas stream, the apparent reaction order with respect to the H2 partial pressure was increased to 1.7. It is demonstrated for the first time that the presence of internal mass transport phenomena when large catalytic particles (2–3 mm in size) are used in a semi-batch mode reactor operation results in the reduction of the selectivity of reaction with respect to NH4 + and NO2 − products, and at the same time in the increase of N2 product selectivity, whereas the opposite is true for the rate of reaction. Moreover, it was found that the reaction rate, the NO3 − conversion (%), and the reaction product selectivity are affected significantly by the presence of oxygen in the reducing feed gas stream, and by the Pd and Cumetal loading used in catalyst’s composition.

In this work, a detailed first principle study of the mechanism of the heterogeneous catalytic water-gas shift (WGS) reaction on a rhodium cluster is presented. A large number of possible reaction mechanisms relevant to the WGS reaction are explored, and as many as 34 possible pathways are located due to the unsaturated nature of the metal cluster and the multitude of intermediate species binding configurations. Brønsted-Evans-Polanyi relationships and the Sabatier principle are used to locate the kinetic and thermodynamic paths occurring on the rhodium cluster. A detailed potential energy diagram of the kinetically favored mechanism is presented that shows that the RDS of the reaction are the water dissociation, formate association and formate decomposition elementary reactions, with free energy barriers (ΔG‡) of 24.2, 25.9, and 27.0 kcal/mol, respectively. The poisoning effect of coadsorbed CO and the beneficial effect of preadsorbed water on the kinetic rate of this reaction is demonstrated and a water-mediated mechanism is proposed. In the watermediated mechanism favorable H-bonding interactions stabilize Zundel-cations adsorbed to the metal cluster, which manifest a lower energy path for the dissociation of water. Participation of coadsorbed water in this mechanism explains the promoting effect water vapor pressure has on the reaction kinetics (positive reaction order with respect to water) by lowering the free energy of the rate-determining step barrier by 4.0 kcal/mol and causing a 10-fold increase of the reaction rate for the water dissociation elementary reaction step. In addition a presumable preadsorbed water-mediated mechanism is shown to have an even lower free energy barrier (16,9 kcal/mol) causing a 2000-fold increase of the elementary reaction rate for water dissociation. It is suggested that operation of heterogeneous WGS catalysts in a cyclic fashion where water is first preadsorbed might enhance current catalyst performance and CO conversion turnover frequencies. © 2008 American Chemical Society. 112 48 19030 19039 Cited By :23

[EN] Three MIL-100 (Fe) samples differing in average crystal size (from 60-70 to >400 nm) have been synthesized by microwave heating using three HF/Fe 3+ ratios. Oxidation of diphenylmethane with tert-butylhydroperoxide (TBHP) and thiophenol with oxygen are catalyzed by three MIL-100 (Fe) samples with similar reaction rates regardless of its average particle size. In contrast, the activity of the three MIL-100 (Fe) samples for the oxidation of bulky triphenylmethane by TBHP largely depends on the average crystal size of the sample: the smaller the average particle size, the larger the initial reaction rate of triphenylmethane oxidation. These results show that diffusion limitation takes place on MOF catalysis depending on the substrate size and provides indirect evidence that these reactions take place inside the intracrystalline space of the porous catalysts. © 2011 The Royal Society of Chemistry., Financial support by the Spanish DGI (CTQ2009-11587 and CTQ2010-18671 and Consolider MULTICAT) is gratefully acknowledged. Prof. Jong-San Chang and his group are thanked for providing us with sufficient quantities of the three MIL-100 (Fe) samples. Korean researchers acknowledge the financial support of KICOS and KRICT for the international collaboration program.

La siliciuration de rubans de niobium par décomposition hétérogène de SiH 4 résulte d'une compétition entre l'apport du silicium en surface et sa diffusion à l'intérieur du métal. On a étudié la cinétique de décomposition de SiH4 sur le silicium et sur les siliciures, ainsi que l'évolution de la surface pendant la siliciuration. En chauffant des rubans siliciurés ou recouverts de silicium en présence d'oxygène, on a mis en évidence la transition brutale de l'état de passivation du métal, protégé par une couche de SiO2, à un régime stationnaire de combustion rapide du silicium, résultant de la vaporisation de l'oxyde SiO ; dans le cas de rubans constitués d'une solution solide du silicium dans le niobium, on a déterminé la cinétique de réaction de l'oxygène sur la couche d'oxyde de niobium non volatil formée en surface.

The reaction-rate ratio C8/Ftot, neutron captures in 238U to total fissions, has been measured in 80 out of 96 fuel rods of a Westinghouse SVEA-96+ boiling water reactor fuel assembly. High-resolution gamma spectroscopy was performed on individual fuel rods, withdraw: from the SVEA-96+ assembly after irradiation at low power in the center of the LWR-PROTEUS reactor core. Absolute experimental errors of 1.7% and relative errors of 0.6% (for rod-to-rod ratios) were achieved. The experimental results were used as a database for validation of four different calculational tools: CASMO-4 and HELIOS as commercial assembly codes, the Paul Scherrer Institute in-house code BOXER, and the Monte Carlo transport code MCNPX. In general, on the level of a few percent, there is good agreement between experiment and calculations, the use of a recently proposed 239Np gamma-ray emission probability improving even further the agreement. However, the highly heterogeneous design of the SVEA-96+ assembly (both in terms of material compositions and neutron moderation conditions) causes some problems. Clear deviations from assembly mean values are found among the burnable absorber fuel rods that are grouped in clusters (direct neighbors), a unique feature of this assembly design. For these rods the codes overpredict C8/F tot by several percent, including MCNPX. Additional trends, not present in the results from the Monte Carlo calculation which generally shows the best overall agreement with experiment, are identified for the deterministic codes.

Radial distributions of the total fission rate and the238U- capture-to-total-fission (C 8/F tot) ratio were measured in SVEA-96+ and SVEA-96 Optimal assemblies during the LWR-PROTEUS program. Fission rates predicted using MCNPX with JEFF-3.1 cross sections underestimated the measured values in the gadolinium-poisoned pins of the SVEA-96 Optima2 assembly; similarly, C 8/F tot ratios were overestimated in some gadolinium-poisoned pins of the SVEA-96+ assembly. A considerable effort was invested at the Paul Scherrer Institut to explain the discrepancies in gadolinium pins, without success. Recently, gadolinium cross sections were measured at the Rensselaer Polytechnic Institute by Leinweber et al. and differed significantly from current library values. ENDF/B-VII.0 gadolinium cross sections have currently been modified to include the new measurements, and these data have been processed with NJOY to yield files usable by MCNPX. Fission rates in the gadolinium-poisoned fuel pins of the SVEA-96 Optimal pins were increased by 1.4 to 2.0% using the newly produced cross sections, yielding to a better agreement with the experimental values. Predicted C 8/F tot ratios were decreased on average by 1.7% in both clustered and unclustered groups of gadolinium-poisoned fuel pins of the SVEA-96 + assembly correcting the overpredictions previously reported in the clustered gadolinium pins. Earlier reported discrepancies observed in PROTEUS integral experiments, between measured and calculated reaction rates in the gadolinium-poisoned pins, might thus be due to inaccurate gadolinium cross sections. The PROTEUS results support the new thermal and epithermal gadolinium data measured by Leinweber et al.

High resolution gamma-ray spectroscopy measurements were performed on 61 rods of an SCWR-like fuel lattice, after irradiation in the central test zone of the PROTEUS zero-power research reactor at the Paul Scherrer Institute in Switzerland. The derived reaction rates are the capture rate in U-238 (C-8) and the total fission rate (F-tot), and also the reaction rate ratio C-8/F-tot. Each of these has been mapped rod-wise on the lattice and compared to calculated results from whole-reactor Monte Carlo simulations with MCNPX. Ratios of calculated to experimental values (C/E's) have been assessed for the C-8, F-tot, and C-8/F-tot distributions across the lattice. These C/E's show excellent agreement between the calculations and the measurements. For the U-238 capture rate distribution, the 1 sigma level in the comparisons corresponds to an uncertainty of +/- 0.8%, while for the total fission rate the corresponding value is +/- 0.4%. The uncertainty for C-8/F-tot, assessed as a reaction rate ratio characterizing each individual rod position in the test lattice, is significantly higher at +/- 2.2%. To determine the reproducibility of these results, the measurements were performed twice, once in 2006 and again in 2009. The agreement between these two measurement sets is within the respective statistical uncertainties. (C) 2010 Elsevier Ltd. All rights reserved.