Reaction Rate Measurements Research Articles

Experimental validation of the neutronic parameters in the Jordan subcritical assembly

Following the construction of the Jordan Subcritical Assembly (JSA) on the campus of the Jordan University of Science and Technology (JUST); a detailed experimental study of the facility was performed to measure different reactor parameters. In this paper, the reactivity and activation reaction rate in JSA were determined experimentally and then validated with the calculated results from Monte Carlo Simulation Radaideh et al. (2018a). The reactivity and criticality analysis were implemented using the source jerk method, while the foil activation technique was used for activation reaction rate and flux characterization. The experimental subcritical reactivity in dollars was −5.498 ± 0.045 while keff for the facility was 0.96476 ± 0.00771. The multiplication factor calculated using Monte Carlo codes of JSA was 0.96553 ± 0.00002. In addition, it was found that the source jerk results had large uncertainties due to the counting error. 115In foils were used to measure the activation reaction rate. The absolute axial and radial reaction rates were measured by averaging the measured counts for three photo-peaks of 116mIn. The experimental results were validated by the calculated reaction rates from MCNP5. The error in the activation reaction rate measurements were dominated by the peak area measurement error. The calculated and measured activation reaction rate results demonstrated very good agreement with a maximum deviation of approximately 11% in both directions. In this work, the neutron source strength was experimentally determined using the foil activation technique. The value estimated for the source was 1.02×106±0.9% (n/s) which is very close to the reported value.

Reaction rate theory for supramolecular kinetics: application to protein aggregation

ABSTRACTProbing reaction mechanisms of supramolecular processes in soft and biological matter, such as protein aggregation, is inherently challenging. This is because these processes involve multiple molecular mechanisms that are associated with the rearrangement of large numbers of weak bonds, resulting in complex free energy landscapes with many kinetic barriers. Reaction rate measurements at different temperatures can offer unprecedented insights into the underlying molecular mechanisms. However, to be able to interpret such measurements, a key challenge is to establish which properties of the complex free energy landscapes are probed by the reaction rate. Here, we present a reaction rate theory for supramolecular kinetics based on Kramers theory of diffusive reactions over multiple kinetic barriers. We find that reaction rates for protein aggregation are of the Arrhenius–Eyring type and that the associated activation energies probe only one relevant barrier along the respective free energy landscapes. We apply this advancement to interpret, in experiments and in coarse-grained computer simulations, reaction rates of amyloid aggregation in terms of molecular mechanisms and associated thermodynamic signatures. These results suggest a practical extension of the concept of rate-determining steps for complex supramolecular processes and establish a general platform for probing the underlying energy landscape using kinetic measurements.

Assessment of kinetic model for ceria oxidation for chemical-looping CO2 dissociation

Chemical looping technologies are identified as to have an excellent potential for CO2 capture and fuels synthesis. Oxygen carriers are the fundamental component of a chemical looping process, and the choice of stable and efficient carriers with fast redox kinetics is the key to the successful design of the process. Hence, understanding the reaction kinetics is of paramount importance for the selection of an appropriate oxygen carrier material. This work provides a method for kinetic model selection based on a statistical approach to identify the reaction mechanism. The study experimentally investigates the oxidation kinetics of CeO2−δ by CO2 and applies a statistical method for the selection of the best-fitting kinetic model for the reaction. The kinetic study is performed in the temperature range of 700–1000 °C with a CO2 concentration between 20 and 40 vol% in the feed. The measured peak rates of CO production on ceria were influenced both by temperature and concentration of reactant. The total CO production was more influenced by the temperature than by CO2 concentration, with a maximum CO yield of 33.66 ml/g at 1000 °C and 40% CO2. The identification of the oxidation kinetic model is performed by fitting different reactions models to the measured reaction rates and statistically comparing them using the Residual sum of squares (RSS), Akaike information criterion (AICc) and the F-test for the selection of the best-fitting one. Models corresponding to the nucleation and grain growth reaction mechanism provided a good fit of the data, with the Sestak-Berggren (SB) model showing the best approximation of the measured rate of reaction with an evaluated activation energy of 79.1 ± 6.5 kJ/mol for the CO2 oxidation.

Development of a rapidly responding fluidized bed reactor by theoretical and experimental evaluation of combustion reactions

The development of a rapidly responding lab-scale fluidized bed reactor (FBR) optimized for the investigation into combustion reactions of solid fuels is presented. The experimental setup represents a novelty in FBR systems, as it quantitatively captures reactions with an apparent 90% carbon conversion time t90 of <3 s, which is a third of the time of comparable setups described in literature. The reactor design is discussed in detail and improvements undertaken to shorten the systems’ response time are explained. The shortened response time results in the possibility to investigate reactions at higher temperatures.Original and improved design are modeled with a 1-D finite volume approach to gain insights into the ideal operating conditions and operating limits. In a second step, the theoretical reaction rate limits are compared with experimental combustion experiments. Reaction rate measurements with chars from two pulverized Colombian coals (Calenturitas, Mina Norte) and one pulverized biomass (beech wood) are undertaken in N2/O2 atmosphere at different temperatures ranging from 823 to 1273 K.It is found that results of the 1-D simulation are a good guidance for actual experiments.

OH+ Formation in the Low-temperature O+(4S) + H2 Reaction

Abstract Formation of OH+ in collisions of ground-state O+(4S) ions with normal H2 has been studied using a variable temperature 22-pole RF ion trap. From 300 to 30 K the measured reaction rate coefficient is temperature-independent, with a small decrease toward 15 K. The recent wave packet calculation predicts a slightly steeper temperature dependence. The rate coefficients at 300 and 15 K are almost the same, (1.4 ± 0.3) × 10−9 cm3 s−1 and (1.3 ± 0.3) × 10−9 cm3 s−1, respectively. The influence of traces of the two metastable ions, O+(2D) and O+(2P), has been examined by monitoring the H+ products of their reactions with H2, as well as by chemically probing them with N2 reactant gas.

In Situ Observation of Chymotrypsin Catalytic Activity Change Actuated by Nonheating Low-Frequency Magnetic Field.

Magnetomechanical modulation of biochemical processes is a promising instrument for bioengineering and nanomedicine. This work demonstrates two approaches to control activity of an enzyme, α-chymotrypsin immobilized on the surface of gold-coated magnetite magnetic nanoparticles (GM-MNPs) using a nonheating low-frequency magnetic field (LF MF). The measurement of the enzyme reaction rate was carried out in situ during exposure to the magnetic field. The first approach involves α-chymotrypsin-GM-MNPs conjugates, in which the enzyme undergoes mechanical deformations with the reorientation of the MNPs under LF MF (16-410 Hz frequency, 88 mT flux density). Such mechanical deformations result in conformational changes in α-chymotrypsin structure, as confirmed by infrared spectroscopy and molecular modeling, and lead to a 63% decrease of enzyme initial activity. The second approach involves an α-chymotrypsin-GM-MNPs/trypsin inhibitor-GM-MNPs complex, in which the activity of the enzyme is partially inhibited. In this case the reorientation of MNPs in the field leads to disruption of the enzyme-inhibitor complex and an almost 2-fold increase of enzyme activity. The results further demonstrate the utility of magnetomechanical actuation at the nanoscale for the remote modulation of biochemical reactions.

Surface-promoted hydrolysis of 2,4,6-trinitrotoluene and 2,4-dinitroanisole on pyrogenic carbonaceous matter

This study investigates the fate of sorbed nitroaromatics on the surface of pyrogenic carbonaceous matter (PCM) to assess the feasibility of a PCM-promoted hydrolysis. The degradation of two nitroaromatic compounds, 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole, was observed at pH 7 in the presence of graphite powder, a model PCM. By contrast, no decay occurred without graphite. Using TNT as a model compound, our results suggest that TNT decay demonstrated a strong pH dependence, with no reaction at pH 3–5 but rapid degradation at pH 6–10. Moreover, by fitting TNT decay at different pH conditions along with its sorption kinetics to the Langmuir Kinetic Model, our results suggest that the base-catalyzed hydrolysis was important. The activation energy for TNT decay was obtained by measuring reaction rates at different temperatures with or without graphite and no significant difference was observed. However, the addition of tetramethylammonium cation was able to promote TNT decay possibly due to its ability to attract more OH− from the aqueous solution, leading to an increase in the sorbed OH− concentrations. Nitrite and a Meisenheimer complex were identified as degradation products for TNT. Other PCM, such as biochar, also demonstrated a comparable ability in promoting TNT decay at pH 7. Furthermore, a rapid degradation of TNT at pH 7 was observed when biochar was used as a soil amendment (4% by weight). Our results suggest that PCM can facilitate TNT and 2,4-dinitroanisole decay via a surface-promoted hydrolysis at neutral pH conditions, suggesting a promising alternative for in situ soil remediation.

Kinetics of CO+ and CO2+ with N and O atoms.

We have measured reaction rate constants for CO+ and CO2+ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10-11 cm3 s-1 and 4.0 ± 1.6 × 10-11 cm3 s-1 for the reactions of CO+ with N and O atoms, respectively, and 8.0 ± 3.0 × 10-12 cm3 s-1 and 2.0 ± 0.8 × 10-11 cm3 s-1 for the reactions of CO2+ with N and O atoms, respectively. The reaction of CO2+ + O is observed to yield O2+ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere.

Spectrophotometric determination of reaction rates and kinetic parameters of a BAHD acyltransferase using DTNB (5,5′-dithio-bis-[2-nitrobenzoic acid

Hydroxycinnamoyl-Coenzyme A (CoA) hydroxycinnamoyl transferases are BAHD family acyltransferases that transfer hydroxycinnamoyl moieties from a CoA-thioester to an acceptor amine or alcohol to form an N-hydroxycinnamoyl amide or O-hydroxycinnamoyl ester, respectively, with the concomitant release of free CoA. One approach to measure reaction rates for these enzymes is to quantify the hydroxycinnamoyl amide or ester reaction product following chromatographic separation of reaction components. This approach can be labor-intensive and time-consuming. As an alternative, we examined the use of 5,5′-dithio-bis-(2-nitrobenzoic acid) (DTNB, Ellman’s reagent) to spectrophotometrically quantify, in real time, the release of free CoA during the transferase reaction. Using a hydroxycinnamoyl-CoA:l-DOPA hydroxycinnamoyl transferase as a model, we show that DTNB has little to no effect on the transferase reaction and can be used to provide a good estimate of hydroxycinnamoyl amide formation, thus allowing for the quick and easy collection of reaction rate data and determination of transferase kinetic parameters. This approach should be applicable to a wide range of hydroxycinnamoyl-CoA and other BAHD acyltransferases.

Acceleration of a ground-state reaction by selective femtosecond-infrared-laser-pulse excitation

Infrared (IR) excitation of vibrations that participate in the reaction coordinate of an otherwise thermally driven chemical reaction are believed to lead to its acceleration. Attempts at the practical realization of this concept have been hampered so far by competing processes leading to sample heating. Here we demonstrate, using femtosecond IR-pump IR-probe experiments, the acceleration of urethane and polyurethane formation due to vibrational excitation of the reactants for 1:1 mixtures of phenylisocyanate and cyclohexanol, and toluene-2,4-diisocyanate and 2,2,2-trichloroethane-1,1-diol, respectively. We measured reaction rate changes upon selective vibrational excitation with negligible heating of the sample and observed an increase of the reaction rate up to 24%. The observation is rationalized using reactant and transition-state structures obtained from quantum chemical calculations. We subsequently used IR-driven reaction acceleration to write a polyurethane square on sample windows using a femtosecond IR pulse.

Field Reaction Experiments of Carbonate Minerals in Spring Waters: Natural Analogue of Geologic CO2 Storage

To diminish the uncertainty of the mineral trapping rate during geologic CO2 storage, the growth rate of carbonate minerals was measured in CO2-containing spring waters, which can be regarded as a natural analogue of geologic CO2 storage. The authors’ approach, using nanoscale analysis of seed crystal surfaces after immersion into spring waters, enables rapid and accurate measurement of mineral reaction rates. The results show that calcite growth rates in spring waters were lower by 1–3 orders than the values given in a database of laboratory experiment results. We verified the traditional paradigm that Mg2+ controls carbonate reaction kinetics. An increase of the Mg/Ca ratio to around 5 by adding Mg2+ to spring waters markedly reduced the calcite growth rate. However, even if effects of Mg2+ and flow rate are considered, we were unable to explain satisfactorily the difference of the calcite growth rates between those of spring waters and laboratory experiments. Therefore, other factors might also be related to the slow growth rate in nature. The present results, including the fact such that neither dolomite nor magnesite was formed even at the high Mg/Ca ratio, are expected to provide an important constraint to overestimation of the mineral trapping rate.

Modeling Bimolecular Reactive Transport With Mixing‐Limitation: Theory and Application to Column Experiments

AbstractThe challenge of determining mixing extent of solutions undergoing advective‐dispersive‐diffusive transport is well known. In particular, reaction extent between displacing and displaced solutes depends on mixing at the pore scale, that is, generally smaller than continuum scale quantification that relies on dispersive fluxes. Here a novel mobile‐mobile mass transfer approach is developed to distinguish diffusive mixing from dispersive spreading in one‐dimensional transport involving small‐scale velocity variations with some correlation, such as occurs in hydrodynamic dispersion, in which short‐range ballistic transports give rise to dispersed but not mixed segregation zones, termed here ballisticules. When considering transport of a single solution, this approach distinguishes self‐diffusive mixing from spreading, and in the case of displacement of one solution by another, each containing a participant reactant of an irreversible bimolecular reaction, this results in time‐delayed diffusive mixing of reactants. The approach generates models for both kinetically controlled and equilibrium irreversible reaction cases, while honoring independently measured reaction rates and dispersivities. The mathematical solution for the equilibrium case is a simple analytical expression. The approach is applied to published experimental data on bimolecular reactions for homogeneous porous media under postasymptotic dispersive conditions with good results.

NEUTRON FIELD MEASUREMENT OF P(35)+Be SOURCE USING THE MULTI-FOIL ACTIVATION METHOD.

Neutron field from the p+Be interaction was investigated at the NPI CAS for a proton beam energy of 35 MeV and thick beryllium target. Broad neutron spectra at close source-to-sample distances were determined using the multi-foil activation technique. Two large sets of dosimetry foils containing the Ni, Co, Au, In, Ti, Al, Y, Lu, Nb and Fe were irradiated at a distance of 74 mm at direct neutron beam axis and at a distance of 34 mm from beam axis. Supporting Monte-Carlo MCNPX calculations of the irradiation system were performed as well. From measured reaction rates, the neutron energy spectra at both positions were reconstructed employing the modified version of the SAND-II unfolding code and activation cross-section data from the EAF-2010 library. At the position of irradiated samples, the total fast neutron flux reaches the value up to 1010 cm-2 s-1, and the neutron field is utilizable for radiation hardness study and integral benchmark experiments within the International Fusion Material Irradiation Facility (IFMIF) program.

Limitation of thermogravimetry for oxy-combustion analysis of coal chars

A kinetic study of the oxy-combustion of chars obtained from two Polish steam coals was conducted. A comparative study was focused on the data analysis concerning oxy-combustion of coal chars collected from two thermobalances, which differed in construction, design of crucibles, sample size and gas flow direction. The influence of individual factors such as temperature, O2 concentration, the dimensions of the crucibles and the dimensions of the char bed on the reaction rate was analysed. On the basis of these results, a global kinetic model was elaborated showing the influence of all factors, particularly mass transfer conditions, on reaction rates. The developed model can be extended to other mass transfer conditions or fluidised bed conditions and can be applied to other fast reactions to ensure they occur in the chemical reaction kinetics control regime during TG tests. It was concluded that the gas–solid fuel contact characteristics are crucial for the measured reaction rate and for the interpretation of kinetic data.

Bio-sensing of metal ions by a novel 3D-printable smartphone spectrometer

The first lens-free smartphone spectrometer capable of observation and analysis of reaction kinetics and point measurements without additional electronics is demonstrated in this paper. The reaction rate is calculated and plotted in adjustable intervals by a smartphone app. The only additional materials needed for the spectrometer are produced by 3D-printing. The flashlight LED and the ambient light sensor of the smartphone are used for the absorbance measurements. The smartphone spectrometer is capable of measuring reaction rates by monitoring changes in the absorbance at wavelengths of approximately 425–560 and 625–675nm. The restriction on specific wavelength is caused by the nonhomogeneous emission spectra of the smartphone’s flashlights LED. As a demonstration, the spectrometer and the individually programmed analysis software are used for reading and processing the signal of a urease bioassay developed for the detection and measurement of enzyme inhibitors. The inhibition caused by heavy metal ions is assessed by the smartphone through measurement of urease activity to a minimum activity of approximately 1Uml−1. For a detailed investigation of cumulative inhibition effects by low-cost analytical smartphone equipment, the results of mixed inhibition by heavy metals is compared to Bliss independence and Loewe additivity model theories. The study shows that cumulative inhibition effects of multi-component heavy metal solutions and different surface and supply water samples do not follow the conventional models of mixed inhibition effects. However, toxicity of mixed and single heavy metal ions in multi-component solutions, surface water samples, and supply water samples can be determined by the urease bioassay and the smartphone spectrometer. General water quality can also be rated using the same bioassay and equipment.

Electromagnetic and Heat-Transfer Simulation of the Catalytic Dehydrogenation of Ethylbenzene under Microwave Irradiation

Electromagnetic and heat-transfer simulations were used to study the effects of microwave-generated nonuniform temperature distributions in a catalyst bed on the rate enhancement of a fixed-bed flow reaction. We used the dehydrogenation of ethylbenzene over a magnetite catalyst as a model reaction. During the microwave reaction, a temperature gradient was generated in the catalyst bed; the highest temperature occurred at the core of the catalyst bed, and it parabolically decreased toward the surface. Using these simulation results and Arrhenius parameters, the reaction rates were estimated by considering the nonuniform temperature distribution. The measured reaction rate was 36% larger than the simulated value, indicating that the rate enhancement under microwaves can not only be attributed to the nonuniform temperature distribution in the catalyst bed. This could be due to nonequilibrium local heating (the so-called hot spot) in the very small region around the catalyst particle.

Lead Benchmark Experiment with DT Neutrons at JAEA/FNS

A new benchmark experiment on lead with DT neutrons was designed and carried out with a large lead assembly covered with Li2O blocks at JAEA/FNS to validate nuclear data of lead for measurement of reaction rates without impact of background neutrons. The experiment was analyzed by using the MCNP5-1.40 code with the latest nuclear data libraries, ENDF/B-VII.1, JEFF-3.2 and JENDL-4.0. The calculated reaction rates underestimated the measured ones with the depth. Moreover, the tendencies of C/Es were different among the nuclear data libraries. In order to find out the reasons of the differences, we examined reaction cross-sections of lead in the nuclear data libraries in detail. The potential reactions to cause the underestimation issue of the calculated reaction rates were indicated through this study.

Mechanistic Insight into the Reactivity of Chlorine-Derived Radicals in the Aqueous-Phase UV-Chlorine Advanced Oxidation Process: Quantum Mechanical Calculations.

The combined ultraviolet (UV) and free chlorine (UV-chlorine) advanced oxidation process that produces highly reactive hydroxyl radicals (HO•) and chlorine radicals (Cl•) is an attractive alternative to UV alone or chlorination for disinfection because of the destruction of a wide variety of organic compounds. However, concerns about the potential formation of chlorinated transformation products require an understanding of the radical-induced elementary reaction mechanisms and their reaction-rate constants. While many studies have revealed the reactivity of oxygenated radicals, the reaction mechanisms of chlorine-derived radicals have not been elucidated due to the data scarcity and discrepancies among experimental observations. We found a linear free-energy relationship quantum mechanically calculated free energies of reaction and the literature-reported experimentally measured reaction rate constants, kexp, for 22 chlorine-derived inorganic radical reactions in the UV-chlorine process. This relationship highlights the discrepancy among literature-reported rate constants and aids in the determination of the rate constant using quantum mechanical calculations. We also found linear correlations between the theoretically calculated free energies of activation and kexp for 31 reactions of Cl• with organic compounds. The correlation suggests that H-abstraction and Cl-adduct formation are the major reaction mechanisms. This is the first comprehensive study on chlorine-derived radical reactions, and it provides mechanistic insight into the reaction mechanisms for the development of an elementary reaction-based kinetic model.

Measurement of 89Y(n,2n) spectral averaged cross section in LR-0 special core reactor spectrum

The present paper describes reaction rate measurement of 89Y(n,2n)88Y in a well-defined reactor spectrum of a special core assembled in the LR-0 reactor and compares this value with results of simulation. The reaction rate is derived from the measurement of activity of 88Y using gamma-ray spectrometry of irradiated Y2O3 sample. The resulting cross section value averaged in spectrum is 43.9 ± 1.5 µb, averaged in the 235U spectrum is 0.172 ± 0.006mb. This cross-section is important as it is used as high energy neutron monitor and is therefore included in the International Reactor Dosimetry and Fusion File. Calculations of reaction rates were performed with the MCNP6 code using ENDF/B-VII.0, JEFF-3.1, JEFF-3.2, JENDL-3.3, JENDL-4, ROSFOND-2010, CENDL-3.1 and IRDFF nuclear data libraries. The agreement with uranium description by CIELO library is very good, while in ENDF/B-VII.0 description of uranium, underprediction about 10% in average can be observed.

UV and visible hydrogen photo-production using Pt promoted Nb-doped TiO2 photo-catalysts: Interpreting quantum efficiency

Nb-doped titania nanopowders active under UV and visible light were used as supports of platinum and tested in the photo-production of hydrogen using methanol as sacrificial agent in liquid phase. Samples were characterized using X-ray diffraction and photoelectron spectroscopy, UV–vis and photoluminescence spectroscopies, electron paramagnetic resonance, and transmission electron microscopy. The catalytic performance was evaluated taking into account the measurement of the corresponding reaction rates and the computation of the quantum efficiency values. Optimization of the catalytic output upon UV and visible excitation was carried out as a function of the Nb content of the material and using an experimental design taking as factors the methanol:water ratio, and catalyst concentration at liquid phase. The performance of the materials and the optimum for a catalyst containing a 2.5mol% of Nb upon UV, visible light and sunlight was examined. The characterization results concerning both the charge carrier species recombination and the fate of such species at the catalytic surface were used to elucidate the physico-chemical roots for maximizing activity.