Reaction Rate Measurements Research Articles

Isotope Exchange Reaction of OH− Anion with HD at Temperatures from 15 K up to 300 K: Ion Trap Study

This paper presents the results of an experimental study of the reaction of OH− anions with HD molecules leading to the formation of OD− anions. The study’s main goal was to obtain the temperature dependence of the reaction rate coefficient and determine the reaction’s enthalpy. This study was carried out at astrophysically relevant temperatures from 15 to 300 K. The reaction was studied using a temperature-variable cryogenic linear 22-pole radio-frequency ion trap. The rotational temperature of the OH− anions in the ion trap was characterized by near-threshold photodetachment spectroscopy. At 15 K, the measured reaction rate coefficient is 5 × 10−10 cm3 s−1. With increasing temperature, the reaction rate coefficient decreases monotonically to 5 × 10−11 cm3 s−1 at 300 K. Comparing with the previously determined rate coefficient of the reverse reaction of OD− anions with H2, we obtained the temperature dependence of the equilibrium constant. The enthalpy and entropy of the title reaction were determined in the studied temperature range as ΔH = (−23.9 ± 0.7 ± 2.6sys) meV and ΔS = (−8.5 ± 1.2 ± 1.4sys) J mol−1 K−1, respectively.

Effect of equilibrium constant for carbon dioxide recombination in hypersonic flow analysis

An equilibrium constant is an important parameter in regard to determining the backward reaction rate constant in chemical kinetics modeling for a hypersonic flow. Three common approaches for the equilibrium constant determination are based on the partition function, Gibbs free energy, and the experimental reaction rate measurement. The present study conducted a computational fluid dynamics (CFD) analysis with different equilibrium constant formulations in a thermochemical nonequilibrium hypersonic flow in order to study the influence of the equilibrium constant in carbon dioxide flow during the Martian entry. The equilibrium constant for the carbon dioxide molecule dissociation differs from one method to another among the reactions that are considered in the carbon dioxide flow. Three different flow conditions, which are based on the experimental data that is provided in the literature, are considered in the detailed comparison analysis using CFD. The variation of the flow properties in terms of pressure, temperature, and mass fraction along the stagnation line is compared for different cases of the equilibrium constant computation. The results that are obtained from the present study confirm that the equilibrium constant influences the numerical computation in the thermochemical nonequilibrium flow especially for the non-catalytic wall boundary condition.

Data integration across conditions improves turnover number estimates and metabolic predictions

Turnover numbers characterize a key property of enzymes, and their usage in constraint-based metabolic modeling is expected to increase the prediction accuracy of diverse cellular phenotypes. In vivo turnover numbers can be obtained by integrating reaction rate and enzyme abundance measurements from individual experiments. Yet, their contribution to improving predictions of condition-specific cellular phenotypes remains elusive. Here, we show that available in vitro and in vivo turnover numbers lead to poor prediction of condition-specific growth rates with protein-constrained models of Escherichia coli and Saccharomyces cerevisiae, particularly when protein abundances are considered. We demonstrate that correction of turnover numbers by simultaneous consideration of proteomics and physiological data leads to improved predictions of condition-specific growth rates. Moreover, the obtained estimates are more precise than corresponding in vitro turnover numbers. Therefore, our approach provides the means to correct turnover numbers and paves the way towards cataloguing kcatomes of other organisms.

Prediction of heavy metal biosorption mechanism through studying isotherm kinetic equations

The kinetic constants for free and immobilized cells were determined by measuring reaction rates at different metal concentrations at the optimum reaction conditions. (Kmax and Vmax) were calculated from the slope and intercept of the straight lines. The pseudo-second-order rate constants are derived based on the sorption capacity of the solid phase, where K2 is the rate constant for the pseudo-second-order model. Determined experimentally by plotting t/q against t. The mean free vitality of adsorption (E) was figured as 2.62 kJ mol−1 and the extent of E communicated gives data on the adsorption mechanism. An E value ranging from 1 to 8 indicates physisorption and 8–16 kJ mol−1 predicts ion exchange. Thus, the evaluated value of 2.62 kJ mol−1 predicts the phenomenon of physisorption, which suggests that metal ions were favorably adsorbed by this biosorbent in a multi-layer fashion. The overall result suggested that 98.2% of U (VI) by biosorption of U in the mechanism of adsorption will include chemisorption mechanistic pathway: Langmuir, Freundlich, equations and the values of Kf 5.791 where KL 3.9 were determined from the linear plot of log qe vs. log Ce at 30 °C, indicating that metal ions were favorably adsorbed by this biosorbent in a multi-layer fashion and instrumentation of beads characterizing novel Binding sites using FTIR & SEM beside change in peaks position which assigned for its groups confirm biosorption of metal.

Operando kinetics of hydrogen evolution and elemental dissolution I: The dissolution of galvanized steel in hydrochloric acid

A new approach to the simultaneous measurement of the hydrogen evolution reaction (HER) and elemental dissolution rates is presented and applied to the reaction of galvanized steel coatings with HCl solution. Dissolution profiles revealed several kinetic regimes including the dissolution of the intact coating, the dissolution of the nanometric Fe2Al5 intermetallic layer, and the corrosion of the steel substrate. The HER and Zn dissolution rates were in excellent agreement, and the quantities of interfacial Fe and Al were consistent with the predicted 50 nm thick Fe2Al5 intermetallic film, demonstrating the analytical capabilities of the atomic emission spectroelectrochemistry (AESEC) technique.

Selective hydrogenation via precise hydrogen bond interactions on catalytic scaffolds

The active site environment in enzymes has been known to affect catalyst performance through weak interactions with a substrate, but precise synthetic control of enzyme inspired heterogeneous catalysts remains challenging. Here, we synthesize hyper-crosslinked porous polymer (HCPs) with solely -OH or -CH3 groups on the polymer scaffold to tune the environment of active sites. Reaction rate measurements, spectroscopic techniques, along with DFT calculations show that HCP-OH catalysts enhance the hydrogenation rate of H-acceptor substrates containing carbonyl groups whereas hydrophobic HCP- CH3 ones promote non-H bond substrate activation. The functional groups go beyond enhancing substrate adsorption to partially activate the C = O bond and tune the catalytic sites. They also expose selectivity control in the hydrogenation of multifunctional substrates through preferential substrate functional group adsorption. The proposed synthetic strategy opens a new class of porous polymers for selective catalysis.

Determination of the swimming mechanism of Au@TiO2 active matter and implications on active-passive interactions.

Non-equilibrium dynamic assembly attracts considerable attention due to the possibility of forming diverse structures that can potentially lead to functional materials. Despite significant progress in understanding and modelling, the complexity of the system implies that different phases of the assembly formation are governed by different interactions. It is clear that both, hydrodynamic and chemical interactions stem from the activity of the particle, but correlation to specific chemical species remains not yet understood. Here, we investigate the origin of the main driving forces for light-driven Au@TiO2 micromotors and look at the implication this causes for the interactions between active and passive particles. We develop precision experimental measurements of the photochemical reaction rate, which are correlated with the observed speed of Au@TiO2 micromotors. The comparison with two distinct models allows the conclusion that the dominant propulsion mechanism of the active particles is self-electrophoresis based on the self-generated H+ gradient. We verify this assumption by adding salt and confirm the dependence of the expected swimming behaviour on salt concentration and investigate the consequences for raft formation in COMSOL simulations.

Low-temperature kinetics for the N + NO reaction: experiment guides the way.

The reaction N(4S) + NO(X2Π) → O(3P) + N2(X1Σ+g) plays a pivotal role in the conversion of atomic to molecular nitrogen in dense interstellar clouds and in the atmosphere. Here we report a joint experimental and computational investigation of the N + NO reaction with the aim of providing improved constraints on its low temperature reactivity. Thermal rates were measured over the 50 to 296 K range in a continuous supersonic flow reactor coupled with pulsed laser photolysis and laser induced fluorescence for the production and detection of N(4S) atoms, respectively. With decreasing temperature, the experimentally measured reaction rate was found to monotonously increase up to a value of (6.6 ± 1.3) × 10-11 cm3 s-1 at 50 K. To confirm this finding, quasi-classical trajectory simulations were carried out on a previously validated, full-dimensional potential energy surface (PES). However, around 50 K the computed rates decreased which required re-evaluation of the reactive PES in the long-range part due to a small spurious barrier with a height of ∼40 K in the entrance channel. By exploring different correction schemes the measured thermal rates can be adequately reproduced, displaying a clear negative temperature dependence over the entire temperature range. The possible astrochemical implications of an increased reaction rate at low temperature are also discussed.

Reaction of N+ Ion with H2, HD, and D2 at Low Temperatures: Experimental Study of the Pathway to Deuterated Nitrogen-containing Molecules in the Interstellar Medium

This work is motivated by many observations of nitrogen hydrides including their isotopologues in the interstellar space. We studied the formation of NH+ and ND+ ions in the nearly thermoneutral hydrogen abstraction reactions of N+ ions with H2, HD, and D2 at temperatures from 300 K down to 15 K using a variable-temperature 22-pole radio frequency ion trap. For the reaction of N+ with HD, the branching ratios for production of ND+ and NH+ ions were also determined. The activation energies of all four reaction channels were determined from the temperature dependencies of the measured reaction rate coefficients. Under the assumption of no energy barriers on the reaction paths, we derive the vibrationless energy change (i.e., the difference of equilibrium Born–Oppenheimer potential energies of products and reactants) in the reactions as ΔE e = (103 ± 3) meV.

Bayesian Calibration of Kinetic Parameters in the CH Chemistry Toward Accurate Prompt-NO Modelling

Abstract Significant efforts made by the gas turbine industry have helped reduce nitrogen oxides (NOx) emissions considerably. To meet and surpass the increasingly stringent regulations, accurate and robust thermochemical mechanisms are needed to help design future sub-10 ppm combustion systems. Uncertainty in kinetic modeling, however, can result in large prediction uncertainty and significant discrepancy between models that hinder the identification of promising combustors with confidence. Direct reaction rate measurements are seldom available for some reactions, especially when involving short-lived radicals such as methylidyne, CH. As the main precursor to the prompt-NO formation pathway, its large parametric uncertainty directly propagates through the nitrogen chemistry preventing accurate and precise emissions predictions. Recent independent CH concentration measurements obtained at various operating conditions are used as indirect rate measurements to perform statistical, or Bayesian, calibration. A subset of important reactions in the CH chemistry affecting peak-CH concentration is identified through uncertainty-weighted sensitivity analysis to first constrain the parametric space of this prompt-NO precursor. Spectral expansion provides the surrogate model used in the Markov-Chain Monte Carlo method to evaluate the posterior kinetic distribution. The resulting constrained CH-chemistry better captures experimental measurements while providing smaller prediction uncertainty of a similar order as the uncertainty of the measurements, which can increase the confidence in simulation results to identify promising future low-emissions configurations. For the quasi-steady-state species CH, fuel decomposition reactions leading to CH production are constrained while little impact is observed for intermediate reactions within the CH-chemistry. The reduction in prediction uncertainty results mainly from the constrained correlations between parameters which greatly limit the set of feasible reaction rate combinations. Additional independent direct and indirect measurements would be necessary to further constrain rate parameters in the CH chemistry, but this calibration demonstrates that predictions of radical species can be improved by assimilating enough data.

Elucidating the Roles of Local and Nonlocal Rate Enhancement Mechanisms in Plasmonic Catalysis.

Plasmonic metal nanoparticles (e.g., Ag, Au, and Cu) constitute a class of materials that interact with light via the excitation of localized surface plasmon resonance (LSPR). Numerous studies have reported substantial enhancements in the rates of chemical reactions on illuminated plasmonic nanoparticle catalysts compared to corresponding systems in the absence of illumination. There are two mechanisms that have been proposed to explain the LSPR-induced chemical reactivity. One mechanism assumes a local plasmon-induced hot charge-carrier-mediated activation of the reactants, while the other assumes an LSPR-induced equilibrium heating of the catalyst, which leads to energy transfer to and chemical reaction of the adsorbed reactants. In this contribution, we developed a setup amenable to accurate in situ catalyst temperature and kinetic reaction rate measurements. We employed this setup to study the LSPR-induced rate enhancement in a case study of the CO oxidation reaction on plasmonic, monometallic Ag nanoparticle catalysts supported on α-Al2O3. We explored various Ag loadings and clustering levels. Our data show that the equilibrium heating of the catalyst cannot fully explain the illumination-induced plasmonic rate enhancements. This is the case even for high loading and clustering of Ag nanoparticles, where the equilibrium heating significantly increases. Based on the analysis, we propose that local effects, related to the plasmon-induced activation of adsorbates (reactants) via electronic excitation of the reactant or photothermal heating of the reactants that is highly localized to the individual nanoparticles, play a critical role in driving LSPR-induced chemical reactions.

Towards solar measurements of nuclear reaction rates

ABSTRACT Nuclear reaction rates are a fundamental yet uncertain ingredient in stellar evolution models. The astrophysical S-factor pertaining to the initial reaction in the proton–proton (pp) chain is uncertain at the 1 per cent level, which contributes a systematic but generally unpropagated error of similar order in the theoretical ages of stars. In this work, we study the prospect of improving the measurement of this and other reaction rates in the pp chain and CNO cycle using helioseismology and solar neutrinos. We show that when other aspects of the solar model are improved, then it shall be possible using current solar data to improve the precision of this measurement by nearly an order of magnitude and hence the corresponding uncertainty on the ages of low-mass stars by a similar amount.

A reaction rate measurement experimental program of China experimental fast reactor

The study of the nuclear reaction rate distribution in the MOX core of China Experimental Fast Reactor is a necessary condition for obtaining the operating license. From the characteristics of core structure, both the principle of experimental and experimental system are studies, emphasized on the scheme design such as the size of foils, the irradiation power and counting rate. Firstly, based on the Monte Carlo Code（MCNP）and NJOY codes, the distribution of reaction rates is obtained. MCNP is a general-purpose Monte Carlo N-Particle code that can be used for neutron, photon, electron, and so on. The NJOY nuclear data processing system is a comprehensive computer code package for producing to cross sections and related nuclear parameters from ENDF/B VIII.0 evaluated nuclear data. Secondly, the activity measurement of foils was obtained by HPGE, then the reaction rate is determined by data processing. It is worth mentioning that the experimental reactivity introduction has a negligible effect on the reactor core by MCNP. Finally, a set of activation experiment scheme that is suitable for CEFR MOX core is formed, which serves as a guide to carry out activation method experiments.

A Geo‐Structurally Based Correction Factor for Apparent Dissolution Rates in Fractured Media

AbstractField measurements of apparent geochemical weathering reaction rates in subsurface fractured porous media are known to deviate from laboratory measurements by multiple orders of magnitude. To date, there is no geologically based explanation for this discrepancy that can be used to predict reaction rates in field systems. Proposed correction factors are typically based on ad hoc characterizations related to geochemical kinetic models. Through a series of high‐fidelity reactive transport simulations of mineral dissolution within explicit 3D discrete fracture networks, we are able to link the geo‐structural attributes with reactive transport observations. We develop a correction factor to linear transition state theory for the prediction of the apparent dissolution rate based on measurable geological properties. The modified rate law shows excellent agreement with numerical simulations, indicating that geological structure could be a primary reason for the discrepancy between laboratory and field observations of apparent dissolution rates in fractured media.

Ability to Inhibit H+ Transmission of Gemini Surfactants with Different Chain Lengths under Different Ca2+ Circumstances.

To study the ability to inhibit ion transmission of the Gemini surfactant under different Ca2+ circumstances, three kinds of Gemini surfactants with different alkyl chain lengths are synthesized (Cn-4-Cn, n = 12, 14, and 16), which are characterized using 1H NMR, 13C NMR, and Fourier transform infrared spectroscopy. To analyze the property of inhibition of the acid–rock reaction rate, surface tension and contact angle measurements and atomic force microscopy (AFM) results are obtained with different surfactants and under different Ca2+ concentrations. Inhibition rates with different alkyl chain lengths and an acid-etched surface morphology are also studied carefully. The result shows that all cationic Gemini surfactants significantly impact the control of the reaction rate, and the reaction rate decreased remarkably by 44.4% after adding 12-4-12. The ΔG and WA indicate that 12-4-12 has the best adsorption ability on the rock with added Ca2+ compared with the other two Gemini surfactants. It is revealed through the AFM that Ca2+ can significantly change the adsorption morphology of the surfactant. The surfactant adsorption area decreased when Ca2+ is dispersed in the solution as well. These two phenomena can lead to the reduced ability to block H+ of 14-4-14 and 16-4-16. However, the presence of Ca2+ affects the adsorption area of 12-4-12 slightly. Thus, the reaction rate, including that of 12-4-12, is almost unchanged. Because 12-4-12 is adsorbed tightly on the rock surface, H+ can only react with the rock on the unabsorbed dot, resulting in rock surface nonuniformity after being etched, which is beneficial for maintaining the conductivity of the crack.

Direct measurement of reaction rate for decomposition of diisopropyl methylphosphonate at high temperature using shock tube and laser absorption

AbstractDiisopropyl methylphosphonate (DIMP) is an organophosphorus compound used as a simulant for the chemical weapon Sarin. One of the means of destruction of Sarin is incineration at high temperature, which requires a clear understanding of the high‐temperature reaction kinetics of Sarin. In this work, we study the reaction rate for thermal decomposition of DIMP in the temperature range of 762–1059 K at a pressure near 1 atm. Test conditions are carefully chosen to minimize interferences from other reactions and intermediates formed during the decomposition process. The experimental rates obtained have been fitted for the Arrhenius rate expression to get the rate parameters of the reaction r1, DIMP (C7H17O3P) = IMP (C4H11O3P) + propene (C3H6) as The rate obtained in this work extended the temperature range of previous measurements (up to 800 K) and can be used in reaction kinetic models for combustion of DIMP.

Kinetic isotope effects for dissociative recombination of tritiated ketenyl ion (3HCCO+): A surface-hopping ab initio molecular dynamics study

Dissociative recombination (DR) reactions are important when modeling charged species in the presence of free electrons. While experimental measurements of DR reaction rates are challenging, surface hopping ab initio molecular dynamics (SH-AIMD) simulations provide an attractive alternative. SH-AIMD is especially well-suited for estimating branching ratios, i.e., the relative rates of competing production channels, for DR reactions. Although the radiolysis of diatomic tritium has been studied experimentally, previous attempts to model these systems have failed to account for isotope effects in DR reactions. Previous SH-AIMD studies have also not investigated tritium isotope effects for the branching ratios of DR reactions. In this study, we compute the DR branching ratios of the protiated and tritiated ketenyl ion. Comparison with literature values for the protiated branching ratios provides confidence in the reliability of our SH-AIMD results. Our simulations predict a significant increase of the HC + CO branching ratio for the tritiated system.

Acid stimulation in carbonates: Microfluidics allows accurate measurement of acidic fluid reaction rates in carbonate rocks by quantifying the produced CO2 gas

Acid stimulation has been the interest of researchers for many years due to its high efficiency in improving the near-wellbore permeability caused by the formation damage. The productivity enhancement stems from the creation of extremely conductive pathways called wormholes as a result of the interaction between the acidic fluid and carbonate rocks. Therefore, it is crucial to quantify the reaction rate between the acidic fluid and the carbonate rocks to ensure achieving the desired outcome. In this study, we will utilize microfluidics technology to accurately calculate the reaction rate of hydrochloric acid (HCl) with carbonate rocks under flow conditions by quantifying the produced CO2 gas. This approach, unlike conventional methods, allows for accurate measurement, short experiment duration, reduction of chemical usage, and power consumption. In this study, the reaction rate of the HCl with Silurian dolomite was successfully measured by quantifying the produced CO2 gas over time and found to be a surface reaction limited regime. Additionally, the reaction rate of HCl and Indiana limestone was also measured for various HCl concentrations and found to be a mass transfer limit regime. The findings of this study allow for accurate reaction rate measurement of reactive fluid with carbonate rocks which will ultimately improve the design and prediction of carbonate acidizing process. Furthermore, the proposed method enables direct observation of the multi-physics process of porous media flow through a geo-material by facilitating the visualization and understanding of rock-fluid interactions. Lastly, the cell presented here offers a promising technique to investigate a real reservoir rock for fast and reliable laboratory tests. Multiple fluid-solid chemical reactions can be assessed, allowing direct observations of the flow and transport in porous and fractured media.

Experiment and theory elucidate the pathways for H3+formation in the ultrafast double ionization in methanol

Experiment and theory elucidate the pathways for H₃⁺formation in the ultrafast double ionization in methanol

Pozzolanic reactivity and drying shrinkage characteristics of optimized blended cementitious composites comprising of Nano-Silica particles

Measurement of reaction rate amid Pozzolans and portlandite (Ca(OH2) in the pore solution of cementitious system is the essential mechanism that need to be quantified for any blended cementitious system. So, in this study pozzolanic reactivity of multi-blended cementitious composites (binary, ternary and quaternary) was determined and corroborated using three different techniques i.e., strength activity index (SAI), selective dissolution method (SDM) and thermogravimetric analysis (TGA). In addition, efforts were made to correlate the measured drying shrinkage values of multi-blended cementitious mix with pozzolanic reactivity indices. Theory of particle packing which works on the basis of modified Andreasen and Andersen model was adopted to design the optimized blended cementitious mixes. It is observed that pozzolanic reactivity was found to be the highest for 3% nano-silica admixed binary cementitious mix, however, this binary mix reported that associated drying shrinkage is a cause of concern. Further measurement on ternary and quaternary blended mix revealed that SCMs at triplet scale (quaternary blended) is found to be equally pozzolanic and also sustainable with respect to the phenomenon of drying shrinkage. This is attributed to the fact that synergic effect of multiple SCMs (quaternary blend) triggered the pozzolanic activity by enhancing the CH consumption rate from the pore solution. From the results of the experimental investigation, it is also proposed that there exists a “three-phase drying system” for all kind of cementitious mixes. Drying shrinkage results also showed best fit in correspondence to measured pozzolanic reactivity indices.