Chemical Rate Constants Research Articles

Hydrodynamic Rocking Disc Electrode Study of the TEMPO‐mediated Catalytic Oxidation of Primary Alcohols

AbstractThe hydrodynamically thinned diffusion layer at a sinusoidally rocking disc is approximately uniform and can be expressed in terms of a diffusion layer thickness with D, the diffusion coefficient of the redox active species, v, the kinematic viscosity, Θ, the total rocking angle (here 90 degrees), and f, the rocking frequency (ranging here from 0.83 to 25 Hz). For the one‐electron oxidation of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) in aqueous carbonate buffer pH 9.5, it is shown that there is quantitative agreement between the expression for the diffusion layer thickness and experimental data. Next, for seven primary alcohols, the catalytic TEMPO‐mediated oxidation mechanism is investigated under rocking disc conditions. Chemical rate constants are evaluated (by varying the diffusion layer thickness) employing the DigiElch4.F simulation package. Trends in the chemical rate constants (compared with DFT calculations) suggest enhanced reactivity for methanol versus higher primary alcohols and for aromatic versus non‐aromatic primary alcohols. Rocking disc voltammetry allows quantitative mechanistic analysis in the laminar flow regime.

Primary yields and reaction sets with corresponding rate constants for computer simulation of water radiolysis at elevated temperature

ABSTRACTThe chemical conditions of coolant water in light water reactors are important factors affecting the corrosive environment and the integrity of the structural materials. Computer simulations have commonly been used to predict chemical environment changes due to water radiolysis under extreme conditions. Initial parameters, including the chemical reactions, rate constants, and primary yields of water decomposition by radiolysis, have been reported by several groups in the past few decades. Using a FACSIMILE simulation, the variations caused by utilizing different sets of primary yields of water radiolysis, including γ-rays and fast neutrons, along with the chemical reactions and rate constants at 285 °C were evaluated in this study. The rate constant of the reverse reaction of OH + H2 → H + H2O was found to be the main factor controlling the variation between the different sets of chemical reactions and rate constants.

Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts.

The hydrogen production electrocatalyst Ni(P(Ph)2N(Ph)2)2(2+) (1) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF(H)(+), the mechanism of H2 formation by 1 changes from an ECEC to an EECC mechanism as the potential approaches the Ni(I/0) couple. Two electrochemical methods, current-potential analysis and foot-of-the-wave analysis (FOWA), were performed on 1 to measure detailed kinetics of the competing ECEC and EECC pathways. A sensitivity analysis was performed on the methods using digital simulations to understand their strengths and limitations. Chemical rate constants were significantly underestimated when not accounting for electron-transfer kinetics, even when electron transfer was fast enough to afford a reversible noncatalytic wave. The EECC pathway of 1 was faster than the ECEC pathway under all conditions studied. Buffered DMF:DMF(H)(+) mixtures afforded an increase in the catalytic rate constant (k(obs)) of the EECC pathway, but k(obs) for the ECEC pathway did not change when using buffered acid. Further kinetic analysis of the ECEC path revealed that base increases the rate of isomerization from exo-protonated Ni(0) isomers to the catalytically active endo-isomers, but decreases the rate of protonation of Ni(I). FOWA did not provide accurate rate constants, but FOWA was used to estimate the reduction potential of the previously undetected exo-protonated Ni(I) intermediate. Comparison of catalytic Tafel plots for 1 under different conditions reveals substantial inaccuracies in the turnover frequency at zero overpotential when the kinetic and thermodynamic effects of the conjugate base are not accounted for properly.

Discrimination of Kinetic Models by a Combination of Microirradiation and Fluorescence Photobleaching

Fluorescence recovery after photobleaching (FRAP) is an excellent tool to measure the chemical rate constants of fluorescently labeled proteins in living cells. Usually FRAP experiments are conducted with the protein concentrations being in a steady state, i.e., when the association and dissociation of the proteins are equilibrated. This is a strong limitation because situations in which rate constants change with time are of great scientific interest. In this study, we present an approach in which FRAP is used shortly after DNA damage introducing laser microirradiation, which results in the recruitment of the DNA clamp protein proliferating cell nuclear antigen (PCNA) to DNA lesions. We establish different kinetic models that are compatible with the observed PCNA recruitment data if FRAP is not used. By using FRAP at different time points during protein accumulation, we can not only exclude two out of three models, but we can also determine the rate constants with increased reliability. This study thus demonstrates the feasibility of using FRAP during protein recruitment and its application in the discrimination of possible kinetic models.

Refinement of the inversion-transfer NMR experiment for faster characterization of chemical exchange.

It is shown theoretically that the inversion-transfer experiment used to estimate the value of unidirectional rate constants in chemical exchange systems can be performed faster via a reduction of the recovery delay. The chemical exchange rate constants can then be estimated accurately with a formula close to that of standard inversion transfer and easy to use, after a justified approximation. A function was developed to determine the optimal value of the recovery delay for an optimal inversion-transfer sequence. The validity of these theoretical results was checked experimentally with a solution of N,N-dimethylacetamide in which chemical exchange arises from internal hindered rotation.

Simple method for determining metal power oxidation kinetics with a zirconia sensor

A basic zirconia oxygen sensor was utilized to monitor the gas–solid oxidation reaction characteristics of metal/metal-oxide powder beds. The method described allowed for simple determination of chemical reaction rate constants for the oxidation reactions. The signal output of the sensor was analyzed for oxidation of the powder bed and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed, and a discussion of relevant thermodynamic and kinetic theory was provided. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor. Combining the experimental data with relevant kinetic theory, chemical reaction rate constants were calculated from plots of sensor ocv versus time for each reaction step during the metal oxidation sequences.

Effect of temperature in titanium chemical mechanical planarization

The effect of temperature on the tribological and kinetic attributes of Ti chemical mechanical planarization (CMP) was investigated. Results indicated that processes at platen temperatures of 25 and 50 °C behaved similarly in terms of their tribological mechanism. At both temperatures, average coefficient of friction (COF) ranged from 0.19 to 0.41, indicating that boundary lubrication was the dominant tribological mechanism. Results also showed that average COF decreased with increasing platen temperature likely due to softening of pad asperities and lower slurry viscosity at the higher temperature. Due to exponentially accelerated chemical effects, Ti removal rate was higher when platen temperature was set at 50 °C. A two-step modified Langmuir–Hinshelwood model was used to simulate Ti removal rate and the chemical and mechanical rate constants under different polishing conditions. Simulated values of removal rate agreed well with experimental data. Simulated chemical rate to mechanical rate constant ratios suggested that the removal mechanism shifted from a more chemically-controlled to a more mechanically-controlled process as platen temperature was raised.

Thermal Decomposition of 2(3H) and 2(5H) Furanones: Theoretical Aspects.

The thermal decomposition reactions of 2(3H) and 2(5H) furanones and their methyl derivatives are explored. Theoretical calculations of the barriers, reaction enthalpies, and the properties of these and intermediate species are reported using the composite model chemistry CBS-QB3 and also the functional M06-2X allied to the 6-311++G(d,p) basis set. Thus, the bond dissociation enthalpies, ionization energies, and unimolecular chemical kinetic rate constants in the high-pressure limit were computed. We show that flow reactor experiments that intimated that heating the 2(3H) furanone converts it to the isomeric 2(5H) furanone occurs via a 1 → 2 H-transfer reaction to an open ring ketenoic aldehyde. The latter can then ring close to the other isomeric structure. The final products acrolein and carbon monoxide are only formed from 2(3H), and acrolein will further decompose to ethylene and CO. Comparable channels explain the interconversion of 5-methyl-2(3H) furanone to its 2(5H) isomer and to the formation of methyl vinyl ketone and CO. The influence of the methyl group at other positions on the ring is hardly of significance except in the case of 5-methyl-2(5H) furanone where a hydrogen atom transfer from the methyl group leads to the formation of a doubly unsaturated carboxylic compound, 2,4-pentadienoic acid. Studies of the UV photolysis of the parent compounds in both low-temperature inert argon matrices and in solution are broadly in accord with the thermal findings insofar as product formation is concerned and with our theoretical calculations. The dominant features of the early decomposition chemistry of these compounds are simple hydrogen transfer and simultaneous ring opening reactions, which do however result in some quite unusual species.

Saponification reaction system: a detailed mass transfer coefficient determination

The saponification of an aromatic ester with an aqueous sodium hydroxide was studied within a heterogeneous reaction medium in order to determine the overall kinetics of the selected system. The extended thermo-kinetic model was developed compared to the previously used simple one. The reaction rate within a heterogeneous liquid-liquid system incorporates a chemical kinetics term as well as mass transfer between both phases. Chemical rate constant was obtained from experiments within a homogeneous medium, whilst the mass-transfer coefficient was determined separately. The measured thermal profiles were then the bases for determining the overall reaction-rate. This study presents the development of an extended kinetic model for considering mass transfer regarding the saponification of ethyl benzoate with sodium hydroxide within a heterogeneous reaction medium. The time-dependences are presented for the mass transfer coefficient and the interfacial areas at different heterogeneous stages and temperatures. The results indicated an important role of reliable kinetic model, as significant difference in k(L)a product was obtained with extended and simple approach.

Kinetic Study of the Oxygen Reduction Reaction on α-Ni(OH)2 and α-Ni(OH)2 Supported on Graphene Oxide

The kinetics of the oxygen reduction reaction on α-Ni(OH)2 and α-Ni(OH)2 supported on graphene oxide (α-Ni(OH)2/GO) were investigated using rotating disk linear sweep voltammetry in alkaline solutions of varying oxygen and hydroxyl concentrations. Over the full hydroxyl concentration range (0.05 M to 0.5M), α-Ni(OH)2/GO displayed higher activity than unsupported α-Ni(OH)2. The electron transfer numbers were 2.9 ± 0.2 for α-Ni(OH)2, 3.4 ± 0.1 for α-Ni(OH)2/GO at low [OH−], and 3.8–3.9 for α-Ni(OH)2/GO at high [OH−]. Compared to α-Ni(OH)2, α-Ni(OH)2/GO displayed higher chemical reaction rate constants and higher electron transfer rate constants. These differences suggest that the synergy between the α-Ni(OH)2 catalyst and the graphene oxide support is related to increases in both the speed of the rate determining chemical step and the facility for electron transfer. The order of reaction with respect to oxygen was ∼1 for α-Ni(OH)2 and α-Ni(OH)2/GO. The order of reaction with respect to hydroxyl was ∼0 for α-Ni(OH)2 and α-Ni(OH)2/GO. The first order reaction with respect to oxygen is precedented, but the zero reaction order with respect to hydroxyl is particular to the α-Ni(OH)2 catalysts. The zero reaction order is explained by possible decoupling of solution and catalyst surface hydroxyl concentrations.

Applications of power-law transition state theory to the reaction of a hydroxyl radical with molecular hydrogen and the respiration in Camellia Japonica

The reaction of a hydroxyl radical with molecular hydrogen and the respiration in Camellia Japonica are used as examples for studying chemical reaction rate constants by the power-law transition state theory. In the first example, the rate constants are numerically obtained at different temperature and activation energies based on the Morse potential at the potential well and improved potential energy surface 5 (I5B) at the saddle point respectively, and the relationship between the rate constants and the power-law parameter is plotted as well. In the second one, especially, the power-law parameter presents an abrupt change at temperature 290 K around, which may be explained that the Camellia Japonica is the ombrophyte, and the activity of biological enzyme reaches the maximum at this temperature. This theory allows accordance with experimental values by a power-law parameter. It is found that the values of the power-law parameter in both examples deviate from 1 but not too much and lie in the 0–1 range and proved that power-law transition state theory shows a good agreement with experiments.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds for energy storage applications. Energy is stored in the metallic state and released during oxidation. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the sensor. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor for actual energy storage applications.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed. A model metal/metal-oxide system is examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility.

Analysis of Polymerase-DNA Interactions and Polymerase Activity with Electrically Actuated DNA Nanolevers on a Chip

Next generation DNA sequencing technologies are on the verge of revolutionizing research and clinical genomics. An important cornerstone for ever faster, less expensive and more accurate genomic information are highly optimized and error-free DNA polymerases.Here we present a novel method to study DNA polymerases and their interaction with nucleic acids on a chip. DNA molecules, which are end-tethered to microelectrodes on a chip, are set in motion by alternating electric fields, and the molecular dynamics of their oscillation (orientation switching) are measured by fluorescence energy transfer. The association and dissociation of various DNA polymerases are monitored in real-time by analyzing changes in the DNA motion that occur due to polymerase binding. Chemical rate constants of association and dissociation and affinity constants are determined.For the polymerases phi29, Taq, and the Klenow fragment, the influences of temperature, mono- and divalent ion concentration, and presence/absence of dNTPs on polymerase binding kinetics are investigated. The polymerization activity is evaluated and even exonuclease activity can be observed in real-time. Simultaneously, information on the size of the DNA-polymerase complex is obtained with sub-nanometer accuracy and conformational changes in the ternary polymerase-dNTP-DNA complex are revealed (match/mismatch situation).The method is label-free, uses a parallel microelectrode format for multiplexed assays and microfluidics for low sample consumption. It bears great potential as a powerful tool for the characterization of polymerases and facilitates the engineering of polymerases for, e.g., more efficient sequencing technologies.

Computational Infrared and Two-Dimensional Infrared Photon Echo Spectroscopy of Both Wild-Type and Double Mutant Myoglobin-CO Proteins

The CO stretching mode of both wild-type and double mutant ( T67R / S92D ) MbCO (carbonmonoxymyoglobin) proteins is an ideal infrared (IR) probe for studying the local electrostatic environment inside the myoglobin heme pocket. Recently, to elucidate the conformational switching dynamics between two distinguishable states, extensive IR absorption, IR pump-probe, and two-dimensional (2D) IR spectroscopic studies for various mutant MbCO's have been performed by the Fayer group. They showed that the 2D IR spectroscopy of the double mutant, which has a peroxidase enzyme activity, reveals a rapid chemical exchange between two distinct states, whereas that of the wild-type does not. Despite the fact that a few simulation studies on these systems were already performed and reported, such complicated experimental results have not been fully reproduced nor described in terms of conformational state-to-state transition processes. Here, we first develop a distributed vibrational solvatochromic charge model for describing the CO stretch frequency shift reflecting local electric potential changes. Then, by carrying out molecular dynamic simulations of the two MbCO's and examining their CO frequency trajectories, it becomes possible to identify a proper reaction coordinate consisting of His64 imidazole ring rotation and its distance to the CO ligand. From the 2D surfaces of the resulting potential of mean forces, the spectroscopically distinguished A1 and A3 states of the wild-type as well as two more substates of the double mutant are identified and their vibrational frequencies and distributions are separately examined. Our simulated IR absorption and 2D IR spectra of the two MbCO's are directly compared with the previous experimental results reported by the Fayer group. The chemical exchange rate constants extracted from the two-state kinetic analyses of the simulated 2D IR spectra are in excellent agreement with the experimental values. On the basis of the quantitative agreement between the simulated spectra and experimental ones, we further examine the conformational differences in the heme pockets of the two proteins and show that the double mutation, T67R / S92D , suppresses the A1 population, restricts the imidazole ring rotation, and increases hydrogen-bond strength between the imidazole Nε-H and the oxygen atom of the CO ligand. It is believed that such delicate change of distal His64 imidazole ring dynamics induced by the double mutation may be responsible for its enhanced peroxidase catalytic activity as compared to the wild-type myoglobin.

The Approximation of the Chemical Reaction Rate by Solving the Integral Equation

The paper discusses possible methods of approximation of the chemical reaction rate con- stant for the range of values that lie outside of the experimental temperature range: direct approximation of chemical reaction rate constants obtained by pro- cessing experimental values; approximation based on an analytical model of dependence of the integrated process cross-section on energy; and approximation based on the direct solution of the chemical reaction rate constant equation with arbitrary dependence of the integrated process cross-section on energy. The second-order reactions CH4 + M ! CH3 + H + M, CH3 +M! CH2 +H +M, CH3 +M! CH +H2 +M were explored. To solve the integrated equation, the variational Tikhonov's regularization method was used. It was shown that this method allowed both estimating the threshold energy value and re-establishing the cross- section form. By using the calculated cross-section we can obtain estimated chemical reaction rate constants over a wide temperature range. The data obtained can be used in various calculations in applied elds, in particular, in hypersonic gas dynamics problems, as well as for lling information system databases.

Kinetics of pseudo-boehmite sol coagulation

The kinetics of coagulation of spherical drops of the sol obtained in an ammonia solution by peptization of pseudo-boehmite with nitric acid has been investigated. The chemical reaction rate constants and coefficients of the ammonia cations diffusion in the formed gel were calculated from the experimental data in accordance with the model of a particle with noninteracting core.

Sources and Processes Contributing to Nitrogen Deposition: An Adjoint Model Analysis Applied to Biodiversity Hotspots Worldwide

Anthropogenic enrichment of reactive nitrogen (Nr) deposition is an ecological concern. We use the adjoint of a global 3-D chemical transport model (GEOS-Chem) to identify the sources and processes that control Nr deposition to an ensemble of biodiversity hotspots worldwide and two U.S. national parks (Cuyahoga and Rocky Mountain). We find that anthropogenic sources dominate deposition at all continental sites and are mainly regional (less than 1000 km) in origin. In Hawaii, Nr supply is controlled by oceanic emissions of ammonia (50%) and anthropogenic sources (50%), with important contributions from Asia and North America. Nr deposition is also sensitive in complicated ways to emissions of SO2, which affect Nr gas-aerosol partitioning, and of volatile organic compounds (VOCs), which affect oxidant concentrations and produce organic nitrate reservoirs. For example, VOC emissions generally inhibit deposition of locally emitted NOx but significantly increase Nr deposition downwind. However, in polluted boreal regions, anthropogenic VOC emissions can promote Nr deposition in winter. Uncertainties in chemical rate constants for OH + NO2 and NO2 hydrolysis also complicate the determination of source-receptor relationships for polluted sites in winter. Application of our adjoint sensitivities to the representative concentration pathways (RCPs) scenarios for 2010-2050 indicates that future decreases in Nr deposition due to NOx emission controls will be offset by concurrent increases in ammonia emissions from agriculture.

Kinetics and heat transfer analysis of carbon catalyzed solar cracking process

Solar thermochemical processing offers production of many fuels and commodities via reduced emission footprint. Although solar reactor design and flow configuration play a key role in process efficiency, use of the right catalyst further enhances the overall efficiency. Solar cracking of methane provides an excellent example of the direct effect of carbon catalyst on the heat transfer which in turn affects the feedstock decomposition rate. In this paper, a compilation of our research results on the testing of carbon catalyst using thermogravimetry, and its impact on the heat transfer are summarized along with a thorough kinetics analysis of methane decomposition. It is seen that carbon seeding uniforms the reactor temperature and the volumetric heating caused by the suspended carbon particles substantially improves the reactor performance. 37 chemical gas-phase reactions and rate constants were considered to simulate the non-catalyzed methane cracking, whereas 8 chemical surface reactions and rate constants were considered for methane cracking with carbon catalyst. As for the heat transfer analysis, thermal interaction of gas-particle flow and the thermal hydraulics between gas flow and particle were studied by two way coupled Euler-Lagrange approach. Discrete ordinate model was used to solve radiative transport between reactor walls and entrained particles. The results show that when carbon loading is increased from 0.2 g/min to 0.6 g/min, the product gas temperature reduces from 1121 K to 1010 K, whereas mass fractions of methane shows 30% increase in efficiency.

Kinetic study on the reaction between CO2-CO and wustite using the isotope exchange method

To investigate the reaction between CO2-CO and wustite using the isotope exchange method at 1073, 1173, 1273, and 1373 K, the experiment apparatus was designed to simulate the fluidized bed. The chemical rate constant was estimated by considering the effect of gas phase mass transfer on the reaction. It is found that the chemical rate constant is inversely decreased with the increase in the ratio of CO2/CO by volume. The activation energy of reaction is in a linear relationship with the ratio of CO2/CO by volume, and the average activation energy is 155.37 kJ/mol.