Showing papers on "Reaction rate constant published in 2020"

Atom-Tunneling in Chemistry

Abstract: Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom-tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. On the contrary, using computational investigations the influence of tunneling on the reaction rates can directly be monitored. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.

Highly efficient oxidative desulfurization of dibenzothiophene using Ni modified MoO3 catalyst

Abstract: Ni modified MoO3 (Ni-MoO3) had been synthesized by a facile one-step hydrothermal technique and was used for oxidative desulfurization (ODS) of dibenzothiophene (DBT) in the decalin/acetonitrile biphasic system with H2O2 as oxidant, the effect of different operating conditions was investigated. Under the optimal reaction condition, Ni-MoO3 catalyst showed excellent ODS performance toward DBT, the highest sulfur removal efficiency can be up to 99.8% and sulfur content was wiped out from 5000 to 10 ppm, which is more effective than the recent reported MoO3-based catalysts. The reaction kinetics obeyed the pseudo-first-order equation with an apparent rate constant of 0.076 min−1, which is twice that of pure MoO3 (0.035 min−1). The ODS mechanism of DBT with Ni-MoO3 was explored by combining radical scavenger, FT-IR experiments and theoretical analysis, proving that surface oxygen vacancies and Lewis acid sites play important roles in the high-efficiency ODS reaction with Ni-MoO3 catalyst.

Structure and solvation of confined water and water-ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis.

Abstract: Aqueous-phase reactions within microporous Bronsted acids occur at active centers comprised of water-reactant-clustered hydronium ions, solvated within extended hydrogen-bonded water networks that tend to stabilize reactive intermediates and transition states differently The effects of these diverse clustered and networked structures were disentangled here by measuring turnover rates of gas-phase ethanol dehydration to diethyl ether (DEE) on H-form zeolites as water pressure was increased to the point of intrapore condensation, causing protons to become solvated in larger clusters that subsequently become solvated by extended hydrogen-bonded water networks, according to in situ IR spectra Measured first-order rate constants in ethanol quantify the stability of SN2 transition states that eliminate DEE relative to (C2H5OH)(H+)(H2O)n clusters of increasing molecularity, whose structures were respectively determined using metadynamics and ab initio molecular dynamics simulations At low water pressures (2–10 kPa H2O), rate inhibition by water (−1 reaction order) reflects the need to displace one water by ethanol in the cluster en route to the DEE-formation transition state, which resides at the periphery of water–ethanol clusters At higher water pressures (10–75 kPa H2O), water–ethanol clusters reach their maximum stable size ((C2H5OH)(H+)(H2O)4–5), and water begins to form extended hydrogen-bonded networks; concomitantly, rate inhibition by water (up to −3 reaction order) becomes stronger than expected from the molecularity of the reaction, reflecting the more extensive disruption of hydrogen bonds at DEE-formation transition states that contain an additional solvated non-polar ethyl group compared to the relevant reactant cluster, as described by non-ideal thermodynamic formalisms of reaction rates Microporous voids of different hydrophilic binding site density (Beta; varying H+ and Si–OH density) and different size and shape (Beta, MFI, TON, CHA, AEI, FAU), influence the relative extents to which intermediates and transition states disrupt their confined water networks, which manifest as different kinetic orders of inhibition at high water pressures The confinement of water within sub-nanometer spaces influences the structures and dynamics of the complexes and extended networks formed, and in turn their ability to accommodate the evolution in polarity and hydrogen-bonding capacity as reactive intermediates become transition states in Bronsted acid-catalyzed reactions

Kinetic Analysis of 4-Nitrophenol Reduction by "Water-Soluble" Palladium Nanoparticles.

Abstract: The most important model catalytic reaction to test the catalytic activity of metal nanoparticles is the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride as it can be precisely monitored by UV-vis spectroscopy with high accuracy. This work presents the catalytic reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Amp) in the presence of Pd nanoparticles and sodium borohydride as reductants in water. We first evaluate the kinetics using classical pseudo first-order kinetics. We report the effects of different initial 4-Nip and NaBH4 concentrations, reaction temperatures, and mass of Pd nanoparticles used for catalytic reduction. The thermodynamic parameters (activation energy, enthalpy, and entropy) were also determined. Results show that the kinetics are highly dependent on the reactant ratio and that pseudo first-order simplification is not always fit to describe the kinetics of the reaction. Assuming that all steps of this reaction proceed only on the surface of Pd nanoparticles, we applied a Langmuir-Hinshelwood model to describe the kinetics of the reaction. Experimental data of the decay rate of 4-nitrophenol were successfully fitted to the theoretical values obtained from the Langmuir-Hinshelwood model and all thermodynamic parameters, the true rate constant k, as well as the adsorption constants of 4-Nip, and BH4- (K4-Nip and KBH4-) were determined for each temperature.

Structural and photochemical properties of Fe-doped ZrO2 and their application as photocatalysts with TiO2 for chromate reduction

Abstract: In this work, we are interested by the photocatalytic activity under sunlight irradiation of Fe-doped ZrO2 prepared by co-precipitation. The structural and photophysical properties of the catalysts have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), BET method and UV–Vis diffuse reflectance. The results show that Fe-doped ZrO2 exhibits higher photocatalytic activity than that of both ZrO2 and commercial TiO2 P25. The best photocatalytic activity is obtained with the 50 mol% Fe-content calcined at 600 °C/ 3 h. A total photoreduction of Cr(VI) is achieved within 90 min under solar light with the new hetero-junction Fe-ZrO2/TiO2. The kinetic of Cr(VI) photoreduction is well described by the Langmuir-Hinshelwood (L H) model which obeys to pseudo-first-order kinetics with a rate constant of 0.031 min−1. On the basis of the energy band positions of the hetero-junction Fe-ZrO2/TiO2, the detailed reaction mechanism of reduction of inorganic pollutant has been discussed.

Sono-photo activation of percarbonate for the degradation of organic dye: The effect of water matrix and identification of by-products

Abstract: Several efforts have been conducted to reach an efficient and safe process for degradation of organic pollutants in water environments. Percarbonate (PC) or solid hydrogen peroxide has gained great attention as an alternative for hydrogen peroxide in advanced oxidation processes. In current work, sono-photo activation method was used to PC activation for degradation of Acid Orange 7 (AO7). Ultrasound (US) and ultraviolet light emitted diode (UVC-LEDs) were prosperous in the generation of free radicals in presence of PC. Under optimum conditions (pH = 6.0, PC = 1.5 mM, 100 W US power and 90 reaction time), 93% of AO7 was eliminated while its rate constant was 0.0315 min−1. There was a promotional effect for chloride and nitrate ions at high concentration in PC/US/UVC-LEDs process. Nitrate ions photolysis and sonolysis could degrade AO7 indicating that hydroxyl radical are generated in absent of PC. A major role for hydroxyl radical and minor role for superoxide radical were determined by trapping experiments. PC/US/UVC-LEDs was also applied for an actual matrix which was not successful as well as synthetic solution. By-products of AO7 degradation was identified and proposed reaction was illustrated. This article conclusively states that PC/US/UVC-LEDs can be a new method for the degradation of organic pollutants in water.

Facile synthesis of Bi2WO6/C3N4/Ti3C2 composite as Z-scheme photocatalyst for efficient ciprofloxacin degradation and H2 production

Abstract: In this study, a novel 2D/2D/2D Bi2WO6/C3N4/Ti3C2 composite photocatalyst is synthesized through a one-step hydrothermal method. The ternary photocatalyst exhibits remarkable CIP photodegradation activity with an apparent rate constant of 0.058 min−1 (4.78 times the rate of Bi2WO6), and the photocatalytic H2 production rate reaches 54.4 μmol/g/h. The H2 evolution with simultaneous CIP degradation can also be achieved via prepared ternary catalyst, with H2 production rate of 37.8 μmol/g/h. The well-matched band structure favors the formation of Z-scheme heterojunction, which is affirmed by the ESR and radicals scavenger testing results. The mechanism and driving force of suggested Z-scheme heterojunction is discussed based on the investigation of charge effective mass and surface electrostatic potential. Moreover, Ti3C2 co-catalysts can further facilitate charge transfer and improve redox abilities of binary Bi2WO6/C3N4 composites, leading to a faster active radicals generation rate. This work highlights the prospective role of Ti3C2 in construction of Z-scheme photocatalysts.