Estimation of rate constants for near-diffusion-controlled reactions in water at high temperatures

Abstract

Rateconstants measured over the temperature range 2Cb2OO"C are reported for the following reactions: (a) reaction of the hydrated electron with oxygen, the proton, hydrogen peroxide, nitrate, nitrite, nitrobenzene and methyl viologen ; (b) reaction of the hydroxyl radical with another hydroxyl radical and ferrocyanide; (c) reaction of the hydrogen atom with permanganate and oxygen. To evaluate methods of estimating rate constants at high temperatures these rate constants and others in the literature have been fitted to the following equation: kobs = kdiff/(l + kdiff/kreact)where kobs is the measured rate constant for the bimolecular reaction in solution, kdiff is the encounter rate constant of the two reacting species, and k,,,,, is the rate constant that would be measured if diffusion of the species was not rate influencing. With the exception of reactions of the hydrated electron with nitrate and nitrite ions and nitrous oxide, good fits have been obtained to the above equation, and the results demonstrate that few, if any, of the reactions which are pertinent to water radiolysis are truly diffusion controlled at elevated temperatures.

Photo-catalytic oxidation of cyclohexane over TiO2: a novel interpretation of temperature dependent performance

“…From pulse radiolysis it was found that for the reaction of organic molecules with OH radicals the typical activation energy is in the range of 5-13 kJ mol À1 ; 28 similarly, an activation energy for the formation of the superoxide anion was estimated as 11 kJ mol À1 . 29 It is striking that our study, based on activity measurements using ATR-FTIR analysis, provided an estimate of 0 kJ mol À1 for the apparent activation energy for the selective cyclohexane photo-oxidation to cyclohexanone ( Table 3), implying that this reaction is not thermally activated. Herrmann et al estimated an apparent activation energy of 10.5 kJ mol À1 between 23-55 1C for the same reaction.…”

Solvent effects on the reactivity of solvated electrons with ions in tert-butanol/water mixtures

“…Therefore, for the reactions of e; with NO;, and H: in propanols (21) and butanols (13, 14, 21b), fixed values of Rr were used for f factor calculation in eqs. [4] and [5], namely, R,(e; + NO;,) = 1.5 nm, R,(e; + H: ) = R,(e; + NHd,,) = 1.0 nm. Then K, the probability that reaction occurs during one encounter, was inserted into [3] (21): [lo]…”

“…The reactivity of solvated electrons e; depends mainly on the natures of the coreactant and solvent (1)(2)(3)(4)(5). The study of rates of reaction of e; with solutes in different solvents provides useful information about solvent effects, which helps us understand the behaviour of electrons and the nature of solvents.…”

In situmeasurement of corrosion of type 316L stainless steel in 553 K pure water via the electrical resistance of a thin wire

“…Corrosion of stainless steel in pure water at 553 K is thought to be controlled by a parabolic rate law or a logarithmic rate law described as Equation (19) or Equation (20), respectively [7,9]. Consequently, the corrosion depth data were fitted with the equations:…”

“…The shape of test specimens will affect the diffusion amount of hydrogen peroxide or hydrogen to the metal surfaces and the diffusion amount of metal ion from metal surfaces. Diffusion layer thickness (d) was calculated to be 169 mm from Equation (13) using the mass transfer coefficient of straight pipe (K m , Equation (14)), diffusion coefficient of hydrogen peroxide (D), equivalent diameter of flow path (d), Reynolds number (Re) and Schmidt number (Sc) [15,19,20]. Diffusion coefficient of hydrogen peroxide (D) at 553 K was obtained from the Einstein-Stokes equation (Equation (15)) using viscosity of water (Z), Boltzmann constant (k b ) and radius of chemical species (r C ) [15].…”