Water radiolysis temperature dependent rate constants

It is known that more than 30 reactions are needed to reproduce the radiation-induced reactions occurring in pure water. Intensive measurements with a pulse radiolysis method have been done at elevated temperature up to 300°C [25 2], and the temperature dependence of some reactions does not exhibit a straight line but a curved one in Arrhenius plot. These examples are the reactions of the hydrated electron with N2O, NOJ, NO2, phenol, Se04, 8203 , and Mn [33,35], and two examples, egq + NOJ and ejq -i- NOJ, are shown in Fig. 2. The rate constant for the reaction of hydrated electron with NOJ is near diffusion-controlled reaction at room temperature and is increasing with increasing temperature. Above 100°C, the rate does not increase and reaches the maximum at 150°C, and then decreases. Therefore the curve is concave upward in Arrhenius plot. 

As for the rate constants above 300 °C, little experimental results have been reported. Again, not only temperature, but also pressure dependence was reported. One of the important issues is the decrease of the dielectric constant of water in supercritical water, where it is less than 10 and much lower than the value of 79 at room temperature. It was pointed out that the Coulombic interaction becomes important and the radiolysis of supercritical water resembles that of the organic liquids with low dielectric constant [88]. In addition, it should be noted that the solubility of the solute is quite low and ion pairing would have a significant role [89-91], which reflects the difficulties for the sample preparation in the actual experiment. 

Measmement of the rate constants for the reactions listed in Table 1 has been achieved using pulse radiolysis up to 200 °C or 300 C, depending on the apparatus available the pressures needed to prevent water boiling at these temperatures are 20 atmospheres and 100 atmospheres, respectively. Where it has not been possible to make measurements at 300 °C, values of the rate... 

It should be noted that, in the reaction mechanisms that are currently used for modeling the radiolysis of water in subcritical systems,most of the bimolecular reactions are at the diffusion limit. Because the temperature dependence of an activation controlled reaction is normally greater than that of a diffusion controlled reaction, any reaction that is diffusion controlled at subcritical temperatures is almost certainly diffusion controlled at supercritical temperatures and many that are activation controlled at subcritical temperatures will become diffusion controlled at supercritical temperatures. Accordingly, the rate constants for many reactions between radiolytic species in any mechanism adopted for the radiolysis of water in supercritical water might be reasonably estimated. The challenge exists with the unimolecular reactions and those bimolecular reactions whose rates are below the diffusion limit. Nevertheless, the author s opinion is that a good chance exists that an acceptable set of rate constants for a reaction mechanism could be developed for use at supercritical temperatures. 

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