Recombination of the hydrated electron at high temperature and pressure in hydrogenated alkaline water

Pulse radiolysis experiments were performed on hydrogenated, alkaline water at high temperatures and pressures to obtain rate constants for the reaction of hydrated electrons with hydrogen atoms (H• + e-aq → H2 + OH-, reaction 1) and the bimolecular reaction of two hydrated electrons (e-aq + e-aq → H2 + 2 OH-, reaction 2). Values for the reaction 1 rate constant, k1, were obtained from 100 − 325 °C, and those for the reaction 2 rate constant, k2, were obtained from 100 − 250 °C, both in increments of 25 °C. Both k1 and k2 show non-Arrhenius behavior over the entire temperature range studied. k1 shows a rapid increase with increasing temperature, where k1 = 9.3 × 1010 M-1 s-1 at 100 °C and 1.2 × 1012 M-1 s-1 at 325 °C. This behavior is interpreted in terms of a long-range electron-transfer model, and we conclude that e-aq diffusion has a very high activation energy above 150 °C. The behavior of k2 is similar to that previously reported, reaching a maximum value of 5.9 × 1010 M-1 s-1 at 150 °C in the presence of 1.5 × 10-3 m hydroxide. At higher temperatures, the value of k2 decreases rapidly and above 250 °C is too small to measure reliably. We suggest that reaction 2 is a two-step reaction, where the first step is a proton transfer stimulated by the proximity of two hydrated electrons, followed immediately by reaction 1.