Spectra of the hydrated electron in pressurized light and heavy water at temperatures up to and beyond the critical temperature are reported, for wavelengths between 0.4 and 1.7 microm. In agreement with previous work, spectra can be approximately represented by a Gaussian function on the low-energy side, and a Lorentzian function on the high-energy side in subcritical water, but deviations from this form are very clear above 200 degrees C. The spectrum shifts strongly to the red as temperature rises. At supercritical temperatures, the spectrum shifts slightly to the red as density decreases, and the Gaussian-Lorentzian form is a very poor description. Application of spectral moment theory allows one to make an estimate of the average size of the electron wave function and of its kinetic energy. It appears that for water densities below about 0.6 g/cc, and down to below 0.1 g/cc, the average radius of gyration for the electron remains constant at around 3.4 angstroms, and its absorption maximum is near 0.9 eV. For higher densities, the electron is squeezed into a smaller cavity and the spectrum is shifted to the blue. The enthalpy and free energy of electron hydration are derived as a function of temperature on the basis of existing equilibrium data and absolute proton hydration energies derived from the cluster-based common point method. In a discussion, we compare the effective "size" of the hydrated electron derived from both methods.

Reaction rates of solvated electrons with oxygen and with sulfur hexafluoride were measured in hydrothermal and supercritical water using transient absorption spectroscopy and electron pulse radiolysis. Under alkaline conditions, the reaction of hydrogen atoms with hydroxide ions to generate solvated electrons was also observed in the presence of the SF6 scavenger. At temperatures below 300 °C, the rate constants for scavenging by O2 or SF6 follow Arrhenius behavior but become increasingly dependent on water density (pressure) at higher temperatures. Above 100 °C, the rate constant for the H reaction with OH- falls well below the numbers extrapolated from the Arrhenius behavior in the one atmosphere liquid. At a fixed temperature above the water critical temperature (380 °C, T/T c=1.01), rate constants for all three reactions reach a distinct minimum near 0.45 g/cm3. We propose an explanation for this behavior in terms of the potential of mean force separating an ion (OH- or (e-)aq) from a hydrophobic species (H, O2, or SF6) in the compressible fluid. The data also reveal an increasing initial yield of atomic hydrogen relative to solvated electrons as water density decreases. The initial yield of H appears to surpass that of solvated electrons when the water density is below 0.6 g/cm3 at 380 °C.

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There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point (374 °C and 22.4 MPa for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While Ni and Ni-based alloys are generally considered important for severe service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking and dealloying, are seen in some environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, some evidence suggests the potential for corrosion control by judicious feed modification. Various alloys were exposed in a SCWO system at 600 °C for 66.2 h. After exposure, samples were coated with a thick outer salt layer and an inner oxide layer. It is considered likely that, at the high supercritical temperature employed during this test, the salt was molten and contained a substantial quantity of gas. The inner oxide layer revealed the presence of numerous defects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys tested, G-30 exhibited the highest corrosion resistance. Experiments in which a C-276 tube was instrumented with thermocouples and exposed to a HCl feed indicate for this simple non-salt-forming influent that there is a strong correlation between temperature and the extent and form of corrosion, with the most pronounced degradation being at high subcritical temperatures. These experiments corroborate previous results from a failure analysis for C-276, suggesting a corrosion maximum in the subcritical region.

The rate constants for the reactions of nitrobenzene with the hydroxyl radical (OH•) and hydrated electron ((e-)aq) in water have been measured from room temperature to 400 °C using electron pulse radiolysis and transient absorption spectroscopy. The diffusion-limited reaction of nitrobenzene with (e-)aq exhibits temperature-insensitive activation energy up to 300 °C, indicating that the activation energy for electron diffusion remains high over this range. The (e-)aq reactivity is explained as a long-range electron transfer, and the results are interpreted in terms of extended Marcus theory and Smoluchowski relationships. At 380 °C, the rate constant has a density dependence similar to that previously reported for other (e-)aq scavenging reactions. The reaction rate of nitrobenzene with OH• is very insensitive to temperature from room temperature up to 300 °C, in agreement with previous studies. Above 300 °C, the rate constant increases as the critical temperature is approached and exceeded. Time-resolved electronic absorption spectra of the nitrobenzene radiolysis transients reveal complex kinetics involving multiple absorbing species.

A high-temperature high-pressure optical cell for general-purpose spectrometers designed for supercritical water experiments Rev. Sci. Instrum. 72, 3605 (2001); 10.1063/1.1389494Supercritical-fluid cell with device of variable optical path length giving fringe-free terahertz spectra Rev. Sci. Instrum. 71, 4061 (2000)The design of a flow cell that is applicable to pulse radiolysis/transient absorption experiments on supercritical water is described. The cell is designed to minimize dead volume and prevent the accumulation of radiolytic products. It is also necessary to minimize emission and absorption of sapphire windows from high energy electron beam irradiation. To obtain an optical throughput of f /4, the inner diameter is 6 mm, and distance between windows is 25 mm. The effective optical path length is 20 mm for irradiation from the side through a thin Hastelloy wall. Belleville spring washers were used to keep a constant force on the 3 mm sapphire windows, which were sealed to the Hastelloy body with copper gaskets. An application of this cell to measurements of solvated electrons in supercritical water is demonstrated.

We present the first ab initio density functional theory study of the oxygen-terminated Cr2O3 (0001) surface within the local spin-density approximation (LSDA). We find that spin plays a critical role for even the most basic properties of Cr2O3 such as the structure and mechanical response of the bulk material. The surface exhibits strong relaxations and changes in electronic and magnetic structure with important implications for the chemical reactivity and unusual spin-dependent catalytic activity of the surface. Unlike the bulk, the outermost chromium bilayer is ferromagnetically ordered, and the surface oxygen layer exhibits appreciable net spin polarization in the opposite sense. Surprisingly, despite this ferrimagnetic order, the chemically important states near the Fermi level exhibit ferromagnetic order and thus favor electronic spin alignment of species interacting with the surface. Finally, we also find a high density of unoccupied electronic surface states available to participate in the chemical reactivity of the surface.

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