Structure, Dynamics, and Reactions of Supercritical Water Studied by NMR and Computer Simulation

The structure and properties of supercritical water is investigated in relation to the energy and environmental issues of the 21 century. Molecular interpretations are given to the temperature dependence of the hydrogen-bonding structure, dynamics, and chemical reactions of supercritical water. The three-dimensional network structure, characteristic of ambient water, is broken down in hot expanded water at temperatures higher than ~200 ̊C and densities lower than ~0.9 g cm. The number of hydrogen bonds per molecule has been determined by the NMR method combined with computer simulation; it decreases from ~4 for ambient water to 1-2 for supercritical water at 400 ̊C and the critical density (~0.32 g cm). The NMR rotational correlation time ( 2R) for supercritical water at the medium densities is in the range of 50-70 fs, two orders of magnitude smaller than the ambient value. New hydrothermal organic reactions without catalyst are developed for green chemistry. In hot water, carbon monoxide, carbon dioxide, and hydrogen are linked through formic acid. Formic acid can serve as a chemical tank for the compact transportation and storage in the clean hydrogen fuel technology.