D/H fractionation in the H-2-H2O system at supercritical water conditions: Compositional and hydrogen bonding effects

A series of experiments has been conducted in the H-2-D-2-D2O-H2O-Ti-TiO2 system at temperatures ranging from 300 to 800 degrees C and pressures between similar to 0.3 and 1.3 GPa in a hydrothermal diamond anvil cell, utilizing Raman spectroscopy as a quantitative tool to explore the relative distribution of hydrogen and deuterium isotopologues of the H-2 and H2O in supercritical fluids. In detail, H2O-D2O solutions (1: 1) were reacted with Ti metal (3-9 h) in the diamond cell, leading to formation of H-2, D-2, HD, and HDO species through Ti oxidation and H-D isotope exchange reactions. Experimental results obtained in situ and at ambient conditions on quenched samples indicate significant differences from the theoretical estimates of the equilibrium thermodynamic properties of the H-D exchange reactions. In fact, the estimated enthalpy for the H-2(aq)-D-2(aq) disproportionation reaction (Delta H-rxn) is about -3.4 kcal/mol, which differs greatly from the +0.16 kcal/mol predicted for the exchange reaction in the gas phase by statistical mechanics models.