Effect of water density on hydrogen peroxide dissociation in supercriticalwater. 1. Reaction equilibrium

Recent experiments showed that the rate of dissociation of H2O2 in supercri tical water (SCW) is density dependent and faster than its high-pressure li mit rate in the gas phase. These observations suggest that water molecules play a role in this reaction in SCW. We performed density functional theory (DFT) calculations and molecular dynamics simulations to investigate the r ole of water in H2O2 dissociation. We generated the potential energy surfac e for H2O2-water and OH-water complexes by DFT calculations to determine th e parameters in an analytical intermolecular potential model, which was sub sequently employed in the molecular dynamics simulations. These simulations were performed at different water densities. They provided the structural properties (pair correlation functions) of dilute mixtures of H2O2 and OH i n SCW, from which we were able to calculate the number of excess solvent mo lecules and partial molar volumes for each solute. We used the partial mola r volumes for H2O2 and OH to calculate the reaction volume for H2O2 = 2OH a nd thereby determined the density dependence of the equilibrium constant fo r this reaction. The results show that at the reduced temperature of T-r = 1.15 (695 K) the equilibrium constant for H2O2 dissociation is a function o f the water density. The mean value of the equilibrium constant changes by less than 5% between 0.25 < rho(r) < 1, but it decreases by an order of mag nitude between 1 < rho(r) < 2.75. Knowing the density dependence of the equ ilibrium constant for this reaction will allow more accurate mechanism-base d models of supercritical water oxidation chemistry to be developed. The co mputational approach applied herein for H2O2 dissociation is general and ca n be profitably employed to discern the density dependence of the equilibri um constant of any elementary reaction in SCW. There is currently no experi mental approach that will provide this information for reactions involving free radicals.