Water Density Effects on Homogeneous Water-Gas Shift Reaction Kinetics

Abstract

Several years ago, using ab initio calculations, Melius et al. examined an interesting pathway for the homogeneous gas-phase water-gas shift reaction. They predicted that an enhancement in the rate of this reaction occurs at high water density due to changes in the generation of formic acid as an intermediate. To investigate this possibility, we have examined the conversion of CO and H2O to CO2 and H2 in the absence of a catalyst in supercritical water at conditions from 410 to 520 °C and 2.0−60 MPa. In these experiments, Raman spectroscopy produced in situ, real-time measurements of reaction rates in an optically accessible, constant-volume reactor. The most rapid homogeneous rate measured, at 59 MPa and 450 °C, corresponds to an effective first-order reaction rate constant of 0.0033 s-1. The data show that the measured reaction rates increase far faster than linearly as water density is raised above 0.35 g/cm3, and the pressure dependence of the rate constant indicates that the transition state is chara...