Thermodynamic Analysis of Catalyst Stability in Hydrothermal Reaction Media

Hydrothermal solutions are important media for the conversion of biomass-derived species to useful chemicals and the destruction of environmental pollutants. These solutions are aggressive and can degrade heterogeneous catalysts. This article describes a framework for understanding the hydrothermal stability of heterogeneous catalyst materials with respect to oxidation and dissolution. We applied the revised Helgeson-Kirkham-Flowers thermodynamic equation of state to determine the oxidation states and solubilities of metals and oxides in water at 150-550 °C and 22-50 MPa. Design criteria for catalyst compositions were determined through correlations between metal solubility and electronegativity and between oxide solubility and cation electronegativity, ionic-covalent parameter, and polarizing power. Design criteria for aqueous solution compositions were determined by constructing oxygen fugacity-pH diagrams, which illustrate material phase changes in response to changes in pH and the oxidative or reductive strength of the solution. Combined, these criteria facilitate design of stable catalytic materials for hydrothermal reactions.