Heterogeneous Catalyst Stability in Hydrothermal Media

Abstract

Catalytic reactions in hydrothermal media are important for the conversion of biomass-derived species to useful chemicals and the destruction of environmental pollutants, however these aqueous solutions are aggressive and can degrade heterogeneous catalysts. This dissertation describes the study of heterogeneous catalyst stability during hydrothermal reactions and the application of thermodynamic modeling to predict dissolution of catalysts and changes in their oxidation states. Agreement between experimental results for catalytic materials (Co, Ni, Pd, Ru, CeO2, TiO2, ZrO2, Mo2C, W2C, MoN, WN) in supercritical water at 400 degrees Celsius and 24-40 MPa and the revised Helgeson-Kirkham-Flowers thermodynamic model showed that the model is a good predictor of the oxidation and dissolution behavior of metal, oxide, carbide and nitride catalytic materials in supercritical water reactor systems. The model was applied to additional hydrothermal temperatures and pressures (150-550 degrees Celsius, 22-50 MPa) and catalytic materials and the results were used to identify relationships between material properties and catalyst solubility. The solubility of metals at 400 degrees Celsius and 50 MPa correlated strongly with electronegativity and the solubility of oxides at 400 degrees Celsius and 50 MPa was correlated with cation electronegativity, ionic-covalent parameter, and polarizing power. These relationships suggest that changes to the composition of the catalyst which alter these properties (e.g., alloying, doping) may improve stability. Catalyst stability is also affected by the pH and oxygen fugacity of hydrothermal solutions, which are controlled by the reactants and products present in solution. The pH and oxygen fugacity can also change during a reaction as various solutes are generated or consumed and their concentrations change. Oxygen fugacity-pH diagrams were constructed using the revised Helgeson-Kirkham-Flowers thermodynamic model and were used to predict catalyst stability as a function of temperature, pressure, pH, and oxygen fugacity. Equilibrium calculations were also performed on different concentrations of solutes that are common in hydrothermal reactions (e.g., CO2, CH4, NH3, formic acid) to predict the pH and oxygen fugacity of the overall solution. Using these tools, one can learn how best to engineer the composition of the catalyst and the hydrothermal medium to ensure catalyst stability. As an illustration, Pt/TiO2 was predicted to be hydrothermally stable at the temperatures, pressures, and solution compositions typical of hydrodenitrogenation reactions in supercritical water. Flow experiments in aqueous solutions of formic acid and ammonia at 50 MPa and 380 and 500 degrees Celsius resulted in little (≤ 2 wt%) or no dissolution of Pt and TiO2 and verified the predicted stability. While Pt/TiO2 did not undergo dissolution or changes in oxidation state, flow experiments in aqueous solutions of formic acid and either propylamine or pyridine at 420 degrees Celsius and 30 MPa showed that Pt/TiO2 was only active for denitrogenation of propylamine.