Catalyst Oxidation and Dissolution in Supercritical Water

We use thermodynamic models to predict catalyst oxidation and dissolution in supercritical water (SCW) and use experiments to assess the viability of the models for practical SCW reaction systems and provide relative rates for these mechanisms. We examined the oxidation and dissolution of noble and transition metals, metal oxide catalyst supports, and transition metal carbides and nitrides under SCW conditions. The materials were tested in batch reactors at 400 °C for 60 min, and the SCW density was varied from 0 to 0.5 g/mL to observe the influence of the solvent properties on stability. Oxidation and dissolution were determined by comparing the initial catalyst composition and structure with those of the catalysts recovered from the reactors after exposure to the SCW environment. The gas-phase recovered from the reactors was analyzed for H₂ produced from oxidation. The aqueous phase was analyzed for metals from dissolution. The ΔG_rxn for oxidation and the solubility of the catalysts in SCW at the experimental conditions were calculated for comparison. Overall, the thermodynamic calculations agreed with the experimentally observed oxidation and dissolution. We conclude that thermodynamic modeling is an effective tool for efficiently screening the stability of catalytic materials in SCW and for estimating long-term hydrothermal catalyst stability.