(782g) Physical Properties Controlling Heterogeneous Catalyst Stability in Supercritical Water

Catalysis in supercritical water (SCW) offers a potentially “greener” approach than many alternatives for carrying out reactions such as gasification of wet biomass, biofuel production, desulfurization of heavy crude oil, and destruction of environmental pollutants. Near the critical point of H₂O (374°C, 22.1 MPa), its ion product and dielectric constant are highly tunable with temperature and density. These properties can strongly influence reaction rates and selectivities [1]. By coupling the tunable solvent properties of SCW with a heterogeneous catalyst, we aim to expand the design space for engineering novel reaction pathways for energy production and waste destruction. A key challenge for applying heterogeneous catalysts in SCW is the limited understanding of catalyst stability across conditions that correspond with the significant changes in the solvent properties [2]. While thermodynamic models can predict process conditions where metal oxidation and dissolution would be problematic [3-4], experiments are needed to validate the models and provide rates for the instability mechanisms.

We examined the stabilities of noble and transition metals, metal oxide catalyst supports, and transition metal carbides and nitrides. The catalysts were tested in both batch and flow conditions and the SCW density was varied from 0-0.5g/mL to observe the influence of the solvent properties on stability. The composition and structure of the fresh catalyst samples were characterized using x-ray diffraction, N₂ physisorption, and electron microscopy techniques and then characterized again after exposure to the SCW environment. The aqueous and gaseous compositions exiting the flow reactor were analyzed using inductively coupled plasma atomic emission spectroscopy and gas chromatography, respectively. Overall, the experimentally observed hydrothermal stabilities agreed with trends predicted in the thermodynamic models. In addition, we found that the oxidative or reductive strength of the reaction substrate will influence the dissolution rate and maximum solubility of the catalyst. These results provide design criteria for creating high-stability catalysts for hydrothermal applications.

References

1. N. Akiya, P.E. Savage, Chem. Rev. 102 (2002) 2725-2750.

2. T.M. Yeh, J.G. Dickinson, A. Franck, S. Linic, L.T. Thompson, P.E. Savage, Technol. Biotechnol. 88 (2013) 12-24.

3. E.L. Shock and H.C. Helgeson, Geochimica et Cosmochimica Acta 52 (1988) 2009-2036.

4. K. Sue, T. Adschiri, K. Arai, Ind. Eng. Chem. Res. 41 (2002) 3298-3306.