Green catalysts preparation using supercritical CO2 as an antisolvent.

Using rapid supercritical CO2 antisolvent precipitation, a set of nanoscaled oxide catalyst precursors have been prepared, which were further calcined to give metal oxides, such as CuMnOx(hopcalite), CeC>2, Ti02 and ZnO etc. CuMnOx(hopcalite) was used as catalyst without further modification and other single metal oxides were used as supports for gold catalysts. These materials have been characterized using a full range of techniques: XRD, Raman, FT-IR, BET surface area, SEM, DTA/TGA and XPS. Element distribution in the as-precipitated materials was evaluated by TEM-EDX. The catalytic activity of our products was evaluated by low temperature (0 25 C) CO oxidation. For CuMnOx (hopcalites), an amorphous homogenous precursor mainly composed of acetates has been prepared. Following calcinations, separated copper nanocrystals supported on manganese oxide (Cu/MnOx) has been obtained. The preliminary catalytic data show the intrinsic activity for CO oxidation of the catalyst derived from this precursor is considerably higher than the conventional CuMn204 catalysts prepared by coprecipitation, and also currently available commercial catalysts. The results clearly show that a catalyst with enhanced activity can be prepared without the presence of intimately mixed copper and manganese oxide components. In addition, using mixed solvents, such as water-ethanol and water-DMF, crystalline heterogeneous precursors have been produced. With the addition of more water to the precursor solution, there appears to be a reaction between the metal acetates CO2, and H2O. Therefore, carbonates of the metals are precipitated instead of the acetate composition. Following calcination, less crystalline or even amorphous phase-separated nanostructure final catalysts retain the high surface area, which leads higher catalytic activities than that of the current commercial hopcalite catalysts. Furthermore, using 30% H2O2 as an oxidizer and ethanol as solvent, catalysts have been prepared as well. All the as-prepared catalysts exhibit higher catalytic activities on CO oxidation when compared to those from solvents in the absence of H2O2. Novel nano-polycrystalline Ce02 was produced. When it was used as a support for gold and gold palladium nanoparticles, the catalytic data show that the activity and catalyst lifetime for CO oxidation of a gold catalyst supported on this material is much greater than that for gold supported on regular Ce02 derived from the direct calcination of cerium acetylacetonate. In addition, the Au-Pd catalysts supported on Ce02 prepared using supercritical antisolvent precipitation are amongst the most active catalysts yet reported for the selective oxidation of alcohols and the direct oxidation of hydrogen to hydrogen peroxide. Similarly, T1O2 has been produced by supercritical process. When using it as a support for gold nanoparticles, the activity and stability for CO oxidation of a gold catalyst supported on this material is much greater than that for gold supported on regular Ti02 derived from the direct calcination of titanium oxide acetylacetonate. Finally, ZnO was prepared using supercritical process and then was used as supports for gold particles as well. The catalytic data show that it can give very high activity for CO oxidation