H. G. Niessen, A. Eichhorn, K. Woelk, and J. Bargon, J. Mol. Catal. A. 182-183, 463-470 (2002).

• As an intermediate form between homogeneous and heterogeneous catalysis, catalysts based on transition metal colloids have drawn a lot of attention in recent years. Mating advantages from different concepts of catalysis, they turn out to be highly active, selective, and, in addition, easily separated from other reaction components.

• Even though colloidal catalysts are widely used in conventional solvents, hardly anything is known about their reactivity in supercritical fluids such as supercritical carbon dioxide (scCO₂). Furthermore, scCO₂ is an especially attractive, non-toxic, and environmentally benign solvent for chemical reactions.

• When hydrogenations were conducted using polymer-supported colloidal Pd nanoparticles as catalysts in scCO₂, we found turnover frequencies (TOFs) as high as 4 000 000 h^-1 even at a reasonably low hydrogen pressures of 15 bar and temperatures of 50 °C. To our surprise, their reactivity turned out to be much higher than those of most other catalysts reported in the literature.

• The kinetics of their catalytic reactions in supercritical fluids has been investigated using in situ NMR in combination with a toroid cavity autoclave (TCA).

K. Woelk, October 1, 2002