Preparation and catalytic properties of NR 4 + ‐stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR   4 + ] with the reducing agent [BEt 3 H − ] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR 4 X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl) 4 Br to Pd(ac) 2 in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR   4 + ‐stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl) 4 ] 2 PdCl 2 Br 2 with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10 3 catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10 3 catalytic cycles.