The building of a Catal.ytic metal particle one atom at a time: solvated metal atom dispersed Catalysts

Abstract

Free metal atoms and free metal clusters have profoundly different reactivities compared with clean metal surfaces or ligand-stabilized atoms and clusters. As evidence of this, Al atoms react with methane at 10 K and bare Ni clusters react with pentane at 140 K. By dispering metal atoms in excess organic solvent at low temperature, solvated metal atoms can be prepared and used as a source of metal particles for Catal.yst preparation. Both unsupported metal powders and supported metal Catal.ysts can be prepared, and each exhibits very unusual physical and Catal.ytic properties. The dispersing solvent affects the Catal.yst's activity/selectivity. Likewise, the Catal.yst support has an effect on the Catal.yst properties, although in unexpected ways. For example, small electronic support effects were realized when Ni/MgO, Ni/Al2O3, Ni/SiO2 and Ni/C solvated metal atom dispersed (SMAD) Catal.ysts were employed for a series of test reactions. However, the support does affect the metal particle nucleation and deposition differently for Co vs. Ni supported systems.Important conclusions are, first, that the surface acidic sites affect the decomposition of the organic solvation sphere around the metal atom/cluster and thus affect the deposition/nucleation onto the support surface, and second, that the metal clusters contain large amounts of carbonaceous species (C, CH, CH2, etc.) and are probably attached to the Catal.yst supports through carbonaceous linkages. These carbonaceous fragments can be minimized by using a solution metal atom technique for metal deposition. The solution method yields more highly dispersed, more active Catal.ysts than the normal codeposition method, but these Catal.ysts are shorter lived.