NANOCLUSTER FORMATION SYNTHETIC, KINETIC, AND MECHANISTIC STUDIES - THE DETECTION OF, AND THEN METHODS TO AVOID, HYDROGEN MASS-TRANSFER LIMITATIONS IN THE SYNTHESIS OF POLYOXOANION-STABILIZED AND TETRABUTYLAMMONIUM-STABILIZED, NEAR-MONODISPERSE 40+ -6 ANGSTROM RH(0) NANOCLUSTERS/

Previously we reported P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilize d, 20 +/- 3 Angstrom, Ir(0),(similar to 300) nanoclusters. These nanos copic materials are synthesized from the reduction of [(C4H9)(4)N](5)N a-3[(1,5-COD)M . P2W15Nb3O62] (M = Ir) by H-2 in acetone and are isola ble, highly catalytically active, with an unprecedented catalytic life time in solution. However, an initial attempt to synthesize Rh(0) nano clusters from the analogous M = Ph precursor, and under conditions oth erwise identical to the M = Ir synthesis, led to polydisperse 16 to 11 6 Angstrom Rh(0) nanoclusters. The above results led, in turn, to the discovery that H-2 gas-to-solution mass-transfer limitations (MTL) wer e responsible for the failure of the initial synthesis, an important f inding since H2 is one of the most common reducing agents in syntheses of modern, near-monodisperse transition metal nanoclusters. The findi ng of a hydrogen MTL regime is fortified by stirring-rate-dependence d ata, kinetic data [demonstrating a catalyst nondependent (MTL) regime, and a catalyst-dependent (chemical-reaction rate-limiting) regime], a nd transmission electron microscopy used as a mechanistic probe. The r esults provided evidence for a growth mechanism involving parallel aut ocatalytic surface growth (leading to near-monodisperse nanoclusters) in competition with diffusive agglomeration (leading to polydisperse n anoclusters). The above mechanistic insights were then used, in turn, to design conditions where only the autocatalytic surface-growth pathw ay occurs, conditions which led to the successful synthesis of the des ired near-monodisperse, 40 +/- 6 Rh(0) P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilized nanoclusters. The resultant Rh(0) nanoclusters are o nly the second example of polyoxoanion- and Bu4N+-stabilized transitio n metal nanoclusters.