Nanocluster Nucleation, Growth, and Then Agglomeration Kinetic and Mechanistic Studies: A More General, Four-Step Mechanism Involving Double Autocatalysis

The discovery of the four-step, double autocatalytic mechanism by which transition-metal organometallic and metal-salt precursors self-assemble into zerovalent transition-metal nanoclusters under reductive conditions is reported. The prototype system investigated is (1,5-COD)PtCl2 reduction under hydrogen plus 2 equiv of Bu3N and 2 equiv of Proton Sponge (1,8-bis(dimethylamino)naphthalene). The reaction stoichiometry is established by TEM, XPS, NMR, and GLC. A concomitant, fast, cyclohexene hydrogenation reporter reaction is employed to monitor the kinetics of Pt0 product/catalyst formation and agglomeration. After 15 alternative mechanisms were ruled out, a minimalistic (“Ockham's Razor”) mechanism is proposed consisting of four steps:  slow continuous nucleation, A → B (rate constant k1), fast autocatalytic surface growth, A + B → 2B (rate constant k2), bimolecular agglomeration, B + B → C (rate constant k3), and a new, unprecedented autocatalytic agglomeration step between small (B) and larger, bulk-me...