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 k 1), fast autocatalytic surface growth, A + B → 2B (rate constant k 2), bimolecular agglomeration, B + B → C (rate constant k 3), and a new, unprecedented autocatalytic agglomeration step between small (B) and larger, bulk-metal-like (C) particles, B + C → 1.5C (rate constant k 4). The results provide the following: a rare case of a mechanism with two autocatalytic steps in the same reaction scheme (“double autocatalysis”); the most general mechanism to date by which transition-metal nanoparticles nucleate, grow, and agglomerate to bulk metal under reductive conditions; probably the best understood self-assembly mechanism to date for such a large system in which the extensive kinetic studies required for reliable mechanistic deduction also exist; kinetic curves that can have step-function-like shapes; and insights for the synthesis of nanoclusters vs bulk-metal films (notably that higher temperature and lower concentrations favor nanocluster formation, while the opposite conditions favor bulk-metal production). A summary section details the main conclusions plus caveats and remaining questions/future research goals.